CDR technologies for wastewater treatment plants
The use of CDR (Carbon Dioxide Removal) technologies is regarded as a promising approach to generating ‘negative emissions’. This article explains CDR strategies that can be implemented in wastewater treatment plants (WWTP).
In municipal WWTPs, the majority of the organic matter in the wastewater comes from natural sources, namely plants, and is excreted by humans and animals via the food chain. When the amount of CO₂ generated during the biological decomposition of organic wastewater ingredients or during sewage sludge treatment cannot escape into the atmosphere but is permanently stored, this is referred to as CDR.
CDR options in wastewater treatment
The Carbon Gap study on the potential of CDR in wastewater and waste treatment, published in April 2026, identifies four different areas for WWTPs:
- The alkalinity management of wastewater,
- The conversion of sewage sludge to bio-oil, biochar or syngas,
- CO2 capture from the biogas that is produced during sludge digestion,
- CO2 capture from the exhaust gas of sludge combustion.
For the two capture options the subsequent storage of CO2 is mandatory to fulfil the requirements of CDR.
All above-mentioned CDR options differ in terms of their technological readiness levels (TRL) and the costs of plant components, energy and operating resources.
With conversion of sewage sludge into bio-oil oder biochar, combustion is no CDR compliant utilization of these products, for this would result in CO2 emissions into the atmosphere. Combustion of bio-oil or biochar could be CO2 neutral in the best case, but it does not bring about negative emissions.
In contrast, bio-oil, biochar and synthesis gas offer opportunities for value creation; for example, biochar can be used as a soil improver, whilst bio-oil and synthesis gas can serve as feedstocks for the chemical industry. This could offset the higher capital investment required for plant technology, such as pyrolysis.
Producing biogas from sewage sludge has been state-of-the-art for a longer time. Depending on the size of the WWTP and the presence of additional capacities for power generation by photovoltaic systems onsite, several WWTPs are at least energy neutral on balance sheet terms. If CO2 is captured from the biogas and stored, this would account for a negative emission.
CDR in the aeration tank: alkalinity management
Controlling the pH of wastewater in the biological treatment stage is not a new concept, but has been practised to mitigate significant pH fluctuations and maintain stable nitrification performance for a longer time. Stable nitrification requires an alkalinity of at least 50 mg/l calcium carbonate. Buffer substances such as limestone (calcium carbonate) or chalk suspension are added to adjust the alkalinity.
Now the capability of calcium carbonate to form bicarbonate in the presence of CO2 and water takes the centre stage:
The bicarbonate is transported from the WWTP through rivers to the ocean, where the carbon is likely to remain over thousands of years.
The function principle of alkalinity management as described here is similar to the principle of treating alkaline industrial wastewater by injecting CO2, as Prof. Helmuth Thomas, head of the Hereon Institute of Carbon Cycles, and his co-authors outlined in Environmental Science & Technology Letters. A summary of their study is also given here.
Applying CDR as alkalinity management has the advantage that additional costs for dosing limestone to aeration tank amounts to less than 1 % of the total costs for wastewater treatment, say the authors of the Carbon Gap study. Thus the impact on the consumer prize is very low.
Taking into account, that alkalinity management for buffering pH fluctuations is a known process and the costs for dosing limestone and monitoring the effect seem to be not too high, the low TRL of 6 – 7 given in the study is surprising.
CDR pioneers: WWTP Breisgauer Bucht
At IFAT 2026, Nicholas Nelson of Omya International, together with Anne Schulze from AZV Breisgauer Bucht, presented details on the implementation of CDR through alkalinity management at the Breisgauer Bucht WWTP in the Southwest of Germany. This plant treats wastewater for a total of around 381,000 residents of Freiburg and the surrounding municipalities. It has a total of eight treatment lines for biological treatment. Two of these were only commissioned in 2022. These two new lines, with identical flow rates, served as test beds for the CDR process. In one line, the alkalinity was set to 200 mg CaCO3/l, whilst the other line operated with increased alkalinity.
Inter alia, the presentation offered insights to the characteristics and function principle of a particular natural calcium carbonate product delivered by Omya. Besides its central role as pH buffer and CO2 capturer according to the above-mentioned chemical reaction, help the positively charged calcium ions forming larger and denser sludge flocs. They sediment easily, so that sludge dewatering is improved and can be carried out with less polymer than without the CaCO3 dosing.
How much CO2 can demonstrably be removed?
Although the chemistry behind CO₂ sequestration using bicarbonates is well understood, a comprehensive CO₂ accounting framework for the described use of calcium carbonate had previously been lacking. Now the protocol for monitoring, reporting and verification (MRV) of CO₂ removal, along with recommendations for its optimisation, was provided by the company isometric.
In accordance with this protocol, the trials at the Breisgauer Bucht WWTP were analysed and the CO₂ removal achieved was validated. Based on the data collected so far from a trial period of approximately five months (November 2024 to March 2025), a gross CO2 removal of around 90 tonnes was determined.
Meanwhile, the trials were extended to the remaining aeration tanks. Although the data set is still too small overall, these initial results suggest a CDR potential of over 1,000 tonnes of CO2 per year for the entire plant comprising the eight lines.
Sources:
This report is based on the study “No wasted opportunities – Embedding carbon removal in the management of wastewater, concrete and mine waste”, which was produced by Carbon Gap in collaboration with Deloitte North and South Europe.
The information regarding the CDR trials at the Breisgauer Bucht wastewater treatment plant origins from the presentation by Nicholas Nelson, Vice President of Sales & Marketing Life Science at Omya International AG, at IFAT 2026 in Munich. I would like to thank him for making this contribution possible.
Featured image: jhenning-beauty of nature / Pixabay
This photo shows part of an unknown sewage treatment plant; it is not the Breisgauer Bucht wastewater treatment plant.
Republishing without the author’s express permission and a link to this source is not permitted.