Solids Loading Rate (Clarifier)
Calculate the solids loading rate (SLR) of a secondary clarifier, SLR = Q × X ÷ A, multiplying the flow (m³/day) by the mixed-liquor solids concentration (mg/L, converted to kg/m³) and dividing by the surface area (m²). The result, in kg/(m²·day), is a design criterion independent of the surface overflow (hydraulic) rate: an activated-sludge secondary clarifier must satisfy both the hydraulic limit and the solids loading limit, since it receives a concentrated mixed liquor that must thicken at the bottom. Excessive solids loading causes sludge to wash out with the effluent. Enter the flow, the solids concentration and the clarifier area.
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Solids loading rate (SLR)
The secondary clarifier of an activated sludge system has a double duty: to clarify the effluent (deliver a clean supernatant) and to thicken the sludge at the bottom for return to the process. That is why it has to be checked against two independent criteria. The first is the surface overflow rate (hydraulic), Q/A, tied to clarification. The second is the solids loading rate (SLR), SLR = Q × X ÷ A, which multiplies the flow by the mixed liquor suspended solids concentration (X) and divides by the surface area — in other words, the mass of solids reaching each m² of clarifier per day, in kg/(m²·day). This criterion exists because, unlike a primary clarifier (which handles almost clean water), the secondary one receives a mixed liquor loaded with biomass that has to thicken at the bottom; if the solids load exceeds the thickening capacity, the sludge blanket rises and escapes over the weir, degrading the effluent and washing biomass out of the system. Correct sizing adopts the area that satisfies the hydraulic limit and the solids limit at the same time — usually the more restrictive of the two governs. Enter the flow, the solids concentration and the clarifier area.
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