Nutrients, particularly nitrogen and phosphorus (N and P), can negatively affect water quality in rivers if concentrations become too high. N and P are found naturally within freshwater riverine systems and can enter through normal environmental process such as:
There can also be enhanced sources of N and P that can enter rivers through human activities, these can be a combination of diffuse and point sources, such as:
Water companies, like Wessex Water, play a role in a much larger effort to improve river water quality. However, success relies on all catchment stakeholders working together towards shared aims via a catchment-based approach.
This shows that there are a multitude of factors affecting water quality. Each diffuse source can be triggered or be exacerbated by various conditions e.g. heavy or prolonged rainfall or time of year/season. Excess nutrients within river systems can negatively affect the water quality of a river or waterbody; when this happens, it is called eutrophication. Key impacts on water quality are set out below:
Chemicals entering the sewers can come from food and drink, domestic cleaning products and personal care products. These chemicals end up at Water Recycling Centres (WRCs) where some are removed. Some WRCs have enhanced phosphorus removal to limit the effect of nutrients on the river downstream depending on the sensitivity of the receiving watercourse.
Sediments are a natural component of rivers. The active transport of sediment within rivers is key to the functioning of several freshwater, terrestrial and marine ecosystems. Sediments are essential in the formation of key river habitats, such as sand bars, and provide diversity to freshwater habitat giving growing medium for many aquatic plants. Human activities can elevate the influx of fine sediments and surface water into watercourses by accelerating erosion rates. Agricultural methods and impermeable surfaces are common contributors to sediment runoff into waterbodies. Excess sediments in watercourses can smother important gravel habitat on riverbeds which are important habitat for invertebrates or spawning. Farm runoff can encompass various sources you may encounter:
Various pollutants are deposited on the surface of roads from passing and stationary vehicles. These include rubber from worn tyres, motor oil, fuel, particulates and heavy metals. When it rains, these chemicals can flow directly into rivers through surface drains. The most obvious indicator to look out for are oil streaks which leave a shiny multi-coloured sheen on the surface of the road or on the waters surface. It is important to report instances of rivers with oil streaks to the Environment Agency.
In areas that are not collected to a public sewerage system, septic tanks are frequently used to treat wastewater. This is done by separating solids from liquids and then draining the liquid into the ground. Domestic wastewater flows into a tank where settling and bacterial decomposition of larger particles occurs. The treated liquid then filters into the soil. When septic tanks are not maintained, failures can occur. Untreated wastewater and sewage can be introduced into groundwater or nearby streams and waterbodies causing localised pollution.
Litter refers to any human-made material that has been improperly discarded outside of designated bins or refuse areas. Cleaning up litter along rivers is crucial as it pollutes the ecosystem and poses a significant threat to wildlife. A recent report found that 90% of the plastic in the ocean is transported there by rivers. We encourage you to collect any litter you encounter while visiting. Instances of fly tipping or hazardous materials should be reported to your local council.
Increased concentrations of nutrients, including phosphate and nitrate, can lead to eutrophication. Phosphorus is the limiting nutrient in river systems (in estuarine systems, nitrogen is the limiting factor). Therefore, with more phosphorus input to rivers via point and diffuse sources in the catchment, algae will grow rapidly under the right conditions as there is more nutrients available which can cause algal blooms. As the algal bloom grows this can prevent sunlight penetrating the waterbody which can cause plants and other vegetation to die.
As algae eventually dies off and sinks, oxygen levels in the river drop as other organisms consume the dead algae and other plants, reducing the amount of oxygen for aquatic life. On the river bed, dead algae can smother sensitive fish eggs and larvae, further affecting the rivers. The graphic below shows this process cycle.
How eutrophication takes place over time
Eutrophication is a long-term impact. Some chemicals have significant short-term impacts and need to be controlled to prevent short-term toxicity. Short-term elevated levels of ammonia, biochemical or chemical oxygen demand can have devastating impacted on the ecological of stream and rivers. Potentially causes fish-kills which can take years for the environment to recover from. These chemicals are highly regulated to avoid such short-term impacts. In the next section, we set out how the Environment Agency regulates Wessex Water to ensure that streams and river and not impacted from such chemicals.
