Pre-Treatment
- 1. General
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Pretreatment is critical in MBR plant design to ensure adequate protection of membranes from physical damage.
All MBR systems require fine screening and grit removal to prevent membrane damage from abrasive particles common in influent sewage.
Removal of fibrous or stringy material is also important. This material can become entangled and wrap around
the hollow fibers or stuck within the gaps between membrane flat plates. This can plug the membrane
scour aeration systems leading to problems with operation of and potential damage to the system.
If the influent oil and grease concentration exceeds 100 mg/L, or if historic problems with fats, oils and grease (FOG)
exist within the community, oil and grease removal may also be necessary to prevent membranes from being coated.
- 2. Bar Screens
- A mechanically cleaned bar screen with a manually cleaned emergency bypass bar screen or self-cleaning screening device should be provided.
- 3. Grit Removal
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Permanent grit removal facilities meeting the requirements should be provided for
combined sewer systems and sewer systems receiving substantial amount of grit.
If a plant serving a separate sewer system is designed without grit facilities,
consideration shall be given to possible damaging effects on pump, comminutors,
other preceding equipment, and the need for additional storage capacity in treatment units where grit is likely to accumulate.
- 4. Primary Clarification
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Primary clarification must be considered for all plants. Proposals that do not include primary
clarification should justify why primary clarifiers are not practical due to facility
size constraints or limited benefit in comparison to the cost of handling primary solids.
- 6. Fine screens
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6-1 Location
Fine screens should be located downstream of primary clarifiers.6-2 Design
Coarse screens followed by fine screens may be used to minimize the complications of
fine screening due to high flow restrictions and increased solid waste handling
Fine screens should be rotary drum or traveling band screen with either perforated plate or wire mesh
Fine screens should have an opening size between 0.25 mm ? 2.0 mm for hollow-fiber
and tubular membranes, or 2.0 ? 3.0 mm screening for flat plate membranes.6-3 Number of Units
A minimum of two fine screens should be provided, with each unit being capable of independent operation.
Capacity should be provided to treat peak design flows with one unit out of service.6-4 Electrical Fixtures and Controls
Electrical fixtures and screening areas where hazardous gases may accumulate should meet the requirements of the National Electric Code.6-5 Servicing
Hosing equipment should be provided to facilitate cleaning.
Provisions should be made for isolating or removing units from location for servicing.