Three numbers are quoted for every settling basin, and you now have two of them. Detention time uses the tank's volume and says how long the water stays. The surface overflow rate uses its plan area and says how fast the water rises. The third uses neither. It looks at the very last thing the water does, which is spill over a weir into the effluent launder.

$W_L = \dfrac{Q}{L_w}$, read aloud W-L equals Q over L-w. $W_L$ is the weir loading rate, the quantity you are solving for, in cubic metres per day per metre of weir, written m³/(d·m). $Q$ is the flow leaving over the weir in m³/d. $L_w$ is the total length of weir crest in service, in metres. Rearranged, $L_w = \dfrac{Q}{W_L}$ sizes the weir for a limit, and $Q = W_L\,L_w$ gives the flow ceiling of a weir that already exists.

Why it matters. All the flow in the tank converges on the crest, and water speeds up as it converges. Put too much flow over too little weir and the upward velocity near the lip is enough to lift settled floc and carry it out with the effluent. The usual ceilings: about 250 m³/(d·m) for a water plant's settling basin, and nearer 125 to 185 for a secondary clarifier, because activated sludge floc is far easier to drag over a lip than alum floc.

Count the weir honestly, because that is where this number usually goes wrong. An inboard launder collects over both faces, so its length counts twice. A peripheral weir on a round tank is one circumference, $\pi D$, with $\pi = 3.14159$ and never 3. A V-notch plate is measured along the crest line and not around the notches. Weir that is out of service or drowned is not weir length at all.

Check it at peak flow, not at the average. A plant that meets 250 on its annual average and doubles every morning is loaded at 500 for the hours that matter, and those are the hours the solids go over.

And hold the figure a little loosely. It is an average along the whole crest. A weir only a few millimetres out of level sends most of the flow over its low end, which multiplies the real local loading several times while this calculation goes on reporting the average. A level crest is worth more than a long one.