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An assessment of an anaerobic filter packed with a low-cost material for treating domestic wastewater a
b
a
Alberto López-López , María Guadalupe Albarrán-Rivas , Leonel Hernández-Mena & Elizabeth León-Becerril
a
a
Centro de Investigación y Asistencia en Tecnología y Diseño del Estado de Jalisco (CIATEJ) Guadalajara , Jal., México b
Posgrado Interinstitucional de Ciencia y Tecnología (PICyT-CIATEJ) Guadalajara , Jal., México Accepted author version posted online: 25 Oct 2012.Published online: 19 Nov 2012.
To cite this article: Alberto López-López , María Guadalupe Albarrán-Rivas , Leonel Hernández-Mena & Elizabeth LeónBecerril (2013) An assessment of an anaerobic filter packed with a low-cost material for treating domestic wastewater, Environmental Technology, 34:9, 1151-1159, DOI: 10.1080/09593330.2012.743588 To link to this article: http://dx.doi.org/10.1080/09593330.2012.743588
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, No. 9, 1151–1159, http://dx.doi.org/10.1080/09593330.2012.743588
An assessment of an anaerobic filter packed with a low-cost material for treating domestic wastewater Alberto López-Lópeza∗ , María Guadalupe Albarrán-Rivasb , Leonel Hernández-Menaa and Elizabeth León-Becerrila a Centro
de Investigación y Asistencia en Tecnología y Diseño del Estado de Jalisco (CIATEJ) Guadalajara, Jal. México; b Posgrado Interinstitucional de Ciencia y Tecnología (PICyT-CIATEJ) Guadalajara, Jal. México
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(Received 31 October 2011; final version received 14 October 2012 ) The lack of available technologies that assembled both the technical and economical characteristics for domestic wastewater treatment is a major problem for rural communities. In response to this issue, a technical assessment of an up-flow anaerobic filter (UAF) was carried out in the laboratory to treat domestic wastewater. Tezontle, a volcanic rock, was used as the packing media and as the support for the biofilm. It was selected due to its abundance, low cost and high porosity. The UAF assessment was based on an experimental design of two variables, hydraulic retention time (HRT) and temperature, with three and four levels of operation, respectively. Each test at HRT of 12, 18 and 24 h was carried out at temperatures of 20, 25, 30 and 35◦ C. Methane production rates were 32.4, 110 and 191 mL of CH4 /g of chemical oxygen demand removed for HRT of 12, 18 and 24 h, respectively, at a temperature of 35◦ C. In order to reach a high removal of organic matter (higher than 80%) the optimum operational conditions for the UAF were HRT = 24 h and T = 35◦ C. The temperature was the determining factor for achieving the greatest removal efficiencies in the UAF. Keywords: domestic wastewater; rural communities; anaerobic filter; methane; Tezontle
Introduction In Mexico, wastewaters generated by rural communities are directly discharged without any previous treatment into surface water resources such as lakes, rivers, creeks or soil. These direct releases are in violation with National Regulations for wastewater discharge in Mexico [1,2]. Domestic wastewater is a potential risk for public health due to the high concentration of pathogenic microorganisms related with illnesses such as typhoid fever, dysentery, cholera, hepatitis, etc. [3]. Commercially, a variety of biological aerobic treatment systems for wastewater of domestic origin are available. However, these technologies present high investments as well as high operating and maintenance costs, making acquisition impossible for City Councils or rural communities that are personally in charge of their wastewater treatment [4,5]. This situation prompts the necessity to assess, validate and adopt treatment processes that are both technically and economically viable. In this context, research has been carried out on robust and practical technologies, such as facultative lagoons, septic tank, up-flow anaerobic sludge blanket reactors (UASB), anaerobic filters of upward or horizontal flow, UASBhybrid reactors and natural and constructed wetlands for suspended and soluble organic matter removal present in domestic and municipal wastewaters [5–8].
∗ Corresponding
One of the most attractive processes for this situation is an anaerobic system, due to the low investment costs, easy operation, minimal or null energy consumption, low sludge generation and the possibility of generating biogas (methane) as an alternative energy source, among other advantages that can be attributed to an aerobic process [9,10]. The efficiency of an anaerobic process is highly dependent on temperature and pH. The anaerobic conversion of organic matter has a maximum efficiency at 35–37◦ C for mesophilic conditions and close to 55◦ C for thermophilic conditions [10–12]. Therefore, the performance of anaerobic processes is related to temperature, in a range of 10 to 45◦ C and no important changes in the microbial ecosystem are observed [10,12]. Indeed, low temperatures reduce the hydrolysis performance (yield) and consequently the removal rate of total suspended solids (TSS) and, in general, the organic matter in the water is reduced. However, satisfactory efficiencies have been achieved at lower temperature ranges (10 to 15◦ C) for an UASB-hybrid with diluted primary sludge (total solids about 22 g/L) [8,13,14]. The performance of an anaerobic treatment is also affected by changes in pH outside of the optimal range (6.8–7.4), where acidogenic (produce acid) and methanogenic (produce methane) bacteria exist. The success of the anaerobic wastewater treatment
author. Email: [email protected]; [email protected]
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A. López-López et al.
