The performance of a laboratory-scale anaerobic acidogenic fermenter fed with a mixture of blended kitchen food-waste and primary sludge from a sewage treatment plant was investigated for the production of volatile fatty acids (VFA). The operating variables for acidogenic fermentation were kitchen food-waste content (10 and 25 wt %), hydraulic retention time (HRT: 1, 3 and 5 days), temperature (ambient: 18 ± 2• C, and mesophilic: 35 ± 2 • C) and pH (varied from 5.2 to 6.7). The experimental results indicated that effluent VFA concentrations and VFA production rates were higher at ambient temperature than at mesophilic conditions. The net amount of VFA with 10 wt % food-waste increased up to 920 mg dm −3 with an increase of HRT, but contrasting results (a decrease of 2610 mg dm −3 ) were found due to the conversion of VFA into biogas in the case of 25 wt % food-waste, which increased significantly at HRT of 3-5 days. In terms of biogas composition (CO 2 and CH 4 ), the organic matter was converted into CO 2 through the oxidative pathway by facultative species at low temperature while mesophilic temperature and optimum pH (6.3-7.8) played a pivotal role in increasing rate of conversion of VFA into biogas by methanogenesis. Rates of VFA production and their conversion are dependent on the food-waste content in the mixture. Yet, the higher concentration of food-waste (25% compared with 10%) did not produce VFA proportionally due to the increased rate of conversion of VFA into gaseous products. The maximum VFA production rate (0.318 g VFA produced g −1 VS fed day −1 ) was achieved in the 10 wt % food-waste at ambient temperature and at a 5-day HRT.

Laboratory scale batch experiments were conducted at 20 degrees C to investigate the acidogenic fermentation for the conversion of wasted sludge into short chain fatty acids (SCFA) to be utilized as a carbon source in the denitrification process. Hydraulic retention time (HRT), volatile solid (VS) loading rate and pH were studied as these are the important parameters governing the production of volatile fatty acids (VFA). Four different phases were investigated by varying these parameters. HRT was varied from 2.7 to 8.2 days whereas VS loading rate was varied from 1.2 to 3.6 g d(-1). VFA production decreased with the increase in HRT above 2.7 days. 538.37+/-19.39 mg VFA(produced) x d(-1) (0.176+/-0.010 mg VFA(produced) mg(-1) VS(feed) was found as the maximum value of VFA at 2.7 days. The present results based on wasted sludge showed that almost 0.0483+/-0.0016 mg VFA (as COD mg(-1) initial COD) and about 5% of soluble COD production were achieved, which are slightly less than the results reported for primary sludges. The rates of VFA production increased with the increase in VS, however, opposite results were obtained when pH was increased in the reactor. SCFA/FA ratios during fermentation were found in the range of 67-73%. The specific denitrification rates (SDNR) of methanol (2.20+/-0.44 mg NO3-N g(-1) MLVSS x h(-1)) and the fermenter supernatant (2.00+/-0.45 mg NO3-N g(-1) MLVSS x h(-1)) were found to be comparable. Fermenter supernatant, therefore, has the potential to be utilized as a carbon source. However, the results need to be investigated further on a larger scale to ascertain their validity.

The performance of a novel high-rate anaerobic process, the anaerobic digestion elutriated phased treatment (ADEPT) process, for treating a slurry-type piggery waste (55 g COD/L and 37 g TS/L) was investigated. The ADEPT process consists of an acid elutriation slurry reactor for hydrolysis and acidification, followed by an upflow anaerobic sludge bed reactor for methanification. This process provides stable and high system performance with short HRT (7.4 d) and better effluent quality (2 g SCOD/L and 0.68 g VSS/L) due to the alkaline pH condition for hydrolysis/acidification phase, high refractory solids removal and ammonia toxicity reduction. The optimum pH and HRT for hydrolysis/acidogenesis of the piggery waste were 9 and 5 days at both 35 degrees C and 55 degrees C conditions. The hydrolysis and acidification rate in the mesophilic reactor were 0.05 d(-1) and 0.11 d(-1), meaning that hydrolysis was a limiting step. SCOD production by the hydrolysis was about 0.26 g SCOD/g VS(fed) (3.6 g SCOD/g VS reduction). Methane production and content in the system were 0.3 L CH4/g VS(fed) (0.67 L CH4/g VS destroyed) and 80%, respectively, corresponding to 0.23 L CH4/g COD removal (@STP).