Discharging trade effluent

How to use septic tanks, package treatment plants and cesspools


If your business is in an area where no mains sewers are available, you may need to treat your effluent on site before discharging it to surface waters or groundwater. You may need to use a wastewater treatment system such as a package treatment plant or septic tank.

Septic tanks and package treatment plants

Septic tanks provide basic sewage treatment. You should make sure the effluent from your septic tank is further treated - eg by a filter or reed bed - before you dispose of it to a watercourse.

You may be able to dispose of the septic tank effluent by soaking it away to ground, for example by using a herringbone drainage field, if you have sufficient land and the drainage conditions are favourable. Permission to do this will depend on the distance of your septic tank from local groundwater sources and other protected areas. You should contact the Northern Ireland Environment Agency (NIEA) for advice.

Correctly operated and well-maintained package treatment plants produce a higher quality effluent which you may be able to discharge to surface water or groundwater.

You will need to carry out a percolation test if you want to use a soakaway or drainage field to dispose of the effluent from your septic tank or package treatment plant. This will confirm whether the area is suitable for a soakaway and will determine the size of the drainage area you need.

If you plan to install a new system or alter your existing system of sewage disposal, you should discuss your plans at an early stage by contacting NIEA.

Authorisation for your septic tank or package treatment plant

You must have a discharge consent, groundwater authorisation or pollution prevention and control permit from NIEA before you discharge any sewage, effluent or contaminated run-off to surface waters or groundwater. NIEA will not normally give you authorisation to discharge if it is reasonable for you to connect to the public sewer.

You may need a waste management license from NIEA if you treat effluent from another business' site.

If you need to construct a new outfall structure for your discharge you will need consent from DfI Rivers.


A cesspool is a watertight tank with no outlet.

You will need to empty your cesspool regularly to prevent it overflowing. Fit an alarm that will go off when your cesspool is nearly full. This is an emergency back-up and you should check the tank regularly.

Maintaining your cesspool or wastewater treatment system

Follow the manufacturer's operating and maintenance instructions to make sure that your wastewater treatment system operates effectively.

You must use a registered waste carrier to empty your cesspool or to remove sludge from your septic tank or treatment plant - you can search NIEA's registered waste carriers database.

You should make sure that clean surface water, eg from roofs and parking areas, does not enter your treatment system. The extra water will reduce the effective capacity of the system and may flush solids out.