- Email: [email protected]
Effect of alkalinity on the performance of a simulated landfill bioreactor digesting organic solid wastes
Chemosphere 59 (2005) 871–879 www.elsevier.com/locate/chemosphere
Effect of alkalinity on the performance of a simulated landfill bioreactor digesting organic solid wastes Osman Nuri Ag˘dag˘, Delia Teresa Sponza
*
Department of Environmental Engineering, Engineering Faculty, Dokuz Eylu¨l University, Buca Kaynaklar Campus, Tınaztepe-Izmir 35160, Turkey Received 1 March 2004; received in revised form 27 October 2004; accepted 17 November 2004
Abstract This study investigated the effects of alkalinity on the anaerobic treatment of the organic solid wastes collected from _ the kitchen of Engineering Faculty in Dokuz Eylu¨l University, Izmir, Turkey and the leachate characteristics treated in three simulated landfill anaerobic bioreactors. All of the reactors were operated with leachate recirculation. One reactor was operated without alkalinity addition. The second reactor was operated by the addition of 3 g l 1 d 1 of NaHCO3 alkalinity to the leachate and the third reactor was operated by the addition of 6 g l 1 d 1 NaHCO3 alkalinity to the leachate. After 65 d of anaerobic incubation, it was observed that the chemical oxygen demand (COD), volatile fatty acids (VFA) concentrations, and biochemical oxygen demand to chemical oxygen demand (BOD5/COD) ratios in the leachate samples produced from the alkalinity added reactors were lower than the control reactor while the pH values were higher than the control reactor. The COD values were measured as 18 900, 3800 and 2900 mg l 1 while the VFA concentrations were 6900, 1400 and 1290 mg l 1, respectively, in the leachate samples of the control, and reactors containing 3 g l 1 NaHCO3 and 6 g l 1 NaHCO3 after 65 d of anaerobic incubation. The total nitrogen (TN), total phosphorus (TP) and ammonium nitrogen (NH4-N) concentrations in organic solid waste (OSW) significantly reduced in the reactor containing 6 g l 1 NaHCO3 by d 65. The values of pH were 6.54, 7.19 and 7.31, after 65 d of anaerobic incubation, respectively, in the aforementioned reactors results in neutral environmental conditions in alkalinity added reactors. Methane percentage of the control, reactors containing 3 g l 1 NaHCO3 and 6 g l 1 NaHCO3 were 37%, 64% and 65%, respectively, after 65 d of incubation. BOD5/COD ratios of 0.27 and 0.25 were achieved in the 3 and 6 g l 1 NaHCO3 containing reactors, indicating a better OSW stabilization. Alkalinity addition reduced the waste quantity, the organic content of the solid waste and the biodegradation time. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Organic solid waste; Bioreactor; Anaerobic treatment; Leachate recirculation; Alkalinity addition
1. Introduction *
Corresponding author. Tel.: +90 232 453 1008; fax: +90 232 453 1153. E-mail addresses: [email protected] (O.N. Ag˘dag˘), [email protected] (D.T. Sponza).
The bioreactor landfill provides a similar approach and treatment to that utilized in organic solid waste digestion. Factors affecting the treatment efficiency of municipal solid waste (MSW) in bioreactors are leachate
0045-6535/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2004.11.017
872
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879
recirculation, recirculation volume, waste shredding, waste compaction, pH adjustment, aeration, and nutrient and alkalinity additions (Reinhart et al., 2002). The alkalinity of water is a measure of its capacity to neutralize acids and is due primarily to the salts of weak acids (Quasim and Chiang, 1994). If the acid concentrations (H2CO3 and VFA) exceed the available alkalinity, the landfilling bioreactor will ‘‘sour’’. This will be severely inhibiting the microbial activity, especially the methanogens. When methane production becomes ceases the VFA may continue to accumulate. Methanogens prefer nearly neutral pH conditions with a generally accepted optimum range of approximately 6.5–8.2 (Anderson and Yang, 1992; Speece, 1996). The continuous hydrolysis of solid waste, followed by the microbial conversion of biodegradable organic content resulting in the production of intermediate VFAs at high concentrations. In the methane fermentation phase, the pH value is elevated, being controlled by the bicarbonate buffering system (Reinhart and Al-Yousfi, 1996). A balance between acid production and acid consumption is essential for a stable anaerobic process running at the highest possible rate. Plaza et al. (1996) showed that, pH was controlled by the addition of sodium bicarbonate as a buffer at a minimum buffer/substrate ratio of 0.06 kg kg 1 TS. Adequate alkalinity, or buffer capacity, is necessary to maintain a stable pH in the digester for optimal biological activity (Cobb and Hill, 1990). An alkalinity level ranging from 1000 to 5000 mg CaCO3 l 1 was recommended by Tchobanoglous and Burton (1979). Alkaline pretreatment in the anaerobic digestion of co-mingled municipal solid waste significantly increases the biodegradability of the waste mixture (Hamzawi et al., 1998). Limited studies have been performed investigating the effect of alkalinity on the anaerobic treatment of the organic solid waste (OSW) in bioreactors. In the study realized by Warith (2002) it was shown that the highest pH was observed in the sludge added reactor and a highly reduced COD was observed in the pH buffered and nutrient added reactor. The study carried out by San and Onay (2001) showed that a four times per week recirculation strategy with a pH control provided the highest degree of stabilization. Dinamarca et al. (2003) studied the influence of pH on the anaerobic digestion of the organic fraction of the urban solid waste
in a two phase anaerobic reactor. The higher degradation of total suspended solid (TSS) and volatile suspended solid (VSS) were obtained in the reactors operated at pH 7 and 8. The effect of alkalinity addition has not been well reported for simulated landfill anaerobic reactors treating OSW in literature. This paper reports the results of an examination of a landfill simulated anaerobic reactor treating OSW at varying NaHCO3 alkalinity. The influence of bicarbonate alkalinity on leachate characteristics and on the degradation of OSW was investigated.
2. Materials and methods 2.1. Lab-scale simulated landfill bioreactor To treat the municipal solid wastes and to collect the biogas, stainless-steel cylindrical bioreactors with a 10 cm diameter and 30 cm in height were constructed. These bioreactors were operated in batch mode at a temperature of 35–40 °C under anaerobic conditions. The leachate was collected at the bottom section of the solid waste reactor and the effluent was recycled to the top of the reactor with a peristaltic pump. There were three separate ports on the top of the reactor for the addition of simulated rain water, measurement of the methane, total gas productions, percentage of methane and for recirculation of the leachate. 2.2. Operating conditions for simulated anaerobic landfill reactors All the reactors were loaded with solid waste but with different operational modes. The first reactor was operated with leachate recirculation (control-no alkalinity addition), the second reactor was operated with recirculation 3 g l 1 NaHCO3 alkalinity was added and the third was operated with leachate recirculation and 6 g l 1 NaHCO3 alkalinity was added. Fifty milliliters of anaerobic sludge was added to all the reactors and mixed in order to provide methanogenic conditions. Approximately 20 ml of NaHCO3 was added on the top of the reactors daily. Table 1 shows the operating protocol for the all reactors. The organic solid wastes
Table 1 Operating protocol for simulated landfill bioreactors in alkalinity study
Quantity of waste (g) Recirculation vol. (ml d 1) Alk. addition (NaHCO3) Operation time (d)
Control reactor
3gl
1000 300 No 65
1000 300 3 65
1
alkalinity added reactor
6gl 1000 300 6 65
1
alkalinity added reactor
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879
COD conc. (mg.l-1)
40000 30000 20000 10000 0 25000
(b) VFA conc. (mg.l -1)
Organic matters, water content in OSWs were measured by the method proposed by Kocasoy (1994). Carbon (C) in OSWs was measured following the method developed by Sorgun (1987). The COD concentrations in leachate samples were detected by using closed reflux colorimetric method following standard methods (APHA, 1992). BOD5 in leachate samples was measured using the WTW Oxi Top IS 12 system. TN, TP in OSWs were measured using spectroquant kits numbered 14 537, 14 543 in a photometer Merck SQ 300 after OSW was dried at 100 °C and crashed. In this process 20 g of OSW was mixed with 200 ml distilled water and was kept through 48 h in distilled water (USEPA, 1986). Ammonia-nitrogen in leachate samples was measured using spectroquant kit numbered 14 752 in a photometer Merck SQ 300. The pH in leachate samples was determined immediately after sampling to avoid any change due to CO2 stripping, using a pH meter, type NEL pH 890. Total volatile fatty acid (TVFA) concentrations in the leachate samples were measured using Anderson and Yang (1992) method. Gas productions were measured by liquid displacement method. Total gas was measured by passing it through a liquid containing 2% (v/v) H2SO4 and 10% (w/v) NaCl (Beydilli et al., 1998). Methane gas was detected using a liquid solution containing 3% NaOH (w/v) (Razo-Flores et al., 1997). Methane percentage was monitored with a digital methane meter (Drager Pac Ex).
(a)
20000 15000 10000 5000 0 8 7 6 5
pH
2.3. Analytical procedure
50000
4 3 2
(c)
1 0
Ammonium conc. (mg.l -1)
collected were from the kitchen of the Engineering Faculty in Dokuz Eylu¨l University Campus.
873
1000 800 600 400 200
(d)
0 0
10
20 30 40 Operation time (d)
50
60
70
3. Results and discussion 3.1. COD variations in leachate samples produced from the control and alkalinity supplemented simulated landfill bioreactors The initial COD concentration in leachate samples collected from the control, and reactors containing 3 g l 1 and 6 g l 1 NaHCO3 were approximately 16 000 mg l 1, respectively (Fig. 1a). The COD value of the leachate in the control reactor increased to 40 700 mg l 1 until d 21, and then started to decrease. The COD values of the leachate taken from the 3 g l 1 and 6 g l 1 NaHCO3 containing reactors increased to 30 000 and 23 000 mg l 1 until d 7. After d 7, the COD concentrations in these reactors started to decrease. The reason for this decrease in COD level may possibly have been the quick degradation of solid wastes in the lab-scale anaerobic OSW reactor and the positive effect of alkalinity on anaerobic degradation. The COD values of the leachate from the control, and reactors containing
Fig. 1. Trends in COD, VFA, pH and NH4-N concentrations (a–d) in leachate from the control (—d—), 3 g l 1 (—m—) and 6 g l 1 (——) NaHCO3 containing reactors.
