Natalie Lamb and the overview of UK and Ireland water company ownership, operation and regulation

While every tap delivers the exact same clean water, the organisations running the networks behind the scenes of the UK and Ireland water sector are completely different. Here's my overview of how the different water companies in the UK and Ireland operate and are regulated, as of 2026.

Who Owns the Pipes?

• England treats water as a private business. The water companies are run to make a profit and are owned by international investment firms and big corporations.
• Wales uses a private, not-for-profit model. The main company has no owners or shareholders. Any extra money it makes cannot be given away as a payout- it must be spent on fixing the pipes or lowering customer bills.
• Scotland, Northern Ireland and Ireland view water as a public service. Their networks are completely owned by the government.

How Upgrades are Funded

• Borrowing from banks and investors: England and Wales get the sums of cash they need by taking out corporate loans and selling bonds to global investors. In England, the companies also get cash from private investors who expect a share of the profits (dividends) in return.
• Paying through the government: Scotland, Northern Ireland, and Ireland rely on public money. Scotland takes out long term infrastructure loans directly from the government. Northern Ireland and Ireland fund their big building projects directly out of the national budget using tax money.

What Happens in Financial Crisis?

• Going bankrupt: Because English water companies are normal private businesses, they can actually run out of money. If one collapses, the government has to step in, use special courts to take it over and find a new buyer.
• Using a rainy day fund: Because Welsh Water has no shareholders or government safety net to rescue it, it keeps a stash of emergency cash hidden away to keep itself running if times get tough.
• Government protection: Utilities in Scotland, Northern Ireland, and Ireland can never go bankrupt. Because they belong to the public, the government treasury guarantees their debts. If they run out of cash, it just means politicians have to discuss the state budget.

Operation	Ownership Model	Funding Sources	Customer Billing
England	Privately owned, for profit	• Customer Billing • Commercial Debt • Shareholder Equity	Direct water bill from the local water company (metered or unmetered)
Wales (Dŵr Cymru Welsh Water)	Privately owned, not for profit	• Customer Billing • Commercial Debt	Direct water bill from the local water company (metered or unmetered)
Scotland (Scottish Water)	Publicly Owned	• Customer Billing • Government Loans	Water and wastewater charges collected through the local authority council tax bill
Northern Ireland (Northern Ireland Water)	Publicly Owned	• Government Subsidies • Customer Billing (Commercial only)	Paid indirectly through general taxation. No separate domestic water bill
Republic of Ireland (Uisce Éireann)	Publicly Owned	• Government Subsidies • Customer Billing (Commercial only)	Paid indirectly through general taxation. No separate domestic water bill

How the Timelines Work

Every water company is forced to plan its budget and targets in strict blocks of time to make sure they are doing their jobs.

• Five year cycles: England and Wales are grouped together in strict five year blocks. The regulator sets a firm limit on how much they can charge and what targets they must hit for things like leaks or pollution over those five years.
• Six year cycles: Scotland and Northern Ireland use slightly longer six year timelines. This is designed to match up with the long term funding plans of their local devolved governments.
• Five year plans with yearly reviews: Ireland sets a five year spending limit for its utility, but the regulator checks the company's paperwork and performance data every single year to keep them on track.

Regulation	Economic Regulator	Drinking Water Quality Regulator	Environmental Regulator	Regulatory Period
England	Ofwat	Drinking Water Inspectorate (DWI)	Environment Agency (EA)	5 years
Wales (Dŵr Cymru Welsh Water)	Ofwat	Drinking Water Inspectorate (DWI)	Natural Resources Wales (NRW)	5 years
Scotland (Scottish Water)	Water Industry Commission for Scotland (WICS)	Drinking Water Quality Regulator for Scotland (DWQR)	Scottish Environment Protection Agency (SEPA)	6 years
Northern Ireland (Northern Ireland Water)	Northern Ireland Authority for Utility Regulation	Drinking Water Inspectorate Northern Ireland (DWI NI)	Northern Ireland Environment Agency (NIEA)	6 years
Republic of Ireland (Uisce Éireann)	Commission for Regulation of Utilities (CRU)	Environmental Protection Agency (EPA Ireland)	Environmental Protection Agency (EPA Ireland)	5 year plans with yearly regulator reviews

Natalie Lamb and the Cunliffe Review

What is the Cunliffe Review?

