Guidance

Waste water treatment works: treatment monitoring and compliance limits

Updated 17 January 2019

The Environment Agency regulates waste water treatment works (WWTW) by assessing the quality of the waste water they discharge against set compliance limits.

The level of treatment and monitoring that’s required is based on the population the WWTW serves and where the sewage is discharged.

This guide covers discharges to surface waters and to groundwater.

You must also read the guide for site-specific quality numeric permit limits.

Population equivalent compliance

The level of treatment and monitoring that a WWTW must provide depends on the population equivalent (PE) of the ‘agglomeration’ it serves.

An agglomeration is an area where the population and economic activities are sufficiently concentrated for urban waste water collection. The waste water is then taken for treatment to a WWTW or to a final discharge point.

When areas have permanent residential areas and a PE higher than thresholds specified in this guide, the WWTW must:

• meet the appropriate level of treatment
• be subject to the required monitoring and reporting
• meet the specified numeric compliance limits

The PE must represent the highest biochemical oxygen demand (BOD) load that enters the WWTW. It must include any trade and tourist PE.

The Environment Agency will normally accept the PE of WWTW estimated by water companies for Ofwat reporting purposes. However, if the PE is close to a relevant Urban Waste Water Treatment Regulations (UWWTR) PE threshold you may calculate the PE in the following way:

The maximum average weekly load entering the treatment plant during the year, where 60g of BOD is equivalent to 1 person per day.

You may decide to calculate the PE in this way, or we may ask you to.

Treatment requirements

You must use ‘appropriate treatment’ for discharges from agglomerations of:

• less than 2,000 PE to freshwaters (including groundwater) or estuaries
• less than 10,000 PE to coastal waters

Appropriate treatment is the treatment of urban waste water by any process or disposal system which, after discharge, allows the receiving waters to meet the relevant quality objectives and the relevant provisions of the UWWTR and other relevant regulations.

The conditions in your environmental permit will make sure the relevant quality objectives in the receiving water are met.

Secondary treatment

You must use secondary treatment on all discharges from agglomerations of:

• more than 2,000 PE to freshwaters (including groundwater) or estuaries
• more than 10,000 PE to coastal water

Secondary treatment is the treatment of urban waste water by a process generally involving biological treatment with a secondary settlement.

The secondary treatment process must meet the relevant look-up table (LUT) compliance limits and maximum compliance limits for BOD and chemical oxygen demand (COD).

Advanced treatment

You must use more advanced tertiary treatment for agglomerations of more than 10,000 PE in sensitive areas and their catchments where the discharge contributes significantly to nutrient load.

Sensitive areas requirements

The following water bodies are known as sensitive areas:

• freshwater bodies, estuaries and coastal waters which are eutrophic (meaning rich in nutrients, supporting a dense plant or algae population) or which may become eutrophic if protective action is not taken
• nitrate sensitive areas – surface freshwaters meant for the abstraction of drinking water that contain or are likely to contain more than 50 mg/l of nitrates
• areas where further treatment is necessary to comply with related EU directives such as the bathing water directive

The Environment Agency will tell you if your WWTW is within a sensitive area.

We check discharges that contribute a significant load to eutrophic sensitive areas for compliance with annual average and percentage reduction for phosphorus or nitrogen.

For discharges to other sensitive area water body types your permit may require optimised treatment processes to meet requirements of other regulations and the relevant water quality objectives.

You must have nutrient removal in place within 7 years of a eutrophic sensitive area being designated. For discharges into existing sensitive areas, nutrient removal must be in place when the PE crosses the relevant threshold.

Industrial waste water discharges

Certain industries discharging biodegradable waste water directly to receiving waters or to ground from plants representing 4,000 PE or more must not cause a failure of water quality standards. Environmental permits will include conditions to make sure receiving waters meet these standards. For example, discharge specific effluent quality numeric limits for parameters such as BOD and ammoniacal nitrogen.

These industries are:

• milk processing
• manufacture of fruit and vegetable products
• manufacture and bottling of soft drinks
• potato processing
• meat
• breweries
• production of alcohol or alcoholic beverages
• animal feed manufacture – from plant products
• gelatine and glue manufacture – from animal hides, skin and bones
• malt houses
• fish processing

Monitor discharges

You must monitor discharges of treated sewage effluent from WWTW serving agglomerations above set PE thresholds.

You can carry out the monitoring yourself or you can get an MCERTS-accredited organisation to carry out the monitoring for you. MCERTS is the Environment Agency’s monitoring certification scheme.

