Anaerobic co-digestion of algal sludge and waste paper to produce methane

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1 Bioresource Technology 98 (2007) Anaerobic co-digestion of algal sludge and waste paper to produce methane Hong-Wei Yen a,, David E. Brune b a Department of Chemical Engineering, Tunghai University, Taiwan, ROC b Department of Biosystems Engineering, Clemson University, SC, USA Received 6 June 2005; received in revised form 11 November 2005; accepted 12 November 2005 Available online 4 January 2006 Abstract The unbalanced nutrients of algal sludge (low C/N ratio) were regarded as an important limitation factor to anaerobic digestion process. Adding high carbon content of waste paper in algal sludge feedstock to have a balanced C/N ratio was undertaken in this study. The results showed adding 50% (based on volatile solid) of waste paper in algal sludge feedstock increased the methane production rate to ml/l day, as compared to ml/l day of algal sludge digestion alone, both operated at 4 g VS/l day, 35 C and 10 days HRT. The maximum methane production rate of ml/l day was observed at a combined 5 g VS/l day loading rate with 60% (VS based) of paper adding in algal sludge feedstock. Results suggested an optimum C/N ratio for co-digestion of algal sludge and waste paper was in the range of 20 25/ Elsevier Ltd. All rights reserved. Keywords: Anaerobic digestion; Co-digestion; C/N; Methane; Algal sludge 1. Introduction The recently patented Partitioned Aquaculture System (PAS) has been proven to be able to increase aquaculture production capacity by improving pond waste nutrient treatment (Brune et al., 2001). In the PAS, algae play a dual role, as both an oxygen supplier to the Wsh and as a waste nutrient cleaner. The capacity of Wsh production in the PAS has been demonstrated to be four times greater than that of conventional aquaculture and also at the same time the wastewater discharges are signiwcantly decreased. However concentrated algal sludge production from the PAS must be continually removed from the ponds to control algal density and water column respiration. The large amount of algal sludge represents a potential source of fuel and recovered N and P fertilizer (Mulbry et al., 2005). Anaerobic digestion of algal sludge produced from the PAS can not * Corresponding author. Tel.: x209; fax: address: hwyen@thu.edu.tw (H.-W. Yen). only decrease the amount of waste to be handled, but also yields methane to ovset the energy needs of the PAS achieving a sustainable aquaculture production system. The solar energy stored in the algal biomass in results of the photosynthesis reaction could be released as methane through the anaerobic digestion. This concept was originally proposed over 40 years ago in a paper by Oswald and Golueke (1960), describing integrated processes of largescale raceway pond cultivation of microalgae and wastewater treatment, followed by fermentation of algal biomass to methane fuel. Due to the recalcitrant of algal sludge to biodegradation, Chen and Oswald (1998) found the heat pretreatment of algal sludge at 100 C for 8 h could improve the eyciency of methane fermentation a maximum at 33%. However, the improvement on the methane energy produced would not be economically competitive to the energy lost on the heat pretreatment. Except for the resistant to biodegradation, the low C/N ratio of algal sludge is also a serious problem to the anaerobic digestion. Although, an optimum C/N range in feedstock for the anaerobic digestion is still debatable in the /$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi: /j.biortech

2 H.-W. Yen, D.E. Brune / Bioresource Technology 98 (2007) literature, 20/1 30/1 is a most acceptable range (Parkin and Owen, 1986). The C/N ratio in algal sludge is about 6/1, which is too low for the digestion. Low C/N ratio feedstock could result in high total ammonia nitrogen (TAN) released and high volatile fatty acids (VFAs) accumulated in the digester. The TAN and VFAs both are important intermediates and potential inhibitors in the anaerobic digestion process (Parkin and Owen, 1986). High concentration of TAN and VFAs in the digester would decrease the methanogen activity and further accumulation could fail the anaerobic digestion. One method to avoid excessive ammonia accumulation is to adjust low feedstock C/N ratios by adding high carbon content materials, thereby improving the digestion performance. This practice has been used for co-digestion of sewage sludge and municipal solid waste (MSW) (Sosnowaki