Membrane manufactures typically specify that RO feedwater should have a SDI (Silt Density Index) of < 5.0 and a turbidity value <0.20 NTU. Low SDI and turbidity values are believed to mean that the potential for colloidal fouling is reduced. With proper application, design, and operation, a multi media filter (MMF) can achieve these goals.
Turbidity
When a MMF is put on-line following a backwash, the turbidity value can sometimes be similar to the feedwater turbidity. Over a brief period of time, the turbidity should drop and stabilize to an acceptable level. As the system operates, the turbidity rises. When the turbidity increases by 10%, a backwash should be initiated. If the MMF is not backwashed, the turbidity in the effluent may eventually exceed the feedwater turbidity.
Pressure Drop (delta–P)
It is important to monitor the delta-P of the MMF. Increases in delta-P signal the filter is working and removing particles. The MMF should be backwashed at or before the delta-P reaches ten (10).
Delta-P and Turbidity are Independent
It is important to understand that delta-P and turbidity breakthrough can be independent of each other. The MMF should be backwashed based on whichever occurs first.
Turbidity vs. Dosage
When dosing a coagulant, high turbidity does not always indicate an inadequate dosage. It is important to understand that both underdosing and overdosing can cause turbidity to rise. Additionally, significant overdosing can cause the effluent turbidity to exceed the feedwater NTU. The optimal dosing range at the bottom of the curve usually has a spread of 1 to 2 ppm.
Important Operating Tips
- Frequently test MMF feed and effluent turbidity.
- Use a drawdown tube to verify coagulant dosages.
- Use reliable pressure gauges to measure MMF delta-P.
- Initiate MMF backwash based on turbidity, delta-P or time.
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