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Υλικά φίλτρανσης ……
Κάνοντας την σωστή επιλογή

Αυτή η συζήτηση των μέσων φίλτρου προορίζεται να είναι μια συγκριτική σύνοψη των προϊόντων που χρησιμοποιούνται συνήθως σε οικιακές και εμπορικές εγκαταστάσεις διήθησης υπό πίεση.
Για μια σύγκριση των μέσων φίλτρου, δείτε τον παρακάτω πίνακα. Υπάρχουν και άλλα μέσα που θα μπορούσαν να συμπεριληφθούν σε αυτήν τη λίστα, αλλά προσφέρει μια βασική κατανόηση των αρχών που εμπλέκονται.

Οι Βασικοί Μηχανισμοί
Η διήθηση γίνεται με δύο μηχανισμούς. Η πρώτη είναι απλώς μια ενέργεια κοσκινίσματος για την απομάκρυνση της αιωρούμενης ύλης. Οτιδήποτε μεγαλύτερο από τα κενά μεταξύ των κόκκων των μέσων παγιδεύεται. Τα μέσα φίλτρου αφαιρούν τα αιωρούμενα σωματίδια αλλά φιλτράρουν επίσης σε μέγεθος μικρότερο από τα κενά μεταξύ των κόκκων. Οι κόκκοι των μέσων φίλτρου είναι συνήθως στο 300-1. Μέγεθος 700 micron με κενά μεταξύ τους 25 - 500 micron. Η δυνατότητα φιλτραρίσματος κάτω από το πραγματικό μέγεθος του χώρου μεταξύ των σωματιδίων οφείλεται στην επιφάνεια του μέσου φίλτρου. Αυτή η διαδικασία επηρεάζεται από μια σειρά μεταβλητών, όπως η βαρυτική ρύθμιση, η διάχυση, η αναχαίτιση και η υδροδυναμική. Αυτές οι μεταβλητές επηρεάζονται με τη σειρά τους από φυσικές ιδιότητες όπως το μέγεθος του φίλτρου, ο ρυθμός διηθήματος, η θερμοκρασία του υγρού, η πυκνότητα, το μέγεθος και το σχήμα της αιωρούμενης ύλης. Επίσης, μην ξεχνάτε τις ηλεκτροστατικές απωστικές δυνάμεις και τις δυνάμεις του Van der Waal, όπου τα ηλεκτρόνια συσχετίζουν τη διπολική κίνηση των υποατομικών σωματιδίων που επηρεάζουν την έλξη. Επιπλέον, χρησιμοποιούμε μέσα φίλτρου που έχουν σχεδιαστεί με πρόσθετα χαρακτηριστικά, όπως οξείδωση, αναγωγή και εξουδετέρωση. Όλα τα μέσα φίλτρου απαιτούν αντίστροφη πλύση, μερικά χρησιμοποιούν αναγεννητικό και πολλά έχουν περιορισμούς ή απαιτήσεις σε συστατικά του νερού.
Ως συνέπεια της πολυπλοκότητας της διαδικασίας φιλτραρίσματος, οι ρυθμοί ροής υπηρεσιών στη βιβλιογραφία προϊόντων εκτιμώνται στην καλύτερη περίπτωση. Η ροή της υπηρεσίας πρέπει να ταιριάζει με τα κριτήρια προσρόφησης. Οι ρύποι στο νερό μπορεί να έχουν χαμηλή, μεσαία ή υψηλή επιφάνεια χαμηλής, μεσαίας ή μεγάλης επιφάνειας, επηρεάζοντας έτσι την ικανότητά τους να προσροφούν ορισμένα συστατικά. Εάν ο ρυθμός ροής υπερβαίνει τον ρυθμό σχεδιασμού, είναι πιθανό να υπάρχει κακή διήθηση. Οι ρυθμοί αντίστροφης πλύσης έχουν σχεδιαστεί για να ρευστοποιούν (ανυψώνουν) την κλίνη φίλτρου με βάση την πυκνότητα του μέσου φίλτρου. Οι χαμηλοί ρυθμοί αντίστροφης πλύσης έχουν ως αποτέλεσμα ατελή καθαρισμό του κρεβατιού και πρόωρη αστοχία του φίλτρου. Οι πολύ υψηλοί ρυθμοί αντίστροφης πλύσης μπορεί να έχουν ως αποτέλεσμα την απώλεια των μέσων φίλτρου.

