Why does foam generate in biochemical water treatment process?

Why does foam generate in biochemical water treatment process?

Chemical foam (starting foam)

It is formed by the agitating and blowing off the surface active substances in the wastewater. In the initial stage of activated sludge cultivation, there are many chemical foams, and sometimes foam hills up to several meters are formed on the surface of the aeration tank. This is mainly because the initial activated sludge has not yet formed, and all the foam-generating substances form foam under the action of aeration. With the increase of activated sludge, a large amount of surface active substances are absorbed and decomposed by microorganisms, and the foam will gradually disappear. Chemical foam is milky white, easy to handle, can be filled with water to defoam, or defoamer has been added.

Denitrifying foam

If the nitrification reaction is carried out in the sewage plant, denitrification will occur in the sedimentation tank or in insufficiently aerated areas, and bubbles such as nitrogen will be generated to drive some of the sludge to float and cause foaming.

Bio-foam

Under the influence of various factors, the filamentous bacteria, actinomycetes and other microorganisms grow abnormally. The specific growth rate of filamentous bacteria is higher than that of micelle bacteria, and the specific surface area of ​​filamentous bacteria is large. Bacteria are much more advantageous than bacteria micelles in obtaining the oxygen required for BOD and oxidizing BOD in sewage. As a result, filamentous bacteria in the aeration tank become dominant bacteria and increase in value, resulting in the generation of biological foam.

(1) Causes of bio-foam

  Retention time of sludge. Because the microorganisms that generate foam generally have a low growth rate and a long growth cycle, long sludge residence time will be conducive to the growth of these microorganisms.

  pH value. When the pH value drops from 7.0 to 5.0 to 5.6, it can effectively reduce the formation of foam.

  Dissolved oxygen (DO). Actinomycetes are not easy to grow under anoxic or anaerobic conditions, but they do not die. Filamentous bacteria can tolerate hypoxia.

  Temperature. Fungi related to the formation of biological foam have their own suitable growth temperature and optimal temperature. When the environment or water temperature is conducive to their growth, foaming may occur.

  Alternating changes of air temperature, air pressure, and water temperature. The foam in the spring and summer alternating is mainly the outbreak of filamentous bacteria. The filamentous bacteria grow in large numbers and spread out; while in the autumn and winter, the inactivated filamentous bacteria are wrapped in the same inactivated bacterium micelle to form a float foam. It is generally believed that when the season (temperature, air pressure) changes, the microorganisms will be affected, but the adaptability of filamentous bacteria is stronger than some flocculent bacteria, and the growth temperature of some filamentous bacteria can be between 8 ~ 35 ℃ , And more suitable for growing in low temperature environment. When the environment is not conducive to the growth of microorganisms, the hyphae of filamentous fungi will stretch from the micelles to increase the surface area for nutrient intake, and the growth rate is higher than other microorganisms.

Other reasons include hydrophobic substances and aeration methods.

(2) Hazards of biological foam

  Foam is generally viscous. It will draw a large amount of solid materials such as activated sludge into the floating foam layer of the aeration tank. The foam layer will tumbling on the surface of the aeration tank, preventing oxygen from entering the aeration tank mixture and reducing the oxygenation efficiency (In particular, it has the greatest impact on mechanical aeration methods).

  When the mixed liquid of the aeration tank mixed with foam enters the secondary sedimentation tank, the solid materials such as activated sludge wrapped with foam will increase the content of suspended solids in the effluent and cause the deterioration of the effluent water quality. At the same time, a large amount of scum will form on the surface of the secondary sedimentation tank. In winter when the temperature is low, the suction (scraping) mud machine of the second settling tank will be affected by the freezing.

  Biological foam spread to the aisle board, affecting patrol inspection and equipment maintenance. In the summer, bio-foam drifts with the wind, causing a series of environmental health problems, and the medical community also believes that Nocardia forming bio-foam is most likely a human pathogen. After the foam freezes in winter, it is difficult to clean up, and it may slip and slip inspection and maintenance personnel.

  The foam contained in the returned sludge will cause a phenomenon similar to flotation and damage the normal performance of the sludge. Biological foam enters the mud area along with the sludge discharge, interfering with the smoothing of sludge concentration and sludge nitrification.

Control strategy of biological control foam

i.           Spray water, etc. to increase surface agitation. This is one of the most commonly used physical methods. Reduce the bubbles by spraying water flow or water droplets to break the air bubbles floating on the water surface. The scattered sludge particles partially regained their sedimentation performance, but the filamentous bacteria still existed in the mixed liquid, so the foam phenomenon could not be eliminated at all.

ii.           Add bactericide or defoamer. Can use strong oxidizing fungicides, such as chlorine, ozone and peroxide. There are also commercially available chemicals produced using polyethylene glycol and silicone, as well as a mixture of ferric chloride and copper pickling solutions. The effect of the medicament can only reduce the growth of foam, but cannot eliminate the formation of foam. The widely used fungicides generally have negative effects, because excessive or improper dosing position will greatly reduce the number of flocculent bacteria and the total amount of organisms in the reaction tank.

iii.           Reduce sludge age. Generally, the residence time of sludge in the aeration tank is reduced to inhibit the growth of actinomycetes with a longer growth period. Practice has proved that when the sludge residence time is 5-6 days, it can effectively control the growth of Nocardia spp. To avoid the foam problem caused by it. However, there are many unsuitable aspects of reducing the age of sludge: when nitrification is required, the sludge residence time needs at least 6 days in the cold season, which is contradictory to this method; in addition, Microthrix parvicella and some filamentous bacteria are not affected Effects of changes in mud age.

iv.           Reflux the supernatant from the anaerobic digester. Existing experiments have shown that the method of using anaerobic digestion tank supernatant to return to the aeration tank can control the formation of air bubbles on the surface of the aeration tank. The main function of the anaerobic digester supernatant is to inhibit Rhodococcus bacteria, but when this method is used in the actual operation of several sewage treatment plants, it has not been as successful as the laboratory. Since the supernatant of the anaerobic digester contains high concentrations of aerobic substrate and ammonia nitrogen, they will affect the final effluent quality and should be used with caution.

v.           Add special microorganisms. Studies have suggested that some special strains can eliminate the vitality of Nocardia bacteria, including protozoan nephridia. In addition, the increase in predatory and antagonistic microorganisms can control some foaming bacteria.

vi.           Project the loading body (filler) into the aeration reactor. Adding a moving bed or fixed packing to some activated sludge systems can make some microorganisms that are prone to sludge expansion and foam fixation grow, which can not only increase the biomass in the aeration tank, improve the treatment effect, but also reduce or control the generation of foam.

vii.           Add chemical agent: Add flocculant PAC and cationic PAM to the system. For example, after adding PAC, the stable foam on the surface of the mixed liquid loses stability, and then the filamentous bacteria are dispersed and re-entered into the activated sludge.

viii.           Selector. The selector is to create various reaction environments (oxygen, organic load or sludge concentration, etc.) to select the microorganisms that preferentially grow and eliminate other microorganisms. It has been reported that the aerobic selector can control M. parvicella to a certain extent, but it has no great effect on the Nocardia spp .; the hypoxic selector has a control effect on the Nocardia spp., But has no effect on M. parvicella. There are many reasons, and it depends on the specific situation for a fundamental solution.