The Central Pollution Control Board (CPCB) has issued guidelines on the production process and environmental management of Waste-to-Energy (WtE) plants that sheds light on the complex operations required to process municipal solid waste (MSW) and generate power. The guidelines detail the integrated system used in a typical WtE facility and highlight key environmental challenges that must be carefully managed to ensure public health and regulatory compliance.
The WtE Production Process
The process begins with waste reception, where MSW is delivered in closed compactors and unloaded into an enclosed storage pit. To manage odor, the pit is kept under negative air pressure, and the extracted air is directed to the boiler for combustion at over 950°C. Herbal disinfectants are also sprayed to control odors and flies.
From the storage pit, the waste moves to a pre-processing system for manual and mechanical segregation. This is a critical step, as unsegregated waste is typically heterogeneous. Specialized machinery, including trommels (for size-based separation), magnetic separators (for ferrous metals), and shredders, divides the waste into three main fractions:
- Compostable Organics (8-10%): This fraction, deemed unsuitable for direct combustion due to high moisture, is processed into compost using methods like open windrow composting.
- Refuse Derived Fuel (RDF) (65-70%): The high-calorific fraction is directed to the incinerator.
- Inerts and Recyclables: These are sent to engineered landfills or authorized recycling facilities.
The RDF is then fed into a moving grate furnace where it undergoes combustion at temperatures of 950–1100°C. The heat generated produces high-pressure steam, which is fed to a turbine to generate electricity.
Environmental Challenges and Management
The guidelines cover four key environmental issues related to WtE plants: stack emissions, solid residue, leachate, and odor.
- Boiler Stack Emissions: The flue gas contains pollutants such as Particulate Matter (PM), Oxides of Nitrogen (NOx), Sulphur Dioxide (SO₂), and Dioxins & Furans. While the analysis of four WtE plants shows they can comply with most standards under the Solid Waste Management (SWM) Rules, 2016, Dioxin & Furan emissions remain a primary concern. Air Pollution Control Devices (APCDs), including bag filters and systems for injecting alkaline reagents and activated carbon, are installed to treat these gases before they are released through a tall stack.
- Solid Residue: Incineration produces bottom ash (20-30% by weight) and fly ash (2-3% by weight). The report’s analysis found that while bottom ash from all four plants met heavy metal standards, fly ash in three of the four plants exceeded the stipulated norms. This highlights the need for strict testing as per SWM Rules before any beneficial reuse of ash in applications like construction.
- Leachate Management: Leachate, a contaminated liquid from waste piles, poses a threat to groundwater. To manage this, WtE plants are equipped with impervious flooring, drainage systems, and a dedicated Leachate Treatment Plant (LTP). Advanced technologies like Multi-Effect Evaporators (MEE) and Mechanical Vapour Recompression Evaporators (MVRE) are used to concentrate pollutants and achieve Zero Liquid Discharge (ZLD), with the treated water being reused in the plant.
- Odor Control: Odor, primarily from the decomposition of organic waste, is managed by maintaining a negative air pressure system in the storage pit and directing the odorous air to the boiler for high-temperature combustion. Periodical spraying of herbal disinfectants further controls flies and odor.