Wastewater process calculators.
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SRT Calculator (Solids Retention Time)
Solids retention time (SRT), also called sludge age or MCRT, estimates how many days solids stay in the aeration basin. It compares the solids inventory with the solids leaving each day through wasting and the effluent.
SRT (days) = (V × MLSS) ÷ (Q_WAS × TSS_WAS + Q_eff × TSS_eff)Open calculator F/M Ratio Calculator (Food to Microorganism)
The F/M ratio compares the organic load coming into the plant (food) with the volatile solids inventory in the aeration basin (microorganisms).
F/M (lb BOD/lb MLVSS/day) = (Q × BOD) ÷ (V × MLVSS)Open calculator HRT Calculator (Hydraulic Retention Time)
Nominal hydraulic retention time (HRT) estimates how long wastewater stays in a defined treatment volume, based on the volume and the flow through it.
HRT (hours) = V / Q × 24Open calculator RAS Calculator (Return Activated Sludge Flow)
Return activated sludge (RAS) carries settled solids from the secondary clarifier back to the aeration basin. This simplified steady-state mass balance estimates the RAS flow needed to hold your MLSS.
Q_RAS = Q × MLSS ÷ (RAS TSS − MLSS) RAS (%) = Q_RAS ÷ Q × 100Open calculator SVI Calculator (Sludge Volume Index)
Sludge volume index (SVI) shows how well activated sludge settles and compacts. It is the volume in milliliters that one gram of solids takes up after 30 minutes of settling.
SVI (mL/g) = SV30 × 1,000 ÷ MLSSOpen calculator BOD Removal Efficiency Calculator
Removal efficiency compares the average influent and effluent concentrations for the parameter and reporting period your permit specifies.
Removal (%) = (C influent,avg − C effluent,avg) ÷ C influent,avg × 100Open calculator Stop running this math by hand
ComplyFlo for wastewater plants can calculate and trend SRT, F/M, HRT, RAS, SVI and removal efficiency from your daily readings, next to your plant dashboard and DMR-ready reports.
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TPDES Permit Compliance Checklist
Review permit-specific DMR submissions, sampling, SSO reporting, renewal milestones, and applicable pretreatment responsibilities.
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