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Anaerobic digestion of alkaline bleaching wastewater from a kraft pulp and paper mill using UASB technique a
b
a
ab
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Madeleine Larsson , Xu-Bin Truong , Annika Björn , Jörgen Ejlertsson , David Bastviken , Bo a
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H. Svensson & Anna Karlsson a
Department of Thematic Studies, Environmental Change, Linköping University, 581 83 Linköping, Sweden b
Scandinavian Biogas Fuels AB, Holländargatan 21A, 111 60 Stockholm, Sweden Accepted author version posted online: 02 Dec 2014.Published online: 05 Mar 2015.
Click for updates To cite this article: Madeleine Larsson, Xu-Bin Truong, Annika Björn, Jörgen Ejlertsson, David Bastviken, Bo H. Svensson & Anna Karlsson (2015) Anaerobic digestion of alkaline bleaching wastewater from a kraft pulp and paper mill using UASB technique, Environmental Technology, 36:12, 1489-1498, DOI: 10.1080/09593330.2014.994042 To link to this article: http://dx.doi.org/10.1080/09593330.2014.994042
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Environmental Technology, 2015 Vol. 36, No. 12, 1489–1498, http://dx.doi.org/10.1080/09593330.2014.994042
Anaerobic digestion of alkaline bleaching wastewater from a kraft pulp and paper mill using UASB technique Madeleine Larssona , Xu-Bin Truongb , Annika Björna , Jörgen Ejlertssona,b , David Bastvikena , Bo H. Svenssona and Anna Karlssonb∗ a Department
of Thematic Studies, Environmental Change, Linköping University, 581 83 Linköping, Sweden; b Scandinavian Biogas Fuels AB, Holländargatan 21A, 111 60 Stockholm, Sweden
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(Received 18 July 2014; accepted 27 November 2014 ) Anaerobic digestion of alkaline kraft elemental chlorine-free bleaching wastewater in two mesophilic, lab-scale upflow anaerobic sludge bed reactors resulted in significantly higher biogas production (250 ± 50 vs. 120 ± 30 NmL g TOC−1 IN ) and reduction of filtered total organic carbon (fTOC) (60 ± 5 vs. 43 ± 6%) for wastewater from processing of hardwood (HW) compared with softwood (SW). In all cases, the gas production was likely underestimated due to poor gas separation in the reactors. Despite changes in wastewater characteristics, a stable anaerobic process was maintained with hydraulic retention times (HRTs) between 7 and 14 h. Lowering the HRT (from 13.5 to 8.5 h) did not significantly affect the process, and the stable performance at 8.5 h leaves room for further decreases in HRT. The results show that this type of wastewater is suitable for a full-scale implementation, but the difference in methane potential between SW and HW is important to consider both regarding process dimensioning and biogas yield optimization. Keywords: anaerobic digestion; UASB; alkaline kraft ECF bleaching wastewater; hardwood; softwood
1. Introduction The pulp and paper industry is a high energydemanding, water-utilizing and wood resource-consuming industry.[1,2] The large water consumption implies a large production of wastewater, which is highly polluted with organic material from the wood and chemicals used in the processes.[1,3] Instead of treating these wastewaters in the activated sludge processes, which often is the case today, the organic material can be considered a resource possible to explore in anaerobic digestion (AD). Implementation of anaerobic wastewater treatment and biogas production as a part of the conventional wastewater treatment at a mill would reduce its energy usage, sludge production and nutrient consumption.[1,4–7] There is a large potential for biogas production within the pulp and paper industry, but it should be noted that the type of process, raw materials and wastewater effluents will largely affect the outcome.[8] AD has been implemented successfully in full scale for bleached/unbleached thermomechanical pulp,[9] for chemical thermo-mechanical pulp effluents, as well as for neutral sulphite semi-chemical effluents and kraft/sulphite mill condensates.[6,10,11] However, the majority of full-scale installations are found among mills for recycled paper and board (130 out of 203;[6]). More than 80% of the installations in the pulp and paper industry are high-rate anaerobic sludge bed
systems. Of these, more than half are upflow anaerobic sludge bed (UASB) reactors.[6] This technique is suitable for large flows with a high concentration of dissolved organic material and a low concentration of suspended solids (SS).[12] Kraft pulping is the main method for pulp production in Sweden [13] and kraft pulp and paper mill (PPM) effluents represent a large biogas production potential if combined with efficient AD techniques. These streams are, however, regarded as challenging in terms of substrate quality for methanogenesis and due to the large effluent flow rates. The challenges include, for example, spent black liquor, wood extractives, lignin- and lignin-derived compounds together with the high sulphate-to-carbon ratio.[14–18] In addition, kraft bleaching wastewaters are often high in concentration of chlorinated hydrocarbons, mainly found in the acidic bleaching effluents. These kinds of compounds have been shown to interact negatively with the AD process.[3,14–16] However, Ekstrand et al. [8] demonstrated high methane yields for alkaline elemental chlorinefree (ECF) bleaching wastewaters in batch tests, especially when hardwood (HW) is used as raw material in the pulping process (330 and 360 NmL CH4 g total organic carbon (TOC)−1 corresponding to up to 38 ± 2% of the theoretical methane potential for HW and 0–130 NmL CH4 g TOC−1 corresponding to 14% of the theoretical methane
*Corresponding author. Emails: [email protected], [email protected] © 2014 Taylor & Francis
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potential for softwood (SW;[8]). These wastewater streams are characterized by relatively high concentrations of dissolved organic material (170–730 mg TOC L−1 ; according to [8]) normally combined with levels of SS below those 500 mg L−1 often recommended for UASB treatment. These characteristics thus make the streams suitable for UASB technique and opens up for AD at low hydraulic retention times (HRTs), hence minimizing the size of the installation, and maintaining high process efficiency, which is important for the overall economy of a full-scale implementation. A UASB-installation on a bleaching stream can be of advantage compared with an application on the mill’s total effluent, as the bleaching stream normally holds a higher concentration of organic material, meaning that less water volumes need to be treated to obtain the same effect. A rough estimation of the Swedish kraft PPMs shows that the alkaline ECF bleaching wastewaters holds 50–60% of the mills total organic load and more than 75% of their total methane potential (personal communication, Fredrik Nilsson, Pöyry AB). So far, only a few studies indicate that continuous highrate, AD treatment of kraft bleaching effluents is feasible, that is, removing large fractions of the organic material at low HRTs. However, most studies focused on wastewater treatment and toxicity reduction, and not on the production of biogas. Buzzini et al. [19] treated a synthetic wastewater, simulating the composite effluent from a mill producing bleached and unbleached eucalyptus pulp, in two UASBs with chemical oxygen demand (COD) removals of 76 ± 5% vs. 81 ± 2%. Eucalyptus was also the raw material used in a study by Chaparro and Pires [20] evaluating the biodegradability for a mixture of alkaline and acid bleaching wastewaters in a horizontally packed-bed anaerobic reactor. The AD in this case resulted in a COD removal of 52–55%. Lin et al. [21] used a packed-bed AD column to compare treatability of different mixtures of wastewaters (e.g. alkaline and acidic bleaching wastewater) generated from HW pulp (mainly oak) production and obtained a COD removal of 41–57%. The aim of the present study was to investigate the possibilities of obtaining a stable and high-performing biogas production when treating alkaline kraft ECF bleaching wastewaters with UASB technique. This included investigations of the inherent variations in quality of the bleaching wastewater due to frequent shifts between SW and HW as raw material for pulp production as well as other process variations/disturbances at the mill. We also wanted to obtain a process with the lowest possible HRT to facilitate treatment of large flows of wastewater during a short period of time. All tests were performed in lab scale. 2. Material and methods 2.1. Experimental set-up and sampling of wastewater Two identical 4-L UASB reactors (Figure 1; custom made by Humi-Glas, Södra Sandby, Sweden), designated L1 and
Figure 1. Schematic overview of the UASB reactor set-up. The wastewater is pumped into the reactor at the bottom and the liquid is distributed by the help of glass marbles. The wastewater passes through the bed of granules (shaded area) and exits at the top of the reactor at the gas–liquid–solid separator (GLS. SEP.) To create a fluidized bed, a high upflow velocity is generated by recirculating the wastewater by an internal circulation (IC) pump. The UASB reactor is heated to 35 °C by a water jacket connected to a water bath.
L2, were operated at 35 °C for 106 and 144 days, respectively. The two reactors were inoculated with granules from a pilot-scale UASB reactor treating clarified sewage wastewater. The fluidized granular bed volume was 1.5–2 L and an internal circulation (IC) loop was used to fluidize the granular bed. A gas–liquid–solid phase separator was installed at the top of the reactor (cf. Figure 1). A volume of 150–200 L alkaline bleaching wastewater (EOP; alkaline extraction (E) in the presence of oxygen (O) and hydrogen peroxide (P)) from the ECF bleaching sequence D (EOP) D P (where D indicates chlorine dioxide) at a kraft mill was collected and transported (1-hour drive) to the laboratory every week (batches 1–22). The wastewater samples were stored at + 4 °C for at most 15 days before used as substrate in the UASB reactors. The wastewater samples covered normal process shifts between SW and HW used as raw material for the pulp production. Upon arrival at the laboratory, each batch of wastewater was analysed for total and filtered COD, TOC and SO2− 4
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Wastewater characteristics.
