Sewage, Sondes and Section 82

By the end of 2023, EDMs were installed at 100% of storm overflows in England. Now, WaSCs must tackle compliance with Section 82, which requires continuous monitoring of the water quality upstream and downstream of all storm overflow and sewage treatment works discharges. This will require the rollout of water quality measuring sondes that have multiple sensors for simultaneously measuring a number of water quality parameters in situ.

As stated in the technical guidance for the Continuous Water Quality Monitoring Programme the objectives of the programme are to: 

• Quantify the local water quality impacts of sewerage undertaker assets on a watercourse,
• Increase stakeholder and public understanding of the impact on water quality of discharges from sewerage undertaker assets,
• Inform sewerage undertaker improvement programmes to meet the Storm Overflow Discharge Reduction Plan targets; and,
• Inform regulatory action.

Ultimately, the continuous water quality programme will provide a widescale insight into the impact of humans on the environment that we have never had before. However, the implementation of Section 82 comes with several challenges, which will be discussed throughout this article.

Site Challenges

Establishing the most appropriate locations for the sondes is a key challenge. WaSCs will need to find sites where a sonde can be securely installed in the watercourse that has even flow, sufficient depth (any watercourse with a permanent depth below 4cm is exempt) and is clear of significant debris. For each discharge, this must be within 500m of the point of cross-sectional mixing, where pollutants or substances introduced into the flow are evenly distributed across the entire cross-section of the watercourse. 

Even if in a remote location, each sonde site must have reliable power (likely solar and battery operated), have good network signal and meet health and safety requirements for personnel.  Crucially, sites must be legally accessible, but landowners may be reluctant to allow long-term installation of monitors on their property, especially since maintenance visits are required at least once a month.

With roughly 27,500 discharges to be monitored, reducing the number of monitors required is practical for cost and maintenance requirements. Section 82 allows for clustering; where discharge outlets are within 250m of one another they may be clustered and monitored by one pair of monitors. If not clustered, monitors can serve as dual purpose for individual discharges, with one monitor reading conditions downstream of one discharge and upstream of another.

The two key parameters for water quality are: 

1. Dissolved Oxygen (DO):

• What it indicates: How much oxygen is available in the water for fish and other aquatic life.
• What to look out for: Low DO levels can harm aquatic organisms, especially if it drops below about 5 mg/L. Very low levels (near 0 mg/L) can cause fish kills.

2. Ammonium and Ammonia:

• What it indicates: Ammonium (NH4) is a form of ammonia present in water, mostly when the water is cooler and more acidic. It’s less harmful than the unionised ammonia (NH3) form, which is toxic to aquatic life.
• What to look out for: High levels of ammonium (NH4) can still affect water quality and aquatic life. If pH or temperature increase, ammonium can change into the more toxic ammonia (NH3) form. 

The perception may be that a significant proportion of changes in water quality are due to sewage discharges, and it will be challenging to isolate and quantify the impact of water company discharges amongst other recorded pollution sources. Sewer models can be utilised to understand the behaviour of overflows and support sonde data analysis, but this is not accessible for the public. Instead, EDM data can viewed in conjunction with sonde data to indicate whether a sonde response correlates with a WaSC discharge, although this correlation alone does not definitively confirm the true source of impact. 

Looking ahead, there is potential for the legislation to expand beyond sewage pollution which could help broaden public awareness of the various contributors to river pollution aside from WaSC discharges. It could be straightforward to monitor additional point sources such as industrial and highway discharges. However, it will be a challenge to plan monitors that could isolate the impact from diffuse pollution sources such as agricultural and urban runoff.  

Aside from the site challenges that also affect WaSC discharges, it is also important to consider who would bear the costs of installing and maintaining these sondes. In some cases, pollution could be caused by a small business (e.g. local farmer or factory owner) who may not be able to fund the significant investment required for implementation and maintenance of a water quality sonde. Some pollution sources may lack a clear or identifiable stakeholder, making it difficult to assign responsibility. Without extensive coverage of sondes throughout England’s watercourses, we cannot accurately assess the true extent of water quality impacts.

Sonde Calibration, Data Reliability and FIS Assessment

Whilst SOAF is being used to highlight the assets which need priority investment, longer term, this data will likely be used by regulators and WaSCs to identify locations requiring additional water quality investigations and schemes to meet the SODRP target.  

The sondes are resource-intensive equipment, requiring frequent maintenance and calibration activities, and even then they may not produce consistent data. Particularly for ammonia, data drifting is typical but not predictable. Steps in data between calibrations are often visible but it is not recommended to retrospectively calibrate the data. The typical sensor for ammonium can measure to a 2 mg/l or 10% accuracy and 0.01 mg/l resolution, so smaller impact events may be undetectable, and data drifting may be mistaken for impact, especially if this aligns with a spill event. Whilst the sondes could be a valuable tool for identifying where improvements should be targeted, these limitations  reduce confidence in using sonde data to monitor FIS exceedances and assess compliance with the Environment Act in real time.

Research has been conducted into a location's sensitivity to instrument-related uncertainty, and improvements in technology are likely to reduce uncertainty and improve data accuracy. However, with the current rollout of sondes there is an opportunity for integrating machine learning tools to help with real time data cleanup by automating anomaly detection, identifying trends, filtering noise, and improving data quality, enabling faster and more accurate analysis and reporting. 

Ultimately, the true test will be whether WaSCs can design user-friendly interfaces that clearly communicate the nuances of multiple pollution sources and visible impact versus a pollution event that exceeds FIS thresholds. Critically, it must be clear that a single recorded exceedance or sonde response does not automatically translate to ecological damage.