Klärwerk Werhölzli, Zürich


Zurich_viewZurich lake with Alps.
Author photo.


When you think of Zürich, you may think of the lake, the Old City, the local mountain, Uetliberg, with its views to the Eiger and Jungfrau, its many museums, its exceptional transport system, or its rich cultural and street life. You probably don't imagine that Zurich is also home to Switzerland’s largest sewage works (‘Klärwerk’), which is located at Werdhölzli, next to the Limmat, one of Zurich’s popular swimming rivers. Zurich City have made a short film about the works, but given our current crisis of river, lake, and sea pollution by raw sewage, and given our own on-going NRN project to document the water quality of our local watercourses,  I was interested to see for myself how Werdhölzli compares to Cassington’s Sewage Treatment Works, the subject of a previous NRN ‘Long Read’.

Kevan Martin

(Note to nervous readers: this one is a much shorter read!)


I was fortunate enough to be given a personal tour of Werdhölzli by two of the process engineers who work there. They began by giving me a brief history of waste water treatment in Switzerland and what they told me was surprising: up until the 1950’s waste was dumped directly into rivers and lakes. This included domestic and toxic industrial waste, in quantities enough to kill fish and lead to bans on swimming in some lakes and rivers. This was clearly a problem since virtually all cities and major towns in Switzerland are situated on rivers and lakes.


SolothunBathing prohibited in the Solothun Lido because the Aare river was too polluted. 1966.
Photo Eawag.


Although some cities like Zürich, Luzern, and Bern had installed rudimentary water treatment plants by the mid-19 century, the first Federal law governing water treatment only came into force in 1957 (after a referendum where 81% voted for an initiative for water protection). Uptake amongst the cantons, however was slow because of insufficient subsidy; in 1961 the Neue Zuricher Zeitung reported that, "up to now, water pollution control has mainly been on paper, and what had been done up to then was only a drop in the dirty broth that most of our public waters have become today."


Tod'Tod Im Wasserglas'
Poster by Hans Erni


Through the 1960’s water pollution was a constant topic in the Swiss media. Fish mortality, algal bloom, bathing prohibitions and chemical spills were presented in vivid images underlined with almost apocalyptic texts. Calls for stronger water protection led to demonstrations in Lucerne and the peoples’ fears were captured in a poster by Hans Erni, "Tod im Wasserglas"(Death in a glass of water) which apparently was very popular. The media fuelled the public demonstrations and political debate, which created additional pressure on the federal government.


Sewage_connectionConnecting the Swiss population to central sewage treatment works 1965-2005.
Swiss Department of the Environment.


By 1965 still only 14% of the population of Switzerland were connected to a waste water treatment plant, most relying on septic tanks at best. The eutrophication of rivers and lakes by waste water was such that ‘sea-cow’ boats were used to dredge the algae from lakes. Of course, illnesses like diarrhea after swallowing lake water were not uncommon, but it took another typhus epidemic in 1963 in the mountain resort of Zermatt to trigger a radical change. In that outbreak, 450 people fell ill and 3 died due contamination of drinking water contaminated by sewage from a camp for workers who were building the Grand Dixence dam in the valley above Zermatt. This was the spur to action for the federal government and cantons, who then voted to subsidise the construction of wastewater systems in local communes.

As with much Swiss legislation, the call for better water protection came from the public through the mechanism of direct democracy. The people’s initiative of 1967, ‘Protection of waters against pollution’, led to changes in Swiss laws about wastewater treatment. By 2005, and at a cost of SFR 50 billion, the infrastructure was built to connect 97% of the population to a central sewage treatment plant. This involved building a 130 000km sewer network connected to 800 treatment plants. There is now an ongoing program to consolidate the smaller plants built throughout Switzerland.

The Swiss Federal Institute of Aquatic Science and Technology (Eawag) has created an infographic that traces the history of Swiss water legislation and actions.



WerdholzliWerdhölzli Klärwerk - Zurich's Sewage Treatment Works.
Because Werdhölzli is built on an aquifer, only the basins and connecting pipes are below ground; all the rest is above ground and easily visible. A service spine runs through the middle of the whole site.
Photo Swiss Info.


The Werdhölzli Sewage Treatment Works, which is a public utility, now treats the waste water from over 650 000 people in Zurich and surrounding communities. It has the capacity to treat 6 cubic meters of waste water per second, but typically the loading is 2 cubic meter per second – which amounts to about a supertanker load per day. The Zurich sewer system combines waste water and the storm water e.g. from road drains, as does most of the UK sewer network, although new legislation requires that stormwater and sewage are carried in separate sewers.

