Wastewater Treatment Terms Explained: Biological Nitrogen Removal

Biological nitrogen removal converts the nitrogenous pollutants in wastewater into nitrogen gas through the combined action of several groups of microorganisms. The process chain consists of ammonification, nitrification and denitrification, each carried out by a different group of bacteria under different conditions.

Ammonification

Ammonification is the process in which nitrogen-containing organic matter is decomposed by ammonifying bacteria and converted into ammonium (NH4+). Organic nitrogen can be broken down by the corresponding microorganisms under both aerobic and anaerobic conditions, releasing ammonia.

Nitrification

Nitrification is carried out by aerobic autotrophic microorganisms. In the presence of oxygen they use inorganic nitrogen as their nitrogen source, oxidising NH4+ to NO2- and then to NO3-. The process takes place in two stages: in the first, ammonia-oxidising bacteria convert ammonia nitrogen into nitrite (NO2-); in the second, nitrite-oxidising bacteria convert nitrite into nitrate (NO3-).

Denitrification

Denitrification takes place under anoxic conditions, when denitrifying bacteria reduce nitrite nitrogen and nitrate nitrogen to gaseous nitrogen (N2). Denitrifying bacteria are heterotrophic and mostly facultative: under anoxic conditions they use the oxygen bound in nitrate as the electron acceptor and organic matter (the BOD fraction of the wastewater) as the electron donor, obtaining energy while the organic matter is oxidised and stabilised.

Key control parameters

Temperature

The optimum temperature for biological nitrification is 20–30 °C. The nitrification rate falls below 15 °C and essentially stops at 5 °C. Denitrification performs best at 20–40 °C, and its rate also declines below 15 °C. In practice, denitrification in biofilm systems is less affected by temperature than in suspended-sludge systems, and fluidised-bed denitrification is far less temperature-sensitive than rotating biological contactors or suspended sludge.

Dissolved oxygen

Nitrification uses molecular oxygen as the terminal electron acceptor, so it can only occur when dissolved oxygen is present. To achieve normal nitrification in an activated sludge plant, the DO level must be kept above 2 mg/L, typically 2–3 mg/L. At lower DO levels nitrification is restricted or stops altogether.

Nitrification and denitrification are opposed in their oxygen requirements. Conventional denitrification needs a strictly anoxic environment, because DO and NO3- compete as electron acceptors: when DO is present it not only suppresses the synthesis and activity of nitrate reductase but also leads denitrifiers to use DO preferentially as the terminal electron acceptor while degrading organic matter. In real plants, however, oxygen-transfer limitations leave anoxic zones inside the sludge flocs, so denitrification can still occur even where some DO is present in the aeration tank. Studies show that keeping DO in the anoxic tank below 0.5 mg/L is enough to promote denitrification and deliver good results.

pH

pH is one of the most important parameters in biological nitrogen removal. Most experiments show that the functional bacteria are highly sensitive to pH change. The optimum pH for nitrifying bacteria is 8.0–8.4; outside the range of 6.0–9.6, that is above 9.6 or below 6.0, nitrification is inhibited and stops.

BOD5/TKN ratio

Nitrification and denitrification form an opposing yet unified system, determined by the nature of the two bacterial groups. Nitrifiers are autotrophic and do not need organic matter for their metabolism; when organic concentrations are high, their ability to compete for nutrients is far weaker than that of heterotrophs and they are inhibited, so nitrification is limited as the nitrifying population shrinks. The smaller the influent BOD5/TKN ratio, therefore, the larger the relative share of nitrifiers and the more favourable nitrification becomes. Denitrifiers are heterotrophic and need an organic carbon source to supply reaction electrons, so genuine biological nitrogen removal requires sufficient organic carbon. Research indicates that when the influent BOD5/TKN is 4 to 6 or higher, the carbon source can be considered sufficient and no external carbon dosing is needed.

Sludge retention time (SRT)

SRT is a key control parameter in a biological nitrogen removal system. In general the SRT must be longer than the minimum specific growth rate of the nitrifiers, because their specific growth rate is an order of magnitude lower than that of the heterotrophs in an activated sludge system. Only then can nitrifiers survive in a continuous-flow system, allowing nitrification and nitrogen conversion to take place.

Internal recycle ratio (IR)

Recycling plays a vital role: it supplies nitrate nitrogen to the anoxic reactor as substrate for denitrification, so that it can be reduced to N2. The internal recycle ratio affects both denitrification performance and the pumping energy consumed, which makes it a parameter of real practical significance.

A/O and SBR processes

In the A/O biological nitrogen removal process the anoxic zone is placed at the front of the system. The alkalinity produced by denitrification there partly offsets the alkalinity demand of nitrification. Because denitrification in the anoxic tank uses organic matter from the raw wastewater as its carbon source, external carbon dosing can be reduced or eliminated altogether. Internal recycle carries the nitrate produced by nitrification back to the anoxic tank for denitrification; the oxygen in the nitrate serves as the electron acceptor for the respiration and metabolism of the denitrifiers, completing nitrogen removal. In the A/O process the internal recycle ratio strongly influences performance: too low a ratio leaves insufficient nitrate for denitrification and nitrification becomes incomplete, while too high a ratio shortens the contact time between the recycled liquor and the denitrifying biomass and lowers efficiency. An appropriate ratio must therefore be maintained in operation.

Conventional biological nitrogen removal technology dates from the 1930s and came into real application in the 1970s. Since the introduction of Barth's three-stage process, the A/O process, the SBR process and other nitrogen removal processes have been developed. The SBR process follows essentially the same nitrogen removal mechanism as the A/O process. It operates in batch mode, using a single reaction tank instead of the several reaction zones with different functions that make up a conventional A/O reactor. Alternating phases in time create the anoxic and aerobic conditions that a conventional plant creates in space, and its simple structure and flexible operation have attracted considerable research interest.