Biochar Production from Municipal Sludge for Energy Security and Sustainable Agriculture
India faces a severe sewage waste management challenge, with over 60% of sewage waste (sludge) being entirely untreated or inadequately treated. This untreated sludge, rich in organic matter, poses significant environmental and public health risks. It contributes to greenhouse gas emissions and contaminates water bodies, exacerbating urban pollution issues.
In Mumbai for instance, the city generates a staggering 30-40 tons of municipal sludge daily on a dry basis from its STPs. While the Municipal Corporation of Greater Mumbai (MCGM), has established biogas digesters to process this waste, it has led to an unintended challenge—the sludge digestate’s increased moisture content has ballooned the output to 8,000-10,000 tons daily, intensifying disposal difficulties.
This issue is further compounded by using sludge-based fertilizers, which have high contamination of PFAS (per- and polyfluoroalkyl substances). These “forever chemicals” resist breakdown, contaminate soil and water, enter the food chain, and pose severe health risks, such as cancer, immune system disruption, liver damage, and developmental impairments. Municipal sludge-based fertilisers contaminated with PFAS have already contaminated several regions across Europe and the U.S.
Moreover, Municipal corporations face operational, financial, and technical difficulties, including limited sludge disposal infrastructure, high treatment costs, and the need for trained personnel to handle the massive volume of sludge effectively. These issues contribute to increased pollution and environmental degradation, particularly when heavy rains lead to contamination of water bodies.
The Promise of Varhad Gasifier
A promising solution lies in innovative gasifier technology like The Varhad Gasifier which revolutionises sludge management by converting sludge into biochar, heat, and power while addressing the PFAS contamination challenge. This integrated technology reduces waste disposal issues, provides energy security, and supports sustainable agriculture through biochar-based bio-fertilizers.
Biochar and Heat & Energy Production Process
The Varhad Gasifier involves a multi-stage process to convert municipal sludge into biochar and combined heat and power (CHP) while removing PFAS contamination-
1. Pre-treatment
Excess water is removed from the sludge, reducing its moisture content to no more than 20%. Impurities, such as metal fragments and other non-biodegradable materials, are filtered out, ensuring that only suitable sludge enters the gasifier.
2. Heating and Pyrolysis
The treated sludge is heated in the gasifier to temperatures between 650–700°C. During this pyrolysis process, the organic material undergoes thermal decomposition in the absence of oxygen, converting it into two primary outputs:
- Biochar: A carbon-rich byproduct with multiple applications.
- Syngas: A gaseous mixture of hydrogen, carbon monoxide, and methane.
3. Utilization of Syngas
The syngas generated during pyrolysis are captured and utilized to enhance the system’s energy efficiency. It is combusted to produce heat, which powers the gasifier’s operations and supports downstream processes such as:
- Generating steam to drive turbines and produce electricity.
- Supplementing heat requirements for sludge drying or other thermal applications.
This closed-loop system minimizes waste and reduces the reliance on external energy sources, making the process more sustainable and cost-effective.
4. PFAS Removal
The Varhad Gasifier is particularly effective in removing PFAS (per- and poly-fluoroalkyl substances), achieving up to 99% removal through:
- Thermal Decomposition: Breaking down PFAS molecules at temperatures above 600°C.
- Volatilization: Vaporizing PFAS compounds during pyrolysis.
- Adsorption: Trapping residual PFAS in the biochar’s porous structure.
Flue gases from the process are further heated to 950°C, ensuring complete destruction of volatile contaminants, leaving the biochar safe for agricultural and other applications.
5. Heat and Power Generation
Heat captured during pyrolysis and from syngas combustion is converted into steam to generate electricity. This energy can be used to power the gasifier itself, make the sewage treatment plant (STP) energy-independent, and potentially export surplus energy for other municipal needs.
6. Cooling and Packaging
The biochar is cooled to 20°C, stabilized, and packed for distribution. The final product is ready for use in agriculture, soil improvement, water filtration, and other sustainable applications.
This integrated approach ensures optimal use of resources, turning municipal sludge into valuable outputs while addressing critical environmental concerns such as PFAS contamination and energy consumption.
Benefits of Conversion of municipal sludge into biochar/ heat and power:
The Varhad Gasifier offers a comprehensive range of benefits for municipal governments and sewage treatment plant (STP) operators:
- Operational Benefits
- Handles up to 30-40 tons per day, ensuring efficient sludge processing on-site.
- Compact design minimizes space requirements, optimizing utilization at STP facilities.
- Eliminates the need to transport sludge to dumping sites, reducing logistics challenges and costs.
- Energy Independence Benefits
- Produces renewable heat and electricity, reducing reliance on grid power or fossil fuels like diesel.
- Surplus energy can support advanced treatment processes or other municipal applications, promoting operational self-sufficiency.