Wessex Water is the main water and sewerage company (WaSC) in the Bristol Avon catchment. Wessex Water operates 121 water recycling centres (WRCs) in the Bristol Avon catchment. In the following sections are some key information on what sewage is, how sewage is treated, how WRCs are permitted by the Environment Agency and other useful information to assist with interpreting data from RiverHub.
Whenever a toilet is flushed or liquid is poured down the drain, sewage (or wastewater) is produced. Other sewage we treat includes rainwater from roads, roofs and gardens as well as industrial effluent. Our sewerage network carries this sewage to our water recycling centres where the water recycling process begins.
A water recycling centre (WRC) is a location where sewage is treated. They can vary in size depending on how big a population it serves, this is referred to as the population equivalent (PE). The PE is the average pollution load released by one person in one day. Discharges from non-household sources e.g. trade effluent, are given a PE value to determine the strength of discharge depending on its constituent load, this is then accounted for in the WRC size.
Wessex Water provides sewerage services to 2.9 million customers, some of our largest WRCs include:
When sewage arrives at water recycling centres, debris, rags and large objects are removed using screens. It then flows into tanks where the solids that could not be removed during the screening process sink to the bottom and are removed as sludge.
Next, the sewage is treated biologically by passing it through stone or plastic filters which have a layer of bacteria on them. These bacteria feed off the waste, helping to clean the water. As the wastewater comes through these filters, it enters settlement tanks and humus sludge is produced. These are the bodies of the bacteria as they die which are removed through this final settlement stage.
The cleaned wastewater (effluent) then leaves our water recycling centres and flows into local rivers or the sea.
Storm overflows are part of an older type of sewer system called a combined sewer system. These sewer systems carry both surface water (rainwater from roof gutters, patios, driveways and some highways) and foul water (waste from homes and industry) together in one pipe. The combined sewage is then transported to a water recycling centre to be treated.
During a storm event, heavy or prolonged rainfall can rapidly increase the flow in a combined sewer and cause it to become overwhelmed. Storm overflows are licensed by the EA and designed to release this excess stormwater into rivers or the sea to prevent sewer flooding.
Media reports suggest water companies 'dump raw sewage', but this isn't accurate. This implies we can control when they operate and what is released is highly polluting. However, they operate automatically and the discharge is heavily diluted by rainwater.
Diagram of how a storm overflow works
Video on how a storm overflow works
At 93 of our WRCs where the population equivalent is greater than 250, flow is measured using MCERTS accredited flow meters. This gives an accurate measurement of either flow coming into the WRC, or flow coming out of the WRC. Some sites will have flow monitors on both the inflow and outflow of the WRC.
More information on MCerts flow meter accreditation:
Minimum requirements for self-monitoring of flow:
MCERTS performance standard - GOV.UK (www.gov.uk)
This leaves 28 WRCs which are less than 250 PE which do not have a requirement for MCERTS flow measurements. To estimate an average daily flow for these WRC we use an industry standard methodology of the dry weather flow (DWF) permit multiplied by 1.25. Some WRCs do not have DWF permits, for these 14 sites, we estimate an average daily flow by multiplying 90% of the average Wessex Water customer water usage rate of 140 litre per day by the population served by the WRC. We use 90% of the customer usage rate as not all water used is returned to a sewer. For example, when clothes are washed, some water will stay on clothes as they dry.
The Environment Agency sets permits for each WRC, controlling the amount of chemicals that can be put back into nature. WRCs are permitted in relation to the amount of treated effluent it can discharge and the concentration of chemicals. Typical chemicals that may be permitted at a WRC include levels of ammonia, with is toxic to fish, phosphorus and nitrates, which are the main causes of algal blooming and excess weed growth, which harm fish and other species.
WRC compliance falls into two main categories:
Qualitative compliance for 95th percentile limits (BOD, SS and ammonia):
A certain number of measurements (spot sample results) are allowed to exceed these limits before the EA classifies the WRC as ‘failing’ its discharge permit for the year. This recognises that water companies do not have direct control over most discharges to sewer. These generally apply to biochemical oxygen demand (BOD), suspended solids (SS) and ammonia.