process, operating inside an acceptable pH range, is controlled and stabilized mainly by alkalinity and is produced by the same processes, which has to be enough to counteract the production of volatile fatty acids (VFAs) [10–12]. The design and operation of anaerobic reactors was basically oriented towards the treatment of wastewater containing high organic matter concentrations (>3000 mg/L chemical oxygen demand (COD)). However, many authors [15,16] have presented an analysis of the technical viability of the anaerobic treatment of municipal wastewaters in developing countries having a tropical climate such as Colombia, India, and Brazil, using UASB reactors as an alternative treatment, and have obtained good results in large-scale reactors [5,7,16]. This result has contributed to a change in the criteria for application of these types of reactors. Halalsheh et al. [17] operated a 96 m3 UASB reactor for 2.5 years under different conditions to assess the feasibility for treating strong sewage (COD = 1531 mg/L) at ambient temperature (average temperatures of 18 and 25◦ C for winter and summer, respectively). During the first year, the reactor was operated as a two-stage system at organic loading rates (OLR) in the range of 3.6–5.0 kg COD/m3 /d for the first stage and 2.9–4.6 kg COD/m3 /d for the second one. The results of the first stage showed average removals of 51% for COD without significant effect of temperature; however the second stage reactor was unstable. The temperature affected sludge stabilization. During the second year, the first stage was operated as a single-stage UASB reactor at half of the previous loading rates. The results showed an average removal efficiency of 62% for COD during summer, while it dropped to 51% during wintertime. Another type of reactor used for this type of process is the anaerobic filter (AF). This filter is basically used to treat wastewaters with high concentrations of organic matter [10,18]. The use of the AF reactor for the treatment of wastewater was initially proposed in the late 1960’s and the interest in the study of this technology continues over the years [9,19]. The AF reactor consists of a column filled with packing media or material that works as support for the reproduction and growth of anaerobic bacteria as a biofilm. The wastewater flows upwards or downwards with a piston flow regime, in contact with the biofilm [10,13,19,20]. The process requires neither mechanical energy nor complex instrumentation, which is reflected in low investment, and low operating and maintenance costs. The AF also buffers the small variations in pH and it is capable of removing slowly decomposing substrates. It is highly tolerant to changes in organic loading and it is easily adaptable to variations in temperature. However, it does have its disadvantages and one of the most important is clogging as a consequence of a deficient maintenance program and the sludge has to be wasted [10,18,21]. The performance of an AF and anaerobic hybrid (AH) reactor, both working in parallel, for the treatment of domestic wastewaters at 13◦ C and HRT = 4 h, was investigated by
Elmitwalli1 et al. [13]. The AF and the top of the AH reactor consisted of vertical sheets of reticulated polyurethane foam with knobs. The AF reactor showed a significantly higher removal of COD than the AH reactor, 55% and 34%, respectively. However, if both reactors work in series, the removal of COD is higher than 71%, these efficiencies were similar to other values reported in tropical areas. Elmitwalli et al. [22] tested different types of packing material such as seashell, synthetic material, vitrified material and river stone.. The best packing media was seashell, obtaining COD removals of 89.7% for organic loads of 3.7 to 5.1 kg COD/m3 /d and HRT = 26 h. The specific surface area, porosity, surface roughness, porous size and orientation of the packing materials play an important role in the AF reactor performance. In this way, the performance of an AF reactor could be improved with high specific surface area and porosity, large porous size and rough surface for packing material [23]. Also, the morphology of the packing material has an influence in the restart performance of the AF reactors that have remained inactive for long time [19]. The aim of this work was to assess the performance at a laboratory scale of a UAF using Tezontle (a volcanic rock native to Mexico) as the packing material for the treatment of domestic wastewaters at different operating conditions (HRT and temperature). Reducing the organic matter is the principal aim of the UAF in the complete system of treatment of wastewater. Materials and methods Sampling and wastewater analysis The wastewater was taken from the influent of a domestic wastewater treatment plant after screening. Wastewater sampling was carried out once a week for 3 months, collecting grab samples of 0.5 L every 2 h during an 8 h lapse to obtain a composite sample of 4 L, which was analyzed according to standardized methods [24]. Parallel to this, wastewater volumes of about 20 L were collected to carry out treatability studies in the UAF. Experimental system The experimental system consisted of an UAF reactor (Armfield MOD: W8) with a total volume of 5 L. Tezontle (volcanic rock) was used as a supporting medium for the biofilm, it has an interstitial porosity ε = 0.65. The reactor was operated with an effective volume of 3.25 L. A storage tank of 2 L was filled twice daily with wastewater kept at 4◦ C, then when the wastewater has reached room temperature, it was fed to the reactor and this kept the physicochemical characteristics of the wastewater supply constant. The UAF was fed from a wastewater storage tank using a peristaltic pump previously calibrated and the effluent and biogas generated (basically CO2 and CH4 ) were recovered at the upper part of the UAF. The biogas was passed by
Environmental Technology
NaOH Effluent
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Anaerobic filter
Feed tank
Figure 1.
Peristaltic pump
Experimental device used in this study.
a tramp consisting of 3 M sodium hydroxide to retain the CO2 ; the CH4 that passed through the tramp was measured using a water displacement gas meter. A thermostat was used to maintain a constant temperature (Figure 1). UAF packing material or biofilm support When considering the comparative properties of the different materials presented in Table 1, Tezontle was selected due to its high porosity, which is around 65% of the volume of the pore for a grain size of 10–20 mm. Its porous structure generates a relationship of surface area per volume of material of 390 m2 /m3 , which is similar to that of some synthetic materials that present an average of 200 m2 /m3 [18,25]. The chemical composition of Tezontle is based mainly on silicates, in combination with aluminium and ferrous oxides.
Table 1.
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Its structure also contains sodium, calcium and potassium groups. More information about the composition of the Tezontle used is presented in López-López et al. [18]. Above all, Tezontle is a low cost material that is also abundant in Mexico with respect to synthetic materials. Its performance as a packing material has been proved in different wastewater treatment processes, such as filtration in artificial wetlands and also in aerobic processes at pilot scales [25].
Inoculation and UAF start-up The UAF start-up began with its inoculation, using 1 L (20% from the UAF total volume) of the anaerobic sludge coming from an anaerobic vinasse treatment facility. The concentration of volatile suspended solids (VSS) and TSS of the inoculum were 18 000 mg/L and 15 500, respectively, so the ratio VSS/TSS = 0.86 and this ratio indicates high biological activity [10]. Then, the wastewater was manually added to the reactor, having an initial COD concentration of 100 mg/L.
