pHlocrite - Self-Regulating Dolomitic Filter
SELF-REGULATING- pHlocrite™ dolomitic filter medium acts both chemically and physically raising the pH, hardness (calcium & magnesium) and alkalinity, and diminishing acidity (excessive carbon dioxide) until the point of saturation is established (SI=0). The treated water deposits a highly protective, complex and dense 'rust lime' film which protects ferrous pipes and tanks against corrosion and cuts contamination by lead, zinc and copper. Also, due to the irregular shape of the porous grains, pHlocrite™ provides maximum surface area per unit volume which improves the removal of turbid matter.
Product Details
HEAVY METAL REMOVAL is achieved due to the alkaline milieu on the surface of the pHlocrite™. Metallic ions (if present) are trapped in the form of their insoluble hydroxides including lead, cadmium, copper and zinc.
GRADES- The advantage of our synthetic dolomite mix over the natural product is that pHlocrite™ results in optimum performance through positive and precise pH adjustment. All grades are very competitively priced and extremely economical to use, an additional bonus being the lower running costs of the magnesia grades which extract more carbon dioxide per unit weight. It requires no dosing equipment and fits into any gravity or pressure filter tank, placed as a layer on top of a conventional sand bed.
How pHlocrite Works
- Hlocrite™ dolomitic filter material is characterised by the following physical and chemical properties:
- Outer appearance: white and grey angular porous grains
- Granulation: 2 - 4.5mm (4/8 BS Mesh)
- Apparent density: approx. 170 lb/ft3 (2.72 g/ml)
- Bulk Density: approx. 72 lb/ft3 (1.15 g/ml)
- Hardness (Mohs' scale): 3.5 - 4.0
pHlocrite™ synthetic dolomitic filter medium is a combination of magnesium carbonate, calcined to convert it to the oxide (MgO), and high purity calcium carbonate (CaCO3), sieved to appropriate granulation.
It is a special feature of the pHlocrite™ manufacturing process that the calcining temperature is controlled to acheive the desired degree of 'hardness' of the MgO without producing the undesirable free lime or lack of MgO conversion that can result from straying from the critical calcining temperature required for the natural dolomites which tend to contain around 0.2% of free lime even when calcined at their optimum conversion temperature.
The reaction of pHlocrite™ is alkaline and therefore able to neutralise the excessive portion of free CO2 according to the following equation :-The overall result is that the excess free CO2 (if present) is chemically transformed into water soluble bicarbonates of calcium and magnesium, raising the total hardness as well as the alkalinity. One might expect the MgO to react exclusively with the excess CO2, eventually leaving a 'skeleton' of inert CaCO3 behind. However, due to the porosity of pHlocrite™ filter grains, the CaCO3 component is active enough to join in the neutralising reaction. Many years of experience with dolomitic filter media as used for water treatment have confirmed the validity of the above equation in general.
It has been experienced that for each 10 mg/l excess CO2, approximately 18 mg/l alkalinity as CaCO3 (0.36 meq/l) is produced and the total hardness raised accordingly. The above equation generally holds true but, in the case of extremely soft waters, the activity of the CaCO3 component is accelerated, whereas in hard waters with a significant calcium concentration the CaCO3 activity slows down, giving a preference to the MgO component. It is obvious that there will be a consumption of pHlocrite™ almost entirely due to the amount of excess CO2 neutralised.
Following the above equation, a theoretical consumption of 1.06 parts of pHlocrite™ per part CO2 neutralised would result. However, as a small amount of material is lost during backwashing of the filter, it has become an accepted practice to operate with a consumption of 1.3 parts of pHlocrite™ per part of CO2 removed.
Application of pHlocrite™ Dolomitic Filter Material
pHlocrite™ filter material is suitable for the anti-corrosion treatment of aggressive water characterised by low carbonate hardness or low pH resulting in a negative Saturation Index. Such waters may occur as:-
- Ground water, Spring water
- Surface water
- Swimming pool water
- Sea water distillates
- Deionates
- Reverse Osmosis and other Demineralising processes product
pHlocrite™ can be used in virtually any gravity or pressure filter tank and does not require additional equipment to that which is already standard on most conventional water filters. The filters should preferably be equipped with an air scour as part of the backwash system. Upflow or downflow systems may be used. Typical systems are illustrated here.
