Sewage Talk
Let’s talk about all things sewage and data transparency…
History of England's Sewer System
The history of England’s sewage systems spans centuries and it illustrates the progression of societal requirements, technological progress and the creation of governmental regulations pertaining to water quality and sanitation requirements.
Roman Era
During the Roman Era, the issue of sewage was limited as complex sewer systems were created. Cities such as London and Bath showcased these primarily as urbanisation began to increase. However, after the fall of the Roman Empire, these systems fell into disrepair and were largely abandoned.
17th to 18th Century
England’s population grew further which allowed the introduction of proper waste management. But urban areas still faced serious sanitation issues as waste was left in streets and rivers. This created major issues such as contamination, foul odour and the spread of diseases. To combat these issues, a rudimentary drainage systems were introduced, consisting of basic channels and ditches designed to divert wastewater from populated areas. However, the scale of the issue was too great for the solution that was created.
20th Century
The development of sewer systems in other parts of England followed a similar pattern. By the end of the 19th century, most major cities had some form of sewerage system. However, it was not until the mid-20th century that universal sewerage coverage was achieved in England – this was called a combined sewer system.
Along with the creation of sewage treatment plants meant that the removal of pollutants and faeces from sewage before discharges into rivers, increased water quality and improved aquatic systems.
Ancient/Medieval Times
During this period, people often used natural water bodies such as rivers and streams to carry waste away. This meant that the water bodies in which they also sourced water from became contaminated. Urban areas then became used as sites to manage waste, such as ditches. These methods quickly became insufficient in densely populates regions and created unsanitary conditions.
19th Century
The Industrial Revolution brought about a new set of challenges for sewage disposal. With the rapid growth of cities, there was a growing need for more efficient ways to collect and dispose of waste. In the early 19th century, some cities began to build rudimentary sewer systems. However, these systems were often inadequate and could not cope with the growing volume of waste.
The Great Stink (1858) was a turning point in the history of sewage disposal in England. This event, in which the River Thames became so polluted that it could not be used for drinking water, led to a public outcry and calls for action. In response, the government commissioned a new sewer system for London. This was completed in 1865 and is still in use today.
21st Century
England’s sewage systems continue to evolve, characterised by a commitment to sustainability, efficiency, and ecological preservation. Contemporary systems integrate cutting-edge technologies for sewage treatment and waste management. Additionally, climate change considerations necessitate adaptive sewage infrastructure to contend with heightened rainfall and potential flooding.
What Type of Sewer System does England Have?
England has a large sewer network consisting of around 100, 000km of combined sewers. They are a complex and integral component of its urban infrastructure and a testament to modern engineering and environmental stewardship. In certain areas, a modern system may be present called a single sewer system, this is where the sewage and storm water each travel through separate pipes, one leading to a waterway and the other to a sewage treatment works.
The combined sewer system combines two types of water flow within a single network of pipes:
- Domestic and Industry wastewater
- Rainwater runoff (stormwater)
Under normal conditions, when there is no rain, sewage flows from homes and businesses through underground sewer pipes to sewage treatment works, where it undergoes multiple stages of treatment to remove pollutants and debris before it is discharged into water bodies.
How is Sewage being Released into Waterways?
Sewage and wastewater is released into a waterbody in two forms; either treated or untreated. This is dependant on the type of weather during the day and whether the company has permits to spill. Expand the weather to see…
Dry Weather
Dry Day:
During a typical day, when the weather is dry, sewage is processed through a normal channel within underground sewage pipes. It is directed to the nearest sewage treatment works so it can be filtered to remove pollutants before it is discharged into the nearest water way.
As there is no rainfall, there is no need for any Combined Sewer Overflows (CSO) therefore, there is no raw or partially treated sewage entering the waterway.
