LARGE-SCALE PILOT TRIAL OF ANAEROBIC BAFFLED REACTOR: ASSESSMENT OF KEY PERFORMANCE PARAMETERS AND COST-BENEFIT ANALYSIS OF FULL-SCALE RETROFIT

An Anaerobic Baffled Reactor is a simple compartmentalized reactor, designed for retrofit inside an existing primary clarifier. The novel design approach can significantly reduce sludge production by encouraging in situ anaerobic biological degradation of the primary sludge. The key to the successful operation of the Anaerobic Baffled Reactor (ABR) is to separate the solids retention time (days) from the hydraulic retention time (hours). A sludge blanket is established in each baffled zone and the overall effect is to provide primary and some secondary treatment in a single basin.Atkins has been involved in the development of the ABR technology since 2000. A consortium of UK water companies, and Orange County Sanitation District (OCSD) in the US funded the original work in the United Kingdom. However, conditions for operation are different in Southern California to the UK, specifically temperature, hydraulic retention time of primary tanks and use of chemical treatment. Therefore it was decided to operate an ABR under OCSD-specific conditions. The pilot plant at Orange County Sanitation District operated with an HRT of 3.0 to 4.4 hours between March 2003 and June 2004.A detailed mass balance confirmed that the volatile solids were reduced by up to 39% across the ABR. Key operational parameters included the depth of the sludge blanket within the reactor and the rate at which sludge is withdrawn from the baffled compartments. The increase in concentration of volatile fatty acids across the length of the reactor and the production of methane gas confirmed hydrolysis of solids and methanogenic activity.A pilot-scale primary tank was not available to act as a control therefore full-scale primary tank performance was used for comparison. It was observed that the soluble BOD fraction of the ABR effluent increased when compared to the primary tank effluent soluble BOD, an observation that had not been made in previous ABR studies in the UK.The comparative cost of retrofitting baffles to circular and rectangular clarifiers was found to have a significant impact on the overall cost-benefit. The cost-benefit analysis, based on the OCSD biosolids master plan indicated a potential net present saving of 35 million over a twenty year period.