Routine spot audit samples of continuous discharges on a random basis are self-monitored by Wessex Water. These audit samples are analysed for the substances included in the discharge permit. Only these planned audit samples are used to assess look-up compliance. This is done by comparing the number of measurements which have exceeded their look-up numeric permit limits with the number of exceedances allowed in the standard look-up table for audit samples taken in any 12-month period.
Qualitative compliance for phosphorus:
WRCs are permitted to comply with various environmental regulations. The most common are for Urban Wastewater Treatment Directive (UWWTD), Habitats Directive and Water Framework Directive (WFD) purposes. A WRC may have multiple permits resulting from these different regulations. Sampling methods may also vary depending on depending on the environmental regulations governing a WRC. For instance, WRC permitted for UWWTD or Habitats Directive compliance, 24-hour composite samples are utilised. This monitoring technique takes a sample of treated effluent at regular intervals during the day and analysis is performed on the combined sample. For WFD permits, if there are sufficient composite samples available to assess compliance then this sampling method is use. If there are not enough, additional samples are necessary to assess WFD permit compliance, then the required number of spot samples are taken throughout the year. This is because regulations do not allow WFD permit compliance to be assessed with a mixture of composite and spot samples.
UWWTD phosphorus permits are assessed using 12-month calendar mean average (Jan-Dec). Habitats directive and WFD phosphorus permits are assessed using a 12 month rolling average.
The key permits at Wessex Water's WRC including those mentioned above are shown in the table below:
| WRC permit | Permit description | How do we meet the permit requirement? | How does the Environment Agency ensure we comply with the permit? |
|---|---|---|---|
| Flow to full treatment (FFT) | FFT is a measure of how much wastewater a treatment works must be able to treat at any time. | Installation of flumes are used to ensure flow is maintained at the correct levels. | Compliance with FFT permit conditions is reported annually. If reports show exceedance, then numerical limits can tighten, and more upgrades may be required at the WRC. |
| Dry weather flow (DWF) | The average daily flow to the treatment works during seven consecutive days without rain or where rain does not exceed 0.25ml of rain fall in any day. | Outlet flow measurement. | Compliance with DWF permit conditions is reported annually. If reports show exceedance, then numerical limits can tighten, and more upgrades may be required at the WRC. |
| Ammoniacal nitrogen | Ammoniacal nitrogen is the sum of the concentration of both ammonia and ammonium. | Ammoniacal limits set in mg/l are set and regular reporting is set out to be met. | Set monitoring periods (number of samples taken is proportionate to the population equivalent of the WRC and impact it has on receiving environment). |
| Biochemical oxygen demand (BOD) | BOD is a measure of the amount of pollutant dissolved in the sewage. The bacteria feeding on the pollutants will use up the oxygen in the sewage over a period of time (usually five days). The amount of oxygen used up by the bacteria is the BOD and is measured in milligrams per litre (mg/l). | Primary treatment is an important part of a WRC, removing some 50% of the suspended solids (SS) together with around 35% of the pollution load of raw sewage with its associated BOD. | A typical numerical permit would be 20/30/10 (i.e. 20 mg/l of BOD, 30 mg/l of suspended solids, 10 mg/l of ammonia as nitrogen). Note that look-up table compliance can be assessed at any time. |
| Phosphorus | The Urban Waste Water Treatment Directive, Water Framework Directive and the Habitats Directive can all be invoked by the Environment Agency to apply a reduction in phosphorus concentration on certain watercourses. | Phosphorus can be removed from sewage by biochemical or by chemical means. Enhanced biological treatment is not currently practised by Wessex Water. Chemical precipitation is feasible by the addition of iron or aluminium compounds, but at Wessex Water sites only iron compounds are used. | Phosphorus removal permits differ from other permits in that they are based on annual averages over a 12-month period. Compliance can be judged on a rolling basis or at the end of the year. Phosphorus permits can also be expressed as load limits (kg/d). |