3 g l 1 and 6 g l 1 NaHCO3 were approximately 19 000, 3900 and 3000 mg l 1 respectively, on d 65 (Appendix A). This indicates that the addition of alkalinity to the solid waste bioreactors has a positive effect on the anaerobic degradation of organic solid wastes. The organic matter easily converts to methane through the methanogenesis. However, the degradation process occurred slowly in the control reactor, because of unsuitable alkalinity for methanogen bacteria. The time required for stabilization of COD was shortened by the addition of alkalinity. Inadequate alkalinity/leachate COD ratio in the anaerobic reactors may cause minimum pH in the anaerobic reactor to fall below 6.2 which can lead to failure of the system. Speece (1996) and Anderson and Yang
874
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879
(1992) reported that if the acid concentrations exceed the available alkalinity, a drop in pH could occurs, When methane production becomes ceases the VFA may continue to accumulate. Methanogens prefer nearly neutral pH conditions with a generally accepted optimum range of approximately 6.5–8.2. Alkalinity addition was used in numerous studies to neutralize the pH the anaerobic treatment of MSW (San and Onay, 2001; Warith, 2002). Souza et al. (1992) and Moosbruger et al. (1993) found that an alkalinity/COD ratio 0.5 in the influent decreased the pH to 6.6, which is considered as the lower limit value recommended for anaerobic digestion processes. In our study, these ratios were 0.22, 0.98 and 1.3 in the control, 3 g l 1 and 6 g l 1 NaHCO3 supplemented reactors, respectively. Since 6 g l 1 of NaHCO3 concentration provided an optimum buffering capacity to convert effectively the leachate COD to methane and VFA, it could be suggested as an optimum bicarbonate alkalinity concentration in influent to maintain the pH above 6.6. This value corresponds to an Alk./ COD ratio of 1.3 for optimum operation which is higher than the ratio proposed by Gonzales et al. (1998) (0.4), Souza et al. (1992) and Moosbruger et al. (1993) (0.5) and Speece (1996) (1.2). 3.2. VFA variations in leachate samples produced from the control, 3 g l 1 and 6 g l 1 NaHCO3 alkalinity supplemented reactors The initial VFA concentrations in leachate samples were approximately 10 000 mg l 1 in all reactors (Fig. 1b). A strong linear correlation between COD and VFA concentrations was obtained in all reactors (r = 0.96, 086, and 0.85 in control and reactors containing 3 g l 1 and 6 g l 1 NaHCO3). The VFA values of the reactors containing 3 g l 1 and 6 g l 1 NaHCO3 increased to 30 000 and 23 000 mg l 1 on d 7. However, they were down to about 1400 and 1300 mg l 1 by d 65 (Appendix A). The VFA measurements showed that organic solid wastes in control reactor (no alkalinity addition) degraded slowly. Adequate alkalinity, or buffer capacity, is necessary to maintain a stable pH in the digester for optimal biological activity (Cobb and Hill, 1990). An alkalinity varying between 1000 and 5000 mg CaCO3 l 1, was recommended for anaerobic treatment depending on COD and VFA produced (Tchobanoglous and Burton, 1979). Traditionally, the total alkalinity in an anaerobic digester includes all the bicarbonate alkalinity and approximately 80% of the VFA (Anderson and Yang, 1992). When the system is in balance, the methanogens could be inactivated by unfavorable environmental conditions, e.g., pH drop, accumulation of VFA, intermetabolites and toxicity of aromatic amines due to their toxic properties (Kuai et al., 1998).
Behling et al. (1997) reported that, if an UASB reactor is stable, the TVFA/B.Alk. ratio should be between 0.4 and 0.8. In our study, TVFA/B.Alk. ratios were 3.00, 1.20, 1.00, 0.70 and 0.50 on d 5, 28, 31, 65 and 100 in control reactor. TVFA/B.Alk. ratios were 2.00, 0.70, 0.30, 0.40, 0.40 and 0.50 on d 5, 28, 31, 52, 65 and 100 in reactor containing 3 g l 1 NaHCO3 while the same ratios were 1.50, 1.10, 0.30, 0.40, 0.60 and 0.50 in reactor containing 6 g l 1 NaHCO3 on the aforementioned d. The low TVFA/B.Alk. ratios could be attributed to the deficient of B. alkalinity on d 31 and 52 in reactors containing 3 g l 1 and 6 g l 1 NaHCO3. The B. Alk. neutralize the CO2/H2CO3 with only the excess available for neutralizing an increase in VFA. 3.3. pH variations in leachate samples produced from the control, 3 g l 1 and 6 g l 1 NaHCO3 alkalinity supplemented reactors During the first two weeks, the pH of the leachates from the control, and reactors containing 3 g l 1 and 6 g l 1 NaHCO3 were approximately 5.30, respectively, on the acidic side of the pH scale (Fig. 1c). In the control reactor, the pH values decreased to 5 within 7 d. This caused an the extension of the time required for the stabilization of organic fraction of the waste to stabilize since methane forming anaerobes are known to be very sensitive to low pH (Bolzonella et al., 2003). The methane gas productions were 3900, 5900 and 6900 ml d 1 on d 5 (see Fig. 2a–c). The methane gas productions at a pH as low as 5 could be attributed to hydrogenotrrophic methanogens present in OSWs (Paulo et al., 2003). The so-called ‘‘acid habituation’’ or the ‘‘adaptive acid tolerance response’’ phenomenon, described by Hall et al. (1995) might explain the resistance of anaerobic microorganisms in OSWs. Lens et al. (2003) found that the lack of bicarbonate delayed the conversion of OSWs to methane resulting in low methane productions and percentages in control reactor. The other reason for methane production at pH 5 in control reactor could be attributed to ammonium bicarbonate alkalinity which in turn maintained a pH close to neutral inside cells (Speece, 1996). This called ‘‘metabolism generated alkalinity’’ inside cells. The degradation of cation releasing nitrogenous organics (proteins) would double the alkalinity concentration generated during biodegradation of proteins in OSWs (Speece, 1996). On the other hand, VFA alkalinity contributes to the buffering of H2CO3, but is transient since the VFA varies and therefore cannot be consistently relied upon. The pH level of the control reactor was 6.54 on d 65 (Appendix A). The conversion of fatty acids caused an increase in the pH levels within the alkalinity supplemented reactors. In this study the measured pH levels was found to be higher than the data obtained by Plaza et al. (1996) which used CaCO3 for alkalinity.
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879 70
16000
50
12000 10000
40
8000
30
6000
20
4000
(a)
2000 0
70 60
20000
50 15000
40
10000
30 20
5000
Methane percent (%)
Cumulative methane (ml)
10 0
25000
10
(b)
0
Cumulative methane (ml)
Methane percent (%)
60
14000
30000
70
25000
60 50
20000
40 15000 30 10000
20
5000
(c)
0 0
20
40
60 80 Operation time (d)
100
nitrogenous organic substances could be attributed to ammonium bicarbonate alkalinity, which in turn maintained a pH close to neutral in the anaerobic solid waste bioreactor (Speece, 1996; Jokela et al., 2002b). NH4-N loss through stripping was considered to be negligible because the pH in the reactors containing 3 and 6 g l 1 NaHCO3 was below 8 (Diamadopoulos, 1996; Yılmaz ¨ ztu¨rk, 2001). and O Since the NH4-N concentration in the leachate samples is high, the generated ammonium bicarbonate alkalinity contributes to total alkalinity in effluent samples as reported by Dinamarca et al. (2003). On the other hand, since the reserve alkalinity is the bicarbonate alkalinity maintaining pH above 6.2–6.6 in the anaerobic simulated reactor; it can be assumed that this alkalinity was provided in reactors containing 3 and 6 g l 1 NaHCO3 since the lower pH was recorded as 6.4 apart from the initial pHs.
0
Methane percent (%)
Cumulative methane (ml)
18000
875
10 0 120
Fig. 2. Cumulative methane gas productions (—d—) and methane percentage (—h—) of control reactor (a), 3 g l 1 (b) and 6 g l 1 (c) NaHCO3 containing reactors.
