When water contains suspended solids, floating material or debris, it is natural to ask whether an online analyser needs a filter. The more useful question comes first: what water does the site need the analyser to represent? A filter can help deliver a cleaner sample to an analyser, but it may also change the fraction of water presented for measurement. That makes filtration a measurement-design decision, not a generic add-on.
This matters in current UK operations. The Environment Agency’s 2025-2030 Water Industry National Environment Programme places major emphasis on environmental delivery, while its wastewater guidance retains specific permit and accredited-laboratory requirements. More operational data can be useful, but the installed sample system must be clear about the purpose of its data. It should never be assumed that filtering an online sample makes it equivalent to a permit sample or a laboratory determination.
Decide what the analyser result should represent
Start by naming the analytical question. Is the result intended to show a dissolved fraction, a treated-water trend, a relatively clean process stream, an early warning of a change or an investigation of a particular stage? The answer determines whether a filtered sample is meaningful.
Avoid vague requirements such as “filter the dirty water”. Instead, document the expected solids, their likely size and behaviour, the potential for fibres, air, oil or scale, and whether the condition changes during normal operation, cleaning or storms. Also record whether an existing manual or laboratory method filters, digests or otherwise prepares its sample. A comparison is only useful when those differences are understood.
The online water analyser versus laboratory results guide gives the wider comparison framework. This article concentrates on the sample-system decision before the analyser.
What the supplied 3S filtration unit is designed to do
The supplied 3S filtration-unit technical data sheet describes a system connected to a 3S analyser that filters suspended solids to provide a cleaner sample. It uses automated backflush, controlled by a programmable timer, to clean the filter element. The data sheet lists mesh options from 10 to 500 microns, selectable cleaning intervals and installation conditions including sample-line pressure, flow and compressed-air or water requirements.
Those are available configuration facts. They are not a universal mesh-selection table. Choosing 10, 50, 100, 300 or 500 microns requires an application review because the choice affects both the material excluded and the risk of fouling. The same data sheet should be used for installation planning, not copied into a specification without confirming the current site conditions.
Four questions before choosing a mesh
1. What needs to stay in the sample?
If the monitoring objective includes material that a proposed filter would remove, filtration can produce a result that does not answer the question. State the required determination and reporting basis first, then decide what sample presentation is appropriate.
2. What needs to be kept out of the analyser?
The answer may include particles that block small lines, coat a measurement cell or make delivery unstable. It may also include floating material or air that requires a different sample-handling arrangement. A filter is one possible part of the solution, not automatically the whole solution.
3. How variable is the water?
Design for plausible upset conditions, not merely a calm daytime sample. Consider wash-water events, high flows, rainfall-related variation, process start-up, backwash returns and intermittent discharges. The goal is to identify a configuration that can be maintained safely and predictably.
4. How will the team prove the sample system is still working?
Automated backflush reduces the need for constant manual intervention; it does not remove the need for inspection and routine checks. Decide who will inspect the arrangement, what signs of loss of flow or fouling trigger investigation, and how the team will record maintenance.
Make room for the complete sample system
Filtration is part of a route: take-off, isolation, sample line, possible fast loop or reservoir, filtration, analyser connection and safe drain. The supplied 3S-CL brochure describes the analyser sample arriving at a pressure-free reservoir with overflow to drain, and lists sample-related accessories including reservoirs, filtration units and external pumps. The final arrangement must still be reviewed for access, safe drainage, utilities, pressure and maintenance space.
Map the route on a process drawing. Mark whether a backflush discharge is acceptable, where compressed air or water is available if required, and who can access isolation points. If the application is wastewater, remember that the Environment Agency’s compliance framework has its own sampling and analysis requirements; an operational online arrangement must not be relabelled as the permit route without formal confirmation.
Use trends thoughtfully
A stable online trend can be more actionable than an isolated number, but only if the team understands what sample it represents. Record when a filter is cleaned, when the sample conditions change and when process events occur. Then operators can distinguish a potential water-quality change from a sample-system change.
The 3S-CL brochure lists automatic calibration, validation and cleaning as standard analyser functions. These help support operation of the configured analyser, but they do not validate a blocked take-off, incorrect mesh, failed external utility or non-representative sample point. Keep analyser routines and sample-system checks in the same operational plan.
A practical filtration-review brief
Before requesting a proposal, gather:
- the analyte, reporting basis and decision the result must support;
- a process sketch showing the proposed take-off, return and drain;
- representative normal and upset samples or reliable solids information;
- expected flow, pressure, temperature and any air, oil or chemical exposure;
- available compressed air or water and where backflush discharge can go;
- required access, isolation, cleaning and maintenance arrangements; and
- details of any laboratory comparison method.
PMA can use this information to assess a 3S filtration option, a reservoir, an external pump or another sample-conditioning arrangement. It is better than selecting a component by mesh size alone.
Contact PMA about sample filtration
For an online water-analysis project with suspended solids, share the process conditions and the decision the result must support. PMA can review the 3S online colorimetric analyser and the sample-conditioning guide with the complete sample route in mind. Contact PMA to discuss the application.
Frequently asked questions
Does every online analyser need a filter?
No. The need depends on the water matrix, required determination, sample point and configured application. A clean sample stream may need a different arrangement.
Is a finer mesh always better?
No. A finer mesh may change the sample fraction and can affect fouling behaviour. Mesh selection should follow an application review.
Does automated backflush remove the need for maintenance?
No. It is an automated cleaning feature. The sample system still needs defined inspection, access and response arrangements.