(Zoca et al., 2012). Padua Ferreira et al. (2011) gives a value of 28%. Biodigestability is 70 l/kg (Frederiks,

2012) . Methane capture can be economic using a cheap biodigester and long retention times. The coffee factories can use the methane for electricity production. Coffee farmers can install small digesters and use the methane for cooking.

The residue from grain crops amounts to nearly 3000 million tons per year (Table 13.8). The most important are maize, wheat and rice. A fraction of this is used as animal fodder or animal bedding. Ani­mal fodder turns into manure and bedding becomes a bedding manure mixture. The fraction that is left in the fields can be used for anaerobic digestion. During threshing the straws are deposited in the fields in swaths and can be picked up after a few days. Losses are between 20% and 50%. The costs for collecting the residues are high. Schmaltschinski (2008) estimates 75 V per ton of shredded straw on the truck at the field in Germany. Mo et al. (2011) report a value of 50 V in Poland. Baled straw sells for 140 V in the Netherlands. Substrate costs are 0.30 V/m3 methane for shredded straw and 0.50 V/m3 for baled straw at 100 days reten­tion times (Table 13.8).

SPENT BEDDING

Straw used as animal bedding can be collected and digested. It is then an organic fertilizer. The costs for this are mainly transport costs. The use of spent bedding means that all straw can contribute to methane produc­tion. Barsega et al. (1994) digested cattle bedding from wheat straw.

Spent bedding of rice husks from piggery housings gave less than 20% reduction in VS (Tait et al., 2009). Weathering in a pig shed improved the biodegradability. Bonilla et al. (1985) successfully digested spent poultry litter of rice chaff.

Spent wheat bedding from ducks gave a methane yield of 310 l/kg VS (Buisonje, 2009).

KITCHEN AND GARDEN WASTE

Around 100 kg/person kitchen and garden waste are separately collected in Germany (Kern et al., 2008). VS are around 40%. Extrapolation to the world population would give 150 million tons of VS per year.

AQUATIC WEEDS

Sewage from most urban areas in tropical countries is discharged untreated in rivers and lakes. This leads to eutrofication. Fast growing aquatic weeds use the nutri­ents and form thick mats hindering fishing and naviga­tion. Lake Victoria is a prime example. Biological control of the waterweeds is seen as one method to reduce the problems, but does not address the root cause of

eutrofication. Controlling of one type of waterweed will give others the opportunity to become a pest. There are a number of instances where aquatic weeds are used as an organic fertilizer (Jandl, 2010), but this practice is not widespread.

Aquatic weeds are digested in Luzira prison Kampala, Uganda, (Lindsay et al., 2000) and at the Songhai agricul­tural training centre in Porto Novo Benin (Jandl, 2010).

Harvesting of aquatic weeds is expensive. Antunuassi et al. (2002) calculate a cost of 15,000 V/ha or 0.15 V/kg VS. Veitch (2007) suggest significant cost re­ductions using outboard motor-powered launches with a rake and a land-based backhoe.

Anaerobic digestion of whole plants is not common. There are a number of tests with chopped (10—60 mm) water hyacinth (Table 13.9). Gas yields are high for the experiments of Wolverton et al. (1979), Vaidyanathan et al. (1985), and Almoustapha et al. (2008). Swine manure is not well suited as a seed for water hyacinth digestion as it has little biogas bacteria and this explains the low yield of Moorhead et al. (1993). Duration of the tests by Ofoefule et al. (2009) is too low.

Moorhead et al., 1993 have done tests with ground (1.6 mm) water hyacinth and chopped water hyacinth (12.6 mm), resulting in a 15% lower gas yield for the chopped water hyacinth. The results for the digestion of whole plants will be also lower than for ground water hyacinth.