Combined sewer overflow monitoring: the 2026 UK regulation, IoT sensor, and compliance guide

On World Oceans Day 2026, the link between inland sewers and ocean health has never been clearer. Combined sewer overflow monitoring uses IoT sensors to detect surcharging inside CSO chambers before sewage spills into rivers and coastal waters. England recorded 290,000 storm overflow discharges totalling 1.87 million hours in 2025, and the government’s Storm Overflows Discharge Reduction Plan demands a 45% cut in average spills by 2030. Continuous level and flow monitoring inside CSO chambers gives duty teams the minutes they need to intervene, and it gives regulators the evidence trail they require. This guide explains what CSO monitoring involves, what the law now requires, and how AQUAIOT’s sensor-to-cloud stack delivers it.

Key takeaways

• England recorded 290,000 storm overflow discharges in 2025, totalling 1.87 million hours of raw sewage entering waterways.
• The Storm Overflows Discharge Reduction Plan targets a 45% reduction in average CSO spills by 2030, with elimination by 2050.
• The Water (Special Measures) Act 2025 requires near-real-time monitoring of emergency overflows and introduces automatic financial penalties.
• IoT radar level sensors inside CSO chambers detect pre-spill surcharging and alert duty teams minutes before a consent breach.
• AQUAIOT Radar provides non-contact 60 GHz level monitoring accurate to within plus or minus 2 mm, with ATEX variants for hazardous sewer environments.
• Pairing Event Duration Monitoring with continuous level data turns a compliance tick-box into an operational early-warning system.

Contents

• What is a combined sewer overflow and why does it threaten UK rivers and coasts?
• How many CSO spills happen across England each year?
• What does the law now require for combined sewer overflow monitoring?
• How does IoT sensor technology monitor CSO chambers in real time?
• The full AQUAIOT sewer overflow monitoring capability stack
• What does a combined sewer overflow monitoring deployment look like?
• Frequently asked questions

What is a combined sewer overflow and why does it threaten UK rivers and coasts?

A combined sewer overflow is a designed relief point in a sewer network that carries both foul sewage and surface rainwater in the same pipe. When heavy rainfall overwhelms the pipe’s capacity, the CSO opens and discharges a mix of untreated sewage, rainwater, and debris directly into a receiving watercourse, river, estuary, or coastal outfall. England has

approximately 15,000 storm overflows

, most of them combined sewer overflows inherited from Victorian-era infrastructure that was never designed for today’s population density or rainfall intensity.

On World Oceans Day 2026, the United Nations theme “Reimagine” calls for a new relationship with the ocean. For the UK, that relationship starts upstream: every CSO spill that enters a river eventually reaches the estuary or coast. In 2025, water companies discharged sewage for 7,885 hours into bathing waters on dry days alone, and 1,236 people reported illness after entering the water. CSO monitoring is the operational layer that catches surcharging before it becomes a spill, protecting rivers, coasts, and the communities that depend on them.

The regulatory framework treats CSOs as permitted discharges, but only within strict consent conditions. When those conditions are breached, the consequences include prosecution, financial penalties, and reputational damage. That is why continuous monitoring at the chamber level, not just at the treatment works, has become the baseline expectation under both the Water (Special Measures) Act 2025 and the Storm Overflows Discharge Reduction Plan.

How many CSO spills happen across England each year?

England recorded 290,000 storm overflow discharges from combined sewer overflows in 2025, lasting a combined 1.87 million hours. That figure dropped significantly from 450,398 spills and 3.61 million hours in 2024, but the Environment Agency attributed most of the improvement to unusually dry conditions rather than structural reform. In a wet year, the numbers climb straight back.

By the end of 2023, 100% of storm overflows in England had Event Duration Monitoring (EDM) devices fitted. EDM records when an overflow starts and stops, giving regulators a count and a duration. What it does not provide is the level trend inside the CSO chamber in the hours before a spill. That gap is where continuous level monitoring inside CSO chambers adds operational value: it turns a backward-looking compliance record into a forward-looking early-warning signal.

The numbers carry direct consequences. In 2025, 87% of designated bathing waters met “Excellent” or “Good” classification, but 19 of the 34 waters designated since 2021 were classified as Poor. Behind those classifications sit real health outcomes: 1,236 illness reports from bathers, 74% of which occurred at waters rated “Good” or “Excellent”, and 233 linked to confirmed sewage discharges. Monitoring CSO chambers upstream is the most direct way to reduce the spill count that drives those figures.

What does the law now require for combined sewer overflow monitoring?