In October 2024, the UK Government and the Welsh Government commissioned the Cunliffe Review – an independent assessment of how the water system in England and Wales is run, regulated and held to account. It has been described as the most comprehensive review of the water sector since privatisation and is named after its chair, Sir John Cunliffe.

At first glance, the Cunliffe Review may look like another technical examination of a heavily regulated industry. In reality, it represents a pivotal moment for how essential public services are governed. The Cunliffe Review makes 88 recommendations aimed at reforming the system. The Government has already indicated support for several of the Cunliffe Review’s directions of travel and has committed to publishing a White Paper and introducing a Water Reform Bill. In the meantime, water companies and regulators are beginning to prepare for potentially significant change.

What did the Cunliffe Review say?

The Cunliffe Review examined why the water system has struggled to deliver clean rivers, resilient infrastructure and affordable bills at the same time. Its central conclusion was not that a single organisation or group had failed, but that the system as a whole is fragmented and poorly aligned.

Over decades, responsibility for water has been divided between multiple regulators, government departments and public bodies, each setting their own priorities. The result is a system that often demands everything at once (for example lower bills, cleaner rivers, net-zero emissions, drought resilience and rapid housing growth), without clearly deciding what should take precedence when those goals inevitably conflict.

While the Cunliffe Review contains many detailed proposals, its core message is simple- complex public services need clearer leadership, better coordination and more honest choices about trade-offs.

What will change in the water sector?

If the Cunliffe Review’s recommendations are implemented, several practical changes are likely to follow. Below are 5 key changes which may happen in water:

1. A single water regulator. Instead of four separate regulators (Ofwat, the Environment Agency, Natural England and the Drinking Water Inspectorate) with overlapping and sometimes conflicting responsibilities, water related regulatory functions would be brought together into one integrated regulator for England, with a separate regulator for Wales.

2. A more forward-looking approach to price regulation. Five-yearly price reviews would continue, but with a shift away from a heavily data-driven, desk-based process towards a more supervisory and forward-looking approach, focused on long term company resilience and investment planning.

3. Open, real-time monitoring of wastewater. Rather than relying primarily on samples taken by companies and reported to regulators, the Cunliffe Review recommends open monitoring, real-time monitoring of the wastewater system, with data made publicly available online to improve transparency and rebuild trust.

4. A new statutory Water Ombudsman. The Cunliffe Review proposes replacing the existing Consumer Council for Water with a statutory Water Ombudsman, providing stronger, more independent oversight of customer complaints and redress.

5. A long-term National Water Strategy. Finally, the Cunliffe Review recommends a government-produced, 25-year National Water Strategy for both England and Wales. This would be cross-sectoral and systems-focused, intended to align water policy with wider objectives such as environmental protection, climate adaptation and economic growth.

Natalie Lamb and the history of the water sector

Water supply and sewerage services used to be provided on a local basis by a mixture of local authorities (e.g. Wrexham Rural District Council) and joint organisations with multiple local authorities (e.g. Doncaster and District Joint Water Board). This set up was quite fragmented and there was no “source to tap” or “source to source” thinking. In the late 1960s and early 1970s there were problems with meeting the future demands for these services, prompting a restructure.

The Water Act 1973 established 10 new regional water authorities. These authorities were responsible for managing water resources and supplying water and sewerage services on a fully integrated basis. The regional water authorities took control of the services that local authorities had previously been supplying. Central government set financial constraints, and performance aims for each authority.

The European Union introduced stricter legislation on river, bathing, coastal, and drinking water quality, which the regional water authorities were struggling to meet in 1985. Estimates of the capital expenditure required to achieve EU standards and meet the existing backlog in infrastructure maintenance ranged from £24 to £30 billion, a sum not possible to achieve with government funding.

The 10 new regional water authorities were sold (“privatised”) from being government-owned to being privately owned in 1989. This was done because privatised companies are able to access different funding routes (e.g. they can raise finance through the stock market , they can access private capital etc) so they had more money to invest in infrastructure to achieve the new EU standards. 

