The impact of solids removed based on the starting point of efficiency.
The impact of solids removed based on the starting point of efficiency.

Sand filtration has been applied to water processes for over 200 years. Despite this there is still confusion as to what the process can achieve. Yet, by employing traditional microscopic particle filtration using sand and combining this with advances in modern technology, it can bring cost savings, quality and efficiency.

The numerous ways in which sand filtration can be combined mean that if we 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 a user 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.  
Market confusion

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 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 the customer can expect to have to wade through a minefield at worst they are potentially heading for a commercial disaster. 

Complex questions
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’?

The graph (Figure 1) shows the impact of solids removed based on the starting point of efficiency. 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.
If the challenge is 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 assess, 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.
Efficiency percentage

Amiad UK, which specialises in developing and marketing environmentally-friendly water filtration solutions, encourages its clients to ask the efficiency percentage removal question and they are often bombarded with reasons as to why that information cannot be predicted. Despite Amiad having conducted its extensive internal research and development testing, as part of its engagement with the UK market, the company asked Chester University to independently validate the performance of its high efficiency media filter technology and in particular our Double Vortex Filter (DVF).

The purpose of the test was to confirm and validate a number of Amiad’s 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 four 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.
The 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 the 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.

Conclusions 
1. Chester University confirmed that operation of the filter at a flux rate of 40m3/m2 had no impact on filter performance. This had been confirmed previously by the Amiad research and development department, meaning smaller footprints were possible for comparative flow of conventional media filters. This goes against traditional filter design belief.
2. The impact of the patented inlet was contributory to extending the periods between the requirement to backwash, with the bed accepting significant mineral solids loading levels. 
3. Mineral TSS loads of up to 292mg/l were applied without unexpected impact on backwash frequency.
4. A combination of Chester laboratory and Amiad UK field results concluded the DVF was an effective 1 micron filter with efficiency ranges of removal at 1 micron of up to 80% being viable with some clients reporting higher than this.
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 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. 

Best profile
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 and 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. 

Maintaining the vortex
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 aims to ensure that 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 the manufacturer uses 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 and can reduce the performance and limit the life of the filter bed, often requiring sanitation or 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, longer bed life and is 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. 

The manufacturer now incorporates a newly designed backwash nozzle to minimise backwash volumes 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 confidence.

Conclusion
By employing a time-honoured process of microscopic particle filtration using the traditions of sand and combining this with the advancement of modern technology, Amiad UK claims to 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, through its external research with Chester University, Amiad UK is 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.