FULL SCALE DEMONSTRATION AND ASSESSMENT OF ENZYMIC HYDROLYSIS PRE-TREATMENT FOR MESOPHILIC ANAEROBIC DIGESTION OF MUNICIPAL WASTEWATER TREATMENT SLUDGE

A full-scale monitoring programme was undertaken to assess acidogenic pre-treatment as a bolton biological upgrade to mesophilic anaerobic digestion (MAD). The United Utilities Enhanced Enzymic Hydrolysis (EEH) MAD pre-treatment process was installed by Monsal Limited at the Blackburn (UK) wastewater treatment plant. This process combines mesophilic acid phase digestion with pasteurization at 55°C using low grade boiler heat derived from biogas production. The monitoring programme started in January 2006 and ended in June 2007. The monitoring data will be utilized in a comprehensive evaluation of this pre-treatment technology. The purpose of this paper is to show process performance data of scientific significance based on selected monitoring results that are being evaluated as a first step in the larger technical and economic assessment. Enhanced Enzymic Hydrolysis (EEH) achieved 6 log E.coli removal and elimination of Salmonella. The pre-treatment provided a consistent feed to MAD with the anticipated outcome of more stable methane production. The upgrade has enhanced VS destruction by around 10% with concomitant improvements of 24% in biogas production. The potential for VS destruction and gas production are currently limited by low influent TS levels (5 %DS) and the need for optimization of the MAD mixing regime. VFA production during EEH at Blackburn is limited by hydrolysis kinetics and by inhibition when VFA levels are in excess of 9 g VFA-COD/L. The final biosolids product was of consistent quality with metal levels well below the norms established by the EU and EPA. Further research is required to better characterize factors limiting the potential for VFA production during EH and to understand how the EH acidogenic environment contributes to pathogen elimination.