Friends in the field of dry sewage treatment may have encountered situations where the activated sludge load is low. Sometimes I feel that even at a low load point, it doesn't seem like much, the water output looks quite clear, and it seems to be easy to operate. But in reality, this matter is like boiling a frog in warm water. The surface is calm, but there are already hidden currents beneath. Over time, various problems will emerge, and dealing with them will be much more troublesome than controlling the load from the beginning. Today, let's have a good talk about the hazards of low load operation of activated sludge, so that everyone has a clear idea.
First of all, we need to understand what "low load activated sludge" means? Simply put, we feed too little organic matter to the activated sludge in the aeration tank, and the sludge cannot eat enough. Under normal circumstances, microorganisms in activated sludge rely on organic matter such as BOD (biochemical oxygen demand) and COD (chemical oxygen demand) in wastewater to survive and reproduce in order to decompose pollutants. But once the load is low, the microbial "food" is not enough, and the balance of the entire system is disrupted, the next trouble comes.
The first unavoidable problem is that microbial activity is getting worse and the processing capacity is declining sharply. Think about it, if a person can't eat enough for a long time, their strength will definitely decrease and they won't have the energy to work; Microorganisms are also in a state of "hunger" for a long time, and their ability to decompose organic matter will gradually deteriorate. At first, it may not be apparent because there are not many pollutants in the sewage itself, and even if the microbial activity is low, it can be barely treated to meet the standards. But once there is a fluctuation in the quality of sewage, such as a sudden influx of water with high organic matter concentration, these "hungry and scared" microorganisms cannot withstand it and immediately "stop eating". The COD and BOD of the effluent immediately exceed the standard, and adjustments will be made later. It is not something that can be solved in one or two days.
Moreover, when microorganisms are not fully fed, they may also experience autolysis. It is some microorganisms with weak constitution that can be decomposed by other microorganisms as "food", or they can rupture and die on their own. In this way, the amount of sludge in the aeration tank will gradually decrease, and most of the remaining sludge is of poor activity to the elderly, weak, sick, and disabled. Even if we want to increase the load later, we need to first retrain the sludge with high activity, which not only takes time but also requires more manpower and resources, and the gains are not worth the losses.
The second major hazard is that it can easily cause sludge swelling and completely "paralyze" the sedimentation tank. This is definitely a major problem for sewage treatment plants, and low load operation is one of the important causes of sludge expansion. When the sludge load is low, microorganisms not only cannot eat enough, but also crazily reproduce those microorganisms that can adapt to low nutrient environments, such as filamentous bacteria, because they have nothing to do. Filamentous bacteria are particularly annoying. They can spread like spider webs in the aeration tank and wrap around the normal activated sludge flocs.
Normal activated sludge flocs have a tight structure and a high specific gravity, and can quickly settle in the sedimentation tank, allowing for smooth separation of clean water. But once the filamentous bacteria multiply in large numbers, the sludge flocs will become loose and their density will become lighter, floating like clusters of "cotton" in the sedimentation tank and unable to sink no matter what. At this point, the phenomenon of "sludge floating up" will occur, and the effluent from the sedimentation tank will carry a large amount of sludge. Not only will the effluent quality exceed the standard, but the lost sludge will also block the subsequent pipelines and equipment, making it dirty and tiring to clean. More seriously, once the sludge swells and forms a vicious cycle, the entire sewage treatment system must be shut down for rectification, ranging from a few days to several weeks, resulting in significant losses.
I have heard of a sewage treatment plant before, but due to long-term low load operation, it was not adjusted in a timely manner. As a result, all the sludge in the sedimentation tank turned into "floating sludge", and the SS (suspended solids) in the effluent exceeded the standard several times. When the environmental protection department inspected it, the red light was on, and it had to be shut down for half a month. New sludge was added to cultivate activity and gradually returned to normal. This rectification alone resulted in losses of hundreds of thousands of yuan.
The third hazard is that operating costs secretly "skyrocket" and money is spent on "useless work". Some people may think that during low load operation, the aeration tank does not need to have such a large aeration volume, and the chemicals do not need to be added so much, so the cost should be lower. Actually, the 'hidden costs' here are quite high.
Let's talk about the cost of aeration first. Although microorganisms require less oxygen at low loads, in order to prevent sludge from settling at the bottom of the aeration tank and avoid local hypoxia, the aeration rate cannot be set too low. Moreover, due to the low activity of sludge, decomposing the same amount of organic matter requires a longer reaction time and a corresponding increase in aeration time, and the total aeration energy consumption has not decreased much. More importantly, these energy consumptions result in low processing efficiency, equivalent to "high input, low output", with particularly poor cost-effectiveness.
