LEACHING OF HEAVY METALS, NITRATE AND ORGANIC MICROPOLLUTANTS AFTER APPLICATION OF SEWAGE SLUDGE TO SUGARCANE IN MAURITIUS

摘要

Stringent regulations have emerged to protect the environment from sewage sludge and other organic wastes. Of the disposal options available, only re-use on agricultural land produces economic benefits, as sewage sludge contains plant nutrients and acts as a soil conditioner. However, care is necessary because sewage sludge contains heavy metals and organic micro-pollutants. In addition, there is a risk of greater than normal movement of nitrate to surface waters and ground water. To clarify these issues, agricultural re-use of sewage sludge in sugarcane was studied in concrete lysimeters under rain-fed conditions at two sites in Mauritius. Sewage sludge rates ranged from 0 to 60 t/ha and was enriched with heavy metals (cadmium, mercury and lead) and organic micro-pollutants (fluoranthene, phenanthrene and decachlorobiphenyl) to maximum levels permissible by the United States Environment Protection Agency. Subsequent analysis of leachates collected at one metre depth after each rainfall event failed to detect micro-pollutants (limit of detection being 0.1 μg/L for PAHs and PCBs) or heavy metals, even when the spiked sewage sludge was applied at 60 t/ha. Sewage sludge disposal in sugarcane soils in Mauritius therefore seems to present little risk of groundwater contamination by heavy metals or organic micro-pollutants. Furthermore, concentrations of heavy metals in sugarcane treated with sludge at 60 t/ha were not higher than in the control plot. Additionally, sewage sludge at 60 t/ha produced, except on a few occasions, nitrate concentrations in the leachates well below the maximum limit of 50 mg NO_3/L recommended for drinking water. This study suggests that sugarcane fields can provide a safe outlet for the disposal of sewage sludge and other organic wastes without the sustainability of the sugar industry being undermined.