How to evaluate the performance of activated sludge?

Composition of Activated Sludge
　　Activated sludge contains bacteria, fungi, protozoa, and metazoa. Aerobic bacteria are the main decomposers of organic matter.
　　The total number of bacteria in 1 mL of aeration tank mixed liquor is approximately 1 × 10^8.
　　Fungi are mainly filamentous molds. In normal activated sludge, fungi are not dominant. If filamentous bacteria grow significantly, the settling performance of activated sludge will deteriorate.
　　Protozoa and bacteria work together in wastewater purification. In 1 mL of normal activated sludge mixed liquor, there are generally 5 × 10^3 to 2 × 10^4 protozoa, of which 70% to 90% are ciliates. Protozoa promote bacterial flocculation and improve bacterial settling efficiency. Protozoa feed on bacteria and can remove free bacteria.
　　Metazoa in activated sludge usually include rotifers and nematodes. These metazoa feed on bacteria, protozoa, and activated sludge fragments.

Material Composition of Activated Sludge
　　Ma: Microbial population with metabolic function
　　Me: Microbial residue (mainly bacterial endogenous metabolism, self-oxidation products)
　　Mi: Inert organic matter from the original wastewater that is difficult to degrade by bacteria
　　Mii: Inorganic matter from wastewater

Activated Sludge Evaluation Indicators
　　1. MLSS
　　Mixed Liquor Suspended Solids Concentration Refers to the dry weight of suspended solids contained in 1 L of aeration tank mixed liquor. It is an indicator for measuring the amount of activated sludge in the reactor. It includes microbial cells (Ma), microbial auto-oxidation products (Me), organic matter (Mi) adsorbed on sludge flocs that cannot be degraded by microorganisms, and inorganic matter (Mii).
　　Because MLSS is relatively easy to measure, it is often used in engineering as an indicator to estimate the number of microorganisms in activated sludge. In engineering design, it is hoped to maintain a higher MLSS to reduce the volume of the aeration tank, saving land and investment, but the MLSS concentration cannot be too high, otherwise it will lead to insufficient oxygen supply. The sludge concentration in the reactor is generally controlled at 2000～6000 mg/L.
　　2. MLVSS
　　Mixed Liquor Volatile Suspended Solids Concentration Refers to the volatile suspended solids content in 1 L of aeration tank mixed liquor. It only includes microbial cells (Ma), microbial auto-oxidation products (Me), and organic matter (Mi) adsorbed on sludge flocs that cannot be degraded by microorganisms, not including inorganic matter (Mii). Therefore, MLVSS can more accurately reflect the number of microorganisms in the reactor.
　　Generally, the activated sludge for treating domestic sewage The MLVSS/MLSS ratio is around 0.75. For industrial wastewater, it varies depending on the water quality, and the MLVSS/MLSS ratio varies greatly.
　　3. SV
　　Sludge settling ratio. The mixed liquor in the aeration tank is allowed to stand in a graduated cylinder After 30 min, the ratio of the volume occupied by the sludge to the original volume of the mixed liquor. Normal activated sludge can reach its maximum density after settling for 30 min, so during normal operation, SV% roughly reflects the amount of sludge in the reactor and can be used to control sludge discharge.
　　Generally, in the aeration tank, The normal value of SV% is 20%～30%. Changes in SV% can also reflect abnormal situations such as sludge bulking in a timely manner. Therefore, SV% is an important indicator for controlling the operation of the activated sludge process.

4. SVI
　　Sludge volume index, refers to the aeration tank mixed liquor after 30 min of static settling, the volume occupied by 1 g of dry sludge, the unit is ml/g.
　　SVI = (SV% × 100) / MLSS
　　SVI reflects the looseness and flocculation performance of the sludge. If SVI is too low, it indicates that the sludge particles are small and dense, with a high content of inorganic matter and a small number of microorganisms. In this case, the sludge lacks activity and adsorption capacity. If SVI is too high, it indicates that the sludge structure is loose and difficult to settle and separate, which is about to bulge or has already bulged.
　　Theoretically, The SV value is generally 15%～30%, and the SVI value is generally 70～100. Generally:
　　SVI ≤ 100 Good sludge settling performance
100 < SVI < 200 General sludge settling performance
　　SVI ≥ 200 Poor sludge settling performance
　　The water quality of urban domestic sewage is relatively stable, SVI is controlled at around 50～150. The water quality of industrial wastewater varies greatly. For example, in some industrial wastewater, COD is mainly dissolved organic matter, which is very easy to synthesize sludge, and the sludge ash content is low, and the number of microorganisms is large, so although its SVI is high, it is not true sludge bulking. Conversely, if the wastewater contains a lot of inorganic suspended matter, the sludge density is large, and the SVI is low, but its activity and adsorption capacity are not necessarily poor.

