Bioresource Technology xxx (2015) xxx–xxx
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Usage of pumice as bulking agent in sewage sludge composting Chuandong Wu b, Weiguang Li a,b, Ke Wang b,⇑, Yunbei Li c a
State Key Laboratory of Urban Water Resource and Environment (SKLUWER), Harbin Institute of Technology, Harbin 150090, China School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China c School of Environment, Henan Normal University, Xinxiang 453007, China b
h i g h l i g h t s Water absorption characteristics of pumice were studied. Pumice can hold water during composting. Reused pumice promoted degradation of organic matter and reduced nitrogen loss. Sucrose-decorated pumice was used to reduce nitrogen loss.
a r t i c l e
i n f o
Article history: Received 5 January 2015 Received in revised form 22 March 2015 Accepted 23 March 2015 Available online xxxx Keywords: Sludge composting Pumice Water absorb Reuse Sucrose
a b s t r a c t In this study, the impacts of reused and sucrose-decorated pumice as bulking agents on the composting of sewage sludge were evaluated in the lab-scale reactor. The variations of temperature, pH, NH3 and CO2 emission rate, moisture content (MC), volatile solid, dissolved organic carbon, C/N and the water absorption characteristics of pumice were detected during the 25 days composting. The MC of pumice achieved 65.23% of the 24 h water absorptivity within the first 2 h at the mass ratio of 0.6:1 (pumice:sewage sludge). Reused pumice increased 23.68% of CO2 production and reduced 21.25% of NH3 emission. The sucrose-decorated pumice reduced 43.37% of nitrogen loss. These results suggested that adding pumice and sucrose-decorated pumice in sludge composting matrix could not only adjust the MC of materials, but also improve the degradation of organic matters and reduce nitrogen loss. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction With the upgrading and expansion of wastewater treatment plants in China, increasing amounts of sewage sludge are produced. Indirect severe risk to human health exists due to the possibility of pollutants migration to soil and groundwater. Composting is one of the most popular biological technologies to treat organic solid wastes including animal matures, agricultural residues, sewage sludge, food waste etc. (Doublet et al., 2010; Li et al., 2011; Jiang et al., 2014). Nutrients such as humics, nitrogen and phosphorous in sewage sludge can be recycled for plant growth and soil fertility improvement (Cheng et al., 2007; Wang et al., 2014). During composting process, aeration is one of the most important factors in affecting aerobic biodegradation efficiency and product quality. However, the high moisture content ⇑ Corresponding author at: School of Municipal and Environmental Engineering, Harbin Institute of Technology, Box No. 2602, 73 Huanghe Road, Harbin, Heilongjiang 150090, China. Tel.: +86 15636826515; fax: +86 451 86283003. E-mail address: [email protected] (K. Wang).
(MC) of dewatered sewage sludge (about 80%) caused poor air permeability (Eftoda and McCartney, 2004). Accordingly, bulking agent (BA) is needed in the feedstock to adjust the MC and porosity of materials before composting. BA could be divided into active and inert materials according to whether the BA is involved in the biological reactions in the composting process (Zhou et al., 2014). Active BA, such as wheat straw, rice straw, rice husk, woodchips and sawdust, have been widely applied in composting plant (Gea et al., 2007; Adhikari et al., 2009; Shao et al., 2014). However, the active BAs were degradable and compacted in the incubation process (Jolanun and Towprayoon, 2010), leading to the high cost and poor mass transfer in the composting matrix during the later stage. Additionally, the application of the organic BAs was also limited because of the difficulties in the collection, transportation and secure storage (Zhou et al., 2014). On the other hand, the inert BAs (e.g. zeolite, coal fly ash and peat) had the better performance on heavy metal passivation, improving the physical structure and the MC of composting materials (Zorpas and Loizidou, 2008). However, the inert BAs cannot increase C/N ratio of the mixture
http://dx.doi.org/10.1016/j.biortech.2015.03.104 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wu, C., et al. Usage of pumice as bulking agent in sewage sludge composting. Bioresour. Technol. (2015), http:// dx.doi.org/10.1016/j.biortech.2015.03.104
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C. Wu et al. / Bioresource Technology xxx (2015) xxx–xxx
materials to achieve desired nutrient condition for microorganism growth. It has been confirmed that sucrose could contribute to reduction of nitrogen loss during sewage sludge composting (Li et al., 2013). However, adding sucrose in the feedstock could increase osmotic pressure and viscosity of materials, which would not only impact the microbial activity and oxygen transfer efficiency in composting matrix, but also increased operating cost of sludge treatment. Therefore, an applicable way of sucrose addition which could ensure indirect contact to sludge and slow consumption was needed. A series of research experiences evaluating the recyclable BAs and their influence on the composting process has been reported in recent years. Nagao et al. (2008) compared synthetic polyethylene terephthalate (PET) flake with woodchip as the bulking agent. Due to the high abrasion resistance and non-biodegradability, PET showed the greater viability than the woodchip. Zhou et al. (2014) used a recyclable plastic bulking agent (RPBA) to reduce costs. The improvement of temperature, oxygen transfer, and water removal was demonstrated by adding RPBA. Adding some natural minerals (clinoptilolite and pumice) could significantly increase organic degradation rate, material porosity, NH3 adsorption and water holding capacity according to Hulya (2012). However, a large amount of matured compost retained on the surface of BAs was involved in the composting when the recyclable BAs were reused. Few studies have investigated the influence of matured compost on this process. Pumice, a lightweight porous volcanic rock (Wang et al., 2013), which could be screened and reused, was expected to carry more matured compost due to the rough surface and porous structure. But the impact of recycling on the composting process and the water absorption characteristics of pumice were still unclear. As an inert BA, pumice hardly contains any organic matters to improve the C/N ratio of composting materials. Therefore, decorating pumice by sucrose would be an efficient and convenient method which could ensure sucrose’s indirect contact to sludge simultaneously. The purpose of this study is to investigate the effect of reused pumice on the composting process in terms of nitrogen loss and organic degradation rate. Sucrose-decorated pumice was introduced as the bulking agent to reduce nitrogen loss during composting. Laboratory scale comparative experiments were conducted under controlled conditions. Moreover, the water absorption characteristics of pumice were evaluated.
