Circular sedimentation tank, population 35675

Assumptions: sewage $=0.8\times 135 = 108$ lpcd, so the average flow is $35675\times 108/1000 = 3852.9$ m³/day; the design flow is the maximum, $2.7\times$ average. Detention 2 h at maximum flow, depth 3 m.

Step 1: Design flow

$$Q = 35675\times 135\times 0.8\times 2.7/1000 = 10402.8\ \text{m}^3/\text{day}$$

Step 2: Capacity

Adopt detention period $t = 2.0$ h and side water depth $H = 3.0$ m (typical for plain sedimentation: 2-3 h, 2.5-4 m).

$$V = Q\,t = \frac{10402.8}{24}\times 2.0 = 866.9\ \text{m}^3,\qquad A = \frac{V}{H} = \frac{866.9}{3.0} = 289.0\ \text{m}^2$$

Step 3: Diameter

Provide one circular tank, so $A_1=A$.

$$D = \sqrt{\frac{4A_1}{\pi}} = \sqrt{\frac{4\times 289.0}{\pi}} = 19.18\ \text{m}\ \Rightarrow\ \text{adopt } D = 19.5\ \text{m}$$

Adopt a total depth of $H + 0.5\ (\text{free board}) + 0.5\ (\text{sludge zone}) = 4.0$ m, with a central inlet well and a peripheral weir.

Step 4: Checks

$$\begin{aligned} \text{Surface loading} &= \frac{Q/N}{\pi D^2/4} = \frac{10402.8}{298.6} = 34.8\ \text{m}^3/\text{m}^2\text{/day}\ (\text{usual 25-40 for primary tanks})\\ \text{Weir loading} &= \frac{Q/N}{\pi D} = \frac{10402.8}{\pi\times 19.5} = 169.8\ \text{m}^3/\text{m/day}\ (\text{should be below about 300})\\ \text{Actual detention} &= \frac{(\pi D^2/4)H}{Q/N}\times 24 = 2.07\ \text{h} \end{aligned}$$

The surface loading lies within the usual range for primary settling (25-40 m³/m²/day), so the tank is satisfactory. The inlet well, scum board and sludge scraper are provided as shown.

   Section (circular, radial flow)
      inlet well        weir/launder
   ->  |__|~~~~~~~~~~~~~~~~~~~~~|_
       |  \                  /  | H = 3.0 m
       |   \______    ______/   | + sludge zone
       +----------\__/----------+
                  sludge hopper -> sludge pipe
        <---------- D = 19.5 m ---------->

Answer: 1 circular sedimentation tank of diameter 19.5 m, side water depth 3.0 m (total depth about 4.0 m), detention 2.0 h, surface loading 34.8 m³/m²/day.