Regulating sewage discharges

Where do discharges come from?

Discharges may come from the following:

• Waste Water Treatment Works (WWTWs), commonly known as sewage treatment works, the primary source of Water Utility Sector discharges
• Combined Sewer Overflows (CSOs) cause intermittent discharges from sewers which carry both foul sewage and rainfall run-off water
• Emergency Overflows (EOs) from sewage pumping stations - sewage may be discharged under emergency conditions from a sewage pumping station
• Water Treatment Works (WTWs) where waste waters may arise as a by-product of the drinking water purification processes

Consents are also set for discharges under the Water and Sewerage Services (NI) Order 2006 Section 226/227 to enable Northern Ireland Water (NIW) or their contractors to discharge water from pipes, wells, reservoirs and boreholes while carrying out construction, maintenance or repair works.

The Water Utility Regulation Group keeps public registers of WWTWs and WTWs.

Role of Northern Ireland Water Limited

Since April 1 2007, Northern Ireland Water Limited (NIW) has been responsible for regulating discharges from Waste Water Treatment Works under the Water (Northern Ireland) Order 1999.

Water Order permissions have been issued to over 1100 waste water treatment works, 30 water treatment works, and 300 sewer systems. Every permission granted (known as a consent) outlines the required quality and quantity of the discharge into the water environment. These are drawn-up with a number of factors taken into account, including the requirements of relevant European directives.

NIW are fully responsible for non-compliance and any pollution incidents caused by a failure to properly maintain and operate its water infrastructure. The summary compliance report for the Water Utility Sector in 2013  is now available and contains a compliance assessment of those works operated by NIW and those under public private partnership contracts.

• Summary Compliance Report for the Water Utility Sector 2014 and 2015
• Summary Compliance Report for the Water Utility Sector 2013

Sewer systems

Both Combined Sewer Overflows (CSOs) and Emergency Overflows (EOs) are part of Northern Ireland's sewerage system. Along with NIW, the Water Utility Regulation Group (WURG) is assessing proposed upgrades to the sewerage systems of towns and cities across the region.

This is a long-term programme which addresses the impact of CSOs to complete sewerage systems and is guided by what is known as Urban Pollution Management methodology. Discharge standards for CSOs include minimum flows that the sewer must be capable of before spilling. For EOs, discharge standards relate to measures to be taken to ensure that there is no spillage following a breakdown.

Scientific and technical advice is provided on planning consultation issues in relation to the sewerage infrastructure

Sewage sludge

Where sewage sludge is applied to agricultural land, NIW have to meet the requirements of the Sludge (Use in Agriculture) Regulations (N.I.) 1990 and comply with the Code of Practice for Agricultural Use of Sewage Sludge. The WURG monitors information held by NIW. In practice, however, little sewage sludge is presently being spread on agricultural land.

Waste Water Treatment Works Public Register

The waste water treatment works register was initiated in 1997 for works serving populations greater than 1000. As of 1 January 2006 all works are placed on the public register. Below is a list of general information which you can find in the Waste Water Treatment Works Register:

• works name
• Population Equivalent (PE) served by the works*
• grid reference of the works
• grid reference of the discharge point
• Dry Weather Flow (DWF) from the works in cubic meters per day**
• the receiving water for the discharge

* PE is a term specified by the European Commission (EC) Urban Waste Water Treatment Directive. 1 PE is equivalent to 60 grams of BOD (Biochemical Oxygen Demand) per head per day. In this way the loadings from both domestic and industrial inputs may be equated together. Much of the PE information, however, has been assessed by a variety of other means such as house counts. PE information is supplied by Northern Ireland Water (NIW).

**  For existing works, this is the average daily flow to the sewage works during seven consecutive days without rain (excluding holidays) following seven days in which the rainfall has not exceeded 0.25mm on any day. At present, there is little actual flow data available. Most of the DWF information on the register, therefore, is estimated based on a flow of 250 litres per PE per day.

