How to deal with sludge bulking?
Preface：Every year, the sewage treatment system experiences expansion, which is very periodic! In fact, many sewage treatment systems do not experience serious sludge expansion in summer with high temperatures or in winter with cold temperatures. It often occurs during the seasonal transitions between spring and summer, as well as autumn and winter. That is, in an environment of alternating air temperature, water temperature, and air pressure.
After analyzing the statistical data of some sewage treatment plants, the period of foam occurrence is: when the water temperature changes from higher than the air temperature to lower than the air temperature (from March to April) and when the water temperature changes from lower than the air temperature to higher than the air temperature (from October to November).
Analysis of the Reasons for Expansion during Seasonal Transitions
Due to the change of the ecological environment, the growth and composition of microorganisms have changed. From past operational runs, it has been found that without changing other conditions, the foam phenomenon will gradually disappear after a period of time (10 to 20 days), and the sewage treatment system will automatically repair itself.
Through microscopic examination, it was found that during the spring and summer transitions, the main outbreak was of filamentous bacteria, which grew vigorously and extended; while during the autumn and winter transitions, the inactive filamentous bacteria were wrapped in the inactive zoogloea to form floating foam.
Its principle still needs further study. Generally, it is believed that when the season (temperature, air pressure) changes, microorganisms will all be affected, but the adaptability of filamentous bacteria is stronger than that of some floc-forming bacteria. For example, the growth temperature of Microthrix parvicella can range from 8 to 35 °C, and it is more suitable for growing in a low-temperature environment.
When the environment is not conducive to the growth of microorganisms, the filaments of filamentous bacteria will extend from the zoogloea to increase their surface area for nutrient uptake, and their growth rate is higher than that of other microorganisms.
When there is a spring and summer transition, the activity of the sludge all declines. In domestic sewage, a large amount of synthetic detergents and oils cannot be degraded, while some filamentous bacteria are still active. They like to use these substances as food and grow rapidly, which leads to the outbreak of filamentous bacteria and the formation of foam.
During the autumn and winter transitions, the main formation is floating sludge (which is different from the previous one). In the floating sludge and foam, it is difficult to find extended filamentous bacteria. Under the microscope, fine bubbles can be seen wrapped in the floating sludge. It is estimated that when the environment changes, the zoogloea becomes dispersed and fine, and when combined with aeration bubbles, the density decreases, resulting in floating.
Control Methods for Expansion during Seasonal Transitions
After summarizing the law of foam formation, it is beneficial to take control measures. For example, for the foam during the spring and summer transitions, mechanical cleaning and scraping methods can be used. Because these foams contain a large number of filamentous bacteria and should not be left in the mixed liquid to avoid re-causing the foam phenomenon. The disposal methods are as follows. For the floating sludge and foam during the autumn and winter transitions, high-pressure water gun spraying can be used to alleviate it, because most of the floating sludge is still composed of floc-forming bacteria, and after being broken, it can return to the mixed liquid.
Adding coagulants: Adding synthetic organic polymers, iron salts, aluminum salts and other coagulants can all improve the compaction of the sludge and increase the specific gravity of the sludge through their coagulation effect; adding kaolin, calcium carbonate, calcium hydroxide, etc. can also improve the sedimentation performance of the sludge through increasing the compaction of the sludge. Practice has proved that sewage treatment plants without primary sedimentation tanks have relatively low SVI values. Therefore, when sludge expansion occurs in sewage treatment plants with primary sedimentation tanks, sending part of the sewage directly to the aeration tank is also a method of controlling sludge expansion.
Adding Cl₂ or bleaching powder: The traditional oxidant used to control sludge expansion is Cl₂. Cl₂, HOCl, and hypochlorite ions with oxidizing ability can penetrate into the cell and destroy the enzyme system in the cell, resulting in cell death. Most filamentous bacteria can be controlled by adding chlorine.
It is generally added to the returned sludge, and the dosage is crucial. Excessive addition is likely to kill all the normal zoogloea, while too little addition is likely to not have any effect, but instead make the effluent of the biochemical system deteriorate. Confirm the optimal dosage of bleaching powder by adding different proportions and observing the inhibition of activated sludge. When the SVI value gradually decreases and the expansion continues to be alleviated, the dosage of the drug should be gradually reduced.
Introducing inert substances to inhibit filamentous bacteria expansion: We have found in practice that highly and extremely expanded filamentous bacteria are difficult to effectively prevent through simple methods, but inert substances can be introduced to inhibit the expansion of filamentous bacteria. Sufficient inert substances flowing into the biochemical system not only strengthen the relative sedimentation performance of the active sludge, but also play a role in destroying the structure of the filamentous bacteria, reducing its expansion degree, and also reducing its breeding speed. Therefore, through the introduction of inert substances, the inhibitory effect on filamentous bacteria is obvious.
Adjusting the pH value to inhibit filamentous bacteria expansion: Using wastewater with a high pH value to inhibit filamentous bacteria expansion, because the specific surface area of filamentous bacteria is larger than that of the zoogloea. In theory, filamentous bacteria should generally be less able to withstand acute environmental deterioration than the zoogloea, especially tolerate high pH values and have an inhibitory effect on the active sludge. However, the control range of the pH value using this method is very important. According to experience, in the entire aeration tank, controlling the pH value at around 10 and lasting for 4 to 8 hours can have a significant inhibitory effect on filamentous bacteria. Too high a pH value has an impact on the active sludge, and too low a pH value does not work well.
