Bio-gas augmentation and waste minimization by co-digestion process in anaerobic digestion system of a municipal waste water treatment plant

Abstract

Anaerobic digesters of Tehran waste water treatment plant (WWTP) working in mono-digestion operation are investigated for not only methane augmentation but also for waste minimization by the use of co-digestion process. Three types of co-substrate wastes including slaughterhouse flotation greases, biowaste, and cattle manure were selected because they were available in large amounts. A model was required to predict the volume flow rates of feeding co-substrate wastes into digesters during operation time for providing sufficient biogas in order to generate maximum gas engine power output. Thus, Anaerobic Digestion Model Number 1 (ADM1) for mono-substrate is modified so that it could be used with characterization of primary sludge, secondary sludge, and several co-substrate waste inputs into digesters. Then, appropriate coefficients (hydrolysis coefficient and bio-degradability factor) in this model were determined by the use of sludge biochemical methane potential (BMP) tests. After complete modification, calibration and validation of ADM1 method with experimental values of primary and secondary sludge for anaerobic digestion (which is the current reality in Tehran WWTP), the developed model is integrated with a co-substrate feeding strategy for estimation of required feed flow rates of co-substrate wastes. Results indicated that feeding grease, biowaste and manure increased the average generated biogas from 10524 m3/day (62.1% CH4) to 29161 m3/day (61.5% CH4), 30183 m3/day (59.5% CH4), and 32531 m3/day (53.8% CH4), respectively. Also, the gas engine power output increased from an average value of 906 kW to about 2.5 MW (during the studied operation time period). Low hydrolysis coefficient and low degradability of manure caused high bio-degradable feed flow rate of this co-substrate waste to about 58,200 kgCODb/day in average. This value for biowaste and grease was 38,800 and 36,100 kgCODb/day, respectively. Feeding grease and biowaste into digesters increased COD removal efficiency from 37% to 58% and 54%, respectively, while feeding manure dropped it to 31%. Furthermore, values of pH and VFA/Alk indicators showed stable conditions in digesters. Moreover, it was found that all three co-digestion processes did not impose a new load on other WWTP equipment for reducing nitrogen in the effluent stream of WWTP. Finally, it was concluded that biowaste is an appropriate co-substrate for our case study.