Oxygen-consumption contaminants and nutritious substances present in the water, such as various organic carbon (C), nitrogen (N) and phosphorus (P), are the main pollutants causing deterioration of natural water quality. The most widely used method for organic carbon (COD or BOD) removal is the activated sludge process, i.e., secondary biological wastewater treatment process, which was invented between 1898 and 1914. The removal efficiency of organic carbon reaches 90-95%. In this biological treatment process, organic substances are oxidized and decomposed by heterotrophs. Part of the carbon, nitrogen, phosphorus and sulfur are assimilated to bacterial cells and are discharged in the form of excess sludge; the remaining organic carbon is oxidized to CO2 by dissimilation and then removed. The energy produced in the process is required by the growth and metabolism of the heterotrophs. The rest of the inorganic substances such as nitrogen, phosphorus and sulfur are discharged along with the water in the form of NH3, NO2−, NO3−, PO43−, SO42− etc.
Conventional biological methods aiming at removing organic carbon (COD) are insufficient for ammonia removal. The ratio of carbon, nitrogen and phosphorus in the effluent of traditional secondary treatment process is approximately C (BOD):N:P=10:20:1. Therefore the process is able to remove 90% of BOD, but only about 20%-30% of nitrogen. The 70%-80% soluble nitrogen remained in wastewater is one of the causative factors of eutrophication.
It has been commonly recognized that the threats of ammonia to the water ecosystem are just second to organic carbon. And even though large municipal wastewater treatment facilities have been constructed and operated to remove organic carbon, the contamination of ammonia still causes a problem.
The biological method has already been proved to be effective for organic carbon removal, but how to remove nitrogen efficiently and economically in large scale still need to be investigated.
Conventional wastewater treatment technologies for removing organic carbon and nitrogen are based on the microbiological theory and technological principles that combine three processes: degradation of organic carbon and “ammonification” of organic nitrogen by heterotrophs, “nitrification” of ammonia and nitrite carried out by autotrophs, and “denitrification” by anaerobic (facultative) heterotrophs. The three processes above can be demonstrated as follows:

Some main features of the three steps are listed as follow:
{circle around (1)} Ammonification is facilitated by the growth of heterotrophs of various genera in which organic nitrogen is converted to inorganic nitrogen, i.e. ammonia;
{circle around (2)} Nitrification is facilitated by the growth of obligate aerobic autotrophs of various genera in which ammonia is oxidized to nitrite and nitrite is further oxidized to nitrate; Nitrosomonas and Nitrobacter are typical of these chemolithotrophic species that carry out the two oxidation processes, respectively.{circle around (3)} Denitrification is facilitated by the growth of heterotrophs of various genera in which nitrate is reduced to nitrogen gas.
Therefore, from the microbiological point, the mechanism of nitrogen and carbon removal follows a model as heterotrophic bacterial utilization→autotrophic bacterial utilization→heterotrophic bacterial utilization.
From a nitrogen removal perspective, the conventional activated sludge system in which organic substances removal and ammonification take place in the same reactor, can be considered as a single-stage nitrification process. According to the above model, nitrification is facilitated by the growth of autotrophs, and denitrification is facilitated by the growth of heterotrophs. In this single-stage nitrification process, the growth rate and the oxygen and nutrient utilization rate of the heterotrophs involved in oxidizing organic carbon are greater than the nitrifying autotrophs, therefore the heterotrophs predominate over the autotrophs, which ultimately leads to low efficient nitrification.
The phenomenon of low efficient nitrification is often observed in the secondary treatment process, which seemingly strengthens the fact that nitrifying bacteria is indeed autotrophic in nature. Researchers undoubtedly believe that organic substances inhibit the growth and physiological activity of autotrophic ammonia oxidizing bacteria in the waste water treatment system aiming at the removal of C and N pollutants.
Owing to this theory, two-stage and multistage activated sludge treatment processes are brought forth in order to eliminate the adverse effects of organic substances on nitrification by separating organic removal process and nitrification (and denitrification) in two (or three) separated reactors. However, the multistage activated sludge treatment processes have failed to achieve wide application due to its high investment and operation cost.
It is therefore understandable that before the breakthrough of theory, engineers and designers have conceived of a range of improved single-stage activated sludge technologies to remove nitrogen. These processes combine the aerobic nitrification zone and the anoxic denitrification zone into a single system such as PHOREDOX (A/O), A2/O, UCT (or MUCT) and VIP etc. However, the operations of these systems are still complicated although they have improved carbon and nitrogen removal.
