Using Vacuum-UV, Ozone, and Activated Carbon for micropollutant removal in wastewater treatment

A MUDP project (The Environmental Technology Development and Demonstration Program) at the Slagelse Wastewater Treatment Plant, shows that low-dose ozonation and vacuum-UV can extend the lifespan of used activated carbon filters, so they can purify micropollutants, such as industrial chemicals and medicine residues while reducing carbon footprint.

One of the key financial supporters of the project was the Ministry of Environment of Denmark, as they believe that Denmark may face difficulties in terms of meeting the upcoming stringent requirements for micropollutants, defined by the EU’s proposal for an urban wastewater directive, as the price of activated carbon is high and needs to be replaced relatively frequently.

The results from the MUDP project at the Slagelse Wastewater Treatment Plant show that it is possible to use old spend activated carbon (four-year-old coal) in combination with low-dose ozone or vacuum UV – thus reducing cleaning costs – while the removal rate of the substances defined in the EU wastewater directive is over 90%.

With vacuum-UV water treatment, UV irradiation of the water occurs using a short wavelength (185 nm), which results in free reactive radicals being formed from the water molecules. These radicals can degrade micropollutants through Advanced Oxidation Processes (AOP), or Advanced Reduction Processes (ARP).

The old spend activated carbon, which only removes 5-40% of the substances defined in the EU directive-proposal, is now down to a price of about 0.15 DKK/m3. The preliminary operating statements show that low-dose ozone and vacuum-UV each cost around or slightly over 1 DKK/m3.

“Experiences from Slagelse show that if you want to remove at least 80% of the directive proposal’s substances with activated carbon, the price becomes roughly 2 DKK/m3″ says Ulf Nielsen, Consulting Manager at ULTRAAQUA.

The purpose of the project has been to test new, cost-effective solutions for removing medicines and other micropollutant substances as well as antibiotic-resistant bacteria. After a 15 month period, the results show that the installed plant can extend the lifespan of a used carbon filter with both low-dose ozonation and vacuum-UV.

Combining all three technologies is most effective

By combining all three polishing technologies – ozone, used activated carbon, and vacuum UV – and with a high dose of ozone, nearly 100% of the directive proposal’s substances (precisely 99.7%) are removed.

The combination of low-dose ozone and activated carbon removes about 90% of the directive proposal’s substances – although with minor exceedances of Predicted No-Effect Concentration (PNEC) for individual micropollutant, while the combination of used activated carbon and vacuum-UV removes more than 95% – and without exceedances of environmental effect concentrations.

“The vacuum-UV technique has a higher energy consumption, but we are working to optimize the process” says Ulf Nielsen.

He also points out that higher ozone dosing potentially means bromate formation – a problem many Danish wastewater treatment plants struggle with – while there is no bromate formation using low-dose ozone nor with vacuum-UV.

The practical experiences from this project have thus provided valuable input on which polishing technologies should be chosen for an upcoming full-scale solution.

Additionally, the concentrations of micropollutants in the nearby stream, which the treatment plant discharges into, needs to be reduced. Here, the plant has shown that very effective removals of the micropollutants can be achieved using the three water purification technologies.

Environmental Manager Jan Jørgensen from Slagelse Municipality says: “Measurements show that there are increased concentrations of, for example, bisphenol A and diclofenac in the nearby stream. The project has shown that it is possible to combine used activated carbon with oxidation methods and thus achieve very effective removals of the micropollutants. And the combination with vacuum-UV is an interesting new method, which – without the use of chemicals – only by UV light – can remove the substances proposed by the EU directive.”