Other uses of biomass, such as tapping the energy potential of wastes and residues, likewise offer sustainable options. Agricultural activities, for example, produce both marketable food crops and large quantities of stalks, cobs, straws and stems that could be used for energy. By products from the timber industry, such as the tops and limbs of trees, sawdust, bark and other scraps of wood, can be collected and used for energy. As well, more than 60% of municipal solid waste is composed of organic materials such as paper, cardboard, wood and food wastes that can be burned to recover their energy rather than merely taking up space in a landfill (once buried, however, organic wastes can also be tapped for their methane gas).
A Danish firm has developed a system that converts pig slurry into water, gas and manure. The system is undergoing tests (January 2000) in central Denmark, processing waste from 20,000 pigs while providing energy to a local electricity works and heating to five farms in addition to providing manure and potable water.