You must analyse the composite samples for these parameters:

• biochemical oxygen demand (BOD-ATU)
• chemical oxygen demand (COD)

You will also sometimes need to analyse samples for:

• total phosphorus (P)
• total nitrogen (N)

Total nitrogen is the sum of total nitrogen Kjeldahl nitrogen (organic N + NH₃), nitrate (NO₃)-nitrogen and nitrite (NO₂)-nitrogen.

The analysis must be done by a laboratory with appropriate accreditation such as the MCERTS performance standard for organisations undertaking sampling and chemical testing of water (part 1).

Population equivalent thresholds for analytical parameters

You must carry out certain analyses on discharges depending on the PE and the type of water body it’s discharged to.

Freshwaters and estuarine waters

The performance analyses you must carry out on sewage discharges to freshwaters and estuarine waters:

• normal water body with PE more than 2,000 – analyse for BOD and COD
• sensitive water body with PE more than 10,000 – analyse for BOD and COD, P and N

Coastal waters

The performance analyses you must carry out on sewage discharges to coastal waters:

• normal water body with PE more than 10,000 – analyse for BOD and COD
• sensitive water body with PE more than 10,000 – analyse for BOD and COD, P and N

Sampling frequency

The frequency of sampling for a discharge is based on the WWTW PE.

PE 2,000 to 9,999

Take at least 12 samples in the first year of sampling and 4 samples in other years.

However, you must take at least 12 samples in a calendar year if any of the following occur in the previous calendar year:

• failure of a numeric limit
• an individual LUT exceedance of a limit
• failure to collect and report all results for the number of samples required

These limits are set out in this guide.

You must take weekend samples:

• once a year where you take 12 samples per year
• once every 3 years where you take 4 samples per year

PE 10,000 to 49,999

Take at least 12 samples every year. One of these samples must be taken at a weekend.

PE 50,000 and greater

Take at least 24 samples every year. At least 2, but no more than 3, of these samples must be taken at a weekend.

How to take samples

Samples are usually taken in pairs – influent and effluent. Paired samples must always be taken on the same day to calculate percentage reduction through the treatment process.

You only have to collect an inlet sample if you need to calculate the percentage reduction compliance.

Take the samples at the locations specified in your permit using automatic samplers. Inlet sample points must be representative of the sewage entering the WWTW by being upstream of any sludge returns or treatment processes.

Discharge sample points must be representative of the treated sewage effluent discharge. If there are multiple outlets or other complex discharge arrangements, you may need to agree your sample point locations with the Environment Agency before they can be specified in your permit. There may be combined sample points before pipes split, or each outlet may be sampled separately with mass balance calculations used to determine overall compliance.

Samples are taken as 24-hour composite samples so the sample point must allow a representative mixed sample to be taken at all times.

The sampling equipment must be MCERTS certified, with temperature control. The automatic sampler should be certified for UWWTR purposes and to an appropriate lift height. As a minimum you must follow the manufacturer’s maintenance and cleaning procedures.

Store samples

Refrigerate samples in automatic sampling devices, including during retention time after sampling, intermediate depot fridges and during transportation.

Maintain the storage environment between 1°C and 8°C, with the average temperature over the storage period not exceeding 5°C. An organisation carrying out sampling will have appropriate procedures for demonstrating this. It’s recognised that some time may be required to bring the sample temperature to within this range. No ice should form on the sample.

The temperature range is to allow for the cycling of the refrigeration devices, their opening and closing during normal operation and effects of adding a number of warm samples. For most analytical purposes, you should keep the samples at a constant temperature of not more than 5°C.

Sampling programme

You must give the Environment Agency your annual pre-scheduled sampling programme for each relevant WWTW discharge before the end of November each year. You may be able to get written agreement to provide it on a quarterly basis.

Samples must be pre-programmed and collected at regular and randomised intervals throughout the year. Regular and randomised means approximately equal intervals during the year and includes samples from different days of the week.

Sample events that end on Saturday or Sunday are counted as weekend samples and must account for 10% of the overall programme.

Missed samples

If you miss, or cannot confirm compliance from, a programmed sample or individual parameter you must:

• notify the Environment Agency within 1 working day
• reprogramme the missed sample or parameter to be taken as soon as practicable

Where samples are collected but a result for a parameter cannot be reported, then only that parameter must be rescheduled, not the whole sampling event. For pre-scheduled paired samples, you must reschedule both the influent and effluent individual parameter or the whole sampling event.

You can only report a less than or greater than result for BOD or COD if sampling allows LUT compliance to be assessed. If it is not possible to assess whether the LUT limits have been met then it is a missed parameter and must be rescheduled to be taken as soon as practicable.

The number of samples required in a calendar year must be taken in that calendar year. If you have missing samples or missing results for any parameters in a calendar year then it will be reported as failing the UWWTR and any relevant permit condition.