3.4. NH4-N variations in leachate produced from the control and alkalinity supplemented simulated landfill reactors The highest NH4-N concentrations were measured to be 770, 760 and 660 mg l 1 in control, and reactors containing 3 g l 1 and 6 g l 1 NaHCO3 on d 20, 30 and 25 through the mineralization of organic nitrogenous compounds (Fig. 1d). The NH4-N concentrations were 750, 720 and 650 mg l 1 in the aforementioned reactors after 65 d of operation period (Appendix A). In this study, it was observed that there were no significant differences in NH4-N concentrations among the control, and reactors containing 3 g l 1 and 6 g l 1 NaHCO3. The low NH4-N removals between d 20–30 and 65 could be attributed to the utilization of NH4-N through assimilation of anaer¨ ztu¨rk, 2001). As reported by obic bacteria (Yılmaz and O Jokela et al. (2002a) a small amount of ammonium nitrogen released through degradation of proteinic and nitrogenous compounds may be incorporated into cell biomass. The decreases of NH4-N released from the
3.5. Methane gas productions and methane percentages in simulated anaerobic solid waste bioreactors The maximum cumulative methane gas production was recorded as 12 l, 19 l and 23 l in control, and reactors containing 3 g l 1 and 6 g l 1 NaHCO3, respectively at the end of 65 d, while the maximum cumulative methane gas productions were measured at 16 l, 20 l and 24 l after 100 d of operation period (Fig. 2a–c). In order to compare the stability of the control reactor the methane gas productions and methane percentages were given for 100 d. The reason for the high cumulative methane in 6 g l 1 reactor is the fast degradation of municipal organic solid wastes in this reactor through rapid methanogenesis on d 65. The methane gas percentages were 58%, 60% and 61% on d 65 while the methane percentages were measured to be 30% after 100 d of anaerobic incubation in control, and reactors containing 3 g l 1 and 6 g l 1 NaHCO3. It was observed that the anaerobic degradation terminated since the methane gas productions decreased significantly by d 65 in the reactors containing 3 g l 1 and 6 g l 1 NaHCO3. The biodegradation rate was higher in reactors containing 3 and 6 g l 1 NaHCO3. It is important to note that the increases percentages of cumulative methane gas productions was lower in reactors containing 3 and 6 g l 1 NaHCO3 (E = 5.00% and 6.20%, respectively) while the increase in the percentages of cumulative methane gas productions were higher in control reactor (E = 36%) between d 65 and 100. This shows that the ultimate mineralization of OSW in the control reactor was carried out between d 65 and 100. In this study, the methane percentages measured in alkalinity added reactors were higher than the study realized by San and Onay (2001). Methane gas productions and methane percentages in simulated anaerobic solid waste bioreactors show that the alkalinity addition has a positive effect on
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879
876
Table 2 Comparison of characteristics of the simulated landfilling bioreactors Initial
Water content (%) Organic matter (%) (in DS) % C (in DS) TN (mg g 1) (in waste) TP (mg g 1) (in waste) NH4-N (mg g 1) (in waste) Waste quantity (g)
Final 1
1
Control
3 g l NaHCO3 added R.
6 g l NaHCO3 added R.
Control
3 g l 1 NaHCO3 added R.
6 g l 1 NaHCO3 added R.
90 97 54 4350 1050 155 1000
90 97 54 4350 1050 155 1000
90 97 54 4350 1050 155 1000
91 65 36 610 53 38 213
92 58 32 220 46 36 140
92 51 28 213 43 35 120
biodegradation of municipal solid waste in simulated bioreactors due to high methane gas productions and high methane percentages. 3.6. BOD5 concentrations and BOD5/COD ratios in the leachate samples from the simulated anaerobic solid waste bioreactors In order to study in greater detail the proportion of biodegradable organic carbon in the leachate it was decided to determine the BOD5/COD ratios. The BOD5 values decreased substantially by 69%, 95% and 96% in the aforementioned reactors, respectively (data not shown). The experimental results indicate that a higher MSW stabilization rate was achieved in the reactor containing 6 g l 1 NaHCO3 than that of control and 3 g l 1 NaHCO3 containing reactors. Initially, all the reactors had high BOD5/COD ratios such as 0.9, indicating the high degradability of leachate (Quasim and Chiang, 1994) (data not shown). On d 52, BOD5/COD ratio decreased to approximately 0.25 from 0.78 in the reactor containing 6 g l 1 NaHCO3. This ratio implies a low biodegradable leachate. As the biodegradation of organic content of OSW occurred, the BOD5/COD ratio decreased. Low BOD5/COD ratio in 6 g l 1 alkalinity added reactor shows that MSW converted rapidly to methane via methanogenesis. If the BOD5/COD ratio was between 0.02 and 0.13, this implies that the leachate has a low biodegradability through anaerobic phase. When this ratio is between 0.4 and 0.8, this implies the high biodegradability of the leachate (Otieno, 1994). The BOD5/COD ratio measured in the study shows similar data to the findings of Ledakowicz and Kaczarek (2002). 3.7. Comparison of characteristics of the simulated anaerobic landfilling reactors The initial organic matter content of OSWs were 97% in all reactors while the organic matter ratios of OSWs were 65%, 58%, 51% organic in control, and reactors
containing 3 g l 1 and 6 g l 1 NaHCO3, respectively, at the end of anaerobic incubation (Table 2). The reductions in the organic matter were 33%, 40% and 47%, in control, reactors containing 3 g l 1 and 6 g l 1 NaHCO3, respectively, through 65 d of operation period. The results showed that more organic material is transferred into the gaseous phase in the reactor containing 6 g l 1 NaHCO3 under methanogenic conditions. The lack of NaHCO3 dropped the methane produced and the conversion of organic matter in OSWs. If the acid concentration (H2CO3 and VFA) exceeds the available alkalinity, this severely inhibits the microbial activity, especially the methanogens in OSWs. The reduction in C content of OSW in the reactor containing 6 g l 1 NaHCO3 was higher than the other reactors. The alkaline conditions considerably increased the carbon conversion rates in OSWs. On d 65, the gradual decline of the carbon content of OSWs in the reactor containing 6 g l 1 NaHCO3 can most probably be attributed to the absence of un-ionized acids since the buffer capacity of the reactor is high. In the absence of NaHCO3 the total carbon content in OSWs was not successfully used by the methanogens through the low conversion rate of organic substances compared to alkalinity containing anaerobic simulated reactors. It was observed that the reduction of waste quantity was high in the reactor containing 6 g l 1 NaHCO3. Seventy eight percent, 99% and 99.9% reductions in OSWs quantity were observed in control, and reactors containing 3 g l 1 and 6 g l 1 NaHCO3. This parameter is very important since it would optimize the land utilization. A depletion of high volume in OSWs would reduce the landfilling sites and could be used for the management of anaerobic degradation of OSWs in simulated bioreactors. TN removal efficiencies of OSWs were 86%, 95% and 96% in control, and reactors 3 g l 1 and 6 g l 1 NaHCO3 while TP removal efficiencies were 95%, 96% and 96% in the same reactors, respectively. The proteins and organic substances in OSW degraded to NH4-N and were released to the leachate. The organic phosphorus and phosphate in OSW were used by the acetogenic
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879
and methanogenic bacteria as nutrient and energy source through anaerobic metabolism. 3.8. COD mass balance and degree of stabilization of solid wastes As the waste stabilization is directly related to the amount of methane produced and methane percentage of the total gas, the amount of methane generated per kg of organic matter stabilized is taken to be an indicator of waste stabilization degree. About 12, 19 and 23 l of methane was regenerated from the control, and reactors with added 3 g l 1 NaHCO3 and 6 g l 1 NaHCO3, respectively, on d 65. These results are comparable high with the study performed by San and Onay (2001) and Ledakowicz and Kaczarek (2002). In order to determine the stabilization degree, the maximum mass of COD released was determined by taking into consideration the maximum released COD and the available moisture in all the reactors for d 57. It was found that the maximum mass of COD released from the OMS to the leachate was equal to 40, 30 and 23 g in the control, and in the reactors added 3 g l 1 NaHCO3 and 6 g l 1 NaHCO3, respectively. Twenty eight percent of COD removed in OSW from the control reactor was converted into methane while 44% and 65% of COD removed in OSW was converted to methane in the reactors added 3 g l 1 NaHCO3 and 6 g l 1 NaHCO3, respectively, on d 57. In our study the COD amount converted to into methane was comparably higher in the reactor with added 6 g l 1 NaHCO3 compared to the study performed by San and Onay (2001). As the alkalinity supplemented reactors have a higher capacity for extracting the COD from the OSW compared to control reactor, addition of alkalinity increased the specific methane yield to about 0.19 and 0.23 m3 kg 1 TSadded waste in 3 g l 1 NaHCO3 and 6 g l 1 NaHCO3 added reactors, respectively, whereas the methane yield in the control reactor was 0.13 m3 kg 1 TS on d 65. The specific methane yield in the control reactor was low since the biodegradation of OSWs and therefore the methane gas production were not completed by d 65. The specific methane yield increased to 0.19 m3 kg 1 TS in the control reactor on d 100 indicating the anaerobic degradation of remaining organic substances through methanogenesis. The specific methane yields were 0.36 and 0.50 m3 kg 1 TS in reactors containing 3 g l 1 NaHCO3 and 6 g l 1 NaHCO3 depending to CH4-COD remaining and TS in simulated anaerobic reactors after 100 d. These results are significantly higher than the studies performed by Chugh et al. (1998) and Mata-Alvarez et al. (2000). The methane production rates were 0.033 l d 1, 0.102 l d 1 and 0.155 l d 1 in control, and the reactors with added 3 and 6 g l 1 NaHCO3 on d 5 while the same parameter were 0.118 l d 1, 0.290 l d 1 and 0.349 l d 1
877
in the aforementioned reactors on d 65. Maximum methane production rates were reached on d 75 and 35 in control and the other two reactors. On d 100 the methane gas production rates decreased to 0.199 l d 1 and 0.238 l d 1 in the reactors with added 3 and 6 g l 1 NaHCO3, respectively.
