Biogas is a gaseous fuel that mostly consists of hydrocarbon methane. Biogas can be recovered from wastewater treatment plants, garbage dumps or other places where biodegradable material exists. It can be produced locally and the emissions are low, with no soot. The energy efficiency compared to diesel is only 20% and currently only small volumes are produced. Buses powered by biogas are carbon neutral. Volvo Buses offers the 9-liter gas-fuelled engine with emission levels that are below Euro V and EEV standards. Biogas can be substituted with natural gas (CNG) which - although a fossil fuel - produces considerably less CO2 than diesel.