Guidance

Water companies: operator self monitoring (OSM) environmental permits

Updated 17 March 2026

Applies to England

• Publication for Northern Ireland
• Publication for Scotland
• Publication for Wales

Your permit states if you need to carry out OSM.

This guide sets out the minimum standards you need to meet the requirements of OSM permit conditions. You must develop your own management systems and procedures to meet OSM requirements.

The Environment Agency carries out audits and inspections to assess how effective your systems and procedures are. When we identify activities that could reasonably be improved, we expect you to act on our recommendations. Our main auditing tool is the operator monitoring assessment (OMA).

You must meet the minimum monitoring requirements in this guide, unless agreed with the Environment Agency in writing.

1. Sampling frequency

The normal sampling frequency is either 12 or 24 samples per year. This is based on whether the discharge meets the requirements of the higher (OSM table 1) or lower (OSM table 2) sampling frequency. Your permit includes the sampling frequency as a condition of the permit.

The OSM higher sampling frequency used to be called OSM Opra tier 3. The lower frequency was called OSM Opra tier 2. Opra was used to assess activities’ environmental risk.

When a discharge complies with its discharge quality numeric limits for the previous 12 consecutive months you can follow the reduced sampling frequency. When this change is within a calendar year, you can pro rata the samples for the remainder of the year.

When a discharge fails any permitted discharge quality numeric limit you must return to its normal sampling frequency as soon as reasonably practicable. You must continue at the normal frequency until you have reported 12 consecutive months of numeric compliance.

Numeric compliance includes:

• look-up table (LUT) compliance limits
• maximum and minimum compliance limits, such as upper tier, lower tier and absolute limits
• mean compliance limits
• percentile compliance limits
• comparative compliance limits
• Urban Waste Water Treatment Regulations (UWWTR) numeric compliance limits

It also includes any other discharge quality numeric limits specified within an environmental permit.

The Environment Agency will only agree to you continuing at the reduced rate following a numeric failure in exceptional cases. We must be satisfied that the failure was not caused by the operator.

The Environment agency may agree to sampling frequencies other than those specified in this guide in exceptional circumstances.

1.1 Higher sampling frequency: OSM table 1

The higher sampling frequency applies to any continuous discharge from a wastewater treatment works (WWTW) that requires disinfection. It also applies to discharges from WWTW with numeric effluent quality limits other than oil and grease, biochemical oxygen demand (BOD), suspended solids or pH, and the permitted discharge volume, when assessed in the following order, is greater than 50m³ per day for:

• permitted maximum discharge volume
• 3 x the dry weather flow
• 2.4 x the daily mean flow

1.2 OSM table 1

Parameter	Normal sampling frequency (samples per year)	Reduced sampling frequency (samples per year or pro rata over the remainder of a year)
Sanitary	24	12
Non sanitary	12	12

1.3 Lower sampling frequency: OSM table 2

The lower sampling frequency applies to:

• water treatment works (WTW)
• WWTW with numeric quality limits that are not subject to the higher sampling frequency – OSM table 1

1.4 OSM table 2

Sample frequency thresholds	Parameter	Normal sampling frequency (samples per year)	Reduced sampling frequency (samples per year or pro rata over the remainder of a year)
>5 m3/d (not in table 1) with numeric limits for ammonia, nutrients, hazardous pollutants [see note 1] or UWWTR self monitoring	Sanitary	12	6
>5 m3/d (not in table 1) with numeric limits for ammonia, nutrients, hazardous pollutants [see note 1] or UWWTR self monitoring	Non sanitary	6	6
>20 m3/d (not in table 1) with numeric limits for one or more of BOD, suspended solids pH, temperature or oil and grease only	Sanitary	12	4

Note 1: Hazardous pollutants is used in relation to surface water discharge activities only. For discharges to groundwater the assessment relates to hazardous substances and non-hazardous pollutants.

WTW discharges that meet the volume and numeric quality limit parameters in table 2 but are only permitted to occur in an emergency will not normally have OSM permit requirements. Where emergency discharges have OSM requirements, you may only need to sample when a discharge occurs rather than at the specified frequency.

1.5 Out-of-hours samples

Your sampling must include the required number of out-of-hours samples. Out-of-hours samples are samples taken outside the normal sampling window, which is 9am to 3pm, Monday to Friday.

When your sampling frequency is:

• 24 samples per year – take 2 out-of-hours samples
• 12 samples per year – take 1 out-of-hours sample
• 6 samples per year – take 1 out-of-hours sample every 2 years on average
• 4 samples per year – take 1 out-of-hours sample every 3 years on average

2. Sampling programme

The monitoring programme runs over a full calendar year. You must schedule the days of sampling for each discharge before the start of the following calendar year.