SW (n = 17) HW (n = 5)
TOC (mg L−1 )
fTOC (mg L−1 )
SO2− 4 (mg L−1 )
fSO2− 4 (mg L−1 )
SS (mg L−1 )
VFA (mM)
640 ± 80 650 ± 70
630 ± 80 610 ± 80
260 ± 40 180 ± 50
250 ± 40 170 ± 50
7±2 35 ± 8
< 0.6 < 0.6
Notes: COD values can be obtained by multiplying the TOC value with 2.4 ± 0.1 and 2.6 ± 0.2 for SW and HW, respectively. Analyses made on filtered samples are denoted as fTOC and fSO2− 4 . Data presented are mean values ± standard deviation.
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Table 2. Concentrations (mg L−1 ) of macro- and micro-nutrients in two batches of alkaline bleaching wastewater, one with SW (batch 17) and the other with HW (batch 16).
Nutrient
SW
HW
Cl− 110 110 NH+ 0.18 0.22 4 −N Kjeldahl-N < 2.0 < 2.0 P 0.23 0.26 S 95 70 Na 640 730 K 4.4 4.1 Ca 16 16 Fe 0.18 0.21 Mg 21 5.1 Mn 0.26 0.24 Sb < 0.0010 < 0.0010 Pb 0.00071 0.0022 Cd 0.00055 0.00088 Co < 0.0010 < 0.0010 Cu 0.0040 0.0051 Cr 0.0041 0.0035 Hg < 0.00010 < 0.00010 Mo < 0.0010 0.0011 Ni 0.0011 0.0012 V 0.0028 0.0023 Zn 0.044 0.062 Al 0.18 0.20 Se < 0.0030 < 0.0030 Ag < 0.00050 < 0.00050 W < 1.0 – B 0.051 0.052 Ti (μg L−1 ) < 50 < 51
Error in measurement ( ± %) 15 15 10 10 20 20 20 15 20 20 20 25 35 30 25 15 25 20 40 40 30 15 15 35 20 20 20 20
Note: Concentration and measurement errors were reported by Eurofins Environment Sweden AB.
(analyses of filtered samples are denoted fCOD, fTOC and fSO2− 4 ), SS, volatile fatty acids (VFA) and pH (Table 1). Due to the high pH of the wastewater (11.2 ± 0.6), the pH was adjusted to 7.5 with 1 M HCl directly before being fed to the reactors (approx. 10 L of wastewater adjusted at a time). To get an overview of the amount of chlorinated hydrocarbons in the wastewater, 5 of the 22 batches (SW: batches 7, 17 and 20; HW: batches 12 and 16) were analysed for adsorbable organic halogen (AOX) content. One
batch for each raw material (batches 16 and 17) was analysed for macro- and micro-nutrient content (Table 2) by Eurofins Environment Sweden AB according to Swedish standard methods. During a 22-day start-up phase, the HRT for the two reactors was 12–14 h. The two reactors (L1 and L2) were then run at a HRT of 13.5 h for 10 days (days 23–32) before the HRT in L1 was decreased to 10.5 h and then further to 8.5 h by day 48. The HRT in L2 was maintained at 13.5 h until day 112, when it was decreased successively to 10.5 h, but temporarily brought back to 13.5 h during days 118–124 due to wastewater shortage. The HRT of L2 was lowered to 8.5 h on day 132 and finally to 7 h on day 135. The organic loading rates (OLR) ranged from 0.9 to 2.2 g TOC L−1 day−1 for both L1 and L2 according to the shifts in HRT and wastewater characteristics. The composition of the ingoing wastewaters is given in Table 1. Due to the low nutrient content in the wastewaters (Table 2), macro-nutrients ((NH2 )2 CO and Na2 HPO4 ) and trace elements (CoCl2 , CuCl2 , ZnCl2 , NiCl2 , (NH4 )6 Mo7 O24 , Na2 SeO3 and Na2 WO4 ) were added to the substrate throughout the experiment to enable biomass growth, a well-functioning fluidized bed and an efficient AD. N and P were added at a ratio of COD:N:P 350:5:1. The trace element solution was prepared from two stock solutions; solution A (CoCl2 , CuCl2 , ZnCl2 , NiCl2 and (NH4 )6 Mo7 O24 dissolved in milli-Q water) and solution B (Na2 SeO3 and Na2 WO4 dissolved in 10 mM NaOH and further diluted in milli-Q water). The trace element solution was added directly to the reactor in 24 pulses during one day each week amounting to 20 μmol of each metal. The dosing strategy was chosen to expose the granular bed to a higher concentration compared with when dosing the trace elements continuously with the substrate.[22,23]
2.2.
Evaluation of reactor performance
The performance of the UASB reactors was evaluated by analysing the biogas quantity and its methane content together with the composition of the reactor effluents in terms of fCOD, fTOC, fSO2− 4 , SS, VFA and pH twice a week. Additionally, the effluents were analysed for AOX on days 41, 69, 103, 111 and 125 corresponding to the wastewater batches: 7, 12, 16, 17 and 20. The reactor
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performance and wastewater quality are mainly presented as/in relation to TOC and fTOC, but are recalculated to COD and fCOD when necessary to enable comparisons with other studies. 2.2.1. Analytical procedure The gas production was monitored online by means of gas meters based on the principle of water displacement (MGC-1 V3.0 PMMA; Ritter, Germany) and the methane content of the biogas was determined twice a week by gas chromatography according to Karlsson et al.[24] COD, TOC and SO2− 4 were measured spectrophotometrically using kits from Hach Lange, Germany (LCK014, LCK514; LCK386, LCK387; LCK153, LCK353) according to the manufacturer’s instructions. Prior to analysis of fCOD, fTOC and fSO2− 4 , the samples were filtered (MGA grade, pore size 1.6 μm; Munktell). SS were analysed according to Swedish standard (SS-EN 872:2005; MGA grade, pore size 1.6 μm; Munktell; the standard deviation was at the most 5% for the triplicate samples) and pH was measured using a PHM93 meter (Radiometer, Copenhagen, 166 Denmark). The VFA concentrations (acetate, propionate, butyrate, iso-butyrate, valeriate, iso-valeriate, capronate and iso-capronate) were analysed according to Jonsson and Borén.[25] The amount of AOX was determined for 15 thawed, triplicate samples according to a method modified from standard methods [26] and described by Asplund et al.[27] Prior to analyses, the samples were diluted at 1:100. The method comprised (i) adsorption of the organic compounds in the sample on activated carbon; (ii) removal of inorganic halides from the carbon by ion-exchange by nitrate; (iii) combustion of the carbon in oxygen atmosphere at 1000 °C and (iv) micro-coulometric titration of formed hydrogen halides. The AOX content was determined using an EDAK AOX-analyser (model ECS 3000). All chemicals used were of analytical grade. 2.3. Theoretical CH4 yield per TOC (NmL g−1 ) To calculate the theoretical methane yield, the TOC content was assumed to be 100% carbohydrates. A complete anaerobic degradation of carbohydrates gives a molar ratio of 1:1 for CH4 and CO2 , which implies that 0.042 mol CH4 should be produced per g of TOC. Using the ideal gas law at standard pressure and temperature (STP) (1 atm and 273 K), this corresponds to 940 NmL of CH4 per g TOC. This value was used for comparison with experimentally determined CH4 yields. 2.4. Statistical analysis A two-way analysis of variance (ANOVA) with a confidence level of 95% was performed to evaluate the effect of the raw material (SW or HW) and/or the HRT on the
Table 3. Ingoing wastewater composition (SW vs. HW). L1 Days 1–22
SW/HW
L2 Days
100% SW (start-up 1–22 phase) 23–61 100% SW 23–62 63–64 Mixture 63–67 65–73 ≥ 95% HW 68–73 74–75 Mixture 74–79 76–84 ≥ 95% SW 80–84 85–86 mixture 85–89 87–104 ≥ 95% HW 90–104 105 mixture 105–106 107–114 115 116–121 122–125 126–143
SW/HW 100% SW (start-up phase) 100% SW Mixture ≥ 95% HW Mixture ≥ 95% SW Mixture ≥ 95% HW Mixture ≥ 95% SW Mixture ≥ 95% HW Mixture ≥ 95% SW
Notes: When a new batch of wastewater is introduced, a mixture of SW and HW goes into the systems; these periods are thus denoted ‘mixture’ in the table. Periods with ≥ 95% of SW or HW were used in the comparison of the impact of wastewater quality on the process performance.
process performance with data from both reactors. The process performance variables included in the evaluation were: biogas production per ingoing TOC, biogas production per degraded fTOC, methane content of the biogas, reduction of fTOC and reduction of fSO2− 4 . The following considerations were made: • The start-up phase, days 1–22, was excluded for both reactors. • The SW and HW periods were defined based on ingoing wastewater composition: ≥ 95 vol-% SW was considered as SW and ≥ 95 vol-% of HW was considered as HW (Table 3). • For evaluation on the effect of HRT, the periods of 13.5 h and 8.5 h were used in the statistical analysis.