Adaptation stage The adaptation stage of the inoculum or microbial mass to the new substrate contained in domestic wastewater took two weeks. The concentration of COD used was approximately 100 mg/L and the raw wastewater was diluted with distilled water. The reactor was operated continuously with a HRT of 48 h, at an average room temperature of 24◦ C.
Growth stage During the following four weeks, the COD concentration was gradually increased from 100 to 700 mg/L and feeding was carried out in a continuous manner with a HRT of 48 h and at an average room temperature of 24◦ C. The wastewater dilution was carried out using demineralized water.
Properties of different biofilm packing media (values adapted from [3,18,25]).
Types of packing media
Nominal size (cm)
Specific surface area (m2 /m3 )
Void spaces (%)
Costa (USD/m3 )
Tezontle Pumice Stone Gravel River Stone River Stone Plastic conventional Plastic high-specific surface Redwood Random pack
1–2 0–0.9 2.5–7.5 2.5–10 10–13 60 × 60 × 120b 60 × 60 × 120b 120 × 120 × 50b 2.5–8.9
390 360–600 45 56–70 40–164 80–100 100–200 40–50 125–280
∼65 ∼85 ∼50 40–50 50–60 94–97 94–97 70–80 90–95
10 10 22 13 13 150 250 — 585
Note: a Prices in USD, 2010. b Module size.
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A. López-López et al. Table 2.
Experimental conditions for AF trials.
Stage
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Adaptation Growth Stabilization
Operating period (d)
COD (mg/L)
HRT (h)
OLR kg/m3 /d
Temperature (◦ C)
14 28 182
100 100–700 400–700
48 48 12
0.05 0.05–0.35 0.80–1.40
18
0.53–0.93
24
0.40–0.70
Room temperature Room temperature 20 25 30 35 20 25 30 35 20 25 30 35
Stabilization stage After six weeks, a constant COD removal was observed, along with a constant and stable biogas generation. These operating characteristics were taken as indicators of the UAF stabilization [10,15], the assessment of the UAF lasted 32 weeks (224 days). Once the reactor was stabilized, 12 wastewater treatment trials were carried out, following an experimental design of two variables (HRT and temperature) of three and four levels of operation, respectively, i.e. each test at HRT of 12, 18 and 24 h [9,19] was carried out at 20, 25, 30 and 35◦ C (average temperatures in Mexico). The UAF operating conditions are summarized in Table 2. The response or monitoring parameters during the UAF operation were: COD, pH, alkalinity and CH4 production. Each operational condition reached a stabilization stage and consequently a new steady state in a period of time of three weeks. Results and discussion The characteristics of the wastewater before the treatment are reported in Table 3, which was analyzed according to standardized methods [24]. Figure 2 shows the removal efficiency profiles of organic matter measured as COD in the AF. Two important stages of the operating period of the AF are observed: (i) an adaptation and growth stage, and (ii) a stabilization stage. (i) During the first six weeks of adaptation and growth of the biofilm, COD removals higher than 50% were gradually achieved. This fact was attributed more to the physical phenomena of filtration and sedimentation rather than to biologic means. Tezontle worked as a mean of filtration, since the biofilm began its formation and biogas production was minimal [18]. Because of the increasing of the concentration of the organic matter in the feeding of the reactor, an
important growth of the biofilm on Tezontle was visually observed. (ii) COD removal efficiencies and biogas production were stable from the 6th week, with both indicators showing the stabilization of the reactor. In this stabilization stage, as the temperature increased, for each HRT, the COD removal efficiencies improved, except for the experiments carried out at HRT of 12 h and 25◦ C and at HRT of 18 h and 30◦ C, where the efficiencies were against the previously established criteria. The performances of the reactor at those conditions are linked to climate and technical problems beyond our reach (energy supply failure). However, it was also observed that for an established temperature, the efficiencies improved when the HRT were higher (24 h). Table 4 shows a summary of the COD removal efficiencies during the operation of the stabilized UAF; the values presented are an average of five sampling points. The excess sludge produced was wasted in a volume of 200 mL of sludge once a month, or when it was observed an amount of sludge deposited in the reactor. This practice maintained the correct performance of the reactor. TSS concentration of the sludge was quantified as 11 500 mg/L. COD removal as a function of temperature Figure 3 shows the COD removal profiles of organic matter measured as COD as a function of temperature at different HRT. The COD removal efficiency improves considerably as the temperature increases for the three HRT used. For a HRT of 24 h, the COD removal increased by an order of 28.7 (shift of 57 to 85.7 %) when the temperature shifted from 20 to 35◦ C; whereas, for a HRT of 12 and 24 h, the efficiency only increased 20%. Bodík et al. [26] found average removal efficiencies between 46–92% with real municipal
Environmental Technology Table 3.
Characterization of the composite samples of domestic wastewaters.
Parameter
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pH Turbidity BOD COD Fats and oils TS TSS VSS Sed. S MBAS Total N Total P Total coliforms Faecal coliforms Salmonella
Unity
Value
Ref. [24]
U.P. U.T. mg/L mg/L mg/L mg/L mg/L mg/L ml/L mg/L mg/L mg/L MPN/100 mL MPN/100 mL in 25 mL of sample
7.13 ± 0.23 42.0 ± 15.3 354.0 ± 74.4 574.6 ± 92.1 49.2 ± 4.8 1038.0 ± 215.3 235.3 ± 49.4 213.8 ± 39.3 10.2 ± 3.4 45.9 ± 7.1 45.0 ± 9.2 9.2 ± 3.3 6.7 × 106 ± 5.5 × 106 4.6 × 104 ± 4.6 × 103 Positive
Method 4500-H+ Method 2130 A Method 5210 B Method 5220 D Method 5520 B Method 2540 B Method 2540 D Method 2540 E Method 2540 F UV spectrophotometry Method 4500-N Method 4500 P Method 9221 B Method 4500 E NOM-114-SSA1-1994
Note: BOD: Biochemical Oxygen Demand, COD: Chemical Oxygen Demand, Fats and oils, TS: Total Solids, TSS: Total Suspended Solids, VSS: Volatile Suspended Solids, Sed. S: Sedimentable Solids, Total N: total organic Nitrogen, Total P: Total phosphorus, MBAS: Methylene Blue Active Substances, MPN: Most Probable Number.