• Anglian Water (previously Anglian Water Authority)
• Dŵr Cymru Welsh Water (previously Welsh Water Authority)
• North West Water (previously North West Water Authority)
• Northumbrian Water (previously Northumbrian Water Authority)
• Severn Trent Water (previously Severn Trent Water Authority)
• Southern Water (previously Southern Water Authority)
• South West Water (previously South West Water Authority)
• Thames Water (previously Thames Water Authority)
• Wessex Water (previously Wessex Water Authority)
• Yorkshire Water (previously Yorkshire Water Authority)

Also established were some regulators: 

• The National Rivers Authority – the environmental regulator
• The Drinking Water Inspectorate – the drinking water quality regulator
• The Director General of Water Services supported by the Office of Water Services (Ofwat) –the economic regulator 

The 10 new regional water authorities. Source: Sun, Jingrui. (2021). Connectivity restoration for fishes in post-industrial rivers of North East England. 

Natalie Lamb and the Danish case study

I recently attended the IWA World Water Congress 2022 in Copenhagen, Denmark, as part of the Young Water Professionals Water Camp. It was a massive congress with over 10,000 delegates from around the world. The below is a summary of Denmark water utilities of things I learned during the event, with figures supplemented from DANVA (2022), Water in Figures: Statistics and Benchmarking, as well as a series of site visits with the water utility FORS.

Water Facts: Denmark vs UK

• Source water: 100% groundwater, although there is a small desalination plant in Christiansø. In England, 30% of drinking water sources are from groundwater and 70% from surface water.
• Drinking water treatment works: 2600. 1,069.
• Litres of water per person per day: 105. 142.
• Water loss: 7.22%. 23%.
• Drinking water pipes: 45,000 km. 347,636 km.
• Pipe network material: 50.9% PVC, 37.2% PE, 4.4% other, 3.4% grey cast iron, 2.6% eternit and 1.5% ductile cast iron. 50% cast iron, 16% PVC, 12% PE, 11% cement and some steel, copper, glass fibre reinforced plastics and lead. The pipe network material in the UK is not always known by the utility and not information usually shared between different utilities.
• Pipes have an expected service life of 75 years. Average pipe age in the UK is 75-80 years old but some pipes still in use today have been in use for over 100 years.
• Drinking water provided: 310 million m³ every year. 16.6 billion litres every day.
• Chlorine: 0 mg/l. ~1 mg/l at the water treatment works to achieve 0.5 mg/l at the tap.
• Denmark has a goal of becoming energy and climate neutral by 2030. In 2019, water companies in England joined forces to make a pledge to reach net zero on operational emissions by 2030.
• Separated (non-combined) sewer systems: 68%. ~17.5%.
• Sewer network: 85,850 km. 571,424 km.
• Wastewater plants: 701. 6,327.
• Discharged wastewater: 683 million m³ in a year. 11 billion litres of waste water in a day.

Finacials and Ownership

The cost of water in Denmark varies throughout the country. But half a litre on average costs €0.50. That equates to €9.85 per m³, estimated using a household size of 2.12 people and a consumption of 105 litres. This is ~1.41% of the average annual household expenses. By law, water utilities are able to charge a fixed annual administration fee per household as well as a charge per m³ of water consumed, although some utilities only charge for the latter. In all, a water utility’s expenditure and income must balance, with 100% of that income being sourced from customers, of which there is a population in Denmark of 5.8 million. It is also required that if, for example, a water utility would like to install a UV system, they require a permit from the government to do so (this is not the case in England).

The cost for water in UK also varies by location, with 1 litre of water costing less than 1p. The 11 water and wastewater utilities in England (a population of 56 million) are privately owned and are designed to generate a profit. Both Scotland (which has a population of 5.5 million) and Northern Ireland (population 2 million) have only one public sector-owned utility, Scottish Water and Northern Ireland Water, respectively. The utility in Wales (population 3 million), Dŵr Cymru Welsh Water, is owned by a company but is not for profit.  

