A project by the environmental technology development and demonstration programme at the Slagelse Wastewater Treatment Plant (WWTP) in Denmark 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.
A key financial supporter of the project was the Ministry of Environment of Denmark as the upcoming stringent requirements for micropollutants defined by EU proposal for an urban wastewater directive may be difficult for the nation to face. The price of activated carbon is high and needs to be replaced frequently.
The results from this project at the Slagelse WWTP showed that it was possible to use old spend activated carbon which is four-year-old coal in combination with low-dose ozone or vacuum UV — thus reducing cleaning costs — while the removal rate of 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 of 185nm 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 Danish krone/m3. The preliminary operating statements show that low-dose ozone and vacuum UV each cost around or slightly over 1 Danish krone/m3.
“If you want to remove at least 80% of the directive proposal’s substances with activated carbon, the price becomes roughly 2 Danish krone/m3,” said Ulf Nielsen, Ultraaqua consulting manager.
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 showed 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, 99.7% of the directive proposal substances are removed without exceeding environmental effect concentrations though it minimally exceeds predicted no-effect concentration (PNEC) for individual micropollutants.
“The vacuum UV technique has a higher energy consumption, but we are working to optimise the process,” added Nielsen.
He also pointed out that higher ozone dosing potentially means bromate formation — a problem many Danish WWTPs struggle with — while there is no bromate formation using low-dose ozone nor with vacuum UV.
Additionally, concentrations of micropollutants which the treatment plant discharges into the nearby stream need to be reduced. Here, the plant has shown that effective removals of the micropollutants can be achieved using three water purification technologies.
Environmental manager Jan Jørgensen from Slagelse municipality said: “Measurements showed that there are increased concentrations of, for example, bisphenol A and diclofenac in the nearby stream. It is possible to combine used activated carbon with oxidation methods [to] achieve effective removals of the micropollutants. The combination with vacuum UV is an interesting new method which only by UV light can remove the substances proposed by the EU directive.”