Rajasthan's Biochar Plants Show Promise Amid Mercury and Pathogen Risks: Report

A new study of 39 faecal sludge treatment plants across Rajasthan by the Centre for Science and Environment (CSE) found that a technology designed to turn human waste into clean water, energy and farm-ready biochar is largely working but is falling short on key safety benchmarks, including pathogen destruction and heavy-metal contamination.

The study evaluated plants commissioned in 2023-24 and operated by MVR Technology, ranging in treatment capacity from 10 to 35 kilolitres a day, spread across central and eastern Rajasthan.

Most Households Rely on Septic Tanks Rather Than Sewer Networks

According to the report, Rajasthan has officially achieved open-defecation-free status under the Swachh Bharat Mission, but roughly seven in ten urban households still rely on septic tanks rather than sewer networks, generating nearly 3,870 kilolitres of faecal sludge daily that existing infrastructure only partly treats. 

That gap has pushed the state toward plants using "pyrolysis-based Omni Processor" technology, which separates solid and liquid waste through a screw press, treats the liquid in a bioreactor, and heats the solids in a low-oxygen furnace to produce biochar, a stable, carbon-rich material that locks carbon in soil for centuries while serving as organic fertilizer.

Treatment Side of the Process Worked At Most Sites

The report found the treatment side of the process working effectively at most sites. Outlet pH stayed within the required 5.5-9.0 band across all plants, and suspended-solids removal topped 90% at many facilities, meeting national discharge limits. 

Organic pollutant removal exceeded 95% at most sites, with the majority discharging effluent that met even the strictest national standards. Disinfection using UV treatment also kept faecal coliform levels in the treated liquid within permissible limits at most locations.

The biochar and biosolids produced also showed nutrient value: both maintained pH and salinity levels compliant with India's Fertilizer Control Order standards for organic manure, contained more than 14 per cent organic carbon, and had combined nitrogen-phosphorus-potassium content exceeding the regulatory minimum, making them, workable substitutes for chemical fertilizer.

The report said plants at Kishangarh Renwal and Khairthal showed nitrogen well above limits, while phosphorus exceeded caps at several sites, including Kishangarh Bas. A few plants, such as Lakheri and Losal, showed unusually high dissolved-solids levels needing direct intervention.

Arsenic, cadmium, and lead stayed within safe limits, but mercury and zinc frequently exceeded fertilizer-safety standards in both biosolids and biochar,  mercury alone 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, unfit for unrestricted use. The report showed none of the 25 larger plants met USEPA or WHO pathogen limits for open agricultural use. Even biochar, despite pyrolysis cutting pathogen loads sharply, still exceeded USEPA's Class A limit for helminth eggs at multiple sites.

Recommendations

The researchers proposed better dissolved-oxygen and carbon-to-nitrogen control for nitrogen removal, chemical precipitation for phosphorus, and reverse osmosis for dissolved-solids problems. For solids, they recommend optimized belt-dryer conditions, pulverizing biosolids to aid pathogen kill-off, and secure long-term storage and tighter pyrolysis temperature and residence-time control for biochar.