Figure 2.
Removal profiles of organic matter during treatability trials.
Table 4.
COD removal efficiencies in the UAF. HRT (h) 12
T (◦ C) CODi (mg/L) CODe (mg/L) COD removal (%)
18
24
20
25
30
35
20
25
30
35
20
25
30
35
475.2 284.6 59.9
535.2 266.8 50.1
584.8 216.6 62.8
624.6 161.6 74.1
529.4 233.2 56.0
592.0 159.6 73.0
436.6 218.8 50.0
579.2 147.0 74.6
616.6 265.0 57.0
555.2 122.8 77.9
718.0 173.4 75.0
626.4 90.0 85.7
Note: i = inlet; e = outlet.
wastewater for an UAF operating at a temperature range of 9 to 23◦ C and at different HRTs from 6 to 46 h. The results show higher COD removal rates achieved at higher temperatures and higher hydraulic retention times. Therefore, the temperature was the determining factor for achieving the
COD removal efficiencies. Halalsheh et al. [17] operated an UASB reactor during two years and in the winter and summer the COD removal efficiencies were 51% and 62%, respectively. The difference was attributed to the average temperatures for each time of year.
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A. López-López et al. Table 5 shows the difference in the mean values of pH and alkalinity in the inlet and outlet flows of the UAF as a function of temperature and HRT. Particularly, at a HRT of 24 h and at different temperatures, the outlet pH values are 0.5 ± 0.1 higher than the inlet pH and the outlet alkalinity is 60 ± 30 CaCO3 mg/L higher than the inlet alkalinity. The variation in the net alkalinity production is associated with water quality of the inlet flow, but above all, it is associated with the good performance of the AF, i.e. the nature of the process generates net alkalinities in quantities that are equal to the difference in alkalinity between the inlet and outlet flows of the AF and proportional to the performance of the AF [10,12].
100 90
COD removed (%)
80
70 60 50 40
HRT = 12 h
30
HRT = 18 h
20
HRT = 24 h
10
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0 10
15
20
25
30
35
40
45
Temperature (°C)
Figure 3.
COD removal at different temperatures.
The effect of pH and alkalinity Figure 4 shows the pH and alkalinity profiles in the inlet and outlet flow of the UAF during the stabilization period. The pH of the outlet wastewater was slightly higher compared to the inlet pH (at any cases with at least p < 0.008 in all cases), except for the higher inlets pH at HRT of 12 and 18 h and 20◦ C (p < 0.009 at both cases) and HRT of 18 h and 30◦ C (p > 0.05), where a pH drop was observed. These exceptions are associated with the technical problems mentioned above. Increasing of alkalinity and pH levels observed in the reactor effluent rare due to the organically-bound nitrogen degradation and of ammonium bicarbonate (ammonium and bicarbonate ions) formation in the anaerobic process [18,19].
Figure 4.
pH and alkalinity profiles during treatability trials.
Biogas generation in the UAF During the trials carried out for a specific HRT and at different temperatures, the volume of CH4 generated and accumulated was proportional to the amount of COD removed during the UAF operation. Figure 5 shows the accumulated values of the CH4 volume generated as a function of the COD removed by day, corresponding to the experiments carried out at 35◦ C, for three different HRT. After the linear regression analysis by least-squares, the respective straight line equations were obtained, having a R2 coefficient of determination higher than 0.98, where the slope of each of the straight lines represents the methane production rate in mL per gram of COD removed (PR-CH4 : CH4 mL/gCOD). The values obtained for PR-CH4 were 32.4, 110 and 191 CH4 mL/g COD for HRT of 12, 18 and 24 h, respectively (25.0; 84.8; 147.5 CH4 mL/g at STP). These are lower than the expected theoretical value of 350 mL CH4 /g COD removed [10]. The latter indicates that for a HRT of 24 h, the anaerobic degradation process of
Environmental Technology Table 5.
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Alkalinity and pH behaviour as a function of HRT and inlet and outlet temperature of the AF. HRT 12
18 Alkalinitya
pH
24 Alkalinitya
pH
Alkalinitya
pH
T (◦ C)
i
e
i
e
i
e
i
e
i
e
i
e
20 25 30 35 Mean
7.26 7.28 7.11 7.08 7.18
7.0 7.55 7.55 7.58 7.42
402 449 290 343 371
359 439 330 384 378
7.37 7.11 6.96 7.16 7.15
7.06 7.78 6.87 7.78 7.37
387 336 356 355 359
358 376 367 406 377
7.41 7.23 7.16 6.97 7.19
7.99 7.99 7.78 7.50 7.82
321 360 327 303 328
364 435 350 425 394
1800
y = 191x R2 = 0.99
HRT = 12
1600
HRT = 18
1400
CH4 generated (mL)
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Note: a Alkalinity units in CaCO3 mg/L.
y = 110x R2 = 0.98
HRT = 24
1200 1000
800 y = 32.4x R² = 0.98
600 400 200 0 0
2
4
6
8
10
12
COD removed (g)
Figure 5. (35◦ C).
between the substrate and nutrients with the microorganisms. Similar profiles were obtained with the experiments carried out at 20, 25 and 30◦ C; whereas, the PR-CH4 was lower than those obtained at 35◦ C. This phenomenon is associated with the drop in the efficiencies of a process of anaerobic treatment, when there is a temperature drop. Low values of PR-CH4 were obtained in this work because the hydrolysis rate is greater than the methanogenesis rate, especially when wastewater and have low concentration of soluble substrates, i.e. anaerobic systems operating at
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
An assessment of an anaerobic filter packed with a low-cost material for treating domestic wastewater a
b
a
Alberto López-López , María Guadalupe Albarrán-Rivas , Leonel Hernández-Mena & Elizabeth León-Becerril
a
a
Centro de Investigación y Asistencia en Tecnología y Diseño del Estado de Jalisco (CIATEJ) Guadalajara , Jal., México b
Posgrado Interinstitucional de Ciencia y Tecnología (PICyT-CIATEJ) Guadalajara , Jal., México Accepted author version posted online: 25 Oct 2012.Published online: 19 Nov 2012.