The average annual water utility bill in England was £408 a year, or £34 a month in 2021/22. In England, bills are either calculated unmetered (a set amount based on the rateable value of the home, a figure determined by the government that estimates the value of a property) or metered (the domestic water readings are taken from a water meter and consumers pay for the units of water used).

Drinking Water Sampling

The Documented Drinking Water Safety (DDS) consists of tests in selected chemical parameters (e.g. iron, manganese) and microbiological parameters (e.g. E. coli and bacterial counts) in Denmark. The frequency of samples depends on the size of the utility. 99.4% of 77 companies in 2021 met these regulations, from 14,030 accredited samples. 6 companies had to issue boil notices due to microbiological exceedances, which impacted 11,405 households in 2021. The water treatment works that I visited during my stay was Hornsherred Waterworks, which supplies Roskilde, ~82,000 people. At the site I visited, final water was sampled 2 x a week for microbial parameters and 1 x a fortnight for chemical parameters.

In the UK, water treatment works are sampled every day at the Final Water (the final sample point before the pipe network) and is also sampled at the tap, with frequency depending on the population of the zone. During 2017 to 2019, public water supply compliance with the drinking water regulations was 99.95%. The below table shows the UK regulatory frequency of customer tap sampling.

Drinking Water Treatment

There is an emphasis in Denmark on having “simple treatment” although in recent years there has been some debate as to what this means. The below figure provides a drinking water treatment example in Denmark from FORS.

There can be advanced treatment process used in the UK, especially for surface sourced water treatment works. The below figure provides a drinking water treatment example in UK from Thames Water.

 A Final Short Note on Culture

Danish drinking water production is considered in a similar way to food production. For example, plastic overshoes are worn when in the treatment plant, with different coloured lines on the floor indicating if that is a covered shoe only zone or if you are not allowed in a specific area at all. This is not something that happens in the UK.

Natalie Lamb and the nitrous oxide explainer

Currently, medium to large sewage treatment works break down sewage using activated sludge (Martin, 2022). This involves bubbling air through wastewater to support the growth of bacteria. This method has been used for over 100 years and has played a huge global role in improving public health (Martin, 2022). However, it takes a lot of energy to pump air through the wastewater, 30-40% of a water company’s carbon emissions (Martin, 2022). This is because of the release of nitrous oxide, a greenhouse gas with a global warming potential 265 times greater than that of carbon dioxide (IPCC, 2013).

Nitrous oxide (N₂O) is produced as a byproduct of the microbial nitrogen transformation processes in a wastewater treatment plant (Law et al., 2012) [Figure 1]. For instance, it is a known obligatory intermediate in the heterotrophic denitrification pathway and is also produced by autotrophic nitrifying bacteria, mainly ammonia-oxidizing bacteria (AOB) as a by-product (Kampschreur et al., 2008) (Law et al., 2012).

 

Figure 1: Microbial Nitrogen Transformation Processes (Martin, 2022)

Utilities know N₂O is being emitted but they do not know how much is being emitted so they do not know the magnitude of the issue (Martin, 2022).

One idea to decrease the N₂O is to not give the microorganisms oxygen anymore. Anaerobic treatment uses bacteria that do not nitrify so nitrous oxide is not produced. Instead, byproducts include methane (which can be recovered for energy production) and ammonia (in a recoverable form). While it does require additional processes for solids and nutrient removal, compared to activated sludge, this technique could reduce emissions by 98% and energy consumption by 80% (Martin, 2022).

 

References

IPCC, G. Myhre, D. Shindell, Anthropogenic and natural radiative forcing, T.F. Stocker, D. Qin, G.-K. Plattner, M.M.B. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, P.M. Midgley (Eds.), Climate Change 2013: the Physical Science Basis, Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press (2013) (Chapter 8)

Law Y, Ye L, Pan Y, Yuan Z. Nitrous oxide emissions from wastewater treatment processes. Philos Trans R Soc Lond B Biol Sci. 2012 May 5;367(1593):1265-77. doi: 10.1098/rstb.2011.0317. PMID: 22451112; PMCID: PMC3306625.

Martin, B. 2022. Pioneering cold anaerobic digestion: decarbonising wastewater by reducing nitrous oxide emissions, Birmingham: Utility Week Live 2022.

