Don't Bury your Head in the Sand when it comes to Process Water Filtration [read more]

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Don't Bury your Head in the Sand when it comes to Process Water Filtration

Sand filtration has been applied to municipal and industrial water processes for over 200 years. Despite this there is still confusion as to what sand filtration technology can actually achieve this being mainly due to lack of reliable evidence. 

The numerous ways in which sand filtration can be combined mean that if you ask ‘how fine does it filter', which is a question that we would all agree is pertinent and to the point, you would expect to get a straight forward answer if the supplier knows his stuff.  What you wouldn't then want to face is barrage of questions such as ‘is your load mineral or organic?'; ‘what is the filtration flux rate (velocity through the filter bed)?' etc. Eventually, if we take the time to dig deep enough, some suppliers will give a filtration performance figure usually expressed in micron size as to the expected solid size that will be removed, or an anticipated reduction in turbidity. This very lack lustre, possibly unscientific response, then defines our selection process against what could be a significant investment.  

All of this causes market confusion as to what to specify and what to expect in terms of operational results. Whether to procure a basic shallow bed filter often seen in domestic swimming pools, or a deep multimedia system designed specifically for a set purpose, the choices are innumerable. Combine this with some unscrupulous suppliers who will state that a filter has 5 micron performance (when it only actually removes a few 5 micron particles with every pass) together with the need to consider if the performance claimed is based on a single pass or multipass though the filter (in which case the difference in comparative performance is immense) at best you can expect to have to wade through a minefield at worst you are potentially heading for a commercial disaster. And of course there are many more complex questions that need to be asked when specifying process water filtration technology. Questions such as ‘what time during the filtration cycle is the performance measured, i.e when the bed is at its least efficient, directly after backwashing' or ‘when is it at its optimum just before backwashing'?

Which is why Amiad UK supports its clients with a laboratory facility here in the UK that answers these awkward questions for you. The Graph below shows the impact of solids removed based on the starting point of efficiency. You will see both filters are similar in performance just before backwashing but just after there is a considerable difference in what is removed by the higher efficiency filter.  



Given all of this, if our challenge is therefore to remove suspended solids, what we really need to be asking is, ‘at a given micron size performance what is the efficiency of removal'? This is usually expressed as a percentage i.e 5 micron removal at a typical efficiency of 50%, or as a percentage of the total solids removed.  If we assume either a mineral or organic load, a constant TSS load, this gives us a real performance measure when comparing media filter technologies. 

Materials of construction are far easier to asses, if we consider the nature of the liquid we are filtering in that in industry, most of us have to make material compatibility decisions daily.

We also need to consider how much water we put to waste overtime to regenerate the filters as this brings with it considerable saving in terms of waste stream processing or disposal. Combine this with media bed replacement and disposal costs, we begin to get a true picture of the real OPEX costs of the investment.

At Amiad UK, we encourage our clients to ask the efficiency % removal question of our competitors and they are often bombarded with reasons as to why that information cannot be predicted.

Despite having conducted our own extensive internal research and development testing, as part of our engagement with the UK market, we decided to ask Chester University to independently validate the performance of our high efficiency media filter technology and in particular our Double Vortex Filter (DVF).

A test program was initiated using a test rig as described in the schematic below. 



The purpose of the test was to confirm and validate a number of our own research and development findings, the main ones being:
1. The removal efficiency of the DVF filter system at varying filtration velocities including up to 4 times that of a standard media filter.
2. The removal efficiency in a single pass through the filter by sampling inlet and outlet water flow.
3. Establishing the impact that our patented inlet system had on the filtration cycle period over time, quantifying the loading capability of the filter.
4. Examining the Total Suspended Solids (TSS) removal rate as well as the influence the filter would have on differing particle size ranges. This was carried out through particle analysis of the samples as well as the standard industry gravimetric test.

Our tests were carried out to industry standards which meant using Arizona test dust as a source contaminant. The standard test dust used throughout conformed to ISO 12103-1:2016 A2 Fine - Arizona Simulate (Particle Technology Ltd, UK) and contained silica particles in the 1 - 100 μm range (SG 2.65).The particle analysis equipment used was a Spextrex laser particle counter, size/count analysis in the 1 - 100 μm (Spectrex Inc, USA). This was chosen to mimic the in-house laboratory facility in Amiad UK so data from field operational plants with organic challenges as well as Chester laboratory results could be combined to establish practical predictive performance for clients.Some interesting conclusions were drawn:-


What has also become clear is that every application is different and differing challenges produce different results. However what is important to us is the ability to effectively predict performance when presented with a client's individual process conditions.

At this point, we have to be clear that the DVF filter system is not a standard media filter. Rather than using a flat filter bed, the DVF activates the top layer of the filter bed releasing the lighter contaminates and allowing them to be retained in a vortex which operates above the filter bed surface.

It has long been recognised that a flat filter bed produces the best profile for filtration. However, traditional media filtration with challenging influent are prone to biofouling due to the static nature of the bed. This also limits the size of the filter media that can be incorporated as finer media only accentuates this problem and promotes the biofouling process. Nevertheless, flatbed configurations can optimise performance.

Over recent years, tangential inlets have been introduced to create a vortex, activate the bed surface and create a surface scouring action. However, this re-profiles the bed and fluid, taking the easiest route, shortcuts the deepest part of the bed where the optimum filtration takes place. A number of devices have been incorporated to continuously flatten the bed using various designs of nozzles but these also bring with them their own issues. The nozzles very often incorporate fine jets and, as the water they distribute contains the solids from the filter inlet, this results in additional maintenance to maintain the nozzles performance.

Creating a vortex above the filter bed can have its own issues in that a quiet zone is created above the vortex where often microbiological growth can proliferate. Amiad UK's patented inlet design incorporates a vortex generating system. This ensures the vortex effect is maintained throughout the upper part of the filter vessel, ensuring balanced dynamics within the whole top section of the vessels without causing a quiet zone. In addition, the vortex generating nozzles have large orifices so no blockage is possible whilst an active regenerating filter bed is still maintained. By designing an inlet which allows a vortex to be developed even at minimal flows we have also effectively eliminated the previous issue relating to minimum flow requirement of technologies with vortex generating inlet designs.

Sand media is a traditional material for media filters and has good filtration properties the filter media that we use is activated glass as this resists microbiological fouling. Because the structure of sand is irregular with rough surfaces such media can act as a substrate for microbiological growth. This is proliferated in a sand bed can reduce the performance and limit the life of the filter bed often requiring sanitation nor even complete replacement. Glass has a much smoother spherical surface and in some cases can be manufactured with a surface charge that attracts fine particulate. Overall glass delivers better performance and longer bed life and is thus less susceptible to biofouling. All the advantages of a traditional sand filter design are incorporated in terms of the bed height so optimum performance can be achieved. The inlet system allows very fine media to be incorporated so the optimum of performance can be achieved repeatedly.

We also now incorporate a newly designed backwash nozzle to minimise backwash volumes yet further. These also act as a physical barrier to prevent media migrating downstream with the finest media being larger than the retention capability of the nozzles. The media is drinking water approved so the system can be incorporated in the most sensitive applications with the confidence of excellent performance.

In summary, by using a time honoured process of microscopic particle filtration using the traditions of sand and combining this with the advancement of modern technology we have proven not only the benefits of using a quality media but also the way in which it can be used can be qualified to clients in terms of cost savings, quality and efficiency. Most importantly is the fact that, through our external research with Chester University, we are also now in a position to be able to provide our customers with the facts and figures on what they should expect from their filtration processes in terms of efficiencies, cost savings and return on investment.

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