To cite this article: Alberto López-López , María Guadalupe Albarrán-Rivas , Leonel Hernández-Mena & Elizabeth LeónBecerril (2013) An assessment of an anaerobic filter packed with a low-cost material for treating domestic wastewater, Environmental Technology, 34:9, 1151-1159, DOI: 10.1080/09593330.2012.743588 To link to this article: http://dx.doi.org/10.1080/09593330.2012.743588
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, No. 9, 1151–1159, http://dx.doi.org/10.1080/09593330.2012.743588
An assessment of an anaerobic filter packed with a low-cost material for treating domestic wastewater Alberto López-Lópeza∗ , María Guadalupe Albarrán-Rivasb , Leonel Hernández-Menaa and Elizabeth León-Becerrila a Centro
de Investigación y Asistencia en Tecnología y Diseño del Estado de Jalisco (CIATEJ) Guadalajara, Jal. México; b Posgrado Interinstitucional de Ciencia y Tecnología (PICyT-CIATEJ) Guadalajara, Jal. México
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(Received 31 October 2011; final version received 14 October 2012 ) The lack of available technologies that assembled both the technical and economical characteristics for domestic wastewater treatment is a major problem for rural communities. In response to this issue, a technical assessment of an up-flow anaerobic filter (UAF) was carried out in the laboratory to treat domestic wastewater. Tezontle, a volcanic rock, was used as the packing media and as the support for the biofilm. It was selected due to its abundance, low cost and high porosity. The UAF assessment was based on an experimental design of two variables, hydraulic retention time (HRT) and temperature, with three and four levels of operation, respectively. Each test at HRT of 12, 18 and 24 h was carried out at temperatures of 20, 25, 30 and 35◦ C. Methane production rates were 32.4, 110 and 191 mL of CH4 /g of chemical oxygen demand removed for HRT of 12, 18 and 24 h, respectively, at a temperature of 35◦ C. In order to reach a high removal of organic matter (higher than 80%) the optimum operational conditions for the UAF were HRT = 24 h and T = 35◦ C. The temperature was the determining factor for achieving the greatest removal efficiencies in the UAF. Keywords: domestic wastewater; rural communities; anaerobic filter; methane; Tezontle
Introduction In Mexico, wastewaters generated by rural communities are directly discharged without any previous treatment into surface water resources such as lakes, rivers, creeks or soil. These direct releases are in violation with National Regulations for wastewater discharge in Mexico [1,2]. Domestic wastewater is a potential risk for public health due to the high concentration of pathogenic microorganisms related with illnesses such as typhoid fever, dysentery, cholera, hepatitis, etc. [3]. Commercially, a variety of biological aerobic treatment systems for wastewater of domestic origin are available. However, these technologies present high investments as well as high operating and maintenance costs, making acquisition impossible for City Councils or rural communities that are personally in charge of their wastewater treatment [4,5]. This situation prompts the necessity to assess, validate and adopt treatment processes that are both technically and economically viable. In this context, research has been carried out on robust and practical technologies, such as facultative lagoons, septic tank, up-flow anaerobic sludge blanket reactors (UASB), anaerobic filters of upward or horizontal flow, UASBhybrid reactors and natural and constructed wetlands for suspended and soluble organic matter removal present in domestic and municipal wastewaters [5–8].
∗ Corresponding
One of the most attractive processes for this situation is an anaerobic system, due to the low investment costs, easy operation, minimal or null energy consumption, low sludge generation and the possibility of generating biogas (methane) as an alternative energy source, among other advantages that can be attributed to an aerobic process [9,10]. The efficiency of an anaerobic process is highly dependent on temperature and pH. The anaerobic conversion of organic matter has a maximum efficiency at 35–37◦ C for mesophilic conditions and close to 55◦ C for thermophilic conditions [10–12]. Therefore, the performance of anaerobic processes is related to temperature, in a range of 10 to 45◦ C and no important changes in the microbial ecosystem are observed [10,12]. Indeed, low temperatures reduce the hydrolysis performance (yield) and consequently the removal rate of total suspended solids (TSS) and, in general, the organic matter in the water is reduced. However, satisfactory efficiencies have been achieved at lower temperature ranges (10 to 15◦ C) for an UASB-hybrid with diluted primary sludge (total solids about 22 g/L) [8,13,14]. The performance of an anaerobic treatment is also affected by changes in pH outside of the optimal range (6.8–7.4), where acidogenic (produce acid) and methanogenic (produce methane) bacteria exist. The success of the anaerobic wastewater treatment
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© 2013 Taylor & Francis
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process, operating inside an acceptable pH range, is controlled and stabilized mainly by alkalinity and is produced by the same processes, which has to be enough to counteract the production of volatile fatty acids (VFAs) [10–12]. The design and operation of anaerobic reactors was basically oriented towards the treatment of wastewater containing high organic matter concentrations (>3000 mg/L chemical oxygen demand (COD)). However, many authors [15,16] have presented an analysis of the technical viability of the anaerobic treatment of municipal wastewaters in developing countries having a tropical climate such as Colombia, India, and Brazil, using UASB reactors as an alternative treatment, and have obtained good results in large-scale reactors [5,7,16]. This result has contributed to a change in the criteria for application of these types of reactors. Halalsheh et al. [17] operated a 96 m3 UASB reactor for 2.5 years under different conditions to assess the feasibility for treating strong sewage (COD = 1531 mg/L) at ambient temperature (average temperatures of 18 and 25◦ C for winter and summer, respectively). During the first year, the reactor was operated as a two-stage system at organic loading rates (OLR) in the range of 3.6–5.0 kg COD/m3 /d for the first stage and 2.9–4.6 kg COD/m3 /d for the second one. The results of the first stage showed