It's normal to ask questions like, "how can a technology best meet my needs?" or "why should I pay so much for this technology?" In this article, you will be provided with answers to your questions about 5 popular filtration technologies. It is important to note that the comparison of these technologies is very difficult since small changes in the use of the system can affect its efficiency, its costs or any other aspect that can be a selection criterion.
Therefore, in order to be able to compare the different technologies, we have chosen to elaborate 3 relatively simple situations, but which will strongly affect one of the most important aspects in the selection of a system, the CAPEX/OPEX:
- Situation 1: Prefiltration for a reverse osmosis system with clean water as an influent ;
- Situation 2: Prefiltration for a RO system with moderate water (TSS 5PPM) as influent
- Situation 3: Prefiltration for a RO system with dirty water (TSS 50PPM) as influent
Note that we will discuss the situations in more detail below. To begin, here are some details about the different technologies compared.
Cartridge Filters
Bien qu’il existe une grande variété de cartouches, elles fonctionnent toutes sensiblement de la même manière. D’ailleurs, afin de limiter la longueur du texte, les filtres à sac/poches ne seront pas abordés distinctement puisqu’ils présentent pratiquement les mêmes caractéristiques que les filtres à cartouches. Bref, que l’on parle de microfiltration ou de nanofiltration, la base du fonctionnement des filtres à cartouches et de ceux à sac est que l’eau passe au travers d’une barrière physique que les impuretés ne traversent pas.
Cartridge filters offer some interesting advantages. First, the large variety of products allows for the optimization of filtration since there is most likely a type of cartridge made for your situation. Among the major families of cartridges, we often find depth cartridge filters and pleated cartridge filters.
Depth cartridge filters can come from a variety of manufacturers. In general, they are made of either molten polypropylene or polypropylene fibre. Pleated cartridge filters are also known as "accordion filters" because the filter element resembles the bellows of an accordion. This type of cartridge is generally more expensive because they are more challenging to manufacture.
As mentioned above, the wide variety of cartridges allows you to adapt the parts used according to your situation. Depending on the type of cartridge adopted, different characteristics can be found. One can think of chlorine resistance, heat resistance or even extended durability. By varying the materials, pore sizes and adhesives used, cartridge filters can provide optimal filtration that meets the standards required of you.
For your information, the basic reference for filter cartridges is 10 inches long. On average, these lengths offer a flow rate of 5 gpm and their prices can vary between $10 and $1000 per unit. Finally, this type of filtration offers pores ranging from 0.03 micron to 100 microns.
Media Filters
This type of filter captures the impurities present in the water thanks to the media that can be found in their tanks. These media can be quartz sand, anthracite coal, grenat, or even cationic and anionic resins. Often referred to as "multimedia filters" since they are usually made up of several layers of different media, this treatment technology generally works on the principle of filtration and adsorption.
The purpose of using several types of media is justified by the fact that they do not all have the same effectiveness against different types of contaminants. For example, round media removes more material than angular media; this is because the spacing between round media is smaller since they stack better.
As such, media filters have several interesting advantages. However, they are not suitable for turbid water because they tend to clog quickly, which can cause shutdowns or a decrease in flow. In addition to the reduced flow, when the cleaning cycle is initiated, a large amount of water is required and must be discharged afterwards.
AMF Filters
From the family of self-cleaning filters, AMF or automatic microfibre filters are a technology that has been perfected by Amiad. Although there are other types of self-cleaning filters, AMFs offer the best filtration quality and a high flow rate. Their unique, complex and easily recognizable housings make them the technology of choice for highly turbid waters.
Inside the housing is the filtration unit, which consists of several plastic plates called cassettes. These cassettes are wrapped in thin layers of fabric. The type of threads, tension and number of layers will determine the quantity and size of the pores (20, 10, 7, 3 or 2 microns).
Like most self-cleaning filters, the AMF is equipped with a control unit to automate the system and its cleaning. This unit can be programmed to activate cleaning at time intervals or when the internal system pressure reaches a certain point due to filter blockage.
Typically used in the pre-filtration stage, AMFs protect downstream equipment with filtration down to 2 microns, while providing a filtration rate of up to 880 gpm. And because they are automatic, they are practically maintenance free and almost never stop providing quality influent. In fact, once installed and programmed according to the customer's needs, AMFs are self-sufficient.
Self-cleaning Disc Filter
Ces filtres particuliers peuvent facilement être confondus avec des filtres à cartouches traditionnels, car les boitiers sont très similaires. Cependant, les unités de filtration à l’intérieur de ceux-ci sont très différentes. Comme leur nom le dit, les « cartouches » sont fabriquées de disques
To make disc filter cartridges, polypropylene discs are stacked and pressed together. Each disc is engraved with grooves to achieve the desired filtration. The superposition of different groove sizes allows the adjustment of the micron offered by the filter. At Amiad, where Arkal disc filters are found, they use a colour code to identify the micron of each disc: blue (400 microns), yellow (200 microns), red (130 microns) and black (100 microns). By adjusting the grooves accordingly, it is possible to achieve filtration down to 20 microns.
