1Faculty of Agriculture and Environment, Agricultural University of Tirana
*Corresponding author e-mail: firstname.lastname@example.org
Strategic studies are necessary to determine the best practicable environmental option (BPEO) for the management of sludge (and effluent), i.e. an integrated and holistic approach to planning treatment and reuse or disposal that is adapted to the local conditions. Sewage treatment transfers much of the pollution load contain in sewage to sludge, so that clean effluent can be discharged safely to the environment. The pollutants include nutrients and organic matter which are of potential agronomic value but also other contaminants, particularly heavy metals which may be potentially harmful if sludge is used on land without control and concentration in soil allowed to accumulate to critical levels. A wide range of synthetic organic compounds may also be found in sludge, but most of these are relatively easily degraded and are of limited concern for sludge use or disposal. The heavy metals of concern in sludge are zinc, copper, nickel, cadmium, lead, chromium and mercury, although other potentially toxic elements may be of local concern, such as selenium, molybdenum, arsenic and fluoride. While heavy metals are regarded as a major problem for sludge use on land, it should be borne in mind that at normal agronomic rates of sludge application, the accumulation of heavy metals in soil is very slow and may take several decades or even centuries of application to reach soil concentrations of concern. However, because of the strong retention in soil, concentrations of heavy metals in sewage and sludge need to be effectively controlled to protect the long-term sustainability of soil for crop production. The chemical composition of sludge is not strictly controllable but specific measures can reduce the occurrence of specific contaminants and these are necessary in industrialised catchments where the discharge of heavy metals may give rise to concentrations in sludge that may restrict the potential use of sludge in agriculture. In most countries, industrial effluent control regulations and continual improvements in manufacturing technologies that are less polluting, have progressively reduced the occurrence of point sources of heavy metal inputs to sewage. Where run-off from paved areas is combined with wastewater, atmospheric deposition of heavy metals, such as lead from car exhaust (although the introduction of lead-free petrol can effectively reduce concentrations), may be washed into the sewerage system.
Keywords: waste; sewage; sludges; heavt metals.