Zhang Xianzhong, Shanghai Urban Construction Design and Research Institute: Reflections on the relationship between disinfection of urban sewage treatment plants and natural water microecology

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Under the requirements of the current standards, the environmental protection department has higher requirements for the microbial indicators discharged from sewage treatment plants. In order to ensure that the effluent meets the standard, sewage treatment plants generally use disinfection for disposal. However, some disinfectants follow the effluent into the water body, causing damage to the microecology of the natural water body and not conducive to the improvement of the quality of the water environment. In this paper, combined with the engineering experience of Zhang Xianzhong, executive deputy director of Shanghai Urban Construction Design and Research Institute/Shanghai Rainwater Engineering Research Center, and the research results of Dean Dai Xiaohu of the School of Environment of Tongji University, this paper analyzes and thinks about how to balance the problem between water plant discharge standards and water ecology.

In urban sewage treatment discharge, in order to prevent pathogenic microorganisms from causing harm to the health of people and livestock and the ecological environment, the effluent is often disinfected to reduce the number of pathogenic microorganisms in the wastewater. At present, pathogenic microorganisms in wastewater are mainly evaluated by the number of coliforms (feces). Coli flora generally refers to the Escherichia coli bacteria related to fecal pollution, including Escherichia coli (commonly known as "Escherichia coli"), Salmonella, Enterobacter gustis and Klebsiella gas production, etc., most of which are pathogenic bacteria or condition pathogenic bacteria, if they are high in domestic water, it may cause wound infection, food poisoning and other hazards, for the weak, immunodeficient and other special groups, but also cause some serious complications.

The primary A discharge standard of urban sewage treatment plants stipulated in China has a limit of 103 coliforms/L, a limit of 104/L for the first-level B and secondary discharge standards, and no limit requirements for the third-level standard. However, Germany, Finland, Sweden, Denmark and other European countries do not uniformly limit the number of coliform bacteria in the effluent, implement different standards in different regions, and some areas also prohibit the disinfection of effluent.

In view of the new round of sewage upgrading and transformation, how to deal with the contradiction between the unended coli flora and the environment and human and animal health, whether China's standard specifications can be adjusted for disinfection indicators such as E. coli, this paper discusses this.

The current situation of effluent disinfection in sewage plants

At present, urban sewage treatment plants mostly use the following methods to disinfect effluent.

(1) Ultraviolet rays

The use of ultraviolet rays in the UV-C band to overcurrent irradiation of water bodies can cause the nucleic acids of most microorganisms in the water body to be irreversiblely destroyed, enzymatically denatured or passivated, so as to achieve the purpose of sterilization. This method is simple and fast, without additional dosing agents, but its bactericidal effect is greatly affected by factors such as the flow rate of the effluent and the turbidity of the effluent.

(2) Liquid chlorine

Chlorine can be dissolved in water to generate hypochlorous acid with strong oxidation, which has a more direct killing effect on pathogenic microorganisms, but compared with other disinfection measures at the same concentration, the effect is not good, especially the killing effect of parasitic spores is not ideal. And liquid chlorine has great toxicity and corrosiveness, which is not easy to transport and manage.

(3) Ozone

Ozone is also a strong oxidant, its oxidation is strong, the bactericidal effect and the broad spectrum of sterilization far exceeds that of common disinfectants, and the killing effect on pathogenic microorganisms is hundreds of times that of other disinfectants. However, at present, its preparation efficiency is not very high, and the energy consumption is large.

(4) Other methods and effect evaluation

In addition to the above methods, there are chlorine dioxide, sodium hypochlorite, photocatalysis and other disinfection methods. These numerous methods have their own advantages and disadvantages, and have also achieved good application results in many wastewater treatment plants, but according to the census of sewage plant effluent disinfection indicators, the overall effect is still not good.

Mechanism of self-extinction of harmful flora

The coliform itself is difficult to become a dominant strain or even survive in the natural water body, and its influence on the natural water body is limited, mainly due to the following reasons.

