Sedimentation tank and oxidation pond design

Data

$Q = 10000\times 100/1000 = 1000$ m³/day; raw BOD $=250$ mg/l; required effluent BOD $<30$ mg/l.

Part A: Sedimentation tank (circular)

Adopt detention $t=2.5$ h and side water depth $H=2.5$ m.

$$\begin{aligned} V &= \frac{1000}{24}\times 2.5 = 104.2\ \text{m}^3\\ A &= \frac{V}{H} = \frac{104.2}{2.5} = 41.7\ \text{m}^2\\ D &= \sqrt{\frac{4A}{\pi}} = 7.28\ \text{m}\ \Rightarrow\ \text{adopt } D=7.5\ \text{m}\\ \text{Surface loading} &= \frac{1000}{44.2} = 22.6\ \text{m}^3/\text{m}^2\text{/day} \end{aligned}$$

The surface loading is a little below the usual 25-40 m³/m²/day. A lower overflow rate only means better settling, so the tank is on the safe side and acceptable. (If a loading within the range is wanted, adopt $t = 2$ h, giving $D \approx 6.5$ m and about 30 m³/m²/day.)

Provide total depth $= 2.5+0.5+0.5=3.5$ m. Primary sedimentation removes about 30% of the BOD, so the BOD leaving it is $250\times0.7=175$ mg/l.

Part B: Oxidation pond (facultative)

Adopt a surface organic loading $\lambda_s = 250$ kg BOD/ha/day (suitable for a warm climate; usual range 200-400) and depth 1.2 m.

$$\begin{aligned} \text{BOD load} &= Q\times BOD = 1000\times 175\times10^{-3} = 175.0\ \text{kg/day}\\ \text{Area} &= \frac{175.0}{250} = 0.70\ \text{ha} = 7000\ \text{m}^2\\ \text{Volume} &= 7000\times 1.2 = 8400\ \text{m}^3\\ \text{Detention time} &= \frac{V}{Q} = \frac{8400}{1000} = 8.4\ \text{days (usual 5-10 days)} \end{aligned}$$

Take length : width $=2:1$: $W=\sqrt{7000/2}=59.2$ m, $L=2W=118.3$ m. Provide 2 ponds in parallel (each half this area) so that one can be desludged. Embankment side slope 1 : 2 (inside) to 1 : 1.5 (outside), free board 0.5 m, top width 2.5-3 m.

Efficiency check

Required BOD removal in the pond $= \dfrac{175-30}{175}\times100 = 82.9\%$, which is within the 80-90% removal normally achieved at this loading and detention time, so the effluent will be below 30 mg/l.

Answer: sedimentation tank D = 7.5 m, depth 2.5 m (+1 m); oxidation pond area 7000 m² (about 0.70 ha), depth 1.2 m, size about 59.2 m x 118.3 m, detention 8.4 days.