Sampling programme annual submission format

Use a spreadsheet format with these column headers:

• WWTW name
• unique reference number (URN) – Environment Agency or operator
• population equivalent (method confirmed)
• sample frequency
• parameters (BOD, COD, N, P)
• % reduction and concentration or concentration only
• UWWTR monitoring threshold crossed since last submission (up or down, PE or monitoring under sensitive areas designation taking effect)
• number of samples Monday
• number of samples Tuesday
• number of samples Wednesday
• number of samples Thursday
• number of samples Friday
• number of samples weekend
• dates in January
• dates in February
• dates in March
• dates in April
• dates in May
• dates in June
• dates in July
• dates in August
• dates in September
• dates in October
• dates in November
• dates in December

Use a separate line for each WWTW.

Include the PE for all WWTW.

Clearly show any changes that affect the UWWTR monitoring of a WWTW compared to the previous year’s monitoring.

Crossing a population equivalent threshold

You must be aware of factors such as planned developments that will affect the PE of agglomerations served by your WWTW.

If your annual pre-scheduled sampling programme shows the PE will increase above a relevant threshold then you must begin the appropriate level of BOD, COD, P and N monitoring from 1 January.

If your annual pre-scheduled sampling programme shows the PE will drop below a relevant threshold then you can amend the sampling programme for that WWTW from 1 January to reflect the change.

If you are required to begin UWWTR monitoring or your monitoring requirements change you must apply to change your permit to include the new monitoring requirements. You do not need to change your permit if it already specifies your UWWTR monitoring requirements by reference to the PE thresholds.

If you cross any PE thresholds during the year you must report it to the Environment Agency in November as part of your annual submission. By 1 January the following year you will need to:

• provide the required level of treatment
• comply with relevant quality limits
• carry out the relevant UWWTR monitoring

Submit results

You must submit the monthly results from your sampling programme to the Environment Agency. You must do this within 28 days of the end of each month.

Results must be in the format we have specified.

Analytical results for each sample event should be reported to at least 3 significant figures where it is practical to do so.

Compliance assessment

The Environment Agency checks that samples comply with the UWWTR. The compliance checks are done in a variety of ways. Some of the ways a discharge can be non-compliant include:

• not providing treatment by a statutory deadline
• not reporting the required number of samples
• failing LUT compliance limits for BOD and COD
• failing maximum compliance limits for BOD and COD
• failing compliance limits for P and N

Provide treatment by a statutory deadline

If a WWTW does not have the level of treatment required by the UWWTR then it’s reported as failing the UWWTR and any relevant permit condition.

For example, if a WWTW does not have the required tertiary nutrient removal following the designation of a sensitive area (eutrophic) by a specified date.

Report the required number of samples

The discharge fails the UWWTR and its permit condition if you do not submit the required number of discharge results for each parameter (BOD, COD and where needed P and N) in a calendar year.

LUT compliance limits for BOD and COD

Parameter	Concentration limit	Minimum percentage reduction (in relation to the load of the influent)
Biochemical oxygen demand (BOD5 at 20°C) without nitrification. This parameter can be replaced by another parameter: total organic carbon or total oxygen demand, if a relationship can be established between BOD5 and the substitute parameter.	25 mg/l O2	70 - 90
Chemical oxygen demand (COD)	125 mg/l O2	75

If only final effluent sample results are available for each sampling event then these are compared against the BOD and COD concentration limits. A sample passes if it’s less than or equal to the concentration limit.

Where both influent and effluent sample results are available for each sampling event they’re compared against both the concentration and the minimum percentage reduction limits. A sample passes if either of the following is true:

• the sample result is less than or equal to the concentration limit
• the percentage reduction value exceeds or is equal to the minimum percentage reduction limit

At the end of the calendar year, the number of samples exceeding the limit for each parameter are counted. The number of exceedances allowed in a calendar year are listed in the look-up table.

Look-up table

Number of samples taken in the calendar year	Maximum number of samples permitted to exceed for given parameter
4 - 7	1
8 - 16	2
17 - 28	3

If the number of recorded LUT exceedances is greater than the number allowed by the LUT then the BOD or COD fails. The discharge is reported as failing the UWWTR and its permit limit for that parameter.

You must notify the Environment Agency of all BOD or COD LUT exceedances as soon as is reasonably practicable.

Maximum compliance limits for BOD and COD

Parameter	Maximum permitted concentration
Biochemical oxygen demand (BOD5 at 20°C) without nitrification	50 mg/l O2
Chemical oxygen demand (COD)	250 mg/l O2

Maximum compliance is only assessed on BOD or COD effluent sample results that exceed the limits in the Look-up table (LUT) compliance limits for BOD and COD.

