Discussion on overexposure of sewage plant

Over-aeration of a sewage plant is an operating condition that occurs during operation. Under normal operation, sewage gas (aeration volume), water (influent water quality, water volume), and mud (activated sludge concentration) are three A dynamic balance, but this balance will be constantly broken by the constant changes of external environmental factors, and then enter a new balance. The defined state of aeration means that in operation, the aeration system provides the amount of oxygen that exceeds the amount of oxygen required by the microorganisms in the activated sludge system to degrade the organics in the sewage, resulting in the enrichment of oxygen in the activated sludge system, and the detection of the biological tank A state where the dissolved oxygen content is too high (4mg/L or more).

A general idea in the minds of operators is that in the overexposed state, after the microorganisms in the activated sludge have degraded the organic matter in the sewage, they will begin to consume themselves, which means that the endogenous respiration period of the activated sludge is reached. The activated sludge is self-digested, thereby reducing the output of activated sludge.

In the traditional activated sludge yield calculation formula:

ΔX=Y (S0-Se) Q-KdVXV

ΔX: is the daily increase in the amount of volatile sludge (VSS)

Q: is the daily water intake

S0-Se: is the difference between BOD5 of organic matter entering and leaving the water daily. COD can also be used, but it must be multiplied by a B/C coefficient.

Kd: The self-oxidation rate of activated sludge microorganisms. Domestic sewage is generally low, about 0.07.

Xv: the concentration of volatile suspended solids in the aeration tank, MLVSS

V: Volume of aeration tank.

It can be seen from the formula that the growth of sludge mainly comes from the removal of organic matter and the self-oxidation of sludge. The oxidation rate of sludge is relatively low as the value of Kd in domestic sewage is 0.07, and industrial wastewater is relatively low. High, 0.5~0.65. These two values are relatively low values for the previous increase. Under normal full-load operation, the self-oxidation of activated sludge will be relatively small, but in the sewage treatment plant In the early stage of operation; insufficient water inflow; insufficient organic load, etc., the value of the front end of the formula will be relatively small, and the value of the latter section will be relatively large. At this time, the sludge growth will be small or even negative. Note that this situation is determined by the quality and volume of the influent water, and it is also a manifestation of overexposure in the sense of balance.

The endogenous respiration period of activated sludge is that the organic matter in the external sewage of the activated sludge flocs can no longer meet the nutrients required for the growth of microorganisms, and the microorganisms begin to decompose and metabolize their own cellular materials to maintain life activities. In this process, the microorganisms are still proliferating weakly, but the rate is much lower than their own oxidation rate, and the amount of activated sludge begins to decrease, but the endogenous respiration degrades the easily degradable part of the cell, for the hard-to-degrade cell wall and cytoplasm, etc. The substance cannot be used, so it is impossible for activated sludge to completely disappear by relying on endogenous respiration. It can also be seen from the figure below that after entering the endogenous respiration period, the amount of sludge has declined, but the downward trend is gentle, far from reaching the increase in the logarithmic proliferation period.

In the influent water of the actual sewage treatment plant, the sludge output not only comes from the self-reproduction of microorganisms in the degradation process of organic matter, but also a part of the inorganic particles in the influent water, most of which exist in the form of SS, especially in SS The inorganic components of the sludge are not biodegradable and constitute another main source of the sludge output of the sewage treatment plant. This part of the inorganic components cannot be removed regardless of the excessive aeration. Therefore, in the case of over-exposure, this part of the sludge is It can't be reduced. During the over-exposure period, part of the refractory organic matter cannot be removed by conventional aeration. In some special industrial wastewater, even though aeration for 40-50 hours, some refractory COD still exists, and activated fouling The amount of mud was not significantly reduced as expected. In the process adjustment of wastewater treatment plants, excessive aeration is used to greatly reduce the sludge output. The practice is seldom applied to engineering practice, and more is through fine process management, reasonable biological control, and effective removal of inorganic particles. Exploit biological effects and other methods to reduce sludge production, etc.

Some studies have shown that when activated sludge has a low F/M, when the microorganisms in the aeration tank deplete the food in the sewage, a viscous film or EPS will be excreted around the cells. The suspended state of the activated sludge held in the aeration tank makes the EPS around the cells collide and finally bond together to form larger and more floccules. This is also a manifestation of non-filamentary swelling. In appearance It is the occurrence of a large number of biological foams in the sludge, combined with the environmental factors at the time, it is easy to induce sludge expansion.

Excessive aeration not only causes problems of sludge proliferation or reduction, but also causes more problems for process operation. In addition to the above-mentioned sludge foam and non-filamentous bacteria expansion problems, excessive aeration of sewage plants can cause internal problems. The returned nitrification solution contains a large amount of free oxygen, which is not conducive to the formation of anoxic environment in the denitrification zone. Excessive aeration can also cause sludge aging. The pond forms cloud-like suspended solids, which causes the deterioration of the effluent water quality, and most importantly, excessive aeration makes the energy consumption of the sewage treatment plant remain high, resulting in a waste of energy.

In some sewage plants, a reasonable combination of fans can be used to reduce aeration. However, some sewage plants do not have multiple combinations of fan equipment. Excessive aeration can be carried out by intermittently operating aeration fans. Many sewage plants worry that intermittent aeration will cause The aeration device (aeration pipe, aeration plate) is blocked, or the suspended state of activated sludge cannot be formed. In practice, some sewage plants have been operated with intermittent aeration, including some process designs that use intermittent aeration. , It has been shown that intermittent operation will not change the basic characteristics of activated sludge and can be used in practice.

The process operation problems caused by over-aeration are the source of most process problems in daily operation, and the operators of the sewage treatment plant need to face them seriously, and take sufficient attention to adjust the aeration volume reasonably. Over-aeration should not be used for simple reasons. The reduction of mud volume brings more complex process operation problems. Reasonable, adaptable, accurate, and energy-saving aeration volume control is the main direction for the operation of wastewater treatment plants in the future.