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New Zealand has a relatively large primary agricultural industry with many opportunities for producing biogas from animal manure. Potential methane resource generated from piggery waste alone at farm scale could potentially provide up to 0.05PJ (14G Wh) a year, equivalent to the amount of electricity used by around 1,700 houses (EECA 2005). There is also further potential for biogas from the poultry and dairy industry in New Zealand. The potential production of biogas and energy output from various fresh feedstocks in New Zealand has not yet been evaluated. The yield of biogas, however, varies significantly from one feedstock to another due to the percentage of dry matter (or solids) and, in turn, to the percentage of volatile solids within the feedstock. However, given New Zealand has an estimated cattle population of > 5 million cattle, the total amount of waste that can be converted to produce biogas energy is enormous. Therefore it would be prudent to explore opportunities for biogas production and its utilization for energy supply especially within the rural sectors in New Zealand.

The production and use of biogas as an alternative fuel in many developing countries is fast growing, with India currently leading the way. Biogas digesters to produce methane for cooking, lighting and heating have become widespread in China and India following progressive national policies to aid their adoption (Ministry of Agriculture 2000, 2001; Somashekhar et al. 2000; Wenhua 2001). Animal waste and plant material are added to the digester to produce methane, and the remaining sediment, which is high in organic matter, is returned to the soil. There are 8.48 million biogas digesters in China, up from 4.5 million in 1990, with projections for an additional million per year for 2010 (Shuhong 1998; Ministry of Agriculture 2000).

In India, there are some 2.5 million family-sized biogas plants, constructed by the National Project on Biogas Development, plus another 500 larger community biogas systems (Ravindranath and Ramakrishna 1997; Shukla 1998; Somashekhar et al. 2000). Shukla (1998) estimated that nationwide there are 60% are functioning well (1.5 million units). Depending on the size of these domestic digesters, and their efficiency and productive period during the year, each can save the annual combustion of 1.5-4.0 tonnes of fuelwood, equivalent to an avoided emission of 0.75-2.0 tC per digester (Shuhong 1998; Shukla 1998; Wenhua 2001).

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