In the event of heavy rainfall, the excess flow is buffered in large underground reservoirs that have a total capacity of 40 000 cubic meters. This temporary store of water is then pumped out to be treated as normal. When the reservoirs are emptied, they are flushed and cleaned. When the reservoirs are full to the brim, however, any excess inflow is released directly into the Limmat river as untreated sewage. My guides told me that because the capacity of the reservoirs is well-matched to the load in heavy rainfall, the release of raw sewage is rare event.  


ETH_SARSScientists of the Swiss Federal Institute of Technology (ETH) sampling raw sewage
to detect variants of SARS-CoV-2.
Source Eawag/Esther Michel



The raw sewage is regularly sampled and analysed. When I was there one of the tests they performed on the raw sewage was to detect genetic material from the SARS-CoV-2 virus, which appears in the faeces of infected people. These analyses show give an early and accurate indication of how high the virus load is in the population and which virus variants are prevalent. For example, the omicron variant was detected in Zurich sewage when it was thought to be present only in South Africa.



screeningSieves (left) are scraped clean and the solids are removed by conveyor (right).
Author photo.

The raw sewage first enters a screening process, which removes stones and debris and solids that are more than 10mm in size. These solids are coarse and fibrous materials (textiles, plastic, toiletries, cell phones, larger rocks, branches, etc.), as well as large pieces of faeces. This extracted material is incinerated – more of which much later – and the liquid fraction, which contains only smaller solids, is passed to long tanks to trap the sand, oil and fat.


Newt_shadeOil and sand trap. The stainless steel box (covered with cloth sunshade)
has a ladder (diagonal box) to allow amphibia to climb out of the tank.
The elevated brown structure in the background is the service spine.
Author photo.


In the oil and sand trap lateral air jets are used to create a vertical circulating current in which the fine sand falls to the bottom and, in a non-aerated section of the tank, the oil and fat rises to the surface to be skimmed off and added to the coarse screened fraction to be incinerated.  An intriguing feature of these oil/sand separation tanks was the existence of partitioned, unstirred zones at the input end of each tank. These pools had little ladders leading to a closed box, which turned out, were escape ladders for salamanders and other amphibians who found themselves being swept into the sewers by storm water and who had made it through the initial screening. The boxes were checked regularly and the self-rescued amphibians were then released back into the wild.  

With large solids, oil, grease and sand now removed, the liquid fraction containing suspended solids then passes to the large circular primary settling tanks, where the solids like paper and food scraps, faeces, etc. are settled out and removed as a sludge.

This sludge is thickened by passing it into a separate set of thickening tanks where the sludge is slowly stirred, and thickens as it settles. The liquid fraction from these thickening tanks (the ‘supernatant’) is returned to the primary settling tanks for further treatment. This gravity process of gravity concentration of the sludge takes about 10 hours, so to speed up the process, the thickening tanks are currently being replaced by centrifuges 


primary_digestionAnaerobic sludge digestion tanks.
Author photo.


The treatment of the thickened sludge is of particular interest because it is made to do work, which happens at only few plants in the UK – e.g. Minworth Sewage Treatment Works at Birmingham.

The strategy is to pass the sludge into primary digestion tanks, where it is warmed to 38 deg. C and allowed to ferment under anaerobic conditions for 3 weeks. The microorganisms that digest the sludge generate methane, carbon dioxide and water. The gas is collected in gasometers and purified and the methane is used to heat about 5000 homes. 

The digested sludge still contains organic matter, so it is passed to a secondary anaerobic digestion stage before being further dewatered by centrifugation, which increases the solid fraction of the sludge from 3% to 30%. Archimedes screws then transfer the mud-like solids to disc dryers, which heat and dry the sludge, before passing it to a fluidised bed incinerator. This ingenious device is able to incinerate low grade fuels, like sludge, and do it with high combustion efficiency at relatively low temperatures, which means that it does not generate sulphur and nitrogen oxides. The flue gases are scrubbed through a multistage process to remove gaseous pollutants before released to the atmosphere. The ash is removed and presently accumulated until an efficient method is found for extracting the valuable phosphorus and other useful compounds. Sludge from surrounding plants is also brought to Werhölzli for incineration. The heat generated is used to heat the whole Werdhölzli plant.


