This paper describes the case history of a two-stage, low-rate anaerobic treatment system at a South American alcochemical and citric acid plant. Environmental concerns associated with land irrigation of a high-strength industrial wastewater forced the plant to implement wastewater treatment. This paper will also outline results from an on-site pilot study which was conducted to develop the design criteria for the full-scale system.

The installed full-scale system will be described in detail, including the plant s physical layout, design parameters, system performance, biogas production and utilization, and some of its unique aspects. Among these is the use of a floating biofilter (bagasse) on the second stage to eliminate any nuisance odours from off gases.

All biogas generated in the first stage is collected, with a significant portion drawn off and burned in a retrofitted 8000 kW boiler to generate process steam. The excess biogas is flared. Effluent from the treatment system is combined with other low-strength process plant streams and discharged to a river.

During the first year of operation, performance has been very good, with chemical oxygen demand (COD), biochemical oxygen demand (BOD), and total suspended solids (TSS) removals averaging 74 percent, 93 percent, and 70 percent, respectively. The corresponding COD, BOD, and TSS results obtained in the pilot study were 74 percent, 96 percent, and 70 percent, respectively, and agree very well with those being obtained in full scale.

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