One of the NRNs major survey projects has been the monthly testing of the water quality of various selected sites around Eynsham and now also at South Leigh. We undertook this because unpolluted waterways are a vital necessity if we are to achieve nature recovery in and around our watercourses. 

At the forefront of this project is the vexed issue of the pollution of the Thames and its tributaries, one of which being the Limb Brook, which runs beween South Leigh and Eynsham.

Sewage from Eynsham and surrounding villages is processed at the Cassington Sewage Treatment Works (STW), which is operated by Thames Water. Cassington STW is one of the many sewage treatment works that repeatedly release raw sewage directly into the Thames and its tributaries.

A small group of Concerned Parties were offered a guided tour of the Works, which took place in mid-December 2021 and it seemed an ideal opportunity to understand more about how the sewage from 17000 souls is processed.

This Article began as Kevan Martin's report about that visit. Somehow it morphed into Long Mead's Long Read for New Year 2022. Please feel free to dip in and out - or just look at the pictures.


Walking in to Cassington's Sewage Treatment Works (STW) and seeing the reactors and settling tanks was a déjà vu experience for me – probably for two reasons: Firstly, Long Mead has its own package sewage treatment plant that operates on exactly the same principles (although we release its effluent into a drainage field, not into the Thames), and secondly, during my misspent youth I did research for my Master’s degree in Civil Engineering in a Water Resources lab. The title of my thesis was, ‘ The kinetics of enhanced phosphorus removal in the activated sludge system.’

What is ‘activated sludge?’ you ask, 'And what's it got to do with sewage in the Thames?’ Well, if you read on you will find many answers, but since you are sitting comfybold and two-square...let me begin by perambulating around the delicate and historical problem of how we dispose of human waste.

Its just fertilizer!

First, why treat sewage at all? Why not just use it as fertilizer and put it directly on arable fields as humanure? This is of course what farmers traditionally did, and many still do. Indeed, the million or so tonnes of sludge from treated sewage we generate each year is used in agriculture, because of the nutrients it still contains. In towns, however, disposing of raw sewage by making it into fertilizer for agriculture is not an easy option, but ‘night soil’ (i.e. urine and faeces) collected in a ‘honey bucket’ is still practised in towns and cities worldwide for use as fertilizer.

In China the collection in honey buckets was delicately called ‘emptying nocturnal fragrance’.  In Japan, the night soil of rich households fetched higher prices as fertilizer, presumably because rich people’s diet was more varied and more nutrients were present in their night soil. In Mexico, the Aztecs built wicker containers called chinampas – effectively compost heaps - to receive sediment, night soil and decaying vegetation, upon which they planted the crops of a market garden; some of these still exist. In the Amazon, the terra preta de indio or ‘black earth of the indians’ probably reflects a similar ancient practice. In Tudor England a ‘gong farmer’ collected the contents of privies and cesspits at night. Later a privy midden involving an outhouse and associated midden was used to dispose of waste. These in turn were superseded by pail closets, which required emptying, and water closets, in which the waste was flushed into a sewer system.


ChinampaChinampas, Mexico.
Source U.N. FAO


Sewers, Part I.


Sewers have a long history. Ancient Athens piped its sewage to a reservoir and then channelled it to the Cessiphus river for use as fertilizer. Ancient Romans built the cloaca maxima to take their waste water to the Tiber river; it still functions. In London, inadequate and leaking sewers and cesspits led to contamination of the drinking water and repeated outbreaks of cholera and typhoid.

That the Thames was badly contaminated was already long-appreciated – in 1372 Edward III forbade throwing waste into the Thames, and in 1388 an Act of Parliament, ‘forbade the throwing of waste into ditches, rivers, and watercourses.’   Although the increase in flush toilets might have improved household hygiene, it had the inadvertent consequence of contaminating the aquifers and water courses from which drinking water was drawn beecause the pipes leaked. Diseases were almost inevitable, as the Thames itself was used as an open sewer, receiving untreated human waste, discharge from industries, and slaughterhouse effluent.

Dr. John Snow (1813-1858) pioneered an epidemiological approach to discover the source of a cholera outbreak. His statistical mapping of the addresses of the fatal cases led him to identify a water pump in Broad Street, Soho as the source of the major cholera outbreak of 1854. The solution was to remove the pump's handle  (a replica pump is still there as his monument). Although he could not identify the cholera bacteria in the pump water, he produced some of the earliest scientific evidence that cholera was water-borne, and not due to ‘miasma’ as the Victorians generally supposed.



John-Snows-1854-Broad-Street-epidemic Dr. John Snow's map of showing the cluster of cholera cases around the water pump in Broad Street.


The Pause that Refreshes: Drinking Thames Water.


Dr. Snow’s research had shown convincingly that waterborne contaminants from sewage could enter drinking water (Snow himself always drank boiled water). Provision of safe drinking water, in cities at least, where sewage contamination was concentrated, meant finding some method of purification. John Doulton had a pottery at Lambeth and as early as 1827 had developed a ceramic water filter. His son Henry, was commissioned by Queen Victoria in 1835 to produce a water filter for use in the royal household and finessed his father’s prototype. The pores in the ceramic ‘candles’ he developed are small enough to filter out all bacteria, viruses, and probably prions as well.