Σύγκριση μέσων φίλτρων

Anthracite coal when crushed and graded makes an ideal medium weight filter media. Because of its irregular shape, sediment penetrates deeper into the bed resulting in longer service runs. In recent times, anthracite has been primarily used in dual or multi- media filters.

Density (kg/dm³): 0,80
Bed depth (mm): 600 – 900
Service flow (m/h): 12,5
Backwash flow (m/h): 30 - 45

Birm is a plastic filter media (see Filter AG) coated with magnesium oxide. The result is a lightweight product that has catalytic oxidizing capabilities. The catalytic activity is between dissolved oxygen in water and iron, and manganese in the water supply. The chemical relation causes the iron and / or manganese to precipitate (change from dissolved to a particulate) and the particulate then adsorbs to the surface of the media.

Density (kg/dm³): 0,75
Bed depth (mm): 750 - 900
Service flow (m/h): 8 - 13
Backwash flow (m/h): 25 - 30

Manganese Greensand

Manganese greensand is naturally occurring glauconitic greensand coated with manganese resulting in a purple-black media. The intended use is as a catalyst to precipitation of iron, manganese and hydrogen sulfide. Greensand is continuously regenerated (CR) with KMnO4 (potassium permanganate) and/or chlorine. Greensand may also be intermittently regenerated (IR) with KMnO4. CR capacity is 350 parts per million (ppm) per liter when regenerated with 50 – 100g of potassium permanganate per cft. and is calculated by KMnO4 demand; where iron = 1:1, manganese = 1:2, and hydrogen sulfide = 1:4 ppm. That translates as removal of 350 ppm iron alone, 175 ppm manganese alone, or 90 ppm hydrogen sulfide alone. IR has half the capacity of CR or 175 ppm. It’s common practice to place a 400 mm layer of anthracite on top of the greensand in CR applications.

Density (kg/dm³): 1,35
Bed depth (mm): 750 - 900
Service flow (m/h): 5 - 12
Backwash flow (m/h): 30 - 35
Reg. Level 1,5 – 3 g KMnO4 / ltr MG

MTM is a granular manganese dioxide filtering media used for reducing iron manganese and hydrogen sulfide from water. MTM works in the same fashion as greensand including regeneration with KMnO4 and/or chlorine. The primary difference between MTM and greensand is MTM manufactured with a lightweight (plastic) core. This lightweight property reduces backwash rates and pressure drop of service flow.

Density (kg/dm³): 0,44
Bed depth (mm): 600 - 900
Service flow (m/h): 7,5 – 12,5
Backwash flow (m/h): 20 - 25

Corosex is a highly reactive magnesium oxide used to neutralize free carbon dioxide in water. Corosex is used in high flow and / or high pH correction situations. Limitations include the propensity to overcorrect in low flow or intermittent use applications. In waters containing medium to high hardness, calcium may precipitate resulting in cementing of the media.
Density (kg/dm³): 1,60
Bed depth (mm): 600 - 700
Service flow (m/h): 12 - 15
Backwash flow (m/h): 25 – 30

Pyrolox is a mined ore (manganese dioxide) used for iron, manganese and hydrogen sulfide reduction. Pyrolox – like Birm, greensand and MTM – acts as a catalyst to oxidation. Waters low in dissolved oxygen cannot use the catalytic properties of Pyrolox. It doesn’t require a regenerant but must be backwashed aggessively. Backwashing causes the media to abrade itself resulting in a new exposed surface. Inadequate backwash rates and low dissolved oxygen are the two primary causes of filter failure when using Pyrolox.

Density (kg/dm³): 2,0
Bed depth (mm): 600+
Service flow (m/h): 12
Backwash flow (m/h): 60 - 75

Filter-Ag is a non hydrous silicon dioxide material designed to be a lightweight “sand”. Ag weighs 0,40 kg/dm³ vs. Sand at 1,60 kg/dm³. It has an irregular surface, which increases surface area over many natural silica products. Ag is intended for removal of suspended solids (turbidity).
Density (kg/dm³): 0,40
Bed depth (mm): 600 - 900
Service flow (m/h): 12
Backwash flow (m/h): 20 - 25

Garnet is a high density and small mesh size filter media. Due to its density, it classifies with the media fines on the top, resulting in very fine filtration in the 10-20 micron range. Because of this fine top layer, filtration garnet is seldom used alone as filter runs would be very short. Garnet is used in dual or multi-media filters where larger particles are filtered by larger mesh media.

