Slaley sewage treatment works, run by Northumbrian Water, has 2 monitored storm overflows discharging to Flothers Burn. In 2025 they recorded 78 spills totalling 218 hours, against 40 spills and 77 hours in 2024. By hours spilling, it ranks 88 of 183 Northumbrian Water works, where 1 spilled longest.
Every spill in 2025
Each blue mark is one spill, placed by when it started and sized by how long it lasted. Hover over a mark for the date and duration.
Hours spilling by month, 2025
Hours, measured by the event duration monitor.
Longest single spills
Started (GMT)
Overflow
Duration
12 Nov 2025, 21:50
Slaley STW
17.2 h
12 Nov 2025, 22:38
Slaley STW
14.3 h
20 Sep 2025, 23:48
Slaley STW
12.0 h
19 Jul 2025, 14:57
Slaley STW
8.6 h
13 Nov 2025, 20:14
Slaley STW
8.3 h
Estimated 2025 spill volume and pollution
Estimate, not a measurement. Monitors record how long an overflow spills, not how much. This calculator back-calculates a spill rate from the size of the works, using the Environment Agency's permit formulae, and multiplies it by published storm-sewage concentrations. How it works.
This works serves fewer than 2,000 people, or was not matched to a UWWTD report, so its size is not published. The population equivalent above is a placeholder of 1,000. Enter the population served if you know it.
Volume, central estimate3,727 m³range 1,484 – 9,356 m³
In litres3.7 millionrange 1.5 million – 9.4 million L
Olympic swimming pools1.5at 2,500 m³ each
Organic load, as people's raw sewage5,590person-days of untreated BOD
Assumed spill rate while spilling: 1.9 – 11.9 L/s (central 4.8 L/s), from a dry-weather flow of 1.9 L/s.
1.9 / 6.3 / 9.3 mg/L Low: Germany, Brombach 2005; typical/high: Paris range 3.3–9.3 midpoint and top, Gasperi 2012 (via botturi); no UK value found
Total phosphorus
1.8 kg
9.7 kg
51 kg
1.2 / 2.6 / 5.4 mg/L Low/high: Paris, Gasperi 2012; typical: Slovakia (via botturi). UK figure of 10 mg/L treated as an outlier
E. coli
148.4 billion organisms
3.7 trillion organisms
93.6 trillion organisms
10,000 / 100,000 / 1.0 million per 100 mL Median range at CSO and retention-tank outlets, Stott et al. 2018 (via botturi); typical = log midpoint
Low combines the low spill rate with low concentrations, and high combines high with high, so the true figure is very likely inside the range. Long spills become more diluted as they go on, so the central figure tends to overstate the load of long events. The calculator does not estimate the effect on the receiving water, which depends on river flow or tide at the time of the spill.
Slaley STW
Inlet overflow at sewage works · discharges to Flothers Burn
Spills 2025
36 (159 hours) · up 100% on 2024
Spills 2024
18 (9.7 hours)
Long-term average
35.4 spills a year (monitored since 2021)
Monitor uptime
99% of the year
High-spill reason (company)
Not asset maintenance - Hydraulic capacity
Investigation
Env Act (SODRP) investigation ongoing
Improvement
No improvement action in reporting period
WFD catchment
March Burn Catchment (trib of Tyne) (GB103023075650)
Dry-weather flow (DWF) = population × water use per person × (1 + infiltration). Water use is 136.5 L per person per day, England's per-capita consumption for 2024–25 [EA]. Infiltration of groundwater into sewers is taken as 0–40% of that flow [Escritt, via HBF]. The Environment Agency defines DWF as PG + I + E [EA].
Population. The size of each works comes from the load entering it as reported under the Urban Waste Water Treatment Directive, in population equivalents (1 p.e. = 60 g of BOD a day) [UWWTD]. We treat population equivalent as population. That overstates flow where trade effluent adds load, and no official source links the two. Works serving fewer than 2,000 p.e. are not reported, so their size has to be entered by hand.
Low spill rate = DWF. This is the method used by Giakoumis & Voulvoulis (2026), which they note likely underestimates storm loads [G&V].
High spill rate = Formula A − flow to full treatment = (DWF + 1360P) − 3DWF. This is the largest flow the permit expects the storm tanks to handle before it passes on to treatment or overflows [EA]. The central rate is the geometric mean of the low and high rates.
Volume = spill rate × hours spilling, with hours taken from the event duration monitor.
Pollution = volume × published storm-sewage concentrations [Botturi et al.][US EPA]. Organic load is also shown as person-days of untreated sewage at 60 g of BOD per person per day [UWWTD].
What this cannot tell you. It gives no real flow for any single spill, and no effect on the river or sea, which depends on the flow, tide and the other pressures on the water at the time. It gives no figure for spills from overflows on the sewer network, which serve catchments of unknown size. It is a way to put hours into rough physical terms, not a substitute for flow monitoring.