3. Results and discussion 3.1. Overall reactor performance The process performance of the two reactors in terms of biogas production and fTOC are illustrated in Figures 2 and 3. The pH was stable at 8.0 ± 0.3 for both reactors during the entire study (data not shown).The biogas produced had a methane content of 75 ± 5%. Acetic and propionic acids were the only VFAs detected, with concentrations not exciding 1.0 mM (data not shown). The methane yield ranged 10–21% of the theoretical methane potential based on the assumptions given in paragraph 2.3 above. The amount of SS in the effluent was closely related to the SS of the ingoing wastewater except from day 96 and onwards for L1 (see details below). The ingoing concentration of SS
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Environmental Technology
Figure 2. Biogas production (NmL g TOC−1 IN ) for the UASB reactors L1 and L2. The methane content of the produced biogas is 75 ± 5% for both reactors. Periods with HW wastewater are indicated with a horizontal line for L1 and a horizontal, dotted line for L2. Days 1–22 are considered as the start-up phase. Reactor L1 was terminated on day 105 while L2 was terminated on day 143. Missing data points (days 54–55 and 78 for L1 and days 55, 78, 89, 105, 113 and 129 for L2) are due to wastewater shortage, power or pump failure or gas leakage.
was 7 ± 2 mg L−1 for SW and 35 ± 8 mg L−1 for HW and the outgoing SS was 14 ± 3 and 45 ± 17 mg L−1 , respectively, for L1 (day 96 and onwards excluded) and 19 ± 9 and 50 ± 25 mg L−1 , respectively, for L2. The stable pH and VFA concentrations over time indicated that neither of the raw materials (SW or HW), the shifts between them or the lowered HRT (from 14 to 7 h) affected the process stability negatively. Thus, all results show that UASB is a suitable technique for applying AD on alkaline bleaching wastewater within kraft PPMs. The high amounts of SS in the L1 effluent from day 96 and onwards were due to a malfunctioning pump for IC of the wastewater. From day 89 and onwards, only intermittent circulation could be performed and after every shutdown of the pump, a sharp increase in the circulation flow had to be applied to get the granular bed fluidized again. After one week of repeated shutdowns and sharp increases, the granular bed had turned into sludge and much of the biomass was lost with the effluent (3360 mg L−1 of SS on day 96). As a remedial action, Ca (in the form of CaCl2 ), known to be important for granulation,[28] was added to the reactor on days 96 and 98. The SS concentration in the effluent decreased after the additions (220 mg L−1 on day 97 and 77 mg L−1 on day 99). The CaCl2 -additions, however, caused the formation of a gel structure, which led to problems with gas separation and floating biomass, the reason why the system was terminated on day 105. During this
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Figure 3. fTOC reduction (%) for the UASB reactors L1 and L2. Days 1–22 are considered as the start-up phase. Reactor L1 was terminated on day 105 while L2 was terminated on day 143.
period (days 89–105), no negative effect was observed on the biogas production or the effluent quality, except for the SS increase from day 96.
3.2. Carbon mass balance Mass balance calculations were performed for SW and HW at HRT of 8.5 and 13.5 h (Table 4). The calculations were based on ingoing wastewater fTOC, effluent fTOC, CH4 production, the theoretical amount of methane dissolved in the effluent (0.0175 g CH4 L−1 effluent at 35 °C), CO2 production (which was calculated from the methane production, both measured and dissolved, and from the sulphate reduction; see discussion below) and finally biomass maintenance/growth (estimated to 10% of the reduced fTOC). The calculations show that 58 ± 8.6% (SW) vs. 61 ± 6.6% (HW) of the reduced fTOC could be accounted for by the above parameters. A substantial part of the deficit in the carbon mass balances is likely due to a poor separation of the produced biogas in the gas– liquid–solid phase separator. In addition, fTOC might have been retained in the reactor as particles adsorbed on, for example, the granules. While the slip of dissolved methane can be accounted for by theoretical calculations, it was not possible to quantify the methane escaping the system as small bubbles in the effluent. Our experience is that higher biogas production rates result in better carbon balances and, thus, likely better phase separation in the UASB reactor. It should be noted that the methane potential determined in batch tests, 130 ± 51 and 330 ± 22 NmL g TOC−1 for SW and HW, respectively [8] is higher than that obtained in the UASB reactors.
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Parameter Ingoing Reactor Outgoing
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Carbon mass balance
fTOC Biomass growtha fTOC CH4 CH4 -slip (dissolved)b (CO2 )c (CO2 )d ‘Missing carbon’ fTOCDEG explained fTOCDEG explained (average)
Unit mmol mmol mmol mmol mmol mmol mmol mmol % %
Hardwood
8.5 h
13.5 h
8.5 h
13.5 h
n=4
n = 13
n=6
n=8
± 4.8 ± 0.44 ± 3.7 ± 1.1 1.1 4.2 ± 1.1 2.1 ± 0.5 10 ± 1.8 57 ± 4.9 58 ± 8.6 53 2.2 31 3.1
± 6.2 ± 0.27 ± 5.9 ± 1.0 1.1 3.7 ± 1.0 3.4 ± 0.8 9.4 ± 2.5 58 ± 10 51 2.2 29 2.6
± 7.8 ± 0.29 ± 5.7 ± 1.0 1.1 6.9 ± 1.0 2.9 ± 0.7 12 ± 1.6 62 ± 5.2 61 ± 6.6 56 3.2 24 5.8
± 7.6 ± 0.36 ± 4.7 ± 1.9 1.1 7.1 ± 1.9 2.4 ± 1.0 13 ± 1.6 60 ± 7.7 54 3.3 21 6.0
Notes: One litre of wastewater is considered and data are presented as mean values ± standard deviation. ‘Missing carbon’ is calculated by subtracting the carbon retained in the reactor together with the outgoing carbon from the ingoing carbon. a Calculated, estimated to be 10% out of fTOC DEG . b Calculated, based on 0.0175 g CH L−1 effluent at 35 °C. 4 c Calculated, based on the measured and calculated CH content of the biogas 4 d Calculated, based on the measured fSO2− reduction. One mole of reduced fSO2− corresponds to the production of 4 4 two moles of CO2 .
It should also be noted that microbial sulphate reduction affects the mass balance of the system. In AD of alkaline bleaching wastewaters, rich in sulphate (260 ± 40 and 180 ± 50 mg L−1 for SW and HW respectively, Table 1), part of the substrate will be degraded by the action of sulphate-reducing bacteria, which results in a lower methane yield.[21,29,30] This was accounted for in the mass balance calculation (Table 4). 3.3.
Impact of raw material and HRT on reactor performance
The gas production was influenced by the type of raw material fed to the UASB reactors, that is, it was significantly higher for HW-wastewater than for SW-wastewater according to the ANOVA (p < 0.001, Table 5). This was shown for the periods covering days 63–73, 85–104 and 115–121, which peaked in relation to the lower production where SW dominated (Figure 2). The pattern is confirmed by the linear increase in biogas production as a function of the proportion of HW in the ingoing wastewater (R2 = 0.66 and 0.75 for L1 and L2, respectively; Figure 4). No relation could be found between fSO2− 4 reduction and raw material variations (p > 0.05, Table 5), neither by percentage nor by absolute numbers (data not shown), implying that the sulphate present is consumed by the sulphate-reducing bacteria regardless of raw material. However, for the SW periods, the sulphate reduction was higher at a HRT of 13.5 h compared with 8.5 h (61 ± 16 vs. 44 ± 13%; not statistically significant), whereas it was
Figure 4. Biogas production (NmL g TOC−1 IN ) vs. vol-% HW in ingoing wastewater. Data points included are in the range of 6–94 vol-% HW independent of HRT. Trend lines are presented for both reactors L1 and L2 (dotted) with corresponding equations and R2 .
not so for HW (59 ± 17 vs. 64 ± 10%). The mechanisms behind this observation cannot be explained with the present data set, but an increase in the importance of sulphate reduction at increasing HRT has earlier been observed by Mizuno et al.[31] The higher methane yield per reduced fTOC for HW (Table 5) may partly be due to the higher amount of TOC in relation to sulphate in the wastewater compared with SW (TOC/sulphate: 2.4 ± 0.3 and 3.8 ± 1.1 for SW vs. HW). Theoretically, the sulphate reduction present
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Table 5. Process performance presented for the two types of ingoing wastewater (SW and HW), regardless of reactor, but separated by HRT (8.5 and 13.5 h). Softwood
Hardwood
8.5 h Biogas production (NmL g TOC−1 IN ) Biogas production (NmL g fTOC−1 DEG ) ) CH4 production (NmL g TOC−1 IN CH4 production (NmL g fTOC−1 DEG ) fTOC reduction (%) fSO2− 4 reduction (%)
130 330 100 250 41 44
± ± ± ± ± ±
28 (n = 24) 46 (n = 5) 32 (n = 4) 50 (n = 4) 6 (n = 5) 13 (n = 5)
13.5 h 120 290 90 210 43 61
± ± ± ± ± ±
30 (n = 61) 70 (n = 16) 26 (n = 14) 70 (n = 13) 6 (n = 16) 16 (n = 16)
8.5 h 260 450 200 340 58 64
± ± ± ± ± ±
38 (n = 27) 52 (n = 6) 25 (n = 7) 41 (n = 6) 5 (n = 6) 10 (n = 6)
13.5 h 250 440 200 340 61 59
± ± ± ± ± ±
64 (n = 27) 88 (n = 8) 50 (n = 8) 82 (n = 8) 4 (n = 8) 17 (n = 8)
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Notes: Data are presented as mean values ± standard deviation. COD/TOC for the ingoing wastewater is 2.4 ± 0.1 and 2.6 ± 0.2 for SW and HW, respectively, while for the effluent, the ratio is 2.6 ± 0.3 for both raw materials.
