Biological nitrogen removal process of low temperature wastewater

Effect of low temperature on nitrogen removal process

Temperature is an important environmental condition that affects the growth and metabolism of bacteria. The normal growth temperature of most microorganisms is 20~35℃.

Temperature mainly affects the growth and metabolic rate of microorganisms by affecting the activity of some enzymes in microbial cells, and then affects the sludge yield and pollutant removal efficiency and rate. Temperature also affects the degradation path of pollutants, the formation of intermediate products, and the solubility of various substances in solution, and may affect gas production and composition.

Low temperature weakens the fluidity of cytoplasm in microorganisms, thereby affecting metabolic processes such as material transport, and it is generally believed that low temperature will lead to the decline of adsorption and settling performance of activated sludge, as well as changes in microbial communities. The inhibition of microbial activity at low temperatures, unlike the devastating effects of high temperatures, is usually recoverable.

1.1 Nitrification process

The biological nitrification reaction can be carried out in the temperature range of 4~45℃.

The optimum growth temperature of ammonia-oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) was 25~30℃. Temperature not only affects the growth of nitrobacteria, but also affects the activity of nitrobacteria. Studies have shown that the most suitable growth temperature of nitrifying bacteria is 25~30℃. When the temperature is less than 15℃, the nitrification rate significantly decreases, and the activity of nitrifying bacteria is also greatly reduced. When the temperature is less than 5℃, the life activity of nitrifying bacteria almost stops.

A large number of studies have shown that nitrification is seriously affected by temperature, especially the impact of temperature shock. It is common to fail to achieve stable operation of nitrification process due to low temperature in winter. U.Sudarno et al. investigated the effect of temperature change on nitrification, and the results showed that the ammonia oxidation rate increased when the temperature increased from 12.5℃ to 40℃, but when the temperature dropped to 6℃, the activity of nitrifying bacteria was very low.

With the continuous development of nitrogen removal process, people have put forward higher requirements for nitrification process, hoping to control the reaction product of nitrification in the nitrite stage, as a pre-treatment technology for denitrification or anaerobic ammonia oxidation, which can save aeration energy consumption and add alkali amount. Through more understanding of two classes of nitrifying bacteria (AOB and NOB), the short-range nitrification process has emerged.

The core of the process is to selectively enrich AOB, first inhibit and then restrict NOB, and finally flush out NOB, so that AOB has a high amount and NOB is eliminated, so as to maintain stable nitrite accumulation. The short-range nitrification process is usually achieved by controlling temperature, dissolved oxygen, and pH. The basis of temperature controlled short-cut nitrification is that the two types of nitrifying bacteria have different sensitivity to temperature. When the temperature is above 25℃, the maximum specific growth rate of AOB is greater than that of NOB.

Therefore, the SHARON process, the world's first industrial application of short-range nitrification process (temperature set at 30~40℃ (1)), was proposed. Therefore, achieving short-range nitrification at low temperatures is a challenge.

1.2 Denitrification process

Low temperature has a significant inhibitory effect on denitrification. The study of denitrification in the sediments of Taihu Lake showed that the denitrification rate showed seasonal changes after several months of experimental analysis. The performance of the denitrification process at low temperature (3-20 ℃) was investigated. The results showed that the denitrification rate of the reactor at 3℃ was only 55% of that at 15℃.

Compared with the traditional anoxic denitrification, the effect of temperature on the denitrification efficiency of aerobic denitrification was not significant, and a strain of aerobic denitrifying bacteria screened could reach a nitrogen removal efficiency greater than 78% at 25~35℃.

1.3 Anaerobic ammonia oxidation process

Some scholars have shown that the temperature range of anaerobic ammonia oxidation reaction can be 6~43℃, and the best temperature is 28~40℃. In the biological treatment of wastewater, the activation energy usually ranges from 8.37 to 83.68kJ/mol, while the activation energy of anaerobic ammonia oxidation is 70kJ/mol.

Therefore, anaerobic ammoX is a reaction type sensitive to temperature change, and the decrease of temperature has obvious inhibition effect on it.

Low temperature has a great influence on anaerobic ammoX, and it takes a long time to recover after being inhibited by low temperature. When the operating temperature of the anaerobic ammox process is reduced from 18℃ to 15℃, nitrite cannot be completely removed, resulting in the accumulation of nitrite, which has a significant inhibitory effect on the anaerobic ammox process, thereby causing a chain effect and inactivation of anaerobic ammox bacteria [6,25].