Royal Doulton water filter
Royal Doulton's ceramic water filter.
Photo Merrill's Auctioneers, Vermont.


Doulton’s ceramic filters are now incorporated as a modification of the Berkefeld filter, a German invention that used diatomaceous earth as the filter. The British Berkefeld gravity filter accompanied British missionaries across the world to provide them with safe drinking water. (We are not told what water their converts drank). Although not missionaries, we also use a British Berkefeld on Horseshoe Island to filter our drinking water, which we draw directly from the Thames. The addition of silver and charcoal to Doulton’s ceramic filter provide a further disinfection step and the charcoal removes most pesticides, herbicides and organic solvents. Arguably, our filtered Thames water is purer (and better tasting) than Oxford’s water, 80% of which drawn from the Thames and is therefore highly chlorinated to reduce the concentration of waterborne pathogens (that’s ‘reduce’ not ‘eliminate’).


Sewers, Part II


Despite repeated outbreaks of cholera in London, it took the hot summer of 1858 to produce a sufficiently ‘Great Stink’ immediately outside the Houses of Parliament before the politicians were moved to do something radical about Thames pollution and they urgently voted in a Bill to build a new sewer system. The chief engineer of London’s Metropolitan Board of Works, Sir Joseph Bazalgette (1819-1891), was put in charge and given a £3million budget (raised by a tax levy) to build a sewer network that would capture London’s sewage and surface water from rain and release it further downstream to be swept out to the estuary on the ebbing tide.


Bazalgette_sewernetworkBazalgette's sewer network of 1858


To hide the northern one of the two major 'intercepting' sewers he located on either side of the Thames, Bazalgette built the new embankments of Albert, Victoria, and Chelsea. The final outfall into the Thames was on both sides of the river: at Crossness on the South bank and Beckton on the North bank.


Crossness_PeterScrimshawCrossness pumping station - a gem of Victorian engineering.
Photo Peter Scrimshaw


Significantly, nowhere in Bazalgette’s plan was any intention of separating the surface water arising from rain from the sewage; both of these were piped through the same sewer system - the so-called 'combined sewer' design.


Difference-between-a-combined-and-a-separated-sewer-systemCombined vs. separated sewer designs.


Nor did Bazalgette have any intention to treat the raw sewage before discharging it into the Thames: he only made provision to hold back 6 hrs worth of sewage at the Southern and Northern Sewer Outfalls by pumping it into 27 million-gallon reservoirs so it could then be discharged into the Thames on the outgoing tide. Of course, the incoming tide then brought much of the sewage back to where it started and it took days for a given discharge to reach the estuary. Essentially then Bazalgette's costly solution was simply to move the 'Great Stink' further downstream, away from the MPs sitting at Westminster, but this strategy quickly turned and bit him.

Bazalgette's new outfall of London’s sewage was the site of a tragedy, In 1878, the pleasure steamer, Princess Alice, was returning from Sheerness in Kent and collided with a collier, the Bywell Castle. Londoners on their 'moonlight trip' were pitched into the fetid water and over 600 people died, some from ingesting foul water due to the millions of gallons of raw sewage that had been released into the Thames an hour before the collision.


Ticket_for_the_Princess_Alice,_1878Ticket for the Princess Alice 1878.
Photo Greenwich Maritime Museum


This tragedy, the worst loss of life ever in UK inland waters, led to a Royal Commission in 1882, ‘to inquire into and report upon the system under which sewage is discharged into the Thames…whether any evil effects result therefrom; and, in that case, what measure can be applied for remedying or preventing the same.’

The main conclusions of the Royal Commission's findings sound very contemporary:

1. London’s sewage should not be discharged in its raw state into any part of the Thames,

2. solids and liquids should be separated by some process of deposition or precipitation, and

3. liquids ‘might, as a temporary measure’ be discharged into the Thames.

Clearly the Commission did not think that the liquid fraction was free from ‘evil effects’ and intended that some tertiary treatment should be applied to the liquid, perhaps by sand filtering or chemical treatment, which is what they did by building a precipitation works at Barking and Crossness (Humphreys, 1930). To deal with immediate problem of separating the liquids and solids, sedimentation tanks were added to the holding reservoirs of the Southern and Northern Sewer Outfalls. The settled sludge was loaded into purpose-built vessels and dumped further out to sea, a practice that was only discontinued in 1998.


Technologies for treating sewage.


The prime goal of all treatments of domestic sewage is to digest as much of the biological nutrient content as possible. Preferably this is done using biological processes rather than artificial chemical means, which are expensive and require close monitoring. Preferably too it needs to be done rapidly and at scale, which rules out septic tank-type systems, which operate anaerobically (which is slow) and produce a poor quality effluent (which is why septic tank effluent cannot legally be discharged into a watercourse).