(1) Environmental stress

The main source of coliform bacteria is human and animal manure, and its original living environment is the digestive tract of human animals. However, when the coliform bacteria enter the natural water body, various environmental factors are far from the original living environment and the facility environment of the sewage treatment plant, and the reproduction of the coliform bacteria will be inhibited; even if some strains survive under environmental pressure, it is difficult to form a dominant flora in the environment, the main reason is the suppression of "net bacterial force". On the other hand, from the perspective of microecology, the large number of coliforms and the formation of dominant microflora need to rely on some special substances secreted by the host, that is, the digestive tract of humans and animals, and this condition is also extremely difficult to meet in natural water bodies.

(2) Net bacteria pressing

There may be antagonistic inhibition between a certain microorganism and other microorganisms, but the current level of understanding of it is not high. The "net bacteriological force" proposed by Bishyafu is used to describe the inhibition ability of beneficial microorganisms on pathogenic microorganisms and measure their strength, after the coliform flora is discharged into the natural water body, it will be inhibited by the net bacterial force from environmental microorganisms, and its mechanisms are mainly as follows.

(1) Placeholder and nutrition competition

The beneficial microbiota inherent in the environment or artificially inoculated multiply in large quantities, occupying the place where microorganisms in the water body can be attached, such as aquatic weeds, stone tablets, sediment, etc., so that the harmful microflora including the coliform flora has no foothold, thus unable to reproduce, and eventually dissolves due to hunger.

(2) Direct contact killing

Bacteria of some Bacillus species can lyse certain other bacteria that come into contact with themselves, because the harmful flora gives them stimulation and causes them to release lytic substances. Vibrio vermiculo bacteria have a bacteriophage-like effect, but their specificity to the host organism is not obvious, and it can parasitize and lyse most harmful microorganisms, and the elimination effect on Escherichia coli, Salmonella, etc. is particularly significant. In addition, many proto- and post-species animals, namely grasshoppers, rotifers, water lice, etc., can directly feed on harmful microorganisms.

(3) Allelopathic effect

Allelopathy refers to the inhibition between different kinds of plants or microorganisms, commonly known as "symbiotic phase", and its mechanism lies mainly in some bacteriostatic metabolites produced by beneficial microorganisms, including organic acids, bacteriocins (antibiotics) and the like. For example, bacteria of the genus Lactobacillus can produce lactic acid, lactic acid bacteriocin, hydrogen peroxide and other products; Bacillus subtilis, Bacillus licheniformis, etc. can produce bacitracin, polymyxin and other products, which have an inhibitory effect on harmful microorganisms in the environment such as sediment.

(3) Conversion to being eaten

In addition to proto-and-postzoans, benthic animals such as snails, river mussels, river clams, and water earthworms can also feed and filter harmful flora, including coliforms, thus greatly reducing their number in the environment.

Negative effects of effluent disinfection

In order to meet the requirements of the disinfection index limit of the sewage plant, there are certain drawbacks in the existing disinfection method, and the specific reasons are as follows.

(1) Energy consumption is large, and resources are wasted

Large and medium-sized sewage treatment plants using chlorine, ultraviolet light or ozone and other equipment used for effluent disinfection energy consumption of up to tens or even hundreds of kilowatts, electric energy consumption is larger, and the service life of the above equipment is generally not long, if the waste equipment can not be reasonably disposed of, it may also cause secondary pollution dominated by heavy metal pollution.

(2) Secondary production, causing cancer

Disinfection with disinfectant may produce some harmful, refractory secondary products or some substances that are not obvious in harm but are not environmentally friendly, commonly known as "disinfection by-products". For example, the use of liquid chlorine disinfection may produce halogenated hydrocarbons, halogenated furans; the use of ozone disinfection may produce bromate and the like. In addition, there are many potential or direct hazard disinfection by-products that can be produced, and more than 600 have been reported so far.

(3) It seems to be hygienic, but it is not

Most of the pathogenic microorganisms represented by the coliform bacteria in sewage are not compatible with natural water bodies, and when they enter the natural water body, they will naturally die. In general, the water intake point of the municipal water supply is far from the discharge point of the wastewater treatment plant, and the water plant will also reliably disinfect the water leaving the factory, so the necessity of strict disinfection of the effluent of the sewage plant is controversial.

