Faecal coliform die-off in wastewater storage and treatment reservoirs

Abstract

This paper describes faecal coliform (FC) removal in pilot-scale Wastewater Storage and Treatment Reservoirs (WSTR) in northeast Brazil. FC decay during the filling phase of WSTR was very reduced, suggesting that these reactors should be filled as quickly as possible, and subjected to the highest organic loading that will not cause odour emissions. The highest loading employed in this study was 659 kgBOD5/ha.d, causing no nuisance conditions. During the resting phase, FC decay rate decreased exponentially and Chick's law was modified. The time into the resting phase for FC to reach 103 cfu/100 mL ranged from 15 to 25 days as WSTR depth varied from 2.00 to 6.50 m. The performance of sequential batch-fed waste stabilization ponds (SBFWSP) in removing FC was compared to that of waste stabilization ponds (WSP) operated in series. It was shown that, in general, SBFWSP were cheaper than WSP in series, with the bacteriological quality of the effluent in the former being more reliably predicted than in the latter operational regime. When provision of volume to store the winter effluents is considered, a WSP system presents a higher benefit/cost ratio than a hybrid WSP-WSTR system, but the adoption of the latter can double the annual net return for a rainy season of 5 months for instance. In the case of use of the hybrid WSP-WSTR system, the net return would be US$ 0.17 – 0.22 per cubic metre of available wastewater, depending on the length of the rainy season (3 – 5 months considered) as compared to US$ 0.12 from the WSP system alone.