average removals of 51% for COD without significant effect of temperature; however the second stage reactor was unstable. The temperature affected sludge stabilization. During the second year, the first stage was operated as a single-stage UASB reactor at half of the previous loading rates. The results showed an average removal efficiency of 62% for COD during summer, while it dropped to 51% during wintertime. Another type of reactor used for this type of process is the anaerobic filter (AF). This filter is basically used to treat wastewaters with high concentrations of organic matter [10,18]. The use of the AF reactor for the treatment of wastewater was initially proposed in the late 1960’s and the interest in the study of this technology continues over the years [9,19]. The AF reactor consists of a column filled with packing media or material that works as support for the reproduction and growth of anaerobic bacteria as a biofilm. The wastewater flows upwards or downwards with a piston flow regime, in contact with the biofilm [10,13,19,20]. The process requires neither mechanical energy nor complex instrumentation, which is reflected in low investment, and low operating and maintenance costs. The AF also buffers the small variations in pH and it is capable of removing slowly decomposing substrates. It is highly tolerant to changes in organic loading and it is easily adaptable to variations in temperature. However, it does have its disadvantages and one of the most important is clogging as a consequence of a deficient maintenance program and the sludge has to be wasted [10,18,21]. The performance of an AF and anaerobic hybrid (AH) reactor, both working in parallel, for the treatment of domestic wastewaters at 13◦ C and HRT = 4 h, was investigated by
Elmitwalli1 et al. [13]. The AF and the top of the AH reactor consisted of vertical sheets of reticulated polyurethane foam with knobs. The AF reactor showed a significantly higher removal of COD than the AH reactor, 55% and 34%, respectively. However, if both reactors work in series, the removal of COD is higher than 71%, these efficiencies were similar to other values reported in tropical areas. Elmitwalli et al. [22] tested different types of packing material such as seashell, synthetic material, vitrified material and river stone.. The best packing media was seashell, obtaining COD removals of 89.7% for organic loads of 3.7 to 5.1 kg COD/m3 /d and HRT = 26 h. The specific surface area, porosity, surface roughness, porous size and orientation of the packing materials play an important role in the AF reactor performance. In this way, the performance of an AF reactor could be improved with high specific surface area and porosity, large porous size and rough surface for packing material [23]. Also, the morphology of the packing material has an influence in the restart performance of the AF reactors that have remained inactive for long time [19]. The aim of this work was to assess the performance at a laboratory scale of a UAF using Tezontle (a volcanic rock native to Mexico) as the packing material for the treatment of domestic wastewaters at different operating conditions (HRT and temperature). Reducing the organic matter is the principal aim of the UAF in the complete system of treatment of wastewater. Materials and methods Sampling and wastewater analysis The wastewater was taken from the influent of a domestic wastewater treatment plant after screening. Wastewater sampling was carried out once a week for 3 months, collecting grab samples of 0.5 L every 2 h during an 8 h lapse to obtain a composite sample of 4 L, which was analyzed according to standardized methods [24]. Parallel to this, wastewater volumes of about 20 L were collected to carry out treatability studies in the UAF. Experimental system The experimental system consisted of an UAF reactor (Armfield MOD: W8) with a total volume of 5 L. Tezontle (volcanic rock) was used as a supporting medium for the biofilm, it has an interstitial porosity ε = 0.65. The reactor was operated with an effective volume of 3.25 L. A storage tank of 2 L was filled twice daily with wastewater kept at 4◦ C, then when the wastewater has reached room temperature, it was fed to the reactor and this kept the physicochemical characteristics of the wastewater supply constant. The UAF was fed from a wastewater storage tank using a peristaltic pump previously calibrated and the effluent and biogas generated (basically CO2 and CH4 ) were recovered at the upper part of the UAF. The biogas was passed by
Environmental Technology
NaOH Effluent
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Anaerobic filter
Feed tank
Figure 1.
Peristaltic pump
Experimental device used in this study.
a tramp consisting of 3 M sodium hydroxide to retain the CO2 ; the CH4 that passed through the tramp was measured using a water displacement gas meter. A thermostat was used to maintain a constant temperature (Figure 1). UAF packing material or biofilm support When considering the comparative properties of the different materials presented in Table 1, Tezontle was selected due to its high porosity, which is around 65% of the volume of the pore for a grain size of 10–20 mm. Its porous structure generates a relationship of surface area per volume of material of 390 m2 /m3 , which is similar to that of some synthetic materials that present an average of 200 m2 /m3 [18,25]. The chemical composition of Tezontle is based mainly on silicates, in combination with aluminium and ferrous oxides.
Table 1.
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Its structure also contains sodium, calcium and potassium groups. More information about the composition of the Tezontle used is presented in López-López et al. [18]. Above all, Tezontle is a low cost material that is also abundant in Mexico with respect to synthetic materials. Its performance as a packing material has been proved in different wastewater treatment processes, such as filtration in artificial wetlands and also in aerobic processes at pilot scales [25].
Inoculation and UAF start-up The UAF start-up began with its inoculation, using 1 L (20% from the UAF total volume) of the anaerobic sludge coming from an anaerobic vinasse treatment facility. The concentration of volatile suspended solids (VSS) and TSS of the inoculum were 18 000 mg/L and 15 500, respectively, so the ratio VSS/TSS = 0.86 and this ratio indicates high biological activity [10]. Then, the wastewater was manually added to the reactor, having an initial COD concentration of 100 mg/L.