et al., 2003). Most MSW consists of paper material (including oyce and newspaper), which has a C/N ratio ranging from 173/1 to greater than 1000/1 while typical sewage sludge has a C/N ratio ranging from 6/1 to 16/1 (Stroot et al., 2001). Co-digestion of cattle manure slurry with fruit, vegetable wastes and chicken manures is another example of successful blending of high C/N and low C/N feedstocks to improve digester performance (Callaghan et al., 2002). Co-digestion of sisal pulp and Wsh wastes had shown a 59 94% increased in the methane production yield as compared to sisal pulp and Wsh wastes digestion alone (Mshandete et al., 2004). The bene- Wts of co-digestion include: dilution of potentially toxic ammonia, allowing for increased loading rate and improved biogas yield (Sosnowaki et al., 2003). The purpose of this work was to assess the possibility of co-digestion of algal sludge and high carbon content of waste paper at diverent fraction to produce methane and evaluate the waste paper adding evects on the methane production. 2. Methods 2.1. Anaerobic digestion experiments Bench-top anaerobic digesters with gas storage tanks were fabricated from 25 mm Plexiglas sheet and contained with a temperature-controlled water bath at 35 1 C. Digesters were connected to gas storage tanks and gas sampling ports using silicone tube. The digester and gas storage tank measured cm and cm, respectively (working volume, 4 l). Semi-continuously feeding type was adopted and digesters were fed once per day following the removal of the same volume of ezuent. Hand mixing of the digester was performed with a permanently installed mixing rod prior to ezuent removal and immediately after loading. The digestion was operated at 10 days HRT. The ph was 6.5 and was controlled (based on the sample s ph) by adding 5 N sodium hydroxide solution, if necessary. The daily biogas production was recorded by measurement of water displacement. Daily samples were stored at 4 C for analysis. The digesters were operated for 20 days and then assumed to be in steady-state (variation in daily biogas production was within 10% of average production). Furthermore, a NR ratio (D TKN in /TKN out ) between 0.95 and 1.05 was used as a measure of steady-state conditions (Cobbs and Hill, 1990) Characteristics of substrates Algal sludge and waste paper were used as feeding substrates in this investigation. Algal sludge was harvested from Partition Aquaculture System, Clemson University. The species of algae in the sludge would change by seasons, but most of the species would be Scenedesmus spp. and Chlorella spp. Waste paper was collected from recycle bins at the Clemson Computing and Information Technology labs (DCIT). This paper was used in laser printer with one side printed mostly and was cut by shredder or scissor into cm pieces before mixing with algal sludge for feeding Analytical methods Biogas was collected from the digester gas sampling ports with gas sampler tube. The tubes were allowed to sample for s before shutting the outlet valve to insure that the sample was representative of digester gas. The biogas composition was determined using a gas chromatograph (SRI-8610C, SRI instruments) with a thermal conductivity detector (TCD). A 0.5 ml of gas sample was injected into the chromatograph with a column temperature of 45 C, using helium as the carrier gas. The sample gas concentration was compared to a standard gas mixture consisting of 40% methane and 60% carbon dioxide, and 60% methane and 40% carbon dioxide. Glucose-release rate from conversion of carbomethycellulose (CMC) by cellulase during a predetermined time period was used as measurement method of cellulase activity. Cellulase activity is expressed as mg of glucose-released per ml of sample per minute at 38 C. Soluble total ammonia nitrogen (TAN) was measured using an ammonia sensor (ORION-720A). Volatile fatty acids (VFAs) and total kjeldahl nitrogen (TKN) were determined following standard methods (APHA, 1995). 3. Results and discussion Total carbon and kjeldahl nitrogen was 10,500 mg/l and 2000 mg/l, and 406 mg/g and 182 mg/kg in algal sludge and waste paper, respectively. The C/N ratios were about 5.3/1 in algal sludge and 2000/1 in waste paper Algal sludge digestion Digesters fed with algal sludge alone at 2, 4 and 6 g VS/ l day loading rate were undertaken to investigate the digestibility of algal sludge and following methane production rate. As seen in Table 1, methane production rates