could lower the methane production by 34 ± 6.8% and 17 ± 0.8% for SW and HW, respectively. A recalculation of the methane yield per reduced fTOC thus narrows the difference between SW and HW substantially; methane yields of 350 ± 80 vs. 410 ± 70 NmL g TOC−1 DEG , respectively, were obtained. The same recalculations for ingoing TOC did not have the same impact, a large difference in methane yield here remains (150 ± 30 and 240 ± 40 NmL g TOC−1 IN for SW vs. HW) even when the sulphate reduction is taken into account. The above estimations thus show that the activity of sulphate-reducing bacteria cannot alone explain the difference in methane production per ingoing TOC between SW and HW. This is further supported by the non-significant difference in sulphate reduction between the HW and SW periods (Table 5). Hence, the difference in methane yield is also likely to depend on a difference in the TOC quality. HW has earlier been reported to be more easily fractionated than SW (cf. [8]). This means that more low-molecular, bioavailable TOC is likely released into the wastewater and thus accessible for biogas production after processing of HW than after processing of SW. In addition, wood extractives, which may have a negative impact on AD, from SW have been shown to be more easily dissolved than from HW, implying higher concentrations in wastewaters for SW. To the authors’ knowledge, no studies similar to ours are available in the literature. However, Lin et al. [21] applied a pilot-scale study of a combined anaerobic and aerobic treatment of alkaline bleaching wastewater with oak as the raw material for pulp production. The HRT and the OLR for their anaerobic packed-bed reactor was around 2.4 days and 2.3 kg COD m−3 day−1 , respectively, and slightly higher COD reduction and methane yield were obtained compared with ours (56 ± 5% vs. 51 ± 3% and 219 NmL g CODred−1 vs. 150 ± 25 NmL g CODred−1 , respectively). It should, however, be noticed that the HRT of the Lin study was 4–6 times longer than in the present study (2.4 days vs. 8.5–13.5 h). In addition, the OLR of the Lin study was lower (2.3 vs. 3.9 ± 1.1 kg COD m−3 day−1 ). Hence, a fivefold increase in the HRT (to match the
study by Lin et al.) might have resulted in a higher treatment efficiency and biogas production per kg COD treated in the present set-up, but with the large volumes of wastewater to be treated, HRTs as long as two days is likely not reasonable in full-scale implementations. The limited fTOC reduction, which in the best case (wastewater from HW) reached around 60% in our study, is likely due to the presence of highly recalcitrant lignin compounds, possibly including low molecular chlorinated aromatics and also wood extractives such as wood resins and tannins. All these compounds might negatively affect the TOC reduction.[14,15,17] Vidal et al.,[17] who studied the biodegradability and toxicity of ECF and total chlorine-free bleaching wastewaters, concluded that wood resin compounds, which are extracted under alkaline conditions, contributed to the wastewaters toxicity. Prior to our continuous reactor study, a batch of the alkaline bleaching wastewater (SW) was analysed for its organic material composition. The TOC was 750 mg L−1 and the amounts of lignin and wood extractives were 490 and 110 mg L−1 , respectively. Hence, the persistency and possible toxicity of lignin and wood extractives may explain the low degradability of this wastewater. The differences in degradability and biogas production of wastewater from SW and HW confirm earlier investigations by Yang et al. [32] and Ekstrand et al.[8] Both studies explain the lower degradability of SW wastewater by its higher content of wood extractives. Yang et al. [32] more specifically links the low biogas yields to dehydroabietic acid and tannin–lignin compounds. Lowering the HRT (13.5 vs. 8.5 h) did not significantly affect the process as judged from the evaluated parameters (Table 4), and indicates that there is room for a further decrease in HRT for both SW and HW wastewaters. No significant interaction effects were identified for raw material and HRT for any of the process performance variables included in the test (p > 0.05). The stable process performance throughout the shifts in wastewater quality by raw material shows that this wastewater is suitable for a full-scale implementation. However,
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Figure 5. AOX concentrations (μg ClORG L−1 ) for five batches of ingoing wastewater and corresponding effluents. Data are presented as mean values ± standard deviation.
the difference in methane potential for SW vs. HW needs to be considered when dimensioning the system, since the produced gas volumes will vary over time. This difference also means that the biogas production, per ingoing TOC, partly can be governed by the raw material used for the pulp and paper production.
3.4.
Adsorbable organic halogens
The results from the analysis of the AOX concentrations in the ingoing wastewater and reactor effluents are presented in Figure 5. For SW, similar concentrations of AOX were observed for the different batches of ingoing wastewaters (4300 ± 380, 4300 ± 110 and 4100 ± 630 μg ClORG L−1 ) as well as for the corresponding effluents (3700 ± 290 μg ClORG L−1 for L1; 3700 ± 320, 3900 ± 110 and 3800 ± 240 μg ClORG L−1 for L2). Calculated AOX removals were 13% for L1 (only one sampling occasion, so no standard deviation could be calculated for this reactor) and 10 ± 3% for L2. The concentration in the HW wastewater deviated more between the substrate batches (6600 ± 1300 and 3500 ± 140 μg ClORG L−1 ) as wells as in the effluents (5700 ± 900 and 3700 ± 190 μg ClORG L−1 for L1; 4400 ± 340 and 3800 ± 190 μg ClORG L−1 for L2) compared with SW. On day 69, the calculated AOX removal was 11% and 33% for L1 and L2, respectively. At the second sampling (day 103), the AOX concentration was higher in the effluent than in the ingoing wastewater. Although the amounts of samples were not enough for statistical analysis, the results may indicate that a reversible adsorption had taken place in the reactors or that new AOX compounds were formed in the process. Further investigation is, however, needed to elucidate the background for this observation.
Recalculations of the AOX concentrations, in relation to TOC on a mole basis, indicate that the organic material in all cases was more chlorinated leaving the UASB reactors than entering it and that wastewater from HW contained more chlorinated material per unit of TOC than wastewater from SW. For the incoming wastewater, the mole ratios were 2.4 ± 0.3‰ and 3.0 ± 1.2‰ for SW and HW, respectively, while for the effluents, the corresponding ratios were 3.1‰ and 3.4 ± 0.4‰ for L1 and L2, respectively, when treating SW wastewaters, and 7.3 ± 3.4‰ and 6.4 ± 2.0‰ for L1 and L2, respectively, when treating HW wastewaters. These results indicate a higher chlorination of the organic material present in HW wastewater and implies that chlorinated TOC is less degradable than non-chlorinated. AOX is often assessed in AD of kraft bleaching effluents and the removal efficiency varies between 39% and 99%.[19,20] Despite the varying results, the above studies indicate that the presence of AOX lowers the COD reduction and methane production. The relatively high AOX-removal efficiencies given for wastewaters with an AOX content of 2.5–42 mg L−1 [19,20,33] made us to expect a high removal in our reactors (AOX levels of 3.5– 6.6 mg L−1 ). One reason for our low AOX removal might be the low HRT of 7–14 h compared with 25 h,[20] 40 h [19] and 20 days.[33] Another reason might be a shortage of electron donors, which has been shown crucial to maintain a high reductive dechlorination.[33] Despite the low AOX-removal efficiency, the process did not seem negatively affected by the present levels of chlorinated compounds as judged from the low VFA levels in the system. According to literature, the acetoclastic methanogens, specifically, are sensitive to chlorinated compounds [34] and an inhibition most probably would have been indicated by an accumulation of acetate.[35]
4.
Conclusions • A stable anaerobic process was maintained at HRTs of 7–14 h. Thus, the changes in wastewater characteristics did not affect the process stability. • A significant higher biogas production and fTOC reduction was obtained with wastewater from HW compared to SW. • Lowering the HRT (13.5 vs. 8.5 h) did not significantly affect the process as judged from the evaluated parameters. This shows that there is likely room for a further decrease in HRT for both SW and HW wastewaters. • The biogas production in our reactors did not seem to be inhibited by present levels of AOX.
Acknowledgements The authors wish to thank the personnel at the mill for assistance during sampling and for providing information.
Environmental Technology Disclosure statement
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No potential conflict of interest was reported by the author(s).
[16]
Funding
[17]
This study was funded by the Swedish Energy Agency [project No. 32802-1], Linköping University, Scandinavian Biogas Fuels AB, Pöyry Sweden AB, BillerudKorsnäs Sweden AB and SCA.
[18]
References
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[32] Yang MI, Edwards EA, Allen DG. Anaerobic treatability and biogas production potential of selected in-mill streams. Water Sci Technol. 2010;62:2427–2434. [33] Deshmukh NS, Lapsiya KL, Savant DV, Chiplonkar SA, Yeole TY, Dhakephalkar PK, Ranade DR. Upflow anaerobic filter for the degradation of adsorbable organic halides (AOX) from bleach composite wastewater of pulp and paper industry. Chemosphere. 2009;75:1179–1185.
