Case Study 3 Metal fluxes from households to STPs sludge and agricultural soils

One of the most important routes for trace metal transport from the anthroposphere to sensitive ecosystems and to human targets is via municipal sewers to sewage treatment plants and further on to sludge, which may be recycled to agricultural land and, thus, back to the crop/food production system. It is therefore important to identify the dominant sources of potentially harmful trace metals occurring in the sewage treatment process, to undertake a speciation of these metals and assess their availability for uptake into crops and their toxicity to soil micro-organisms. Several broad reviews of these problems have been published in the last few years, with a special focus on Cu.

The release of Cu from Cu pipes to the tap water in houses has attracted considerable attention in recent years, mainly due to the rising concern for Cu build-up in agricultural soils when Cu-containing sewage sludge is utilized as a soil conditioner. Based on both equilibrium calculations and chemical analyses, it was concluded that at the pH and alkalinity ranges that are common in Swedish drinking waters, CuCO3 (aq) is the main dissolved inorganic copper compound, and CuO the main solid one. The concentration of dissolved cupric ions is, in general, much lower than the measured total dissolved Cu concentration. Other common Cu species in tap waters are various adsorbed forms on the surfaces of hydroxides and oxides and those occurring as organic complexes or bound to colloidal organic particles. In the drinking water of the city of Uppsala, with usually quite high total Cu concentrations, it was found that free cupric ions in solution amounted at 3-11% of the total Cu content. High Cu levels in this drinking water were usually related to high concentrations of dissolved organic carbon (DOC), supporting the hypothesis on the formation of organic Cu complexes.

Application of sewage sludge of "good quality " to arable lands, parks, forests or reclaimed areas may provide many agronomic benefits and contribute to solve the problem of nutrient and trace element deficiency. It is, however, important to stress that too high concentrations of trace metals and other elements or (organic) compounds may render the sludge less useful for application on arable land. Therefore, a set of limits for metals, including Cu, in sludges to be used for land application have been enacted both on the EU and the national level.

Long-term studies (up to 18 years) in several countries (e. g. in southern Sweden) were used to evaluate the impacts caused by yearly addition to arable land of Cu-containing sewage sludge (on Cu accumulation in the soil, on soil fauna and microbes, on crop yields and Cu-contents in crops). Based on such studies and on experiments with direct addition of dissolved Cu to agricultural soils, the conclusion was reached that the most sensitive sub-system is the microbial activity in the topsoil. As an overall conclusion, it was stated that an application rate of 1.4 kg Cu/ha, year with sludge would be a safe and acceptable load for Cu on most soils, where cereals are grown. A three times higher application rate, 4.2 kg Cu/ha, year, was not found to have any detrimental effects on crops or on the soil ecosystem, but a significant increase in soil Cu levels was demonstrated both in the topsoil and in the subsoil. Therefore, the latter loading rate cannot be considered as sustainable on the soil types investigated. A provisional level of acceptable Cu loading with sludge would, for most common types of agricultural soils, be somewhere in the range of 1.4 - 2.0 kg Cu/ha, year.

In the quality declaration of the sludge, with respect to the metal content, it is recommended to give it as milligram metal per kg of phosphorus (i.e. mg Cu/kg P), instead of mg Cu/kg sludge dry matter, because the main rational for using sludge in agriculture is to recycle phosphorus back to the soil (for more details on metal bioavailability in sludge see also section 5.5.7 below).

0 0

Post a comment