Fourth Treatment Stage for WWTW’s
by Rami Elias Kremesti M.Sc., CSci, CEnv, CWEM
Kremesti Environmental Consulting Ltd.
Transmutare Substantiarum Basium In Aurum TM
Introduction
I have worked in the water treatment sector for 20 years and waste water treatment is the closest that I find to Alchemy because it turns black water into something pure. Waste water treatment works normally have primary, secondary and tertiary treatment stages that involve pre-settling, aeration, coagulant dosing, finial settling then optional disinfection. This normally removes up to 99% of Ammonia, Phosphate, BOD, COD and SS. Bacteria are reduced up to 99.9% through the disinfection step. However this does not remove pharmaceuticals (like antibiotics), pesticides, pfas, EDC’s (like BPA) and preservatives used in cosmetics and personal cleaning products. To remove these recalcitrant chemicals, WWTW’s need a Fourth Treatment Stage also known as the Quaternary Treatment Step or Micro-Pollutant or MP stage. Below is a schematic of some of the categories of these micropollutants and their environmental and health impact. The diagram does not show that antibiotics released into the environment increase the risk of the formation of superbugs, or anti-biotic resistant bacteria – which according to WHO is very likely to be the source of the next global epidemic.
Micropollutants, Sources and Toxicity
The Fourth Treatment Stage
To remove recalcitrant organic chemicals from treatment sewage effluent in an economic way, a combination of ozone + Granular Activated Carbon is used + cloth filters or some other advanced oxidation processes + filtration. Non Thermal Plasmas have been researched in the EU by institutes like WETSUS to remove recalcitrant organics. This would work but the question is can it be scaled economically. GAC/Ozone treatments have already been implemented in countries like Germany, Holland and Switzerland and are proven to work. NF and RO can be used but these membrane technologies are expensive. MBR is a sewage treatment technology that combines ASP with a UF membrane that can remove up to 99.999% of bacteria and viruses as well as many organic molecules. Short chain PFAS don’t stick too well on GAC therefore Ion Exchange can be a more effective technology for their removal.
Fourth Treatment Stage Plants in Germany
Germany is actively implementing fourth wastewater treatment stages (or quaternary treatment) in its Wastewater Treatment Works (WwTW/WWTP) to remove micropollutants, such as pharmaceutical residues, pesticides, and household/cosmetics chemicals, which are not captured by conventional (secondary/tertiary) treatment.
Westerheim (Swabian Alb): One of the first, operating since 2015 using Huber’s SANDFILTER CONTIFLOW®.
Uhldingen-Mühlhofen: Put into operation in 2023, representing one of Germany’s largest combined 4th treatment stage methods (ozonation and activated carbon).
Bickenbach: Put into operation in April 2025, considered one of the most modern in Hesse. The system integrates three main components: an upstream two-line cloth filtration (HUBER RotaFilt®) for pre-treatment, an ozonation stage to break down pollutants, and a four-line activated carbon filter (16 HUBER CONTIFLOW® GAC units) to adsorb remaining substances.
Tübingen: Inaugurated in 2021. The plant utilizes ozonation combined with downstream filtration (activated carbon).
The wastewater treatment plant (WWTW) in Mörfelden-Walldorf, Germany, utilizes an advanced four-stage process to remove trace substances like pharmaceuticals, pesticides, and industrial chemicals. The fourth treatment stage is based on Ozonation, powdered activated carbon adsorption and cloth filtration.
Fourth Treatment Stages in Switzerland
Switzerland introduced a mandatory fourth WWTW treatment stage requirement in 2016, while projects such as the one in Bickenbach WWTW’s in Germany have been implemented in the European Union on a voluntary basis and supported by subsidies.
The Villette wastewater treatment plant (WWTP) in Switzerland is a significant infrastructure project located in Thônex, Canton of Geneva. It is designed to handle population growth and, from 2023, has implemented advanced solutions to treat wastewater and remove micropollutants.
AFRY (Pöyry Schweiz AG) is currently planning further micropollutant elimination stages at Swiss wastewater treatment plants Aire Geneva (1,000,000 p.e.), Bern (500,000 p.e.), Monthey-Cimo (500,000 p.e.), Kloten-Opfikon (125,000 p.e.), Rosenbergsau (110,000 p.e.), Lucens (70,000 p.e.), Delémont (50,000 p.e.) and Hinwil (30,000 p.e.).
References:
https://afry.com/en/insight/switzerland-pioneering-in-micropollutants-removal-wastewater
https://www.stereau.com/en/csr-initiatives/sustainability-at-the-heart-of-our-installations/
Fourth Treatment Stage Plants in Holland
- WWTP Emmen: Utilizes the BODAC® system, which combines BioActive® Carbon and Ozone Strong Water (OSW). BODAC = Biological Oxygen-Dosed Activated Carbon
- WWTP Winterswijk: Employs an O3+GAC (Ozone + Granular Activated Carbon) process.
- WWTP Leiden-Noord: Utilizes PACAS (Powdered Activated Carbon in Activated Sludge).