[34] Chen Y, Cheng JJ, Creamer KS. Inhibition of anaerobic digestion process: A review. Biores Technol. 2008;99: 4044–4064. [35] Ejlertsson J, Johansson E, Karlsson A, Meyerson U, Svensson BH. Anaerobic degradation of xenobiotics by organisms from municipal solid waste under landfilling conditions. Antoine van Leeuwenhoek. 1996;69: 67–74.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Anaerobic digestion of alkaline bleaching wastewater from a kraft pulp and paper mill using UASB technique a
b
a
ab
a
Madeleine Larsson , Xu-Bin Truong , Annika Björn , Jörgen Ejlertsson , David Bastviken , Bo a
b
H. Svensson & Anna Karlsson a
Department of Thematic Studies, Environmental Change, Linköping University, 581 83 Linköping, Sweden b
Scandinavian Biogas Fuels AB, Holländargatan 21A, 111 60 Stockholm, Sweden Accepted author version posted online: 02 Dec 2014.Published online: 05 Mar 2015.
Click for updates To cite this article: Madeleine Larsson, Xu-Bin Truong, Annika Björn, Jörgen Ejlertsson, David Bastviken, Bo H. Svensson & Anna Karlsson (2015) Anaerobic digestion of alkaline bleaching wastewater from a kraft pulp and paper mill using UASB technique, Environmental Technology, 36:12, 1489-1498, DOI: 10.1080/09593330.2014.994042 To link to this article: http://dx.doi.org/10.1080/09593330.2014.994042
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Environmental Technology, 2015 Vol. 36, No. 12, 1489–1498, http://dx.doi.org/10.1080/09593330.2014.994042
Anaerobic digestion of alkaline bleaching wastewater from a kraft pulp and paper mill using UASB technique Madeleine Larssona , Xu-Bin Truongb , Annika Björna , Jörgen Ejlertssona,b , David Bastvikena , Bo H. Svenssona and Anna Karlssonb∗ a Department
of Thematic Studies, Environmental Change, Linköping University, 581 83 Linköping, Sweden; b Scandinavian Biogas Fuels AB, Holländargatan 21A, 111 60 Stockholm, Sweden
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(Received 18 July 2014; accepted 27 November 2014 ) Anaerobic digestion of alkaline kraft elemental chlorine-free bleaching wastewater in two mesophilic, lab-scale upflow anaerobic sludge bed reactors resulted in significantly higher biogas production (250 ± 50 vs. 120 ± 30 NmL g TOC−1 IN ) and reduction of filtered total organic carbon (fTOC) (60 ± 5 vs. 43 ± 6%) for wastewater from processing of hardwood (HW) compared with softwood (SW). In all cases, the gas production was likely underestimated due to poor gas separation in the reactors. Despite changes in wastewater characteristics, a stable anaerobic process was maintained with hydraulic retention times (HRTs) between 7 and 14 h. Lowering the HRT (from 13.5 to 8.5 h) did not significantly affect the process, and the stable performance at 8.5 h leaves room for further decreases in HRT. The results show that this type of wastewater is suitable for a full-scale implementation, but the difference in methane potential between SW and HW is important to consider both regarding process dimensioning and biogas yield optimization. Keywords: anaerobic digestion; UASB; alkaline kraft ECF bleaching wastewater; hardwood; softwood
1. Introduction The pulp and paper industry is a high energydemanding, water-utilizing and wood resource-consuming industry.[1,2] The large water consumption implies a large production of wastewater, which is highly polluted with organic material from the wood and chemicals used in the processes.[1,3] Instead of treating these wastewaters in the activated sludge processes, which often is the case today, the organic material can be considered a resource possible to explore in anaerobic digestion (AD). Implementation of anaerobic wastewater treatment and biogas production as a part of the conventional wastewater treatment at a mill would reduce its energy usage, sludge production and nutrient consumption.[1,4–7] There is a large potential for biogas production within the pulp and paper industry, but it should be noted that the type of process, raw materials and wastewater effluents will largely affect the outcome.[8] AD has been implemented successfully in full scale for bleached/unbleached thermomechanical pulp,[9] for chemical thermo-mechanical pulp effluents, as well as for neutral sulphite semi-chemical effluents and kraft/sulphite mill condensates.[6,10,11] However, the majority of full-scale installations are found among mills for recycled paper and board (130 out of 203;[6]). More than 80% of the installations in the pulp and paper industry are high-rate anaerobic sludge bed
systems. Of these, more than half are upflow anaerobic sludge bed (UASB) reactors.[6] This technique is suitable for large flows with a high concentration of dissolved organic material and a low concentration of suspended solids (SS).[12] Kraft pulping is the main method for pulp production in Sweden [13] and kraft pulp and paper mill (PPM) effluents represent a large biogas production potential if combined with efficient AD techniques. These streams are, however, regarded as challenging in terms of substrate quality for methanogenesis and due to the large effluent flow rates. The challenges include, for example, spent black liquor, wood extractives, lignin- and lignin-derived compounds together with the high sulphate-to-carbon ratio.[14–18] In addition, kraft bleaching wastewaters are often high in concentration of chlorinated hydrocarbons, mainly found in the acidic bleaching effluents. These kinds of compounds have been shown to interact negatively with the AD process.[3,14–16] However, Ekstrand et al. [8] demonstrated high methane yields for alkaline elemental chlorinefree (ECF) bleaching wastewaters in batch tests, especially when hardwood (HW) is used as raw material in the pulping process (330 and 360 NmL CH4 g total organic carbon (TOC)−1 corresponding to up to 38 ± 2% of the theoretical methane potential for HW and 0–130 NmL CH4 g TOC−1 corresponding to 14% of the theoretical methane
*Corresponding author. Emails: [email protected], [email protected] © 2014 Taylor & Francis
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potential for softwood (SW;[8]). These wastewater streams are characterized by relatively high concentrations of dissolved organic material (170–730 mg TOC L−1 ; according to [8]) normally combined with levels of SS below those 500 mg L−1 often recommended for UASB treatment. These characteristics thus make the streams suitable for UASB technique and opens up for AD at low hydraulic retention times (HRTs), hence minimizing the size of the installation, and maintaining high process efficiency, which is important for the overall economy of a full-scale implementation. A UASB-installation on a bleaching stream can be of advantage compared with an application on the mill’s total effluent, as the bleaching stream normally holds a higher concentration of organic material, meaning that less water volumes need to be treated to obtain the same effect. A rough estimation of the Swedish kraft PPMs shows that the alkaline ECF bleaching wastewaters holds 50–60% of the mills total organic load and more than 75% of their total methane potential (personal communication, Fredrik Nilsson, Pöyry AB). So far, only a few studies indicate that continuous highrate, AD treatment of kraft bleaching effluents is feasible, that is, removing large fractions of the organic material at low HRTs. However, most studies focused on wastewater treatment and toxicity reduction, and not on the production of biogas. Buzzini et al. [19] treated a synthetic wastewater, simulating the composite effluent from a mill producing bleached and unbleached eucalyptus pulp, in two UASBs with chemical oxygen demand (COD) removals of 76 ± 5% vs. 81 ± 2%. Eucalyptus was also the raw material used in a study by Chaparro and Pires [20] evaluating the biodegradability for a mixture of alkaline and acid bleaching wastewaters in a horizontally packed-bed anaerobic reactor. The AD in this case resulted in a COD removal of 52–55%. Lin et al. [21] used a packed-bed AD column to compare treatability of different mixtures of wastewaters (e.g. alkaline and acidic bleaching wastewater) generated from HW pulp (mainly oak) production and obtained a COD removal of 41–57%. The aim of the present study was to investigate the possibilities of obtaining a stable and high-performing biogas production when treating alkaline kraft ECF bleaching wastewaters with UASB technique. This included investigations of the inherent variations in quality of the bleaching wastewater due to frequent shifts between SW and HW as raw material for pulp production as well as other process variations/disturbances at the mill. We also wanted to obtain a process with the lowest possible HRT to facilitate treatment of large flows of wastewater during a short period of time. All tests were performed in lab scale. 2. Material and methods 2.1. Experimental set-up and sampling of wastewater Two identical 4-L UASB reactors (Figure 1; custom made by Humi-Glas, Södra Sandby, Sweden), designated L1 and
Figure 1. Schematic overview of the UASB reactor set-up. The wastewater is pumped into the reactor at the bottom and the liquid is distributed by the help of glass marbles. The wastewater passes through the bed of granules (shaded area) and exits at the top of the reactor at the gas–liquid–solid separator (GLS. SEP.) To create a fluidized bed, a high upflow velocity is generated by recirculating the wastewater by an internal circulation (IC) pump. The UASB reactor is heated to 35 °C by a water jacket connected to a water bath.
