Robert M. Sterritt

Formation and conditional stability constants of complexes formed between heavy metals and bacterial extracellular polymers

The strength and nature of the binding of heavy metal ions to bacterial extracellular polymers has been investigated. The conditional stability constants (Kt) for complexes formed between extracted Klebsiella aerogenes polymer and copper, cadmium, cobalt and nickel were determined, using a gel chromatographic technique, and log Kt values of 7.69, 5.16, 5.48 and 5.49 respectively were obtained. Adsorption isotherms constructed for copper, cadmium and cobalt indicated that metal uptake occurred after the initial complexation capacity had been exceeded, suggesting the presence of more than one binding site, but nickel adsorption ceased when the complexation capacity was reached. Nickel was found to be associated predominantly with the soluble form of polymer, and copper and cadmium with the colloidal fraction when metals were added to the polymer simultaneously, rather than individually. The overall specific metal uptake by polymers extracted from activated sludge was approximately ten times higher than that by K. aerogenes polymer.

Significance and behaviour of heavy metals in waste water treatment processes II. Sludge treatment and disposal

Abstract The literature concerning the significance and behaviour of heavy metals in sewage sludge treatment and following sludge disposal is reviewed. Particular attention is given to metal form during anaerobic digestion and following sludge disposal to agricultural land and to sea, and the influence of metal form on toxicity. It is evident that anaerobic digestion results in a strong association between sludge particulates and heavy metals due to the formation of inorganic precipitates, organo-metallic interactions and by association with the biomass. Toxic inhibition of anaerobic digestion by metals is concluded to be a direct result of the free metal ion concentration and thus mechanisms designed to reduce metal toxicity are based on artificial modification of heavy metal speciation in the digester. Guidelines and legislation designed to limit the application of sludge to agricultural land and the disposal of sludge to sea are reviewed and discussed. Following the application of sludge to agricultural land significant changes occur in heavy metal speciation, which ultimately determine metal availability to plants and potential contamination of groundwaters. Guidelines for application to land currently take limited account of metal form, although the importance of metal speciation is being recognised. Sludge disposal to sea has received relatively little attention with efforts being directed towards monitoring rather than more fundamental research. However, it is apparent that significant heavy metal solubilisation may occur following sludge discharge to the marine environment. It is concluded that sludge disposal to agricultural land and sea, which are increasingly becoming subject to more stringent international pressures, should take account of the ultimate forms of heavy metals in the receiving environment and that a more unified approach should be adopted in the formulation of standards for all forms of sludge disposal.

Characterisation of metal forms in sewage sludge by chemical extraction and progressive acidification

Sequential chemical extraction, acidification to pH 4, 2 and 0.5, and repetitive CaCl2 (0.05 M) extractions were used to evaluate the forms of Cd, Cu, Ni, Pb and Zn in seven sewage sludges. There was more variation in the fractionation profiles of different metals in the same sludge than of the same metal in different sludges. Speciation in air-dried and liquid forms of sludges differed slightly, but the former gave more reproducible results. The sequential extraction scheme indicated that the predominant fractions of Pb and Zn were extractable in 0.1 M Na4P2O7, corresponding to organic and some insoluble inorganic forms. The largest Cd and Ni fractions were extractable in 0.1 M sodium ethylenediaminetetraacetate (pH 6.5), corresponding to the carbonates. The major forms of Cu were probably the sulphide and organic phases, both extractable in 6 MHNO3. Progressive acidification of liquid sludges mobilised significant quantities of Zn at pH 4, of Cd and Pb at pH 2 and of Ni at all pH values investigated. Copper was relatively immobile throughout. Application of the sequential extraction scheme to the residues obtained following acidification indicated that Cu, Cd and Pb forms were relatively stable, but that Ni and Zn speciation was likely to change with decreasing pH. Repeated CaCl2 extractions resulted in a low degree of metal mobilisation and had little effect on the speciation of the metals remaining in the sludge.

