Compliance rests on three things: the system being registered with the local authority, being operated and maintained so it does not cause a risk to human health or the environment, and — where inspected — having satisfied the inspector or completed any works required by an advisory notice.
Owners of domestic wastewater treatment systems have a duty to ensure the system does not cause a risk to human health or the environment, and to maintain it properly. Local authorities carry out inspections under the EPA's National Inspection Plan, and where a system fails, an advisory notice sets out the works required and a timeframe.
There is no single 'compliance certificate' for an existing system. For older systems installed before current standards, compliance is judged on performance and risk rather than on whether the installation matches today's design guidance.