First, several factors affecting the stable operation of sewage treatment plants in winter
(1) temperature
Water temperature is one of the most important factors in activated sludge treatment. In a certain range, with the increase of temperature, the rate of microbial biochemical reaction is accelerated, and the reproduction rate is also accelerated. However, when the temperature suddenly increases or decreases and exceeds a certain limit, some components of the temperature-sensitive battery will be irreversibly damaged, thus seriously affecting the efficiency of sewage treatment.
(2) Dissolved oxygen
Aerobic process should always keep enough dissolved oxygen content in the treatment equipment, and usually need aeration auxiliary equipment to keep dissolved oxygen greater than 2 mg/L; In the anaerobic process, the content of dissolved oxygen should be strictly controlled, and usually the dissolved oxygen should be controlled below 0.5 mg/L.
(3) pH value
Generally, the optimum pH of aerobic microorganisms is between 6.5 and 8.5, and the pH is too small (
(4) nutrients
Generally, aerobic process and anaerobic process should add N and P according to BOD: N: P = 100: 5: 1 and COD: N: P = 200: 5: L, and sometimes some other inorganic nutrients (K, Mg, Ca, S, Na, etc.) should be added. ) and trace elements (iron, sodium, etc.
(5) Organic load
Both aerobic and anaerobic processes need to ensure a certain organic load, and anaerobic processes require more, but when there is too much organic matter, it will also have an adverse impact on microbial growth.
(vi) Redox potential
The most suitable redox potential for aerobic microorganisms is +300-400mV, at least greater than+1000 mv; For anaerobic microorganisms, it is required to be less than+1100mv, and for strict anaerobic microorganisms, it is required to be less than-1100mv, or even less than -300mV.
(7) Toxic substances (inhibitory substances)
Both aerobic and anaerobic processes are affected by some toxic substances. Such as heavy metals, cyanide, H2S, halogen elements and their compounds, phenols, alcohols, aldehydes, etc.
Second, the operation of sewage treatment plant at low temperature
(A) the structure can not work normally
Low temperature causes freezing, icing and cracking of sewage treatment structures (grid, grit chamber, sludge tank, etc.). ), interrupt or even damage the sewage treatment process and equipment, seriously affecting the normal production and operation and effluent quality.
(B) activated sludge adsorption and organic matter degradation rate decreased
Activated sludge is the main part of sewage treatment in sewage treatment plant. Low temperature will make its adsorption worse and the degradation rate of organic matter will decrease. At low temperature (below 5oC), the catalysis of extracellular polymers and enzymes secreted by psychrophilic microorganisms decreases, which slows down the speed of biochemical reaction and makes the organic matter adsorbed on the surface of activated sludge unable to degrade quickly, thus reducing the degradation efficiency of activated sludge. At the same time, the speed of biochemical reaction also slows down the hydrolysis and uptake of organic matter adsorbed on the surface of activated sludge. To a certain extent, the surface activity of microorganisms wrapped by polysaccharide mucus layer decreases, and undegraded substances accumulate on the adsorption surface of activated sludge, which also inhibits the recovery of sludge surface activity, thus reducing the adsorption of activated sludge.
(3) Sludge bulking
The activated sludge of sewage treatment is easy to swell at low temperature, and Microcystis will multiply in large quantities at low temperature, which is long and hydrophobic, and excessive growth will lead to sludge swelling in cold areas.
(d) affecting sludge dewatering
The appearance of filamentous bacteria at low temperature leads to the loosening of sludge flocs and the decrease of sludge density, which in turn leads to the increase of sludge specific resistance and sedimentation index. In addition, the extracellular secretion of low-temperature activated sludge contains a lot of viscous substances, which also reduces the compressibility of sludge and seriously affects sludge dehydration.
(5) Denitrification rate is reduced.
Microbial denitrification mainly goes through three processes: ammoniation, nitrification and denitrification. Among them, ammonifying bacteria and nitrifying bacteria are the most important microorganisms in the process of nitrification, and they have high requirements on temperature. When the optimum temperature is 20-30oC, the reaction rate decreases obviously, and when the temperature is lower than 5oC, the reaction almost stops. Therefore, low temperature interrupts the denitrification process and reduces the denitrification rate of effluent.
(VI) The removal rate of suspended particles is reduced.
At low temperature, the increase of viscosity coefficient of sewage, insufficient mixing of suspended particulate matter (SS) with sludge, low hydrolysis efficiency of activated sludge, and easy shedding of adsorbed SS all reduce the removal rate of SS.
Three, the measures taken by the sewage treatment plant in winter operation
(A) improve the operating equipment and parameters
The research shows that reducing sludge load, prolonging sludge age, increasing hydraulic retention time and adopting tank heating or insulation can effectively improve the efficiency of low-temperature sewage treatment. A domestic sewage treatment plant uses solar energy, adopts floating lighting and heat preservation cover on water, effectively solves the problem of water temperature maintenance in winter, reduces the cost and ensures the quality of effluent. It is found that the effect of low temperature nitrification and denitrification can be enhanced by increasing dissolved oxygen concentration, prolonging sludge age, reducing sludge load, controlling dissolved oxygen concentration, increasing reflux ratio of mixed liquor and adding carbon source, thus improving the influence of low temperature on sewage denitrification.
(2) Physical and chemical strengthening measures
Pretreatment of low-temperature sewage by physical and chemical measures is also helpful to improve the efficiency of sewage treatment. For example, pretreatment of refractory wastewater by instantaneous cavitation of ultrasonic wave can degrade refractory macromolecules into biodegradable micromolecules, which can improve the biodegradability of wastewater. The purpose of increasing the contact area between pollutants and active microorganisms and shortening the treatment time can also be to enhance the flocculation and anti-degradation performance of sludge by adding chemicals.
(3) Biological strengthening measures
The use of biological additives or biological synergists refers to a method of accelerating the removal of pollutants and strengthening the effect of biochemical treatment by using itself, exotic biological species or selected microorganisms. Adding polyurethane foam, powdered activated carbon, diatomite and iron salt as carriers in the process of sewage treatment is beneficial to the attachment and growth of microorganisms and the formation of high-tech biofilm. Suspended activated sludge and attached biofilm are used to remove pollutants from low-temperature sewage, which can improve the biomass in the reaction tank, prevent sludge from swelling and improve the separation effect of sludge and water.
(4) Selection and improvement of treatment process
Under the condition of low temperature, the selection of treatment process is the key to the success or failure of engineering construction. Whether the treatment process is reasonable or not is directly related to the treatment effect, operation stability, construction investment and operation cost of the whole treatment system. Therefore, it is necessary to combine the actual situation, comprehensively consider various factors, and carefully choose the appropriate treatment process to achieve the best treatment effect and economic benefits.
Four. Concluding remarks
Most areas in China are in a relatively low-temperature climate for about half a year, which poses a severe and arduous challenge to the treatment of low-temperature sewage. This paper analyzes several factors that affect the stable operation of sewage treatment plant in winter and the operation status of sewage treatment plant at low temperature, and puts forward some suggestions for improvement, which are for reference only. Please correct me if there are any inadequacies.