The effect of ageing on the fertilizer value of sludge from Botswana

This study was designed to characterize sludge of three different ages (36 months old, 3 months old and fresh sludge referred to as Type 1, Type 2 and Type 3 sludge respectively) in an endeavor to appreciate their suitability for use as manure for arable agriculture. Sludge properties including volatile solid and nutrient contents, concentrations of the heavy metals As, Cd, Cr, Co, Cu, Mn, Pb, Ni and Zn as well as coliform population were determined. The concentrations of plant nutrients in the sludge followed the order Type 3 sludge Type 2 Type 1 sludge with nutrient content scores of 1.4, 2.1 and 2.6 respectively. Scores for potential leachability of heavy metals from the three sludge were 2.1, 1.9 and 2.3 for Type 1, 2 and 3 sludge respectively, indicating that Type 2 sludge would release a higher concentration of heavy metals to the environment than the other two sludge types. The faecal coliform population in Type 3 sludge was higher than in Types 2 and 3 sludge with values of 5.6log10MPN/10g dry solid, 4.6log10MPN/10g dry solid and 2.7log10MPN/10g dry solid respectively. These results revealed that the nutrient contents and heavy metal concentrations in the three sludge types may not hinder their application to soil for arable purposes but steps to allow for further pathogen reduction need to be taken to reduce health risks.