2. Methods 2.1. Feedstock composition and composting process Dewatered sewage sludge was obtained from a wastewater treatment plant in Harbin, China. Fresh pumice was used as an inorganic bulking agent, and the sucrose was introduced as carbon source. Reused pumice was collected from previous composting experiment. Sucrose-decorated pumice was treated with sucrose solution. 20 g of sucrose was dissolved into 200 ml distilled water, and then the sucrose solution was sprayed onto 2 kg of pumice evenly. The pumice was dried in an oven at 45 °C for 4 h and then pumice was mixed with 2 kg sewage sludge as the feedstock of composting. The characteristics of the raw materials are presented in Table 1. The composting process was conducted in closed batch reactor with an inner diameter of 300 mm and height of 600 mm. Forced ventilation was supplied by the air pump, and the aeration rate was controlled by a flow meter and maintained at 0.4 L h1. Fresh air was pumped into the reactor from the bottom through
Table 1 Characteristics of the raw materials. Parameters
MC (%)
Volatile solid (%)
pH
C/N
Sewage sludge Pumice Reused pumice Sucrose-decorated pumice
78.86 ± 0.84 0.57 ± 0.12 15.65 ± 0.92 0.42 ± 0.22
55.53 ± 0.91 – 0.53 ± 0.28 1.96 ± 0.55
7.13 ± 0.02 7.24 ± 0.05 7.68 ± 0.04 6.96 ± 0.03
6.42 – – –
a perforation plate. The reactor is completely closed and the exhaust gas passed through the absorption glass jars in which CO2 and NH3 generated by aerobic fermentation were trapped by 4 mol l1 sodium hydroxide and 0.5 mol l1 of boric acid, respectively. 2.2. Experiment design and procedure To investigate the hydroscopicity of the pumice, two water sorption experiments were detailed below. In the water sorption experiment 1, the mass ratios were 1.2:1, 1:1, 0.8:1, 0.6:1, and 0.4:1 (pumice:sludge), respectively. The MC of the pumice after 2 h and 24 h and the MC of mixture after 2 h were measured. In the water sorption experiment 2, the pumice was mixed with the sewage sludge at the mass ratio of 0.6:1 (pumice:sludge), the MC of the pumice was detected every two hours. To evaluate the effects of reused pumice and sucrose-decorated pumice on composting progress, three composting experiments were conducted. In the composting experiment 1, the sewage sludge and the pumice were mixed at the mass ratio of 1:0.6 (pumice:sludge). The MC variation of the sewage sludge, pumice and mixture were detected. In the composting experiment 2, fresh pumice (R1) and reused pumice (R2) were used as the bulking agents, respectively. The amount of the reused pumice was same with the fresh one on volume. In the composting experiment 3, fresh pumice (P1) and sucrose-decorated pumice (P2) were mixed with sewage sludge at the mass ratio of 0.6:1 (pumice:sludge). 2.3. Physical and chemical analysis The amounts of CO2 and NH3 emission were measured by titration according to Alkanani et al. (1992) every 24 h. The difference in the weight between before and after drying at 105 °C for 24 h was concerned as the MC of sample. The pH of samples were detected by a pH meter after 5 g sample was dissolved with 50 ml distilled water (Rihani et al., 2010). The dried sample was further heated at 550 °C for 4 h, the weight difference was determined as the VS content (Wang et al., 2011). The dried sewage sludge samples were pestle completely and screened through a mesh size of 200. 5–10 mg residue was wrapped with aluminum foil and accurately weighed to the nearest 0.001 mg. Then the contents of carbon and nitrogen in the sludge were determined by using elemental analyzer (Vario EL, Germany). For scanning electron microscopy (SEM), samples was dehydrated according to the method described by Dresbøll and Magid (2006), coated with gold in a sputter coater (IB-5), examined in the scanning electron microscope (HITACHI S-20). The MC of pumice was calculated by formulas below. MCP means the MC of pumice, and MPB means the weight of pumice in mixture before drying. MPD means the weight of pumice in mixture after drying (pumice was easy to be separated from sewage sludge after drying). M0 means the weight of mixture before drying. MSD means the weight of mixture before drying. MCS means the MC of sewage sludge.
Please cite this article in press as: Wu, C., et al. Usage of pumice as bulking agent in sewage sludge composting. Bioresour. Technol. (2015), http:// dx.doi.org/10.1016/j.biortech.2015.03.104
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C. Wu et al. / Bioresource Technology xxx (2015) xxx–xxx
M PB
M PB MPD 100% M PB
90
ð1Þ
80
M SD ¼ M0 ð1 MCS Þ
ð2Þ
The value of MC, VS and pH were presented by using mean ± standard deviation. Means were calculated as an average of three replicates of each treatment. The standard deviation of these values and the variance of the MC, VS content, DOC content and nitrogen content were calculated using Excel.
sludge mixture pumice
70
Moisture Content (%)
MCP ¼
60 50 40 30 20 10
3. Results and discussion
0
3.1. Water absorption characteristics of pumice
0
3.1.1. Mixing ratio of pumice and sludge Bulking agent can adjust MC and free air space of composting materials (Petric and Selimbasic, 2008). To study the optimal mixing ratio of pumice (P) and sewage sludge (SS), five mass ratios of mixture were placed into closed battles. The MCs of the pumice after 2 h and 24 h and the mixture after 2 h were measured in water absorption experiment 1 (Table 2). It was showed that the MC of mixture was significantly reduced as the proportion of pumice increased. However, the MCP was not linearly dependent on the mixing ratio of pumice and sludge. The MCP after 2 h and the one after 24 h at the ratio of 0.6:1, both were the highest (18.71% and 27.68%, respectively). It was indicated that the best ratio of mixture was 0.6:1, at which the water absorption capacity of pumice could be utilized fully. 3.1.2. Water absorption dynamics of pumice It was confirmed that pumice has a strong ability to absorb water and its saturated water absorptivity can reach 72.0%. To evaluate the dynamics of pumice’s water absorption, the MCP was detected every two hours during 24 h period, and the mass ratio of 0.6:1 (pumice:sludge) was chosen. The variation of MC could be divided into three stages: (1) 0–2 h, the MCP increased from 0.38% to 18.71%, and the average water absorption rate (AWAR) could reach to 91.66 g (kg h)1. (2) 2–12 h, MCP changed from 18.71% to 25.57% and the AWAR was 6.92 g (kg h)1. (3) 12–24 h, MCP varied from 25.57% to 28.68%, and the AWAR was 2.61 g (kg h)1. This result showed that pumice achieved 65.23% of the 24 h water absorptivity within 2 h. The water absorption capacity of pumice was related to its microstructure. Rich pore structure could be observed from the SEM images of fresh pumice. The pore size varied from 10 to 150 lm, and interconnected pores and closed type air bubble structures coexisted. Thus pumice could absorb the water from sewage sludge under the power of capillarity. As water penetrated into the interior of pumice particle, the capillarity was weakened, and the increasing rate of pumice MC decreased. 3.1.3. Variation of MC of material Fig. 1 showed the MC variation of pumice, sewage sludge and their mixture. At the initial period of composting, a large amount of water was absorbed by the pumice within a short time. After that, MCS dropped from 80.61% to 53.9% on Day-8, while no
2
4
6
8
10
14
18
24
Time (day) Fig. 1. Evolution of materials MC during composting.