In the 1870s the English chemist Edward Frankland discovered that when sewage was passed through a gravel filter bed, one major component of sewage, ammonia (mainly from urine), was oxidised to nitrate. This is a crucial step that remains central to modern-day treatments of domestic sewage. Frankland realised that it might be possible to use biological means to treat sewage - this was a revolutionary insight that led directly to the development of modern sewage treatment plants. In the first stage of the evolution of sewage treatment plants, the raw sewage was piped to tanks containing stones or slate to maximize the surface area available for the microbial growth needed to break down the sewage by oxidation. The effluent of this process was filtered out into the ground. This ‘contact bed’ method of sewage treatment became widespread in UK cities.



Trickling_filter_bedSewage treatment by trickling filter bed using plastic media at a small rural treatment plant. Beddgelert sewage treatment works, Gwynedd, Wales.
Photo Valela.


An improved variant on contact beds was the trickling filter, in which sewage trickles through a bed of rock, slag, or plastic contained in a tank 4-12 meters deep, making the filters look like towers, which are conveniently compact (see picture above, and Stanton Harcourt’s and South Leigh’s sewage treatment works). As in the contact beds, the microorganisms in trickling filters are attached to the medium (rock, slag, plastic etc.) and digest the sewage as it trickles down through the filter. After passing through the filter the effluent is led to a settling tank to separate the solids and liquids. Some of the liquid is returned to the trickle filter to improve the wetting and flushing of the filter medium. These systems provide good digestion, but only if the input rates are low.

The next major advance, which brings us right up to the present, was not long in coming. Gilbert Fowler (1868-1953) was a biochemist who studied bacterial growth and their oxygen requirement. His experiments showed that the sludge was in fact the key factor in the decomposition of sewage and that the process of rapid digestion demanded oxygen, so he devised a system whereby the sludge could be retained and aerated – so-called ‘activated sludge’. Fowler’s system was developed to plant scale by two of his students, E. Ardern and WT. Lockett who published 3 classic papers on their design (Ardern & Locket, 1914a,b; 1915). Their system was first successfully deployed at Salford in 1914 and at Davyhulme in 1915. Their design is essentially the same as that now used at Cassington STW.


Adern_LockettE Ardern (left, standing) and WT Lockett (right, sitting).
Photographer unknown.


There are three key steps in any activated sludge system (see schematic below) : 1. The waste water containing aerobic microorganisms is aerated, 2. solids are removed by sedimentation, 3. biological solids are recycled back into the aeration tank. This third step concentrates those aerobic microorganisms that can be sedimented, while the non-settleable aerobic microorganisms are washed out. The concentrated culture of aerobic microorganisms in the aeration tank ensures that the incoming organic matter in the sewage is oxidised to carbon dioxide and water in a reasonable through-put time in the aeration tank, (usually 8-12 hours in the UK). Longer times would require bigger tanks.

activated sludge schematicSchematic of activated sludge system


So lets elaborate  this simple schematic to capture the steps of the process deployed at Cassington STW. There are a few components we have to add. The first is a primary treatment to remove detritus that we have thrown down the toilet or into our drains. This involves a lot of grease in the form of 'fatbergs' of various dimensions, cloth, plastic, grit, etc. Much of this debris can be removed at the primary sedimentation stage and has to be disposed of separately to landfill or incineration. The now 'pure' raw sewage is then led to an  aeration tank, then on to a settling tank that separates the solids from the liquids. The liquids are then piped to the Thames, and the solids are piped to Sandford for dewatering and disposal. At Cassington STW a coagulant, ferrous chloride, is added at the inlet - this is for removing phosphorus in a process that will be discussed further below in, 'What about Eutrification?'


schematic_cassingtonSchematic of the Cassington STW system. (see Sharma & Sanghi, 2012 for these and many more details on current technologies). 


 We can now map this schematic onto the Cassington STW shown in this bird's eye view:


CassingtonSTW_birdseyeBirds' eye view of Cassington STW. A: Coagulant dosing. B: Primary sedimentation and filters. C: Inlet to aeration basin ('oxidation ditch'). D. Brush aerators. E. Secondary sedimentation in settling tanks. F: Buffer tanks for storing flood water. G: Final monitoring point before discharge to Thames.  Arrows show the direction of flow in the aeration basin.


Sludge Age

What has not yet been mentioned is the critical parameter in the control of activated sludge systems, and that is something called the ‘sludge age’, which is the average time in days that a suspended solid remains in the entire system. The sludge age is typically between 3 and 30 days, depending on local conditions (Cassington STW maintains a sludge age of 15-20 days). Sludge age is not to be confused with the hydraulic retention time - the time a parcel of fluid spends in the tank - typically is 1-2 days. The sludge age is relatively easily maintained by wasting a defined proportion of the reactor volume daily.

