Cadmium - a complex environmental problem. Part II. Cadmium in sludges used as fertilizer.
In intensively populated countries efficient sewage treatment is essential to protect river quality. An inevitable by-product is sewage sludge which has to be disposed of safely and economically. Utilisation of sludge as a fertilizer of agricultural land is the most economic disposal route for inland sewage-treatment works and also benefits farmers by providing a cheap manure. Much of the cadmium in wastewater is concentrated into sludge which consequently contains higher concentrations of cadmium than soil does. It is impracticable to reduce cadmium concentrations in sludge below certain levels. When sludge is used on farmland rates of application must be controlled so that cadmium concentrations in soil never reach levels that could significantly contaminate food crops. Cadmium is a principal factor limiting the use of sludge on land. Nevertheless, it is a local problem since agricultural land in general receives more cadmium from aerial deposition and phosphatic fertilizers. The significance of accumulations of cadmium in soil depends mainly on its availability for crop uptake. Investigations are described which have attempted to identify and to determine the availability of forms of cadmium in soil. There is considerable research interest in cadmium in soil solution which is likely to be directly available for crop uptake. Another area of interest is the apparent disappearance of cadmium from sludge-treated soil. Soil analysis often cannot fully account for the cadmium added in sludge. Apart from the effect of soil conditions, especially pH value, crop uptake varies according to the particular crop examined. Highest concentrations of cadmium occur in tobacco, lettuce, spinach and other leafy vegetables. Using crop uptake data from field trials it is possible to relate potential human dietary intake of cadmium, on which hazard depends, to soil concentrations of cadmium, which can be controlled by regulating applications of sludge. This provides an objective basis for limits for cadmium concentrations in soils receiving sludge. Transfer of cadmium via farm animals to meat and dairy products for human consumption is thought to be minimal, even allowing for some direct ingestion of sludge-treated soil by the animals. Evidence from these and other investigations suggests that a loading rate limit of 5 kg Cd/ha (equivalent to a soil concentration of about 3.5 mg Cd/kg) affords adequate protection to the foodchain where sludge is used on agricultural land. More research work is needed to provide a basis for predicting the long-term availability of cadmium introduced to the soil in sludge.