By: Kate
Email:kate@aquasust.com
Date: 16th December 2024
In a membrane bioreactor, as the operation time goes by, the inner and outer surfaces of the membrane will be polluted to varying degrees, causing the membrane filtration pressure to rise and the membrane operation cycle to shorten. In recent years, many studies have shown that extracellular polymers are the most important biological factor among the many factors of membrane pollution; especially when non-filamentous bacteria sludge expands, the concentration of extracellular polymers rises sharply, seriously affecting the normal operation of the membrane assembly and shortening the replacement cycle of the membrane assembly. So how to deal with sludge expansion in the MBR process?
Flocculation method
The density of expanded activated sludge is generally smaller than that of water. As an emergency treatment measure, it is possible to consider adding coagulants to improve its sedimentation performance. We initially selected the commonly used high molecular coagulant, cationic polyacrylamide, and the inorganic coagulant, ferrous sulfate, for comparative tests.
The relationship between the dosage of polyacrylamide and the sedimentation performance of sludge
The addition of polyacrylamide has a certain effect on the improvement of the sedimentation performance of sludge, and there is an optimal dosage, but the effect is not very ideal. After analysis, the author believes that the reclaimed water reuse system adopts a new submerged composite membrane bioreactor with large aeration volume and strong hydraulic stirring. The aggregated floc particles are easily destroyed, resulting in unsatisfactory coagulation effect; when the dosage is higher than the optimal dosage, in addition to neutralizing the negative charge of the colloid, the excessive positive charge of the flocs makes the colloid ions carry positive charges and stabilize again.
The relationship between the amount of ferrous sulfate added and the sludge settling performance
The effect of cationic polyacrylamide addition is limited by hydraulic conditions and other factors, which is not very ideal. At the same time, its monomer is toxic and difficult to degrade, and there is a secondary pollution problem. The economic benefit is worse than that of adding ferrous sulfate. Ferrous sulfate is cheap, easy to use, has no negative impact on the membrane and sludge, and its effect on sludge density is effective, but it cannot fundamentally solve the problem of nutrient imbalance, so it can only be used as an emergency control measure.
Nutrient adjustment method
In the study of sludge bulking, the recovery and control of sludge bulking is a very important link. During the operation of the reclaimed water reuse project, it was found that after the addition of ferrous sulfate, the sedimentation performance of the activated sludge was once improved. If the addition was stopped and the treatment was continued under the original organic load conditions, the sedimentation performance of the activated sludge would gradually deteriorate and return to the state before the addition after three days. Therefore, it is necessary to find an effective recovery and control method after the expansion of activated sludge. During the operation, we conducted a comparative test on two groups of membrane bioreactors that were running simultaneously: the first group added sufficient nitrogen source to make its BOD5, N average mass ratio about 100:5; the second group, while adding sufficient nitrogen source, increased the organic load of the influent at the same time, and the organic load (in terms of CODCr) was increased to more than 2.0kgCOD/m3˙d. The results showed that when the SVI value of the sludge dropped below 150mL/g, the first group of reactors operated for about one week; the second group of reactors operated for only three to four days. Practical operation experience shows that: first, the fundamental solution to sludge bulking caused by nitrogen deficiency is to adjust the ratio of nutrients. Second, increasing the organic load while maintaining an appropriate nutrient ratio can shorten the time it takes for sludge settling performance to return to normal.
Other control methods
In the case of the most serious sludge viscous expansion (when a container is used to hold some sludge, the sludge always adheres to the surface of the container no matter what method is used), it is possible to consider draining some of the expanded sludge appropriately and then taking some new sludge to reduce the coverage of the sludge by polysaccharides; at the same time, increase the hydraulic retention time so that the organic matter that has not been completely oxidized has enough time to be consumed. Due to the high content of detergent in the raw water and the high aeration intensity, white, viscous foam often appears and accumulates more and more. When the sludge expands, the harm is greater. There was once a foam mountain as high as one meter due to the accumulation of foam, which caused a large amount of sludge to be lost. After this accident, in addition to adding defoaming agents, we adopted the method of hydraulic defoaming. A nozzle is installed above the reaction tank, and the effluent of the MBR reactor is used to spray the upper part of the reaction tank to control the damage of expanded sludge and foam to the reactor, and good results have been achieved.
Conclusion
①In the MBR process with bathing water as the main raw water, ferrous sulfate can be used as an emergency coagulant during the sludge expansion period. The optimal dosage is 60 mg/L, but it can only be used as an emergency control measure because it cannot fundamentally solve the problem of nutrient imbalance.
②For the sludge expansion that occurs during the operation of the reclaimed water reuse project, the fundamental solution is to adjust the proportion of nutrients; at the same time, it is found that, under the premise of ensuring the appropriate nutrient ratio, increasing the organic load can accelerate the recovery of sludge settling performance. Engineering practice has proved that the high viscosity expansion of sludge can be successfully controlled through the above measures. At the same time, it is found that increasing sludge discharge and increasing hydraulic retention time are also effective auxiliary measures.