Establishing procedures for design, operation and maintenance of sewage sludge anaerobic treatment plants

In 1993 the Irish Government commissioned a strategy study to address the difficulties encountered with sludge management and disposal in this country. Many Local Authorities have complied with the recommendations of this strategy study by incorporating Anaerobic Digestion (AD) of sewage solids in the design of new and retrofitted sewage treatment plants. Irish Local Authorities have little experience in anaerobic technologies compared to the more traditional mechanical and aerobic processes. The United States Environmental Protection Agency (US EPA) developed procedures for evaluating sewage treatment plants. These Comprehensive Performance Evaluations (CPE) are described in such publications as "Retrofitting Publicly Owned Treatment Works (1989)". The emphasis in such documentation was on aerobic secondary treatment processes. The present study developed an evaluation procedure for AD plants similar to the CPEs produced by the US EPA for secondary processes. This procedure was applied to existing and newly commissioned AD plants in Ireland in order to deduce process performance. Based on these studies, recommendation for the design, operation and maintenance of AD plants is presented. All plant operators agreed that frequent and accurate monitoring of pH and temperature was essential for adequate process control. The five existing sites were surveyed. Apart from the usual difficulties at start-up, plant operators expressed satisfaction with the anaerobic process at four of the sites. The fifth plant is currently being upgraded.