Colleagues in the field of dry wastewater treatment are likely to have come into contact with wastewater containing organic silicon to some extent - whether it is from chemical parks or discharged from daily chemical and textile factories. Although this may seem insignificant, it has a significant impact on activated sludge systems. Many times, the biochemical pool suddenly experiences abnormal bubbling and a surge in effluent quality. After investigation, the culprit is found to be it. Today, let's talk in plain language about how organic silicon affects activated sludge and how we should deal with it in our daily operations.
First, let's talk about what organic silicon is. What we often refer to as organosilicon is actually a general term for a large class of organic compounds containing silicon, such as silicone oil, silicone rubber, and silicone resin, all of which fall within this category. This thing has outstanding advantages, such as heat resistance, water resistance, and corrosion resistance, so it is widely used in industry. However, its "disadvantages" can be fatal for activated sludge - difficult to degrade and easy to accumulate. Ordinary heterotrophic and nitrifying bacteria simply cannot gnaw on it, which lays the groundwork for subsequent biochemical system failures.
The first impact of organic silicon on activated sludge is to interfere with the aeration system and disrupt mass transfer efficiency. This should be the most intuitive phenomenon. After the organic silicon enters the biochemical pool, it is easy to form a stable layer of foam on the water surface. Unlike ordinary sewage foam, this kind of foam is fine and viscous, with strong adhesion. It is not easy to break even when the wind blows. It will also spill along the pool wall, which is very difficult to clean. What's more, this layer of foam will isolate the contact between air and water. Even if you turn on the fan to the maximum, it is difficult for the aerated oxygen to dissolve into the water, leading to the "virtual low" of dissolved oxygen in the biochemical tank - the online monitor shows a good value, but the sludge at the bottom of the tank has long been in a state of oxygen deficiency. Insufficient dissolved oxygen will lead to a decrease in the efficiency of heterotrophic bacteria in decomposing organic matter, and nitrifying bacteria will simply "lie flat". It is only a matter of time before effluent COD and ammonia nitrogen exceed the standard.
The second impact is that it accumulates on the surface of sludge flocs, hindering microbial metabolism. Microorganisms in activated sludge rely on cell membranes to absorb nutrients from wastewater in order to survive. Organic silicon has strong lipid solubility and is easily adsorbed on the surface of sludge flocs, gradually forming a layer of "silicon film". This layer of membrane is like putting a tight "protective suit" on microorganisms, preventing nutrients from entering and waste generated from metabolism from being discharged, resulting in a decrease in microbial activity. We usually observe under the microscope that the sludge flocs become loose and fine, and the number of protozoa such as nematodes and rotifers sharply decreases, even leading to a large number of deaths. This is a typical manifestation of organic silicon enrichment. Over time, the settling performance of sludge will deteriorate sharply, with SV30 values fluctuating high and low, and problems such as sludge floating and running in the secondary sedimentation tank will also arise one after another.
Another easily overlooked point is that organosilicon can inhibit the activity of specific functional bacteria, especially nitrifying bacteria. Nitrifying bacteria themselves are known as "glass hearts" and are particularly sensitive to environmental changes and toxic substances. Although organic silicon does not directly "poison" nitrifying bacteria, it can interfere with the enzymatic reactions inside nitrifying bacteria when it accumulates to a certain concentration in sludge flocs, slowing down the process of converting ammonia nitrogen into nitrite and nitrate. Many times we find that the effluent ammonia nitrogen exceeds the standard, but COD has not changed much. After checking DO, pH, and sludge age, there are no problems. At this time, we need to think about whether the influent organic silicon exceeds the standard.
In addition, organosilicon can also cause disorder in the secretion of extracellular polymeric substances (EPS) in sludge. EPS is the "adhesive" of sludge flocs, which can allow microorganisms to agglomerate and form structurally stable flocs. The invasion of organic silicon can disrupt the secretion rhythm of microorganisms, or excessive EPS secretion can lead to increased sludge viscosity and easy blockage of pipelines; Either the secretion is insufficient, the sludge flocs are dispersed, and the separation of mud and water is difficult. Both of these situations are quite troublesome for our biochemical system.
Having said so much about the impact, everyone must be concerned about how to deal with it. In fact, the core idea is two-fold: source control+process optimization.
Source control is the most fundamental approach. We need to communicate with polluting enterprises and require them to pretreat wastewater containing organic silicon, such as using coagulation sedimentation and air flotation methods, to remove a portion of the organic silicon first and prevent high concentrations of organic silicon from directly entering the biochemical tank. In daily operation and maintenance, it is also necessary to strengthen the monitoring of incoming water quality. Once the concentration of organic silicon surges, it should be quickly diverted to the regulating tank or the incoming water volume should be reduced to avoid system impact.
Process optimization is a means of making up for lost sheep. If foam caused by organic silicon has appeared in the biochemical pool, an appropriate amount of defoamer can be added, but attention should be paid to the selection of non silicon defoamer, otherwise it is "adding fuel to the fire"; Simultaneously increasing the aeration rate appropriately, enhancing water disturbance, and reducing the accumulation of organic silicon on the liquid surface; Additionally, avoid excessive sludge discharge and ensure sufficient sludge age to allow microorganisms in the system enough time to adapt to low concentration organic silicon environments.
Finally, it should be reminded that the impact of organosilicon on activated sludge is "chronic", and there may be no problem at the initial stage. It will take much more effort to deal with it when foam is everywhere and the water quality exceeds the standard. So in daily operation and maintenance, we must pay more attention to the changes in the composition of the incoming water, and not let this' invisible killer 'destroy our biochemical system.
