Calculate the dry-solids feed
For sludge volume flow Q in m³/h, slurry density ρ in kg/m³ and dry-solids mass fraction f, the dry-solids flow is Q × ρ × f ÷ 1000 tonnes/h. Express a measured 2% mass fraction as 0.02, not 2. If solids are already reported in kg/m³, multiply that concentration by volume flow and divide by 1000.
Use a density appropriate to the material. Treating every slurry as exactly 1000 kg/m³ introduces an assumption; document and check it when accuracy matters. Solids percentage and mass concentration are not interchangeable without density.
Calculate commercial and active dose separately
Commercial dose in kg/t dry solids equals commercial product mass flow in kg/h divided by dry-solids flow in tonnes/h. Active dose equals that commercial dose multiplied by the product’s active mass fraction.
For illustration only, 20 m³/h of sludge at 1000 kg/m³ and 2% dry solids carries 0.4 tonnes/h dry solids. A commercial product feed of 2 kg/h is therefore 5 kg/t dry solids. At a hypothetical active fraction of 0.80, the active dose is 4 kg/t. This example demonstrates arithmetic, not a recommended plant dose.
Convert solution flow without double counting
If the prepared solution concentration is expressed as kg commercial product per litre of solution, multiply it by solution flow in L/h to obtain commercial product mass flow. If concentration is expressed as a mass fraction, include solution density. Know which convention the make-down system uses.
Secondary dilution increases the volume delivered to the injection point but does not add polymer. Applying the original stock concentration to the final diluted flow would overstate consumption.
Check the numbers against actual usage
Compare calculated daily consumption with weighed powder or measured liquid inventory over a meaningful period. Account for tank level changes, flushes, rejected batches and intermittent production. A pump calibration from clean water may not represent the operating polymer solution.
Use synchronized readings. A solids sample from the morning and a flow reading after a major feed change may give a mathematically correct calculation for a condition that never existed.
Use dose to interpret performance
A fall in feed solids at constant solution flow raises dose per dry tonne. The resulting sticky cake or higher cost can be mistaken for a product problem. Conversely, rising solids load can cause underconditioning without any change in pump speed.
Record the useful range established by a representative trial, not a universal number. Carry the mass basis into the purchase comparison and review wet-cake causes alongside the dosing record.