To reitterate: sludge age is the most fundamental and important parameter in the design, operation and control of activated sludge systems. As a rule-of-thumb, the better the effluent and waste sludge quality that is required, the longer the sludge age has to be, the larger the biological reactor needed, and the more the incoming wastewater characteristics need to be known, for the amount of nutrient in the incoming raw sewage determines how much live biomass is generated.

The solids in an activated sludge system consist of a mix of live microorganisms and dead ones, and the longer the sludge age, the lower the fraction of live microorganisms in the mix, so most of the sludge is simply bulk - not doing any work - and this affects the quality of the effluent. In long sludge ages where too many solids have accumulated in the system, a granular sludge particle called ‘pin floc’ is produced, making the effluent turbid, which is one measure of effluent quality monitored at Cassington STW. Short sludge ages may produce a fluffy, buoyant ‘straggler floc’ which settles slowly and may be carried over the settling tank weirs as sludge particles even if the effluent is otherwise clear.


cassington_sewage_works_settledFinal clear effluent being drawn off the settling tank for discharge into the Thames. Cassington STW.
Photo Adrian Arbib.


The sludge age is critical to the nitrification of ammonia, which like all chemical reactions is temperature dependent, but is also highly dependent on the rate of growth of the species of bacteria that actually do the job – more fascinating details on that subject follow in the section: ‘What about eutrification?’

In the UK sewage treatment plants the method of aeration is usually by mechanical agitation (Cassington STW uses rotating wire brushes, see illustration below), while in USA (and Long Mead) it is by diffused aeration (as in a fishtank). The main difference between the two methods is that mechanical agitation aerates the surface liquid well, but the deeper liquid less so, whereas the diffused aeration stirs and aerates all the liquid equally.

On our visit, the dissolved oxygen concentration at a depth of about 1m in the Cassington aeration tank was only 0.6 parts per million (ppm); for comparison, open water has a dissolved oxygen concentration of around 10 ppm, as Dr. Lucy Dickinson and her trusty Oximeter can confirm. This low concentration of dissolved oxygen measured in the Cassington aeration tank shows how much oxygen demand there is and why how untreated raw sewage entering a natural watercourse like the Thames would deoxygenate it.


cassington_sewage_works_aerationRotating metal brushes aerate the mixed liquor and propel it around the oxygenation ditch. Cassington STW.
Photo Adrian Arbib.


The effectiveness of the activated sludge digestion is typically assessed by measuring the ‘biochemical oxygen demand’ (BOD). The BOD is the amount of oxygen needed by the microorganisms to digest (metabolise) the organic material present in a given volume of waste water in a given time (typically measured in the lab. over 5 days at 20 deg. C; this time needed for a  BOD test result is a good reason to use the related method of chemical oxygen demand, ‘COD’ where the analysis can be made in under an hour (which is why I used the COD method during my M.Sc. research).

In their 8th report published in 1912, the Royal Commission on Sewage Disposal specified what has now become the International Standard for discharge into rivers: the '20:30 standard', which allowed, ‘2 parts per hundred thousand’ (i.e. 20ppm) of BOD to be released and ‘3 parts per hundred thousand’ (i.e. 30ppm) of suspended solids. (summarised in Royal Commission final report of 2015).

The BOD value for the wastewater entering Cassington STW is 200-300 ppm and the BOD of the treated effluent they release into the Thames should be no more than 20 ppm. 

What about eutrophication? 

(Health Warning from the Editor: Dear Reader, this is a fascinating and important topic, but if you have an untreatable allergy to biochemistry, please avert your gaze and skip nimbly down to the big blue disc that summarises the Nitrogen Cycle and then on to 'A word about Pathogens')

Having reduced the biodegradable fraction of the oxygen demand and removed the solids by using the activated sludge process, we are still left with two main pollutants in domestic wastewater – nitrogen (N) and phosphorus (P) that can pollute our waterways and lead to eutrophication. Dr. Lucy Dickinson has been measuring nitrate and phosphate levels in our watercourses and, as she reports, their levels are excessively high.


In the activated sludge system, nitrogen in the form of ammonium (NH4+) is converted to nitrate (NO3-) (i.e. nitrified). This is carried out in two stages by specialised microorganisms which need dissolved oxygen.

Ammonium (NH4+) is oxidised to produce nitrite (NO2-); this is mainly carried out by Nitrosomonas type bacteria by the following reaction

2NH4+ + 3O2 --> 2NO2- + 4H+ + 2H2O

In the second stage NO2- is oxidised to produce nitrate (NO3-). This is carried out by bacteria belonging to the Nitrobacter genus

2NO2- + O2  --> 2NO3-

From start to finish, the total oxidation reaction can be summarised as,

NH4+ + 2O2 --> 2NO3- + 2H+ + 2H2

Nitrosomonas and Nitrobacter bacteria are autotrophs (also known as ‘producers’ because the make their own food from raw materials and energy) and their growth is slow compared to that of heterotrophic organisms (known as ‘consumers’ because they consume producers or other consumers). The limiting parameter is the oxidation of ammonium to nitrite by Nitrosomonas, whose slow growth rate requires long sludge ages, especially in winter when temperatures drop. In designing a reactor for nitrification, therefore, it must accommodate the volume of sludge that accumulates at long sludge ages. 

