Thermo-tolerant coliformbacteria decay rates in a full scale waste stabilization pond system in Northeast Brazil

摘要

This paper presents the results for thermo-tolerant coliform (TTC) decay rates (K_b) in a full scale WSP system located in Natal-RN, northeast Brazil. The series comprises a primary facultative pond (2 m deep), followed by two maturation ponds (1.5 m deep) giving a total area of 11 ha. The influent sewage and the pond effluents were monitored weekly during a seven month period. The results showed that the K_b values predicted by the Marais equation assuming a hydraulic regime of complete mixing overestimated TTC die-off rates. The K_b value adopted in the project design was 6.20 d~(-1) but the mean value found for the WSP system during the monitoring programme was only 0.85 d~(-1). This value is low compared to the values cited in the literature for shallow ponds (<1.25 m deep) but similar to values for deeper ponds. The sub optimal TTC removal rate in this WSP system may be caused by the adoption of too high a K_b value at the design stage and the negative influence of high wind conditions on the mixing regime in the water columns of the ponds. Thus values for K_b adopted at the design stage of WSP systems should be coherent with the hydraulic flow model, the type of pond, pond depth, and with the surface organic loading.