Fixing Mumbai

Fixing Mumbai: Electricity from waste

The discussion on running Mumbai as a sustainable city revolves around two major civic requirements and functions: the collection and disposal of about 9,000 tonnes of waste, and a peak power requirement of about 3,500 MW a day.

Rather than seeing them as two separate issues, the aim of civic authorities should be to find a solution that mitigates both burdens. The solution I proposed here is a model of Waste to Energy (WTE) technology that will attempt to do just this.

These solutions recognise a growing concern: the existing method of waste disposal — dumping tonnes of waste in landfills every day — is hazardous to both the environment and to the health of those living in Mumbai. A caveat too: the successful implementation of this model requires, first and foremost, effective and efficient segregation of waste at source by citizens.

Achieving this will require a system of incentives and disincentives, a carrot-and-stick approach, which I will also attempt to explain in this piece.

The garbage problem

Mumbai generates approximately 9,000 TPD (tonnes per day) of municipal solid waste (MSW), or nearly 0.475 kg per person, according to data provided by the BMC. This is expected to exceed 10,000 tonnes per day by 2025.

Most of this gets transported to three dumping grounds at Deonar, Mulund and Gorai. (A new site is proposed at Kanjur Village to accommodate the increase in waste.) At present, Deonar receives about 4,000 TPD of waste from the eastern suburbs, dumped without any treatment; Mulund receives around 600 TPD from the city and eastern suburbs; and Gorai gets about 1,200 TPD from the western suburbs. These sites are nearing the end of their usable life and carrying capacity.

The recyclable components in this waste are mainly plastic, paper, cartons, polystyrene (what we inaccurately call Thermocol or Styrofoam, which are brand names), glass, rubber, leather, and metals. Some of these are combustible and have a significant calorific value (heat content). After separation of recyclables at source and at secondary storage, around 18.6 per cent remains.

In India, the major portion — around 56 per cent — of urban MSW is organic matter. (The actual percentage of recyclables discarded as waste in India is unknown due to informal picking of waste, which is generally not accounted for.) Organic matter is suitable for aerobic digestion where the waste can be used to create compost, or anaerobic digestion and fermentation under controlled conditions, where it can be diverted to produce biomethane. This, in turn, can be used for electricity generation.

Biogas technology has emerged as a key environmental technology for integrated solid and liquid waste treatment concepts and climate protection in both industrialised and developing countries. With anaerobic conversion or fermentation, over 90 per cent of the energy available in the wastes is retained within the biogas as methane, and the rest is sludge. This process takes place in enclosed reactors called digesters. A digester can turn a tonne of MSW into 100 m³ of biogas for electricity production.

The power problem

Mumbai has a unique power requirement and load curves. The average daily requirement of electricity is about 60 million units (MU). The peak power demand is almost 3,500 MW and average demand is about 3,100 MW. Mumbai’s own generation is about 2,200 MW, produced by two generation utilities in Mumbai city, Reliance Infra (capacity: 500 MW) and Tata Power (1,700 MW), and distributed by them as well as the BEST Undertaking (which has no generation capacity).

Being India’s financial capital, Mumbai’s needs 24/7/365 electricity supply. The supply-demand shortfall is met through external power purchases, which are very costly. Hence, the overall retail tariff for power is very high compared to other Indian cities, which is a very serious concern for citizens.

Setting up Waste-To-Energy plants

The gap between Mumbai’s peak power demand (3,500MW) and generation capacity (2,200 MW) can be bridged with WTE plants. The Waste to Energy (WTE) can be configured in many ways, and the best current option has the following components:

Use the existing landfill site areas to set up a solar water preheating platform to preheat water to 90 degrees Celsius.

Transport this hot water to the power generation plant (offshore).

Set up an anaerobic digester to digest the segregated wet waste for the generation of biomethane (or use the existing matured landfill sites for tapping the generated methane).

Transport and burn this methane in a boiler (part of the offshore power plant) to produce superheated steam from the solar preheated water platform.

Run the steam turbine connected to the alternator to produce electricity to be fed to the grid.

The cost of generating green energy from waste is around Rs. 2/kWh. Energy from WTE plants can partially fulfill the city's peak energy demand. Our daily green energy generation potential is about 3.5 MU. This can save the city the cost of about five per cent of external power it currently purchases. An annual generation of 1,300 MU (million units) of energy can be achieved.