The alert among you will have noticed that both these reactions require oxygen, which adds to the total oxygen demand of the aeration basin. We can easily calculate from these equations that the complete oxidation of ammonia nitrogen requires 4.57g O2 per g N-NH4+.

You will also have noted that all we have done is to convert ammonia to nitrate, which is a key ingredient of fertilizer. Commercially, the Haber-Bosch process artificially fixes nitrogen gas from the air to make ammonia, which is then converted to the fertilizer ammonium nitrate (NH4NO3). or urea (CO(NH2)2).

The Haber-Bosch process revolutionised agriculture – and warfare - providing copious amounts of ammonia for the manufacture of artificial fertilizer, which fuelled the Green Revolution, and the raw material for the high explosives used by the German Army during WW1.

Interestingly, we learned on our visit to Cassington STW that Thames Water measure the ammonia in their effluent, but have no statutory duty to the Environment Agency to measure the amount of nitrate, let alone do anything about it. It is just released into the water courses to add to eutrophication.

Is this the best we can do?

As it turns out, we could do better: by a simple modification of the activated sludge process we can denitrify the effluent, turning the nitrate into nitrogen gas.

NO3-  -->  NO2-  --> NO  -->  N2 (nitrogen gas)

There are several ways in which this can be done, but the most common way is through species of heterotrophic bacteria that grow in the absence of dissolved oxygen (i.e. anaerobic conditions), and so use the oxidised nitrogen as their source of oxygen. This reaction also needs biodegradable BOD (e.g. raw sewage). By stripping the oxygen from the nitrogen, we end up with nitrogen gas (N2),  which is released to the air. This important reaction can be realised by adding an anaerobic reactor tank at the head stage of the activated sludge plant.


anaerobic_processes2Schematic of process for nitrificaion and denitrification of sewage. Note the addition of an anaerobic reactor at the input.


At Cassington STW there is no anearobic tank. Aeration is carried out in a large oval ditch containing two horizontal brush aerators on the surface that also circulate the mix of liquid and solids (mixed liquor) around the ditch. This design is commonly installed because it is a robust ‘fit and forget’ technology, easy to maintain and resilient to changing loads. . The sludge age in such ‘oxidation ditch’ designs is typically 12-20 days with a hydraulic retention time of 24-48 hours. In line with the Cassington STW parameters. Unlike completely mixed tanks with diffuser aeration systems attached to the floor (as in the plant at Long Mead) the oxygen concentration in the oxidation ditch varies – high at the surface as the liquor exits from the aerator and lower in intermediate zones and depths as the aerobic bacteria consume the oxygen. This means it is possible that sufficiently anaerobic conditions exist in the aeration basin at Cassington STW for some degree of denitrification. But until the output levels of nitrite and nitrate are actually measured, we will have to leave this for now in the tray marked ‘Known Unknowns’ 

To summarise: the various steps of the nitrogen cycle are illustrated graphically below and show the fixation of nitrigen in the atmosphere by converting it to ammonia, the stages of aerobic nitrification, and the processes of dentitrification that returns the nitrogen gas to the atmosphere.

itrogen_cycleThe Nitrogen Cycle.




Although the kinetics of phosphorus uptake was the subject of my M.Sc. thesis, I promise to be brief-ish.

At Cassington STW, phosphorus (P), in the form of phosphate (PO43-), is removed chemically by adding iron chloride to the influent. Simples. 

The ferrous form of iron chloride (iron II chloride) is used at Cassington STW because it is cheaper than the ferric form, but since ferrous chloride does not precipitate soluble phosphate, it has first to be converted to ferric chloride (iron III chloride), which occurs through oxidation in the aeration tank. Then the reaction is 

Fe3+ + PO43- <--> FePO4 

Ferric phosphate precipitates out of the liquid and is removed with the sludge.  

There is a cunning biological method of removing phosphate that I studied for my M.Sc. It was observed that the anaerobic step needed for denitrification may also evoke a phenomenon known as ‘luxury uptake’ of phosphorus, where phosphorus-accumulating bacteria take up substantially more phosphorus than they need for growth, which is normally only 1.5-2.0% of their mass. Under luxury uptake this fraction can increase ten-fold. Thus, if a sufficient quantity of live bacteria that have undergone luxury uptake of phosphorus can be removed before they die, a substantial reduction in the phosphorus load can be achieved without using chemicals. This is only achieved at very short sludge ages.

The main difficulty in the UK is that our relatively cool climate means that the sludge age cannot be reduced sufficiently to make this a viable and reliable method of phosphorus removal, even if the denitrification step were present.  

A Word about Pathogens


Monster_Soup_WellcomeMonster Soup commonly called Thames Water.