significant decrease was observed on the MC of mixture. It could be explained by the continuing increase of MCP. Except for evaporation, an amount of water transferred from the sewage sludge to the pumice. From Day-8 to the end of composting, MCS gradually stabled to 44.78%, while rapid decrease emerged on the MC of mixture. It could be contributed to the intense decline of the MCP during this period, which could be explained by the transfer of water from the pumice to sewage sludge. These results showed that the pumice had a high capacity of holding water during the composting process. Researchers reported that maturation efficiency was closely related to the MC of materials (Yang et al., 2013). Pumice could be used to avoid the low MCS (below 30%) of materials in the curing stage of composting.
3.2. Reuse of pumice 3.2.1. Compost properties In this study, the peak temperature was detected on Day-6, and then the temperature decreased gradually. The highest temperature in the R2 was 53 °C, which was 2 °C higher than that in the R1. The materials temperature above 50 °C was retained one day longer in the R2 than that in the R1. The differences in the incubation temperature between two reactors were not significant after the thermophilic phase. The materials in the R2 were detected to have the higher MC than that in the R1 (Table 3). As water absorption capacity of the reused pumice was reduced, the adjustment of the pumice on the MC of the composting materials was weakened. The porosity of the materials was not affected seriously, because the reused pumice could support the space structure of the matrix. The initial pH of R2 was improved due to the addition of mature compost with high pH (about 8). The pH of materials decreased during the first 3 days, since the accumulation of acid intermediates was produced from the hydrolysis of easily degradable organic matters (Ndegwa et al., 2000). The pH value of materials in the R2 was consistently higher than that in the R1, probably because the mixture in the R2 absorbed more NH3 produced from proteins hydrolysis.
Table 2 MC of composting materials at different mixing ratios. Mass ratio of mixture (P:SS)
0.4:1
0.6:1
0.8:1
1.0:1
1.2:1
MC of pumice after 2 h (%) MC of pumice after 24 h (%) MC of mixture after 2 h (%)
17.05 ± 0.93 26.75 ± 0.78 57.24 ± 0.85
19.71 ± 0.84 28.68 ± 0.81 50.18 ± 1.17
18.71 ± 0.88 27.68 ± 0.72 45.60 ± 1.09
17.20 ± 0.72 26.83 ± 0.86 40.12 ± 0.78
16.17 ± 0.90 25.66 ± 0.75 36.50 ± 0.80
Please cite this article in press as: Wu, C., et al. Usage of pumice as bulking agent in sewage sludge composting. Bioresour. Technol. (2015), http:// dx.doi.org/10.1016/j.biortech.2015.03.104
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Table 3 The MC and pH of sewage sludge during composting. Time (day)
1
5
10
14
18
26
MC (%)
R1 R2
55.60 ± 0.54 68.90 ± 0.79
52.07 ± 0.83 59.55 ± 0.75
43.00 ± 0.80 48.70 ± 0.98
38.82 ± 0.65 45.20 ± 0.79
35.46 ± 0.71 42.61 ± 0.86
32.61 ± 0.87 40.41 ± 0.72
pH
R1 R2
7.13 ± 0.04 7.53 ± 0.02
7.27 ± 0.03 7.08 ± 0.06
7.91 ± 0.05 8.00 ± 0.02
8.27 ± 0.08 8.26 ± 0.05
8.25 ± 0.03 8.09 ± 0.07
7.84 ± 0.05 8.10 ± 0.04
3.2.2. CO2 emission rate The CO2 emission rate reflected the degradation rate of organic matters in the sludge during composting (Li et al., 2013). Changes of cumulative CO2 emission and VS content in R1 and R2 were showed in Fig. 2a. A rapidly increase in CO2 emission rate was observed during the first 6 days, and then the CO2 emission rate gradually decreased. The differences in CO2 emission rate between R1 and R2 mainly occurred at the prior period of composting. Rapid growth of CO2 emission rate was observed from Day-3 to Day-8 in the R2, whereas no significant difference was found between two runs after the thermophilic phase. Similar situation was also occurred in the evolution of VS content. The VS content of the sludge significantly decreased by 17.87% in the R2 compared with 16.79% in the R1 during the first 9 days, and the VS content of the sludge in the two runs decreased at the similar rate. This could be attributed to a large amount of mesophilic microorganisms carried by the reused pumice. After the pumice was screened from the matured compost, the surface of the pumice was covered with decomposed materials which contained a large amount of mesophilic microorganisms and their propagules (Tang et al., 2004). The mesophilic microorganisms grew and reproduced rapidly after the reused pumice was mixed with the nutrient-rich sludge. Consequently, the degradation of organic matter was promoted and the releasing rate of bio-heat was improved. This could explain why the temperature of R2 was higher than that of R1 at the temperature-rise stage. From this perspective, the influence mechanism of adding reused pumice on the composting process was similar to the inoculation. The composting progress could be accelerated with the addition of matured compost during the initial period. The quantities of the fungi and actinomycosis increased obviously at the beginning (Luo et al., 2014). With the rapid rise in the temperature of R2, the heating process was promoted and the composting period was shortened. Numerous studies have reported that biological diversity and the metabolic activity of mesophilic microorganism would be inhabited at the thermophilic phase of composting
60
55
1200 50 900 45 600 40
CO2 fresh
300
35
0 0
5
10
15
Time (day)
(a)
20
25
IAE fresh IAE Reused
4500
CAE fresh CAE reused
320
3600
240
2700
160
1800
80
900
-1
VS fresh VS reused
CO2 reused
-1
1500
400
Instaneous NH3 emissions(mg kg d.s.)