Three interlocking pieces of legislation and policy now define what is expected of anyone operating a CSO in England. Continuous sewer overflow monitoring sits at the intersection of all three, and the direction of travel is toward more data, faster reporting, and heavier penalties for non-compliance.

The Storm Overflows Discharge Reduction Plan (SODRP). Published in August 2022 under the Environment Act 2021, the plan sets headline targets: a 45% reduction in average spills per storm overflow by 2030 compared with 2021 levels. By 2035, all overflows discharging into or near designated bathing waters must be improved. By 2050, no overflow should discharge above an average of 10 times per year. More than £12 billion is allocated in the AMP8 period (2025 to 2030) to upgrade approximately 2,500 storm overflows across England.

The Water (Special Measures) Act 2025. Receiving Royal Assent on 24 February 2025, this Act introduces near-real-time monitoring of all emergency overflows, with information about a discharge’s start and duration published within one hour. Emergency overflow monitors must reach 50% coverage by 31 March 2030 and 100% by 1 April 2035. The Act also introduces automatic financial penalties for certain pollution offences, applied on the civil standard of proof. Water companies must now publish annual Pollution Incident Reduction Plans (PIRPs).

AMP8 performance commitments. Ofwat’s AMP8 framework ties investment directly to measurable outcomes. Storm overflow reduction is a named performance commitment: companies that miss their spill-count targets face financial penalties. The framework also requires continuous water quality monitoring at high-priority assets during the 2025 to 2030 period. For duty teams on the ground, this means CSO monitoring is no longer optional. It is the mechanism that generates the evidence trail regulators use to assess compliance.

How does IoT sensor technology monitor CSO chambers in real time?

IoT-based CSO monitoring works by placing a non-contact level sensor inside or above the CSO chamber and transmitting continuous readings to a cloud dashboard. The sensor measures the distance to the water surface using radar, ultrasonic, or pressure-based technology. When the level rises above a pre-set threshold, the system fires an alert to the duty team, typically minutes before the overflow weir is breached. That early warning is the operational difference between catching a surge and reading about a spill in the next morning’s EDM report.

Radar-based sensors are the preferred choice for sewer environments because they are not affected by foam, vapour, condensation, or the corrosive gases common in wet wells and CSO chambers. The AQUAIOT Radar uses 60 GHz mmWave technology for non-contact level measurement accurate to within plus or minus 2 mm. It penetrates the harsh conditions inside enclosed underground chambers where ultrasonic sensors often struggle. ATEX variants are available for environments classified as hazardous under the ATEX directive.

Data leaves the sensor via cellular or LoRaWAN telemetry, encrypted in transit, and lands on the AQUAIOT Cloud dashboard within minutes. The dashboard displays live level trends, historical comparisons, and configurable threshold alerts via SMS, email, or voice call. For operations teams managing dozens or hundreds of CSO chambers, the value is not just the alert itself. It is the trend: a gradually rising baseline that flags an upstream blockage or capacity problem days before it causes a consent breach.

Pairing continuous level data with existing EDM creates a layered monitoring approach. EDM tells the regulator that a spill happened and for how long. Level data tells the duty team that a spill is about to happen and gives them time to respond. Together, they satisfy both the backward-looking compliance requirement and the forward-looking operational need that the SODRP and the Water (Special Measures) Act now demand.

The full AQUAIOT sewer overflow monitoring capability stack

A CSO monitoring deployment is more than a single sensor in a chamber. It is a layered system that covers level sensing, flow measurement, surface water management, and a cloud platform that ties everything together. AQUAIOT’s stack spans the full sewer overflow lifecycle, from upstream attenuation to the CSO chamber itself to the downstream receiving water.

Upstream: surface water attenuation

The AQUAIOT Smart Water Butt monitors rainwater storage levels and applies rainfall-aware logic to optimise capacity before storm events. In a SuDS context, smart attenuation reduces the peak inflow that reaches the combined sewer and triggers CSO activation. Fewer peaks mean fewer spills.

CSO chamber: radar level monitoring

The AQUAIOT Radar sits above the chamber and measures level continuously. Its 60 GHz mmWave beam penetrates foam, vapour, and condensation. The sensor detects pre-spill surcharging and fires threshold alerts to the duty team. ATEX-rated variants cover chambers classified as hazardous. Level trends are logged to the AQUAIOT Cloud, building the evidence base that Ofwat and the Environment Agency use to assess performance.

Network flow: clamp-on ultrasonic metering

The Clamp-on Ultrasonic Flow Meter sits on the outside of existing pipework, covering DN8 to DN100, with no pipe cutting and no shutdown. Flow anomaly detection flags unexpected surges that indicate a blockage or illegal discharge upstream of the CSO. Bi-directional flow and total volume data feed into the AQUAIOT Cloud alongside level readings, giving a complete picture of what is entering the chamber and when.