Punch cartoon, Wellcome collection


The most frequently-mentioned of river-water pathogens are Escherichi coli, which are only one of the rich mix of 1000 species of bacteria that live in all of us and which we donate to our sewage.

The bacteria in our colon weigh about the same as a pack of butter. The health of this colony (no pun intended), which is referred to in the trade as the ‘gut microbiota’, is a key contribution to our own health, physical and mental. The list of vitamins that our gut microbiota synthesise sounds like a line of a patter song by Gilbert and Sullivan:  thiamine and folate, biotin and riboflavin, panthothenic acid and vitamin K. Although we usually have an adequate supply of most of these vitamins in our diet, the vitamin factory in our gut provides a buffer when any of these vitamins are low or missing in our diet; indeed, as much as half our daily vitamin K requirement is provided by the gut biota.

The genetic composition of the gut biota (the gut ‘microbiome’) has been analysed and it shows clear links to our health, especially immunity, and even our behaviour.  A recent large-scale study by Tim Spector, professor of genetic epidemiology at Kings College, London, and his colleagues reported that people who eat higher quality diets (i.e. low levels of ultra-processed foods) have healthier gut microbiota and associated better health, including a 10% reduced risk to Covid-19 infection and a 40% reduction in severity of symptoms if they did develop Covid-19. 

Note in passing: Guilia Enders, a German medical microbiologist, wrote a best-seller called ‘Darm mit Charme’ (titled ‘Gut’ in the English translation), in which she gives a passionate and informed account of this amazing organ. Its highly recommended (if the subject now intrigues you). Similarly Tim Spector’s ‘The Diet Myth’ gives a generous and very accessible scientific account of the importance of gut biome and his epidemiological investigations of how diet affects it. For example, the Hadza, a hunter-gatherer tribe of Tanzania, have the richest and most diverse gut biome yet discovered, which Prof. Spector attributes to their foraging lifestyle and the high diversity of plants and animals they eat (about 600 species of plant and animals). When he spent 3 days with them, eating what they ate, that short time was enough to increase his gut biodiversity (temporarily) by 20%. So re-wilding our diet and lifestyle makes medical sense. Prof. Spector’s conclusion is that,

Being more adventurous in your normal cuisine plus reconnecting with nature and its associated microbial life, may be what we all need.

 What's to be done? 

Margaret Thatcher’s government privatised the water industry in 1989, writing off all the debt that the nationalised water industry had accumulated. Perhaps her government had some forlorn hope that these new monopolies would reinvest some of the considerable profits they were going to make as in renewing the infrastructure, but managers and shareholders exploited their monopoly power to take £ billions in profit. In 2020, the companies spent 3.1m hours dumping sewage into rivers. Of course we the consumers and payers of water bills are outraged, but, as Dieter Helm, Professor of Economic Policy at Oxford University, has pointed out, ‘The water companies behaved exactly how we believe a commercial company does behave. The question is, do we expect capitalists to behave like capitalists? What we have seen is a comprehensive regulatory failure [i.e. by Ofwat and the Environment Agency] to control the companies.’ 

So what can we-the-people do?

Our Brief History of pollution in the Thames tells us that our problem is not new: since 1375 there have been injunctions not to pollute the Thames, yet it has continued unabated.

In 'Our Mutual Friend' by Charles Dickens (1812-1870 the Thames is the setting for the plot and so watery phrases abound, such as, "time had come for flushing and flourishing this man down for good". Dickens was certainly well aware of the polluted state of the river, as was his contemporary, the scientist Michael Faraday, who gave us the concept of electrical and magnetic 'fields'. Faraday wrote a letter to The Times, complaining that the water in the River Thames was so polluted that pieces of white card could not be seen 1 inch below the water's surface.



Michael_Faraday,_British_chemist_water_pollutionMichael Faraday (1791-1867), British chemist, giving his business card to Father Thames, who is covered in sewage and surrounded by dead, carcasses.
Punch cartoon 1855

The most decisive action to clean up the Thames came was when  MPs in Westminster voted to improve London’s sewer infrastructure, but it is significant that this only happened because there was a ‘Great Stink’ that made MP's life in the Houses of Parliament unbearable. Thus, one lesson from history is: make a 'Great Stink' for the politicians and hold the regulator's feet to the fire. This is something that many organisations including WASP (Windrush Against Sewage Pollution). Surfers Against Sewage, Thames 21, and the Rivers Trust have been doing very effectively, aided by academics like like Prof. Peter Hammond, and media-savvy activists like George Monbiot and Fergal Sharkey.


Sir Joseph BazalgetteMemorial to Sir Joseph Bazalgette in the Victoria Embankment.
Photo Jacqueline Banerjee, The Victorian Web.


