WWTW and WTW Public Registers
The sewage treatment works register was initiated in 1997 for works serving population equivalents greater than 1000. An initial batch of around 25 works in the 250-999 Population Equivalent (PE) bracket was added in April 1999 with the bulk of the remainder added in 2001. As of 1 January 2006 all works were placed on the public register. Below is listed the information available.
General Information:
- Works name
- Population Equivalent (PE) served by the works - 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 presently available PE information, however, had been assessed by a variety of other means such as house counts. PE information is supplied by Northern Ireland Water (NIW)
- Grid reference of the works
- Grid reference of the discharge point
- The Dry Weather Flow (DWF) from the works in cubic meters per day - for existing works this may be defined as 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; this matter is being actively addressed by us and NIW. Most of the DWF information on the register, therefore, is estimated and is based on a flow of 250 litres per PE per day. It must be emphasised that such figures are only rough estimates
- The receiving water for the discharge.
Water Order Consents and Compliance
The discharge standard for a works is known as its registered standard. Registered standards (hereafter referred to simply as '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 on a 95 percentile compliance basis which means 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 effluent. However, what are known as upper tier standards may also be applied in addition to 95 percentiles. These are limits which should not be exceeded and any exceedence fails the works for the year. They are usually set at the 99.9 percentile limit or as required by the Urban Waster Water Treatment Directive (Waste Water Treatment Works Discharges and the EC Urban Waste Water Treatment Directive(.PDF 18Kb)
. There will be some works for which this extra level of control is not required and for compliance purposes, the 95 percentile standard is the regulatory standard.
Compliance monitoring is carried out under an agreed self monitoring programme, depending on the size of the works. The sampling and analysis is carried out by the Water Utility Sector, who employ agreed laboratory quality schemes, and their laboratories and analytical systems are audited by us.
Compliance is assessed by this group against what is known as the 'look-up table' which allows a specific number of standard exceedences based on the number of samples taken. This table has been statistically derived to assess 95 percentile compliance and was first introduced by the Urban Waster Water Treatment Directive.
The 'look-up' table:
Samples taken in any year |
Maximum permitted number of |
|---|---|
4-7 |
1 |
8-16 |
2 |
17-28 |
3 |
29-40 |
4 |
41-53 |
5 |
54-67 |
6 |
It can be seen, therefore, that if 52 samples are taken in any year that five are allowed to exceed the standard. Standards are assessed on a parameter basis, i.e. the same number of exceedences 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 6 times but never exceeded its SS standard it would fail for the year. On the other hand, if there were 4 BOD and 4 SS exceedences 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 (i.e. eutrophic - prone to adverse effects through nutrient enrichment) under the Urban Waste Water Treatment Directive. 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 percentile 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.
Compliance
Sewage works compliance is formally assessed at the end of each year. It is however, assessed on an ongoing basis during the year and NIW are informed when a works is liable to fail, has already failed or in the case of what is considered an extreme monitoring result. NIW are asked for the reasons for the problems and what action is to be taken. Copies of the correspondence are placed on the Public Register at NIEA WMU, 17 Antrim Road, Lisburn, BT28 3AL.
Regulation Summary Report 1997 - 2000(.PDF11.5Kb)
Regulation of Water Service Discharges Report 2001(.PDF 688Kb)
Regulation of Water Service Discharges Report 2002-2003(.PDF 1.18Mb)
Regulation of Water Service Discharges Report 2004(.PDF 1.27Mb)
Regulation of Water Service Discharges Report 2006(.PDF 880Kb)
WWTW Discharges and the EC Urban Waste Water Treatment Directive(.PDF 1.27Mb)
Viewing the WWTW Public Register: The register may be viewed by arrangement at NIEA Lisburn. Contact details are given at the end of this page.
Water Treatment Works (WTW) Public Register
The register currently includes details of 30 works. This will be expanded to include all works in the future. Below is listed the information available.
General Information:
- 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 maximum volume of effluent that is possible for the works to discharge in a day. The volume of effluent discharged 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 with this being dependent on the treatment processes employed
- The receiving water for the discharge.
Water Order Consents and Compliance
Discharge standards for water treatment works effluents 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.
WTW effluent discharges would be considered more like industrial process discharges than sewage works discharges in that effluent is discharged on a more continuous basis, as opposed to the diurnal flow variation of sewage works discharges. WTW effluent discharges may also be intermittent (i.e. not discharging all the time). For these reasons the compliance regime is different to that employed for sewage discharges. Twelve samples are taken each year and only one sample is allowed to exceed the standard. It makes no difference whether just one or all of the individual parameters in the standard exceed their limits in the 'failing' sample.
Viewing the Water Treatment Works Public Register:
The register may be viewed by arrangement at NIEA WMU Lisburn.
Contact details for viewing the public registers:
| NIEA Water Management Unit 17 Antrim Road Lisburn BT28 3AL |
John McNally Tel: 028 9263 3426 Email: john.mcnally@doeni.gov.uk |