Adaptation stage The adaptation stage of the inoculum or microbial mass to the new substrate contained in domestic wastewater took two weeks. The concentration of COD used was approximately 100 mg/L and the raw wastewater was diluted with distilled water. The reactor was operated continuously with a HRT of 48 h, at an average room temperature of 24◦ C.
Growth stage During the following four weeks, the COD concentration was gradually increased from 100 to 700 mg/L and feeding was carried out in a continuous manner with a HRT of 48 h and at an average room temperature of 24◦ C. The wastewater dilution was carried out using demineralized water.
Properties of different biofilm packing media (values adapted from [3,18,25]).
Types of packing media
Nominal size (cm)
Specific surface area (m2 /m3 )
Void spaces (%)
Costa (USD/m3 )
Tezontle Pumice Stone Gravel River Stone River Stone Plastic conventional Plastic high-specific surface Redwood Random pack
1–2 0–0.9 2.5–7.5 2.5–10 10–13 60 × 60 × 120b 60 × 60 × 120b 120 × 120 × 50b 2.5–8.9
390 360–600 45 56–70 40–164 80–100 100–200 40–50 125–280
∼65 ∼85 ∼50 40–50 50–60 94–97 94–97 70–80 90–95
10 10 22 13 13 150 250 — 585
Note: a Prices in USD, 2010. b Module size.
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A. López-López et al. Table 2.
Experimental conditions for AF trials.
Stage
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Adaptation Growth Stabilization
Operating period (d)
COD (mg/L)
HRT (h)
OLR kg/m3 /d
Temperature (◦ C)
14 28 182
100 100–700 400–700
48 48 12
0.05 0.05–0.35 0.80–1.40
18
0.53–0.93
24
0.40–0.70
Room temperature Room temperature 20 25 30 35 20 25 30 35 20 25 30 35
Stabilization stage After six weeks, a constant COD removal was observed, along with a constant and stable biogas generation. These operating characteristics were taken as indicators of the UAF stabilization [10,15], the assessment of the UAF lasted 32 weeks (224 days). Once the reactor was stabilized, 12 wastewater treatment trials were carried out, following an experimental design of two variables (HRT and temperature) of three and four levels of operation, respectively, i.e. each test at HRT of 12, 18 and 24 h [9,19] was carried out at 20, 25, 30 and 35◦ C (average temperatures in Mexico). The UAF operating conditions are summarized in Table 2. The response or monitoring parameters during the UAF operation were: COD, pH, alkalinity and CH4 production. Each operational condition reached a stabilization stage and consequently a new steady state in a period of time of three weeks. Results and discussion The characteristics of the wastewater before the treatment are reported in Table 3, which was analyzed according to standardized methods [24]. Figure 2 shows the removal efficiency profiles of organic matter measured as COD in the AF. Two important stages of the operating period of the AF are observed: (i) an adaptation and growth stage, and (ii) a stabilization stage. (i) During the first six weeks of adaptation and growth of the biofilm, COD removals higher than 50% were gradually achieved. This fact was attributed more to the physical phenomena of filtration and sedimentation rather than to biologic means. Tezontle worked as a mean of filtration, since the biofilm began its formation and biogas production was minimal [18]. Because of the increasing of the concentration of the organic matter in the feeding of the reactor, an
important growth of the biofilm on Tezontle was visually observed. (ii) COD removal efficiencies and biogas production were stable from the 6th week, with both indicators showing the stabilization of the reactor. In this stabilization stage, as the temperature increased, for each HRT, the COD removal efficiencies improved, except for the experiments carried out at HRT of 12 h and 25◦ C and at HRT of 18 h and 30◦ C, where the efficiencies were against the previously established criteria. The performances of the reactor at those conditions are linked to climate and technical problems beyond our reach (energy supply failure). However, it was also observed that for an established temperature, the efficiencies improved when the HRT were higher (24 h). Table 4 shows a summary of the COD removal efficiencies during the operation of the stabilized UAF; the values presented are an average of five sampling points. The excess sludge produced was wasted in a volume of 200 mL of sludge once a month, or when it was observed an amount of sludge deposited in the reactor. This practice maintained the correct performance of the reactor. TSS concentration of the sludge was quantified as 11 500 mg/L. COD removal as a function of temperature Figure 3 shows the COD removal profiles of organic matter measured as COD as a function of temperature at different HRT. The COD removal efficiency improves considerably as the temperature increases for the three HRT used. For a HRT of 24 h, the COD removal increased by an order of 28.7 (shift of 57 to 85.7 %) when the temperature shifted from 20 to 35◦ C; whereas, for a HRT of 12 and 24 h, the efficiency only increased 20%. Bodík et al. [26] found average removal efficiencies between 46–92% with real municipal
Environmental Technology Table 3.
Characterization of the composite samples of domestic wastewaters.
Parameter
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pH Turbidity BOD COD Fats and oils TS TSS VSS Sed. S MBAS Total N Total P Total coliforms Faecal coliforms Salmonella
Unity
Value
Ref. [24]
U.P. U.T. mg/L mg/L mg/L mg/L mg/L mg/L ml/L mg/L mg/L mg/L MPN/100 mL MPN/100 mL in 25 mL of sample
7.13 ± 0.23 42.0 ± 15.3 354.0 ± 74.4 574.6 ± 92.1 49.2 ± 4.8 1038.0 ± 215.3 235.3 ± 49.4 213.8 ± 39.3 10.2 ± 3.4 45.9 ± 7.1 45.0 ± 9.2 9.2 ± 3.3 6.7 × 106 ± 5.5 × 106 4.6 × 104 ± 4.6 × 103 Positive
Method 4500-H+ Method 2130 A Method 5210 B Method 5220 D Method 5520 B Method 2540 B Method 2540 D Method 2540 E Method 2540 F UV spectrophotometry Method 4500-N Method 4500 P Method 9221 B Method 4500 E NOM-114-SSA1-1994
Note: BOD: Biochemical Oxygen Demand, COD: Chemical Oxygen Demand, Fats and oils, TS: Total Solids, TSS: Total Suspended Solids, VSS: Volatile Suspended Solids, Sed. S: Sedimentable Solids, Total N: total organic Nitrogen, Total P: Total phosphorus, MBAS: Methylene Blue Active Substances, MPN: Most Probable Number.