While Bazalgette’s sewer system is seen as a wonder of Victorian engineering, (and now another London ‘Super Sewer' is under construction  at a cost of £4.1 billion, led by a company named Bazalgette), Bazalgette left us the unfortunate legacy of an inherently flawed design, which is to collect the surface water drainage and sewage into a single combined sewer. This worked for his solution to the 'Great Stink', i.e. simply dump London's raw sewage further downstream, but after the Princess Alice disaster highlighted the need to process the sewage, Bazalgette’s design was suboptimal, and certainly not a future-proof solution. This flaw was recognised way back in 1915 by the Royal Commission on Sewage Treatment and Disposal, who observed that these two streams were better kept separate. But the Royal Commission also thought that it would be too disruptive and expensive to construct a separate sewer system to carry London’s stormwater, Fast forward to the present,  we have heard the same thing  from our MPs.

Unfortunately, Bazalgette’s strategy of combining the stormwater and sewage sewers was repeated throughout the UK, (and indeed throughout the world) and we now live with the consequences. Interestingly, London's new 'Super Sewer' is also a combined surface and sewage sewer, presumably because all its inlets are already combined.

In the UK, the best  the water companies have been able to do to ameliorate the effects of combined sewers is to add stormwater  basins at the sewage treatment works (as at Cassington STW) to hold back some of the extra volume coming into the works in time of heavy rain. No doubt helps moderately, but it is inevitable that the hydraulic retention times needed for processing the sewage will be compromised or fail during high flows, that occur during heavy rain or flood conditions. There is no reason, however,  why raw sewage should ever be released into watercourses when there has been no heavy rain, yet Prof. Peter Hammond's forensic examination of sewage releases countrywide indicate that rogue releases are a far from infrequent occurence.  Why?

In many towns and cities from Japan to the USA, new regulations have been brought in to separate the surface water sewers from the wastewater sewers by building stormwater drains. Were there the political will, the same could be done in the UK at costs estimated by Water UK that are likely to be  less than HS2 (despite the scaremongering numbers of £159 -660bn being claimed by a minister in the recent debate).

Surface Water

stormwaterSchematic showing  separation of stormwater and wastewater sewers.



We should not raise our hopes too high, however, that separating surface water (i.e. stormwater) from wastewater sewers would solve the problem of river pollution. Surface water is also highly polluted - look at any roadside gutter and you will see sediments, litter, leaves, together with much that is unseen, including pesticides, herbicides, fertilzer, etc. (More than half of the phosphorous load in urban stormwater comes from run-off from gardens).  In separate systems the polluted stormwater is typically flushed into the rivers without treatment. 


Limb_brook_floodWharf Stream pollution due to top soil run-off during heavy rains.
Photo Catriona Bass.


Pollution of surface water has been exacerbated by the increasing impermeability of our built environment. Macademised roads were well able to soak up water, but when motor vehicles became common, the need to provide a better, dust-free surface on the macadam by adding a layer of tar or concrete, meant that the road surfaces became impermeable. The huge road-building program over the past century has added to the volumes of water than have to be managed by gutters and drains, rather than simply letting it soak away. In towns this water has to be removed to prevent flooding, so it is led into the sewers, only some of which have  separate stormwater systems. The millions of houses built over the last 50 years have added to the impermeability of villages towns and cities. The severe floods of 2007, which completely overwhelmed the sewer systems, whether combined or separated. It was an accident waiting to happen and it took this disaster to jog the government into action. The Flood and Water Management Act, passed in April 2010, contained the requirement for sustainable urban drainage systems (SuDS), which are intended to mimic the natural drainage of a site before development.

Quite what ‘mimic’ means is open to interpretation, but SuDS include green roofs, more natural features such as ponds, wetlands, and shallow ditches called swales (as at Farmoor on the B4044). Hard engineered elements include permeable paving, canals, treatment channels, attenuation storage and soakaways.

It was only recently (2015) that Oxfordshire made SuDS mandatory in major new developments, which of course does not address the existing infrastructure, nor is there a legal or financial encouragement from e.g. Ofwat to upgrade the sewer systems. There is a compelling argument that says that if a similar level of investment  to that budgeted for HS2 (£100billion and rising), could be made in upgrading our water resources infrastructure, of sewers,  and sewage treatment plants, and in retrofitting SuDs. in cities towns and villages. This upgrade would have far more long-term benefits for people's quality of life - and our efforts for nature recovery.

Does this mean we are doomed to have sewage foul our rivers for ever and anon?

Well, maybe not. There is much that each of us can do now to make the situation less severe.

Charity begins at home, or at least in the kitchen and bathroom and garden.   

There are many common-sense ways in which we as individuals can make a difference to what ends up in our rivers. At Cassington STW we saw detritus like cotton buds, condoms, tampons and wet wipes being removed before it blocked pumps.