Cloud, alerts, and integration

The AQUAIOT Cloud consolidates level, flow, and telemetry data into a single operations-grade dashboard. Role-based access, configurable threshold alerts (SMS, email, voice), trendlines, anomaly detection, and exportable audit trails support both day-to-day operations and regulatory reporting. The platform integrates via Modbus, SCADA, and open APIs into existing asset management, CAFM, and telemetry systems.

What does a combined sewer overflow monitoring deployment look like?

A typical combined sewer overflow monitoring deployment follows a structured sequence from site survey to live dashboard. The process applies whether the operator is monitoring a single high-risk CSO or rolling out across hundreds of chambers in an AMP8 programme. Each step builds on the previous one, so the system is operational before the next storm event.

Step 1: Site survey and risk assessment. An AQUAIOT engineer surveys the CSO chamber, assesses access, atmospheric hazards (confined space, H2S), mounting options, and cellular or LoRaWAN signal strength. The survey identifies whether a standard or ATEX-rated sensor is required and confirms the optimal mounting height above the overflow weir.

Step 2: Sensor installation. The AQUAIOT Radar mounts above the chamber using a bracket or flange. Installation is non-invasive: the sensor never contacts the liquid. A typical single-chamber install takes under two hours with no flow interruption.

Step 3: Telemetry commissioning. The sensor connects via cellular or LoRaWAN to the AQUAIOT Cloud. Encrypted payloads ensure data integrity. The engineer confirms live readings on the dashboard before leaving site.

Step 4: Threshold configuration and alert routing. Pre-spill alert thresholds are set based on the chamber geometry and the overflow weir height. Alerts route to the duty team via SMS, email, or voice call. Escalation rules ensure that a missed first alert triggers a second notification to a supervisor or on-call engineer.

Step 5: Integration and reporting. Level and spill data integrate into existing SCADA, asset management, or regulatory reporting systems via Modbus or API. EDM data and level trends sit side by side in the AQUAIOT Cloud, giving both the operator and the regulator a complete, auditable record of every chamber’s behaviour.

The deployment is retrofit-friendly. There is no need to drain or decommission a chamber. Sensors install on live assets, and the dashboard goes live the same day. For multi-site programmes, AQUAIOT provides project management, training, and post-installation engineering support.

Frequently asked questions

What is the difference between EDM and continuous CSO monitoring?

Event Duration Monitoring records when a storm overflow starts discharging and when it stops, giving regulators a spill count and a duration. Continuous CSO monitoring measures the water level inside the chamber at regular intervals, typically every few minutes, and fires alerts when the level approaches the overflow weir. EDM is backward-looking: it confirms that a spill happened. Continuous monitoring is forward-looking: it warns that a spill is about to happen and gives the duty team time to intervene. The two systems complement each other, and the Water (Special Measures) Act 2025 is pushing operators toward both.

Can IoT sensors operate inside confined and hazardous CSO chambers?

Yes. Non-contact radar sensors mount above the liquid surface and never enter the flow. ATEX-rated variants, such as the AQUAIOT Radar, are certified for use in environments with explosive atmospheres from hydrogen sulphide and methane, which are common in sewer chambers. The sensor’s 60 GHz mmWave beam penetrates foam, vapour, and condensation, so readings remain accurate even in the worst conditions. Installation follows confined-space entry protocols, but once mounted the sensor operates unattended.

How does combined sewer overflow monitoring help meet SODRP targets?

The Storm Overflows Discharge Reduction Plan requires a 45% reduction in average spills by 2030 and progressive elimination by 2050. Continuous level monitoring at CSO chambers enables early intervention: duty teams receive alerts as the level rises and can deploy tankers, activate storage, or adjust upstream flow before the overflow weir is breached. Over time, level trend data also identifies chronic capacity problems and informs capital investment decisions, targeting upgrades where they will have the greatest spill-reduction impact. The evidence trail from continuous monitoring supports regulatory reporting under both the SODRP and AMP8 performance commitments.

What connectivity options are available for remote CSO chambers?

AQUAIOT sensors support both cellular (4G/LTE) and LoRaWAN connectivity. Cellular is the default for isolated chambers where a LoRaWAN gateway is not already in range. LoRaWAN suits dense deployments where dozens of chambers feed into a single gateway, reducing per-unit connectivity cost. Both options deliver encrypted payloads to the AQUAIOT Cloud.

By Gianbattista Porru, Digital and IoT lead at AQUAIOT

Last updated: 8 June 2026