Resource detail

Title: Generation of clean and green energy from waste
Organization: Odisha Power Generation Corporation Ltd
Source:Narang R K (Ed). 2009. Environment Best Practices: Industry Initiative. In series “Cleaner in Cheaper, Vol. 6”. New Delhi, India: The Energy and Resources Institute, 285pp.

Background: Odisha Power Generation Corporation Ltd (OPGC) is a joint venture of the Government of Orissa and AES Corporation, USA. The main objective of the OPGC is to establish, operate and maintain large thermal power generating stations with zero harm to people & environment.

Due to increasing population, urbanization, growth in industries, huge amount of solid waste is generated. This waste needs to be effectively managed in a holistic manner so that it does not pollute the environment nor has an adverse effect on human health. Thus in order to manage the domestic solid waste generated from its kitchens, canteens and guest house, OPGC established a biogas plant in its premises.

Initiative: An efficient way of disposing biodegradable waste is by treating it in a biogas plant to generate biogas. Biogas is a smoke free, soot free, non-polluting, clean and green fuel. It comprises mainly of methane as well as other gases such as carbon dioxide, hydrogen sulphide, nitrogen, hydrogen and carbon monoxide in traces. It is produced by anaerobic decomposition in three phases- Hydroysis, Acid formation and Methane formation.

Structure of the Biogas Plant

The biogas plant is a gravity based structure requiring the maintenance of a level difference in various components of the system. It includes the following components:

♦ A mixer and a premix tanks are located on the platform. There is space available for segregating and feeding the waste and operating the mixer. A 7.5HP motor is installed on the top of the tank, with a belt pulley to rotate the mixer blade inside the tank. The premix tanks have a water supply arrangement for generating slurry.

Solar heating system that has a capacity of 250L/day. It comprises a storage tank solar panels and pipelines. Insulation is provided on pipelines to prevent heat loss.

Predigester tank which is an underground cylindrical tank made up of brick and a concrete block is used as a cover. RCC pipes are provided for the movement of the slurry.

Digester comprises of the main tank, dome, partition wall, an inlet pipe, an outlet pipe and gas outlet connection

Manure pits are four underground tanks made of brick masonry.

Process Details

♦ Waste Collection, segregation and conditioning is the first stage in the process. The kitchen waste is transported to landfill sites and is segregated into sections of biodegradable and non biodegradable waste. The biodegradable waste (except paper and raw vegetative matter) is then carried to the biogas plant for processing in the 7.5HP mixer. Paper is shredded and soaked in one of the premix tanks for three to four days which is then passed through the mixture.

Processing of the conditioned waste is the next step where the conditioned waste is transferred to the mixer for mixing with water in 1:1 ratio for 10-15 minutes to form slurry. The pH of the slurry is maintained as 6—7.5 which then flows to one of the premix tanks in the predigester.

After that comes the task of processing in the predigester. The slurry is subjected to thermophilic bacteria and other micro organisms in the predigester. The growth of these micro-organisms is optimum in the temperature range 55-600C. Hot water supplied by the solar heater is mixed with the waste and the pH of the slurry is dropped to 3 in the tank due to formation of various acids.

After this the slurry is processed in the digester tank, where is undergoes anaerobic degradation due to the action of methanogenic bacteria on the cellulosic material of the waste to produce methane. As the gas generates in the main tank, thee is a rise in the dome that reaches a maximum height of 8 feet, holding 35m3 of gas. This gas is transported to end users through a network of pipeline.

In the last stage the undigested lignocellulosic and hemicellulosic materials are passed to manure pits. They get converted into rich manure which is used in agricultural applications.

There are various factors that affect the biogas plant and its productivity. The temperature has to be just right at around 550C. The higher the temperature, faster the bacteria act on the slurry, after which it needs replacing. At lower temperature gas production reduces and stops at 100C. The pH of the predigester has to be maintained at 6.5-7.5 for maintenance of an optimum number of methanogenic bacteria.

Outcome/Benefits: There have been multiple benefits of installing the biogas plant in the OPGC premises. Waste management has been efficiently carried out and about 200 kg of domestic waste is now used as a resource for production of methane. It has resulted in the saving of Rs 1,64,000/year worth of natural resources in the form of LPG. There is no emission of CO2 or any other potent greenhouse gas that would have been released otherwise. In fact rich manure is produced as a result which is used for improving the health of the soil and increasing agricultural productivity. The net savings per annum due to the production of biogas and manure are due to the tune of Rs 1,08,000.

Commercially tested: Yes