L2, were operated at 35 °C for 106 and 144 days, respectively. The two reactors were inoculated with granules from a pilot-scale UASB reactor treating clarified sewage wastewater. The fluidized granular bed volume was 1.5–2 L and an internal circulation (IC) loop was used to fluidize the granular bed. A gas–liquid–solid phase separator was installed at the top of the reactor (cf. Figure 1). A volume of 150–200 L alkaline bleaching wastewater (EOP; alkaline extraction (E) in the presence of oxygen (O) and hydrogen peroxide (P)) from the ECF bleaching sequence D (EOP) D P (where D indicates chlorine dioxide) at a kraft mill was collected and transported (1-hour drive) to the laboratory every week (batches 1–22). The wastewater samples were stored at + 4 °C for at most 15 days before used as substrate in the UASB reactors. The wastewater samples covered normal process shifts between SW and HW used as raw material for the pulp production. Upon arrival at the laboratory, each batch of wastewater was analysed for total and filtered COD, TOC and SO2− 4
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Wastewater characteristics.
SW (n = 17) HW (n = 5)
TOC (mg L−1 )
fTOC (mg L−1 )
SO2− 4 (mg L−1 )
fSO2− 4 (mg L−1 )
SS (mg L−1 )
VFA (mM)
640 ± 80 650 ± 70
630 ± 80 610 ± 80
260 ± 40 180 ± 50
250 ± 40 170 ± 50
7±2 35 ± 8
< 0.6 < 0.6
Notes: COD values can be obtained by multiplying the TOC value with 2.4 ± 0.1 and 2.6 ± 0.2 for SW and HW, respectively. Analyses made on filtered samples are denoted as fTOC and fSO2− 4 . Data presented are mean values ± standard deviation.
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Table 2. Concentrations (mg L−1 ) of macro- and micro-nutrients in two batches of alkaline bleaching wastewater, one with SW (batch 17) and the other with HW (batch 16).
Nutrient
SW
HW
Cl− 110 110 NH+ 0.18 0.22 4 −N Kjeldahl-N < 2.0 < 2.0 P 0.23 0.26 S 95 70 Na 640 730 K 4.4 4.1 Ca 16 16 Fe 0.18 0.21 Mg 21 5.1 Mn 0.26 0.24 Sb < 0.0010 < 0.0010 Pb 0.00071 0.0022 Cd 0.00055 0.00088 Co < 0.0010 < 0.0010 Cu 0.0040 0.0051 Cr 0.0041 0.0035 Hg < 0.00010 < 0.00010 Mo < 0.0010 0.0011 Ni 0.0011 0.0012 V 0.0028 0.0023 Zn 0.044 0.062 Al 0.18 0.20 Se < 0.0030 < 0.0030 Ag < 0.00050 < 0.00050 W < 1.0 – B 0.051 0.052 Ti (μg L−1 ) < 50 < 51
Error in measurement ( ± %) 15 15 10 10 20 20 20 15 20 20 20 25 35 30 25 15 25 20 40 40 30 15 15 35 20 20 20 20
Note: Concentration and measurement errors were reported by Eurofins Environment Sweden AB.
(analyses of filtered samples are denoted fCOD, fTOC and fSO2− 4 ), SS, volatile fatty acids (VFA) and pH (Table 1). Due to the high pH of the wastewater (11.2 ± 0.6), the pH was adjusted to 7.5 with 1 M HCl directly before being fed to the reactors (approx. 10 L of wastewater adjusted at a time). To get an overview of the amount of chlorinated hydrocarbons in the wastewater, 5 of the 22 batches (SW: batches 7, 17 and 20; HW: batches 12 and 16) were analysed for adsorbable organic halogen (AOX) content. One
batch for each raw material (batches 16 and 17) was analysed for macro- and micro-nutrient content (Table 2) by Eurofins Environment Sweden AB according to Swedish standard methods. During a 22-day start-up phase, the HRT for the two reactors was 12–14 h. The two reactors (L1 and L2) were then run at a HRT of 13.5 h for 10 days (days 23–32) before the HRT in L1 was decreased to 10.5 h and then further to 8.5 h by day 48. The HRT in L2 was maintained at 13.5 h until day 112, when it was decreased successively to 10.5 h, but temporarily brought back to 13.5 h during days 118–124 due to wastewater shortage. The HRT of L2 was lowered to 8.5 h on day 132 and finally to 7 h on day 135. The organic loading rates (OLR) ranged from 0.9 to 2.2 g TOC L−1 day−1 for both L1 and L2 according to the shifts in HRT and wastewater characteristics. The composition of the ingoing wastewaters is given in Table 1. Due to the low nutrient content in the wastewaters (Table 2), macro-nutrients ((NH2 )2 CO and Na2 HPO4 ) and trace elements (CoCl2 , CuCl2 , ZnCl2 , NiCl2 , (NH4 )6 Mo7 O24 , Na2 SeO3 and Na2 WO4 ) were added to the substrate throughout the experiment to enable biomass growth, a well-functioning fluidized bed and an efficient AD. N and P were added at a ratio of COD:N:P 350:5:1. The trace element solution was prepared from two stock solutions; solution A (CoCl2 , CuCl2 , ZnCl2 , NiCl2 and (NH4 )6 Mo7 O24 dissolved in milli-Q water) and solution B (Na2 SeO3 and Na2 WO4 dissolved in 10 mM NaOH and further diluted in milli-Q water). The trace element solution was added directly to the reactor in 24 pulses during one day each week amounting to 20 μmol of each metal. The dosing strategy was chosen to expose the granular bed to a higher concentration compared with when dosing the trace elements continuously with the substrate.[22,23]
2.2.
Evaluation of reactor performance
The performance of the UASB reactors was evaluated by analysing the biogas quantity and its methane content together with the composition of the reactor effluents in terms of fCOD, fTOC, fSO2− 4 , SS, VFA and pH twice a week. Additionally, the effluents were analysed for AOX on days 41, 69, 103, 111 and 125 corresponding to the wastewater batches: 7, 12, 16, 17 and 20. The reactor
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performance and wastewater quality are mainly presented as/in relation to TOC and fTOC, but are recalculated to COD and fCOD when necessary to enable comparisons with other studies. 2.2.1. Analytical procedure The gas production was monitored online by means of gas meters based on the principle of water displacement (MGC-1 V3.0 PMMA; Ritter, Germany) and the methane content of the biogas was determined twice a week by gas chromatography according to Karlsson et al.[24] COD, TOC and SO2− 4 were measured spectrophotometrically using kits from Hach Lange, Germany (LCK014, LCK514; LCK386, LCK387; LCK153, LCK353) according to the manufacturer’s instructions. Prior to analysis of fCOD, fTOC and fSO2− 4 , the samples were filtered (MGA grade, pore size 1.6 μm; Munktell). SS were analysed according to Swedish standard (SS-EN 872:2005; MGA grade, pore size 1.6 μm; Munktell; the standard deviation was at the most 5% for the triplicate samples) and pH was measured using a PHM93 meter (Radiometer, Copenhagen, 166 Denmark). The VFA concentrations (acetate, propionate, butyrate, iso-butyrate, valeriate, iso-valeriate, capronate and iso-capronate) were analysed according to Jonsson and Borén.[25] The amount of AOX was determined for 15 thawed, triplicate samples according to a method modified from standard methods [26] and described by Asplund et al.[27] Prior to analyses, the samples were diluted at 1:100. The method comprised (i) adsorption of the organic compounds in the sample on activated carbon; (ii) removal of inorganic halides from the carbon by ion-exchange by nitrate; (iii) combustion of the carbon in oxygen atmosphere at 1000 °C and (iv) micro-coulometric titration of formed hydrogen halides. The AOX content was determined using an EDAK AOX-analyser (model ECS 3000). All chemicals used were of analytical grade. 2.3. Theoretical CH4 yield per TOC (NmL g−1 ) To calculate the theoretical methane yield, the TOC content was assumed to be 100% carbohydrates. A complete anaerobic degradation of carbohydrates gives a molar ratio of 1:1 for CH4 and CO2 , which implies that 0.042 mol CH4 should be produced per g of TOC. Using the ideal gas law at standard pressure and temperature (STP) (1 atm and 273 K), this corresponds to 940 NmL of CH4 per g TOC. This value was used for comparison with experimentally determined CH4 yields. 2.4. Statistical analysis A two-way analysis of variance (ANOVA) with a confidence level of 95% was performed to evaluate the effect of the raw material (SW or HW) and/or the HRT on the
Table 3. Ingoing wastewater composition (SW vs. HW). L1 Days 1–22
SW/HW
L2 Days
100% SW (start-up 1–22 phase) 23–61 100% SW 23–62 63–64 Mixture 63–67 65–73 ≥ 95% HW 68–73 74–75 Mixture 74–79 76–84 ≥ 95% SW 80–84 85–86 mixture 85–89 87–104 ≥ 95% HW 90–104 105 mixture 105–106 107–114 115 116–121 122–125 126–143
SW/HW 100% SW (start-up phase) 100% SW Mixture ≥ 95% HW Mixture ≥ 95% SW Mixture ≥ 95% HW Mixture ≥ 95% SW Mixture ≥ 95% HW Mixture ≥ 95% SW
Notes: When a new batch of wastewater is introduced, a mixture of SW and HW goes into the systems; these periods are thus denoted ‘mixture’ in the table. Periods with ≥ 95% of SW or HW were used in the comparison of the impact of wastewater quality on the process performance.
process performance with data from both reactors. The process performance variables included in the evaluation were: biogas production per ingoing TOC, biogas production per degraded fTOC, methane content of the biogas, reduction of fTOC and reduction of fSO2− 4 . The following considerations were made: • The start-up phase, days 1–22, was excluded for both reactors. • The SW and HW periods were defined based on ingoing wastewater composition: ≥ 95 vol-% SW was considered as SW and ≥ 95 vol-% of HW was considered as HW (Table 3). • For evaluation on the effect of HRT, the periods of 13.5 h and 8.5 h were used in the statistical analysis.