3 132 H.-W. Yen, D.E. Brune / Bioresource Technology 98 (2007) Table 1 Methane production rates, VFAs, and TAN ( SD) in algal sludge digestion operated at diverent loading rates and 10 days HRT Loading rate (g VS/l day) CH 4 (ml/l day) CO 2 (ml/l day) VFAs (mg/l) TAN (mg/l) increased in proportion to increases in loading rate. The methane production rate was ml/l day at 2 g VS/ l day loading rate, and increased to a maximum methane production rate of ml/l day at 6 g VS/l day loading rate. Both VFAs and TAN concentrations increased with increasing loading rate to mg/l and mg/l at 6 g VS/l day loading rate. It was expected that the VFAs and TAN concentration eventually would achieve the toxicity concentration, if the loading rate kept increasing. Methane production yields were 90, 143 and 136 CH 4 ml/g VS introduced at loading rates of 2, 4 and 6 g VS/ l day, respectively. Methane production yields of CH 4 ml/g VS introduced at 10 days HRT acquired in this investigation were lower than the yield of 260 ml methane/g VS introduced at 30 days HRT acquired by Oswald and Golueke (1960). Apparently, a longer HRT would be detrimental to have a higher methane production yield. However, HRT increase would decrease the loading rate, and the lower loading rate would have the less methane production rate, as seen in Table 1. The reactor capital cost was considered as the major investment in the anaerobic digestion (Rivard, 1993). Therefore, introducing shorter HRT and higher loading rate to minimize the reactor volume required could be a good choice for having a economical competitive digestion process Co-digestion of algal sludge and waste paper at a loading rate of 4 g VS/l day The results of co-digestion of algal sludge and waste paper at diverent blending fractions at 4 g VS/l day loading rate were shown in Table 2. Five ml of trace elements per day and 350 mg-n as ammonium chloride were added each day in the digester of waste paper to prevent from the nutrients limitation. The waste paper fractions in the co-digestion with algal sludge were on the volatile solid basis. Co-digestion of algal sludge and paper blended at 4 g VS/l day loading rate, with paper addition at 50% of VS, led to signiwcant increases in the methane production rate to ml/l day; or two-fold higher than that observed in algal sludge digestion alone. According to the observed methane production rates in Table 2, neither algal sludge alone nor waste paper alone was an optimum substrate for anaerobic digestion. Digester TAN levels decreased with the increased feedstock C/N ratio to the minimum of mg/l at 75% of waste paper fraction. Digesters fed with 50% algal sludge and 50% paper yielded relatively low VFA levels of mg/l compared to mg/l during algal sludge digestion alone. With the increase of C/N ratio to 36.4/1 at a 75% paper and 25% algal sludge, the digester performance turned to be unstable. Possible reasons for this observed instability could be (1) toxicity of high VFAs at 10, mg/l, and (2) low TAN concentrations of mg/l. A balanced C/N ratio in feedstock was likely to be bene- Wcial to the methanogen activity and resulted in VFAs concentration decreased by more VFAs converted to methane. However, at 75% paper fraction in feedstock, the C/N ratio of 36.4/1 was possibly too high for the anaerobic digestion, which led to low TAN levels of mg/l observed. McCarty (1964) reported that mg/l of TAN was required for growth of anaerobic microorganism. Therefore, low TAN levels of mg/l resulting from high C/N ratio feedstock was likely limiting growth of the microbial population in these digesters. Vinzant et al. (1990) reported that paper digestion under aerobic environment was more eycient than the digestion under anaerobic environment. Also, 12 days of HRT in anaerobic digestion of paper was the minimum required for a stable digester operation (Vinzant et al., 1990). Therefore, it was believed 10 days HRT used in this study was too short for the paper digestion alone. In the batch of paper digestion alone, even if the C/N was Table 2 Methane production rates, VFAs and TAN concentrations ( SD) in co-digestion of algal sludge and waste paper at loading rate 4 g VS/l day (with diverent waste paper fractions) and 10 days HRT Feedstock C/N CH 4 (ml/l day) CO 2 (ml/l day) VFAs (mg/l) TAN (mg/l) Algal sludge (25%) of waste paper + algal sludge (50%) of waste paper + algal sludge (75%) of waste paper + algal sludge a , (100%) of waste paper b a Digestion performance is not at stable situation. b Adding trace element and NH 4 Cl 87.5 mg-n/l day as the nitrogen source.