Metal removal by adsorption and precipitation in the activated sludge process

Abstract Metal removal in the activated sludge process has been determined by adding metals to samples of mixed liquor developed in a synthetic sewage, followed by separation of soluble and particulate forms of the metals using membrane filtration. The importance of precipitation in metal removal was evaluated by adding metals to mixed liquor filtrates and removing precipitates by filtration. The removal efficiencies of Pb, Cr(III) and Cu, to a lesser extent, were unaffected by changing sludge age or increasing metal concentration due to their low solubility. The predominantly soluble metals responded to sludge age in similar ways, with maximum removal generally occurring at a sludge age of 12 days. The percentage removals of the soluble metals decreased as increasing concentrations were added, but there was no evidence of saturation of the biomass. Experiments undertaken to investigate synergistic or antagonistic reactions between metals indicated that competition for activated sludge binding sites did not occur to any significant extent, but some interactions between metals having a tendency to precipitate were observed.

An evaluation of four start-up regimes for anaerobic fluidized bed reactors

SummaryAn investigation of four start-up methods for anaerobic fluidized bed reactors is reported. Addition of methanol and various organic loading regimes were evaluated. Stepped organic loading with methanol addition gave the most rapid and satisfactory organic degradation.

The influence of sludge age on heavy metal removal in the activated sludge process

Abstract In a laboratory simulation of the activated sludge process ten heavy metals were added continuously to the system which was allowed to equilibrate at six sludge ages between 3 and 18d. Cobalt, manganese and molybdenum removals were poor and were unaffected by changes in the sludge age. The highest removal efficiencies for the other metals occurred at the 15d sludge age. Chromium (trivalent) and cadmium had the highest removal efficiencies, typically greater than 50%. The behaviour of the majority of the metals which were removed to a significant extent was related to one of the parameters influenced by sludge age, i.e. mixed liquor suspended solids, effluent suspended solids or effluent chemical oxygen demand. The metals which were poorly removed showed little affinity for the activated sludge, while most metals exhibited maximum specific uptake by the mixed liquor at a sludge age of 9–12d. However, the affinity of silver for the mixed liquor continued to increase as the sludge age increased to 18d. An affinity series, based on an arbitrary measure of the specific accumulation of metals by the mixed liquor, indicated that chromium, cadmium and silver were most readily adsorbed by the activated sludge.

Extraction of extracellular polymers from activated sludge

SummaryMethods of deflocculating activated sludge to facilitate separation of extracellular material have been investigated. Total sludge carbohydrate was estimated to be 160 mg g−1 SS, of which 60 mg g−1 SS could be extracellular. An alkaline treatment extracted this quantity but a less rigorous ion-exchange technique was chosen for routine extraction.

Significance and behaviour of heavy metals in waste water treatment processes III. Speciation in waste waters and related complex matrices

Abstract The methods for determining discrete heavy metals species or groups of species in sewage, sewage effluent and sewage sludge samples have been reviewed, with particular reference to the determination of stability constants of the complexes formed with organic ligands. The drawbacks of several methods arising from the complex nature of these types of sample have been discussed together with the inaccuracies which may be caused by the relative non-specificity of some analytical techniques for the metal species of interest. Typical results obtained by these methods have been presented and these indicate that free metal ions are relatively scarce in sewage matrices and that soluble and surface bound organic ligands represent the predominant species in many instances. The poor removal efficiencies of some metals in sewage treatment may be due to the formation of complexes with soluble ligands which are much more stable than solid phase complexes.

Mass balance of heavy metal uptake by encapsulated cultures ofKlebsiella aerogenes

Dialysis was employed as a method of speciating heavy metals in cultures of an extracellular polymer forming strain ofKlebsiella aerogenes. A noncapsulated strain of the same bacterium was used as a control, and a mass balance of copper, cadmium, cobalt, nickel, and manganese in batch culture at pH 4.5 and pH 6.8 and in continuous culture at pH 6.8 was constructed. Copper and cadmium were accumulated by the cell during rapid proliferation whereas all 5 metals were bound nonspecifically by extracellular polymer produced during stationary phase and at low dilution rates. The presence of extracellular polymer appeared to inhibit cellular uptake of nickel. At the lower pH, metal uptake was considerably reduced. The results are discussed in the context of metal removal in the activated sludge process of waste water treatment.

Atomic absorption spectrophotometric analysis of the metal content of waste water samples

Abstract The application of flame and flameless atomic absorption spectrophotometry to the determination of heavy metals in waste water samples has been reviewed, with particular emphasis placed on the sample pretreatment methods used. The relative suitabilities of acid digestion and dry ashing pretreatments have been presented and the feasibility of direct injection of waste water samples after a rapid pretreatment has been discussed.