CO-DIGESTION OF SEWAGE SLUDGE AND PIG MANURE UNDER MESOPHILIC CONDITIONS

Open Access
Author:
Bai, Xuanye
Graduate Program:
Environmental Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
March 30, 2017
Committee Members:
  • Yen-Chih Chen, Thesis Advisor
Keywords:
  • Anaerobic co-digestion
  • pig manure
  • sewage sludge
  • kinetic model
Abstract:
The feasibility and kinetics of anaerobic co-digestion of pig manure (PM) and sewage sludge (SS) were investigated. In this study, bench scale batch reactors were setup under mesophilic conditions (35℃) with SS/PM volatile solid ratios at 7:1, 14:1, 21:1, and 2 controls of pure SS and PM. The ratio selection was based on the SS production of the University Park Wastewater Treatment Plant and PM production by the University Farm. Physical/chemical properties of sludge before and after digestion were analyzed. Daily methane production and cumulative methane yield were recorded, and the cumulative methane yields were fitted with both the first-order kinetic and the modified Gompertz model. All mixing ratios showed stable digestions as indicated by the pH, total ammonia nitrogen (TAN), and volatile fatty acid (VFA)/alkalinity ratios of the final digestates. PM produced more methane than SS and as a result, increasing PM in co-digestion also increased methane yield. The maximum methane yield occurred at SS/PM ratio of 7:1 with a 10% increase at 200 mL CH4/g VSadded compared with SS alone at 182 mL CH4/g VSadded. This is also the maximum co-digestion need from the University Farm in winter seasons. Although residual nitrogen increased in the co-digestion digestates, most of which were in solid form, resulting a decrease in soluble nitrogen. This implies a potential decrease in the nitrogen return which helps relief mainstream treatment. Both the first-order kinetic model and the modified Gompertz model showed good fit to the data produced. However, the modified Gompertz model proved to be the best choice that works for degradation with or without a lag-period. The first-order kinetic model indicated faster gas production kinetics by the 3 co-digestion treatments than the pure controls, which implies the existence of a positive synergistic effect by co-digestion. The modified Gompertz model also showed the highest maximum gas production rate, a decreased lag-period, and a shortened gas production period by co-digestion, which implies a facilitated hydrolysis has occurred from the mixture. This faster kinetics implies a shorter digestion time than the original wastewater sludge digestion which can help accommodate the increased digestion materials from the co-digestion process.