4. Conclusions The alkalinity addition to OSWs has a positive effect on the rate of biological degradation in anaerobic simulated recycled reactors through the degradation of organic fraction of solid wastes collected from the kitchen of the Engineering Faculty in Dokuz Eylu¨l _ University Campus, Izmir, Turkey. The findings of this study showed that the anaerobic digestion of OSW in a simulated landfill reactor correlate with bicarbonate alkalinity. The alkalinity is lost through NH4-N conversion of organic nitrogen, the buffering of H2CO3 acidity by CO2 and VFA generation by the acidogen microorganisms. VFA build up with VFA generated, increased the acidity and reduced the alkalinity. The VFA converted into methane by methanogens particularly in the lower and upper zone of simulated anaerobic OSW reactor and in the whole reactor since the leachate was recirculated from the bottom to the top of the reactor. In other words, the leachate was partially accumulated in the lower part and the upper parts of OSW reactor, resulting in VFA conversion to methane which the medium part of the reactor mostly exhibits only under methanogenic conditions. The alkalinity requirement is 1.4 g NaHCO3 alkalinity/g influent COD providing 87% COD, 87% VFA removals with a methane gas production of about 100 ml d 1 and methane percentage of 59 at a pH 7.31 on d 65 through degradation of OSW in a simulated landfill anaerobic reactor. The observed COD removal efficiencies in reactors containing 3 and 6 g l 1 of NaHCO3 could be due to no-VFA accumulation resulting in activation of methanogenic bacteria at pH around 7.0. The acidogenic phase would generate acids, but this was buffered by sufficient alkalinity in reactors containing alkalinity. When 3 and 6 g l 1 NaHCO3 was added to the feed this alkalinity was not completely consumed. Utilization of anaerobic simulated bioreactor with an alkalinity enhances the feasibility of degradation of OSWs through methanogenesis. As a result, more organic material is transferred into the gaseous phase. Anaerobic simulated bioreactors reduce the landfilling sites since a reduction in the volume of the leachate as well as solid volume was obtained in OSWs. Because degradation of OSWs and leachate produced through the degradation of organic substances is a most important subject in terms of pollution emissions,
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879
878
besides the cost-effective anaerobic treatment process, the additional alkalinity requirement should be kept in mind in terms of financial costs.
Acknowledgments This study was executed as a part of the research activities of the Environmental Microbiology Laboratory of Environmental Engineering Department and the project was partially funded by the Dokuz Eylu¨l University Research Foundation. The authors would like to thank this body for the financial support to projects with grant numbers Fen 021, 051 and 03.KB.Fen.017.
Appendix A 1
NaHCO3 6 g l
1
Days Control
3gl
COD 1 3 7 10 14 17 21 24 28 31 42 52 65
16 004 18 936 36 618 35 712 38 102 39 304 40 658 38 432 35 200 32 744 26 725 20 214 18 964
16 121 17 009 30 353 27 549 26 065 22 405 20 624 14 276 7680 4211 3960 3920 3865
16 200 16 768 22 850 20 459 16 749 13 269 10 648 6984 5250 3845 3152 3057 2908
VFA 1 3 7 10 14 17 21 24 28 31 42 52 65
10 052 9296 17 152 19 432 20 320 21 080 19 304 18 432 17 936 14 208 11 147 9967 6913
10 169 8064 12 100 11 500 11 376 7465 4947 3623 2305 1385 1559 1500 1413
10 103 7344 10 000 8345 5994 5050 4046 2271 1693 1374 1335 1300 1285
pH 1 3 7 10 14 17
5.32 5.33 4.98 5 5.13 5.14
5.16 5.2 5 5.12 5.41 6.34
NaHCO3
5.3 5.35 5.11 5.35 6.74 6.9
Appendix A (continued) Days 21 24 28 31 42 52 65
Control 5.16 5.13 5.26 5.34 5.85 6.04 6.54
Ammonium 1 388 3 416 7 584 10 604 14 736 17 750 21 764 24 704 28 713 31 740 42 752 52 763 65 750
3gl
1
6.98 7.07 7.07 7.07 6.94 7.04 7.19 370 376 524 612 652 664 672 732 750 760 762 750 713
NaHCO3
6gl
1
NaHCO3
7.02 7.13 7.08 7.08 6.99 7.19 7.31 406 464 528 593 608 603 600 636 604 548 602 650 650
References American Public Association, 1992. American Water Works Association, and Water Environment Federation. Standard Methods for the Examination of Water and Wastewater, 17th ed., Washington, DC. Anderson, G., Yang, G., 1992. Determination of bicarbonate and total volatile acid concentration in anaerobic digestion using a simple titration. Wat. Environ. Res. 64, 53–59. Behling, E., Diaz, A., Colina, G., Herrera, M., Gutierrez, E., Chacin, E., Fernandez, E., Forster, C.F., 1997. Domestic wastewater using a simple titration. Wat. Environ. Res. 64, 53–59. Beydilli, M.I., Pavlosathis, S.G., Tinvher, W.C., 1998. Decolorization and toxicity screening of selected reactive azo dyes under methanogenic conditions. Wat. Sci. Tech. 38, 225– 232. Bolzonella, D., Battistoni, P., Mata-Alvarez, J., Cecchi, F., 2003. Anaerobic digestion of solid wastes: process behavior in transient conditions. Wat. Sci. Tech. 48, 1–8. Chugh, S., Clarke, W., Pulammanappallil, P., Rudolph, V., 1998. Effect of recirculated leachate volume on MSW degradation. Waste Manage. Res. 16, 564–573. Cobb, S.A., Hill, D.T., 1990. Volatile fatty acid interference with alkalinity measurements in anaerobic fermenters. International Summer Meeting, sponsored by the ASAE. Diamadopoulos, E., 1996. Characterization and treatment of recirculation-stabilized leachate. Wat. Res. 28, 2439–2445. Dinamarca, S., Aroca, G., Chamy, R., Guerrero, L., 2003. The influence of pH in the hydrolytic stage of anaerobic digestion of the organic fraction of urban solid waste. Wat. Sci. Tech. 48, 249–254.
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879 Gonzales, J.S., Rivera, A., Borja, R., Sanchez, E., 1998. Influence of organic volumetric loading rate, nutrient balance and alkalinity/COD ratio on the anaerobic sludge granulation of an UASB reactor treating sugar cane molasses. Int. Biodet. Biodeg. 41, 127–131. Hall, H.K., Karem, K.L., Foster, J.W., 1995. Molecular responses of microbes to environmental pH stress. Adv. Microb. Physiol. 37, 229–272. Hamzawi, N., Kennedy, K.J., McLean, D.D., 1998. Anaerobic digestion of co-mingled municipal solid waste and sewage sludge. Wat. Sci. Tech. 38, 127–132. Jokela, J.P.Y., Kettunen, R.H., Rintale, J.A., 2002a. Methane and leachate pollutant emission potential from various fraction of municipal solid wastes. Effects of source separation and aerobic treatment. Waste Manage. Res. 20, 424–433. Jokela, J.P.Y., Kettunen, R.H., Sormunen, K.M., 2002b. Biological nitrogen removal from municipal landfill leachate: low cost nitrification in biofilters and laboratory scale in situ denitrification. Wat. Res. 36, 4079–4087. Kocasoy, G., 1994. Analyses methods in solid waste, compost _ and industrial sludge samples, Bebek, Istanbul, Turkey. Kuai, L., De Vreese, L., Vandevivere, P., 1998. GAC-amended UASB reactor for the stable treatment of toxic textile wastewater. Environ. Technol. 19, 1111–1117. Ledakowicz, S., Kaczarek, K., 2002. Laboratory simulation of anaerobic digestion of municipal solid waste. Appropriate Environmental and Solid Waste Management and Technologies for Developing Countries, Turkey, pp. 1139–1146. Lens, P.N.L., Klijn, R., van Lier, J.B., Lettinga, G., 2003. Effect of specific gas loading rate on thermophilic acidifying and sulfate reducing granular sludge reactors. Wat. Res. 37, 1033–1047. Mata-Alvarez, J., Mace, S., Llabres, P., 2000. Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Biores. Technol. 74, 3–16. Moosbruger, R.E., Wentzel, M.C., Ekama, G.A., Marais, G., 1993. Lauterntun waste in UASB systems, feasibility, alkalinity requirements and pH control. Wat. SA 19, 41–52. Otieno, F.A.O., 1994. Stabilization of solid waste through leachate recycling. Waste Manage. Res. 12, 93–100.
879
Paulo, P.L., Villa, G., van Lier, B., Lettinga, G., 2003. The anaerobic conversion of methanol under thermophilic conditions: pH and bicarbonate dependence. J. Biosci. Bioeng. 96, 213–218. Plaza, G., Robredo, P., Pacheco, O., Toledo, A.S., 1996. Anaerobic treatment of municipal solid waste. Wat. Sci. Tech. 33, 169–175. Quasim, S.R., Chiang, W., 1994. Sanitary Landfill Leachate Generation, Control and Treatment. Technomic Publishing Company, USA. Razo-Flores, E., Luijton, M., Donlon, B.A., Lettinga, G., Field, J.A., 1997. Biodegradation of selected azo dye under methanogenic conditions. Wat. Sci. Tech. 36, 65–72. Reinhart, D.R., Al-Yousfi, A.B., 1996. The impact of leachate recirculation on municipal solid waste landfill operating characteristics. Waste Manage. Res. 20, 337–346. Reinhart, D.R., McCreanor, P.T., Townsend, T., 2002. The bioreactor landfill: its status and future. Waste Manage. Res. 20, 172–186. San, I., Onay, T.T., 2001. Impact of various recirculation regimes on municipal solid waste degradation. J. Hazard. Mater. B 87, 259–271. Speece, R.E., 1996. Anaerobic Biotechnology for Industrial Wastewaters. Archae Press, Nashville Tennessee, USA. Sorgun, K., 1987. Solid waste management. Turkish General Directorate of Education and Production Centre, Ministry of Industry and Commerce, Mersin, Turkey. Souza, M.E., Fuzaro, G., Polegato, A.R., 1992. Thermophilic anaerobic digestion of vinasse in pilot plant UASB reactor. Wat. Sci. Tech. 25, 213–222. Tchobanoglous, G., Burton, F.L., 1979. Wastewater Engineering. McGraw-Hill International Editions. USEPA, 1986. Extraction procedure, toxicity test method and structural integrity tests. Method 1310 B, Environmental Protection Agency, Washington, USA. Warith, M., 2002. Bioreactor landfill: experimental and field results. Waste Manage. 22, 7–17. _ 2001. Biological ammonia removal from ¨ ztu¨rk, I, Yılmaz, G., O anaerobically pre-treated landfill leachate in sequencing batch reactors. Wat. Sci. Tech. 43, 307–314.
