ASP: removal of carbonaceous matter; recirculation

Fundamentals of removing dissolved carbonaceous matter in ASP

In the activated sludge process the dissolved organic (carbonaceous) matter measured as BOD is food for aerobic heterotrophic bacteria kept in suspension in the aeration tank as flocs (activated sludge, MLSS 2000-4000 mg/l).

1. Mixing and contact: settled sewage is mixed with return sludge and aerated. The dissolved and colloidal organics are quickly adsorbed on the sticky bacterial flocs (biosorption).
2. Oxidation (energy): a part of the organics is oxidised by the bacteria with the dissolved oxygen supplied by diffused or mechanical aeration:

$$C_xH_yO_z + O_2 \xrightarrow{\text{bacteria}} CO_2 + H_2O + \text{energy}$$

3. Synthesis (growth): another part, with nitrogen and phosphorus, builds new cells:

$$C_xH_yO_z + O_2 + NH_3 \rightarrow C_5H_7NO_2\ (\text{new cells}) + CO_2 + H_2O$$

4. Endogenous respiration: when food is short, the cells oxidise their own mass: $C_5H_7NO_2 + 5O_2 \rightarrow 5CO_2 + 2H_2O + NH_3 + \text{energy}$.
5. Flocculation and settling: the grown cells clump into settleable flocs. In the secondary clarifier they settle, leaving clear effluent with 85-95% less BOD.
6. Controlling factors: $F/M=\dfrac{QS_0}{VX}$ (0.2-0.4 for conventional), sludge age $\theta_c$ (3-15 days), dissolved oxygen above 2 mg/l, pH 6.5-8.5, nutrient ratio BOD : N : P = 100 : 5 : 1, temperature and absence of toxins.

Oxygen needed is about 0.5-1 kg O₂ per kg BOD removed.

Need for sludge recirculation

Return activated sludge (RAS) is the settled sludge pumped from the secondary clarifier back to the inlet of the aeration tank. It is necessary because:

1. It maintains the required concentration of active micro-organisms (MLSS) in the aeration tank; otherwise the biomass would be washed out with the effluent, since the growth rate of bacteria is slow compared with the detention time of a few hours.
2. It keeps the F/M ratio and sludge age at the desired value, so a stable process is obtained.
3. It seeds the incoming sewage with adapted, flocculent organisms that are already in the endogenous or declining-growth phase, which speeds up adsorption and oxidation.
4. It gives good flocculation and settleability; without return sludge, the dispersed growth would be in the log phase and would not settle.
5. It lets the plant treat the same load in a smaller tank.

Example: if 3000 mg/l MLSS is needed and the return sludge has 10000 mg/l, the return ratio is $\dfrac{3000}{10000-3000}=0.43$, i.e. 43% of the inflow is recycled continuously. Without this return, the tank would hold only the few hundred mg/l of bacteria that grow in one pass, and BOD removal would fall sharply.