TSS, total solids, COD and TDS: what each tells a membrane designer

Four measurements, four design questions

TSS helps estimate particle load and retentate handling. TS includes material left after drying under the method used; it is not interchangeable with TSS. COD expresses chemical oxidisability, not the concentration of one compound. Comparing total and soluble COD indicates how much is associated with solids and how much could pass a UF membrane.

TDS and conductivity indicate dissolved salts and osmotic load relevant to RO. UF may clarify water while leaving salts, ammonium and much soluble COD in the permeate.

Use pairs, not a single number

TSS versus TS distinguishes a particle-rich feed from one dominated by salts or dissolved material. Total versus soluble COD distinguishes clarification from treatment of dissolved compounds. Identical COD values can represent chemicals with very different membrane behaviour.

For meaningful comparison, record method, unit, date, sampling point, temperature and production state.

• Add oils, fibres, particle size and viscosity for loaded streams.
• Measure pH, alkalinity and scaling species ahead of RO.
• Characterise both permeate and concentrate in a representative trial.

An interpretation example

If UF lowers feed TSS but the permeate retains high soluble COD, the next question is not automatically whether to add UF area. Identify the soluble fraction and the final water use before selecting another stage.

Keep analytical units and methods beside every value. For example, a concentration in mg/L cannot be compared directly with a loading in kg/day without flow and time. This simple discipline prevents a treatment train being chosen from numbers that describe different samples or operating states.