If the sample result of a parameter reported as a LUT exceedance is greater than the maximum concentration limit then the discharge fails its maximum permit limit for that parameter.

You must notify the Environment Agency of all maximum limit failures as soon as is reasonably practicable.

If a sample has a result greater than 50 mg/l BOD or 250 mg/l COD but meets the minimum percentage reduction requirement for that parameter then it’s not a LUT exceedance or a maximum fail. However when the discharge has such a high concentration that it may cause a significant adverse environmental effect you may need to notify the Environment Agency.

Annual mean compliance limits for phosphorus and nitrogen

Discharges to drinking water sensitive areas will have site-specific total nitrogen limits. We set these limits on a site-specific basis and they will be detailed in the permit. Compliance against these limits is assessed using an optimistic estimate of the mean rather than face value.

Use this table to find the nutrient limits for WWTW that are subject to eutrophic sensitive area nutrient removal requirements.

Parameter	Concentration limit (annual mean)	Minimum percentage reduction in relation to the load of the influent (annual mean)
Total phosphorus	2 mg/l P (10,000 - 100,000 PE) 1 mg/l P (>100,000 PE)	80
Total nitrogen (the sum of total nitrogen Kjeldahl nitrogen (organic N + NH3), nitrate (NO3)-nitrogen and nitrite (NO2)-nitrogen)	15 mg/l N (10,000 - 100,000 PE) 10 mg/l N (>100,000 PE)	70 - 80

Compliance for P and N is assessed on a complete calendar year’s data.

Less than results are used in the calculations at face value (ignoring the qualifier). Report annual average results to at least 3 significant figures or 2 decimal places.

If only final effluent sample results are available for the calendar year, then these are averaged and compared against the P or N mean concentration limits. If the face value annual average concentration is greater than that of the mean concentration limit then the discharge fails the UWWTR and its permit limit for that parameter.

If both influent and effluent sample results are available for the calendar year, then the annual average of the concentration results and the annual average of the percentage reduction for the individual sampling events are calculated. These averages are compared against both the concentration and the minimum percentage reduction limits.

The discharge fails the permit limit for a parameter if both the face value average of the:

• concentration results is greater than the mean limit
• individual percentage reduction results is less than the minimum percentage removal required

Each individual percentage reduction (for the relevant parameter) is calculated from paired influent and effluent samples taken on the same day. The annual average percentage reduction is the average of each individual percentage reduction. If one or more influent samples are missing, then it’s not possible to calculate the annual average percentage reduction.

Abnormal operating conditions

A WWTW is under normal operating conditions except during:

• circumstances described in the Environmental Permitting Regulations 2016 Regulation 40(1) or paragraph 6(5) of Schedule 21
• a variation of the permit authorising the discharge activity, for example during capital works construction
• unusual weather conditions

You can claim that abnormal operating conditions had an adverse impact on the operation of a treatment process when a sample was taken.

You must submit all claims in writing to the Environment Agency within 14 days of becoming aware that an emission limit has been exceeded. For nutrient parameters you can submit a claim for a result you consider was high due to abnormal operating conditions.

Your claim must include a full description of the abnormal operating conditions and their impact on the operation of the works. Provide us with evidence to support your claim such as:

• photographs
• temperature records
• process monitoring records

We will assess the evidence you provide to support your claim. If we agree, then the non-compliant result is counted as a compliant result.

In annual average assessments, sample results taken during abnormal operating conditions are counted as collected but not used in the annual average assessment.

You must take all practicable steps after an adverse operational effect to prevent or reduce the risk of it recurring. If you do not, we may not accept that a recurrence is outside of your control.

Unusual weather conditions

Unusual weather conditions include:

• low ambient temperatures, or the freezing of equipment in the works
• significant snow deposits affecting operation
• substantial tidal or river flooding of the site
• weather-related failures of the electricity distribution system causing loss of power that could not have reasonably been prevented by standby generation facilities

Heavy rainfall alone is not an adverse operational effect. Your treatment system should be designed to cope with this.

You must continue sampling during unusual weather, where possible.

If you cannot collect a pre-scheduled sample, for example because the monitoring point is flooded, you must record evidence of this and reschedule the sample.

Contact the Environment Agency

General enquiries

National Customer Contact Centre
PO Box 544
Rotherham
S60 1BY

Email enquiries@environment-agency.gov.uk

Telephone 03708 506 506

Telephone from outside the UK (Monday to Friday, 8am to 6pm GMT) +44 (0) 114 282 5312

Monday to Friday, 8am to 6pm.