Kampschreur M. J., Tan N. C. G., Kleerebezem R., Picioreanu C., Jetten M. S. M., Van Loosdrecht M. C. M. 2008. Effect of dynamic process conditions on nitrogen oxide emission from a nitrifying culture. Environ. Sci. Technol. 42, 429–435 10.1021/es071667p

 

Natalie Lamb and the introduction to drinking water treatment

This post will provide an introduction to drinking water tretament, using Bristol Water, a water only utility in the UK as a case study.

An overview of Bristol Water

The two largest water treatment works at Bristol Water are Purton and Barrow.

Purton is located in the North of the region and is canal fed, while Barrow is in the South and reservoir fed. Purton Water Treatment Works has a maximum output of 165 million litres per day, providing 1/3 of Bristol Water’s drinking water supply.

The water quality in Barrow is easier to anticipate, for instance, it is known that challenges such as algal blooms are a likelihood in the summer. Purton, meanwhile, is more volatile so is more prone to weather quality shifts rather than seasonal shifts e.g., heavy rainfall and pesticide use leading to high pesticide concentrations. This makes the canal system a more risky water source, resulting in more chemicals and more energy to treat it, as well as relying on the local water quality scientists to provide a quick response to those changes in quality. One example of a response scientists could take could be to blend water with other sources of water or to stop the abstraction of water from the canal for a short period.

Some examples of challenges from the source water can include:

• Nitrate
• Microorganisms such as E. coli and Cryptosporidium
• Algae (although not as much of a risk at Purton than at Barrow)
• Geosmin (pronounced gee-oz-min), which can cause an earthy odour
• Pesticides such as glyphosate and metaldehyde
• Zebra mussels, which accumulate in biomass to narrow pipes and channels

Future treatment challenges for Bristol Water include ever tightening water quality standards, as they will never relax, they will only get tighter and lead, as around 50% of the pipes in Bristol city centre are made from lead (they would be uncomfortable having phosphate dosing for lead control off for 5 days).

 

What have you seen on site and what was it?

Treatment steps, using Purton as a case study site

1. Abstraction, including the pumping in of any raw water if required.

2. Removal. Large materials can also be removed such as shopping trolleys!

3. Blending of other water sources to help mitigate any challenges in source water, such as nitrate.

4. pH correction. This particular site uses a strong (96% sulphuric acid) but small dose. The source water is alkaline (pH 8) so an acid is added to decrease the pH to around pH 7 to ensure the coagulant is able to work efficiently. In hardwater areas (i.e. lots of calcium and minerals), such as this one, more acid is required to get the pH into the desired range. Due to this, the price of treatment of this water source for Bristol Water fluctuates seasonally (even though customer bills do not) as alkalinity shifts throughout the year because of the amount of acid required (high alkalinity = more acid needed).

5. Coagulation. Coagulants such as PAC (polyaluminium chloride) are a bit like a positively charged glue. If you add this to the water in the right conditions (such as the correct pH), particles will be attracted to it and stick together in a fluffy clump which becomes heavier and then can settle out. The clumps can also sometimes look blue, which you can see a little better in the coagulation photos. You might see on site a very fine trickle of the coagulant, this is done to encourage good mixing.  

6. Clarification by settlement. There are various different clarifiers. They are a process to settle out the heavy coagulant particles. These solids are pumped out to sludge. Around 90% of the solids removal happens here.

7. Filtration. Rapid Gravity Filters (RGF) are a bit like giant sand pits filled with anthracite media (like black sand). Water filters through the sand, removing the remaining solids. Depending on head loss (how much they are getting clogged), water is pumped up the wrong way, removing solids stuck in the filter media. This process is known as backwashing.

8. Ozonation. Ozone is a strong oxidant that is good at breaking down things such as metals, taste, odours etc.

If, like at this site, there is bromide in the raw water, it can combine with the ozone to form bromate. As such ozone levels have to be adjusted to ensure bromate levels are within the drinking water regulatory standards of 10 µg/L (this site normally sits at ~4 µg/L).