Even then, we saw some plastic and other non-biodegradeable items floating in the final settling tanks. In times of floods, at least, all these pollutants end up in our rivers. Those of you who have lived with the old septic tanks (systems that use inefficient anaerobic digestion and soakaways) will know the '3P Rule' - only Paper, Pee and Poop should go down the toilet.  Using bacteriocides like bleach to clean the toilet is also a no-no as it kills the bacteria that are doing the hard work of digestion. Only the bare necessities should be flushed, everything else needs to go in the bin. In the kitchen, there are three main culprits that also end up in the sewers – fats, oils, greases. Pouring hot oil and fat down the kitchen sink might seem the easy option, but it just solidifies within the sewers to create 'fatbergs', which seem also to attract wipes, etc., forming a large unsavoury congealed lump that blocks the sewers and pumps and that some poor contractor has to remove. Prevention being better than cure, the solution again is to dispose of used oils, grease etc. in a food bin or general waste.


cassington_primarySome of the detritus removed daily from pumps at the primary sedimentation stage. Cassington STW.
Photo Adrian Arbib


At Cassington STW we saw the need to precipitate the phosphate, which arrives from our washing powders and detergents and other cleaning agents, some of which contain sodium triphosphate to soften hard water, like Oxford’s. (Interestingly, Dr. Lucy Dickinson's water analyses have revealed that the aquifer that feeds Eynsham’s Holy Well has unusually high levels of phosphate, which probably arises from domestic wastewater leaking out of old sewer pipes and garden fertlizers leaching into the soil). 

How much water do each of us send to the sewers? In the UK we use about 150 litres per person per day. Multiply this by 17000 and that is the minimum amount of wastewater that Cassington STW has to process each day. This volume from inside our homes is relatively constant. The variable is the rainwater collected from our roofs and the roads outside our homes, and this is what causes the overflows in times of heavy rain. 

Is there anything we can do to help short of building a separate sewer for surface water?

One common-sense strategy is to find ways to reduce our individual water consumption. In the drought of 2018 Cape Town, South Africa faced the reality of ‘Day Zero’ when all their reservoirs would run dry. They reduced their per person consumption to under 50 litres a day by innumerable and ingenious means, including putting bricks in the toilet cisterns to reduce the volume of a flush, slogans (‘If its yellow, let it mellow, if its brown flush it down’), flannel baths, or at a push, taking 90-second showers, always putting a plug in the sink and wash basin, reusing grey water to flush toilets or water plants, and in public toilets using water sprays instead of running water for  washing hands. Hardware stores quickly made a fortune selling large water butts to capture and store rain water. Hosepipes were banned, cars were washed with grey water, gardens were replanted with indigenous species.

The enduring effect of the prospect of ‘Day Zero’ was to heighten consciousness about water usage, and how easily we squander this precious resource. Even in the UK, water security is a problem.

A second common-sense strategy is to reduce the amount of rainwater that goes into the sewer. Those of us lucky enough to have gardens can separate the rain water from the wastewater by installing a water butt to collect the runoff from the gutters and then use it to water the garden or replenish a pond. Paved areas can be replaced with gravel or grass, or one can put in planting containers on hard surfaces to reduce the water run-off to the road. In short, anything we each can do to prevent rainwater from reaching a combined sewer system will reduce peak flows at the sewage treatment works and limit the number of overflows, which are inevitable with the traditional design of combined sewers.

We might then be on the right road in doing our bit to help clean up our rivers.

The Vision Thing

 The most effective change, however, will be in how we see things, as much as in how we do things.

If we now see the sewer system as a long pipe that takes the wastewater from our kitchens, bathrooms, toilets, roofs and gardens, and delivers it to a river, then we might not so readily use it as a  waste bin. 

Over one-hundred years ago  technologies were invented that use 'nature's way' - biological means - to remove the oxygen-depleting contents of domestic sewage. We placed these 'sewage treatment works' in our waste pipe to intercept and treat our wastewater before it reaches the river. These technologies worked well,  which is why we still use them. These technologies did not, however, anticipate our invention of all the  multiple synthetic materials, fats, detergents, drain and toilet cleaners, miscellaneous chemicals,  drugs, fertilizers, 'cides of all sorts (herbicides, pesticides, insecticides, etc.) and other poisons, (not to mention the chemicals used to purify our 'clean' drinking water) that  we now put into that very convenient pipe that bears all our wastewater away to our rivers.

We humans will always pollute  the waterways where we live. If we want nature to recover  however, then we, as the primary producers of the wastewater, have the primary responsibility actively to reduce our contribution to water pollution to the barest minimum.



Ardern, E., Lockett, W.T. (1914a) Experiments on the Oxidation of Sewage without the Aid of Filters. J. Soc. Chem. Ind., 33, 523.

Ardern, E., Lockett, W.T. (1914b) Experiments on the Oxidation of Sewage without the Aid of Filters, Part II. J. Soc. Chem. Ind., 33, 1122.

Ardern, E., Lockett, W.T. (1915) Experiments on the Oxidation of Sewage without the Aid of Filters, Part III. J. Soc. Chem. Ind., 34, 937.

Humphreys, Sir George W. (1930) Main Drainage of London. London County Council 

Royal Commission (1915) Final report of the commissioners appointed to inquire and report what methods of treating and disposing of sewage may properly be adopted.

Sharma SK., Sanghi R. (2012) Advances in water treatment and pollution prevention. Springer Science & Business Media, pp. 460