The control of the heavy metals health hazard in the reclamation of wastewater sludge as agricultural fertilizer.

Abstract

WASTEWATER SLUDGE production is esti mated at 33 tons, dry weight, per year per 1,000 people living in sewered areas. About a quarter of this sludge is disposed of by means of application to cropland. This disposal means has an important fertilizing advantage due to the high nitrogen content in the sludge — but also a serious disadvantage caused by the presence of poisonous heavy metals in the same sludge, that affect plants, animals, man and water resources. Out of the 14 most common heavy metals in sludge, that include — aluminium, antimony, arsenic, chromium, iron, lead, manganese, mercury and selenium, as well as cadmium, copper, molybdenum, nickel and zinc — the last five are the most hazardous, in particular the cadmium. Most heavy metals are essential in the metabolism processes, but are poison ous in excess quantities. Heavy metals tend to accumu late in leaves, rather than in the fruit of plants — and in the liver and kidneys of animals feeding on the infected plants. Plants absorb heavy metals only in a soluble form. High content clay soils, maintained in high pH values tend to reduce this solubility. Other control means include adequate selection of the raised crops (preferably non-edible plants), adjusting the sludge application rates to the nitrogen demand of the plants, reducing the heavy metals content in the sludge by pre-treating the relevant industrial wastes and regular monitoring and sampling of the heavy metals concent rations in the sludge, soil and plants.