Efficient way to turn waste into resource

Published in The Hindu on October 18, 2012

Biogas - BARC

The Nisargruna biogas plant is based on technology developed by the Mumbai-based Bhabha Atomic Research Centre. – Photo: BARC

A staggering amount of waste is generated every day in every town and city, and the local bodies are grappling with logistics for its disposal. The problem arises as the government and individuals fail to see waste as a potential source of energy and agricultural input in the form of manure.

The Bangalore Corporation, which recently made waste segregation mandatory at the household level, is showing the way for the rest of India. It is setting up 12 Nisargruna biogas plants across the city to convert biodegradable waste into methane and organic manure.

The Nisargruna biogas plant is based on technology developed by the Mumbai-based Bhabha Atomic Research Centre (BARC).

Such plants are being constructed at local levels too. Tata Consultancy Services (TCS) is planning to construct 4 tonnes a day capacity biodegradable waste conversion plant at its sprawling centre at Siruseri IT Park near Chennai. It already has similar plants operating at other locations. IIT Madras has already cleared a project for a plant with 1 tonne per day capacity. The Chennai Corporation recently initiated preliminary discussion with BARC.

These are not the first plants coming up in the country. “Such plants have already been installed at 146 locations,” says Dr. S. P. Kale, Head of Technology Transfer & Collaboration Division at BARC. “We have transferred the technology to 100 private entrepreneurs.”

In a broad sense, the principle is similar to that of gobar gas plants, but all comparisons end there — the technology is much more advanced and more methane is produced.

Better bet

“Conventional gobar gas plants have a single digester and produce biogas containing 55-65 per cent of methane and 45-35 per cent of carbon dioxide. But the Nisargruna plants are biphasic (aerobic followed by anaerobic phase) and produce biogas containing 70-80 per cent of methane and only 30-20 per cent of carbon dioxide,” he says.

More the methane produced, the better is the fuel value. One tonne of biodegradable waste contains only 22-24 per cent of solid matter; the rest is water. And 30 per cent of municipal waste is biodegradable waste. So with one tonne of biodegradable waste (containing 220-240 kg of solid material) it will be possible to produce 25-30 kg of methane, about 150 kg of carbon dioxide and 50-60 kg of organic manure.

The methane enriched biogas can be used directly for heating (instead of LPG) or for generating electricity. Dr. Kale underlines the higher levels of efficiency when it is used directly for heating and strongly recommends it.

Best usage

“One metre cube of biogas has 3,500 kilo calories of heat. When this is used directly, and the efficiency of the heater is 70 per cent, about 2,400 Kcal are effectively used. But one metre cube of biogas can produce only 1.5 to 1.8 units of electricity, accounting only for 1,200 to 1,400 Kcal,” he says. “It is two times more efficient to use methane for heating than for generating electricity. It may be used for electricity generation only where thermal use is not possible.”

Constructing a one tonne waste plant will cost Rs.15 lakh and it can be installed in two months. “If a plant runs at full efficiency and if you claim carbon credit, the payback period is two years,” Dr. Kale explains.

Hazardous waste

The best part of the technology is its ability to generate resource even from hazardous biological sludge. Waste from textile, food and chemical plants contain harmful chemicals. When the waste is treated, the biological sludge too turns hazardous as organisms absorb harmful chemicals.

“The volume of hazardous solids can be reduced by 90 per cent,” he stresses. “There are seven such plants already in operation.” The first to come up was in 2010 in Baddi in Himachal Pradesh at Auro Textiles belonging to the Vardhman Group.

He rattles off the details of one plant after another, and it becomes difficult to understand why this technology has not been adopted in a much bigger way across the country. “The concept of processing biodegradable waste is more talked about but urban local bodies are not keen to do it. Nisargruna technology offers a decentralized way of reducing the waste reaching dumping yards,” he says. “The citizens on their part must segregate the waste resources and urban local bodies must make provision to collect these waste resources in a segregated manner. It needs a huge change in perception by the society.”

So what makes Nisargruna biogas plants so very efficient? To start with, unlike a gobar gas plant, the Nisargruna plant has two digesters — aerobic and anaerobic. Aerobic digester has nine species of Bacillus required for breaking down waste resources.

The waste is first pulverised in a mixer before it enters the aerobic digester, where it remains for 3 to 4 days. “The smaller clumps tend to aggregate to form lumps [despite pulverisation]. These lumps of waste are attacked by aerobic bacteria. Air and hot water (using solar panels) at 70 degree C are added. “Hot water is added to accelerate the digestion process,” Dr. Kale notes.

Though the temperature of the waste after adding hot water is 32 degree C to 35 degree C in winter and 42 degree C to 44 degree C in summer, the bacteria survive as they are thermophilic. “They can function in mesophilic condition as well,” he clarifies.

The pulverised water-mixed waste is passed through many compartments so that the bacteria have a better chance to degrade it. What flows out of the aerobic digester to anaerobic digester is almost liquefied homogenous slurry with a reduced pH of 5-5.5. The solid content in the slurry has been reduced from 23 per cent to 10 per cent. “The solid is converted into organic acids and carbon dioxide,” Dr. Kale explains.

Methane bacteria are predominant in the anaerobic digester. As a rule, methane bacteria are slow in their actions, and hence it takes about 15 days for the waste to be degraded in the anaerobic digester.

“But it has been reduced from 40 days [in the case of gobar gas] to 19 days,” he points out. “This is due to the initial aerobic phase.”

All that is left of the waste is methane, carbon dioxide and manure — a rich resource extracted from it.