4 H.-W. Yen, D.E. Brune / Bioresource Technology 98 (2007) adjusted to 21.5/1 by adding ammonic chloride, it still had a low methane production rate of ml/l day. One possible explanation for this was that algal biomass was not only playing a nitrogen source supplier in this codigestion process, but also supplied nutrients to the digester microxora after the degradation of algal biomass. This might be able to explain why the co-digestion of algal sludge and waste paper had a higher methane production rate than the paper digestion alone, even if the C/N ratio of digester fed paper alone was adjusted to 21.5/1 by adding ammonium chloride. Glu. released mg/l m Cellulase activity 3.3. Cellulase activity in the co-digestion of algal sludge and waste paper at 4 g VS/l day Co-digestion of waste paper and algal sludge could eyciently balance feedstock carbon and nitrogen and a balanced C/N ratio of feedstock was likely to benewt the methane production rate. The increase of cellulase activity resulting from paper addition was another possible reason for explaining increased methane production rate. Cellulase are inducible enzymes which are synthesized and mostly secreted into the environment by microorganisms during their growth on cellulosic materials, oligo and dimmer sugars, including some of their derivates (Busto et al., 1996). Cellulose hydrolysis is considered the rate-limiting step in digesters fed with a high cellulosic content feedstock. It was suggested that paper addition to the digester might induce cellulase excretion by bacteria such as Clostridium themocellum (Suto and Tomita, 2001). In fact, in the digesters fed paper and algal sludge blends at 4 g VS/ l day loading rate, cellulase activity increased, resulting in a positive impact on the methane production rate (Fig. 1). At 75% paper fraction, elevated VFA concentrations suppressed cellulase activity to mg/l min. The highest level of cellulase activity ( mg/l min) was during paper digestion alone. However, the methane production rate was only ml/l day as compared to ml/l day for algal sludge digestion alone. These results suggested that breakdown of algal biomass contributed some key components to the improvement of methanogenic activity. Therefore, a low methane production rate was observed in the digester fed paper alone, even if there were no nutrients limitation and had high cellulase activity in the digester (see Table 3) % 50% 75% 100% Paper fraction in feedstock Fig. 1. Cellulase activity in digesters fed algal sludge and waste paper at a combined loading rate of 4 g VS/l day and 10 days HRT Co-digestion of waste paper and a Wxed amount of algal sludge The evects of varying waste paper loading (0, 1, 2, 3 and 4 g VS/l day) blended with a Wxed amount of algal sludge loading at 2 g VS/l day on co-digestion performance were investigated. Digesters fed at a Wxed algal sludge loading rate of 2 g VS/l day and paper loading rates of 0, 1, 2, 3 and 4 g VS/l day yielded a maximum methane production rate ml/l day at a combined loading rate of 5 g VS/ l day (C/N D 22.6/1). This decreased to ml/l day at a combined loading of 6 g VS/l day (C/N D 27.2/1). With the increase of loading rate, the VFAs increased and TAN levels decreased. Since nitrogen source in this co-digestion was came from the breakdown of algal sludge, at a Wxed algal sludge loading (2 g VS/l day), TAN levels decreased with the increased paper loading. That meant the waste paper in the combined feedstock turned to be the main carbon source instead of algal sludge. Results suggested that at a loading rate of 6 g VS/l day, the paper fraction in feedstock achieved 67% (C/N D 