Effect of alkalinity on the performance of a simulated landfill bioreactor digesting organic solid wastes Osman Nuri Ag˘dag˘, Delia Teresa Sponza
*
Department of Environmental Engineering, Engineering Faculty, Dokuz Eylu¨l University, Buca Kaynaklar Campus, Tınaztepe-Izmir 35160, Turkey Received 1 March 2004; received in revised form 27 October 2004; accepted 17 November 2004
Abstract This study investigated the effects of alkalinity on the anaerobic treatment of the organic solid wastes collected from _ the kitchen of Engineering Faculty in Dokuz Eylu¨l University, Izmir, Turkey and the leachate characteristics treated in three simulated landfill anaerobic bioreactors. All of the reactors were operated with leachate recirculation. One reactor was operated without alkalinity addition. The second reactor was operated by the addition of 3 g l 1 d 1 of NaHCO3 alkalinity to the leachate and the third reactor was operated by the addition of 6 g l 1 d 1 NaHCO3 alkalinity to the leachate. After 65 d of anaerobic incubation, it was observed that the chemical oxygen demand (COD), volatile fatty acids (VFA) concentrations, and biochemical oxygen demand to chemical oxygen demand (BOD5/COD) ratios in the leachate samples produced from the alkalinity added reactors were lower than the control reactor while the pH values were higher than the control reactor. The COD values were measured as 18 900, 3800 and 2900 mg l 1 while the VFA concentrations were 6900, 1400 and 1290 mg l 1, respectively, in the leachate samples of the control, and reactors containing 3 g l 1 NaHCO3 and 6 g l 1 NaHCO3 after 65 d of anaerobic incubation. The total nitrogen (TN), total phosphorus (TP) and ammonium nitrogen (NH4-N) concentrations in organic solid waste (OSW) significantly reduced in the reactor containing 6 g l 1 NaHCO3 by d 65. The values of pH were 6.54, 7.19 and 7.31, after 65 d of anaerobic incubation, respectively, in the aforementioned reactors results in neutral environmental conditions in alkalinity added reactors. Methane percentage of the control, reactors containing 3 g l 1 NaHCO3 and 6 g l 1 NaHCO3 were 37%, 64% and 65%, respectively, after 65 d of incubation. BOD5/COD ratios of 0.27 and 0.25 were achieved in the 3 and 6 g l 1 NaHCO3 containing reactors, indicating a better OSW stabilization. Alkalinity addition reduced the waste quantity, the organic content of the solid waste and the biodegradation time. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Organic solid waste; Bioreactor; Anaerobic treatment; Leachate recirculation; Alkalinity addition
1. Introduction *
Corresponding author. Tel.: +90 232 453 1008; fax: +90 232 453 1153. E-mail addresses: [email protected] (O.N. Ag˘dag˘), [email protected] (D.T. Sponza).
The bioreactor landfill provides a similar approach and treatment to that utilized in organic solid waste digestion. Factors affecting the treatment efficiency of municipal solid waste (MSW) in bioreactors are leachate
0045-6535/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2004.11.017
872
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879
recirculation, recirculation volume, waste shredding, waste compaction, pH adjustment, aeration, and nutrient and alkalinity additions (Reinhart et al., 2002). The alkalinity of water is a measure of its capacity to neutralize acids and is due primarily to the salts of weak acids (Quasim and Chiang, 1994). If the acid concentrations (H2CO3 and VFA) exceed the available alkalinity, the landfilling bioreactor will ‘‘sour’’. This will be severely inhibiting the microbial activity, especially the methanogens. When methane production becomes ceases the VFA may continue to accumulate. Methanogens prefer nearly neutral pH conditions with a generally accepted optimum range of approximately 6.5–8.2 (Anderson and Yang, 1992; Speece, 1996). The continuous hydrolysis of solid waste, followed by the microbial conversion of biodegradable organic content resulting in the production of intermediate VFAs at high concentrations. In the methane fermentation phase, the pH value is elevated, being controlled by the bicarbonate buffering system (Reinhart and Al-Yousfi, 1996). A balance between acid production and acid consumption is essential for a stable anaerobic process running at the highest possible rate. Plaza et al. (1996) showed that, pH was controlled by the addition of sodium bicarbonate as a buffer at a minimum buffer/substrate ratio of 0.06 kg kg 1 TS. Adequate alkalinity, or buffer capacity, is necessary to maintain a stable pH in the digester for optimal biological activity (Cobb and Hill, 1990). An alkalinity level ranging from 1000 to 5000 mg CaCO3 l 1 was recommended by Tchobanoglous and Burton (1979). Alkaline pretreatment in the anaerobic digestion of co-mingled municipal solid waste significantly increases the biodegradability of the waste mixture (Hamzawi et al., 1998). Limited studies have been performed investigating the effect of alkalinity on the anaerobic treatment of the organic solid waste (OSW) in bioreactors. In the study realized by Warith (2002) it was shown that the highest pH was observed in the sludge added reactor and a highly reduced COD was observed in the pH buffered and nutrient added reactor. The study carried out by San and Onay (2001) showed that a four times per week recirculation strategy with a pH control provided the highest degree of stabilization. Dinamarca et al. (2003) studied the influence of pH on the anaerobic digestion of the organic fraction of the urban solid waste
in a two phase anaerobic reactor. The higher degradation of total suspended solid (TSS) and volatile suspended solid (VSS) were obtained in the reactors operated at pH 7 and 8. The effect of alkalinity addition has not been well reported for simulated landfill anaerobic reactors treating OSW in literature. This paper reports the results of an examination of a landfill simulated anaerobic reactor treating OSW at varying NaHCO3 alkalinity. The influence of bicarbonate alkalinity on leachate characteristics and on the degradation of OSW was investigated.
2. Materials and methods 2.1. Lab-scale simulated landfill bioreactor To treat the municipal solid wastes and to collect the biogas, stainless-steel cylindrical bioreactors with a 10 cm diameter and 30 cm in height were constructed. These bioreactors were operated in batch mode at a temperature of 35–40 °C under anaerobic conditions. The leachate was collected at the bottom section of the solid waste reactor and the effluent was recycled to the top of the reactor with a peristaltic pump. There were three separate ports on the top of the reactor for the addition of simulated rain water, measurement of the methane, total gas productions, percentage of methane and for recirculation of the leachate. 2.2. Operating conditions for simulated anaerobic landfill reactors All the reactors were loaded with solid waste but with different operational modes. The first reactor was operated with leachate recirculation (control-no alkalinity addition), the second reactor was operated with recirculation 3 g l 1 NaHCO3 alkalinity was added and the third was operated with leachate recirculation and 6 g l 1 NaHCO3 alkalinity was added. Fifty milliliters of anaerobic sludge was added to all the reactors and mixed in order to provide methanogenic conditions. Approximately 20 ml of NaHCO3 was added on the top of the reactors daily. Table 1 shows the operating protocol for the all reactors. The organic solid wastes
Table 1 Operating protocol for simulated landfill bioreactors in alkalinity study
Quantity of waste (g) Recirculation vol. (ml d 1) Alk. addition (NaHCO3) Operation time (d)
Control reactor
3gl
1000 300 No 65
1000 300 3 65
1
alkalinity added reactor
6gl 1000 300 6 65
1
alkalinity added reactor
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879
COD conc. (mg.l-1)
40000 30000 20000 10000 0 25000
(b) VFA conc. (mg.l -1)
Organic matters, water content in OSWs were measured by the method proposed by Kocasoy (1994). Carbon (C) in OSWs was measured following the method developed by Sorgun (1987). The COD concentrations in leachate samples were detected by using closed reflux colorimetric method following standard methods (APHA, 1992). BOD5 in leachate samples was measured using the WTW Oxi Top IS 12 system. TN, TP in OSWs were measured using spectroquant kits numbered 14 537, 14 543 in a photometer Merck SQ 300 after OSW was dried at 100 °C and crashed. In this process 20 g of OSW was mixed with 200 ml distilled water and was kept through 48 h in distilled water (USEPA, 1986). Ammonia-nitrogen in leachate samples was measured using spectroquant kit numbered 14 752 in a photometer Merck SQ 300. The pH in leachate samples was determined immediately after sampling to avoid any change due to CO2 stripping, using a pH meter, type NEL pH 890. Total volatile fatty acid (TVFA) concentrations in the leachate samples were measured using Anderson and Yang (1992) method. Gas productions were measured by liquid displacement method. Total gas was measured by passing it through a liquid containing 2% (v/v) H2SO4 and 10% (w/v) NaCl (Beydilli et al., 1998). Methane gas was detected using a liquid solution containing 3% NaOH (w/v) (Razo-Flores et al., 1997). Methane percentage was monitored with a digital methane meter (Drager Pac Ex).
(a)
20000 15000 10000 5000 0 8 7 6 5
pH
2.3. Analytical procedure
50000
4 3 2
(c)
1 0
Ammonium conc. (mg.l -1)
collected were from the kitchen of the Engineering Faculty in Dokuz Eylu¨l University Campus.
873
1000 800 600 400 200
(d)
0 0
10
20 30 40 Operation time (d)
50
60
70
3. Results and discussion 3.1. COD variations in leachate samples produced from the control and alkalinity supplemented simulated landfill bioreactors The initial COD concentration in leachate samples collected from the control, and reactors containing 3 g l 1 and 6 g l 1 NaHCO3 were approximately 16 000 mg l 1, respectively (Fig. 1a). The COD value of the leachate in the control reactor increased to 40 700 mg l 1 until d 21, and then started to decrease. The COD values of the leachate taken from the 3 g l 1 and 6 g l 1 NaHCO3 containing reactors increased to 30 000 and 23 000 mg l 1 until d 7. After d 7, the COD concentrations in these reactors started to decrease. The reason for this decrease in COD level may possibly have been the quick degradation of solid wastes in the lab-scale anaerobic OSW reactor and the positive effect of alkalinity on anaerobic degradation. The COD values of the leachate from the control, and reactors containing
Fig. 1. Trends in COD, VFA, pH and NH4-N concentrations (a–d) in leachate from the control (—d—), 3 g l 1 (—m—) and 6 g l 1 (——) NaHCO3 containing reactors.
