Cadmium and lead impact on biological phosphorus removal: metal partition and adsorption evaluation in wastewater treatment processes

Abstract

Understanding the effect and partition of metals in wastewater treatment plants (WWTPs) can aid in designing processes to reduce metal emissions. This study focused on the effects of cadmium and lead, exposed alone (Cd or Pb) and in combination (Cd + Pb), on the enhanced biological phosphorus removal (EBPR) process at lab-scale. Parameters related to the metabolism of propionate, phosphorus (P), polyhydroxyalkanoates, extracellular polymeric substances (EPS) and enzyme activities were compared with those of a normal EBPR process. The effects induced by Cd included an incomplete uptake of propionate and poor P removal. Both Cd and Pb induced higher EPS production and altered the sludge settling properties; the effect of the former being more pronounced. Additionally, both Cd and Pb caused a lower content of phosphorus in the sludge but for different reasons. Cd affected the overall P removal by altering the P-uptake/release rates. Although Pb did not affect P removal, it formed P-containing complexes, likely resulting in less P available to the bacteria of the EBPR process. When present together, Cd and Pb exhibited synergistic effects. Regarding solubility, Cd remained soluble whereas Pb was mostly insoluble prior to starting the EBPR process. Consequently, Cd and Pb partitioned mainly to the effluent and sludge, respectively. To counteract this outcome, a treatment using adsorbents prior to the EBPR process was applied. However, as only soluble metals such as Cd can be effectively removed, only the effluent quality could be potentially improved. Stringent measures for metals such as Pb should be taken upstream WWTPs.

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