L’unité filtrante étant en polypropylène, les filtres à disques présentent des avantages à bien des égards puisque le polypropylène est un thermoplastique hydrophobe semi-rigide. En français, cela signifie que les filtres à disques offrent une résistance élevée contre la corrosion et l’abrasion. De plus, ils sont réputés pour être très efficaces pour le traitement des eaux ayant beaucoup de matières en suspension. La version automatique possède une unité de contrôle qui prend en compte la pression interne du filtre. Lorsque la pression atteint un seuil prédéfini, une pression inverse est appliquée et permet aux disques de se séparer les uns des autres et, ainsi, laisser les particules se déloger et être extraite par un drain. En revanche, il est important de noter que le polypropylène n’est pas résistant à la chaleur et peut être abîmé s’il est constamment opéré à des températures de 60 degrés Celsius et plus.
Self-Cleaning Screen Filters
Toujours dans la famille des filtres autonettoyants, ces filtres sont séparés en trois sections qui commencent par une chambre de tamisage (préfiltration), une chambre de filtration et une chambre de décharge.
Water first enters the housing through the screening chamber where a filter removes coarse material that could clog the smaller filters. Once inside, the water passes through the filtration chamber where a finer filter is installed. The various particles present in the water accumulate on the inside walls of the filter. When the walls are clogged with the particles, the pressure differential at the inlet and outlet activates the self-cleaning cycle. Once the cycle is started, a suction shaft located in the centre of the filter is activated and starts a circular motion to suck out the impurities accumulated on the inner walls of the filter. What is removed from the inner wall of the filter is then sent to the third section, the discharge chamber, which rejects the contaminants through the drain.
Like most technologies, the filtration rate of these filters varies depending on the micron rating and the available filtration area. With a filtration capacity ranging from 50 to 500 microns, self-cleaning screen filters have a very wide flow range.
Setting the Scene for CAPEX/OPEX Estimation
In order to compare apples with apples, the situations analyzed must all have an identical target. Therefore, we have established the following treatment parameters
- Flow rate of 100 gpm
- Filtration up to 5 microns
- Installation costs and labour are not included in the estimates
- The estimated costs are only for the primary filtration system; in other words, if the filtration requirements are different, the costs will change significantly.
- Since the goal is to compare the different primary filtration options, the only parameter used to differentiate the situations is the concentration of Total Suspended Solids (TSS) in the influent.
In addition, since we are aiming for the filtration of 5 microns, self-cleaning screen filters and disc filters must be complemented with an additional filtration unit since they do not reach this micron level by themselves. For these two technologies, we have adjusted the estimates by adding a 5-micron membrane filter offering a flow rate of 100 gpm.
Situation 1 : Clean Water
In this situation, the compared technology will be used as pre-filtration for a reverse osmosis system. The input water is potable and meets the Recommandations canadiennes pour la qualité de l’eau potable au Canada.
As this situation demonstrates quite clearly, taking into account CAPEX and OPEX, not all technologies are advantageous in all situations. Here, AMF is not an interesting option since the water is already very well treated upstream. As for the other types of filtrations, depending on your situation, it may be advantageous to opt for one or the other. For example, if your situation involves difficult and remote access to the site, self-cleaning filters can be interesting to avoid the continuous handling of a cartridge filter.
Situation 2: Moderately Turbid Water
Here, the technology being compared will once again be used as pre-filtration for a reverse osmosis system. However, the influent used is moderately dirty water, which represents a TSS concentration of 5 PPM.
Here we begin to see the importance of choosing the right technology for the right situation. There is a drastic increase in operating costs for cartridge filters. Moreover, it is important to remember that over 25 years, if you change the cartridges twice a week on average, the cost of labour will have a significant impact on the OPEX and will make the OPEX of cartridge filters exceed that of AMFs.
Situation 3 : Very Turbid Water
Again, the technology being compared will be used as pre-filtration for a reverse osmosis system. However, in this situation, the turbidity of the water is very high and its TSS concentration reaches 50 PPM and more.
As you have probably noticed, cartridge filters are not compared here. The reason is that it would be completely pointless to install this type of treatment for this type of water, otherwise you would have to change the cartridges several times a week, if not daily.
Always According to Your Situation
The graphs above demonstrate how a slight change in the influent can have an impact on the selection of your technology. Besides the fact that the parameters that were used for the cost estimation are very basic, we did not consider seasonal changes that can affect the influent. Indeed, snow melt, drought, human activities of any kind can have an impact on water quality.
It is important to note that labour costs have not been included even though they represent a significant expense in the OPEX of a system. This means that all of the estimated operational costs will have other associated expenses, with the exception of AMF, since they are entirely self-sustaining.
With respect to CAPEX, other factors can affect it. These may include government standards, geographic location of the site, or even industry standards.
In short, no matter what technology you are considering, there is no magic formula and to make your investments profitable and ensure optimal operation of your production line, it is important to choose a technology that meets your needs and your situation.
Enfin, si vous avez des questions ou des suggestions en lien avec le traitement d’eau industrielle, nous vous invitons à nous écrire directement ou en commentaire.