3 g l 1 and 6 g l 1 NaHCO3 were approximately 19 000, 3900 and 3000 mg l 1 respectively, on d 65 (Appendix A). This indicates that the addition of alkalinity to the solid waste bioreactors has a positive effect on the anaerobic degradation of organic solid wastes. The organic matter easily converts to methane through the methanogenesis. However, the degradation process occurred slowly in the control reactor, because of unsuitable alkalinity for methanogen bacteria. The time required for stabilization of COD was shortened by the addition of alkalinity. Inadequate alkalinity/leachate COD ratio in the anaerobic reactors may cause minimum pH in the anaerobic reactor to fall below 6.2 which can lead to failure of the system. Speece (1996) and Anderson and Yang
874
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879
(1992) reported that if the acid concentrations exceed the available alkalinity, a drop in pH could occurs, When methane production becomes ceases the VFA may continue to accumulate. Methanogens prefer nearly neutral pH conditions with a generally accepted optimum range of approximately 6.5–8.2. Alkalinity addition was used in numerous studies to neutralize the pH the anaerobic treatment of MSW (San and Onay, 2001; Warith, 2002). Souza et al. (1992) and Moosbruger et al. (1993) found that an alkalinity/COD ratio 0.5 in the influent decreased the pH to 6.6, which is considered as the lower limit value recommended for anaerobic digestion processes. In our study, these ratios were 0.22, 0.98 and 1.3 in the control, 3 g l 1 and 6 g l 1 NaHCO3 supplemented reactors, respectively. Since 6 g l 1 of NaHCO3 concentration provided an optimum buffering capacity to convert effectively the leachate COD to methane and VFA, it could be suggested as an optimum bicarbonate alkalinity concentration in influent to maintain the pH above 6.6. This value corresponds to an Alk./ COD ratio of 1.3 for optimum operation which is higher than the ratio proposed by Gonzales et al. (1998) (0.4), Souza et al. (1992) and Moosbruger et al. (1993) (0.5) and Speece (1996) (1.2). 3.2. VFA variations in leachate samples produced from the control, 3 g l 1 and 6 g l 1 NaHCO3 alkalinity supplemented reactors The initial VFA concentrations in leachate samples were approximately 10 000 mg l 1 in all reactors (Fig. 1b). A strong linear correlation between COD and VFA concentrations was obtained in all reactors (r = 0.96, 086, and 0.85 in control and reactors containing 3 g l 1 and 6 g l 1 NaHCO3). The VFA values of the reactors containing 3 g l 1 and 6 g l 1 NaHCO3 increased to 30 000 and 23 000 mg l 1 on d 7. However, they were down to about 1400 and 1300 mg l 1 by d 65 (Appendix A). The VFA measurements showed that organic solid wastes in control reactor (no alkalinity addition) degraded slowly. Adequate alkalinity, or buffer capacity, is necessary to maintain a stable pH in the digester for optimal biological activity (Cobb and Hill, 1990). An alkalinity varying between 1000 and 5000 mg CaCO3 l 1, was recommended for anaerobic treatment depending on COD and VFA produced (Tchobanoglous and Burton, 1979). Traditionally, the total alkalinity in an anaerobic digester includes all the bicarbonate alkalinity and approximately 80% of the VFA (Anderson and Yang, 1992). When the system is in balance, the methanogens could be inactivated by unfavorable environmental conditions, e.g., pH drop, accumulation of VFA, intermetabolites and toxicity of aromatic amines due to their toxic properties (Kuai et al., 1998).
Behling et al. (1997) reported that, if an UASB reactor is stable, the TVFA/B.Alk. ratio should be between 0.4 and 0.8. In our study, TVFA/B.Alk. ratios were 3.00, 1.20, 1.00, 0.70 and 0.50 on d 5, 28, 31, 65 and 100 in control reactor. TVFA/B.Alk. ratios were 2.00, 0.70, 0.30, 0.40, 0.40 and 0.50 on d 5, 28, 31, 52, 65 and 100 in reactor containing 3 g l 1 NaHCO3 while the same ratios were 1.50, 1.10, 0.30, 0.40, 0.60 and 0.50 in reactor containing 6 g l 1 NaHCO3 on the aforementioned d. The low TVFA/B.Alk. ratios could be attributed to the deficient of B. alkalinity on d 31 and 52 in reactors containing 3 g l 1 and 6 g l 1 NaHCO3. The B. Alk. neutralize the CO2/H2CO3 with only the excess available for neutralizing an increase in VFA. 3.3. pH variations in leachate samples produced from the control, 3 g l 1 and 6 g l 1 NaHCO3 alkalinity supplemented reactors During the first two weeks, the pH of the leachates from the control, and reactors containing 3 g l 1 and 6 g l 1 NaHCO3 were approximately 5.30, respectively, on the acidic side of the pH scale (Fig. 1c). In the control reactor, the pH values decreased to 5 within 7 d. This caused an the extension of the time required for the stabilization of organic fraction of the waste to stabilize since methane forming anaerobes are known to be very sensitive to low pH (Bolzonella et al., 2003). The methane gas productions were 3900, 5900 and 6900 ml d 1 on d 5 (see Fig. 2a–c). The methane gas productions at a pH as low as 5 could be attributed to hydrogenotrrophic methanogens present in OSWs (Paulo et al., 2003). The so-called ‘‘acid habituation’’ or the ‘‘adaptive acid tolerance response’’ phenomenon, described by Hall et al. (1995) might explain the resistance of anaerobic microorganisms in OSWs. Lens et al. (2003) found that the lack of bicarbonate delayed the conversion of OSWs to methane resulting in low methane productions and percentages in control reactor. The other reason for methane production at pH 5 in control reactor could be attributed to ammonium bicarbonate alkalinity which in turn maintained a pH close to neutral inside cells (Speece, 1996). This called ‘‘metabolism generated alkalinity’’ inside cells. The degradation of cation releasing nitrogenous organics (proteins) would double the alkalinity concentration generated during biodegradation of proteins in OSWs (Speece, 1996). On the other hand, VFA alkalinity contributes to the buffering of H2CO3, but is transient since the VFA varies and therefore cannot be consistently relied upon. The pH level of the control reactor was 6.54 on d 65 (Appendix A). The conversion of fatty acids caused an increase in the pH levels within the alkalinity supplemented reactors. In this study the measured pH levels was found to be higher than the data obtained by Plaza et al. (1996) which used CaCO3 for alkalinity.
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879 70
16000
50
12000 10000
40
8000
30
6000
20
4000
(a)
2000 0
70 60
20000
50 15000
40
10000
30 20
5000
Methane percent (%)
Cumulative methane (ml)
10 0
25000
10
(b)
0
Cumulative methane (ml)
Methane percent (%)
60
14000
30000
70
25000
60 50
20000
40 15000 30 10000
20
5000
(c)
0 0
20
40
60 80 Operation time (d)
100
nitrogenous organic substances could be attributed to ammonium bicarbonate alkalinity, which in turn maintained a pH close to neutral in the anaerobic solid waste bioreactor (Speece, 1996; Jokela et al., 2002b). NH4-N loss through stripping was considered to be negligible because the pH in the reactors containing 3 and 6 g l 1 NaHCO3 was below 8 (Diamadopoulos, 1996; Yılmaz ¨ ztu¨rk, 2001). and O Since the NH4-N concentration in the leachate samples is high, the generated ammonium bicarbonate alkalinity contributes to total alkalinity in effluent samples as reported by Dinamarca et al. (2003). On the other hand, since the reserve alkalinity is the bicarbonate alkalinity maintaining pH above 6.2–6.6 in the anaerobic simulated reactor; it can be assumed that this alkalinity was provided in reactors containing 3 and 6 g l 1 NaHCO3 since the lower pH was recorded as 6.4 apart from the initial pHs.
0
Methane percent (%)
Cumulative methane (ml)
18000
875
10 0 120
Fig. 2. Cumulative methane gas productions (—d—) and methane percentage (—h—) of control reactor (a), 3 g l 1 (b) and 6 g l 1 (c) NaHCO3 containing reactors.
