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.
Highlights
The size, footprint and energy consumption of the activated sludge (AS) system is governed by the requirement of the system to remove nitrogen – if nitrogen does not need to be removed by nitrification-denitrification (ND), for example, when 100% source separation of urine is practised, the AS system could be much smaller and consume much less energy (Ekama et al, 2010)
The drive to intensify the activated sludge (AS) system so that it requires less space and consumes less energy without compromising delivery of a high-quality treated effluent has led to some remarkable inventions and developments in biological nutrient removal over the past 2 decades
The main focus of these inventions and developments is to (i) maintain nitrifiers in the system at low sludge ages (Type A), (ii) make the system less sensitive to the capricious sludge settleability (Type B), and (iii) remove more nitrogen with less oxygen and organics (Type C). Six of these inventions and developments have been briefly described in this paper, viz.: (i) the integrated fixed-film activated sludge (IFAS) system (Type A), (ii) external nitrification (Type A), (iii) membrane solid-liquid separation (Type B), (iv) aerobic granulation biological nutrient removal (BNR) systems (Type B), (v) short-circuiting nitrification-denitrification (ND) by preventing nitrate formation and enforcing ND over only nitrite (Type C), and (vi) encouraging the growth of anammox bacteria in the activated sludge (Type C)
Summary
The size, footprint and energy consumption of the activated sludge (AS) system is governed by the requirement of the system to remove nitrogen – if nitrogen does not need to be removed by nitrification-denitrification (ND), for example, when 100% source separation of urine is practised, the AS system could be much smaller and consume much less energy (Ekama et al, 2010). The suspended medium solids retention time (SRT or sludge age) versus wastewater temperature of these plants are shown in Fig. 1 (Ødegaard et al, 2014) All of these plants are operating well below the minimum suspended medium SRT for nitrification recommended by the ATV (Abwasser Technischen Vereinigung) 131 guideline (blue line). If these plants were conventional suspended medium AS systems, the maximum specific growth rate of nitrifiers at 20°C (μAm20) and temperature sensitivity coefficient (θμ) in the minimum sludge age for nitrification (Rsm) equation, Rsm=1/ {μAm20 (θμ)(T-20) − bA20(1.03)(T−20)}, that best fits the red line in Fig. 1 are μAm20 = 1.10 /d and θμ = 1.143. Placing the media in the middle section of the aerobic reactor has several advantages: Significant organic removal will have already taken place, the ammonia concentration is highest in early stages of the reactor favouring the nitrification capacity of the attached biomass, the DO may be reduced in the last compartment of the aerobic reactor so less DO is recycled back to the anoxic reactor, low intensity of mixing in the last compartment improves flocculation, and the last compartment is seeded with nitrifiers from the media increasing the suspended AS nitrification in the last compartment
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
