In the coming years, as stricter environmental requirements are imposed, many European Union wastewater treatment plants (WWTP) need to be expanded and/or upgraded. This requires considerable investments. Optimising the renovation recourses can lead to significant savings. The use of entrapped nitrifying bacteria for upgrading of WWTP towards nutrient removal may be benificial. Long term pilot tests were performed to evaluate a so-called pellet reactor. Differences in performance and microbiological composition of classical activated sludge and the pellet reactor were investigated. FISH analyses showed (i) absence of Nitrobacter cells and (ii) high abundance of Nitrospira in the pilot reactors. Two Belgian WWTP make use of fine bubble aeration and could – theoretically – easily be renovated towards nitrogen removal using encapsulated nitrifiers. Financial aspects are commented on.

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