In the study of the long-term effect of temperature on anaerobic ammox process, the test temperature was adjusted from 30℃ to 15℃, and the effluent quality could be ensured only when the nitrogen volume load (NLR) was significantly reduced from 0.3kg/ (m3•d) to 0.04kg/ (m3•d). Even after 30 days of acclimation, there was no improvement. After the test temperature was adjusted to 30℃ for 75 days, the sludge activity was only 0.02g/ (g•d), which was at a low level.

Improvement of nitrogen removal process at low temperature

2.1 Strain flow

Strain flow and expansion culture technology derived from fermentation process. Strain expansion culture technology is a kind of strain application technology widely used in fermentation industry. In batch fermentation, the multistage amplification of "test tube → triangle bottle → seed pot → fermenter" is generally adopted to make the bacteria quantity meet the production needs.

In the process of wastewater denitrification, in addition to the self-proliferation of bacteria in the plant, the flow addition of bacteria is beneficial to accelerate the accumulation of bacteria. The complexity of wastewater quality, the instability of toxic substances, matrix, pH, temperature and other factors will inhibit functional bacteria. It is difficult for microorganisms to grow under inhibited conditions. Therefore, the advantages of strain flow can be realized.

The mass concentrations of NH4+-N and NO2--N in the wastewater were 120-200mg /L and 160-240mg /L, respectively, and the acceleration rate of bacterial flow was 0.028g/ (L•L•d). Volumetric nitrogen removal load (NRR) was increased from 0.1kg/ (m3•d) to 7.9kg/ (m3•d).

It is also believed that the flow addition of bacteria not only increases the sludge concentration and the proportion of anaerobic ammox bacteria in the reactor, but also introduces some unknown growth factors, so that the efficient operation of anaerobic ammoX can be achieved at such a low flow acceleration rate.

Bacterial flow addition is expected to be an effective countermeasure for bioreactor operation at low temperature. The study shows that the reactor volume needs to be increased to ensure the normal nitrification rate during the low temperature period. By adding nitrifying bacteria to the activated sludge system, the problems of extending the sludge age and increasing the reactor volume during the low temperature period can be effectively solved.

The operation of bacteria flow addition is flexible and does not require long-term adjustment time, which is a fast and effective method to deal with the impact of low temperature. However, it cannot fundamentally solve the problem of low operating efficiency of the reactor at low temperature. It only increases the number of functional bacteria in the reactor and its proportion in the mixed sludge, and alleviates the impact of low temperature on biological treatment.

2.2 Inoculation with cold-tolerant bacteria

Inoculants are of great significance for the start-up of anaerobic reactors under low temperature conditions. Cold-tolerant bacteria can withstand temperature fluctuations and are more suitable for the treatment of low-temperature wastewater.

For example, the denitrifying cold resistant bacteria - Pseudomonas fluorescens can decrease the hydrolysis of phthalic acid under the condition of less than 10℃, and there are cold resistant bacteria can decrease the hydrolysis of toluene, chlorophenol and other difficult to degrade organic matter at low temperatures.

In order to ensure the effective operation of the biological treatment system of sewage in cold areas, cold-tolerant microorganisms were inoculated for the treatment of domestic sewage, and 86.7% COD in sewage was successfully removed at 6~10℃.

The research progress of cryogenic methanogens and their application in anaerobic treatment of wastewater was reviewed from the aspects of isolation and culture, physiological and biochemical characteristics, cryogenic mechanism and molecular biology. It was pointed out that inoculants are important for the start-up of anaerobic reactors under low temperature conditions.

Ammonia-oxidizing archaea (AOA) are a class of archaea that can maintain activity at low temperatures. If AOA can be applied to the biological treatment of low-temperature wastewater, it will promote the development of biological nitrogen removal process. This can be an important direction for future research.

2.3 Biological immobilization

After immobilization treatment, the stress resistance of microorganisms can be improved, and they can withstand changes in the external environment, thus maintaining a high activity. In addition, the retention ability of microorganisms after embedding and fixing is enhanced, which is expected to achieve rapid start-up and efficient and stable operation of the reactor.

The effect of temperature change on nitrification can be weakened by immobilization. The removal efficiency of ammonia nitrogen by immobilized nitrifying bacteria at different temperatures was studied. The nitrifying sludge containing cold resistant bacteria was fixed by polyvinyl alcohol-boric acid embedding method to treat domestic sewage at room temperature and low temperature.