As the performance of pHlocrite™ is dependent only on the contact time with aggressive water, its rapid reaction rate means that filtration velocities can be selected within a wide range:- 5 - 30 m/h.
It is also applicable in the higher rate filters which are common in some swimming pool applications and in such cases, lower specific amounts are needed because the pool water is recirculated and filtration rates as high as 60 m/h. are tolerated.
Additional Features
Heavy metal removal is achieved due to the alkaline milieu on the surface of the pHlocrite™ grains, Fe2+, Fe3+, Mn2+ and Mn4+ ions (if present) are trapped in the form of their insoluble hydroxides and are removed from the grains during the next backwash. lead, cadmium, copper and zinc are also removed.
Trapping very fine suspended matter present in most waters is also achieved because of the angular surface of the pHlocrite™ grains which increases the surface area compared with more spherical grains.
It should also be noted that the purity of pHlocrite™ results in a virtually complete consumption of the filter material during operation and nothing is introduced to the water which is not present in all natural waters.
pHlocrite™ for pH and Alkalinity Control in Swimming Pool Water
Since 1982 when ICI Ltd withdrew chlorine gas for use in swimming pools, sodium and calcium hypochlorite have been used by most municipal pools for the disinfection of the water and, since both of these disinfectants are alkaline, acid is used to reduce the pH to the optimum value for pool water. A few public pools still use chlorine gas or liquid bromine and many smaller pools such as school and hotel pools use trichloro-isocyanurates for disinfection. All of these disinfectants are acidic when dissolved in water and require an alkali to maintain the pH in the correct range.
When chlorine gas dissolves in water, it hydrolyzes forming hydrochloric acid HCl and the weaker hypochlorous acid, HOCl.
Cl2 + H2O 
HCl + HOCl
The acid from the continuous addition of chlorine gas to the pool water reacts with the alkalinity and is chemically transformed to CO2 and chloride.
2HCl + CaHCO3 CaCl2 + H2O + CO2
This equation demonstrates that the decrease of alkalinity and pH is not only a function of chlorine gas but of all chemicals releasing hydrogen ions in water, e.g. the commonly used flocculant aluminium sulphate. Trichloro-isocyanurate releases HOCl and necessitates pH and alkalinity control, although, due to lower bather load and thus lower chlorine input, the effect is not as signifcant as in heavily loaded public pools.
The effect of chlorine gas on alkalinity and pH becomes conslderable when the initial alkalinity of the make-up water is lower than 200 ppm as CaCO3 (4 meq/l). Hence, pH control is of great importance in order to ensure the efficiency of the chlorine residual and the prevention of corrosion of all materials coming into contact with such an aggressive water.
pHlocrite™ features two advantages over other chemicals used for pH and alkalinity control (sodium carbonate, sodium hydrogen carbonate) automatic control of pH and alkalinity. Without any electronic measuring device, the pool water is kept well balanced.
SI = O ± 0.5
No problems with cloudy water due to improper dosing of soda ash or bicarbonate, improves removal of iron and or manganese possibly present in the make-up water.
pHlocrite™ is usually placed as a thin layer on top of the sand layer inside any conventional sand filter (gravity or pressure type). In respect to the necessary freeboard, a portion of the filter sand can be substituted without diminishing the efficiency of the filter, as pHlocrite™ is superior to sand due to the roughness of its grains.
Limitations
The application of pHlocrite™ dolomitic filter material is economical and feasible for corrosion control under the following conditions which should be carefully observed.
Carbonate hardness (often described as temporary hardness) which is the proportion of alkalinity covered by total hardness, should not exceed 180 mg/l as CaCO3 (3.6 meq/l).
Calcium hardness should not exceed 360 mg/l as CaCO3 (7.2 meq/l).
Non-carbonate or permanent hardness due mainly to calcium and /or magnesium chloride, sulphate and nitrate should not exceed 200mg/l.
Sodium as CaCO3 should not be greater than 60% of the sum of the calcium and magnesium as CaCO3.
To avoid unnecessarily hard final water, excess CO2 should normally not exceed 75 mg/l as CO2 but a sufficient amount of excess CO2 should be present in order to attain a minimum final calcium hardness to achieve corrosion control. If needed, the addition of pressurised CO2 should be considered but, usually, the addition of CO2 is confined to distillates and deionates where CO2 removal is inherent in the process and the calcium and alkalinity levels are practically zero.