Water Order consents and compliance

The discharge standard for a works is known as its registered standard. Registered standards (or simply 'standards') normally consist of a numerical standard for BOD and suspended solids (SS) and there may also be an ammonia standard.  The term 'sanitary' is used to describe the BOD, SS and ammonia parameters. In some cases there are no numerical standards for a works and the standard is referred to as 'descriptive'. Currently for the register, these apply to some coastal outfalls and works less than 250 PE.

Numerical standards for sanitary parameters are set so that the works must be compliant for 95% of the time. The reason for setting standards in this way, as opposed to setting limits that should not be exceeded at all, is due to the inherent nature and variability of the sanitary parameters in sewage waste.

Upper tier standards may also be applied in addition to the 95 percentiles. These are limits which should not be exceeded and any breach fails the works for the year. They are usually set at 99.9 percent or as required by the Urban Waste Water Treatment Directive (UWWTD). 

• Waste Water Treatment Works discharges and the EC Urban Waste Water Treatment Directive

Sampling and analysis are carried out by the Water Utility Sector, who employ agreed laboratory quality schemes. Their laboratories and analytical systems are UKAS accredited and hence audited externally annually by UKAS. Sewage works compliance is formally assessed at the end of each year and on an ongoing basis during the year. The Water Utility Sector is asked to account for any problems and detail what action is to be taken. Copies of the correspondence are placed on the Public Register.

Compliance is assessed against what is known as the 'look-up table' which allows a specific number of standard breaches based on the number of samples taken. This table was first introduced by the UWWTD.

The 'look-up' table

Standards are assessed on a parameter basis, in other words, the same number of breaches are allowed for each parameter. For example, if a works with a BOD and SS standard was sampled 52 times per year and exceeded its BOD sample six times but never exceeded its SS standard it would fail for the year. On the other hand, if there were four BOD and four SS breaches, it would pass.

Standards may also include a number of other parameters, depending on requirements, namely:

Nutrients 

nutrient standards are required for works that discharge to areas declared sensitive (in other words, eutrophic - prone to adverse effects through nutrient enrichment) under the UWWTD. Discharges to freshwater sensitive areas will require a total phosphorus standard and discharges to marine sensitive areas will require a total nitrogen standard. This reflects the fact that phosphorus and nitrogen are the limiting nutrients in the respective environments. Compliance with nutrient standards is assessed on an annual average basis as opposed to the 95 per cent compliance for the sanitary parameters. This is because it is more important to control the total load of nutrients entering the receiving waters as opposed to the sanitary parameters for which it is more important to ensure that there are no toxic effects.

Dangerous Substances

These are as defined by the European Union (EU) Directive 'on pollution caused by certain dangerous substances discharged into the aquatic environment of the Community' 76/464/EEC - commonly known as the Dangerous Substances Directive. Standards for dangerous substances are set as maximum allowable concentrations (MACs) which are not to be exceeded. This is an area of work currently under development.

Ultimately all works will have dry weather flow as an integral part of their standard. This is, however, subject to constraints mentioned previously.

Water Treatment Works Public Register

The register currently includes details of 30 works. This will be expanded to include all works in the future. The information available is listed below:

• works name
• grid reference of works
• grid reference of the discharge point
• maximum treatment capacity of the works in megalitres (millions of litres) per day
• volume of discharges*
• the receiving water for the discharge

* the maximum volume of waste that is possible for the works to discharge in a day. This is proportional to the volume of water treated at the works. The maximum volume discharged is calculated as a percentage of the maximum treatment capacity of the works. This depends on the treatment processes employed.

Water Order consents and compliance

The compliance regime is different to that employed for sewage discharges. WTW waste discharges are considered more like industrial process discharges than sewage works discharges since waste is discharged on a more continuous basis, as opposed to the daily flow variation of sewage works discharges. WTW waste discharges may also be intermittent.

Discharge standards for waste from water treatment works will normally consist of numerical standards for pH, suspended solids, aluminium, iron and total available chlorine, although the details will depend on the treatment processes involved. Twelve samples are taken each year and only one sample is allowed to exceed the standard.

Viewing the registers

The WWTW and WTW registers may be viewed by arrangement with Water Management Unit.