9. GAC (Granular Activated Carbon) Filters. These filters are essentially like giant Brita filters. They are also a great way to absorb things, like smell, colour, taste and pesticides.

10. UV. Ultraviolet light is used for disinfection. It was first adopted by Bristol Water in 2017 but it is now found at all of their sites. They mostly use UV for the microorganism Cryptosporidium because it is resistant to chlorine.

UVA is what causes wrinkles, UVB is what causes sun burn, UVC is the reason why you have to wear a mask while welding. UVC is the biocidal range of the UV spectrum. Even if there is a one second contact time, that is sufficient to deactivate microbes in the water.

However, you can’t get a sunburn from UVB if you are covered up because the light has to penetrate your skin- and the same is true for water. The water has to be clear, have high transmissibility, for UV to be able to act as a disinfectant. For instance, coke has a transmissivity of 0% while ultrapure water has a transmissivity of 100%. Bristol Water often has 98%.The higher this value, the less turbid the water and the more effective the disinfection.

11. Chlorination. While UV acts as the main method of disinfection (primary disinfection), chlorine is also added to provide a residual (secondary disinfection). This means that there will be some leftover chlorine presence at the customer tap to prevent microbial regrowth during distribution in the drinking water pipe network. On average, it takes ~24 hours for the water from the treatment works to reach the >400,000 people supplied in Bristol. When leaving the plant, chlorine concentrations are around 1 mg/l, while at the tap this is around 0.5 mg/l, since chlorine decays within the pipe network. In the summer these concentrations decrease quicker because heat speeds up chemical reactions, including those reactions which cause chlorine to decay.

Interestingly, this site produces their own chlorine, known as on-site electro-chlorination (OSEC). As such, liquid chlorine (sodium hypochlorite, chlorine gas dissolved in sodium hydroxide) is used rather than often the norm, chlorine gas, for health and safety reasons. They also intend to roll this out to smaller remote sites in future.

Natalie Lamb and the introduction to wastewater treatment

What is Wastewater?

Wastewater is a complex variable mixture of waste materials in aqueous suspension/solution resultant from domestic and industrial activities, mixed with varying amounts of ground and surface water. It is described using its physical characteristics (e.g. colour, smell, debris), chemical characteristics (e.g. BOD, suspended solids, ammonia, phosphorus) and strength (e.g. septicity).

Sewage works are sampled so the works performance can be monitored and to ensure parameters are within legally consented values. Sample data can be used to identify early warnings of problems and can confirm if the site is adequately loaded, under loaded or overloaded.

On site testing can consist of: clarity (to see how clear the final effluent discharge is and to get information on suspended solids), ammonia, settlement volume, temperature (treatment is not as efficient in lower temperatures) and sludge depths of tanks (to ensure a compact sludge blanket with minimum suspended solids). Lab tests can include: BOD, COD, suspended solids, ammonia, phosphorus and metals such as iron and aluminium.

Wastewater treatment occurs in 5 stages: preliminary, primary, secondary, tertiary and sludge.

The main reduction in pollutants such as ammonia and BOD occur through biological treatment in the secondary treatment step. Other pollutants including suspended solids and phosphorus are removed by mechanic and/or chemical methods.

Overview of Treatment - Wessex Water

Preliminary Treatment

Preliminary treatment consists of storm separation, a screening process, grit removal and a compactor system.  

FFT (flow to full treatment) is the maximum rate of flow that a sewage treatment works can treat before the storm flow system goes into operation. During heavy rain, the flows to a sewage works increase, moving closer to the FFT. When flows exceed the FFT, the flow enters the storm system, normally a series of storm tanks used for storage and settling. When these storage tanks are also filled up, the flow (which by this point is very dilute) enters the surface water, such as surrounding rivers. 

The screening process can consist of a few different technologies to remove RAG, fibrous material that does not break down after being flushed down the toilet. 

Rotating Rake - Wessex Water

The image depicts a rotating rake. In this process, RAG scrapes up the red line (in the left image) and into a trough. A 6 mm mesh blocks the solids. When solids build up at the bottom, higher up the mesh is used instead (like the zombies going over the wall in World War Z!). When the solids build up so far, a wash is triggered to clean the mesh from blinding. During the wash, the screen rotates and the brush will clean any solids and send them to a compactor system. 