Cassington STW has none of these stages; only large items of debris like wipes and sticks are screened and there is no additional attempt to remove oil, fat, or sand (or amphibia) and no infrastructure for dewatering. Waste sludge from Cassington and from Witney sewage treatment works is piped or tankered off-site to  Oxford's Grenoble Road facility where it first undergoes thermal hydloysis to break up the organic matter, then digestion in an anaerobic reactor. The energy contained in the organic matter in the sludge is metabolised by microorganisms that create methane that can is used instead of natural gas at the plant.

O2-meterAerated activated sludge reactor. Dissolved oxygen is monitored continously -
here the monitor reads 2.13 mg/l a a temperature of 17.3 deg. C.
Author photo


Where the processes at Cassington and Werhölzli align is the activated sludge reactors, which have the job of doing the main treatment of the organic fraction.  The activated sludge reactors are where the sewage (raw sewage in the case of Cassington, the liquid fraction (which contains suspended solids) in the case of Werdhölzli, is aerated so that aerobic micro-organisms grow and digest the organic matter and convert ammonia, which is toxic to aquatic life, to nitrate. This activated digestion process, which requires oxygen, is the same at Cassington and Werdhözli, but one noticeable difference was in the manner in which the tanks were aerated. At Werdhölzli the air is supplied from diffusers at the bottom of the tanks, much like an acquarium. The air bubbling from the diffisers agitates and oxygenates the water evenly and effectively, as the dissolved oxygen reading indicated (picture above).


diffusersRows of diffusers at the bottom of an empty reactor.
Air compressors in a chamber beneath the reactor supply the air.
Author photo.


At Cassington, surface brush rotors are used for the same task of oxygenation, but they are located only at two positions in an oval donut-shaped tank and are relatively inefficient such that the deeper liquid is not well aerated or mixed – when I visited Cassington the dissolved oxygen meter read 0.6 ppm.



denitrificationThe reactor on the left is being aerated, which nitrifies the ammonia, in the tank on the right  aeration has paused to allow denitrification to occur. The 'sludge age' is kept at 15-20 days to allow for good nitrification and good sludge settling characteristics.
Author photo.


Another major difference is a ‘denitrification’ step used in the Werhölzli activated sludge basins, but not at Cassington. This step requires the activated sludge to be made anaerobic. This is achieved by intermittently stopping the aeration in the tank, which encourages the growth of another group of micro-organisms that flourish in anaerobic conditions and obtain their oxygen by stripping it from the nitrate (NO2+). This reaction leaves a nitrogen atom, which combines with a second nitrogen atom to form nitrogen gas (N2), which returns to the atmosphere.

Both anaerobic and aerobic micro-organisms derive their energy by digesting the organic matter– the ‘oxygen-demanding’ fraction - in the raw sewage. At Werdhölzli this efficient biological means of denitrification removes 70% of the nitrate from the liquid. At Cassington, and typically in UK sewage treatment works, there is no such denitrification step, so all the nitrate formed from oxidising the ammonia is released to the river. As in the UK, however, there is no statutory requirement in Switzerland to measure the nitrate concentration, but each Canton can set its own water quality standards. At Werdhölzli they do measure nitrate (continuously, using ion sensitive electrodes) and in the final effluent it is less than 5 parts per million (ppm).

Phosphate in detergents was banned in Switzerland in 1986, and this resulted in a significant decrease in the concentration of phosphate in domestic waste water. The phosphate that is present is removed by adding iron salts to a constantly aerated portion of the tank, much as is done at Cassington. Iron phosphate precipitates as an insoluble compound, which is removed with the fraction of the sludge that is passed back to the primary settling tanks and thence to the sludge processing line leading eventually to the incinerator whose ash thus contains the valuable phosphorus. At Cassington, the activated sludge is led to large settling tanks where a constant fraction of sludge is removed to maintain a ‘sludge age’ of between 15-20 days. The remaining the sludge is returned to the aerobic reactors and the liquid fraction arising in the settling tanks is released into the Thames.



secondary_clarifierSchematic of the secondary clarifier. (See text below for explanation).



At Werdhölzli, the settling tanks – the ‘secondary clarifiers’ – consist of long rectangular tanks where an extremely slow cross-current ensures that the solids, including the sludge particles, micro-organisms, and the precipitated phosphate, settle evenly to the bottom of the tanks where they are sucked up by a slow moving giant vacuum cleaner and returned to the activated sludge reactors.

