Chapter II. Literature review of nitrogen removal from wastewater
3. Biological processes for removing ammonium nitrogen from wastewater
3.1 Conventional biological processes or technologies
Wastewater treatment starts with a pre-treatment process depending on the type of wastewater and subsequent treatment systems. Primary treatment in settling tanks allows for settling of the organic suspended solids to form raw sludge. The secondary treatment of wastewater employs various technologies to reduce up to 85% of the nitrogen, BOD, COD and suspended solids.
The technological processes to treat wastewater can be grouped into four general process categories: source separation, physical/chemical processes, biological nitrification/denitrification, and natural systems (Hazen and Sawyer 2009). Hence, according to the selected process category, the design of the system for nitrogen removal must follow different general concepts (Mulder 2003; Carrera et al. 2003; Murat et al. 2003).
Biological treatment systems that can remove ammonia nitrogen from wastewater include:
activated sludge, wastewater lagoons, trickling filter process, submerged fixed bed reactor, biological filters, biological contactor process, moving-bed process, suspended biomass/biofilm, Linpor-CN, constructed wetlands etc., (Davis and Masten 2004: Valigore 2011). Nitrogen exists in different forms because of the different states of oxidation (from -3 to +5). In the environment, changes from one oxidation state to another can be accomplished biologically by living organisms (Sedlak 1991, Madigan et al. 1949; Reed 1984).
Biological nitrification is the process of converting ammonia in wastewater to nitrate using aerobic autotrophic bacteria in the treatment process. Nitrification is actually a two-step process for removing ammonia from wastewater using two different types of autotrophic bacteria that oxidize ammonia to nitrite (Nitrosomonas) and then oxidize nitrite to nitrate (Nitrobacter).
The incorporation of such autotrophic and heterotrophic nitrogen removal processes has been studied by different researchers over the past. They have analysed the biochemistry, microbiology and physiology of the autotrophic and heterotrophic nitrogen removal processes. A number of laboratories and pilot-scale reactors have been operated. However, there were still several questions remaining towards an implementation for obtaining a high efficiency of nitrogen removal and the approach for upgrading the processes to full scale:
- Analysing/optimising the process operating conditions, such as the influence of concentrations, pH, temperature, wind speed or the conditions considering the light regime.
- Defining the ideal residence time in the operating system.
- Listing the necessary methods and suitable conditions to control the process.
- Describing and predicting the nitrogen removal by a certain modelling development.
In the absence of sufficient organic carbon supply, the denitrification process will not completely convert nitrate into nitrogen gas (Gerardi 2002). To avoid this issue in the denitrification zone, most WWTP first send the wastewater to a mixing-basin where it is stirred without air supply before flowing to the aeration tank (pre-denitrification). During this process, nitrification may occur.
3.1.1 Activated sludge process
The activated sludge process is a wastewater treatment method that is widely used around the world. To date, the largest application of the activated sludge process systems has been to treat domestic or industrial wastewater. The highest quality of effluent could be achieved from this process. Both aerobic and anaerobic bacteria may exist in this process, but the predominant species is facultative bacteria.
However, the activated sludge process system is more mechanised in comparison with other wastewater treatment systems. It requires a large energy consumption for aeration and is difficult to apply in low income countries.
The following units in activated sludge process are integral and essential to any continuous-flow (describe in the Fig. 9):
- Aeration tank (main reactor)
- Settling tank (secondary sedimentation tank)
… …
…
…
Influent (primary-treated)
Anoxic zone Denitrification
Aerobic zone COD-elimination
& Nitrification
Effluent
Aeration Recirculation sludge
Return sludge
Excess sludge Clarifier
… …
…
…
Influent (primary-treated)
Anoxic zone Denitrification
Aerobic zone COD-elimination
& Nitrification
Effluent
Aeration Recirculation sludge
Return sludge
Excess sludge Clarifier
… …
…
…
Influent (primary-treated)
Anoxic zone Denitrification
Aerobic zone COD-elimination
& Nitrification
Effluent
Aeration Recirculation sludge
Return sludge
Excess sludge Clarifier
Figure 9. Standard activated sludge process (DWA 2011) 3.1.2 Trickling filter
Trickling filter consists of a fixed biological bed of coarse contactor media such as crushed traprock, granite, limestone, clinkers, wood slats, plastic tubes, corrugated plastic sections, hard coal, or other material over which wastewater is distributed or contacted. Biological slimes form on the media which assimilate and oxidize substances in the wastewater. Various contact beds and trickling filters have been developed for the transfer of dissolved organic matter and fine suspended solids from settled wastewater to contact surfaces (Wang et al.
2009a).
However, an uncovered trickling filter is vulnerable to below freezing weather and it is less effective in the treatment of wastewater containing high concentrations of soluble organics. It has only limited flexibility and control, and needs long recovery time with upsets. The process creates odor problems if improperly operated (Wang et al. 2009a).
3.1.3 Rotating biological contactor
According to Wang et al. (2009a), rotating biological contactor (RBC) is an attached-growth biological process, which consists of a series of rotating plastic media all coated with a layer of biofilm. The biofilm or slime on the media aerobically reacts with substances in a waste stream for bio-oxidation and nitrification, or anaerobically reacts with the substances for denitrification.
The general concept of rotating biological contactors is to let wastewater flow through the tank, and to rotate the medium in the wastewater to be treated, alternatively exposing the
medium (and the attached biological growth) to air and the wastewater. The rotated treatment units are about 40% immersed in the wastewater for aerobic removal of organic waste by the biological film developing on the media. Wastewater treatment efficiency in terms of carbonaceous oxidation and nitrification can be significantly increased by the multiple staging of rotating biological contactors. RBC systems can also be used for biological denitrification.
The most important factor affecting performance of the rotating biological contactors is the biological slime of those microorganisms that grow on a series of thin media, such as discs, mounted side by side on a shaft. The treatment efficiency decreases with decreasing wastewater temperature below 13oC. Other limiting-factors reducing the efficiency of the process are: high organic and hydraulic overload, pH too high or too low, toxic materials in influent. The performance of a typical four stage RBC system with primary and secondary clarifiers is (Wang et al. 2009a):
- BOD5 removal 80% to 90%
- SS removal 80% to 90%
- Phosphorus removal 0% to 30%
- NH4+-N removal up to 95%