27.2) was too high for the anaerobic digestion process, which led to lower methane production rate than 5 g VS/l day loading rate. C/N ratio between 20 and 25/1 was optimum (Fig. 2) for the anaerobic co-digestion of algal sludge and waste paper at 10 days HRT and 35 C. Table 3 Methane and carbon dioxide production rates, VFAs and TAN concentrations ( SD) in digesters at 2 g VS/l day of algal sludge loading and varying loadings of waste paper (0, 1, 2, 3, 4 g VS/l day) and 10 days HRT Combined loading (g VS/l day) C/N CH 4 (ml/l day) CO 2 (ml/l day) VFAs (mg/l) TAN (mg/l)

5 134 H.-W. Yen, D.E. Brune / Bioresource Technology 98 (2007) Methane yield, CH 4 ml/g VS in Fig. 2. Methane yield vs. C/N ratio in digesters with algal sludge and paper loading at combined loading rates of 2, 3, 4, 5 and 6 g VS/l day and 10 days HRT. 4. Conclusions Co-digestion of algal sludge and waste paper was useful and overed two benewts: (1) a balance of C/N ratio. The optimized C/N range for the co-digestion was 20 25/1; (2) increase in cellulase activity. The increase in cellulase activity might be helpful in the biodegradation of algal sludge, which could provide nutrients in the digester, which Wnally would improve methane production rate. References algal sludge + paper paper + NH C/N APHA, Standard methods for the examination of water and wastewater. In: Eaton, A.D., Clesceri, L.S., Greenberg, A.E., (Eds.), Washington, DC. Brune, D.E., Collier, J.A., Schwedler, T.E., Partitioned aquaculture system. US patent Busto, M.D., Ortega, N., Perez-Mateos, M., Location, kinetics and stability of cellulases induced in Trichoderma reesei cultures. Bioresource Technology 57, Callaghan, F.J., Wase, D.A.J., Thayanithy, K., Forster, C.F., Continuous-co-digestion of cattle slurry with fruit and vegetable wastes and chicken manure. Biomass and Bioenergy 27, Chen, P.H., Oswald, W.J., Thermochemical treatment for algal fermentation. Environment International 24, Cobbs, S.A., Hill, D.T., Using nitrogen ratio as an indicator of biomass retention and steady-state in anaerobic fermentation. Transactions of the ASAE 33, McCarty, P.L., Anaerobic waste treatment fundamentals. III. Toxic materials and their control. Public Works 95, Mshandete, A., Kivaisi, A., Rubindamayugi, M., Mattiasson, B., Anaerobic batch co-digestion of sisal pulp and Wsh wastes. Bioresource Technology 95, Mulbry, W., Westhead, E.K., Pizarro, C., Sikora, L., Recycling of manure nutrients: use of algal biomass from dairy manure treatment as a slow release fertilizer. Bioresource Technology 96, Oswald, J.W., Golueke, C.G., Biological transformation of solar energy. Advance Applied Microbiology 2, Parkin, G.F., Owen, W.F., Fundamental of anaerobic-digestion of wastewater sludge. Journal of Environmental Engineering 112, Rivard, C.J., Anaerobic bioconversion of municipal solid waste using a novel high-solids reactor design. Applied Biochemistry and Biotechnology 39/40, Sosnowaki, P., Wieczorek, A., Ledakowicz, S., Anaerobic co-digestion of sewage sludge and organic fraction of municipal solid wastes. Advances in Environmental Research 7, Stroot, P.G., McMahon, K.D., Mackie, R.I., Raskin, L., Anaerobic codigestion of municipal solid waste and biosolids under various mixing condition. I. Digester performance. Water Research 35, Suto, M., Tomita, F., Induction and catabolite repression mechanisms of cellulase in fungi. Journal of Bioscience and Bioengineering 9, Vinzant, T.B., Adney, W.S., Grohmann, K., Rivard, C.J., Aerobic and anaerobic digestion of processed municipal solid waste: evects of retention time on cellulose degradation. Applied Biochemistry and Biotechnology 24/25,