MLSS (g/L) = SV [mL/L]/SVI [mL/g]    or     SVI [mL/g] = SV30 [mL/L]/MLSS (g/L)
Where:<br />
SVI = sludge volume index (mL/g)<br />
SV30 = Volume of settled solids per 1 litre after 30 minutes
In fact SVI is a calculation from two analyses : SV30 and MLSS.
0=(Q+Q)(X)-(QX+QX')
Where: <br />
Q = wastewater flow rate (m/d)<br />
Q = return sludge flow rate (m/d)<br />
X' = MLSS (kg/m)<br />
X' = return sludge concentration (kg/m)<br />
Q = sludge surplus (or excess) flow rate (m/d)<br />
This equilibrium formula for settling tanks is mostly calculated for the initial flows in m3/h.
This formula describes that the incoming amount of MLSS in a settler should be equivalent to the outcoming amount of MLSS via the return sludge flow. This equilibrium is only valid if the effluent water contains a low concentration in suspended solids. In normal conditions the excess amount is very low in comparison to the return amount, and for that reason many times the excess amount is neglected.
In that case the formula will be : (Q+Q)(X) = (QX)