Where will we find the space for WTE plants, considering the extreme paucity of space in Mumbai and the resulting high cost of land?

Assuming the successful implementation of the carrot-and-stick model (described below), effectively segregated municipal waste will be deposited in the existing landfill sites. Their total area of 322 hectares (Deonar 132, Mulund 25, Gorai 24, Kanjur 141) can then be used as solar water-heating platforms in addition to being waste collection centres.

In this proposal, existing landfills are used as part of a solar heating platform. Two WTE power plants can be set up at locations in the sea in the vicinity of the existing landfills, which will make getting boiler water to the WTE plant very easy. And this will not disturb either the existing city infrastructure or the transportation of collected MSW.

Moreover, preheating boiler water by using solar energy will further reduce the overall carbon footprint. Of course, the overall efficiency of the plant can and will be improved as technology advances.

The carrot-and-stick model

Anaerobic digestion is an ideal method for exploitation of electric power from the organic part of MSW. To do this, waste must be segregated first. To achieve this, sorting and separating organic and inorganic waste must be done at the first stage, i.e., where waste is generated. This is why it is important to incentivise segregation at the citizen level.

Several waste segregation initiatives have been attempted, with little success.

What if stakeholders were assigned different tasks and responsibilities? What if financial incentives were to be included with each task? And what if there were some concrete disincentives for defaulters?

For example, if BMC generates electricity from waste, it would be in its interest to encourage segregation. It could do this with a combination of incentives, sharing the benefits with citizens, and penalties.

Each unit of dwelling or commerce — buildings, housing colonies, hotels, restaurants, shops, markets — must register itself with the BMC at local ward offices. These ward offices will then distribute garbage bags — grey for dry waste and green for wet — in proportion to the number of people in each of these units. Every day, everyone must fill their bags correctly. The BMC’s existing network will collect these bags and will also keep a daily record of the number of bags collected. At the end of each month, the total waste collected from each unit colony, as recorded by the local BMC officer, will get converted into credit points for the units — one credit point for every 10 kg of green bag waste from dwelling units and one credit for 15 kg of waste garbage from commercial units. Then, 10 credits would be exchanged for one unit of electricity.

At the same time, those citizens and enterprises who fail to separate their dry and wet waste will be penalised. The garbage of such defaulters will not be collected from their premises. A BMC-authorised officer will warn such defaulters, and then stop collection from them if they fail to comply.

Mumbai can lead the way

Through selling the electricity to the Mumbai grid and providing citizens with credits with which they can pay their electricity bills, our local government can earn profits, which in turn can help them incentivise other primary public services like education, health and transportation within the city.

This case for a sustainable Mumbai has a large implication on policy. The city could be the ideal platform for a WTE project that partially meets peak demand of electricity. Learning from here can help to establish effective similar models across the country. As ever, Mumbai can help, literally and metaphorically, electrify India.

What is the city's waste made of:

*54% Biodegradable wet waste (Food leftover)

*15% Biodegradable dry waste (Wood, cloth etc)

*12% Sand, stone and fine earth particles

*10% Paper, metels,and other reusable waste

*9% Plastic

Mumbai’s garbade pile

*9,000 TPD (tonnes per day) of municipal solid waste (MSW) approximately generated by Mumbai, or around 0.475 kg per person

*10,000 TPD expected increase by 2025

*4,000 TPD Deonar receives waste from the eastern suburbs

*1,200 TPD Gorai gets from the western suburbs

*600 TPD Mulund receives from the city and eastern suburbs

The power scene

*2,200 MW Power generation

*Peak power demand: nearly 3,500 MW

*Average demand: about 3,100 MW

Power producers

*500 MW Reliance Infra

*1,700 MW Tata Power

The gap between peak power demand and generation capacity can be bridged with WTE plants.

City can generate 3.5 million units of green energy generation a day, and save 5% in external power costs.

(Dr. Aniruddha Pandit is a professor of chemical engineering and dean at Institute of Chemical Technology, Matunga. Prof Pandit has authored over 300 publications and five books and has 14 patents to his credit. He is on the editorial board of five international scientific journals. He has guided 38 PhD and 65 master’s students so far. He is a fellow of all the learned academies of India.)

For the complete list of stories in the Fixing Mumbai series, > click here

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Printable version | Jul 18, 2021 9:33:24 PM |

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