Start with the membrane being protected
A pressurised UF system treating make-up water and an MBR filtering mixed liquor receive different feeds. Identify the module, filtration direction and types of debris that can reach it. For inside-out elements, assess fibre inlet passages; for outside-in arrangements, consider accumulation within the fibre bundle and the removal of debris during cleaning.
For an MBR, locate the screening stage on the complete plant diagram. Identify streams entering downstream of that barrier: returns, drain-down flows, auxiliary inputs and equipment wash water. A correctly selected main-inlet screen protects that route. The overall specification must address every path capable of introducing unsuitable debris into the membrane tank. Follow each stream to its actual point of entry.
Define aperture, geometry and capture performance
Aperture specifies a passage dimension, but its shape also matters. Mesh, perforated plate and slots present different geometries to long or flexible material. Describe hair, textile fibres, plastics, grit and other relevant debris in the actual water. Ask the screen supplier to document how the proposed equipment handles that combination, including variation in the incoming load.
Distinguish the screening element's rating from the maximum incoming particle size allowed by the membrane module. These related values require a coordinated selection. State units clearly: 0.4 mm equals 400 µm. A specification that switches between millimetres and microns without identifying them can result in protection very different from what the designer intended. Make both the requirement and the offered rating explicit.
| Order requirement | Information to define |
|---|---|
| Aperture and geometry | Passage dimension, shape and element tolerances |
| Debris to be removed | Nature, shape and variation of incoming material |
| Hydraulic duty | Peak flow with stated headloss and fouling conditions |
| Cleaning | Mechanism, wash-water demand, duration and capacity during cleaning |
| Isolation and bypass | Water route during failure, cleaning and maintenance |
Check capacity under the demanding operating condition
Size for the flow actually passing through the screen and the expected debris load. A clean-water nominal capacity needs to be accompanied by the conditions under which capacity is guaranteed. For a channel screen, check water levels and overflow; for a pressurised strainer, check inlet and outlet pressures, headloss and the upstream supply capability.
Then assess cleaning and maintenance. When one unit is unavailable, record the remaining capacity and the destination of flow above that capacity. Depending on the project, another unit, flow regulation or storage can address the shortfall. The agreed arrangement should maintain the specified membrane protection while providing a defined response to excess incoming water. Include this condition in the operating description.
Integrate cleaning, waste removal and control signals
Captured debris needs a route out of the process. Define removal, washing where applicable and the waste destination, together with required utilities. Check whether wash water returns part of the debris to a location downstream of the screening barrier. The drawing should make both paths clear: screened water towards the membranes and separated material towards waste handling.
Include signals that make protection observable: differential level or pressure, availability, cleaning failure and relevant equipment positions. Assign an operating response to each alarm. If a screen clogs, the operator needs a defined action that protects membranes and manages incoming flow. Provide access for inspection and debris removal, and identify which activities require the screen to be isolated.
Verify the installation before feeding membranes
Inspect the screening element, perimeter seals, closures and potential paths around the barrier. Confirm installation direction and test cleaning with the actual utility supplies. Gaps or an incorrectly seated panel can create an unintended route separate from the specified aperture. Record these checks and the initial condition of the equipment before introducing membrane feed.
During commissioning, observe flow, levels or pressures and removed debris under different incoming conditions. Compare operation against the requested duty and verify alarm responses. For a useful proposal, provide the membrane model, average and peak flows, debris characteristics, available space and existing screen details. Photographs of captured material help describe the problem more precisely than a suspended-solids concentration alone.
Common questions
Can pressurised UF and MBR use the same screen opening?
Selection depends on the feed and the product. A make-up-water prescreen and a mixed-liquor screen can require different apertures, geometries and cleaning systems.
Does an automatic bypass keep the membranes protected?
Only when its flow path includes the required protection. Define what happens when the main screen is unavailable and verify the complete water route.
Is the screen's nominal flow enough for a specification?
Add headloss or water levels, expected debris, peak flow, cleaning and capacity during maintenance. The supplier can then offer a verifiable operating duty.