- WWTP Asten: Actively piloted the NanoX system, which combines nanofiltration and AOP/UV technology.
- WWTP Hapert: Used for pilot testing Upflow GAC filtration.
- WWTPs in the Regional Water Authority Vallei & Veluwe: (Specifically, the Bennekom WWTP, which has been used as a study site for effluent micropollutant removal).
- Utrecht (AGS plant): A full-scale Aerobic Granular Sludge (AGS a.k.a Nereda) plant in Utrecht was identified as a key location for studying micropollutant removal.
Case Study – SIAAP’s WWTW’s in Greater Paris Region, France
Seine Aval WWTW (in Achères) is Europe’s largest, treating WW from 6M people, with a treatment capacity of about 2.9 million m³/day (m³d) or 34 m³/s . It has 930,000 m³ in storm water storage basins and tank tunnels across its network. In comparison, Beckton, London Sewage Treatment Works (STW) has a flow to full treatment of approximately 27 m³/s, with peak storm flows reaching around 32 m³/s. The London Tidal Tunnel has a larger storm water storage capacity at 1.6 million m3 but the main WWTW’s in London, Beckton is very old and there is no disinfection step. Mogden and Crossness are more modern.
Raw sewage first undergoes initial screening (Eau Tamisee in diagram below), then enters large, covered settling tanks with lamellar Actiflo settlers to remove suspended solids, phosphorus, and some carbon pollution, also reducing odours.
80% of the Water moves to biological purification stages, including advanced biofiltration (Biostyr) to convert ammonia to nitrates (nitrification) and further remove carbon. Denitrification removes the nitrates and converts them into N2.
20% of the waste water is treated using MBR.
A % of the treated water is disinfected using UV lamps/PFA before discharge into the Seine river. Per Formic Acid is chosen for its superior disinfection performance and lower environmental footprint compared to traditional methods like chlorination, as it produces minimal harmful disinfection by-products (DBPs). The by-products of PFA are generally non-toxic to aquatic fauna at necessary dosages.
Based on recent upgrades as part of the Paris 2024 Olympic bathing plan, the Seine Valenton (Val-de-Marne) wastewater treatment plant has implemented UV disinfection capabilities for its treated effluent, enabling it to disinfect a large portion, with the goal of treating 100% of its dry-weather flow (up to 600,000 m3/day with UV, particularly for pathogen removal before discharge into the Seine. The Capacity of the WWTW’s is 600,000 m3/day dry weather, and up to 1,500,000 m^3/day in rainy days.
Target: The recent UV installation aims to treat the effluent, reducing bacteria levels to meet safety standards for recreational water use (bathing).
Sludge is digested in AD’s which generate biogas (methane-rich), which is treated, captured and utilized for combined heat and power (CHP) or injected into the grid, making the plant an energy producer.
Seine Aval WWTW Processes for the Greater Paris area
References:
https://www.siaap.fr/siaap-greater-paris-sanitation-authority/
Case Study: Henriksdal Wastewater Treatment Plant, Stockholm
Henriksdal Wastewater Treatment Plant uses a multi-stage process, including preliminary screening/grit removal, primary settling, biological treatment with advanced Membrane Bioreactors (MBRs) for deep nutrient removal, secondary settling, filtration, and disinfection, plus sludge digestion for biogas, all while undergoing a major upgrade to handle Stockholm’s wastewater, becoming cleaner and more efficient by 2029. MBR removes up to 99.999% of bacteria and viruses from the waste water.
The Singapore Case
Singapore’s Wastewater Treatment Works (WWTW) system effectively includes advanced treatment stages that function as a fourth stage (or advanced treatment/reclamation) to produce high-grade reclaimed water known as NEWater. It polishes treated WW effluent using RO.
Benefits to the Environment
Reduction of Antimicrobial Resistance (AMR): The technology (activated carbon, ozonation, or membrane processes) helps to remove resistant bacteria and genes, reducing the risk of “superbugs” in the environment.
Specifically, studies of the Geräthsbach stream in Hesse—which receives effluent from the Mörfelden-Walldorf plant—showed that the fourth stage improved the fertility of organisms and enabled many sensitive species to return within as little as two years.
Below is the impact assessment of the European Commission in regards the Updated Urban Waste Water Treatment Directive:
https://kremesti.com/wp-content/uploads/2026/01/Impact_assessment_accompanying_the_EU_UWWTD.pdf
Conclusion
Fourth Water Treatment stages are the future. Our rivers are polluted world wide and we need to upgrade our WWTW’s before the next superbug epidemic breaks out. Active Carbon companies are poised for significant growth.
About The Author
Rami Elias Kremesti is a UK chartered water and waste water treatment specialist that is passionate about the science and technology of water treatment and environmental protection. He has worked on water treatment projects worldwide and has twenty years of experience in the industry. He loves to solve a good technical problem and to teach and inspire the next generation of environmental professionals. If you enjoy these informative pages, please give us a good review on Google.
Rami Elias Kremesti Portrait