You must make your monitoring programme, and any changes to it, available to the Environment Agency if we ask for it.

Samples for each permitted discharge activity will be spot samples. You must collect samples at regular and random intervals. Regular and random means approximately equal intervals during the year and includes samples from different days of the week and at different times.

You must analyse samples for each water quality parameter that has a numeric limit specified within the permit for the discharge. This does not include any List 1 substances that are included within your permit in a general standards table attached to the permit.

3. Routine reporting

You must report all results from your OSM programme to the Environment Agency. This must be in an electronic format that we have defined.

Send each result as soon as reasonably practicable – this must be at least quarterly. You must report monitoring results within 28 days of the end of each quarter.

3.1 Less than results

Less than results, which have passed laboratory quality control, are reported and recorded as results. They are counted as compliant with numeric limits such as LUT and maximum limits. They are counted as zero in mean compliance assessments.

You must report the result in your routine reporting. Investigate why any less than face value result higher than a numeric limit has been reported. Where practicable, put measures in place to prevent a recurrence.

3.2 Greater than results

Greater than results, which have passed laboratory quality control, with a face value higher than a permit numeric limit are breaches of maximum limits or exceedances of LUT limits. Greater than results are counted as the face value in mean compliance assessments.

You must notify us of any breach of numeric limit or LUT exceedance.

Greater than results are compliant with the limit when the face value is lower than a permit numeric limit.

You must report the result in your routine reporting. Investigate why any greater than face value result has been reported. Where practicable, put measures in place to prevent a recurrence.

3.3 Report failures and exceedances

You must report all numeric limit failures or exceedances, which have passed normal laboratory validation procedures, to us as soon as is reasonably practicable. This should normally be within one working day of the analytical result becoming available. You, or others carrying out OSM activities on your behalf, must not delay notifying us to carry out additional validation or checks.

3.4 Look-up table reporting

The number of allowable exceedances is based on the number of samples taken in the 12-month period. The 12-month period ends on the last day of the month in which the exceedance occurred. If the number of exceedances for the parameter is more than the maximum number allowed, then the discharge has failed the LUT limit for that parameter. The failure is recorded as happening on the date of the most recent exceedance.

3.5 Look-up table

Number of samples taken in any period of 12 months	Maximum number of samples permitted to exceed for given parameter
4 to 7	1
8 to 16	2
17 to 28	3

A discharge will not become non-compliant due to a change in sample frequency alone. LUT compliance is only assessed by looking back 12 months when an individual LUT exceedance occurs. It’s not assessed on a routine monthly basis or whenever a sample is taken.

4. Reschedule missed samples and parameters

A missed sample is any sample that cannot be reported due to a:

• missed event
• lost sample
• failure of laboratory quality control

If you miss a pre-scheduled sample event you must record the reason and reschedule as soon as possible. Missed out-of-hours sample events must be rescheduled to be taken out of hours. Report that the sample was missed in your annual summary.

A missed parameter is where one or more parameter of a pre-scheduled sample cannot be reported. For example, due to:

• a broken bottle
• failure of laboratory quality control

If the result of any parameter cannot be reported you must record the reason and reschedule the sample for that parameter as soon as possible. You must report the results for other parameters as well. Report that the parameter was missed in your annual summary.

5. No-flow sample events

Permits for discharges subject to OSM require the discharge to be made from a specified outlet. The discharge must pass through a specified monitoring point from which a representative pre-scheduled OSM sample can be collected.

A pre-scheduled sample event may coincide with a time of day or time of year when:

• there is no flow
• the flow is so low that a representative sample cannot be collected from the specified monitoring point

This occurs frequently at WTW due to infrequent and intermittent process discharges. But it can also occur at some WWTW due to the:

• size of works
• type of treatment process
• seasonal conditions in the catchment

Some examples of the causes of no-flow events are:

• pronounced daily flow patterns and short-duration discharges at small WWTW
• lower seasonal catchment flows
• batch treatment WWTW that discharge intermittently
• dual discharge WWTW with 2 sample points, where only one point will discharge at any time, never both at the same time
• flow-banded WWTW with 2 sample points, where both discharge only during seasonally high catchment flows

5.1 Measures to prevent no-flow sample events

You must have procedures in place that take account of site-specific discharge regimes such as those listed previously. This is so you can complete sample collection successfully while minimising operational staff’s prior knowledge of the sampling events.

You must collect samples on the pre-scheduled dates or as close to the original pre-scheduled time as practicable.

5.2 No-flow sample events at WTW

If an OSM sample is not available during a pre-scheduled sample visit to a WTW, you must record and retain evidence of this.

