A clarifier is not sized by its volume. It is sized by its surface, and there is a piece of physics behind that worth understanding once and never forgetting.

The surface overflow rate is $v_o = \dfrac{Q}{A}$ — v-oh equals Q over A. $v_o$ is the overflow rate in metres per day, $Q$ is the flow in m³/d, and $A$ is the plan area of the settling zone in m², the water surface seen from above. Divide m³/d by m² and what survives is m/d: a velocity. That is the point of it. The overflow rate is the speed at which water rises through the tank, and any particle that cannot fall faster than $v_o$ is carried out over the weir. Depth appears nowhere — a deeper tank buys detention time and somewhere to keep sludge, but it does not capture one extra particle.

The second test is the solids loading rate, $\text{SLR} = \dfrac{(Q + Q_r)\,X}{A}$, in kilograms of solids per square metre per day. $Q_r$ is the return activated sludge flow in m³/d and $X$ is the mixed liquor suspended solids in mg/L. Note the bracket: a secondary clarifier receives the plant flow and the return, and the return is routinely a third to a half of the plant flow again. Leave it out and the clarifier is under-rated by exactly that much.

The unit bridge is the sanitary identity worth memorising: 1 mg/L = 1 g/m³, so $X$ divided by a thousand is kilograms per cubic metre. Flow in m³/d times kg/m³ is kg/d, and spread over an area it is kg/(m²·d).

Why two tests? Because they fail in different weather. The overflow rate is about clarity — it decides whether fine solids escape. The solids loading is about thickening — it decides whether the blanket can compact fast enough to be drawn out of the bottom. A clarifier can be hydraulically comfortable and still fail on solids the day the MLSS is high or the sludge is bulking, and that is precisely the day it does. Typical design: 30 to 50 m/d overflow on primaries, 16 to 32 m/d on secondaries, with solids loading held under about 150 kg/(m²·d) at peak.