Protecting membranes starts with defining each stage’s role
Silica sand used before reverse osmosis is not the material that desalinates water. It participates in a granular filtration stage when that stage belongs to the chosen treatment chain. Its usefulness must be assessed through the water quality produced, operating stability and compatibility with the filter, then with equipment downstream.
ONEE’s press release of 20 August 2025 places conveyance works within the security of Greater Agadir’s water supply and mentions connections with desalinated water 1. It confirms the regional relevance of these hydraulic chains. It does not establish a particular plant’s pretreatment configuration. This dossier therefore explains a qualification method without attributing a sand bed to an undocumented installation.
The proposed approach connects four records: raw water, equipment, material and operating results. Sand described only by silica content leaves those connections unresolved. Conversely, documented pretreatment explains why a fraction is selected and which observations should lead to a review of that choice.
1. Write down the filter’s intended function
DuPont’s FilmTec manual presents media filtration among the methods of reducing particulate matter before reverse osmosis 2. Useful selection starts by identifying what this stage must handle and what other stages address.
Write a verifiable statement: “this filter must produce water meeting these criteria, at these flows, using these cleaning modes”. This formulation requires a precise measurement point. An outlet requirement is not always identical to the requirement immediately before the membranes, after storage, injection and cartridges.
Do not confuse filtration, scaling control and chemical protection. Water can contain few particles while retaining dissolved substances important to the process. The assessment must therefore distinguish risks rather than summarise all water preparation through a single quality value.
2. Characterise raw water over its actual operating envelope
Define the source, possible changes and events affecting supply. An annual average does not necessarily describe difficult periods. Trial records should identify the waters actually encountered and conditions absent from the trial.
Specify sampling points and the parameters required for treatment engineering. Include methods, units, dates and preservation conditions. Chemical and particulate analyses answer different questions; interpreting them should guide planned operations rather than produce a long list without an associated decision.
Connect results to a flow and operating condition where relevant. Water during startup, after an intake change or following an intervention is not automatically equivalent to stable operation. This traceability identifies conditions under which filtration succeeds or becomes insufficient.
3. Use SDI within its actual scope
The official ASTM D4189 notice explains that SDI indicates a water’s particulate behaviour and can help assess treatment. It also states that it is not an absolute quantity measurement and that temperature and test membrane can affect comparisons 3. A value must therefore retain its measurement conditions.
In qualification records, include the method, duration associated with the index, temperature, sampling point and consumable identification. A line stating “SDI compliant” without a value or conditions creates a weak history. Series obtained using different methods should be examined before combining them.
The acceptance threshold comes from the particular system and membranes. Do not turn an isolated example value into a universal limit. Records should show who established the criterion, for which configuration and what actions follow an exceedance. Other parameters required to protect the installation must also be monitored.
4. Position sand within a complete chain
The Bureau of Reclamation describes a Yuma pilot comprising clarification, dual-media gravity filters and reverse osmosis 4. This institutional example shows combined operations; it is not a design to copy without study for every Souss-Massa water supply.
Draw your chain with normal and cleaning flows. Identify injection points, tanks, cartridges, protection devices and measurement points. Show bypasses and water paths usable during maintenance. Selecting a suitable material does not correct a membrane supply that bypasses the qualified stage.
For each component, record function, outlet criteria and the procedure in case of failure. This map avoids attributing contamination introduced after the filter to the sand. It also shows where an additional sample is needed to locate deterioration.
5. Connect grading to filter conditions
The media’s grading curve must be examined together with filter design. Identify specified fractions, any proportions and the measurement method. A commercial designation does not provide the full distribution, nor replace compatibility with drainage and cleaning.
Request results for an identified batch. Check fines, material outside the interval and preparation conditions before sieving. If a result is expressed on dry material, retain that information for batch comparisons. A poorly prepared sample can suggest differences that do not represent the material.
Also record loading depth, mass and volume. These are not interchangeable without additional data. For grading concepts, consult our guide to silica sand particle size distribution. The final choice concerns the complete bed rather than a grain percentage detached from its equipment.
6. Treat backwashing as a qualification condition
A trial that filters correctly but does not demonstrate bed cleaning leaves an essential question unanswered. Document available washing flow, discharge route, sequences and equipment restrictions. An executed automatic command does not prove that actual flow matched the setting.
Measure the hydraulic reference after commissioning, then compare conditions before and after cleaning at comparable flow. Record interruptions, valve changes and return to filtration. Identify how restart water is routed until outlet criteria are restored.
A fraction change requires reviewing this compatibility. Changing material alone while retaining an unevaluated procedure can turn a bed acceptable when new into a recurring source of difficulty. Qualification must therefore cover at least the intended operating modes and identify those not yet observed.
7. Observe the outlet throughout the cycle
A sample midway through stable operation does not by itself explain restart after washing or the end of a cycle. Place observations where they can distinguish behaviour: startup, established operation, increasing pressure loss and recommissioning.
Combine outlet quality with hydraulics. Falling pressure does not automatically prove improvement; it can accompany a changed flow path. Visually clear water does not demonstrate every requirement before reverse osmosis. Decisions must use the parameters defined in the records.
