Constructional Details

• It consists of a circular R.C.C. tank with hoppered bottom and having a fixed or a floating type of roc over its top.
• The raw sludge is pumped into the tank and when the tank is first put into operation, it is seeded wit the digested sludge from another pump.
• A screw pump with an arrangement for circulating the sludge from bottom to top of the tank or vice versa is used,
• The gases of decomposition are collected in a gas dome
• The digested sludge which settles down to the hoppered bottom of the tank is removed under hydrostatic pressure, periodically, once a week or so. The supernatant liquor lying between the sludge and the scum is removed at suitable elevations, through a number of withdrawal pipes. The supernatant liquor, being higher in BOD and suspended .Cross-section of a typical sludge digestion tank solids contents, is sent back for treatment along with the raw sewage in the treatment plant.
• The scum formed at the top surface of the supernatant liquor is broken by the recirculating flow through the mechanical rakers called scum breakers.

Design Considerations

• If the progress of sludge dignstion is assumed to be linear, then the capacity of the digestion tank is given as: $$ V=\left(\frac{V_{1}+V_{2}}{2}\right) t $$ where, $\quad V=$ Volume of digestion tank $\left(\mathrm{m}^{3}\right)$ $V_{1}=$ Raw sludge added per day $\left(\mathrm{m}^{3} / \mathrm{d}\right)$ $V_{2}=$ Equivalent digested sludge produced per day on completion of digestion, $\mathrm{m}^{3} / \mathrm{d}, \simeq \frac{V_{1}}{3}$ $t=$ Digestion period $(d)$
• When the daily digested sludge could not be removed due to the factors such as monsoon or winter season etc. then separate capacity for its storage should be provided in the tank. The total digester volume is given as $$ V=\left(\frac{V_{1}+V_{2}}{2}\right) t+V_{2} \cdot T $$ where in $T$ is number of days for which the digested sludge is stored and $V_{2} T$ monsoon sludge. - When the change during digestion is assumed to be parabolic rather than linear, the average of digestion sludge will be $\left[V_{1}-\frac{2}{3}\left(V_{1}-V_{2}\right)\right]$ and the required capacity will be given by $V=\left[V_{1}-\frac{2}{3}\left(V_{1}-V_{2}\right)\right] t \ldots$ without monsoon storage $V=\left[V_{1}-\frac{2}{3}\left(V_{1}-V_{2}\right)\right] t+V_{2} T$...with monsoon storage
• Digesțion period = 30 days