Organic carbon and nitrogen removal efficiency is to the root constrained by the biological features of bacteria during nitrification. Since the operation of the wastewater treatment plants is under the guidance of metabolism theory of autotrophic nitrification, major drawbacks exit in the application of these conventional methods: {circle around (1)} Slow cell growth rate, low sludge production and poor sludge settleability of nitrifying bacteria make it difficult to maintain a high biomass concentration of nitrifying bacteria; {circle around (2)} Many activated sludge systems lack effective nitrification, especially during the winter when temperature drops below 15° C., which results in long hydraulic retention time (HRT) and low organic burden on the system; {circle around (3)} Part of the effluent and sludge have to be returned to the tank to achieve higher biomass concentration and more effective nitrogen removal; {circle around (4)} The addition of alkaline to maintain pH level leads to higher operation costs; {circle around (5)} Conventional nitrification processes tend to have extreme results: either no ammonia oxidation at all or complete oxidation into nitrate; {circle around (6)} Conventional methods are often inadequate for nitrogen-enriched waters with nitrogen content exceeding 200 mg/l.
In all, traditional nitrification-denitrification method is inadequate to prevent nitrogen pollution to the environment.
However, extensive and intensive studies on biological N-removal have been carried out in many developed countries, and lead to the breakthrough in both theory and technology which leads to the invention of a range of innovative nitrogen removal techniques with SHARON® as a representative, and has to some extent improved nitrogen removal efficiency and reduced operation costs in wastewater treatment.
Take the SHARON® (Single Reactor High Activity Ammonia Removal Over Nitrite) which is also considered a short-cut nitrification and denitrification technique (European patent EP 0 826 639 A1, Chinese patent application publication No. CN1310692A) as an example:
Conventional nitrification methods completely oxidize ammonia to nitrate instead of nitrite (NH4+→NO2−→NO3−, termed as “complete nitrification”) in order to both eliminate the oxygen consumption potential of nitrogen and prevent nitrite from inhibiting bacterial growth. However, the complete nitrification process is not necessary in nitrogen removal from wastewater, and the process of oxidizing ammonia to nitrite (NH4+→NO2−) can achieve equally promising results. It is possible to eliminate the conversion of NO2− to NO3− during nitrification and NO3− to NO2− during denitrification in biological nitrogen removal. The process of controlling ammonia oxidation at the nitrite stage is called as the Short-cut Nitrification. In 1997 Delft University of Technology developed the Short-cut Nitrification and Denitrification which resolved the difficulties of treating sludge digester effluents which contain high ammonia concentration to some extent.
The key in the SHARON® technique is to optimize operational conditions in order to facilitate the growth of autotrophic ammonia-oxidizing bacteria (Nitrosomonas sp), especially Nitrosomonas europh, and to allow them to become dominant in the reactor. The conditions proposed by SHARON® enable the growth rate of ammonia-oxidizing bacteria to compensate for the sludge loss in the CSTR (Continuous Stirred Tank Reactor), whereas the growths of nitrite-oxidizing bacteria including Nitrobacteria are constrained and then washed out. Under these conditions, ammonia oxidation is controlled and restrained to the nitrite stage and nitrite acts as the electron acceptor in denitrification. Some main features of SHARON® are that: {circle around (1)} It is a shorter process with short-cut nitrification and denitrification being combined in one single reactor; {circle around (2)} There is no retention of biomass in the reactor, therefore only a simple reactor is required; {circle around (3)} It demands high operation temperature (30˜40° C.) to achieves effective treatment results; {circle around (4)} Alkalinity can be adjusted by denitrification and pH is maintained between 7 and 8 without external alkaline addition.
Compared with conventional nitrogen removal technologies, SHARON® has the following advantages: lower investment and operation costs; easier start-up and operation; simpler maintenance; no production of chemical by-products. However, SHARON® has drawbacks, because it is still based on the traditional autotrophic nitrification theory. From the operational perspective, organic carbon removal, nitrogen removal and sludge disposal remain highly disintegrated. The high processing temperature (35° C.) places stringent requirements on reactors and is unable to treat large volume of wastewater with low ammonia concentration. It is difficult to be realized in traditional sequencing batch reactors (SBR). It still requires excess sludge discharge and relatively long hydraulic retention time (HRT) during denitrification compared with nitrification rate.