Theoretically 10mg/l CO2 can produce nearly 23mg/l of hardness as CaCO3 but the actual amount produced would need to be established for each plant when commissioning and the required amount of calcium hardness increase would, of course, depend up on the level of calcium in the untreated water.
Where heavy metal removal is not the main requirement, iron and manganese contents should not exceed 2 ppm as Fe or 0.1 ppm as Mn. For higher levels, pre-treatment of the raw water is highly recommended in order to avoid reduction of neutralising capacity.
Continuity of filter load is desirable although short term deviations in the range of -30% and +10% are without significance. However, a total shutdown of a filter is always to be preferred to an underload operation. If a temporary underload situation becomes unavoidable, special attention must be paid to frequent and proper backwash in order to minimise the risk of clogging.
Other Methods for the Deposition of Calcium Carbonate Films
Like the dolomite process, other methods for deposition of CaCO3 films are based on the addition of alkaline calcium and/or an increase in pH until a state of CaCO3 saturation is reached and the conditions of a well-balanced water are met. Hydrated lime Ca(OH)2, mostly dosed to the water as a slurry but sometimes as a saturated solution, is widely used and marble or calcite chip filter beds to a lesser extent.
Whereas lime is usually the least cost chemical, it requires very accurate control in order to avoid over dosing and the required degree of saturation is seldom possible to achieve without the addition of CO2. Lime saturators or continuously stirred lime slurry preparation tanks are necessary and overall dosing complexity and costs make this the method of choice only for very large water treatment plants.
The required contact time in the latter method is rather lengthy, involving heavy investment in filter tanks to house the large amounts of marble needed to establish a well balanced water. However, the marble process is very safe, as no problems with overdosing are possible and since all of the increase in hardness is due to calcium it is sometimes preferred to the dolomite process when the raw water is extremely soft, does not contain significant amounts of iron or manganese and the slow reaction rate can be accommodated.
The dolomite process is highly recommended for application in small to medium-sized water works, lacking well trained personnel for the operation and maintenance of sophisticated dosing equipment.
Other Methods for the Deposition of Calcium Carbonate Films
Like the dolomite process, other methods for deposition of CaCO3 films are based on the addition of alkaline calcium and/or an increase in pH until a state of CaCO3 saturation is reached and the conditions of a well-balanced water are met. Hydrated lime Ca(OH)2, mostly dosed to the water as a slurry but sometimes as a saturated solution, is widely used and marble or calcite chip filter beds to a lesser extent.
Whereas lime is usually the least cost chemical, it requires very accurate control in order to avoid over dosing and the required degree of saturation is seldom possible to achieve without the addition of CO2. Lime saturators or continuously stirred lime slurry preparation tanks are necessary and overall dosing complexity and costs make this the method of choice only for very large water treatment plants.
The required contact time in the latter method is rather lengthy, involving heavy investment in filter tanks to house the large amounts of marble needed to establish a well balanced water. However, the marble process is very safe, as no problems with overdosing are possible and since all of the increase in hardness is due to calcium it is sometimes preferred to the dolomite process when the raw water is extremely soft, does not contain significant amounts of iron or manganese and the slow reaction rate can be accommodated.
The dolomite process is highly recommended for application in small to medium-sized water works, lacking well trained personnel for the operation and maintenance of sophisticated dosing equipment.
Applications
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pHlocrite for Dolomitic Filter Material - Water and Wastewater - Water Filtration and Separation
pHlocrite™ filter material is suitable for the anti-corrosion treatment of aggressive water characterised by low carbonate hardness or low pH resulting in a negative Saturation Index.
-
pHlocrite for pH and Alkalinity Control in Swimming Pool Water - Water and Wastewater - Swimming Pools
Since 1982 when ICI Ltd withdrew chlorine gas for use in swimming pools, sodium and calcium hypochlorite have been used by most municipal pools for the disinfection of the water and, since both of these disinfectants are alkaline, acid is used to reduce the pH to the optimum value for pool water. A few public pools still use chlorine gas or liquid bromine and many smaller pools such as school and hotel pools use trichloro-isocyanurates for disinfection. All of these disinfectants are acidic when dissolved in water ...
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