(4) Beneficial bacteria are also exhausted

In addition to the coliform flora in the sewage, there are a larger proportion of environmentally friendly microbiota, such as gum fungus, short bacillus, bacillus, helicobacteria, pseudomonas erythrocytes, erythrococcus, erythrococcus and some proto- and postphytes and other microorganisms with water purification function and more powerful functions. In the process of effluent disinfection, in order to kill the coliform bacteria, many beneficial microorganisms have been killed by mistake, which is closely related to the current microecological imbalance of natural water bodies, ecosystem imbalance and then decline in self-net power. In other words, the more thoroughly disinfected, the worse the local water ecosystem may be.

Recommendations for disinfection methods based on the coordination of natural water microecology

In summary, the current disinfection of sewage treatment plants simultaneously destroys the micro-ecological balance, and increases the construction and operation costs of sewage plants, and the practice of disinfection of effluent from urban sewage treatment plants needs to be further explored. From a microecological point of view, the best approach is to take measures that promote beneficial flora while inhibiting coliform and other harmful flora, as follows.

(1) Increase the aeration area

Proto- and protozoans that can swallow harmful flora and Vibrio vermiculo bacteria that can parasitize harmful flora are all obligate aerobic microorganisms, and bacillus bacteria with high net bacterial power multiply vigorously in an aerobic environment; increasing the aeration area can directly increase the amount of the above microorganisms, especially protozoa that can directly swallow coliforms and other harmful flora. For example, bellworm and tetramembranous pear can effectively reduce the number of Escherichia coli, when these two ciliates are present, the amount of Escherichia coli in the sewage is reduced by up to 95%, and some ciliate and flagella metabolites can also promote the growth of beneficial flora.

(2) Add microecological preparations

The addition of fermentation synthesis composite microecological preparations with high net bacterial power to the inlet of the sewage treatment plant or the main facilities such as hydrolysis acidification tanks, biological reaction tanks, etc., such as typical EM (effective microbiota) preparations, can significantly reduce the number of coliform bacteria and other harmful bacteria in the effluent, and can reduce the amount of CODCr, ammonia nitrogen, reduce sludge production, and eliminate odor, etc., which is also of great help to the upgrading of sewage treatment plants. What is more valuable is that EM preparations or other fermentation synthesis microecological preparations with high net bacterial power are a powerful collection of beneficial bacteria with significant environmental friendliness, which are discharged into natural water bodies with effluent and also have a significant role in promoting the self-purification of natural water bodies.

(3) Selective sterilization measures are adopted

The setting of tourmaline filter media or strong magnetic water processor at the outlet can selectively kill harmful bacteria such as coliform bacteria in the effluent, and is safe for the vast majority of beneficial bacteria. This practice has a great positive effect on restoring the balance of environmental microecology.

(4) Strengthen deep processing

In the process of sewage treatment, most of the pathogenic microorganisms in the water adhere to the suspended particles, so processes such as coagulation, precipitation and filtration can also remove a considerable part of the pathogenic microorganisms.

(5) Strengthen ecological construction

Strengthening ecological construction lies in restoring the self-purification power of natural water bodies, while also restoring their biodiversity, so that harmful bacteria such as coliform bacteria can die out naturally and more quickly, thereby reducing the harm to people and animals.

Conclusions and recommendations

With the promotion of the integration of "plant network rivers and lakes", the scale and number of supporting sewage treatment plants will gradually increase, and the construction and operation of facilities such as sewage collection pipe networks, pump stations, and sewage treatment plants will be coordinated, together with the operation, maintenance, scheduling and other related work of surface water bodies such as rivers and lakes within the scope of sewage treatment facilities, so that the "plant network rivers and lakes" will form a virtuous cycle of operation system, supplement the river channel with reclaimed water, and build an urban ecological "sponge body" where adults and water live in harmony.

In short, the problem of disinfection of urban sewage plants is a very important topic, and the next step can be to formulate sub-regional and seasonal standards through full scientific research and national regional surveys. Fully consider the scientific nature of energy saving and disinfection of urban sewage plants, and assess disinfection standards in different areas and different seasons such as discharge into full-body contact, public swimming areas, non-full-body contact, etc., and supporting combined overflow or rainwater purification facilities, relatively flexible purification standards and disinfection indicators can be formulated.

Original title: Reflections on the relationship between disinfection of urban sewage treatment plants and natural water microecology

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