-1
3.2.3. Reduction of nitrogen loss Variation of instantaneous and cumulative NH3 emission was showed in Fig. 2b. During the initial period of composting, a rapid increase in NH3 emission rate was observed. The intense NH3 emission caused large amounts of nitrogen loss at the thermophilic phase (Li et al., 2013). The cumulative NH3 emission was found to be reduced by 21.25% due to the reuse of pumice. The difference in NH3 emission rate between R1 and R2 mainly occurred at the heating and thermophilic phases, which was similar with the variation of CO2 emission rate. The degradation of organic matter was obviously promoted by reused pumice during composting mentioned above. Researchers have proposed that effective utilization of organic matter played an important role in inhibition of NH3 volatilization. Kuroda et al. (2004) isolated thermophilic ammonium-tolerant bacterium and NH3 emission was reduced by enhancing ammonia assimilation. The intracellular process of ammonia assimilation was composed of two routes: glutamine synthetase (GS)/glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH). GS has a higher affinity for NH+4 than GDH (Yan, 2007). However, the synthesis of glutamine from NH+4 and glutamate which GS catalyses is ATP-dependent (Miller and Maier, 2014). In this study, the rapid decline of DOC content was found in R2 during the initial period (Fig. 3). It indicated that a large amount of dissolved organic matters was degraded by microorganisms. The carbon metabolism was accelerated and the energy and metabolites that the ammonia assimilation need were provided at a higher rate (Kumar et al., 2009). Ammonia assimilation was promoted by reused pumice and nitrogen loss was reduced as ammonia was assimilated to organic nitrogen rather than volatilized. As showed in Fig. 3, 22.33% of nitrogen
Cumulative NH3 emissions(mg kg d.s.)
Cumulative CO2 emission(mg kg d.s.)
1800
(Amir et al., 2008; Li et al., 2013). Mesophilic microbial activities in the two runs decreased during the thermophilic phase. Therefore, the difference in CO2 emission rate between the two runs was substantially reduced during the cooling phase.
0
0 0
5
10
15
20
25
Time (day)
(b)
Fig. 2. (a) Cumulative CO2 emission and VS of sewage sludge during composing. (b) Instantaneous and cumulative NH3 emission during composting.
Please cite this article in press as: Wu, C., et al. Usage of pumice as bulking agent in sewage sludge composting. Bioresour. Technol. (2015), http:// dx.doi.org/10.1016/j.biortech.2015.03.104
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Fresh Reused
DOC (mg/kg d.s.)
520
480
4.5
Nitrogen content (%)
560
440
Residual
ammonia could be converted into organic nitrogen which was used as the biological nitrogen compound (Li et al., 2013). Under moist and nutrient-rich conditions, ammonia-assimilating microorganisms might tolerate high ammonia content (Sasaki et al., 2005). It may be possible that the sucrose-decorated pumice provide a valid medium for ammonia-assimilating microorganisms. Kumar et al. (2009) proposed that modulation of carbon skeleton production might be an alternative approach to improve nitrogen assimilation. In this case, sucrose, as the only carbon source, might provide energy and substrate to the ammonia assimilation reactions more directly than the one of organic matter in sewage sludge.
Lost
3.6 2.7 1.8 0.9 0.0
Fresh
5
Reused
400
4. Conclusions 360 0
5
10
15
20
25
30
Time (day) Fig. 3. The change of dissolved organic carbon content and the residual and lost nitrogen after composting.
Nitrogen content (%)
Rsidual
Lost
3
4000
2
3000 1 0
2000 Fresh
Sucrose-decorated
0 0
5
10
15
20
-1
1000
Sucrose-decorated Fresh
Cumlative NH3 emission( mg kg d.s.)
5000
4
25
Time (day)
Pumice has rich pore structure and strong water absorbent capacity. Pumice could rapidly achieve 65.23% of the 24 h water absorptivity in 2 h at the mixing ratio of 0.6:1 (pumice:sewage sludge). Reused pumice could be used as the inoculant to promote the degradation of organic matter and the reduction of NH3 emission significantly. Sucrose-decorated pumice could enhance the ammonia assimilation reactions. The nitrogen loss during composting could be further controlled effectively by using sucrosedecorated pumice. Acknowledgements This research was financially supported by the National Natural Science Foundation of China (51278146, 51408151), Water Environmental Quality Improvement Technology Integration and Comprehensive Demonstration in Control Unit of Songhua River in Harbin City (2013ZX07201007), China Postdoctoral Science Foundation (2012M520755), China Postdoctoral Science Special Foundation (2014T70357), Postdoctoral Science Special Foundation of Hei Longjiang (LBH-TZ0510) and Specialized Research Fund for the Doctoral Program of Higher Education (20132302120075).
Fig. 4. Cumulative NH3 emission and residual and lost nitrogen after composing.
References was lost in R2 compared with 31.71%. This result indicated that 41.84% of nitrogen loss was reduced due to reuse of pumice. 3.3. Sucrose-decorated pumice In this study, sucrose was decorated to pumice as carbon source for the first time to control the nitrogen loss during composting. Cumulative NH3 emission trend was basically same with the experiment of reused pumice (Fig. 4). The NH3 emission rate was low during the initial period of composting, grew rapidly during the thermophilic phase and stabilized at the cooling phase. The difference in ammonia emission rates between P1 and P2 was occurred in the thermophilic phase. During the first 6 days, the cumulative NH3 emission in P1 and P2 were 173.03 mg kg1 dried sludge (d.s.) and 182.96 mg kg1 d.s., respectively, meaning no differences in the amounts of ammonia emission between the two runs. During the first 15 days, the cumulative NH3 emissions reached to 2117.31 mg kg1 d.s. of P1 and 3086.98 mg kg1 d.s. of P2, respectively. It showed that 969.67 mg kg1 d.s. less ammonia volatilized due to the utilization of sucrose-decorated pumice. At the end of the composting, 31.50% of cumulative NH3 emission and 43.37% of nitrogen loss was reduced, respectively. A large amount of microorganisms could be found from the SEM image of the sucrose-decorated pumice on Day-9. After the processing of sucrose, the barren pumice supplied the condition for microbial growth. Researchers reported that microorganisms could assimilate ammonia into glutamate with essential enzymes, and
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Please cite this article in press as: Wu, C., et al. Usage of pumice as bulking agent in sewage sludge composting. Bioresour. Technol. (2015), http:// dx.doi.org/10.1016/j.biortech.2015.03.104
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Usage of pumice as bulking agent in sewage sludge composting Chuandong Wu b, Weiguang Li a,b, Ke Wang b,⇑, Yunbei Li c a
State Key Laboratory of Urban Water Resource and Environment (SKLUWER), Harbin Institute of Technology, Harbin 150090, China School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China c School of Environment, Henan Normal University, Xinxiang 453007, China b
h i g h l i g h t s Water absorption characteristics of pumice were studied. Pumice can hold water during composting. Reused pumice promoted degradation of organic matter and reduced nitrogen loss. Sucrose-decorated pumice was used to reduce nitrogen loss.