3. Results and discussion 3.1. Overall reactor performance The process performance of the two reactors in terms of biogas production and fTOC are illustrated in Figures 2 and 3. The pH was stable at 8.0 ± 0.3 for both reactors during the entire study (data not shown).The biogas produced had a methane content of 75 ± 5%. Acetic and propionic acids were the only VFAs detected, with concentrations not exciding 1.0 mM (data not shown). The methane yield ranged 10–21% of the theoretical methane potential based on the assumptions given in paragraph 2.3 above. The amount of SS in the effluent was closely related to the SS of the ingoing wastewater except from day 96 and onwards for L1 (see details below). The ingoing concentration of SS
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Environmental Technology
Figure 2. Biogas production (NmL g TOC−1 IN ) for the UASB reactors L1 and L2. The methane content of the produced biogas is 75 ± 5% for both reactors. Periods with HW wastewater are indicated with a horizontal line for L1 and a horizontal, dotted line for L2. Days 1–22 are considered as the start-up phase. Reactor L1 was terminated on day 105 while L2 was terminated on day 143. Missing data points (days 54–55 and 78 for L1 and days 55, 78, 89, 105, 113 and 129 for L2) are due to wastewater shortage, power or pump failure or gas leakage.
was 7 ± 2 mg L−1 for SW and 35 ± 8 mg L−1 for HW and the outgoing SS was 14 ± 3 and 45 ± 17 mg L−1 , respectively, for L1 (day 96 and onwards excluded) and 19 ± 9 and 50 ± 25 mg L−1 , respectively, for L2. The stable pH and VFA concentrations over time indicated that neither of the raw materials (SW or HW), the shifts between them or the lowered HRT (from 14 to 7 h) affected the process stability negatively. Thus, all results show that UASB is a suitable technique for applying AD on alkaline bleaching wastewater within kraft PPMs. The high amounts of SS in the L1 effluent from day 96 and onwards were due to a malfunctioning pump for IC of the wastewater. From day 89 and onwards, only intermittent circulation could be performed and after every shutdown of the pump, a sharp increase in the circulation flow had to be applied to get the granular bed fluidized again. After one week of repeated shutdowns and sharp increases, the granular bed had turned into sludge and much of the biomass was lost with the effluent (3360 mg L−1 of SS on day 96). As a remedial action, Ca (in the form of CaCl2 ), known to be important for granulation,[28] was added to the reactor on days 96 and 98. The SS concentration in the effluent decreased after the additions (220 mg L−1 on day 97 and 77 mg L−1 on day 99). The CaCl2 -additions, however, caused the formation of a gel structure, which led to problems with gas separation and floating biomass, the reason why the system was terminated on day 105. During this
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Figure 3. fTOC reduction (%) for the UASB reactors L1 and L2. Days 1–22 are considered as the start-up phase. Reactor L1 was terminated on day 105 while L2 was terminated on day 143.
period (days 89–105), no negative effect was observed on the biogas production or the effluent quality, except for the SS increase from day 96.
3.2. Carbon mass balance Mass balance calculations were performed for SW and HW at HRT of 8.5 and 13.5 h (Table 4). The calculations were based on ingoing wastewater fTOC, effluent fTOC, CH4 production, the theoretical amount of methane dissolved in the effluent (0.0175 g CH4 L−1 effluent at 35 °C), CO2 production (which was calculated from the methane production, both measured and dissolved, and from the sulphate reduction; see discussion below) and finally biomass maintenance/growth (estimated to 10% of the reduced fTOC). The calculations show that 58 ± 8.6% (SW) vs. 61 ± 6.6% (HW) of the reduced fTOC could be accounted for by the above parameters. A substantial part of the deficit in the carbon mass balances is likely due to a poor separation of the produced biogas in the gas– liquid–solid phase separator. In addition, fTOC might have been retained in the reactor as particles adsorbed on, for example, the granules. While the slip of dissolved methane can be accounted for by theoretical calculations, it was not possible to quantify the methane escaping the system as small bubbles in the effluent. Our experience is that higher biogas production rates result in better carbon balances and, thus, likely better phase separation in the UASB reactor. It should be noted that the methane potential determined in batch tests, 130 ± 51 and 330 ± 22 NmL g TOC−1 for SW and HW, respectively [8] is higher than that obtained in the UASB reactors.
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Parameter Ingoing Reactor Outgoing
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Carbon mass balance
fTOC Biomass growtha fTOC CH4 CH4 -slip (dissolved)b (CO2 )c (CO2 )d ‘Missing carbon’ fTOCDEG explained fTOCDEG explained (average)
Unit mmol mmol mmol mmol mmol mmol mmol mmol % %
Hardwood
8.5 h
13.5 h
8.5 h
13.5 h
n=4
n = 13
n=6
n=8
± 4.8 ± 0.44 ± 3.7 ± 1.1 1.1 4.2 ± 1.1 2.1 ± 0.5 10 ± 1.8 57 ± 4.9 58 ± 8.6 53 2.2 31 3.1
± 6.2 ± 0.27 ± 5.9 ± 1.0 1.1 3.7 ± 1.0 3.4 ± 0.8 9.4 ± 2.5 58 ± 10 51 2.2 29 2.6
± 7.8 ± 0.29 ± 5.7 ± 1.0 1.1 6.9 ± 1.0 2.9 ± 0.7 12 ± 1.6 62 ± 5.2 61 ± 6.6 56 3.2 24 5.8
± 7.6 ± 0.36 ± 4.7 ± 1.9 1.1 7.1 ± 1.9 2.4 ± 1.0 13 ± 1.6 60 ± 7.7 54 3.3 21 6.0
Notes: One litre of wastewater is considered and data are presented as mean values ± standard deviation. ‘Missing carbon’ is calculated by subtracting the carbon retained in the reactor together with the outgoing carbon from the ingoing carbon. a Calculated, estimated to be 10% out of fTOC DEG . b Calculated, based on 0.0175 g CH L−1 effluent at 35 °C. 4 c Calculated, based on the measured and calculated CH content of the biogas 4 d Calculated, based on the measured fSO2− reduction. One mole of reduced fSO2− corresponds to the production of 4 4 two moles of CO2 .
It should also be noted that microbial sulphate reduction affects the mass balance of the system. In AD of alkaline bleaching wastewaters, rich in sulphate (260 ± 40 and 180 ± 50 mg L−1 for SW and HW respectively, Table 1), part of the substrate will be degraded by the action of sulphate-reducing bacteria, which results in a lower methane yield.[21,29,30] This was accounted for in the mass balance calculation (Table 4). 3.3.
Impact of raw material and HRT on reactor performance
The gas production was influenced by the type of raw material fed to the UASB reactors, that is, it was significantly higher for HW-wastewater than for SW-wastewater according to the ANOVA (p < 0.001, Table 5). This was shown for the periods covering days 63–73, 85–104 and 115–121, which peaked in relation to the lower production where SW dominated (Figure 2). The pattern is confirmed by the linear increase in biogas production as a function of the proportion of HW in the ingoing wastewater (R2 = 0.66 and 0.75 for L1 and L2, respectively; Figure 4). No relation could be found between fSO2− 4 reduction and raw material variations (p > 0.05, Table 5), neither by percentage nor by absolute numbers (data not shown), implying that the sulphate present is consumed by the sulphate-reducing bacteria regardless of raw material. However, for the SW periods, the sulphate reduction was higher at a HRT of 13.5 h compared with 8.5 h (61 ± 16 vs. 44 ± 13%; not statistically significant), whereas it was
Figure 4. Biogas production (NmL g TOC−1 IN ) vs. vol-% HW in ingoing wastewater. Data points included are in the range of 6–94 vol-% HW independent of HRT. Trend lines are presented for both reactors L1 and L2 (dotted) with corresponding equations and R2 .
not so for HW (59 ± 17 vs. 64 ± 10%). The mechanisms behind this observation cannot be explained with the present data set, but an increase in the importance of sulphate reduction at increasing HRT has earlier been observed by Mizuno et al.[31] The higher methane yield per reduced fTOC for HW (Table 5) may partly be due to the higher amount of TOC in relation to sulphate in the wastewater compared with SW (TOC/sulphate: 2.4 ± 0.3 and 3.8 ± 1.1 for SW vs. HW). Theoretically, the sulphate reduction present
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Table 5. Process performance presented for the two types of ingoing wastewater (SW and HW), regardless of reactor, but separated by HRT (8.5 and 13.5 h). Softwood
Hardwood
8.5 h Biogas production (NmL g TOC−1 IN ) Biogas production (NmL g fTOC−1 DEG ) ) CH4 production (NmL g TOC−1 IN CH4 production (NmL g fTOC−1 DEG ) fTOC reduction (%) fSO2− 4 reduction (%)
130 330 100 250 41 44
± ± ± ± ± ±
28 (n = 24) 46 (n = 5) 32 (n = 4) 50 (n = 4) 6 (n = 5) 13 (n = 5)
13.5 h 120 290 90 210 43 61
± ± ± ± ± ±
30 (n = 61) 70 (n = 16) 26 (n = 14) 70 (n = 13) 6 (n = 16) 16 (n = 16)
8.5 h 260 450 200 340 58 64
± ± ± ± ± ±
38 (n = 27) 52 (n = 6) 25 (n = 7) 41 (n = 6) 5 (n = 6) 10 (n = 6)
13.5 h 250 440 200 340 61 59
± ± ± ± ± ±
64 (n = 27) 88 (n = 8) 50 (n = 8) 82 (n = 8) 4 (n = 8) 17 (n = 8)
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Notes: Data are presented as mean values ± standard deviation. COD/TOC for the ingoing wastewater is 2.4 ± 0.1 and 2.6 ± 0.2 for SW and HW, respectively, while for the effluent, the ratio is 2.6 ± 0.3 for both raw materials.