- Environmental benefits:
- Net-Negative Greenhouse Gas Emissions: The Varhad Gasifier significantly reduces emissions by preventing methane release from sludge decomposition and replacing fossil fuels with renewable energy. Additionally, it enables Carbon Dioxide Removal (CDR) through biochar production and application in soil, achieving a net-negative carbon footprint. This creates Carbon dioxide removal credits when projects are registered with organizations like Carbon Standards International or Puro Earth. For every 3 tons of sludge which goes through pyrolysis can generate 1-2 tons of carbon credits.
- Removal of PFAS contamination: Effectively eliminates up to 99% of PFAS through high-temperature pyrolysis (650-700°C) and advanced flue gas treatment, ensuring biochar is safe for agricultural and environmental use.
- Same-Day, On-Site Sludge Treatment: The gasifier processes sludge at the source, converting it into biochar and combined heat and power (CHP) within the same day. This eliminates the need for transporting waste to landfills, reduces contamination risks, and conserves valuable landfill space.
- Biochar Application Benefits
- Sustainable Agriculture
- Biochar-based fertilizers enhance soil fertility, improve nutrient retention and support healthier crop growth.
- It also increases water retention in soils, a critical benefit for regions, where high water runoff leads to erosion.
- Water Filtration: The produced biochar is highly porous with a surface area between 200-400 m²/g which effectively removes contaminants, serving as a reliable filtration medium in wastewater treatment plants.
- Boosted Biogas Production: Increases microbial activity in digesters, enhancing biogas yield and efficiency in anaerobic digestion systems.
- Financial Benefits
- Generates revenue from the sale of biochar as a soil enhancer and filtration material.
- Earns Carbon Dioxide Removal (CDR) Credits under voluntary carbon markets, enhancing financial sustainability.
- Reduces combined heat and power costs by leveraging renewable energy generated during the process.
The Varhad Gasifier –
A smaller-sized Varhad Gasifier in Norway. Methane is reformed into hydrogen (yellow flame of hydrogen) before burning, which significantly reduces emissions.
Deploying Varhad Gasifier is a Win-Win Situation for STP operators Municipal bodies and citizens.
Salient Features of the Varhad Gasifier are as follows:
- Specifications: The system is designed to process up to 30-40 tons of sludge per day, generating 1.5 MWh of heat and 0.5 MWh of electricity per ton of sludge. The sludge has an energy content of 18 MJ/kg from the digestate, although this value may vary depending on the lipid content. Additionally, the process produces 300 kg of biochar per ton of sludge.
- Flexible Combined Heat and Power: The gasifier allows for flexibility in energy output, providing 1.5 MW of heat, 0.5 MW of electricity, and 300 kg of biochar per ton of sludge. Alternatively, the system can be adjusted to produce 1 MW of electricity and 1 MW of heat, depending on operational requirements. The system’s heat output can be utilized for high-tech thermal hydrolysis and wet air oxidation in wastewater treatment plants, improving overall energy efficiency.
- PFAS Removal: Pyrolysis is performed at temperatures between 650-700°C, removing up to 99% of PFAS and other contaminants from the sludge, making the resulting biochar safe for agricultural use.
- No Need for Syngas Cleaning: The Varhad Gasifier with specialized equipment eliminates the need for syngas cleaning, reducing costs. Syngas is directly heated up to 1,050°C, ensuring cleaner emissions.
- Low Emissions: The gasifier ensures minimal emissions, with methane being reformed into hydrogen before combustion. As a result, handling 3 tons of sludge or producing 1 ton of biochar generates only 400 kg of CO2 emissions.
- Carbon Removal Credits (CORC/CDR): Based on our life cycle assessment (LCA), the Varhad Gasifier generates between 1.3 to 2.1 Carbon dioxide removal credits per ton of biochar produced (per 3 tons of sludge processed), depending on the carbon content. The Varhad Gasifier can be registered for Carbon Dioxide Removal (CDR) credits with Puro Earth or Carbon Standards International.
- Sludge Variability: The Varhad Gasifier is capable of handling sludge with moisture content up to 20%, converting it into combined heat, power, and biochar efficiently, while also adapting to variations in sludge quality.
Deploying Varhad Gasifier is a Win-Win Situation for STP operators Municipal bodies and citizens.
Conclusion:
Converting municipal sludge into biochar (while removing PFAS contamination), heat and power offers a quadruple win: energy security, waste reduction, sustainable agriculture, and climate change mitigation. As the world seeks innovative solutions to address environmental challenges, integrated biochar production is poised to play a critical role in creating a more sustainable future.
Author:
Mr Prasad Dahapute
Founder & Managing Director
Varhad Capital Pvt Ltd
Website: https://varhad.in/
References:
1. “Biochar Production from Municipal Sludge” by International Biochar Initiative
https://biochar-international.org/wp-content/uploads/2023/01/IBI_white_paper_biosolids_biochar_final.pdf
2. “Climate Change Mitigation through Biochar” by IPCC – https://www.ipcc.ch/srccl/chapter/chapter-4/
3. “Biochar: A Review of its Agronomic and Environmental Benefits” by ScienceDirect
https://www.sciencedirect.com/science/article/abs/pii/B9780123855381000032
