Recent developments in biological nutrient removal

Abstract

Biological nitrogen (N) and phosphorus (P) removal from municipal wastewater with the activated sludge (AS) system has been the preferred technology for the last 40 years. While several questions remain to be answered for more consistent, reliable and stable performance for enhanced biological P removal (EBPR), recent developments in this technology have focused on (i) increasing capacity and reducing the plant space footprint and (ii) improving N removal. To increase capacity and reduce AS system space, (a) integrated fixed-film activated sludge (IFAS), (b) external nitrification, (c) membrane, (d) aerobic granulation BNR systems and (e) more efficient N removal bioprocesses (anammox and nitrite shunt) have been developed. With IFAS, fixed media are added to the aerobic activated sludge reactor to make nitrification independent of the suspended AS sludge age. With external nitrification, nitrification is achieved in a side-stream fixed media reactor, which removes the size-defining nitrification process from the suspended AS system and halves its sludge age, improves sludge settleability and increases capacity. With membranes, secondary settling tanks are replaced with in-reactor membranes for solid-liquid separation. With aerobic granulation, the activated sludge process is controlled to form fast-settling granules comprising heterotrophs, nitrifiers, denitrifiers and phosphorus-accumulating organisms (PAOs) in a sequencing batch (SBR) type reactor – the granules not only settle fast but the SBR-type operation also removes the need for secondary settling tanks allowing higher reactor solids concentrations and hence smaller reactors. To achieve N removal more efficiently, methods are being developed to (i) short-circuit nitrification-denitrification (ND) by preventing nitrate formation and enforcing ND over nitrite – this requires less oxygen and organics than ND over nitrate allowing lower N concentrations to be achieved for the same influent organics concentration and oxygen supply, and (ii) encouraging the growth of anammox bacteria in the activated sludge which remove N autotrophically by combining ammonia and nitrite to form nitrogen gas – this halves oxygen demand for nitrification and requires no organics. These recent developments in BNR technology are briefly reviewed in this paper.