NIEA – Guidance on restarting private sewage treatment plants following a period of shutdown or low flows

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Sewage treatment plants use biological processes to remove pollutants from sewage effluent before it is discharged into receiving waters. If the biological treatment process has been adversely affected by low or no flow entering the sewage treatment plant then when sewage flows increase the sewage treatment plant will not be able to effectively treat the sewage effluent before it is discharged, potentially for a number of weeks.

The restart of sewage treatment plants which have either had no flows or very low flows following shutdowns due to COVID-19 will result in discharges of poor quality effluent to receiving waters unless steps are taken by operators to prime the treatment process, minimise the shock it receives when flows return and to ensure that once back up running it continues to work effectively.


Discharges of treated sewage effluent to surface waters from private sewage treatment plants in Northern Ireland are authorised by Consents to Discharge issued by the Northern Ireland Environment Agency, under the Water Order (NI) 1999, as set out at:

Sewage treatment plants use living microorganisms in biological treatment processes to remove pollutants from sewage effluent before it is discharged to a receiving water, these processes can't be simply switched on and off. The sudden restart of a sewage treatment plant or an increase of flows into it following a relaxation of COVID-19 business shutdown rules will very likely result in the discharge of poor quality effluent unless steps are taken by operators of sewage treatment plants to prepare them for increased flows beforehand.

Operators who discharge poor quality effluent risk being in breach of their discharge if their discharges cause pollution. Operators should therefore take steps to ensure that their treatment plant is able to operate effectively as flows into it increase after their businesses reopen.

Some operators may be used to operating their sewage treatment plants on a start/stop or variable flow and load rate basis, for example those who run campsites and tourist attractions. These operators should have procedures which they can follow, allowing them to increase flows to their sewage treatment plant whilst remaining compliant with their consent.

Other operators of sewage treatment plants may not be used to restarting them following a period of very low or no sewage flows. Where an operator doesn't already have procedures in place to ensure their sewage treatment plant becomes operational quickly and with minimal harmful effects on the receiving waters, they will need to seek technical advice and support. This may be provided by:

  • whoever normally maintains their sewage treatment plant;
  • the manufacturer or supplier of their treatment plant;
  • a competent sewage treatment plant maintenance engineer.

Operators can also refer to the British Water website for further guidance.

British Water have published specific advice on restarting packaged wastewater treatment plants:

As well as ensuring that their sewage treatment plant is ready to receive increased flows, operators must also check that any pre-treatment equipment such as fats, oils and grease (FOG) traps are ready to be used again.

Operators should also avoid sending excessive amounts of chemicals and cleaning products to their sewage treatment plant as they can inhibit and harm biological treatment processes. They should also ensure any contaminated drainage from washing and cleaning does not drain to surface water sewers, water courses or groundwater or cause pollution.

Where operators need to dispose of waste beer stored on their premises they must not pour it down drains that lead to their sewage treatment plant or directly to surface water or to ground. This is because beer has a high biochemical oxygen demand (BOD) and will overwhelm a treatment plant resulting in a high strength non-compliant polluting discharge.

Following restart, operators must continue to check that their treatment plant is operating effectively and observe the impact of their discharge on its receiving surface water where it is safe to do so. Where they observe, or become aware that their discharge is causing pollution of a receiving surface water or groundwater, they should notify their usual regulatory contact or call the NIEA 24-hour incident hotline number - 0800 80 70 60.

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