This paper presents a novel process that hybrids contact stabilization and anaerobic digestion to create a novel process that can treat municipal wastewater while generating savings in energy as well as lowering the sludge yield. In this process, the aerobic sludge stabilization tank
and anaerobic sludge digester are connected in parallel and part of the anaerobic digester sludge is recycled back to the either contact tank or stabilization tank to maintain good F: M ratio of biosorption. In this way, a mixture of un-stabilized aerobic sludge and stabilized anaerobic sludge
are used to adsorb and flocculate colloidal and part of the soluble COD. The concentrated COD are biodegraded in the anaerobic digester. Because majority of the COD biodegradation goes through the anaerobic pathway, it offers advantages in comparison with conventional aerobic processes such
as lower sludge yield, higher methane production and lower energy consumption as a result of the lowered oxygen requirement. BioWin simulation predicted the hybrid process will increase biogas production by 28% and 19% reduction in sludge yield. The model assumes the inert volatile suspended
solids in the raw wastewater and cell lyses residual are completely not biodegradable. Pilot test has been conduced at Ulu Pandan WRP in Singapore. Two hybrid process trains were operated in parallel with a control train which is conventional contact stabilization process followed by anaerobic
digestion of waste activated sludge. Since Jan 2010, the COD removal efficiency was 94.4±4.2% and 90.3±4.5% while the conventional process COD removal efficiency 89.7±4.8%. The biogas production of the two hybrid process trains was 37.5±10.8 l/d and 43.1±14.9
l/d, while conventional process 26.1±8.9 l/d. This gave 44∼65% increase. Meanwhile the bio-solids yield of the two hybrid process trains were 0.158 VSS/COD and 0.162 VSS/COD. In compare with conventional process train's sludge yield of 0.25 VSS/COD, that is 35∼37% sludge
reduction. The pilot result better than simulation prediction suggests that the inert volatile suspended solids in the raw wastewater and cell lyses residual are slowly biodegradable in this hybrid process. A COD mass balance of the hybrid process suggested that the effluent, sludge, aerobic
and anaerobic degradation will count for about 8.6%, 25.3%, 22.5% and 43.6% of total influent COD. Assuming 35% of the biogas energy will be converted to electricity, this amount of electricity is about able to run the whole wastewater treatment processes. In another word, the process is approaching
zero energy consumption. Assuming 40% of the biogas energy will be converted to heat (25% energy loss in conversion), the heat is enough to heat up the anaerobic digester to 35°C and also dry the final sludge.
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