Keep individual data and events rather than only daily averages. For an alarm, record duration and action taken. A brief exceedance on restart and continuous deterioration can require different investigations even if their final average is similar.
8. Read cartridges as a downstream indicator
Monitoring downstream protective devices helps connect filtered water quality with what actually reaches reverse osmosis. Identify cartridges, position, installation date, flows and pressures. Service duration without the treated volume is difficult to compare between periods.
During an investigation, retain deposit location and appearance, then select relevant analyses. Material present can originate from pretreatment, a pipe or a downstream operation. Chronological comparison is more useful than immediately blaming sand.
Do not treat frequent consumable replacement as sufficient proof that the system has been corrected. Understand the mechanism loading it and check what happens after intervention. If several changes occur together, the conclusion should concern the modified assembly rather than an isolated component.
9. Prepare a pilot answering a decision
Define the question before testing: replace like for like, change a fraction, add a stage or compare chains. Write down criteria, comparison conditions and information that will remain missing if tested water does not cover the intended envelope.
The Yuma pilot is presented as a research installation connected with the main process design 4. The methodological lesson is to document similarities and differences between the trial and intended system. The word “pilot” alone does not guarantee transferable results.
Keep identified reference material and a candidate batch. Hold constant the factors that must remain constant. When treatment chemistry changes with the sand, describe a configuration comparison. Record costs and incidents needed for the decision without turning brief technical success into a general promise of membrane life.
10. Establish a measured water balance
Filtration has useful flows and cleaning flows. Establish the balance over a defined period, identifying measurement boundaries. Nominal flow multiplied by duration replaces an observed volume only when the constant-flow assumption is explicitly acceptable.
Separate filtered water sent downstream, washing, rinsing and other identified losses. Any proposed water return belongs to the process design: quality, location and possible treatment must be studied. Coming from the same filter is not enough to justify returning that water to the supply.
Compare configurations on a common basis: volume actually treated, required quality and operating period. A system saving water but no longer meeting downstream criteria does not provide the same function. Conversely, fewer washes can be relevant when demonstrated requirements are retained under the conditions investigated.
11. Calculate hydraulic quantities using correct boundaries
Superficial hydraulic loading is written v = Q / A, where Q is the flow through the filter and A its useful area. With Q in cubic metres per hour and A in square metres, v is expressed in metres per hour. This calculation organises comparison; it does not establish an acceptable velocity in place of the designer.
For parallel filters, use the area actually in service. Dividing flow by the total installed area while one filter is stopped produces an artificially favourable value. Also check distribution: parallel branches do not necessarily receive identical flows. An average for the filter bank can conceal different loading on each bed.
The same principle applies during washing. Flow in the common pipe describes the filter being cleaned only when other water paths are known. Record temperature conditions and configuration, then compare the result with the manual. Avoid turning a velocity documented for another media into a setting for your sand without a compatibility assessment.
To compare periods, calculate the volume actually filtered from available records. If flow varies, a single instantaneous value does not represent the whole period. Keep measured values, calculated values and assumptions separate. A sheet making this distinction remains useful when an operator or engineer revisits the records several months later.
Finally, present flow, pressure loss and quality together. One configuration may maintain quality at a flow too low for the intended use. Another may achieve flow while exceeding downstream criteria. Qualification concerns these simultaneous requirements; it does not mean separately selecting the best result from each trial.
Record the status of instruments used in these calculations. If a flowmeter or pressure gauge is replaced, mark the date in the series and check reading comparability. Precision displayed by a calculation does not correct uncertain input data. For a decision close to a limit, that uncertainty deserves examination with those responsible for qualification.
12. Receive and store an identified material
Connect each delivery to its batch, results and packaging. Protect the material against mixing with earlier materials. A fraction compliant at the supplier’s outlet can be contaminated during storage or handling; receiving records should therefore describe observed conditions.
Plan representative sampling before use. Retain a reference sample if it belongs to the selected control practice. Results must connect with the charge introduced into a specific filter. Without that link, a later anomaly cannot be properly compared with the received material’s characteristics.
Finally, record preparation and commissioning required by the equipment. Records should show the configuration achieved rather than just the number of bags delivered. This tracking simplifies discussions between purchaser, operator, integrator and material supplier.
13. Decide using observations that can be attributed
Yuma’s official pretreatment presentation distinguishes upstream operations, dual-media filtration and subsequent stages 5. This organisation reminds us that membrane protection depends on a chain and each component must be assessed according to its function.
Summarise your decision on a sheet: configuration, water range investigated, results, incidents, satisfied criteria and outstanding actions. Separate observation from explanation. “Outlet quality improved after three changes” describes a result; it does not yet identify which change explains the improvement.
A material is useful when it matches a traceable specification and demonstrated operation. Our silica sand page complements this dossier by presenting the mineral family’s fractions. Before reverse osmosis, the useful choice remains the one whose position in the chain, cleaning conditions and downstream criteria are clearly established.