Acceptable evidence includes:

• 2 digital photos (stamped with date, time and location) showing no sample is available at the sample point upon arrival and 15 minutes later
• flow monitoring data, if available and appropriate

You must provide this evidence to the Environment Agency if they ask for it.

You do not need to reschedule the sample. You should complete the process by uploading the no-flow claim code for the site using the ‘generic operator returns’ (GOR) system. The sample will be reported as compliant for that sample event and count toward the OSM sample programme fulfilment.

5.3 No-flow events at WWTW

If you miss, or cannot confirm compliance from, a programmed WWTW OSM sample or individual parameter due to no-flow, you must:

• notify the Environment Agency within 7 working days of the original sample attempt by completing your no-flow tracker, stating the root cause of the no-flow sample event – including small WwTW with population equivalent equal to or less than 250 (PE<250) and flow-banded sites
• reschedule the missed sample or parameter to be taken as soon as practicable
• add all reschedule details to your no-flow tracker within one month of the original OSM sample attempt

If you collect samples but cannot report a result for a parameter, then you must only reschedule that parameter, not the whole sampling event.

You must take the number of samples required in a calendar year in that same calendar year. If you have missing samples or missing results for any parameters in a calendar year, then it will be reported as failing any relevant permit condition.

If you miss WWTW OSM samples due to a no-flow event, you must reschedule at least twice before you can make a no-flow claim. The exception to this is small WWTW, PE<250 with numeric limits and flow-banded WWTW. You do not have to reschedule for these after the first sample attempt. But you should reschedule beyond the minimum of 2 for all other sites.

5.4 WWTW no-flow claims

You can apply for a no-flow claim by completing the form on the water company SharePoint website and uploading your supporting evidence.

The Environment Agency will notify you of acceptance or rejection within 14 working days from the date of submission.

Minimum evidence for a no-flow claim

You must upload at least 2 digital photos (stamped with date, time and location) that show:

• no sample was available at the OSM sample point on arrival 
• no sample was available at the OSM sample point at least 15 minutes after arrival

You must also send an explanation of the root cause of the no-flow event that your claim is based on.

Optional evidence for a no-flow claim

You can also send:

• MCERTS or non-MCERTS flow data illustrating the no-flow
• any relevant additional text or photos to justify the claim

If your no-flow claim is accepted

No further reschedule is required. The sample is reported as compliant for that sample event and counts toward the OSM sample programme fulfilment.

You must then complete the no-flow claim by uploading the no-flow claim code for the site for that month using the GOR system.

If your no-flow claim is rejected

You will need to reschedule the sample.

5.5 Exceptional circumstance sites

There are WWTW where repeated no-flow sample events occur for specified periods in the OSM sampling year because there will be no discharge.

Examples include:  

• dual discharge sites, where there will be no-flow sample events throughout the year from one of the 2 OSM sample points
• WWTW that have been decommissioned and will not discharge, having been replaced by a sewage pumping station or combined sewer overflow
• water industry national environment programme (WINEP) capital schemes
• other planned major capital works with an Environment Agency-agreed compliance action plan in place
• seasonal discharge only, active during specified months in OSM sample year

The Environment Agency aims to exclude exceptional circumstance sites from the normal no-flow procedure because they produce ‘false’ no-flow events. You do not need to include exceptional circumstance sites on your no-flow tracker.

5.6 Exceptional circumstance process

You must apply for an annual exceptional circumstance claim covering the period of no discharge for a site at the start of the OSM sampling year.  

You must complete the form for an exceptional circumstance claim on the water company SharePoint website and upload the relevant evidence.

The Environment Agency will accept or reject the exceptional circumstance claim within 14 working days from the date of submission. That is unless additional evidence is required.

If additional evidence is required, the clock stops until the day after that additional evidence has been uploaded. At that point the clock resets and restarts.

The Environment Agency will notify you when they accept or reject an exceptional circumstance claim.

If your exceptional circumstance claim is accepted

You should then complete the required number of data uploads for the no discharge period using the no-flow claim code. This will equate to a compliant sample pass for each expected OSM sample and count towards the annual OSM sample programme fulfilment.   

If your exceptional circumstance claim is rejected

You must follow the WWTW no-flow procedure for the site.

6. Challenge an OSM result

If you think a result should be discarded and the sample rescheduled, you must present your reasons to the Environment Agency. This includes results notified to us as a numeric limit exceedance or failure. We will decide whether or not the result stands.

You must provide a challenge to a result within 10 working days of notifying us of a result. We will respond within 10 working days of receiving the challenge. These timings can be extended by mutual agreement.

You must increase to the normal sampling frequency as soon as possible after reporting the failure. You must not wait for the outcome of investigations into the sample result. You can only continue on reduced frequency sampling if we agree to it in writing.