a r t i c l e
i n f o
Article history: Received 5 January 2015 Received in revised form 22 March 2015 Accepted 23 March 2015 Available online xxxx Keywords: Sludge composting Pumice Water absorb Reuse Sucrose
a b s t r a c t In this study, the impacts of reused and sucrose-decorated pumice as bulking agents on the composting of sewage sludge were evaluated in the lab-scale reactor. The variations of temperature, pH, NH3 and CO2 emission rate, moisture content (MC), volatile solid, dissolved organic carbon, C/N and the water absorption characteristics of pumice were detected during the 25 days composting. The MC of pumice achieved 65.23% of the 24 h water absorptivity within the first 2 h at the mass ratio of 0.6:1 (pumice:sewage sludge). Reused pumice increased 23.68% of CO2 production and reduced 21.25% of NH3 emission. The sucrose-decorated pumice reduced 43.37% of nitrogen loss. These results suggested that adding pumice and sucrose-decorated pumice in sludge composting matrix could not only adjust the MC of materials, but also improve the degradation of organic matters and reduce nitrogen loss. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction With the upgrading and expansion of wastewater treatment plants in China, increasing amounts of sewage sludge are produced. Indirect severe risk to human health exists due to the possibility of pollutants migration to soil and groundwater. Composting is one of the most popular biological technologies to treat organic solid wastes including animal matures, agricultural residues, sewage sludge, food waste etc. (Doublet et al., 2010; Li et al., 2011; Jiang et al., 2014). Nutrients such as humics, nitrogen and phosphorous in sewage sludge can be recycled for plant growth and soil fertility improvement (Cheng et al., 2007; Wang et al., 2014). During composting process, aeration is one of the most important factors in affecting aerobic biodegradation efficiency and product quality. However, the high moisture content ⇑ Corresponding author at: School of Municipal and Environmental Engineering, Harbin Institute of Technology, Box No. 2602, 73 Huanghe Road, Harbin, Heilongjiang 150090, China. Tel.: +86 15636826515; fax: +86 451 86283003. E-mail address: [email protected] (K. Wang).
(MC) of dewatered sewage sludge (about 80%) caused poor air permeability (Eftoda and McCartney, 2004). Accordingly, bulking agent (BA) is needed in the feedstock to adjust the MC and porosity of materials before composting. BA could be divided into active and inert materials according to whether the BA is involved in the biological reactions in the composting process (Zhou et al., 2014). Active BA, such as wheat straw, rice straw, rice husk, woodchips and sawdust, have been widely applied in composting plant (Gea et al., 2007; Adhikari et al., 2009; Shao et al., 2014). However, the active BAs were degradable and compacted in the incubation process (Jolanun and Towprayoon, 2010), leading to the high cost and poor mass transfer in the composting matrix during the later stage. Additionally, the application of the organic BAs was also limited because of the difficulties in the collection, transportation and secure storage (Zhou et al., 2014). On the other hand, the inert BAs (e.g. zeolite, coal fly ash and peat) had the better performance on heavy metal passivation, improving the physical structure and the MC of composting materials (Zorpas and Loizidou, 2008). However, the inert BAs cannot increase C/N ratio of the mixture
http://dx.doi.org/10.1016/j.biortech.2015.03.104 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wu, C., et al. Usage of pumice as bulking agent in sewage sludge composting. Bioresour. Technol. (2015), http:// dx.doi.org/10.1016/j.biortech.2015.03.104
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materials to achieve desired nutrient condition for microorganism growth. It has been confirmed that sucrose could contribute to reduction of nitrogen loss during sewage sludge composting (Li et al., 2013). However, adding sucrose in the feedstock could increase osmotic pressure and viscosity of materials, which would not only impact the microbial activity and oxygen transfer efficiency in composting matrix, but also increased operating cost of sludge treatment. Therefore, an applicable way of sucrose addition which could ensure indirect contact to sludge and slow consumption was needed. A series of research experiences evaluating the recyclable BAs and their influence on the composting process has been reported in recent years. Nagao et al. (2008) compared synthetic polyethylene terephthalate (PET) flake with woodchip as the bulking agent. Due to the high abrasion resistance and non-biodegradability, PET showed the greater viability than the woodchip. Zhou et al. (2014) used a recyclable plastic bulking agent (RPBA) to reduce costs. The improvement of temperature, oxygen transfer, and water removal was demonstrated by adding RPBA. Adding some natural minerals (clinoptilolite and pumice) could significantly increase organic degradation rate, material porosity, NH3 adsorption and water holding capacity according to Hulya (2012). However, a large amount of matured compost retained on the surface of BAs was involved in the composting when the recyclable BAs were reused. Few studies have investigated the influence of matured compost on this process. Pumice, a lightweight porous volcanic rock (Wang et al., 2013), which could be screened and reused, was expected to carry more matured compost due to the rough surface and porous structure. But the impact of recycling on the composting process and the water absorption characteristics of pumice were still unclear. As an inert BA, pumice hardly contains any organic matters to improve the C/N ratio of composting materials. Therefore, decorating pumice by sucrose would be an efficient and convenient method which could ensure sucrose’s indirect contact to sludge simultaneously. The purpose of this study is to investigate the effect of reused pumice on the composting process in terms of nitrogen loss and organic degradation rate. Sucrose-decorated pumice was introduced as the bulking agent to reduce nitrogen loss during composting. Laboratory scale comparative experiments were conducted under controlled conditions. Moreover, the water absorption characteristics of pumice were evaluated.