The effluent that Cassington Sewage Works releases into the Thames is not pure water. In addition to nitrate and residual phosphate, it contains microbes, viruses, multiple chemicals, including cosmetics, pharmaceuticals, household chemicals, industrial fertilizers, herbicides, and pesticides, fats and oils, along with fragments of non-biodegradable plastic, foam, rubber etc. These collectively are called ‘micropollutants’. To remove these from the effluent that will be released into a watercourse, Werdhölzli performs two more steps.



ozone_generatorAntechamber where ozone is manufactured and piped to sealed tanks through which the effluent flows for ozonation.
Author photo


Firstly, active compounds in the effluent need to be detoxified and disinfected. The ozone is manufactured on site and bubbled through the effluent. This all takes place in completely sealed tanks as ozone is a highly aggressive oxidant that very effectively oxidises and destroys the organic molecules. Any excess ozone is removed and destroyed to ensure that none remains in the effluent. While ozone is a very effective and clean process, it can leave behind harmful by-products like bromate, so the process at Werdhözli is tightly controlled - a high tech. control centre monitors and manages the entire plant day and night.


sand_filterSand filter to remove micropollutants.
These filters are regularly backwashed to keep them efficient.
Author photo.


To eliminate the remaining plastics and other small solids, and fragments of compounds left after ozonation, the effluent is passed through large sand filters.


monitor_effluentTwo of the automated labs.  monitoring the final effluent quality.
Author photo.


The scrubbed and disinfected effluent is finally ready to undergo its final quality control - automated analyses of phosphate, ammonium, nitrate, nitrite, pH etc. - before being released into the Limmat river after its 19-hour journey through the plant.



Clearly the differences between the typical UK sewage treatment plant and Swiss plants like Werdhölzli are stark. In some ways the need for better effluent quality in the UK is more pressing, because our domestic drinking water is drawn from the same rivers and lakes that receive the effluent from all the sewage treatment plants. (When the head of the Environment Agency lectures us - with no hint of irony - that we should not be 'squeamish' about drinking water derived from sewage, he has clearly failed to grasp that due to the poor oversight of his Agency and of Ofwat, that is we already are have to do). In Switzerland, by contrast a large fraction of the drinking water is drawn from springs and aquifers.

As their history indicates, the Swiss voters pushed the Federal government to clean up the rivers and lakes. This process continues. Even at a state-of-art plant like Werdhölzli, upgrades were clearly underway, something of which my guides, as process engineers, were very proud. Indeed, more widely, the Swiss peoples' pride in the quality of Swiss water is palpable.

The engagement of Federal Institutes in the research and development of the treatment processes and the use of the plants for public health monitoring and diagnosis is a striking difference from UK plants, which seem to do only (at best) their statutory minimum. The notion that Thames Water, for example, would continuously monitor their effluent to ensure its quality with the rigour applied at Werdhölzli, seems laughable given their record.

What does give a glimmer of hope is that the infrastructure for improving the waste water treatment in Switzerland was constructed very rapidly once the Federal government was forced by its citizens to enact legislation, to regulate, and to spend tax revenues in subsidies to the Cantons so they could build sewage treatment plants that served large cities and their surrounding communities.

While many other countries - not just Switzerland - forge ahead in developing and implementing technologies to deal with the new challenges of contemporary waste water treatment, the UK, which pioneered the biological treatment of sewage by the activated sludge process, has, by-and-large, not moved much beyond the treatment processes it first trialed at full- scale at  Salford in 1914 and Davyhulme in 1915. Indeed, it is shocking that people living in one of the richest countries in the world still use trickling filter plants to process sewage in their villages and that septic tanks are still widely used in rural communities, and that due to illegal sewage discharges we have to swim and boat in highly polluted water - the same water we extract for our drinking water.

Of course, the investment to upgrade our waste water management is huge - it needs not the hundreds of millions that the water companies try to impress us with as their investment in upgrading, but many billions.  The UK has embarked on megaprojects like HS2 or Crossrail, so its not that a significant upgrade in waste water infrastructure and management cannot be financed, is just that successive UK governments have not prioritised the health and well-being of the whole population above megaprojects that benefit relatively few. 

The growing swell of public concern and disgust at our sewage-polluted rivers, lakes, and seasides may at last persuade our politicians that it is time to drag the UK wastewater industry into the 21st century.

Acknowledgements: My thanks to ERZ (Ensorgung and Recycling Zürich) for hosting me at Werdhölzli Klärwerk and to my guides who kindly shepherded me through the Works and patiently answered my many questions.