Anaerobic Digestion, Waste-to-Energy and the Hierarchy of Sustainable Waste Management

Contents:

1. The Hierarchy: Follow this link

2. Energy Recovery

2.1 Anaerobic Digestion

2.2 Refuse Derived Fuel (RDF)

2.3 Waste-to-Energy (WTE)

Spittelau Waste-to-Energy Plant, Source: josylein

1. Hierarchy of Sustainable Waste Management, Please follow this link

2. Energy Recovery

Energy requirements of a community can be satiated to some extent by energy recovery from wastes as a better alternative to landfilling. Energy recovery is a method of recovering the chemical energy in MSW. Chemical energy stored in wastes is a fraction of input energy expended in making those materials. Due to the difference in resources (materials/energy) that can be recovered, energy recovery falls below material recovery on the hierarchy of waste management.

2.1 Energy and Material Recovery: Anaerobic Digestion (AD)

Other names: Anaerobic Composting, Biogas, Biomethanation

The USEPA defines Anaerobic Digestion (AD) as a process where microorganisms break down organic materials, such as food scraps, manure and sewage sludge, in the absence of oxygen.  In the context of SWM, anaerobic digestion (also called Anaerobic Composting or Biomethanation) is a method to treat source separated organic waste to recover energy in the form of biogas, and compost in the form of a liquid residual. Biogas consists of methane and carbon dioxide and can be used as fuel or, by using a generator it can be converted to electricity on-site. The liquid slurry can be used as organic fertilizer. The ability to recover energy and compost from organics puts AD above aerobic composting on the hierarchy of waste management.

Similar to aerobic composting, AD needs a feed stream of source separated organic wastes. AD of mixed wastes is not recommended because contaminants in the feed can upset the process. Lack of source separated collection systems, and public awareness and involvement strike off large scale AD from feasible SWM options in India. However, AD on a small scale (called small scale biogas) has emerged as an efficient and decentralized method of renewable energy generation, and waste diversion from landfills. It also reduces green house gas emissions by using methane as an energy source which would otherwise be emitted from landfilling waste.

Refer to Section 5.3 in here to check the conformance of small scale anaerobic digestion in India with the hierarchy of sustainable waste management.

2.2 Energy Recovery: Refuse Derived Fuel (RDF)

Refuse Derived Fuel refers to the segregated high calorific fraction of processed MSW. RDF can be defined as the final product from waste materials which have been processed to fulfill guideline, regulatory or industry specifications mainly to achieve a high calorific value to be useful as secondary/substitute fuels in the solid fuel industry (23). RDF is mainly used as a substitute to coal (a fossil fuel) in high-energy industrial processes like power production, cement kilns, steel manufacturing, etc, where RDF’s use can be optimized to enhance economic performance (23).

The organic fraction (including paper) in RDF is considered to be a bio-fuel and is thus renewable. Since the carbon dioxide released by burning the organic fraction of RDF arises from plant and animal material, the net green house gas (GHG) emissions are zero (Section 4.7 in here ). The overall green house emissions from RDF are however not zero. This is due to carbon emissions from burning the plastics fraction left in RDF. The amount of GHG emissions from RDF depends upon the composition or organics and plastics in the MSW stream it is being processed from. Using RDF prevents GHG emissions from landfills, displaces fossil fuels, and reduces the volume of waste that needs to be landfilled, thus increasing their operating life.

On the hierarchy of waste management, RDF is placed below aerobic composting, as a waste to energy technology. It is a slight variant of the waste-to-energy combustion (WTE) technology, which combusts MSW (processed or as it is) to generate electricity. RDF is different because the objective is to increase the calorific value by processing the fuel.

Refer to Section 5.4 in here to check the conformance of RDF technology in India with the hierarchy of sustainable waste management.

2.3 Energy and Material Recovery: Waste-to-Energy (WTE)

Other names: Energy from Waste (EfW, used in Europe), WTE Combustion

Waste-to-Energy combustion (WTE) is defined as a process of controlled combustion, using an enclosed device to thermally breakdown combustible solid waste to an ash residue that contains little or no combustible material and that produces, electricity, steam or other energy as a result (24). Even though both WTE combustion and RDF combust MSW, the objective of WTE combustion is treating MSW to reduce its volume. Generating energy and electricity only adds value to this process.

As discussed in Section 2.4.1 in here, combusting the organic fraction of MSW (a bio-fuel) and releasing carbon dioxide as the end product is a net zero emissions process (Section 4.7 in here). Due to the dominance of organic waste in MSW, MSW is considered as a bio-fuel which can be replenished by agriculture. Also, bio-fuels are renewable. In India, urban MSW contains as much as 60% organic fraction and 10% paper. Therefore, potentially, 70% of energy from WTE plants is renewable energy. Therefore, WTE is recognized as a renewable energy technology by the Government of India (GOI). Australia, Denmark, Japan, Netherlands and the US also recognize WTE as a renewable energy technology (15).

Thermal waste to energy technologies are the only solutions to handling mixed wastes. In whatever way mixed wastes are treated, the impurities in it will pollute air, water and land resources. By aerobically composting mixed wastes, the heavy metals and other impurities leach into the compost and are distributed through the compost supply chain. In contrast, WTE is a point source pollution control technology, where the impurities in the input mixed waste are captured using extensive pollution control technologies (Table 18) and can be handled separately. The bottom ash from WTE combustion contains nothing but inert inorganic materials and minerals which could be used to make bricks and other construction material. The fly ash from WTE contains pollutants from the input stream and needs to be disposed off in sanitary landfills. By controlling the types of materials fed in to the boiler, European and Japanese WTE plants are known to have achieved nearly zero emissions in the fly ash too.

WTE combustion decreases the volume of wastes by up to 90%. Such reduction in volume would prolong the life of a 20 years landfill to 200 years. However, MSW should be combusted after all possible recycling and composting has been done. The input to WTE plants should be the rejects from material recovery and/or composting facilities. Such an integrated system can decrease the amount of wastes landfilled and prolong the life of landfills further. Therefore, WTE combustion is placed below recycling, aerobic and anaerobic digestion on the hierarchy of sustainable waste management.

Refer to Section 5.5 in here to check the conformance of WTE technology in India with the hierarchy of sustainable waste management.

The Hierarchy of Sustainable Waste Management developed by the Earth Engineering Center, Columbia University