7. Monitoring schedule visibility

You must make sure the OSM programme is not visible to operational staff responsible for discharge performance. There might be very exceptional circumstances where this is not possible, for example due to unavoidable health and safety considerations. When operational staff need to be aware of a sampling event you must notify them as late as is reasonably practicable.

You can make an efficiency saving by combining OSM sampling events at a WWTW with its UWWTR monitoring programme. If you carry out combined monitoring you must meet the requirements of each monitoring programme. Read the guidance for monitoring at WWTW. The dates of the UWWTR monitoring must be unknown to operators on site, so you must have procedures for this. If you cannot meet all the requirements of combined programmes you must carry out the monitoring programmes separately.

8. Quality management system (QMS)

You must have a QMS that covers all aspects of OSM. This includes the:

• pre-scheduling of the sampling programme
• sampling
• transport and storage of samples
• analysis of samples
• reporting to the Environment Agency

The QMS is part of your wider management system that relates to your permitted activities.

Your QMS, or the QMS of an organisation carrying out OSM for you, must contain procedures to make sure you:

• become aware of a numeric limit failure or exceedance as soon as possible after a sample has been taken
• report the failure or exceedance to us, in the specified electronic format, as soon as reasonably practicable – normally within one working day of the analytical result becoming available

Your QMS must define responsibilities and accountabilities up to a senior management level.

A QMS makes sure OSM is operated as independently as possible without interference from within the business.

You must make sure all actions and activities carried out to fulfil the requirements of OSM are appropriately recorded. You must give copies to us when we ask for them.

9. Keep records

Records can be paper or electronic.

You must:

• keep records for 6 years from the date they’re made
• make records as soon as reasonably practicable
• make sure records are legible

If you amend records, you must do this to allow both the original and amendments to be retrieved.

10. Monitoring certification scheme: MCERTS

For all OSM sampling and analysis you must use organisations that have accreditation to the MCERTS standard: performance standard for organisations undertaking sampling and chemical testing of water (Part 1). MCERTS is the Environment Agency’s monitoring certification scheme.

11. OSM carried out by another organisation

You can get another organisation to carry out OSM for you. You must set out in your own QMS how you will meet the OSM requirements through the other organisation. The other organisation must also have their own QMS to ensure that OSM is delivered effectively on your behalf.

Organisations providing an OSM service must have the appropriate accreditation for the activities they carry out. The MCERTS rules apply to all organisations involved in OSM.

12. Sample at higher frequencies

This guide sets out the OSM minimum sampling frequencies. If you want to sample at a higher frequency it will cost you more and take up more resources. Do not set higher sampling frequencies unless you have good operational reasons.

You might choose to sample at a higher frequency when:

• you carry out process control or performance monitoring and it’s easier for you to pre-schedule all samples as OSM samples rather than some as pre-scheduled process monitoring samples
• the required sampling frequency for a discharge will change within a calendar year and it’s easier to pre-schedule the samples at the higher frequency for the full year rather than change during the year

OSM sampling must always be pre-scheduled at one of these frequencies: 4, 6, 12, 24 or 48 samples per year. You must reschedule any missed samples and record the reason.

13. Annual summary reports

You must provide a summary report within 2 months of the end of each calendar year.

Your annual OSM summary report must include a list of discharges where:

• there were missed samples or individual parameters
• there were rescheduled samples or individual parameters
• the sample frequency changed

You must provide the information in the format specified by the Environment Agency.

13.1 Missed samples table

Discharge	Number of samples required	Number of samples taken	Date of missed sample or parameter	Reason for missed sample or parameter
WTW or WWTW	12	11	DD/MM/YY	Laboratory error
WTW or WWTW	12	11	DD/MM/YY	Vehicle breakdown

This does not include missed samples or parameters that were rescheduled and taken in the calendar year. Include these in the rescheduled samples table.

13.2 Rescheduled samples table

Discharge	Date of original sample	Rescheduled date	Reason for missed sample or parameter
WTW or WWTW	DD/MM/YY	DD/MM/YY	Laboratory error
WTW or WWTW	DD/MM/YY	DD/MM/YY	Vehicle breakdown

13.3 Changes to sample frequency table

Discharge	Date of change	Previous sample frequency	New sample frequency	Reason for change
WTW or WWTW	DD/MM/YY	12	24	LUT failure on DD/MM/YY

14. Additional monitoring

If the Environment Agency needs more information about a discharge, for example where the discharge might be causing or contributing to an environmental standard failure, we will work with you to investigate. We might take additional samples ourselves, or we might make agreements with you to take additional samples for us.

15. 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.