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ANAEROBIC DIGESTION OF EXCESS MUNICIPAL SLUDGE: OPTIMIZATION FOR INCREASED SOLID DESTRUCTION

by CACHO RIVERO, JESUS ANDRES

Abstract (Summary)
Anaerobic digestion is an economically viable and environmentally friendly process for the reduction of excess municipals sludge. Primary and activated sludge are generated during wastewater treatment. The production of sludge is increasing worldwide and anaerobic digestion presents limitations in terms of solid destruction and long SRT requirements. This study investigates two major aspects of the anaerobic digestion of excess municipal sludge. First the effect of sludge retention time (SRT) on the performance of the anaerobic digestion process was evaluated. Four different SRTs (5, 10, 20 and 40 days) were used in triplicate. The process showed stability and reproducibility throughout the experimental period. Solid destruction was a function of SRT, increasing as the SRT increased. It was modeled as a first order reaction considering the digester as a plug-flow reactor with recycle. The maximum solid destruction achieved in the anaerobic digestion of a mixture of primary and WAS was 55.3% and the first order reaction constant 0.38d-1. The effect of SRT on other effluent quality parameters was also studied. Increasing SRT decreased fecal coliforms density, although only class B biosolids were achieved in all cases. Once the effect of SRT and the limit in solid destruction of anaerobic digestion were determined, the second part of the study investigated a technology that would enhance solid destruction during the anaerobic process. A thermo-oxidative treatment using hydrogen peroxide as oxidant and temperatures below the boiling point of water was evaluated. Several reactor configurations and SRTs were considered as determinant factors in the process performance. Results showed that solid destruction increased as hydrogen peroxide and temperature increased. Higher stability, versatility and effluent quality were observed for the two stage configuration compared to the single stage configuration for the same overall SRT. Increasing SRT increased stability and improved effluent quality. The best cost effective configuration was a two stage configuration with reactors operated at 5 days SRT and thermo-oxidative treatment operated at 90°C and a dosage of 1.0 g H2O2/g VSSinfluent. A VSS removal of 75.7% was achieved with this configuration, which represents a 36.9% increase when compared to the maximum determined for conventional anaerobic digestion of the sludge mixture. This configuration showed eventually complete fecal coliforms removal thus achieving class A biosolids. Anaerobic digestion performance of municipal sludge was significantly enhanced when combined with thermo-oxidative treatment. However this technology can be economically prohibitive due to chemical addition. Further research should take into account a balance between performance enhancement and economic cost.
Bibliographical Information:

Advisor:

School:University of Cincinnati

School Location:USA - Ohio

Source Type:Master's Thesis

Keywords:anaerobic digestion excess municipal sludge thermo oxidative treatment hydrogen peroxide

ISBN:

Date of Publication:01/01/2005

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