Density (kg/dm³): 2,25
Bed depth (mm): 250+
Service flow (m/h): 25
Backwash flow (m/h): 60 - 75

Multi-media (multi-layer)

The practice of layering several different media (usually 3 or 5) results in higher service flow rates and finer filtration down to 10 micron. The media are loaded by density and reverse grading. The most dense media with the smallest mesh size is loaded first and the least dense with the largest mesh size is loaded last with intervening media layered in the same manner. As a result, larger particles are filtered in the first of top layer and successively smaller particles are adsorbed in succeeding layers. The most common media mix from top to bottom is: anthracite, filter sand, garnet 30x40 (0,4 – 0,6 mm), garnet 8x12 (1,7 – 2,4 mm), gravel 1/8 x 1/16 (1,6 – 3,2 mm), and gravel 1/4 x 1/8 (3,2– 6,3 mm). While any combination of media can be labeled multi-media, the above section has come to be called multi-media.

Density (kg/dm³): 1,47
Bed depth (mm): 900
Service flow (m/h): 25
Backwash flow (m/h): 35

Filter sand is naturally occuring, graded and washed sand that’s high in silica and low in calcium. It’s used for sediment filtration and is often part of a multi-media mix. Sand filters may be the oldest man-made filters and they mimic nature’s filtration.

Density (kg/dm³): 1,60
Bed depth (mm): 450 - 750
Service flow (m/h): 7 - 12
Backwash flow (m/h): 35 - 50

Granular activated carbon (GAC)

To manufacture activated carbon, charcoal is crushed to a powder, reconstituted with a binder to the desired mesh size, then treated with high temperature steam. The steam treatment is the activation process, which creates cavities in the charcoal, resulting in a high surface area to volume ratio.

The high surface area gives GAC much greater adsorptive capacity. Charcoal from coal (anthracite), petroleum, animal bones, wood and coconut shells are used to make GAC. Bituminous coal accounts for most of the GAC being used in point-of-use drinking water applications. Acid washing is an optional preparation, which reduces carbon fines and limits pH spikes. Acid or water washing also reduces carbon fines and limits pH spikes. Acid or water washing also reduces any contaminants naturally present in the carbon source. There are several measures of GAC quality including Iodine Number, which is a measure of the amount of iodine adsorbed by weight. Iodine is used as an indicator of the adsorptive capacity but there’s no direct correlation between Iodine and other constituents. GAC is used to reduce organics, volatile organic compounds (VOCs), and taste and odor causing constituents. The best uses of GAC don’t include sediment removal. Most suspended solids will fill the cavities in GAC, thus reducing its surface area and adsorptive capacity.

Density (kg/dm³): 0,40
Bed depth (mm): 600 - 900
Chlorine removal service flow (m/h): 8 - 13
Backwash flow (m/h): 20 - 25

KDF process media are high purity copper-zinc granules (50 percent Cu; 50 percent Zn) that use the redox (the exchange of electrons) to remove chlorine, heavy metals and is bacteriastatic. KDF has an unusually high service flow rate compared to other filter media. KDF cannot be used in aggressive water and is often preceded by some form of neutralization.

Density (kg/dm³): 2,75
Bed depth (mm): 250+
Service flow (m/h): 75
Backwash flow (m/h): 75

KDF process media are high purity copper-zinc (85 percent Cu; 15 percent Zn) granules that use the redox (the exchange of electrons) in products to remove iron, manganese, hydrogen sulfide and is bacteriastatic. KDF has an unusually high service flow rate compared to other filter media. KDF cannot be used in aggressive water and is often preceded by some form of neutralization.

Density (kg/dm³): 2,75
Bed depth (mm): 250+
Service flow (m/h): 35
Backwash flow (m/h): 75

Sizing Calculations

In addition to selecting appropriate media for a filter application, sizing cannot be ignored. Obviously, one must size for normal service flow but peak flow and backwashing requirements must also be considered. Most filter media have negligible pressure drop at the recommended service flow rate and can easily handle peak loads up to double the service flow rate without pressure loss concerns. There are exceptions, however, as evidenced by multi-media where double the service flow rate of 2,5 m³/h would result in a pressure loss of 1,25 bar.

In some water, chemisty quality suffers with any flow disruption. Peak flows should be limited to the gallons of the empty bed volume (EBV) of the filter. In other words, if you have a 10x54-inch filter tank, the peak flow shouldn’t last longer than 18,36 gallons (see Table 2).
Backwash rates are often double the service flow rate causing problems if the filter was sized to handle the total available flow from a well pump. If the and well pump delivers 2,2 m³/h it’s sized for such, it will require 4,5 m³/h for backwash but the well pump is still limited to 2,2 m³/h.