Sludge Color
　　If the aerobic activated sludge of domestic sewage has a high organic matter content, it is generally dark brown. A yellowish color may indicate a higher sand content. The color of industrial sludge is varied and related to the specific influent. However, if the influent is colorless but the sludge color is inconsistent with the normal situation, it indicates that the sludge condition is abnormal. For example, if the sludge in the domestic sewage aeration tank is black, it indicates insufficient aeration, leading to anaerobic conditions.

Sludge Odor
　　Normal municipal sewage sludge will emit an earthy fragrance. The smell of industrial wastewater varies depending on the pollutants, but the odor will not change significantly. If the sludge odor changes, such as an aerobic tank emitting a foul odor, it indicates insufficient aeration leading to anaerobic digestion!

Sludge load
　　Sludge load It is an important parameter in reactor design and operation. It refers to the amount of five-day biochemical oxygen demand that can be removed per unit of activated sludge. The unit is kg BOD5/kg MLSS. During engineering design, the selection of sludge load needs to consider the expected operating efficiency and effluent quality, aeration rate, sludge age, and other parameters.
　　According to the size of the sludge load, it can be divided into three situations
　　Low sludge load 0.1-0.25 kg BOD5/kg MLSS BOD removal rate 90~95%
　　Normal sludge load 0.3-0.6 kg BOD5/kg MLSS BOD removal rate 85~98%
　　High sludge load 1-5 kg BOD5/kg MLSS BOD removal rate 50~60%
　　For the biological treatment of urban domestic sewage, there is no clear boundary between the low sludge load method and the normal sludge load method, while the boundary between the normal sludge load method and the high sludge load method is clear. According to some operating data of sewage treatment plants, when the sludge load is between 0.6~1.0kg BOD5/kg MLSS, filamentous bacteria have a relative growth advantage, and the growth of filamentous bacteria makes the sludge structure loose, eventually leading to sludge bulking. However, when the sludge load is higher than 1.5kg BOD5/kg MLSS, the reactor has sufficient food, and non-filamentous bacteria can also obtain enough nutrients to grow, so the sludge is not easy to swell.

Sludge age
　　Sludge age refers to the average residence time of sludge in the reactor. The unit is d.
　　Sludge age = Total sludge in the reactor / Amount of excess sludge discharged per day (d)
　　Sludge age is related to sludge load. When the organic load is low, most of the organic matter is completely oxidized into CO2 and water, only a small part is used to synthesize microbial cells, so the amount of excess sludge is small, and the sludge age is longer. When the organic load is high, sludge synthesis is faster, the amount of excess sludge is large, and the sludge age is shorter.

Oxygen consumption rate of activated sludge (SOUR)
　　The oxygen consumption rate of activated sludge refers to the amount of dissolved oxygen that can be consumed per unit weight of activated sludge per unit time. It is generally expressed as SOUR, and the unit is usually mgO2/(gMLVSS•h). SOUR is also called the respiration rate or digestion rate of activated sludge, which is an important indicator for measuring the biological activity of activated sludge.
　　If F/M is high, or SRT is small, the biological activity of activated sludge is also high, and its SOUR value is also large. Conversely, if F/M is low and SRT is too large, its SOUR value is also low.
　　The important role of SOUR in operation management is to indicate whether there are too many refractory substances in the influent wastewater, and whether the activated sludge is poisoned. Generally speaking, when there are more refractory substances in the wastewater, or when the activated sludge is poisoned by toxic substances in the wastewater, the SOUR value will decrease sharply. The cause should be analyzed immediately and measures should be taken, otherwise the effluent will exceed the standard. The SOUR of the activated sludge process is generally between 8～20 mgO2/(gMLVSS•h).⑨ For a certain flow rate of wastewater, sufficient tank volume must be ensured to maintain sufficient retention time of wastewater in the aeration tank, otherwise, untreated wastewater may be discharged, affecting the treatment effect.