However, there are situations where dry spills (also known as emergency overflow) can occur. Dry spills are essentially a storm overflow which occurs when there is no rainfall, so sewage spills into a waterway either partially treated or untreated. This is usually illegal unless permitted by the Environment Agency. Open the accordion to discover how these occur…
Equipment and Infrastructure Faults
This is when a sewage treatment works spills raw sewage into the environment if there is a fault within the works. Faults include electrical, technological or mechanical failure (i.e. a CSO valve opening before it should). This is typically illegal to which the Environment Agency must be notified and the issue must be rectified immediately. Overflows of this nature should not be happening but they are occurring more frequently which pose a plethora of risks to anyone that uses the waterbody; such as marine life, animal life, ecology and humans.
Groundwater Infiltration
Groundwater infiltration refers to water that seeps into sewer pipes through holes, cracks or faulty connections. This creates an additional influx of water which can exceed the capacity of the sewage treatment works leading to an overflow.
Drainage Time
The size of a catchment area and the ability for it to drain when it rains or floods is extremely important as this directly affects how much wastewater a sewage treatment works can hold.
Misconnection
c
Wet Weather
Wet Day:
During a wet day when there is heavy rainfall or snow melt, sewage is processed a little differently. Heavy rainfall causes a huge influx of a vast and fast amount of water in the sewage treatment works which means the capacity of the works is exceeded. This can cause many issues including causing the network to back up, causing flooding through manholes, drains and toilets in homes and businesses.
To combat this scenario, sewage treatment works are permitted by the Environment Agency to divert this water into storm tanks within the sewage treatment works which hold the wastewater until it can be processed and treated it. The works may also be allowed to release partially treated or raw sewage directly into waterways – this is called a storm overflow or a combined sewer overflow (CSO). Open the accordion to find out more…
Combined Sewer Overflow (CSO)
Storm Overflows:
These are designed to release excess stormwater from the sewage system into waterways during periods of heavy or prolonged rainfall or snowmelt. Sewage and rainwater are directed into a storm overflow channel which is discharged into the nearest waterway. They are made as a safety valve to ensure the sewage system does not become overwhelmed when it is raining or snowing. Only under these circumstances and with Environment Agency legal permits are water companies allowed to do this.
Event Duration Monitoring (EDM) devices (seen in red) were introduced by the Environment Agency (EA) to monitor them and help protect our waters. However, one major disadvantage is that they are currently not enforced on every sewage overflow in the UK therefore, there are some storm overflows that have been spilling that are not and never have been recorded.
For any key words that are not understood, please view our glossary!
What is FFT and DWF?
The FFT label (seen in yellow in the above diagrams) refers to the Flow to Full Treatment. This is the maximum flow a wastewater treatment works must treat before it is permitted to discharge excess flows to the environment or into storm tanks.
Dry Weather Flow (DWF) refers to the daily average volume (flow) a treatment works can handle during a period of normal, dry weather.
There are two ways to calculate the DWF as seen in the table to the right; using two formulas. The FFT volume is calculated by multiplying the Dry Weather Flow (DWF) by 3.
Wastewater treatment works are built and/or designed based on the estimated volume of flow. This provides an insight into how much the treatment works can handle during a period of typical, dry weather. However, it does not take into account other weather events such as heavy or prolonged rainfall, snowmelt or increasing populations therefore, this must also be taken into account.
Specialised storm tanks (as mentioned in previous content) allow the sewage treatment works to store excess untreated or partially treated wastewater in order not to overwork the works capacity or DWF.
Who Assigns Permits to Spill?
The Environment Agency is in charge of assigning permits to spill sewage into a waterway.
Who Spilled the Most Sewage in 2022?
It is not unknown that the state of England’s water bodies is poor. The 2019 rating puts surface and ground water at an all time low with multiple rations of ‘fail’ and ‘poor’ in relation to ecological status and chemical status. One major factor contributing to this is sewage pollution from storm overflow discharges.
The Environment Agency (EA) has recognised this and has instructed all major water companies to fit monitors on their storm overflows to capture their spill performance. The Event Duration Monitoring (EDM) works by measuring the frequency and duration of sewage spills. The majority, 91% of storm overflows are fitted with monitors, however, the EA has directed all water companies to install them on storm overflows by the end of 2023. The reason being the data is only available annually, but to allow for transparency and accountability the data needs to be live and open to the public.