could lower the methane production by 34 ± 6.8% and 17 ± 0.8% for SW and HW, respectively. A recalculation of the methane yield per reduced fTOC thus narrows the difference between SW and HW substantially; methane yields of 350 ± 80 vs. 410 ± 70 NmL g TOC−1 DEG , respectively, were obtained. The same recalculations for ingoing TOC did not have the same impact, a large difference in methane yield here remains (150 ± 30 and 240 ± 40 NmL g TOC−1 IN for SW vs. HW) even when the sulphate reduction is taken into account. The above estimations thus show that the activity of sulphate-reducing bacteria cannot alone explain the difference in methane production per ingoing TOC between SW and HW. This is further supported by the non-significant difference in sulphate reduction between the HW and SW periods (Table 5). Hence, the difference in methane yield is also likely to depend on a difference in the TOC quality. HW has earlier been reported to be more easily fractionated than SW (cf. [8]). This means that more low-molecular, bioavailable TOC is likely released into the wastewater and thus accessible for biogas production after processing of HW than after processing of SW. In addition, wood extractives, which may have a negative impact on AD, from SW have been shown to be more easily dissolved than from HW, implying higher concentrations in wastewaters for SW. To the authors’ knowledge, no studies similar to ours are available in the literature. However, Lin et al. [21] applied a pilot-scale study of a combined anaerobic and aerobic treatment of alkaline bleaching wastewater with oak as the raw material for pulp production. The HRT and the OLR for their anaerobic packed-bed reactor was around 2.4 days and 2.3 kg COD m−3 day−1 , respectively, and slightly higher COD reduction and methane yield were obtained compared with ours (56 ± 5% vs. 51 ± 3% and 219 NmL g CODred−1 vs. 150 ± 25 NmL g CODred−1 , respectively). It should, however, be noticed that the HRT of the Lin study was 4–6 times longer than in the present study (2.4 days vs. 8.5–13.5 h). In addition, the OLR of the Lin study was lower (2.3 vs. 3.9 ± 1.1 kg COD m−3 day−1 ). Hence, a fivefold increase in the HRT (to match the
study by Lin et al.) might have resulted in a higher treatment efficiency and biogas production per kg COD treated in the present set-up, but with the large volumes of wastewater to be treated, HRTs as long as two days is likely not reasonable in full-scale implementations. The limited fTOC reduction, which in the best case (wastewater from HW) reached around 60% in our study, is likely due to the presence of highly recalcitrant lignin compounds, possibly including low molecular chlorinated aromatics and also wood extractives such as wood resins and tannins. All these compounds might negatively affect the TOC reduction.[14,15,17] Vidal et al.,[17] who studied the biodegradability and toxicity of ECF and total chlorine-free bleaching wastewaters, concluded that wood resin compounds, which are extracted under alkaline conditions, contributed to the wastewaters toxicity. Prior to our continuous reactor study, a batch of the alkaline bleaching wastewater (SW) was analysed for its organic material composition. The TOC was 750 mg L−1 and the amounts of lignin and wood extractives were 490 and 110 mg L−1 , respectively. Hence, the persistency and possible toxicity of lignin and wood extractives may explain the low degradability of this wastewater. The differences in degradability and biogas production of wastewater from SW and HW confirm earlier investigations by Yang et al. [32] and Ekstrand et al.[8] Both studies explain the lower degradability of SW wastewater by its higher content of wood extractives. Yang et al. [32] more specifically links the low biogas yields to dehydroabietic acid and tannin–lignin compounds. Lowering the HRT (13.5 vs. 8.5 h) did not significantly affect the process as judged from the evaluated parameters (Table 4), and indicates that there is room for a further decrease in HRT for both SW and HW wastewaters. No significant interaction effects were identified for raw material and HRT for any of the process performance variables included in the test (p > 0.05). The stable process performance throughout the shifts in wastewater quality by raw material shows that this wastewater is suitable for a full-scale implementation. However,
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Figure 5. AOX concentrations (μg ClORG L−1 ) for five batches of ingoing wastewater and corresponding effluents. Data are presented as mean values ± standard deviation.
the difference in methane potential for SW vs. HW needs to be considered when dimensioning the system, since the produced gas volumes will vary over time. This difference also means that the biogas production, per ingoing TOC, partly can be governed by the raw material used for the pulp and paper production.
3.4.
Adsorbable organic halogens
The results from the analysis of the AOX concentrations in the ingoing wastewater and reactor effluents are presented in Figure 5. For SW, similar concentrations of AOX were observed for the different batches of ingoing wastewaters (4300 ± 380, 4300 ± 110 and 4100 ± 630 μg ClORG L−1 ) as well as for the corresponding effluents (3700 ± 290 μg ClORG L−1 for L1; 3700 ± 320, 3900 ± 110 and 3800 ± 240 μg ClORG L−1 for L2). Calculated AOX removals were 13% for L1 (only one sampling occasion, so no standard deviation could be calculated for this reactor) and 10 ± 3% for L2. The concentration in the HW wastewater deviated more between the substrate batches (6600 ± 1300 and 3500 ± 140 μg ClORG L−1 ) as wells as in the effluents (5700 ± 900 and 3700 ± 190 μg ClORG L−1 for L1; 4400 ± 340 and 3800 ± 190 μg ClORG L−1 for L2) compared with SW. On day 69, the calculated AOX removal was 11% and 33% for L1 and L2, respectively. At the second sampling (day 103), the AOX concentration was higher in the effluent than in the ingoing wastewater. Although the amounts of samples were not enough for statistical analysis, the results may indicate that a reversible adsorption had taken place in the reactors or that new AOX compounds were formed in the process. Further investigation is, however, needed to elucidate the background for this observation.
Recalculations of the AOX concentrations, in relation to TOC on a mole basis, indicate that the organic material in all cases was more chlorinated leaving the UASB reactors than entering it and that wastewater from HW contained more chlorinated material per unit of TOC than wastewater from SW. For the incoming wastewater, the mole ratios were 2.4 ± 0.3‰ and 3.0 ± 1.2‰ for SW and HW, respectively, while for the effluents, the corresponding ratios were 3.1‰ and 3.4 ± 0.4‰ for L1 and L2, respectively, when treating SW wastewaters, and 7.3 ± 3.4‰ and 6.4 ± 2.0‰ for L1 and L2, respectively, when treating HW wastewaters. These results indicate a higher chlorination of the organic material present in HW wastewater and implies that chlorinated TOC is less degradable than non-chlorinated. AOX is often assessed in AD of kraft bleaching effluents and the removal efficiency varies between 39% and 99%.[19,20] Despite the varying results, the above studies indicate that the presence of AOX lowers the COD reduction and methane production. The relatively high AOX-removal efficiencies given for wastewaters with an AOX content of 2.5–42 mg L−1 [19,20,33] made us to expect a high removal in our reactors (AOX levels of 3.5– 6.6 mg L−1 ). One reason for our low AOX removal might be the low HRT of 7–14 h compared with 25 h,[20] 40 h [19] and 20 days.[33] Another reason might be a shortage of electron donors, which has been shown crucial to maintain a high reductive dechlorination.[33] Despite the low AOX-removal efficiency, the process did not seem negatively affected by the present levels of chlorinated compounds as judged from the low VFA levels in the system. According to literature, the acetoclastic methanogens, specifically, are sensitive to chlorinated compounds [34] and an inhibition most probably would have been indicated by an accumulation of acetate.[35]
4.
Conclusions • A stable anaerobic process was maintained at HRTs of 7–14 h. Thus, the changes in wastewater characteristics did not affect the process stability. • A significant higher biogas production and fTOC reduction was obtained with wastewater from HW compared to SW. • Lowering the HRT (13.5 vs. 8.5 h) did not significantly affect the process as judged from the evaluated parameters. This shows that there is likely room for a further decrease in HRT for both SW and HW wastewaters. • The biogas production in our reactors did not seem to be inhibited by present levels of AOX.
Acknowledgements The authors wish to thank the personnel at the mill for assistance during sampling and for providing information.
Environmental Technology Disclosure statement
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No potential conflict of interest was reported by the author(s).
[16]
Funding
[17]
This study was funded by the Swedish Energy Agency [project No. 32802-1], Linköping University, Scandinavian Biogas Fuels AB, Pöyry Sweden AB, BillerudKorsnäs Sweden AB and SCA.
[18]
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