2. Methods 2.1. Feedstock composition and composting process Dewatered sewage sludge was obtained from a wastewater treatment plant in Harbin, China. Fresh pumice was used as an inorganic bulking agent, and the sucrose was introduced as carbon source. Reused pumice was collected from previous composting experiment. Sucrose-decorated pumice was treated with sucrose solution. 20 g of sucrose was dissolved into 200 ml distilled water, and then the sucrose solution was sprayed onto 2 kg of pumice evenly. The pumice was dried in an oven at 45 °C for 4 h and then pumice was mixed with 2 kg sewage sludge as the feedstock of composting. The characteristics of the raw materials are presented in Table 1. The composting process was conducted in closed batch reactor with an inner diameter of 300 mm and height of 600 mm. Forced ventilation was supplied by the air pump, and the aeration rate was controlled by a flow meter and maintained at 0.4 L h1. Fresh air was pumped into the reactor from the bottom through
Table 1 Characteristics of the raw materials. Parameters
MC (%)
Volatile solid (%)
pH
C/N
Sewage sludge Pumice Reused pumice Sucrose-decorated pumice
78.86 ± 0.84 0.57 ± 0.12 15.65 ± 0.92 0.42 ± 0.22
55.53 ± 0.91 – 0.53 ± 0.28 1.96 ± 0.55
7.13 ± 0.02 7.24 ± 0.05 7.68 ± 0.04 6.96 ± 0.03
6.42 – – –
a perforation plate. The reactor is completely closed and the exhaust gas passed through the absorption glass jars in which CO2 and NH3 generated by aerobic fermentation were trapped by 4 mol l1 sodium hydroxide and 0.5 mol l1 of boric acid, respectively. 2.2. Experiment design and procedure To investigate the hydroscopicity of the pumice, two water sorption experiments were detailed below. In the water sorption experiment 1, the mass ratios were 1.2:1, 1:1, 0.8:1, 0.6:1, and 0.4:1 (pumice:sludge), respectively. The MC of the pumice after 2 h and 24 h and the MC of mixture after 2 h were measured. In the water sorption experiment 2, the pumice was mixed with the sewage sludge at the mass ratio of 0.6:1 (pumice:sludge), the MC of the pumice was detected every two hours. To evaluate the effects of reused pumice and sucrose-decorated pumice on composting progress, three composting experiments were conducted. In the composting experiment 1, the sewage sludge and the pumice were mixed at the mass ratio of 1:0.6 (pumice:sludge). The MC variation of the sewage sludge, pumice and mixture were detected. In the composting experiment 2, fresh pumice (R1) and reused pumice (R2) were used as the bulking agents, respectively. The amount of the reused pumice was same with the fresh one on volume. In the composting experiment 3, fresh pumice (P1) and sucrose-decorated pumice (P2) were mixed with sewage sludge at the mass ratio of 0.6:1 (pumice:sludge). 2.3. Physical and chemical analysis The amounts of CO2 and NH3 emission were measured by titration according to Alkanani et al. (1992) every 24 h. The difference in the weight between before and after drying at 105 °C for 24 h was concerned as the MC of sample. The pH of samples were detected by a pH meter after 5 g sample was dissolved with 50 ml distilled water (Rihani et al., 2010). The dried sample was further heated at 550 °C for 4 h, the weight difference was determined as the VS content (Wang et al., 2011). The dried sewage sludge samples were pestle completely and screened through a mesh size of 200. 5–10 mg residue was wrapped with aluminum foil and accurately weighed to the nearest 0.001 mg. Then the contents of carbon and nitrogen in the sludge were determined by using elemental analyzer (Vario EL, Germany). For scanning electron microscopy (SEM), samples was dehydrated according to the method described by Dresbøll and Magid (2006), coated with gold in a sputter coater (IB-5), examined in the scanning electron microscope (HITACHI S-20). The MC of pumice was calculated by formulas below. MCP means the MC of pumice, and MPB means the weight of pumice in mixture before drying. MPD means the weight of pumice in mixture after drying (pumice was easy to be separated from sewage sludge after drying). M0 means the weight of mixture before drying. MSD means the weight of mixture before drying. MCS means the MC of sewage sludge.
Please cite this article in press as: Wu, C., et al. Usage of pumice as bulking agent in sewage sludge composting. Bioresour. Technol. (2015), http:// dx.doi.org/10.1016/j.biortech.2015.03.104
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M PB
M PB MPD 100% M PB
90
ð1Þ
80
M SD ¼ M0 ð1 MCS Þ
ð2Þ
The value of MC, VS and pH were presented by using mean ± standard deviation. Means were calculated as an average of three replicates of each treatment. The standard deviation of these values and the variance of the MC, VS content, DOC content and nitrogen content were calculated using Excel.
sludge mixture pumice
70
Moisture Content (%)
MCP ¼
60 50 40 30 20 10
3. Results and discussion
0
3.1. Water absorption characteristics of pumice
0
3.1.1. Mixing ratio of pumice and sludge Bulking agent can adjust MC and free air space of composting materials (Petric and Selimbasic, 2008). To study the optimal mixing ratio of pumice (P) and sewage sludge (SS), five mass ratios of mixture were placed into closed battles. The MCs of the pumice after 2 h and 24 h and the mixture after 2 h were measured in water absorption experiment 1 (Table 2). It was showed that the MC of mixture was significantly reduced as the proportion of pumice increased. However, the MCP was not linearly dependent on the mixing ratio of pumice and sludge. The MCP after 2 h and the one after 24 h at the ratio of 0.6:1, both were the highest (18.71% and 27.68%, respectively). It was indicated that the best ratio of mixture was 0.6:1, at which the water absorption capacity of pumice could be utilized fully. 3.1.2. Water absorption dynamics of pumice It was confirmed that pumice has a strong ability to absorb water and its saturated water absorptivity can reach 72.0%. To evaluate the dynamics of pumice’s water absorption, the MCP was detected every two hours during 24 h period, and the mass ratio of 0.6:1 (pumice:sludge) was chosen. The variation of MC could be divided into three stages: (1) 0–2 h, the MCP increased from 0.38% to 18.71%, and the average water absorption rate (AWAR) could reach to 91.66 g (kg h)1. (2) 2–12 h, MCP changed from 18.71% to 25.57% and the AWAR was 6.92 g (kg h)1. (3) 12–24 h, MCP varied from 25.57% to 28.68%, and the AWAR was 2.61 g (kg h)1. This result showed that pumice achieved 65.23% of the 24 h water absorptivity within 2 h. The water absorption capacity of pumice was related to its microstructure. Rich pore structure could be observed from the SEM images of fresh pumice. The pore size varied from 10 to 150 lm, and interconnected pores and closed type air bubble structures coexisted. Thus pumice could absorb the water from sewage sludge under the power of capillarity. As water penetrated into the interior of pumice particle, the capillarity was weakened, and the increasing rate of pumice MC decreased. 3.1.3. Variation of MC of material Fig. 1 showed the MC variation of pumice, sewage sludge and their mixture. At the initial period of composting, a large amount of water was absorbed by the pumice within a short time. After that, MCS dropped from 80.61% to 53.9% on Day-8, while no
2
4
6
8
10
14
18
24
Time (day) Fig. 1. Evolution of materials MC during composting.