Paramount Pictures

The compactor system looks like a screw. The system dewaters and compresses waste, with liquid being drained through the lower part of the compactor.

Compactor System - EvoTech

There are various different types of grit removal. Grit removal is the process used to remove sand, silt and grit from water.

Primary Treatment

Primary treatment is a physical phase separation used to remove settleable solids. It normally consists of primary settlement tanks, which settle out solids, in a way similar to storm tanks. The inlet enters through the bottom of the tank, then the flow radiates outwards. Often there is a rotating scraper which scrapes sludge into a hopper. 

Primary Treatment - Wessex Water

Primary Settlement Tank

Secondary Treatment 

Secondary treatment is the removal of biodegradable organic matter using biological processes. In this treatment stage, microorganisms digest sewage in a managed aerobic or anaerobic process depending on the treatment technology, using either a fixed-film or suspended-growth system. 

Biofilm grown on a fixed media:

• Biological Trickling Filter
• Submerged Aerated Filter (SAF)
• Biological Aerated Flooded Filter (BAFF)
• Rotating Biological Contactor (RBC)
• Submerged Biological Contactor (SBC)
• BioClere Package Plants
• High Rate Filter (HRF)

Suspended biofilm media:

• Activated Sludge Plants (ASP)
• UNOX
• Sequence Batch Reactor (SBR)
• Oxidation Ditch

Helpful microorganisms in biological processes which aid settling include: crawling ciliates, free swimming ciliates, rotifers and stalked ciliates. Undesirable microorganisms can consist of: filamentous bacteria, amoeba and nematode worms. 

Settlement volume (SV) is a way to see how healthy sewage is and how well it settles. If it does not settle well, it is likely there are filamentous bacteria, if it does not, there is not likely microorganisms present that are good for treatment.

Biological Trickling Filters are often seen at UK wastewater treatment works. There are nozzles in the rotation arms. Sewage goes through the nozzles which drives the filter around, meaning no power is needed. This sewage then percolates through a filter to under drain channels. 

There is ~2m deep biomass growth on the surface of the media which needs food (sewage), a moist home (the media surface) and oxygen to respire (obtained through air vents). At the top of this biomass there is BOD bacteria, nitrifying bacteria growth in the middle and ammonia treatment happens at bottom. 

Trickling Filter - Water Action Plan

ASP are another treatment technology often seen at UK wastewater treatment works. Sewage enters, is mixed with microorganisms in the selector zone and passes through aerated lanes. These lanes have a certain retention time to give bacteria time to digest the sewage. 

Suspended Growth Porcesses: Wessex Water

BAFF

Final settlement tanks (FSTs) are similar in design to primary settlement tanks and are an integral part of the activated sludge process.

Tertiary Treatment

Tertiary Treatment is used for consents (e.g. suspended solids, BOD, ammonia) and nutrient removal (e.g. phosphorus, iron, aluminium). Examples of treatment processes can include: sand filter, mixed media filter, cloth filter, UV, lagoons, microscreens, reed beds and grassplots.

 

Sludge

Secondary treatment processes can get a solids build up, a surplus of activated sludge (SAS). Some of this activated sludge goes to waste at a sludge treatment centre. 

Solids are collected in primary and final settlement tanks. This is dewatered and then tankered to a sludge treatment centre. The dewatering is helpful to reduce the number of tankers. 

Natalie Lamb and the mixed liquor suspended solids

I came across a new term recently, MLSS (mixed liquor suspended solids). Here are some of the things that I have learned about this measurement in sewage, what it actually means, how operators use it and where the Sentry sensor, a way for operators to measure microbial metabolic activity in real-time, can add value.

Introduction to MLSS

Wastewater is composed of different inorganic and organic substances, including (but not limited to) excrement, detergents, soaps, fats, greases and food particles. These large organic molecules are easily decomposed by bacteria into smaller molecules and eventually into carbon dioxide and water, but oxygen is required. The amount of oxygen required is known as the biochemical oxygen demand or BOD. BOD is often used as a measure of the “strength” of sewage (the amount of biodegradable organic material in sewage).

