Identify what causes the COD
Total COD combines material with very different behaviour. Solids-associated COD may be removed before RO by physical separation; soluble material may pass UF and reach the dense membrane. There is no COD number that guarantees RO feasibility without composition and a defined method.
Assess TSS, turbidity, total/soluble COD, oils, biology, salinity, pH, temperature and scaling species. Cleaning chemistry and oxidants must also be compatible with the offered membrane.
- Compare several production states, including peaks.
- Measure required water quality rather than designing from COD alone.
- Separate organic, colloidal, biological and mineral fouling risks.
Design the train, not just RO
A train may combine equalisation, biological or chemical treatment and solids filtration ahead of RO, depending on the water. UF blocks particles and colloids but does not automatically make high-soluble-COD feed suitable for RO.
RO recovery concentrates salts and pollutants into reject. Osmotic pressure, scaling, fouling or concentrate disposal can limit recovery.
| Question | Test/data |
|---|---|
| What reaches RO? | Analysis after pretreatment |
| Will it foul? | Flux, pressure and cleaning trial |
| How much water is recovered? | Salt and scaling balance |
| What happens to reject? | Flow, composition and management route |
Procurement criterion
Ask proposals to state admissible feed quality, cleaning conditions, net production and reject destination. A laboratory or pilot trial on real wastewater should demonstrate permeate quality and pressure/flux trends at the proposed duty.
Ask the membrane supplier about residual oxidants, required dechlorination and allowed cleaning agents for the exact RO element. These constraints are separate from COD concentration. Monitor the feed after pretreatment because a good raw-water treatment average can still conceal short breakthrough events.
Common questions
Does upstream UF remove all COD?
No. It retains particle- and colloid-associated fractions according to cut-off; soluble COD may remain.