Let's talk about the cost of medication. During low load operation, the sedimentation performance of the sludge in the sedimentation tank is poor. In order to ensure smooth sedimentation of the sludge, it is sometimes necessary to add more flocculants, such as PAC (polyaluminum chloride) and PAM (polyacrylamide). These medicines are not cheap, and in the long run, the cost of medicines is also a significant expense. In addition, due to the easy expansion or loss of sludge, frequent sludge discharge and replenishment are required, and the cost of sludge treatment (such as dewatering and transportation) after discharge will also increase, which is an invisible "waste of money".
There are also labor costs. When operating at low loads, the system is unstable and requires staff to frequently monitor water quality and adjust equipment parameters, such as measuring dissolved oxygen (DO) and sludge settling ratio (SV30) in the aeration tank from time to time. The effluent situation of the settling tank also needs to be monitored at all times. If not careful, problems may arise, and the workload of the staff will be even greater, so the labor cost naturally cannot be reduced.
The fourth hazard is the difficulty of sludge dewatering, which becomes a "difficult problem" for subsequent disposal. After treating wastewater with activated sludge, it must eventually become dehydrated sludge before it can be transported for disposal (such as landfill, incineration, or resource utilization). The sludge generated by low load operation has particularly poor dewatering performance. Why is that? Due to long-term "starvation" of sludge, the floc structure is loose and contains a large amount of colloidal substances and bound water, which are difficult to separate through dewatering equipment.
Under normal circumstances, after sludge dewatering, the moisture content of the sludge cake can be controlled below 80%, which is convenient for transportation and disposal. Sludge that can operate at low loads may have a moisture content of over 85%, or even 90%, after dehydration. The mud cake is like "rotten mud" and cannot be formed at all. When packed, it will continuously drip water and pollute the road during transportation. Moreover, mud cakes with high moisture content also have a large volume, requiring more vehicles for transportation and increasing disposal costs. For example, for the same 10 tons of sludge, a mud cake with a moisture content of 80% and a mud cake with a moisture content of 90%, if the volume can be doubled, the disposal cost will also be doubled. In the long run, this is another significant expense.
What's even more troublesome is that sludge that is difficult to dehydrate can easily clog the filter cloth and belt of the dewatering equipment, requiring frequent cleaning and replacement. This not only affects dewatering efficiency but also increases equipment maintenance costs. Sometimes it cannot be cleaned thoroughly, which can breed bacteria and produce unpleasant odors, affecting the working environment of the workshop and the health of the staff.
Finally, there is another easily overlooked hazard, which is the poor shock resistance of the system, which can "collapse" once there is a fluctuation. The sewage treatment system is like a 'warrior', requiring sufficient 'combat effectiveness' to respond to various emergencies. A system that can operate at low loads is like a "malnourished" warrior with poor physical fitness, unable to withstand even the slightest "wind and rain".
For example, during the rainy season, a large amount of rainwater will flow into the sewage pipe network, causing a sudden increase in water inflow, dilution of sewage concentration, and further reduction of load. At this time, the activity of microorganisms will be worse, and the treatment capacity will further decrease; For example, in industrial wastewater treatment plants, if upstream enterprises suddenly change their production processes and discharge pollutants that have not been seen before, the low load operating system cannot adapt to these changes due to the single type and low activity of microorganisms, which can easily lead to poisoning and death. The entire system will be instantly "paralyzed" and it will take a long time to recover.
Moreover, if the system is in a low load state for a long time, the staff may easily relax their vigilance, and the monitoring of water quality and sludge characteristics may become less timely and detailed. By the time problems arise, they are often already quite serious, missing the best adjustment opportunity and leading to further losses.
Having said so much, everyone should understand that low load operation of activated sludge is not a trivial matter. On the surface, it appears to be "stable", but in reality, there are a lot of hidden dangers. Therefore, in daily operations, it is necessary to closely monitor changes in sludge load, adjust operating parameters in a timely manner based on the inflow and inflow water quality, such as controlling the inflow, adjusting aeration, discharging sludge reasonably, supplementing nutrients (such as nitrogen and phosphorus), etc., so that activated sludge is always in a "full and well fed" state and maintains good activity. Only in this way can the stable operation of the sewage treatment system be ensured, the effluent quality meet the standards, and operating costs be reduced to avoid unnecessary troubles.
In short, sewage treatment is a "delicate task" that cannot tolerate any carelessness. Do not let low load operation become a "big trouble" just because of temporary "ease of mind", and then regret it.
