Acid treatment lowers metal content in compost, improves soil quality: research

Researchers from the Indian Institute of Soil Science (IISS), Bhopal, and the Central Arid Zone Research Institute, Jodhpur, have found a way to bring down the concentration of heavy metals in compost made out of waste.

An analysis of compost samples collected from mixed waste, partially-segregated waste and segregated bio-waste collected across 12 Indian cities showed that the maximum heavy metal concentration was in mixed waste. Among the 5 cities where mixed waste compost was collected, Mumbai scored the highest for concentration of heavy metals.

When the 3-member team washed fine fractions of the mixed waste compost with acid, they found that the heavy metal content reduced significantly. For instance, content of cadmium and zinc came down by 50% to 60% – from 10.4 ppm to 2.9ppm (parts per million) in cadmium, and from 2,020ppm to 750ppm in zinc.

The permissible limit for cadmium is 5ppm, while it’s at less than 1,000ppm for zinc. According to researchers, present technology entails excavating the contaminated soil and then treating it with acid, which helps dissolve  metals. “But if the compost is treated for heavy metals before it is applied to the soil, the quality of the compost that was earlier rejected can be brought up for agriculture,” said IISS professor Jayanta Saha, lead investigator of the study.

The heavy metals come from electronic and electrical waste. Organic content in compost improves soil quality. With less organic matter, the dependence on fertilisers increases. Indian cities generate more than 70 million tonnes of solid waste every year, with non-biodegradable waste – including heavy metals – comprising 40%. “Segregation at the landfill is a complicated management issue with a lot of labour involved. That is why most civic bodies cannot segregate efficiently,” said Saha.


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