Figure 2.
Removal profiles of organic matter during treatability trials.
Table 4.
COD removal efficiencies in the UAF. HRT (h) 12
T (◦ C) CODi (mg/L) CODe (mg/L) COD removal (%)
18
24
20
25
30
35
20
25
30
35
20
25
30
35
475.2 284.6 59.9
535.2 266.8 50.1
584.8 216.6 62.8
624.6 161.6 74.1
529.4 233.2 56.0
592.0 159.6 73.0
436.6 218.8 50.0
579.2 147.0 74.6
616.6 265.0 57.0
555.2 122.8 77.9
718.0 173.4 75.0
626.4 90.0 85.7
Note: i = inlet; e = outlet.
wastewater for an UAF operating at a temperature range of 9 to 23◦ C and at different HRTs from 6 to 46 h. The results show higher COD removal rates achieved at higher temperatures and higher hydraulic retention times. Therefore, the temperature was the determining factor for achieving the
COD removal efficiencies. Halalsheh et al. [17] operated an UASB reactor during two years and in the winter and summer the COD removal efficiencies were 51% and 62%, respectively. The difference was attributed to the average temperatures for each time of year.
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A. López-López et al. Table 5 shows the difference in the mean values of pH and alkalinity in the inlet and outlet flows of the UAF as a function of temperature and HRT. Particularly, at a HRT of 24 h and at different temperatures, the outlet pH values are 0.5 ± 0.1 higher than the inlet pH and the outlet alkalinity is 60 ± 30 CaCO3 mg/L higher than the inlet alkalinity. The variation in the net alkalinity production is associated with water quality of the inlet flow, but above all, it is associated with the good performance of the AF, i.e. the nature of the process generates net alkalinities in quantities that are equal to the difference in alkalinity between the inlet and outlet flows of the AF and proportional to the performance of the AF [10,12].
100 90
COD removed (%)
80
70 60 50 40
HRT = 12 h
30
HRT = 18 h
20
HRT = 24 h
10
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0 10
15
20
25
30
35
40
45
Temperature (°C)
Figure 3.
COD removal at different temperatures.
The effect of pH and alkalinity Figure 4 shows the pH and alkalinity profiles in the inlet and outlet flow of the UAF during the stabilization period. The pH of the outlet wastewater was slightly higher compared to the inlet pH (at any cases with at least p < 0.008 in all cases), except for the higher inlets pH at HRT of 12 and 18 h and 20◦ C (p < 0.009 at both cases) and HRT of 18 h and 30◦ C (p > 0.05), where a pH drop was observed. These exceptions are associated with the technical problems mentioned above. Increasing of alkalinity and pH levels observed in the reactor effluent rare due to the organically-bound nitrogen degradation and of ammonium bicarbonate (ammonium and bicarbonate ions) formation in the anaerobic process [18,19].
Figure 4.
pH and alkalinity profiles during treatability trials.
Biogas generation in the UAF During the trials carried out for a specific HRT and at different temperatures, the volume of CH4 generated and accumulated was proportional to the amount of COD removed during the UAF operation. Figure 5 shows the accumulated values of the CH4 volume generated as a function of the COD removed by day, corresponding to the experiments carried out at 35◦ C, for three different HRT. After the linear regression analysis by least-squares, the respective straight line equations were obtained, having a R2 coefficient of determination higher than 0.98, where the slope of each of the straight lines represents the methane production rate in mL per gram of COD removed (PR-CH4 : CH4 mL/gCOD). The values obtained for PR-CH4 were 32.4, 110 and 191 CH4 mL/g COD for HRT of 12, 18 and 24 h, respectively (25.0; 84.8; 147.5 CH4 mL/g at STP). These are lower than the expected theoretical value of 350 mL CH4 /g COD removed [10]. The latter indicates that for a HRT of 24 h, the anaerobic degradation process of
Environmental Technology Table 5.
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Alkalinity and pH behaviour as a function of HRT and inlet and outlet temperature of the AF. HRT 12
18 Alkalinitya
pH
24 Alkalinitya
pH
Alkalinitya
pH
T (◦ C)
i
e
i
e
i
e
i
e
i
e
i
e
20 25 30 35 Mean
7.26 7.28 7.11 7.08 7.18
7.0 7.55 7.55 7.58 7.42
402 449 290 343 371
359 439 330 384 378
7.37 7.11 6.96 7.16 7.15
7.06 7.78 6.87 7.78 7.37
387 336 356 355 359
358 376 367 406 377
7.41 7.23 7.16 6.97 7.19
7.99 7.99 7.78 7.50 7.82
321 360 327 303 328
364 435 350 425 394
1800
y = 191x R2 = 0.99
HRT = 12
1600
HRT = 18
1400
CH4 generated (mL)
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Note: a Alkalinity units in CaCO3 mg/L.
y = 110x R2 = 0.98
HRT = 24
1200 1000
800 y = 32.4x R² = 0.98
600 400 200 0 0
2
4
6
8
10
12
COD removed (g)
Figure 5. (35◦ C).
between the substrate and nutrients with the microorganisms. Similar profiles were obtained with the experiments carried out at 20, 25 and 30◦ C; whereas, the PR-CH4 was lower than those obtained at 35◦ C. This phenomenon is associated with the drop in the efficiencies of a process of anaerobic treatment, when there is a temperature drop. Low values of PR-CH4 were obtained in this work because the hydrolysis rate is greater than the methanogenesis rate, especially when wastewater and have low concentration of soluble substrates, i.e. anaerobic systems operating at