Wastewater treatment technology mainly utilizes the variety of bacteria metabolism to decompose and remove pollutants. Current carbon and nitrogen removal methods, including new biological nitrogen removal techniques with SHARON® as representative, are all based on the theory developed by Monod. The Monod theory (or cell growth theory) concerns the relationship between cell growth and organic carbon and nitrogen removal. Monod states that cell growth is associated simultaneously with the assimilation of organic carbon and nitrogen and the decomposition of excess substrate to fuel physiological behaviors. This theory has become the mainstream in microbiology and has guided a range of industrial applications, including organic carbon and nitrogen removal. In particular, it has exerted considerable influence in areas of reactor design, process design and operational management etc.
According to Monod theory, in regard with the kinetics of substrate conversion, bacterial growth and substrate utilization rate exhibit the following relationship:
            ⅆ      s              ⅆ      t        =            -              1        Y              ⁢                  ⅆ        x                    ⅆ        t            
Where: ds/dt is the substrate utilization rate; Y is the biomass yield coefficient (biomass produced per mass of substrate utilized); X is the biomass concentration. It can be concluded from the equation that bacterial growth is directly related to substrate utilization, and that by improving bacterial growth rate, substrate utilization can be enhanced.
During inorganic NH4+conversion in the traditional “heterotrophic-autotrophic-heterotrophic bacterial utilization” model, and according to Monod kinetics, bacterial growth rate or substrate utilization rate is extremely low. In theory, bacterial growth rate is 0.29 g/g (VSS/NH4+—N) and 0.084 g/g (VSS/NO2−—N)(McCarty pL. 1964) while experimental results are only 0.04˜0.13 g/g (VSS/NH4+—N) and 0.02˜0.07 g/g (VSS/NO2−—N). The biomass yield coefficient and substrate utilization coefficient of nitrifying autotrophs are 1-2 orders of magnitude slower than heterotrophs which has become the main limiting factors of nitrogen removal efficiency.
When the Monod theory is implemented in the batch reactor, substrate consumption and the accumulation of toxic substances often result in the deterioration of nutrient environment and other environmental conditions, such as extreme acidic or basic conditions, which in turn hinder cell growth or even lead to cell death. To eliminate these influences, industrial applications often adopt the “chemostat” in which fresh medium is continuously added to supplement nutrients and equal amount of culture liquid (biomass and toxic substances) is continuously discharged to reduce the accumulated biomass and toxic substances, and to sustain stable biomass growth and substrate removal.
The principles mentioned above have served as guidance in main technologies of organic carbon and nitrogen removal from wastewater. These principles have determined the configuration of almost all reactors (mostly continuous stirred tank reactor and continuous flow operation), and most importantly, they have led to the inevitable process of sludge accumulation and discharge during organic carbon and nitrogen removal.
Thus the need for the treatment and disposal of sludge is one of the most crucial problems to be solved of conventional biological wastewater treatment technologies.
Due to the autotrophic nature acknowledged in the prior art, the presence of organic substances is deleterious to the growth and physiological behavior of nitrifying bacteria, therefore any attempt to optimize the biological processes involved in organic carbon and nitrogen removal cannot overcome the inherent limitations.
The present inventor realized that the oxidation of NH4+into NO2− was largely related to the physiological behavior of heterotrophs, and thus adopted a method abandoned by the autotrophic theory and successfully isolated different heterotrophs with various ammonia oxidation activities. Certain strains exhibited high NO2− accumulation properties under pure-culture conditions (Chinese Patent No. 03118598.3, “Methods for Separating and Identifying Heterotrophic Nitrifying Bacteria”). He further proposed a method to cultivate highly active nitrifying heterotrophs and applied them to nitrogen removal from water (Chinese Patent No. 03118597.5, “Cultivation and Application of Nitrifying Heterotrophs”), and proposed two different methods to remove ammonia (Chinese Patent No. 03118599.1, “Combination of nitrogen-removing bacteria and their Application”, and Chinese Patent No. 200410005158.4, “Biological Ammonia Removal Methods from Wastewater and Relative Microorganisms”).
However, the research mentioned above was mainly carried out with pure culture as inoculum, especially in single batch test based on the Monod theory. Therefore ammonia oxidation and nitrogen removal was not significantly more effective compared with classical autotrophic ammonia oxidation and denitrification. Another problem was that the growth of highly active heterotrophs was restrained at temperatures under 15° C. and thus ammonia oxidation activity was hard to exhibit. The technologies were unable to resolve the problems of nitrogen removal at low temperatures.
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