Water Company
Sewage Spills
Average Time (per day)
Duration (Hours)
United Utilities
69,245
189.7
425,491
Yorkshire Water
54,158
148.4
232,054
Severn Trent Water
44,765
122.6
249,116
South West Water
37,587
103
290,271
Northumbrian Water
29,697
81.4
107,536
Wessex Water
21,878
59.9
129,957
Southern Water
16,688
45.7
146,819
Anglian Water
16,082
44.1
89,514
Thames Water
8,014
22
74,693
Total
298,094
816.8
1,745,451
The 2022 EDM report showed a decrease in spills in comparison to 2021, but this was largely down to dry weather and not improvements by water companies. The graph above displays the total spill count for 2022; whereby the company with the most sewage outflows (69, 245) was United Utilities, which covers the North West of England. This was followed by Yorkshire water, with 54, 234 overflows. This is a major problem as raw, untreated sewage can not only affect the ecology and marine life within the waterbody, but anyone who comes into contact with it.
Clarification: Spill vs Storm Overflow:
Spill: Sewage spilling at any time due to a number of reasons such as emergency spills (equipment issues), drainage issues, misconnection issues. EA Permits may be granted dependent on the reason but most commonly illegal.
Storm overflow: A sewage spill which can be permitted under EA permits during heavy rainfall or snowmelt.
What Tracks Spills?
Event Duration Monitoring (EDM):
An EDM is a system that uses sensors to monitor the level of flow from a sewage outlet. EDM monitors are typically installed on storm overflows, where excess sewage is released into rivers and watercourses during heavy rainfall or snow melt. The main purpose of an EDM is to bring into light storm overflow performance. However, not all storm overflows are equipped with an EDM, The Environment Agency details that overflows with less than one spill per year, do not require any monitoring as they are of ‘low significance’, therefore, the accurate number of spills is hard to know.
Scientifically, the EDM monitors any flow’s start and end time (frequency and duration), but they do not measure the volume of the flow itself. Amongst EDM, there are other sensors at various points within sewage treatment works which measure if there is outflow occurring through emergency routes, again not measuring the volume of flow, so the exact quantity of sewage that has been spilt cannot be measured.
The frequency of sewage spill monitoring depends on the size of the water treatment site and the risk of sewage spills. The data collected from sewage spill monitoring is used to identify the causes of sewage spills, track the frequency and severity of sewage spills, and notify the public of sewage spills. The Environment Agency is responsible for regulating sewage spills in the UK and has a number of powers to investigate and take action against water companies that are responsible for sewage spills.
How are Spills Counted?
Spills are counted using the 12/24 method. The 12/24 method is a way of counting sewage spills that takes into account the duration of the spill.
Spills are counted in two ways using this method:
- A spill is counted as one event if it lasts 12 hours or less.
- A spill is counted as two events if it lasts longer than 12 hours.
Why is this method used?
This method is used to reflect the fact that longer spills; of two or more have a greater impact on the environment. For example, if a spill starts at 10am and ends at 10pm, it would be counted as two events: one event for the first 12 hours (from 10am to 10pm) and another event for the remaining 12 hours (from 10pm to 10am).
The method is used by the Environment Agency to track the frequency and severity of sewage spills in the UK. This information is used to assess the water company which is spilling the most, as well as the effectiveness of measures to reduce sewage spills and to identify areas where further action is needed.
Are there other methods?
The 12/24 method is not the only method, other methods include;
- The volume-based method, which counts spills based on the volume of sewage that is discharged
- The event-based method, which counts spills based on the number of times that sewage is discharged.
Which is best?
The 12/24 method is the most commonly used method for counting sewage spills in the UK. This is because it is a relatively simple method that is easy to understand and apply.
However, other methods may be more appropriate in some cases, such as when the volume of sewage that is discharged is more important than the duration of the spill.
Where are Sewage Overflows Located?
Sewage overflows are located in various places around England. They are most commonly located at several points along a river, stream or water body. Navigate to search my water the current 2022 sewage overflow locations.
