Three signals supporting three decisions
The purchasing question is which decision each indicator needs to support. TSS helps estimate the load that separation and washing must handle; turbidity helps detect rapid changes; SDI provides information about a water's tendency to block a test filter. The FilmTec manual describes SDI using a 0.45 µm membrane at constant pressure. EPA Method 160.2 determines residue retained on a glass fibre filter and dried to constant weight. These are different tests, so there is no universal conversion between their results.
| Indicator | Reported quantity | Decision it helps support |
|---|---|---|
| TSS | mg/L of residue under the selected method | Solids loading and management of wash streams |
| Turbidity | NTU or the instrument's specified unit | Alarms and pretreatment stability monitoring |
| SDI | Index with test duration stated | Assessing particulate fouling risk before RO |
How to interpret results that disagree
Low turbidity with an unfavourable SDI warrants investigation of fine particles and colloids, alongside checks on how the test was performed. High TSS with a modest optical increase can indicate that the local relationship between mass and signal has changed. A correlation established for one water should not automatically be applied to another season, production line or coagulation chemistry.
These observations guide investigation; they do not identify a contaminant by themselves. If SDI deteriorates after a tank, also investigate particles introduced by corrosion, carryover or blending with another stream. If only online turbidity changes, compare it with a sample and check air, sensor cleanliness and sampling conditions before attributing the change to a membrane.
A protocol that makes proposals comparable
Define one sampling point in the source water and another immediately before RO. Add points after individual treatment steps when you need to locate deterioration. Collect samples associated with the same event and record flow, temperature, chemical dosing, filter status and cycle phase. A sample taken during a transient and one taken under stable conditions do not describe the same operating state.
For TSS, request the method, quantification limit and treatment of results below that limit. For SDI, retain pressure, test duration and measured collection times; a five-minute result must not be labelled SDI15. For turbidity, document the instrument, unit and verification. Include campaigns during source changes and relevant disturbances, as well as average conditions. Variability and peak duration are design information.
Turning diagnosis into a specification
FilmTec specifies turbidity below 1 NTU and recommends SDI15 below 3 to minimise fouling. These manufacturer references need checking against the selected element and project conditions; meeting both values does not guarantee freedom from organic or biological fouling, or scaling.
A specification can require quality before RO within a defined feed envelope, the capacity to handle solids loading and an agreed response to deviations. Define what happens during start-up, washing or failures: storage, recirculation or diversion as appropriate to the project. UF can then be assessed through delivered quality, integrity and auxiliary streams, alongside individual readings. Ask the integrator to explain which indicator triggers each action and what evidence is needed to authorise return to service.