Significance and behaviour of heavy metals in waste water treatment processes I. Sewage treatment and effluent discharge

Abstract

The literature concerning the significance and behaviour of heavy metals in waste waters is reviewed. Factors influencing the partition of heavy metals between the solid and liquid phases in both primary and secondary (biological) treatment are considered in detail. It is evident that during primary treatment only metals present in an insoluble form will be removed, but the removal of such forms is strongly influenced by the factors controlling sedimentation. During secondary treatment metals initially present as soluble forms may be removed by association with the settleable biomass: further removal of insoluble metals also occurs by association with this biomass. Efficient flocculation and settling is critical to the removal of all metal forms during secondary treatment. Extracellular material, principally bacterial extracellular polymers, but also proteins and nucleic acids released as the result of cell lysis, appear to play a very significant role in the removal of soluble metals during secondary treatment. If biological treatment plants are overloaded with metals, toxic effects on bacteria and other micro-organisms may result in poor quality effluents; although the micro-organisms responsible for waste water treatment are able, within limits, to acclimatise to elevated metal concentrations. The impact of heavy metal contaminated effluents on receiving waters has also been reviewed and it is concluded that the most serious problem posed is drinking water production in areas where water re-use is practised.