As one of the judgment bases for sludge expansion, the sludge index (SVI) has good guiding significance, so the correctness of the calculation of SVI is particularly important! To understand the meaning of the Sludge Index (SVI), we need to start with the definition!

1.Definition and calculation of sludge index (SVI).

Sludge index, also known as sludge volume index (SVI), is an index to measure the sedimentation performance of activated sludge. It refers to the volume (in mL) of the corresponding 1g of dry sludge after 30 minutes of precipitation of the aeration tank mixture, unit mL/g. Namely:

SVI reflects the looseness and cohesion of sludge, and too low SVI indicates that the sludge particles are fine and tight, with many inorganic substances and few microorganisms, and the sludge lacks activity and adsorption capacity. If the SVI is too high, it means that the sludge structure is loose, difficult to sediment, and is about to expand or has already occurred.

Theoretically, the SV value is generally 15%~30%, and the SVI value is generally 70~100.

• SVI≤100 sludge sedimentation performance is better

• 100<SVI<200 sludge settling performance is average

• SVI≥200 sludge settling performance is poor

The water quality of urban domestic sewage is relatively stable, and its SVI is controlled at about 50~150. For example, COD in some industrial sewage is mainly dissolved organic matter, which is very easy to synthesize sludge, and the sludge ash content is small and the number of microorganisms is large, so although its SVI is high, it is not real sludge expansion. On the contrary, if the sewage contains a lot of inorganic suspended solids, the sludge has a high density and low SVI, but its activity and adsorption capacity are not necessarily poor.

Calculation of SVI:

The main point of SVI calculation is mainly the problem of unit conversion, if you don't want to convert the unit (sludge concentration is mg/L, SV is %), you can calculate it by deriving a simple formula and directly substituting the value:

Sludge volume = SV/100*1000 = 10SV

Sludge weight = MLSS * 1/1000 = MLSS/1000

SVI = sludge volume / sludge weight = 10000SV/MLSS

Cases that are substituted for the head of the article:

SVI=10000*92/9890=93mL/g

2.TwoSVIThe reason for the value anomaly

(1) The SVI value is too low:

1. The sudden drop in water temperature reduces the activity of microorganisms and reduces the function of decomposing organic matter.

2. The inflow of acidic wastewater makes the PH value of the aeration tank mixture at 3~4 acidic conditions for a long time, eosinophilic filamentous microorganisms multiply, and the periodic shedding of filamentous microbial membranes growing in the pipeline of acidic wastewater will also lead to the proliferation of filamentous microorganisms in the mixture.

3. The proportion of nitrogen and phosphorus nutrients in the influent water is low, while filamentous bacteria can multiply in large numbers in the case of serious lack of nutrients such as nitrogen and phosphorus, and dominate in the mixture, which in turn causes sludge expansion.

4. The high organic load of the aeration tank leads to the deterioration of the coagulation and sedimentation performance of activated sludge, and the SVI value increases.

5. The content of low molecular weight organic matter in the influent is large, and low molecular weight organic matter is the most easily absorbed and utilized component of filamentous bacteria, so that filamentous microorganisms multiply in large numbers and reduce the sedimentation performance of the aeration tank mixture.

6. Insufficient dissolved oxygen in the aeration tank mixture inhibits the growth of floc. However, filamentous fungi can multiply in large numbers below 0.1mg/l, resulting in an increase in the SVI value of activated sludge expansion.

7. The content of toxic and harmful substances in the influent increases, such as phenols, aldehydes, sulfides and other substances suddenly increases, which reduces the cohesion performance of microbial bacteria and dissolves a large number of flocculents, while filamentous bacteria can proliferate and SVI increase.

8. After the high-concentration organic wastewater enters the aeration tank after hypoxic decay, it contains a large amount of low-molecular organic matter and sulfide, so that the filamentous bacteria multiply and the SVI value increases.

9. The digestive supernatant enters the aeration tank in a short time. The high concentration of organic matter in it increases the organic load of the aeration tank and the proliferation of filamentous bacteria.

10. The SS in the influent is low and the proportion of dissolved organic matter is large, which reduces the bulk density of sludge and makes it difficult to separate solid-liquid, resulting in an increase in SVI value.

11. If the sludge stays in the secondary sedimentation tank for too long, it will lead to a decrease in the dissolved oxygen content, and the sludge will deteriorate, which will cause the filamentous bacteria to multiply in the reflux sludge, causing the activated sludge in the aeration tank to expand and the SVI to increase.

(2) SVI value is too low:

1. The water temperature rises

2. Soil, sand and gravel flow in

3. The organic load is too low

3.The relationship between SVI and other indicators

(1) The relationship between SVI and SV value

The SVI value excludes the influence of sludge concentration on the sedimentation volume of sludge, so it can evaluate and reflect the coagulation and sedimentation performance of activated sludge more accurately than the SV value.

Generally speaking, low SVI values indicate that the sludge particles are fine, the content of inorganic matter is high, and the activity is lacking. Too high SVI indicates poor sludge sedimentation, and sludge expansion is about to occur or has occurred. The SVI value of urban sewage treatment plants is generally between 70~100.

The SVI value is related to the sludge load, and if the sludge load is too high or too low, the metabolic performance of the activated sludge will deteriorate, and the SVI value will also become very high, and there is a possibility of sludge expansion.

(2) The relationship between SVI and sludge load Ns

The relationship between SVI and sludge load Ns is of great practical significance in the operation of sewage plants. When the sludge load Ns is between 0.5~1.5KgBOD5/(KgMLSS·d) range, the SVI reaches the highest, and the sludge sedimentation performance is poor, which belongs to the high incidence area of sludge expansion, so the sludge load in this section should be avoided. Ns between 1.5~2.5KgBOD5/(KgMLSS·d) will accelerate the degradation rate of organic pollutants and the growth rate of activated sludge, and it is economically appropriate to reduce the volume of the aeration tank, but the treated water quality may not be able to meet the predetermined requirements. In the low-load section of Ns<0.5KgBOD5/(KgMLSS·d), the degradation rate of organic pollutants and the growth rate of activated sludge will decrease, the volume of the oxidation ditch will increase, and the construction cost will increase, but the treatment water quality can meet the requirements.

(3) The relationship between SVI and outflow SS

The influence of SVI of the mixture on the effluent quality is very obvious, because the higher SVI value of the mixture has poor sedimentation performance in the final sedimentation tank, which can easily cause SS to be lost with the effluent. The experiment showed that when the SVI value was maintained between 50~200ml/g, 95% of the effluent SS< was 20mg/l. When the SVI > 200ml/g, the SS value of effluent is basically greater than 20mg/l, which cannot meet the discharge standard.

(4) The relationship between SVI and DO

Dissolved oxygen is an important control parameter in the operation of activated sludge method, and the level of DO concentration directly affects the removal efficiency of organic matter and the growth of activated sludge. SVI is basically inversely related to DO, that is, low dissolved oxygen can lead to higher SVI values, and correspondingly, high dissolved oxygen can produce low SVI values. Experiments show that when SVI < 100ml/g, the DO value remains at 4~6mg/l, when the SVI value is maintained at 100~150ml/g, most of the DO value remains between 2~4.0mg/l, and when SVI > 200ml/g, the DO value is basically below 2.0mg/l, in which case it is difficult to ensure that the effluent quality meets the standard.