significant decrease was observed on the MC of mixture. It could be explained by the continuing increase of MCP. Except for evaporation, an amount of water transferred from the sewage sludge to the pumice. From Day-8 to the end of composting, MCS gradually stabled to 44.78%, while rapid decrease emerged on the MC of mixture. It could be contributed to the intense decline of the MCP during this period, which could be explained by the transfer of water from the pumice to sewage sludge. These results showed that the pumice had a high capacity of holding water during the composting process. Researchers reported that maturation efficiency was closely related to the MC of materials (Yang et al., 2013). Pumice could be used to avoid the low MCS (below 30%) of materials in the curing stage of composting.
3.2. Reuse of pumice 3.2.1. Compost properties In this study, the peak temperature was detected on Day-6, and then the temperature decreased gradually. The highest temperature in the R2 was 53 °C, which was 2 °C higher than that in the R1. The materials temperature above 50 °C was retained one day longer in the R2 than that in the R1. The differences in the incubation temperature between two reactors were not significant after the thermophilic phase. The materials in the R2 were detected to have the higher MC than that in the R1 (Table 3). As water absorption capacity of the reused pumice was reduced, the adjustment of the pumice on the MC of the composting materials was weakened. The porosity of the materials was not affected seriously, because the reused pumice could support the space structure of the matrix. The initial pH of R2 was improved due to the addition of mature compost with high pH (about 8). The pH of materials decreased during the first 3 days, since the accumulation of acid intermediates was produced from the hydrolysis of easily degradable organic matters (Ndegwa et al., 2000). The pH value of materials in the R2 was consistently higher than that in the R1, probably because the mixture in the R2 absorbed more NH3 produced from proteins hydrolysis.
Table 2 MC of composting materials at different mixing ratios. Mass ratio of mixture (P:SS)
0.4:1
0.6:1
0.8:1
1.0:1
1.2:1
MC of pumice after 2 h (%) MC of pumice after 24 h (%) MC of mixture after 2 h (%)
17.05 ± 0.93 26.75 ± 0.78 57.24 ± 0.85
19.71 ± 0.84 28.68 ± 0.81 50.18 ± 1.17
18.71 ± 0.88 27.68 ± 0.72 45.60 ± 1.09
17.20 ± 0.72 26.83 ± 0.86 40.12 ± 0.78
16.17 ± 0.90 25.66 ± 0.75 36.50 ± 0.80
Please cite this article in press as: Wu, C., et al. Usage of pumice as bulking agent in sewage sludge composting. Bioresour. Technol. (2015), http:// dx.doi.org/10.1016/j.biortech.2015.03.104
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Table 3 The MC and pH of sewage sludge during composting. Time (day)
1
5
10
14
18
26
MC (%)
R1 R2
55.60 ± 0.54 68.90 ± 0.79
52.07 ± 0.83 59.55 ± 0.75
43.00 ± 0.80 48.70 ± 0.98
38.82 ± 0.65 45.20 ± 0.79
35.46 ± 0.71 42.61 ± 0.86
32.61 ± 0.87 40.41 ± 0.72
pH
R1 R2
7.13 ± 0.04 7.53 ± 0.02
7.27 ± 0.03 7.08 ± 0.06
7.91 ± 0.05 8.00 ± 0.02
8.27 ± 0.08 8.26 ± 0.05
8.25 ± 0.03 8.09 ± 0.07
7.84 ± 0.05 8.10 ± 0.04
3.2.2. CO2 emission rate The CO2 emission rate reflected the degradation rate of organic matters in the sludge during composting (Li et al., 2013). Changes of cumulative CO2 emission and VS content in R1 and R2 were showed in Fig. 2a. A rapidly increase in CO2 emission rate was observed during the first 6 days, and then the CO2 emission rate gradually decreased. The differences in CO2 emission rate between R1 and R2 mainly occurred at the prior period of composting. Rapid growth of CO2 emission rate was observed from Day-3 to Day-8 in the R2, whereas no significant difference was found between two runs after the thermophilic phase. Similar situation was also occurred in the evolution of VS content. The VS content of the sludge significantly decreased by 17.87% in the R2 compared with 16.79% in the R1 during the first 9 days, and the VS content of the sludge in the two runs decreased at the similar rate. This could be attributed to a large amount of mesophilic microorganisms carried by the reused pumice. After the pumice was screened from the matured compost, the surface of the pumice was covered with decomposed materials which contained a large amount of mesophilic microorganisms and their propagules (Tang et al., 2004). The mesophilic microorganisms grew and reproduced rapidly after the reused pumice was mixed with the nutrient-rich sludge. Consequently, the degradation of organic matter was promoted and the releasing rate of bio-heat was improved. This could explain why the temperature of R2 was higher than that of R1 at the temperature-rise stage. From this perspective, the influence mechanism of adding reused pumice on the composting process was similar to the inoculation. The composting progress could be accelerated with the addition of matured compost during the initial period. The quantities of the fungi and actinomycosis increased obviously at the beginning (Luo et al., 2014). With the rapid rise in the temperature of R2, the heating process was promoted and the composting period was shortened. Numerous studies have reported that biological diversity and the metabolic activity of mesophilic microorganism would be inhabited at the thermophilic phase of composting
60
55
1200 50 900 45 600 40
CO2 fresh
300
35
0 0
5
10
15
Time (day)
(a)
20
25
IAE fresh IAE Reused
4500
CAE fresh CAE reused
320
3600
240
2700
160
1800
80
900
-1
VS fresh VS reused
CO2 reused
-1
1500
400
Instaneous NH3 emissions(mg kg d.s.)
