AD plants are 24-hour operations and as such they need to be fed regularly. Pumps and other machinery also need to be maintained to ensure production is not interrupted.
There can be noise, dust, and if there are leaks the potential of smells and environmental contamination. However, these issues are strictly controlled by environmental regulations, so should not occur. The liquid part of the digestate contains nitrates and other chemicals which should not be released to water but which can safely be spread to land or processed for wider use.
The use of biogas also releases CO2, which is a greenhouse gas. However, this is offset because the biogas produced in AD replaces fossil fuels when it is used for heat, power or transport. If the waste were landfilled it would naturally rot and release methane, a potent greenhouse gas.