Biphasic Biomethanation of Hemicellulose Rich Effluent

Abstract

In the dissolving pulp industry, a hemiceJluJose rich effluent (wood-hydrolysate) is the major source of environmental pollution Generally, a pretreatrnent in anaerobic lagoon is given in this effluent which results in the release of methane in the atmosphere. This work deals with the systematic study of the biphasic biomclhanation of wood-hydrolysate effluent to develop a practicable technology for the treatment of this effluent and generation of bioenergy. Wood-hydrolysate was found to contain 81,800, 39,300, 1070, 3492, 23250 mg/1 COD, hemicelluloses, glucose, xylose and Mgnin respectively. Firstly, monophasic reactors viz. upflow anaerobic sludge blanket (UASB), and anaerobic filter (AF) reactors were used to treat this effluent. UASB reactor could attain a maximum organic loading rate of I 1.6 g COD/l/d in 3 I I days with feed concentration of 11,5 g COD/l and hydraulic retention time of 0.94 day. Methane content in the biogas was 64-67% and COD reduction was 76%. For a relatively longer period of study, the AF reactor could sustain an organic loading of 14.7 g COD/l/d with a feed concentration of 14.6 g COD/1 and HRT of 0.97 day; specific methane yield of 4.53 I/I reactor volume/d with 64-66% methane and 82.9% COD reduction was observed. Efficiency of these two monophasic reactors was taken for comparison to show in the ultimate analysis, the advantages of the biphasic system Microbial flora from municipal solid waste for both the stages, acidogenesis and melhanogenesis, were acclimatized with the appropriate substrate. Suitable nutrients and pH (4.5-5.5) environs have been provided to promote the growth of active acidogenic microbial flora in the upflow acidogenic reactor, the simplest one. Two separate experimental runs were conducted for 163 and 331 days to study the acidogenic reactions of wood-hydrolysate. The hydraulic retention times (HRT), inlet COD concentrations and organic loading rates for the two runs were 0.22-0,79 and 0.31-1.16 d, 1.07-44,9 and 10-40.5 g/1, 2.04-114.8 and 9.2-96.7 g COD/l/d respectively, HRT and organic loading rate influence the redaction of hcmicelluloses and ligmu, and the production of acid fermentation products. A limiting HRT of 0.42 day has been found to be the optimum for acidificaton. COD loss through the liberated biogas diminishes sharply with the increase of organic loading rate. Based on the statistical analysis of regression and variance (one way), best operating and performance parameters were selected. 82.6-98.5% hemicelluloses and 43.8-62.5% lignin were reduced and 46.3-48.8% acidification was observed upto an organic loading rate of 96,7 g COD/l/d. Both acetic and propionic acids have been found to have an inverse relationship with the organic loading. Butyric and valeric acid production has the opposite trend, Lignin molecules did not inhibit the acidogenic reactions. A structural modification in the lignin molecule has been found to take place during acidogenesis which also reduces the molecular weight of the macromoiecules. Significant difference has been observed in the acidification behaviour of wood-hydrolysate with xylose and glucose in respect of COD reduction, acidification, methane content in the biogas and acid fermentation product distribution. Methanogenesis of acidified effluent which has been carried out in anaerobic filter reactor, containing cross flow plastic filter media, was very prompt. Faster start-up of the reactor was made possible by using synthetic acid as feed substrate. Within 247 days from the start-up, the reactor was able to sustain an organic loading of 30.1 g COD/l/d with 86% COD reduction and 9.8 1/1 reactor voiume/d specific methane yield. 57.3% of the residual hemicelluloses and 65.$% of residual lignin were further degraded in the methanogenic reactor. Methanogenesis has been found to have a great influence on the alteration of the structure of lignin molecules. From the present investigation, it is concluded that the wood-hydrolysatc, a hemicelluloses rich effluent can be successfully treated by biphasic biomethanation having upflow acidogenic and anaerobic filter methanogenic reactor configuration. Overall hydraulic retention time of 1 24 days is good enough to reduce 88.2% of the feed COD, 92.5% hem [celluloses, 82.3% lignin, and to generate specific methane yield of 6.5 l/l reactor volume/d with 75.8% methane in biogas. The relative volume of acidogenic and methanogenic reactors for wood-hydrolysate substrate has been found to be 1:1.95. Relative requirement of reactor volume for biphasic and monophasic filter reactor is I:1.6.