Behaviour of heavy metals during tertiary bio-filtration

More stringent water quality parameters in the Annex of the Water Framework Directive 2000/60/EC led to the introduction of "Maximum Tolerable Risk concentrations" (MTR-values) in the national legislation in The Netherlands (Vierde Nota Waterhuishouding). The MTR-values give limitations for the concentrations of e.g. heavy metals (HM's) but also for nutrients: N-tot < 2.2 mg/l, P-tot < 0.15 mg/l. Investigations of HM removal during denitrifying flocculation filtration are conducted on the effluent of a typical modern Dutch WWTP. Because of low concentrations of HM's in the feed water, a cocktail of copper, nickel and zinc chloride (approximate to 150-200 mu g/l) is dosed before filtration. Preliminary jar tests and filtration tests with media filtration in pilot-scale and lab-scale are conducted. The results show high removal of nickel and zinc during jar tests with dosing of powdered activated carbon. During filtration tests at pilot-scale the bounded fraction of copper and zinc is highly removed. All three HM's are removed in the lab-scale activated carbon filter. After dosing, nickel is found mainly in the dissolved form, but it is removed in the lab-scale activated carbon filter. The removal of HM's via adsorption subsequently leads to a discussion on the toxicity of HM's and their bio-availability.