Operation and effluent quality of a small rural wastewater treatment plant (WWTP) receiving discharges from an abattoir WWTP

The objective of this study was to evaluate the impact of effluent and sludge discharges of an abattoir wastewater treatment plant (WWTP) on the operation of a municipal aerated pond WWTP. Experiments were carried out in Cervera WWTP, located in northeastern Spain, which comprises four ponds operating in series. Cervera WWTP treats a flow rate of 3 100 m3/d of urban sewage mixed with the effluent and the sludge discharged by an abattoir WWTP. Prior to September 1993, the effluents of Cervera WWTP showed poor quality (70 mg TSS/ and 58 mg BOD5/ ) because the abattoir sewage was discharged directly into the municipal sewer. Since September 1993, when the abattoir WWTP was established, effluent quality has improved considerably (24 mg TSS/ and 15 mg BOD5/ ). The sludge discharges of the abattoir WWTP into the municipal sewer did not significantly affect effluent quality of Cervera WWTP, even when the TSS and BOD5 loading rates were 800% and 60% higher than the design loadings respectively. Cervera WWTP clearly had higher sludge production per treated flow (3 to 6 /m3 with a percentage of dry matter of 4.8 to 6.8%) than other similar WWTPs. In conclusion, sludge discharge to WWTPs with high retention times to allow for sedimentation and high capacity for storing does not affect effluent quality. However, these discharges increase sludge management costs. WaterSA Vol.30 (2) 2004: 197-202