The UNEP- Working Group for Cleaner Production in the Food Industry, based at the University of Queensland carried out the study on behalf of Sustainable Energy Development Authority (SEDA). SEDA was set up by the NSW government in 1996 to reduce the level of greenhouse gas emissions in NSW. The organisation is not an energy producer - it offers financial incentives and technical expertise to those investing in the commercialisation and use of sustainable energy technologies.
The study examined the potential for co-generation using wet wastes from animal farming and processing wastes and food and beverage processing wastes. The technologies from cogeneration are mature and include pyrolysis, anaerobic digestion, gasification and direct combustion. The energy produced can be used in a variety of ways, including replacement of other fuels in boilers or water heating systems, as a fuel for space heating or for electricity generation. Depending on the use, these technologies are economically beneficial.
Schemes for using wet wastes are common in Europe. Poultry litter fuels two 12.5 MW power stations in the UK. One of these at Glanford uses about 130,000 tons/year of poultry litter from poultry farms within a 50 kin radius. In the cooler climates of Europe, North America and Latin America, the biogas generated from animal waste is used to heat animal housings. In Denmark, there are already 19 single farm plants and 23 combined plants with several more under construction.
In addition to electricity generation, using bioenergy sources will
have other benefits, such as providing process heat and disposing of waste
products in an environmentally acceptable manner. Added benefits include
increased investment and job generation now.
Potential Energy Sources
Wet wastes from intensive livestock industries (piggeries, dairies,
feedlots and poultry farms) are animal manure, generated in an effluent,
slurry. or semi solid form. Animal manures are an excellent source of nutrients
and organic carbon and are typically applied to land as a soil additive
and fertiliser.
Processing activities that operate on a large scale, such as abattoirs, poultry processing, dairy products processing brewing and starch processing are most likely to benefit economically from methane recovery by anaerobic digestion.
Biological sludges generated from sewage treatment plants also have energy recovery potential and methane recovery and use is already occurring at those sites where it is economically viable.
Case Studies
Six companies representing five different industry sectors were used
as case studies to discover if recovery from wet wastes is as viable as
it is in other countries. The companies were:
The companies' interest in becoming involved in the project has stemmed from a desire to find ways to reduce waste treatment and disposal costs and in some instances to alleviate environmental problems. Some companies were aware of the role that greenhouse credits may play in the future, but greenhouse considerations were not a major consideration.
Due to the high water content of all the waste streams, the only energy recovery application found to be suitable was anaerobic digestion to produce biogas. Therefore all the case studies explore the various methods of anaerobically digesting the wet waste streams and using the biogas.
Of the case study sites, Banter Poultry processing plant and Tooheys Brewery have greatest methane and energy generation potential due to the high organic loads that are available.
Each of the sites has different uses for the energy recovered. Parkville Piggery can provide space heating in the farrowing and weaning sheds or generate electricity for the rest of the site's electricity needs.
Scone Fresh Meats can displace about 16% of LPG used m the existing boiler Mudgee Abattoir can generate electricity to power the aerators in the aerobic treatment lagoon.
AMH Abattoir can generate electricity to power the aerators in the aerobic treatment lagoon.
Bartter can displace 25-35% of natural gas used in the existing boiler.
Streets can displace about 25% of natural gas used in the existing boiler or generate electricity to fully power the sites effluent treatment system.
Tooheys Brewery can displace natural gas in existing boiler or generate electricity.
The cost of energy varies considerably depending on deals that the companies are able to negotiate with energy suppliers. In general the case study sites pay between 5-8¢/kWh for electricity about $6/GJ for natural gas and $10/GJ for LPG.
Economic Returns
The economic viability of wet waste to energy opportunities is very
variable and highly dependent on sitespecific issues. Economic viability
assessments therefore need to be undertaken on a site-by-site basis.
All of the sites, except for the piggery can expect to benefit from at least one of the options proposed. Generally speaking, for sites that have existing gas fired boilers, the greatest return on investment is achieved by co-firing the biogas with the primary gas supply (natural gas of LPG). This is because the infrastructure for using the biogas is already in place and generally only a small additional expenditure is required. Electricity generation on the other hand generally requires additional capital expenditure in the form of a generator set and associated equipment.
For Parkville Piggery none of the options could be justified on economic grounds. Since the site is not a large energy user, it is not possible to take full advantage of all of the available energy, therefore energy savings are quite small. This means that the savings are too small to justify the significant investment that would be required.
One of the options assessed for the Streets site could provide a good return on investment (2 years). This return does not result from energy savings alone however, but from other savings that result from improved effluent treatment efficiency. These include reduced sewer charges, reduced operating costs for the effluent treatment system and reduced sludge disposal costs. This is also the case for the Tooheys site where the return on investment is achieved through significantly reduced sewer charges as well as through energy savings.
The AMH abattoir has the advantage of having biogas generation and collection infrastructure already in place. By using the biogas for electricity to run the aerators in their aerobic treatment pond, the project could be expected to pay for itself in four years.
Greenhouse Benefits
The largest greenhouse benefits are obtained for the Scone Fresh Meats
and Parkville Piggery sites. These large reductions in CO equivalent emissions
are a result of avoiding the discharge of methane to atmosphere from their
existing uncovered anaerobic lagoons. Methane is estimated to have about
21 times the greenhouse impact of CO and by collecting and combusting the
methane to produce CO substantial greenhouse benefits are possible. Obviously
other sites that do not currently emit methane would not reduce greenhouse
emission to the same extent.