THE USE OF PROCESS MODELLING TO OPTIMIZE SLUDGE THICKENING AND DIGESTION

The paper describes the use of pilot testing, process modelling and a 20-year life-cycle cost analysis to optimize the sludge thickening equipment selection and anaerobic digester sizing for the large wastewater treatment facilities in the São Paulo Metropolitan Area. A model of a typical wastewater treatment plant was built using PRO2D, a steady state, whole plant simulator developed by CH2M HILL to perform complete wastewater treatment plant simulations and calculate full-plant mass balances. The model was calibrated using operating data from the 820 ML/d Barueri WWTP in São Paulo. Fifteen alternatives were developed which used different combinations of primary sludge thickening (gravity, rotary drum, gravity belt), and waste activated sludge thickening (dissolved air flotation, rotary drum, gravity belt, and centrifuge with and without polymer). The assumed thickening unit performance used as model inputs was based on experience at the Barueri WWTP, pilot testing, manufacturer's information and experience in similar applications elsewhere. The alternatives were compared using a 20-year life-cycle cost analysis. The results showed that the two alternatives with the lowest NPVs were those that combined mechanical primary sludge thickening (either by GBT or RDT), and centrifuge thickening of the waste activated sludge without polymer addition. This thickening strategy resulted in a significant reduction in the total anaerobic digester volumetric requirements, and a corresponding reduction in the capital and life-cycle costs. Approximately 50 percent of the NPV for the system consisted of the capital cost associated with the construction of the anaerobic digesters. This, in turn, made the life-cycle costs highly dependent on the performance of the primary sludge and WAS thickening units as poor sludge thickening mandated the construction of more digesters to provide the design HRT of 18 days at average annual flows and loads to the plant. Polymer costs for sludge thickening accounted for approximately 10 percent of total NPV over the 20-year period.