Rajasthan biochar plants show promise despite mercury and pathogen risks

A recent study by the Centre for Science and Environment (CSE) assessed 39 faecal sludge treatment plants in Rajasthan, revealing that while the technology to convert human waste into biochar is functional, it falls short on safety benchmarks like pathogen destruction and heavy-metal contamination.

The plants, operated by MVR Technology and commissioned in 2023-24, have treatment capacities ranging from 10 to 35 kilolitres per day. These facilities are spread across central and eastern Rajasthan. Despite Rajasthan's open-defecation-free status under the Swachh Bharat Mission, approximately 70% of urban households still use septic tanks, generating around 3,870 kilolitres of faecal sludge daily, which the current infrastructure only partially treats.

To address this, the state has adopted "pyrolysis-based Omni Processor" technology. This method separates solid and liquid waste, treats the liquid in a bioreactor, and heats the solids in a low-oxygen furnace to produce biochar. Biochar is a stable, carbon-rich material that sequesters carbon in soil for centuries and serves as an organic fertilizer.

The report found that most plants effectively manage the treatment process. Outlet pH levels remained within the required 5.5-9.0 range, and suspended-solids removal exceeded 90% at many facilities, meeting national discharge limits. Organic pollutant removal surpassed 95% at most sites, with effluent meeting stringent national standards. UV treatment kept faecal coliform levels within permissible limits at most locations.

Biochar and biosolids produced at these plants showed nutrient value, complying with India's Fertilizer Control Order standards for organic manure. They contained over 14% organic carbon and had nitrogen-phosphorus-potassium content above regulatory minimums, making them viable substitutes for chemical fertilizers.

However, the report noted issues at specific sites. Plants at Kishangarh Renwal and Khairthal showed nitrogen levels above limits, while phosphorus exceeded caps at several sites, including Kishangarh Bas. High dissolved-solids levels were found at plants like Lakheri and Losal, requiring intervention.

While arsenic, cadmium, and lead remained within safe limits, mercury and zinc often exceeded safety standards in biosolids and biochar. Mercury breached limits at 15 biochar sites and nearly all biosolid locations. Chromium and nickel also exceeded limits at sites such as Khairthal.

Biosolids retained high levels of E. coli, Salmonella, and helminth eggs at nearly every site, making them unsuitable for unrestricted use. None of the 25 larger plants met USEPA or WHO pathogen limits for open agricultural use. Even biochar, despite reduced pathogen loads through pyrolysis, exceeded USEPA's Class A limit for helminth eggs at multiple sites.

Researchers recommend improved dissolved-oxygen and carbon-to-nitrogen control for nitrogen removal, chemical precipitation for phosphorus, and reverse osmosis for dissolved-solids issues. For solids, they suggest optimized belt-dryer conditions, pulverizing biosolids to enhance pathogen kill-off, and secure long-term storage with tighter pyrolysis temperature and residence-time control for biochar.