 For the activated sludge process to operate properly there must be a balance between the food entering the bioreactor (measured as BOD, COD, or TOC) and the microorganisms in the bioreactor (estimated using MLSS). MLSS is actually the concentration of suspended solids in the mixed liquor (mixed liquor= sewage + microbial mass) i.e. the amount of biomass in the system. In the below image, the F:M ratio (food-to-mass or food-to-microorganism) is used to express this relationship.

• F:M ratio
• Food = influent flow x influent BOD or COD concentration
• Microorganisms = aeration system volume x MLVSS or MLSS

 

https://www.thewastewaterblog.com

MLSS Adjustment 

Ideally operators would want to keep MLSS at a constant level in the reactor (for ASP that is generally between 2000-4000 mg/l). Operators can do this by returning part of the activated sludge to the start of the treatment process, called RAS, which would increase the “food” or MLSS, or by not returning leftover sludge, called SAS, to decrease the available “food” or MLSS. It is noted, however, that it takes a while for the new MLSS concentration to establish.

 

There is motivation to maintain MLSS levels because when MLSS is too low, the plant is not operating in an energy efficient way. Equally, when MLSS is too high, the works gets overloaded, DO decreases, organic matter is not fully degraded and the biology dies. In this case, operators would get a bulky sludge that is more difficult to settle in the final settlement tank.

 A further motivator for adjustment is that when the MLSS is increased, technically the BOD is decreased. There is motivation to do this because BOD is a parameter included in Environment Agency monitoring (as well as COD and sometimes total phosphorus and total nitrogen).

The Value of Sentry in MLSS

The value of using a Sentry probe over using MLSS is that while MLSS is the amount of biomass in the system, a normal probe would quantify everything suspended in the liquids, including mass that is not respiring, such as dead biomass. Even MLVSS, which is considered to be a more accurate estimate of the mass of microorganisms than MLSS, includes both microorganisms and organic matter.

Since Sentry quantifies the electrons emitted when microorganisms respire, only the “live” biomass is quantified. In this way, when operators are deciding whether to return part of the activated sludge to the start of the process or whether to discard it, they can get a more accurate picture of the microorganisms present within that sludge to make a more informed choice.

Summary

Thank you very much for taking the time to read this blog post today, to learn more about MLSS, ASP and the value of Sentry, brought to the UK by QCL! Please feel free to get in touch for more information.

Definitions

• MLSS- mixed liquor suspended solids
• Mixed liquor- sewage + microbial mass
• ASP- activated sludge process
• Reactor- a sewage reactor is any tank where biological reactions take place (e.g. ASP) and is not a specific process
• BOD- biochemical oxygen demand i.e. the amount of DO needed by aerobic biological organisms to break down organic material
• RAS- return activated sludge or sludge return flow rate
• SAS- surplus activated sludge
• DO- dissolved oxygen
• F:M ratio- food-to-mass or food-to-microorganism ratio
• MLVSS- mixed liquor volatile suspended solids

Natalie Lamb and the 4 main challenges Anglian Water face regulating trade effluent discharges

  

1. There are more than 3900 consented discharges of trade effluent in the Anglian Water region

• Variety of companies = variety of discharges
• Each industry will have their own consent for the quality and quantity of the discharge
• There are health and safety implications for sampling in different locations in different companies (working at height, lifting, dangerous substances, hot effluent, lone working)

Anglian Water

2. Anglian Water is a relatively rural region 

• There are small WRCs that could have difficulties with a trade effluent type or with capacity
• Rural small traders may be difficult to track

Ander et al. (2013)

3. Collaboration is key

• Water Industry Act 1991
• Brexit - The Urban Waste Water Treatment Directive – will trade effluent become less important in future? 
• Is collaboration more difficult with market separation?
• What is its impact on compliance and customer relations?
• Some companies now take internal samples 

4. Future challenges, both predictable and unpredictable

• The ongoing COVID-19 pandemic has resulted in both supply chain issues and the closure of many business premises
• Changes in consumer habit
• Unregulated discharges of milk, dairy products and dispose of the contents of kegs/casks

Summary