What Is Simultaneous Nitrogen and Phosphorus Removal?
As water quality standards tighten and more operational problems emerge in practice, nitrogen removal alone or phosphorus removal alone has shown clear limitations. This has driven the search for new process schemes and improved technologies capable of removing nitrogen and phosphorus within a single treatment system, giving rise to a family of simultaneous nitrogen and phosphorus removal processes [2]. The main options include the anaerobic-anoxic-aerobic (A2O) process, the Bardenpho process, the UCT process, the Phoredox process and the SBR process [3].
The A2O process takes its name from Anaerobic/Anoxic/Oxic and is currently one of the most common simultaneous nitrogen and phosphorus removal processes. The A2O biological nutrient removal process is an activated sludge process: while removing BOD, COD and SS, it can also achieve biological nitrogen and phosphorus removal, as shown in the flow diagram on the right [4].
In the aerobic zone, nitrifying bacteria convert the ammonia nitrogen in the influent and the ammonia nitrogen formed by ammonification of organic nitrogen into nitrate through biological nitrification. In the anoxic zone, denitrifying bacteria convert the nitrate carried in by internal recirculation into nitrogen gas through biological denitrification, which escapes to the atmosphere and thereby achieves nitrogen removal. In the anaerobic zone, phosphorus-accumulating organisms release phosphorus and take up readily biodegradable organics such as lower fatty acids; in the aerobic zone they take up phosphorus in excess, and the phosphorus is removed through the discharge of waste activated sludge. All three groups of bacteria contribute to BOD5 removal, although BOD5 removal is in practice dominated by the denitrifying bacteria.
(1) The organic combination of three different environmental conditions - anaerobic, anoxic and aerobic - with different microbial populations gives the process the ability to remove organic matter, nitrogen and phosphorus simultaneously.
(2) Among processes for simultaneous nitrogen and phosphorus removal together with organic matter removal, this is the simplest in flow scheme and has a shorter total hydraulic retention time than comparable processes: 3.5-6.0 h for the aerobic zone, and 0.5 h and 1.0 h for the anaerobic and anoxic zones respectively.
(3) Under alternating anaerobic, anoxic and aerobic operation, filamentous bacteria do not proliferate excessively; the SVI is generally below 100 and sludge bulking does not occur.
(4) The sludge has a high phosphorus content, generally above 2.5%.
(5) No external carbon source is required, and only gentle mixing is needed in the anaerobic and anoxic zones, so operating costs are low.
Bardenpho Process
(2) Each reaction is repeated more than twice, and every reaction unit has its own primary function while also serving two or three secondary functions.
Bardenpho process flow diagram
(1) Because a two-stage A/O configuration is adopted, nitrogen and phosphorus removal performance is good, with nitrogen removal efficiency reaching 90%-95%.
The Bardenpho process consists of a two-stage A/O configuration with four reaction tanks in total. Because of sludge recirculation, both the first anaerobic tank and the first aerobic tank contain nitrate nitrogen. In the first anaerobic tank, denitrifying bacteria use the organic carbon in the raw water to reduce the nitrate nitrogen in the recirculated mixed liquor. The effluent from the first anaerobic tank enters the first aerobic tank, where carbonaceous organic matter is oxidatively degraded and nitrification of nitrogenous organic matter takes place, converting organic nitrogen and ammonia nitrogen into nitrate nitrogen. The treated effluent from the first aerobic tank then enters the second anaerobic tank, where nitrate nitrogen in the wastewater is further reduced to nitrogen gas, lowering the total nitrogen in the effluent and improving sludge settling performance. The detailed flow scheme is shown in the diagram on the right.
In the two simultaneous nitrogen and phosphorus removal processes described above, return sludge is sent directly to the anaerobic tank at the front of the process, where it inevitably carries a certain concentration of nitrate. This triggers denitrification in the first anaerobic tank, and the denitrifying bacteria compete with the phosphorus-removing organisms for the organic matter in the wastewater, impairing phosphorus removal. The UCT (University of Cape Town) process was therefore proposed; its flow scheme is shown in the diagram on the right.
(1) The secondary clarifier sludge is returned to the anoxic zone rather than the anaerobic zone. The mixed liquor leaving the anoxic zone has a very low nitrate content, so when it is returned to the anaerobic tank it provides optimal conditions for phosphorus release by the sludge.
(2) The anaerobic zone has a relatively long retention time and organic matter utilisation is high.
(3) It is better suited to raw wastewater with a low BOD5/TP ratio.
Phoredox Process
The process flow scheme is shown in the diagram on the right:
Phoredox process flow diagram
(1) This five-stage system has anaerobic, anoxic and aerobic tanks for phosphorus removal, nitrogen removal and carbon oxidation, with the second anoxic stage used mainly for further denitrification.
(2) The nitrate produced in the aerobic zone serves as the electron acceptor and organic carbon as the electron donor.
(3) Mixed liquor is recirculated from the aerobic zone to the anoxic zone twice.
(4) The process has a long sludge age (about 30-40 d), which enhances carbon oxidation capacity.
SBR (Sequencing Batch Reactor), also known as the intermittent activated sludge process, is a variant of the activated sludge process. One operating cycle of a conventional SBR consists of five stages: Fill, React, Settle, Draw and Idle. The process flow scheme is shown in the diagram on the right.
(1) The SBR process provides alternating anoxic, anaerobic and aerobic conditions in the time sequence, so that denitrification occurs under anoxic conditions, phosphorus release under anaerobic conditions, and nitrification and excess phosphorus uptake under aerobic conditions, achieving effective nitrogen and phosphorus removal.
(2) The SBR process is simple in flow scheme and low in operating cost, gives good solid-liquid separation and good nitrogen and phosphorus removal, and withstands shock loads while effectively preventing sludge bulking.
(3) Conventional SBR does, however, have limitations in application; for example, when the influent flow rate is large the reaction system must be regulated, which increases capital investment. To further improve effluent quality, many SBR variants have been developed, such as the CASS process, the DAT-IAT process, the MSBR process and UNITANK.