3.4. NH4-N variations in leachate produced from the control and alkalinity supplemented simulated landfill reactors The highest NH4-N concentrations were measured to be 770, 760 and 660 mg l 1 in control, and reactors containing 3 g l 1 and 6 g l 1 NaHCO3 on d 20, 30 and 25 through the mineralization of organic nitrogenous compounds (Fig. 1d). The NH4-N concentrations were 750, 720 and 650 mg l 1 in the aforementioned reactors after 65 d of operation period (Appendix A). In this study, it was observed that there were no significant differences in NH4-N concentrations among the control, and reactors containing 3 g l 1 and 6 g l 1 NaHCO3. The low NH4-N removals between d 20–30 and 65 could be attributed to the utilization of NH4-N through assimilation of anaer¨ ztu¨rk, 2001). As reported by obic bacteria (Yılmaz and O Jokela et al. (2002a) a small amount of ammonium nitrogen released through degradation of proteinic and nitrogenous compounds may be incorporated into cell biomass. The decreases of NH4-N released from the
3.5. Methane gas productions and methane percentages in simulated anaerobic solid waste bioreactors The maximum cumulative methane gas production was recorded as 12 l, 19 l and 23 l in control, and reactors containing 3 g l 1 and 6 g l 1 NaHCO3, respectively at the end of 65 d, while the maximum cumulative methane gas productions were measured at 16 l, 20 l and 24 l after 100 d of operation period (Fig. 2a–c). In order to compare the stability of the control reactor the methane gas productions and methane percentages were given for 100 d. The reason for the high cumulative methane in 6 g l 1 reactor is the fast degradation of municipal organic solid wastes in this reactor through rapid methanogenesis on d 65. The methane gas percentages were 58%, 60% and 61% on d 65 while the methane percentages were measured to be 30% after 100 d of anaerobic incubation in control, and reactors containing 3 g l 1 and 6 g l 1 NaHCO3. It was observed that the anaerobic degradation terminated since the methane gas productions decreased significantly by d 65 in the reactors containing 3 g l 1 and 6 g l 1 NaHCO3. The biodegradation rate was higher in reactors containing 3 and 6 g l 1 NaHCO3. It is important to note that the increases percentages of cumulative methane gas productions was lower in reactors containing 3 and 6 g l 1 NaHCO3 (E = 5.00% and 6.20%, respectively) while the increase in the percentages of cumulative methane gas productions were higher in control reactor (E = 36%) between d 65 and 100. This shows that the ultimate mineralization of OSW in the control reactor was carried out between d 65 and 100. In this study, the methane percentages measured in alkalinity added reactors were higher than the study realized by San and Onay (2001). Methane gas productions and methane percentages in simulated anaerobic solid waste bioreactors show that the alkalinity addition has a positive effect on
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879
876
Table 2 Comparison of characteristics of the simulated landfilling bioreactors Initial
Water content (%) Organic matter (%) (in DS) % C (in DS) TN (mg g 1) (in waste) TP (mg g 1) (in waste) NH4-N (mg g 1) (in waste) Waste quantity (g)
Final 1
1
Control
3 g l NaHCO3 added R.
6 g l NaHCO3 added R.
Control
3 g l 1 NaHCO3 added R.
6 g l 1 NaHCO3 added R.
90 97 54 4350 1050 155 1000
90 97 54 4350 1050 155 1000
90 97 54 4350 1050 155 1000
91 65 36 610 53 38 213
92 58 32 220 46 36 140
92 51 28 213 43 35 120
biodegradation of municipal solid waste in simulated bioreactors due to high methane gas productions and high methane percentages. 3.6. BOD5 concentrations and BOD5/COD ratios in the leachate samples from the simulated anaerobic solid waste bioreactors In order to study in greater detail the proportion of biodegradable organic carbon in the leachate it was decided to determine the BOD5/COD ratios. The BOD5 values decreased substantially by 69%, 95% and 96% in the aforementioned reactors, respectively (data not shown). The experimental results indicate that a higher MSW stabilization rate was achieved in the reactor containing 6 g l 1 NaHCO3 than that of control and 3 g l 1 NaHCO3 containing reactors. Initially, all the reactors had high BOD5/COD ratios such as 0.9, indicating the high degradability of leachate (Quasim and Chiang, 1994) (data not shown). On d 52, BOD5/COD ratio decreased to approximately 0.25 from 0.78 in the reactor containing 6 g l 1 NaHCO3. This ratio implies a low biodegradable leachate. As the biodegradation of organic content of OSW occurred, the BOD5/COD ratio decreased. Low BOD5/COD ratio in 6 g l 1 alkalinity added reactor shows that MSW converted rapidly to methane via methanogenesis. If the BOD5/COD ratio was between 0.02 and 0.13, this implies that the leachate has a low biodegradability through anaerobic phase. When this ratio is between 0.4 and 0.8, this implies the high biodegradability of the leachate (Otieno, 1994). The BOD5/COD ratio measured in the study shows similar data to the findings of Ledakowicz and Kaczarek (2002). 3.7. Comparison of characteristics of the simulated anaerobic landfilling reactors The initial organic matter content of OSWs were 97% in all reactors while the organic matter ratios of OSWs were 65%, 58%, 51% organic in control, and reactors
containing 3 g l 1 and 6 g l 1 NaHCO3, respectively, at the end of anaerobic incubation (Table 2). The reductions in the organic matter were 33%, 40% and 47%, in control, reactors containing 3 g l 1 and 6 g l 1 NaHCO3, respectively, through 65 d of operation period. The results showed that more organic material is transferred into the gaseous phase in the reactor containing 6 g l 1 NaHCO3 under methanogenic conditions. The lack of NaHCO3 dropped the methane produced and the conversion of organic matter in OSWs. If the acid concentration (H2CO3 and VFA) exceeds the available alkalinity, this severely inhibits the microbial activity, especially the methanogens in OSWs. The reduction in C content of OSW in the reactor containing 6 g l 1 NaHCO3 was higher than the other reactors. The alkaline conditions considerably increased the carbon conversion rates in OSWs. On d 65, the gradual decline of the carbon content of OSWs in the reactor containing 6 g l 1 NaHCO3 can most probably be attributed to the absence of un-ionized acids since the buffer capacity of the reactor is high. In the absence of NaHCO3 the total carbon content in OSWs was not successfully used by the methanogens through the low conversion rate of organic substances compared to alkalinity containing anaerobic simulated reactors. It was observed that the reduction of waste quantity was high in the reactor containing 6 g l 1 NaHCO3. Seventy eight percent, 99% and 99.9% reductions in OSWs quantity were observed in control, and reactors containing 3 g l 1 and 6 g l 1 NaHCO3. This parameter is very important since it would optimize the land utilization. A depletion of high volume in OSWs would reduce the landfilling sites and could be used for the management of anaerobic degradation of OSWs in simulated bioreactors. TN removal efficiencies of OSWs were 86%, 95% and 96% in control, and reactors 3 g l 1 and 6 g l 1 NaHCO3 while TP removal efficiencies were 95%, 96% and 96% in the same reactors, respectively. The proteins and organic substances in OSW degraded to NH4-N and were released to the leachate. The organic phosphorus and phosphate in OSW were used by the acetogenic
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879
and methanogenic bacteria as nutrient and energy source through anaerobic metabolism. 3.8. COD mass balance and degree of stabilization of solid wastes As the waste stabilization is directly related to the amount of methane produced and methane percentage of the total gas, the amount of methane generated per kg of organic matter stabilized is taken to be an indicator of waste stabilization degree. About 12, 19 and 23 l of methane was regenerated from the control, and reactors with added 3 g l 1 NaHCO3 and 6 g l 1 NaHCO3, respectively, on d 65. These results are comparable high with the study performed by San and Onay (2001) and Ledakowicz and Kaczarek (2002). In order to determine the stabilization degree, the maximum mass of COD released was determined by taking into consideration the maximum released COD and the available moisture in all the reactors for d 57. It was found that the maximum mass of COD released from the OMS to the leachate was equal to 40, 30 and 23 g in the control, and in the reactors added 3 g l 1 NaHCO3 and 6 g l 1 NaHCO3, respectively. Twenty eight percent of COD removed in OSW from the control reactor was converted into methane while 44% and 65% of COD removed in OSW was converted to methane in the reactors added 3 g l 1 NaHCO3 and 6 g l 1 NaHCO3, respectively, on d 57. In our study the COD amount converted to into methane was comparably higher in the reactor with added 6 g l 1 NaHCO3 compared to the study performed by San and Onay (2001). As the alkalinity supplemented reactors have a higher capacity for extracting the COD from the OSW compared to control reactor, addition of alkalinity increased the specific methane yield to about 0.19 and 0.23 m3 kg 1 TSadded waste in 3 g l 1 NaHCO3 and 6 g l 1 NaHCO3 added reactors, respectively, whereas the methane yield in the control reactor was 0.13 m3 kg 1 TS on d 65. The specific methane yield in the control reactor was low since the biodegradation of OSWs and therefore the methane gas production were not completed by d 65. The specific methane yield increased to 0.19 m3 kg 1 TS in the control reactor on d 100 indicating the anaerobic degradation of remaining organic substances through methanogenesis. The specific methane yields were 0.36 and 0.50 m3 kg 1 TS in reactors containing 3 g l 1 NaHCO3 and 6 g l 1 NaHCO3 depending to CH4-COD remaining and TS in simulated anaerobic reactors after 100 d. These results are significantly higher than the studies performed by Chugh et al. (1998) and Mata-Alvarez et al. (2000). The methane production rates were 0.033 l d 1, 0.102 l d 1 and 0.155 l d 1 in control, and the reactors with added 3 and 6 g l 1 NaHCO3 on d 5 while the same parameter were 0.118 l d 1, 0.290 l d 1 and 0.349 l d 1
877
in the aforementioned reactors on d 65. Maximum methane production rates were reached on d 75 and 35 in control and the other two reactors. On d 100 the methane gas production rates decreased to 0.199 l d 1 and 0.238 l d 1 in the reactors with added 3 and 6 g l 1 NaHCO3, respectively.
