Physico-chemical studies of tannery waste and sewage sludge to estimate carbon content in amended soil

Sewage sludge generated from wastewater treatment plants are being merited greater attention in light of their potential for improving soil properties and for providing important nutrient and trace element supplements that are essential for plant growth. Because of the differences in sludge characteristics among sludges that undergo different levels of treatment as well as the extensive and variable nature of pollutant inputs to wastewater, the fertilizer potential and pollutant risk of sewage sludge intended for agricultural application has to be specifically evaluated for each sludge. Sewage sludge generated from seven wastewater treatment plants in Swaziland were analysed for a range of physico-chemical characteristics including organic matter, nutrients, cation exchange capacity, pH and trace elements. Despite the differences in sludge processing and sludge storage ages, the sludge samples generally show high levels of organic matter, nutrients and trace elements needed for plant growth. The potential risk of heavy metal toxicity was evaluated by comparing the levels of heavy metals in the sludge samples with widely quoted and well known regulatory limits of a number of countries and the levels were found to be within acceptable risk level with respect to agricultural application. Chrome tannery sludge applied to agricultural land may have benefits in terms of added N for crop growth. An experiment was designed to compare tannery waste with commercial N fertilizer and investigate the potential of the waste as an alternative or supplement to commercial fertilizer. Soils with 38% and 7% organic C and N content of 1.3% and 0.2%, respectively, were amended with lime, commercial N fertilizer, or tannery sludge containing 1.6% Cr. A portion of the tannery waste was supplemented with additional Cr(+3) salt before adding to the soils. The amended soils were analyzed for total Cr, ammonium acetate extractable Cr, selected nutrient and trace element concentrations. The tannery sludge increased soil pH, total Cr and N, S, Ca, P, Mg, and Na concentrations.