The residual product generated from sewage treatment is termed sludge. Sewage sludge contains heavy metals, organic compounds, and pathogens, in addition to substantial amounts of nutrients. Sludge may be disposed of by depositing, burning, or dumping into the sea or it may be used in forestry and agriculture. On the one hand, spreading sludge onto the land is desirable because the nutrients of sludge would participate in the biogeochemical cycle in ecosystems. However, due to the inherent nature of its composition, sludge involves the risk of harming the environment because the substances may be accumulated in the soil or damage the ecosystem. Thus, health considerations must be considered by environmental regulatory agencies with respect to the use of sludge because it contains pathogens in the form of bacteria, viruses, and parasite eggs. This content can be reduced by stabilization or disinfection of the sludge.
To ensure that the environment and man are fully safeguarded against the harmful effects arising from the uncontrolled use of sludge, it is necessary to regulate the agricultural use of sewage sludge. Therefore, most countries have legislation regulating the use of municipal sewage sludge in forestry and agriculture . These regulations set maximum limits for the content of heavy metals in sludge (except in the U.K.). Limitations are set on the amount of sludge that can be spread on a given area, based on total amount of sludge (calculated as the amount of dry solids in the sludge), heavy metals, or nutrients added to the soil. It is especially with respect to this point that different national regulations vary . In addition, there are restrictions in relation to certain crops and land uses, (it is normally not permissible to apply sludge to land to be used for growing vegetables). Germany prohibits spreading sludge on forest land, but Denmark and the U.K. permit this. Before application of sludge, chemical composition of the soil must be taken into account (pH, texture, and concentration of heavy metals).
Sludge producers are required to make analyses of the sludge. Typically, the frequency of the analysis is determined by the size of the treatment works and is intensified if the composition of the sludge is variable. The frequency varies from a daily sludge sampling to one analysis every second or fourth year; sometimes, with respect to certain substances, analysis is not required at all. Usually, sludge is analyzed for its content of heavy metals, nitrogen, and phosphorus, but identification of other substances may also be required. In Norway and the U.S., samples must also be taken for the control of pathogen content. Furthermore, in the U.S., it must be controlled whether the vector attraction reduction requirements of the sludge are met or not, so that the sludge exerts only a limited attraction to animals, which can transmit infection to other animals as well as to man. The frequency of the analyses and the substances for analysis vary among the different countries, so the use of sludge may be indirectly limited in some countries by the high cost of analysis.
For the purpose of controlling the legislation, the sludge producers must register the production and the agricultural use of their sludge. Therefore, the controls must be drawn up differently in the different countries. In all countries, except the U.K., maximum limits are set for the content of heavy metals in sludge. In addition, some countries have set value limits for arsenic, selenium, and molybdenum. Germany also limits the content of the following organic compounds: polychlorated biphenyles (PCB), polychlorated dibenzoefuranes and polychlorated dibenzoedioxines (PCBP, PCDF, PCDD), and organic halogens (AOX). The German value limits for the content of organic compounds in sludge are not set on the basis of qualified toxicological tests, but they are set out of prudence . In Germany, the present content of PCB in sludge is unlikely to limit use of sludge, but the content of PCDD/PCDB may exceed the fixed limits. Furthermore, the analyses are complicated and costly .
In general, heavy metals and persistent organic compounds cause damage to the environment by accumulating, while the introduction of nutrients may harm the environment through the leaching of nutrients to the aquatic environment. The extent to which organic compounds are accumulated has not yet been clarified. The effects of heavy metals and organic compounds on the soil fauna have not been sufficiently investigated. One of the most important sources of pollution from persistent organic compounds is pesticides; the pollution caused by heavy metals arises from industrial deposition and fertilizers.
When comparing the regulations, remarkably large differences appear concerning the restrictions on heavy metals with respect to their concentrations in sludge and soil and to the quantities that may be spread. The general tendency appears to be that countries that permit the highest concentrations of heavy metals in sludge also permit the largest quantities of sludge to be spread annually and permit the highest concentrations of heavy metals in soil. Countries that have the most stringent requirements for the heavy metal content in sludge also have a tendency to have the lowest value limits for the permissible heavy metal content in soils.
The great variation between the countries with respect to the amount of heavy metals that can be applied to soils and the lesser variation with respect to the permitted heavy metal concentrations in soils where sludge can be applied give rise to very different time horizons for the use of sludge on individual areas. Therefore, the value limits for heavy metal concentrations in soils can be regarded as an "emergency brake" on the use of sludge. However, this emergency brake is only effective if the control of heavy metal concentrations in soils is effective in the countries that risk exceeding the value limits after a few years' use of sludge.
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