Oxidation pond for Terai town, 12,000

Assumptions

Terai has a warm climate, so a surface organic loading of $\lambda_s=300$ kg BOD/ha/day (usual range 200-400) is adopted. Length : width $=2:1$. Operational depth $=1.1$ m (given). Free board 0.5 m, embankment slopes 1 : 2 (inside) and 1 : 1.5 (outside), crest width 3 m.

Step 1: Flow and load

$$\begin{aligned} Q &= 12000\times100/1000 = 1200\ \text{m}^3/\text{day}\\ \text{BOD load} &= 1200\times250/1000 = 300\ \text{kg/day} \end{aligned}$$

Step 2: Area, volume and detention time

$$\begin{aligned} A &= \frac{300}{300}=1.00\ \text{ha}=10000\ \text{m}^2\\ V &= A\times d = 10000\times1.1=11000\ \text{m}^3\\ t &= \frac{V}{Q}=\frac{11000}{1200}=9.17\ \text{days}\ (\text{within 5-30 days: OK}) \end{aligned}$$

Step 3: Dimensions

$$W=\sqrt{\frac{A}{2}}=\sqrt{\frac{10000}{2}}=70.7\ \text{m (adopt 71 m)},\qquad L=2W=141.4\ \text{m (adopt 142 m)}$$

Pond water surface (at mean depth) $=71\ \text{m}\times142\ \text{m}=10082$ m². Total depth $=1.1+0.5=1.6$ m. If possible, provide two ponds in parallel (each about half the area), so that one can be desludged.

Sketch (plan)

 +--------------------------------------------+
 |  buffer zone (trees), 50 m (assumed)       |
 |   +------------------------------------+   |
 |   | crest 3 m                          |   |
 |   |  +------------------------------+  |   |
 |   |  |     POND 71 m x 142 m        |  |   |
 |   |  | inlet ->  d = 1.1 m -> outlet|  |   |
 |   |  +------------------------------+  |   |
 |   +------------------------------------+   |
 +--------------------------------------------+
  Section: crest 3m, slope 1:2, FB 0.5 m, d 1.1 m

Answer: detention time $=9.17$ days; pond area $=10000$ m² ($\approx71\ \text{m}\times142\ \text{m}$ at mean depth), depth 1.1 m, with a buffer zone around the embankment.