4. Conclusions The alkalinity addition to OSWs has a positive effect on the rate of biological degradation in anaerobic simulated recycled reactors through the degradation of organic fraction of solid wastes collected from the kitchen of the Engineering Faculty in Dokuz Eylu¨l _ University Campus, Izmir, Turkey. The findings of this study showed that the anaerobic digestion of OSW in a simulated landfill reactor correlate with bicarbonate alkalinity. The alkalinity is lost through NH4-N conversion of organic nitrogen, the buffering of H2CO3 acidity by CO2 and VFA generation by the acidogen microorganisms. VFA build up with VFA generated, increased the acidity and reduced the alkalinity. The VFA converted into methane by methanogens particularly in the lower and upper zone of simulated anaerobic OSW reactor and in the whole reactor since the leachate was recirculated from the bottom to the top of the reactor. In other words, the leachate was partially accumulated in the lower part and the upper parts of OSW reactor, resulting in VFA conversion to methane which the medium part of the reactor mostly exhibits only under methanogenic conditions. The alkalinity requirement is 1.4 g NaHCO3 alkalinity/g influent COD providing 87% COD, 87% VFA removals with a methane gas production of about 100 ml d 1 and methane percentage of 59 at a pH 7.31 on d 65 through degradation of OSW in a simulated landfill anaerobic reactor. The observed COD removal efficiencies in reactors containing 3 and 6 g l 1 of NaHCO3 could be due to no-VFA accumulation resulting in activation of methanogenic bacteria at pH around 7.0. The acidogenic phase would generate acids, but this was buffered by sufficient alkalinity in reactors containing alkalinity. When 3 and 6 g l 1 NaHCO3 was added to the feed this alkalinity was not completely consumed. Utilization of anaerobic simulated bioreactor with an alkalinity enhances the feasibility of degradation of OSWs through methanogenesis. As a result, more organic material is transferred into the gaseous phase. Anaerobic simulated bioreactors reduce the landfilling sites since a reduction in the volume of the leachate as well as solid volume was obtained in OSWs. Because degradation of OSWs and leachate produced through the degradation of organic substances is a most important subject in terms of pollution emissions,
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879
878
besides the cost-effective anaerobic treatment process, the additional alkalinity requirement should be kept in mind in terms of financial costs.
Acknowledgments This study was executed as a part of the research activities of the Environmental Microbiology Laboratory of Environmental Engineering Department and the project was partially funded by the Dokuz Eylu¨l University Research Foundation. The authors would like to thank this body for the financial support to projects with grant numbers Fen 021, 051 and 03.KB.Fen.017.
Appendix A 1
NaHCO3 6 g l
1
Days Control
3gl
COD 1 3 7 10 14 17 21 24 28 31 42 52 65
16 004 18 936 36 618 35 712 38 102 39 304 40 658 38 432 35 200 32 744 26 725 20 214 18 964
16 121 17 009 30 353 27 549 26 065 22 405 20 624 14 276 7680 4211 3960 3920 3865
16 200 16 768 22 850 20 459 16 749 13 269 10 648 6984 5250 3845 3152 3057 2908
VFA 1 3 7 10 14 17 21 24 28 31 42 52 65
10 052 9296 17 152 19 432 20 320 21 080 19 304 18 432 17 936 14 208 11 147 9967 6913
10 169 8064 12 100 11 500 11 376 7465 4947 3623 2305 1385 1559 1500 1413
10 103 7344 10 000 8345 5994 5050 4046 2271 1693 1374 1335 1300 1285
pH 1 3 7 10 14 17
5.32 5.33 4.98 5 5.13 5.14
5.16 5.2 5 5.12 5.41 6.34
NaHCO3
5.3 5.35 5.11 5.35 6.74 6.9
Appendix A (continued) Days 21 24 28 31 42 52 65
Control 5.16 5.13 5.26 5.34 5.85 6.04 6.54
Ammonium 1 388 3 416 7 584 10 604 14 736 17 750 21 764 24 704 28 713 31 740 42 752 52 763 65 750
3gl
1
6.98 7.07 7.07 7.07 6.94 7.04 7.19 370 376 524 612 652 664 672 732 750 760 762 750 713
NaHCO3
6gl
1
NaHCO3
7.02 7.13 7.08 7.08 6.99 7.19 7.31 406 464 528 593 608 603 600 636 604 548 602 650 650
References American Public Association, 1992. American Water Works Association, and Water Environment Federation. Standard Methods for the Examination of Water and Wastewater, 17th ed., Washington, DC. Anderson, G., Yang, G., 1992. Determination of bicarbonate and total volatile acid concentration in anaerobic digestion using a simple titration. Wat. Environ. Res. 64, 53–59. Behling, E., Diaz, A., Colina, G., Herrera, M., Gutierrez, E., Chacin, E., Fernandez, E., Forster, C.F., 1997. Domestic wastewater using a simple titration. Wat. Environ. Res. 64, 53–59. Beydilli, M.I., Pavlosathis, S.G., Tinvher, W.C., 1998. Decolorization and toxicity screening of selected reactive azo dyes under methanogenic conditions. Wat. Sci. Tech. 38, 225– 232. Bolzonella, D., Battistoni, P., Mata-Alvarez, J., Cecchi, F., 2003. Anaerobic digestion of solid wastes: process behavior in transient conditions. Wat. Sci. Tech. 48, 1–8. Chugh, S., Clarke, W., Pulammanappallil, P., Rudolph, V., 1998. Effect of recirculated leachate volume on MSW degradation. Waste Manage. Res. 16, 564–573. Cobb, S.A., Hill, D.T., 1990. Volatile fatty acid interference with alkalinity measurements in anaerobic fermenters. International Summer Meeting, sponsored by the ASAE. Diamadopoulos, E., 1996. Characterization and treatment of recirculation-stabilized leachate. Wat. Res. 28, 2439–2445. Dinamarca, S., Aroca, G., Chamy, R., Guerrero, L., 2003. The influence of pH in the hydrolytic stage of anaerobic digestion of the organic fraction of urban solid waste. Wat. Sci. Tech. 48, 249–254.
O.N. Ag˘dag˘, D.T. Sponza / Chemosphere 59 (2005) 871–879 Gonzales, J.S., Rivera, A., Borja, R., Sanchez, E., 1998. Influence of organic volumetric loading rate, nutrient balance and alkalinity/COD ratio on the anaerobic sludge granulation of an UASB reactor treating sugar cane molasses. Int. Biodet. Biodeg. 41, 127–131. Hall, H.K., Karem, K.L., Foster, J.W., 1995. Molecular responses of microbes to environmental pH stress. Adv. Microb. Physiol. 37, 229–272. Hamzawi, N., Kennedy, K.J., McLean, D.D., 1998. Anaerobic digestion of co-mingled municipal solid waste and sewage sludge. Wat. Sci. Tech. 38, 127–132. Jokela, J.P.Y., Kettunen, R.H., Rintale, J.A., 2002a. Methane and leachate pollutant emission potential from various fraction of municipal solid wastes. Effects of source separation and aerobic treatment. Waste Manage. Res. 20, 424–433. Jokela, J.P.Y., Kettunen, R.H., Sormunen, K.M., 2002b. Biological nitrogen removal from municipal landfill leachate: low cost nitrification in biofilters and laboratory scale in situ denitrification. Wat. Res. 36, 4079–4087. Kocasoy, G., 1994. Analyses methods in solid waste, compost _ and industrial sludge samples, Bebek, Istanbul, Turkey. Kuai, L., De Vreese, L., Vandevivere, P., 1998. GAC-amended UASB reactor for the stable treatment of toxic textile wastewater. Environ. Technol. 19, 1111–1117. Ledakowicz, S., Kaczarek, K., 2002. Laboratory simulation of anaerobic digestion of municipal solid waste. Appropriate Environmental and Solid Waste Management and Technologies for Developing Countries, Turkey, pp. 1139–1146. Lens, P.N.L., Klijn, R., van Lier, J.B., Lettinga, G., 2003. Effect of specific gas loading rate on thermophilic acidifying and sulfate reducing granular sludge reactors. Wat. Res. 37, 1033–1047. Mata-Alvarez, J., Mace, S., Llabres, P., 2000. Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Biores. Technol. 74, 3–16. Moosbruger, R.E., Wentzel, M.C., Ekama, G.A., Marais, G., 1993. Lauterntun waste in UASB systems, feasibility, alkalinity requirements and pH control. Wat. SA 19, 41–52. Otieno, F.A.O., 1994. Stabilization of solid waste through leachate recycling. Waste Manage. Res. 12, 93–100.
879
Paulo, P.L., Villa, G., van Lier, B., Lettinga, G., 2003. The anaerobic conversion of methanol under thermophilic conditions: pH and bicarbonate dependence. J. Biosci. Bioeng. 96, 213–218. Plaza, G., Robredo, P., Pacheco, O., Toledo, A.S., 1996. Anaerobic treatment of municipal solid waste. Wat. Sci. Tech. 33, 169–175. Quasim, S.R., Chiang, W., 1994. Sanitary Landfill Leachate Generation, Control and Treatment. Technomic Publishing Company, USA. Razo-Flores, E., Luijton, M., Donlon, B.A., Lettinga, G., Field, J.A., 1997. Biodegradation of selected azo dye under methanogenic conditions. Wat. Sci. Tech. 36, 65–72. Reinhart, D.R., Al-Yousfi, A.B., 1996. The impact of leachate recirculation on municipal solid waste landfill operating characteristics. Waste Manage. Res. 20, 337–346. Reinhart, D.R., McCreanor, P.T., Townsend, T., 2002. The bioreactor landfill: its status and future. Waste Manage. Res. 20, 172–186. San, I., Onay, T.T., 2001. Impact of various recirculation regimes on municipal solid waste degradation. J. Hazard. Mater. B 87, 259–271. Speece, R.E., 1996. Anaerobic Biotechnology for Industrial Wastewaters. Archae Press, Nashville Tennessee, USA. Sorgun, K., 1987. Solid waste management. Turkish General Directorate of Education and Production Centre, Ministry of Industry and Commerce, Mersin, Turkey. Souza, M.E., Fuzaro, G., Polegato, A.R., 1992. Thermophilic anaerobic digestion of vinasse in pilot plant UASB reactor. Wat. Sci. Tech. 25, 213–222. Tchobanoglous, G., Burton, F.L., 1979. Wastewater Engineering. McGraw-Hill International Editions. USEPA, 1986. Extraction procedure, toxicity test method and structural integrity tests. Method 1310 B, Environmental Protection Agency, Washington, USA. Warith, M., 2002. Bioreactor landfill: experimental and field results. Waste Manage. 22, 7–17. _ 2001. Biological ammonia removal from ¨ ztu¨rk, I, Yılmaz, G., O anaerobically pre-treated landfill leachate in sequencing batch reactors. Wat. Sci. Tech. 43, 307–314.