Effects of trace zinc, copper and chromium on organic adsorption capacity and organic removal in activated sludge.

摘要

Heavy metals are commonly found in municipal sewage that contains industrial effluents. It is well documented that metal concentrations above toxic levels inhibit biological processes in municipal sewage treatment works and discharge of metals into surface waters can have severe impacts on the environment, ecosystems and public health.; However, the effects of trace metals at sub-toxic concentrations on biological sewage treatment processes and the mechanisms of their effects on COD removal and COD adsorption capacity (CAC) are less well documented. This study investigated and compared the effects of copper-, zinc-, lead- and chromium-laden wastewater at sub-lethal levels on organic removal efficiency in a simulated activated sludge culture, namely a sequencing batch reactor. The characteristics and mechanisms of adsorption of heavy metals at sub-lethal concentrations by activated sludge bioflocs and the effects of metals on COD removal efficiencies and CAC were also studied using batch activated sludge cultures and experiments.; Experimental results showed that adsorption capacity and rate of copper, zinc, lead and chromium on microbial flocs were much higher than that of organic matters. Metals affected not only the adsorption rate of organic matter but also the CAC of the activated sludge. Metal-laden wastewater at sub-lethal levels affected the activated sludge process performance in organic reduction to different extents, depending on the operating hydraulic retention time (HRT). Metal ions acted as a strong competitor against organic matters for active sites on the bioflocs instead of acting as a toxic microbial inhibitor, thus hampering organic adsorption and affecting the COD removal efficiency under shorter HRTs. A mathematical model has been developed to predict values of CAC. Experimental results validated the proposed model.; Copper was found to have the highest effects on CAC and COD removal efficiencies, and therefore the mechanisms of adsorption and the effects of copper on the activated sludge microbes were investigated on a microscopic level. A Gram-negative bacterium, Pseudomonas putida 5-X was isolated from activated sludge. Cells with superficial layer removed, isolated cell envelopes, and the separated peptidoglycan layer were compared for copper adsorption properties with that of fresh cells. The treatment with 0.3 N HCl obviously enhanced the copper adsorption capacity, due to the degradation of the superficial layer-capsule. In intact cells, the partly metal-binding sites are inaccessible for heavy metal adsorption, but the isolated cell envelopes liberated much more metal-binding sites on cell surface and enhanced copper adsorption capacity. Although peptidoglycan layer of P. putida 5-x cell envelope has some copper binding capacity, both the outer membrane and the inner membrane played an important role on the binding and adsorption of copper. Fresh cells, pretreated cells and whole cell envelopes of P. putida 5-x all could be described with Freundlich equation, but the adsorption model of separated peptidoglycan layer could be better described with Langmuir equation. The results indicated that the adsorption property of peptidoglycan layer with copper is different from that of intact cells or whole cell envelope.