The power of poo
Efficiency drives, waste minimisation and reducing greenhouse gas emissions + rising oil prices = power from the strangest places. Element holds its nose and investigates how sewage and other waste could offset the equivalent of more than the electricity
We all make biogas, but we usually blame the dog. It comes from any decomposing wet organic waste. It is mainly methane and carbon dioxide, as well as very small amounts of hydrogen sulphide and other impurities.
The methane is more of a worry than the smell. If released into the atmosphere it has a 20 times greater greenhouse effect than carbon dioxide. But biogas can be collected to create natural gas-type fuels for homes and vehicles. When used as a fuel the methane is converted into the less harmful carbon dioxide. It’s a win-win situation: we get fuel and we reduce the potential for climate change. But it’s not without its limitations.
Wastewater and sewage treatment plants generate biogas as part of normal operation, so they are a great place to harness it. The latest pilot plant doing this in New Zealand is Rotorua District Council’s Wastewater Treatment Plant, and is part of the Crown Research Institute Scion’s Waste 2 Gold programme. As well as harnessing the biogas for fuel, it uses the resulting heat to break the solid waste down into re-useable chemicals and a range of other by-products. Some of these can even be used for fertilisers or in the production of bioplastics.
The Rotorua pilot is initially only set to run for 12 months. If it works, the next stage would be a full-scale facility diverting as much as possible of the 8,500 tonnes of biosolid waste – sewage, food scraps – from the current treatment plant. The project team believes savings from converting the waste to energy, other reduced plant operating costs and income from selling the new products, could create total benefits for the council and community of about $4 million a year. Not bad for a pile of…
This includes not having to spend $900,000 on burying solid sewage waste in landfill. However, the landfills also have a positive as the organic waste buried there also decomposes and produces biogas. Councils in Hamilton, Christchurch, Dunedin, North Shore, Palmerston North and Porirua all operate biogas harnessing systems at landfill sites. For example, Mighty River Power operate four in New Zealand: Greenmount and Rosedale, which together produce 8.2 megawatts of electricity, and Tirohia and Silverstream, which produce two megawatts each.
Producing biogas from waste produced at industrial food processing plants is another possibility. You can even grow crops, including most animal fodder as well as sorghum, specifically to process into fuels in this way, although this is not yet done in New Zealand.
Currently, biogas systems within New Zealand only produce a negligible amount of electricity. However it is most valuable when used to supplement or offset the heat or electricity needs of the host plants in which it is made.
Brian Cox, from the Bioenergy Association of New Zealand, reckons the industry will continue to develop along these lines, as a smart way to get value from waste, rather than a key energy provider to the nation. He also argues that “Biogas’s best value will be in being used as a vehicle fuel, but this is only really efficient when it is used at source, for example, by the trucks operating at or from a landfill that has a biogas digester. Anything like a national distribution network for biogas would be too costly.
Even so, the association estimates that New Zealand could be producing up to 15 petajoules of energy from biogas per year by 2040, or even double that if rising oil prices or supply problems stimulate the growing of “fuel crops” to convert to biogas.
This lower figure is equivalent to one and a quarter times the amount of electricity used in the greater Christchurch area and would help reduce our reliance on imported oil or the temptation to mine the South Island’s lignite coal to convert to diesel.
The Watercare wastewater treatment plant, Mangere.