The results showed that the immobilized nitrifying flora showed higher nitrification efficiency (> 80%) even at low temperature. Some scholars have also carried out research on nitrogen removal by immobilized denitrifying bacteria, and the results show that after immobilization treatment, the adaptability of denitrifying bacteria to temperature is improved, and the tolerance of immobilized denitrifying bacteria to high concentration of ammonium ions and low temperature is increased.

In the study of low temperature anaerobic ammox, low nitrogen wastewater was treated by inoculating immobilized microorganisms and anaerobic granular sludge. The anaerobic ammox was successfully started at 20℃, and the NRR reached 16.22g/ (m3•d), and the total nitrogen removal rate was 92%. L.m. uan et al. [38] used polyvinyl alcohol (PVA) gel and 1% alginic acid as the embedding materials of anaerobic ammox bacteria, and at (25±0.5) ℃, the NRR of anaerobic ammox process reached 8.0kg/ (m3•d).

Immobilization is an effective technical means, but it will also reduce the activity of microorganisms, and after immobilization, the mass transfer resistance will increase, especially the obstruction of oxygen mass transfer, and immobilization can better play its advantages under anaerobic conditions. In addition, the cost is subject to technical and economic assessment.

2.4 Domestication

Domestication is an ancient breeding method that artificially treats a microbial population in a certain environmental condition for a long time, and at the same time continuously transfers them through generations to accumulate and select suitable spontaneous mutants.

The domestication of microorganisms is an important measure to apply nitrogen removal technology to low temperature environment, so that the enzymes and lipid composition of the cell membrane in microorganisms can adapt to low temperature environment and play a role in low temperature conditions. A large number of studies have shown that the low temperature nitrogen removal process can achieve stable operation through appropriate domestication strategies and after a certain period of domestication.

If the operating temperature of AOB is lowered directly from 30 ° C to 5 ° C, it will cause its inactivation. By gradually reducing the operating temperature, AOB adjusts the type of fatty acids in the cell membrane so that they are less likely to freeze at low temperatures. Later studies have come to the opposite conclusion. Therefore, some scholars began to explore the domestication strategy of low temperature.

2.4.1 Gradual domestication

Gradual acclimation means that the process temperature is gradually reduced from the appropriate temperature to the target temperature. After domesticating microorganisms adapt to the current temperature, their temperature is lowered to further domesticate them.

Shang Huilai et al. adopted the domestication method and gradually lowered the temperature, stabilizing it for more than a month every time it dropped 1℃. After half a year, they did not deliberately control the temperature, experienced the low temperature of 10℃ in winter, successfully stabilized the normal temperature and low temperature short-range nitrification and denitrification, and the nitrification rate was always maintained above 78.8%.

Through this method, the anaerobic ammoX process was successfully started and stably operated at 18℃, but the process system became unstable when the temperature was lowered to 15℃. It is suggested that the optimal operation steps should be as follows: first, accumulate enough anaerobic ammox biomass at the optimal temperature, and then slowly acclimate microorganisms to adapt to low temperature conditions.

2.4.2 Direct domestication

Direct domestication is the domestication of the reaction system directly under the target temperature.

Anaerobic ammox was used to achieve efficient nitrogen removal in anaerobic biological filter at moderate low temperature (20~22℃). By directly incubating the inoculated sludge at 20~22℃, the NLR reached 8.1kg/ (m3•d) after 446d. Microbial anaerobic ammox activity was also detected at 6℃. NLR decreases from 2.8kg/ (m3•d) at 22℃ to 0.36kg/ (m3•d) at 6℃.

The start-up time of anaerobic ammox process under two acclimation strategies was compared. The biofilm in the anaerobic sequencing batch biofilm reactor with the synergistic reaction of short-range nitrification and anaerobic ammox as the dominant reaction was inoculated at 31℃ and acclimated in a biochemical incubator at 16℃, and the low-temperature anaerobic ammox was successfully started 56 days later. The same biofilm as the former was inoculated, first placed in a biochemical incubator at 31℃, and then gradually cooled down to 16℃ at a gradient rate of 3℃ every 12 days. The domestication ended at the slowest 70 days. The sign of the domestication ended was that the removal efficiency of ammonia nitrogen at 16℃ ambient temperature remained stable for about 1 week.

Previous studies have shown that microorganisms are more adaptable to the gradual decrease of temperature, and if the temperature drops suddenly, it will easily cause the instability of the system. But more recent studies have shown that the time for domestication may be shorter if the temperature is lowered directly to the target temperature. A systematic study is needed to prove this, and the mechanism behind the experimental phenomenon remains to be revealed.