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In this article we will discuss about:- 1. Rate of Sludge Production 2. Sludge Recycle—Sludge Volume Index (SVI) 3. Wasting of Excess Sludge Q_{w}^{.}

**1. Rate of Sludge Production: **

It is important to know the quantity of sludge produced per day, because it will affect the design of the sludge handling and disposal facilities. The quantity of sludge (or biological solids or microbial mass or volatile suspended solids VSS) produced per day is obtained from following equation-

The above expression represents the volatile suspended solids (VSS) produced in the system. As indicated earlier the concentration of volatile suspended solids (VSS) in the mixed liquor is usually 60 to 85 percent of the total suspended solids. Thus the sludge produced may be estimated as total suspended solids SS = VSS/0.60 to 0.85 and usual value may be taken as SS = VSS/0.8.

**2. Sludge Recycle—Sludge Volume Index (SVI): **

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The most important aspect in the operation of an activated sludge plant is the maintenance of proper F/M ratio which is achieved by increasing or decreasing the MLSS (and hence MLVSS) levels in the aeration tank to suit the influent BOD_{5} loads. The MLSS in the aeration tank can be regulated by controlling the rate of sludge return.

**The rate of sludge return can be determined by the following two methods: **

**Method 1: **

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This is an approximate method in which the sludge return flow is taken equal to the percentage ratio of the volume occupied by the settleable solids from the aeration tank effluent to the volume of clarified liquid (supernatant) after settling for 30 minutes in a 1000 ml graduated cylinder. For example, if the settleable solids occupy a volume of 200 ml in a 1000 ml cylinder, after 30 minutes of settling, the percentage volume will be equal to [200/(1000 – 200)] = 25%.

Thus if the plant flow is 4 m^{3}/s, the return sludge rate should be 0.25 x 4 = 1 m^{3}/s.

**Method 2: **

This is a more rigorous method in which the rate of sludge return is based on sludge volume index (SVl) as indicated below-

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The sludge volume index (SVl) is defined as the volume in millilitres occupied by one gram of activated sludge after a settling period of 30 minutes.

**The sludge volume index (SVI) is determined experimentally as indicated below: **

(i) One litre sample of mixed liquor is collected from the discharge end of the aeration tank in a 1000 ml measuring cylinder.

(ii) The sample collected in the cylinder is allowed to settle for 30 minutes.

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(iii) The settled sludge volume V_{s} in ml is recorded, which represents the quantity of sludge in the mixed liquor in ml/l.

(iv) After remixing the settled solids the above sample of mixed liquor is further tested for MLSS by the available standard methods for measuring the suspended solids in sewage. Let this concentration of suspended solids in the mixed liquor be x_{t} in mg/l.

Then sludge volume index SVI is given by the equation-

Since the concentration of micro-organisms (MLSS) maintained in the reactor or contactor has a direct effect on the oxidation of organic matter, the liquid-solids separation characteristics of these solids are very important. The value of Sludge Volume Index (SVI) indicates the ability of the micro-organisms (MLSS) to be separated from the sewage.

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Usually SVI values of 80 to 150 ml/g are considered satisfactory. The value of SVI is of operational importance since it reflects changes in the treatment system. Any increase in SVI with no increase in MLSS concentration indicates that the settling characteristics of the solids are changing and that a plant upset may occur.

**A relationship between MLSS concentration, SVI and recycle ratio (Q _{r}/Q) may be obtained as indicated below: **

As shown in Fig. 13.14, if it is assumed that the solids-concentration in the influent and the effluent is negligible then the solids-concentration in the aeration tank is equal to that in the returned sludge.

Solids-concentration in the aeration tank

= (Q + Q_{r})x_{1}

Solids-concentration in the returned sludge

In which SVI is in ml/g.

The relationship between MLSS concentration, SVI and recycle ratio (Qr/Q) in respect of equation 13.53 (a) is shown in Fig. 13.15 which is a plot of MLSS concentration (mg/I) versus recycle ratio (Q_{r}/Q) for different values of SVI in ml/g. The amount of recycle flow depends to a great extent on the settling characteristics of MLSS.

For instance, if SVI value is 400 ml/g and the desired MLSS concentration is 2000 mg/l, a recycle ratio (Q_{r}/Q) of 4 is required. On the other hand, if SVI value is 50 ml/g, the recycle ratio required would be about 0.1. This demonstrates that the settling characteristics of the biological solids is of primary importance to the successful operation of the activated sludge process.

Further from equation 13.53 (a), we have-

From equation 13.53 (b) it may be seen that the concentration of the suspended solids in the returned sludge is equal to (10^{6}/SVI) mg/l, when the value of SVI is in mg/l. Thus

**Sludge Density Index (SDI): **

Sometimes the term Sludge Density Index (SDI) is used to determine the quantity of solids concentration in the returned sludge. It is given by the following expression.

Thus it is observed that SDI is numerically equal to the reciprocal of SVI and is expressed in g/ml. The desirable value of SDI for a good sludge is about 1 to 2, and when the value of SDI reduces to 0.3 or so it indicates a poor sludge.

Values of returned sludge ratio r adopted in different types of activated sludge systems are given in Table 13.3.

The returned sludge has always to be pumped, and the pump capacity should be designed for a minimum returned sludge ratio of 0.5 to 0.75 for large plants and 1.0 to 1.5 for smaller plants, irrespective of theoretical requirements. The required capacity should be provided in multiple units to permit variation of returned sludge ratio as found necessary during the operation of the plant.

**3. Wasting of Excess Sludge Q**_{w.}

_{w.}

The sludge generated in the aeration tank has to be partly discharged and wasted out of the plant to maintain a steady level of MLSS (and hence MLVSS) in the system. The excess sludge quantity will increase with increasing F/M ratio and decrease with increasing temperature. Under steady state operation the mass of wasted sludge is given by the following equation-

Thus the mass of excess sludge to be wasted may be determined from equation 13.56. Alternatively the mass of excess sludge to be wasted from may also be determined from the following equation-

In case of domestic sewage, the excess sludge to be wasted will be about 0.35 to 0.50 kg per kg of BOD_{5} removed from the conventional system and about 0.25 to 0.35 kg per kg of BOD_{5} removed in case of extended aeration plants having no primary settling. The volume of sludge to be wasted will depend on the suspended solids concentration in the waste stream.

Excess sludge may be wasted either from the sludge return line or directly from the aeration tank as mixed liquor. The later procedure is to be preferred as the concentration of suspended solids will then be fairly steady in the waste stream providing better control on biomass wasted.

The waste sludge is either discharged into the primary settling tank or thickened in a sludge thickening unit and digested directly. In extended aeration plants the excess sludge is taken to sludge drying beds directly and the sludge filtrate discharged into the effluent stream.