Impact of Cold Temperatures on Nitrogen Removal in Denitrifying Down-Flow Hanging Sponge (DDHS) Reactors

Innovative and low-energy solutions for the removal of nitrogen from domestic wastewater are needed to achieve regulatory ambitions. However, there is a lack of appropriate technologies for use in non-centralised applications, where receiving waterbodies also are potentially sensitive. Denitrifying down-flow hanging sponge (DDHS) reactors are a promising solution but their performance has not been assessed under colder operating conditions pertinent to northern climates. Two DDHS reactor configurations (short and tall anoxic zones) were tested under typical UK winter, summer, and spring/autumn temperatures. At 22 degrees C, both reactors achieved >58% total nitrogen (TN) removal from domestic wastewater with no significant differences in removal rates between configurations. However, denitrification was lost at 13 degrees C in the reactor with the short anoxic zone, and was lost totally in both systems at 6 degrees C. Efficient nitrification was retained at 6 degrees C in both reactors (>90% removal NH4-N), suggesting that while elevated TN removal was not retained under colder conditions, the DDHS systems still effectively removed ammonia under UK winter conditions. DDHS reactors show promise for use under colder temperature conditions, although optimisation is needed, including the derivation of temperature correction factors for nitrogen removal.