-1
3.2.3. Reduction of nitrogen loss Variation of instantaneous and cumulative NH3 emission was showed in Fig. 2b. During the initial period of composting, a rapid increase in NH3 emission rate was observed. The intense NH3 emission caused large amounts of nitrogen loss at the thermophilic phase (Li et al., 2013). The cumulative NH3 emission was found to be reduced by 21.25% due to the reuse of pumice. The difference in NH3 emission rate between R1 and R2 mainly occurred at the heating and thermophilic phases, which was similar with the variation of CO2 emission rate. The degradation of organic matter was obviously promoted by reused pumice during composting mentioned above. Researchers have proposed that effective utilization of organic matter played an important role in inhibition of NH3 volatilization. Kuroda et al. (2004) isolated thermophilic ammonium-tolerant bacterium and NH3 emission was reduced by enhancing ammonia assimilation. The intracellular process of ammonia assimilation was composed of two routes: glutamine synthetase (GS)/glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH). GS has a higher affinity for NH+4 than GDH (Yan, 2007). However, the synthesis of glutamine from NH+4 and glutamate which GS catalyses is ATP-dependent (Miller and Maier, 2014). In this study, the rapid decline of DOC content was found in R2 during the initial period (Fig. 3). It indicated that a large amount of dissolved organic matters was degraded by microorganisms. The carbon metabolism was accelerated and the energy and metabolites that the ammonia assimilation need were provided at a higher rate (Kumar et al., 2009). Ammonia assimilation was promoted by reused pumice and nitrogen loss was reduced as ammonia was assimilated to organic nitrogen rather than volatilized. As showed in Fig. 3, 22.33% of nitrogen
Cumulative NH3 emissions(mg kg d.s.)
Cumulative CO2 emission(mg kg d.s.)
1800
(Amir et al., 2008; Li et al., 2013). Mesophilic microbial activities in the two runs decreased during the thermophilic phase. Therefore, the difference in CO2 emission rate between the two runs was substantially reduced during the cooling phase.
0
0 0
5
10
15
20
25
Time (day)
(b)
Fig. 2. (a) Cumulative CO2 emission and VS of sewage sludge during composing. (b) Instantaneous and cumulative NH3 emission during composting.
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Fresh Reused
DOC (mg/kg d.s.)
520
480
4.5
Nitrogen content (%)
560
440
Residual
ammonia could be converted into organic nitrogen which was used as the biological nitrogen compound (Li et al., 2013). Under moist and nutrient-rich conditions, ammonia-assimilating microorganisms might tolerate high ammonia content (Sasaki et al., 2005). It may be possible that the sucrose-decorated pumice provide a valid medium for ammonia-assimilating microorganisms. Kumar et al. (2009) proposed that modulation of carbon skeleton production might be an alternative approach to improve nitrogen assimilation. In this case, sucrose, as the only carbon source, might provide energy and substrate to the ammonia assimilation reactions more directly than the one of organic matter in sewage sludge.
Lost
3.6 2.7 1.8 0.9 0.0
Fresh
5
Reused
400
4. Conclusions 360 0
5
10
15
20
25
30
Time (day) Fig. 3. The change of dissolved organic carbon content and the residual and lost nitrogen after composting.
Nitrogen content (%)
Rsidual
Lost
3
4000
2
3000 1 0
2000 Fresh
Sucrose-decorated
0 0
5
10
15
20
-1
1000
Sucrose-decorated Fresh
Cumlative NH3 emission( mg kg d.s.)
5000
4
25
Time (day)
Pumice has rich pore structure and strong water absorbent capacity. Pumice could rapidly achieve 65.23% of the 24 h water absorptivity in 2 h at the mixing ratio of 0.6:1 (pumice:sewage sludge). Reused pumice could be used as the inoculant to promote the degradation of organic matter and the reduction of NH3 emission significantly. Sucrose-decorated pumice could enhance the ammonia assimilation reactions. The nitrogen loss during composting could be further controlled effectively by using sucrosedecorated pumice. Acknowledgements This research was financially supported by the National Natural Science Foundation of China (51278146, 51408151), Water Environmental Quality Improvement Technology Integration and Comprehensive Demonstration in Control Unit of Songhua River in Harbin City (2013ZX07201007), China Postdoctoral Science Foundation (2012M520755), China Postdoctoral Science Special Foundation (2014T70357), Postdoctoral Science Special Foundation of Hei Longjiang (LBH-TZ0510) and Specialized Research Fund for the Doctoral Program of Higher Education (20132302120075).
Fig. 4. Cumulative NH3 emission and residual and lost nitrogen after composing.
References was lost in R2 compared with 31.71%. This result indicated that 41.84% of nitrogen loss was reduced due to reuse of pumice. 3.3. Sucrose-decorated pumice In this study, sucrose was decorated to pumice as carbon source for the first time to control the nitrogen loss during composting. Cumulative NH3 emission trend was basically same with the experiment of reused pumice (Fig. 4). The NH3 emission rate was low during the initial period of composting, grew rapidly during the thermophilic phase and stabilized at the cooling phase. The difference in ammonia emission rates between P1 and P2 was occurred in the thermophilic phase. During the first 6 days, the cumulative NH3 emission in P1 and P2 were 173.03 mg kg1 dried sludge (d.s.) and 182.96 mg kg1 d.s., respectively, meaning no differences in the amounts of ammonia emission between the two runs. During the first 15 days, the cumulative NH3 emissions reached to 2117.31 mg kg1 d.s. of P1 and 3086.98 mg kg1 d.s. of P2, respectively. It showed that 969.67 mg kg1 d.s. less ammonia volatilized due to the utilization of sucrose-decorated pumice. At the end of the composting, 31.50% of cumulative NH3 emission and 43.37% of nitrogen loss was reduced, respectively. A large amount of microorganisms could be found from the SEM image of the sucrose-decorated pumice on Day-9. After the processing of sucrose, the barren pumice supplied the condition for microbial growth. Researchers reported that microorganisms could assimilate ammonia into glutamate with essential enzymes, and
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