Determination of the effect of multiple feedstock on biogas production in anaerobic sludge digestion

In the present graduate study, it is aimed to investigate the effect of winter marketplace wastes (fruit (FW) and vegetable (VW) wastes (FVW)) on the process performance and stabilized sludge quality in separate and combined anaerobic sewage sludge digestion system. For this purpose, co-digestion of primary (PS), secondary (SS) and mixed (MS) sludge fractions fed respectively at 1.65, 0.40 and 1.0 kg VS / m3.day volatile loading rates (VLR) was studied at a hydraulic retention time (HRT) of 20 day and 35°C temperature in lab-scale semi-continuous reactors. The grinded FW, VW or FVW were fed in different compositions and proportions as multiple feedstock to PS, SS and MS digestion with parallel control reactor monitoring. The optimum specific methane production (SMP) values obtained in co-digestion were 12 and 5% higher respectively in 0.33 g VS-VW/L.d for PS and MS with 55% surplus in SS digestion at 0.44 g VS-VW/L.d compared to the control reactors. The increase in daily methane production was obtained at 34, 67 and higher than 100% in PS, MS and SS co-digestion with VW, respectively. In FW co-digestion, 5 and 26% increase were obtained in SMP at 0.84 g VS-FW/L.d in PS and SS co-digestion. The effect on daily methane production proceeded at similar levels as 1105±80, 1005±40 and 1050±90 mL/d for PS, SS and MS co-digestion, respectively, at VLR of 1.68 g VS-FW/L.d. Results indicated a higher biodegradability on the VS base for VW. Average SMP values for FW and VW digestion were obtained as 690±100 and 905±150 mL CH4/g VSfed.d, respectively. The highest increase in digestion performance was obtained in SS co-digestion at VLRs 1.68 g VS-FW/L.d and 1.17 g VS-FVW/L.d. VW addition produced a limited feasibility on VLR as high loading produced a toxic effect on methanogenesis. An increase in the VS removal was only obtained as 9% in SS co-digestion at 0.33 g VS-VW/L.d whereas VS accumulation took place in other digesters. Stabilized SS showed a limited feasibility towards use in agriculture with electrical conductivity (EC) level whereas higher EC values were obtained in stabilized PS and MS samples. Average N and P contents were determined as 111, 363 and 202 mg N/g dry solid and 15, 57 and 26 mg P/g dry solid for PS, SS and MS, respectively. Consequently, anaerobic sewage sludge co-digestion with FVW proved a considerable potential in promoting the methane recovery while it increased EC in the stabilized sludge.