Removal of helminth eggs in an advanced primary treatment with sludge blanket

This paper describes the application of a physicochemical system, based on advanced primary treatment (APT) to remove Helminth eggs and faecal colifor ms from domestic wastewater in order to obtain an effluent suitable for agr icultural reuse. The system comprises a coagulation-flocculation unit, with a high rate sedimentation tank and a sludge blanket (Densadeg) along with a system of sand filtration and chlorine disinfection. It was found that th e Densadeg reduced the Helminth eggs from 22.6 HE l(-1) to 1.2 HE l(-1) wit h hydraulic loads of up to 1140 m(3) m(-2) d(-1) (60 m h(-1)) in the sedime ntation tank, which are far greater than those which have been described pr eviously. The water quality of this effluent is suitable for the irrigation of crops which are not consumed raw. To achieve values recommended by the WHO, <1 HE l(-1) and 1000 MPN/100 ml faecal coliforms CFC), for crops which are consumed raw, it is necessary to filter the effluent to obtain 0.27 HE l(-1) and apply 17 mg l(-1) of chlorine with a contact time of one hour. I n the whole treatment system, the organic matter is partially reduced (from 389 to 160 mgCOD l(-1)), as is the nitrogen (from 39 to 17 mgN l(-1)) and phosphorus (from 12 mg P-PT l(-1) to 7 mg l(-1)). Hence the advantages of u sing wastewater for agricultural irrigation are maintained, but without the associated microbiological risks.