Life cycle analysis of AFM & Sand.
Current situation
Activated Filter Media (AFM) is an expensive product and is only for high-end applications that require very stringent quality norms. It is 3-4 times more expensive to sand. The cost of AFM is very high and we need not have so much expenditure on filtration media as sand is available almost free.
Some other most common misconception pertaining to sand are as follows.
- Sand is never required to change in the filter.
- Performance of the sand stays consistent till the life of the filter.
- Every sand is required be replaced every 2-3 years.
- The size of the sand particle stays the same there is no change in the dimensions.
- Quality of sand has no impact on the consumption of coagulants & Biocides.
- Particle size sand can filter stays same for the entire life span of sand.
It would be worthwhile to have the above mentioned and the similar other aspects to have a detailed consideration.
Any type of filter ( PSF, MGF, RG, etc …) usually uses relatively coarse sand and other granular media to remove particles and impurities that have been trapped in a floc through the use of flocculation chemicals–typical salts of aluminum or iron. Water and flocs flow through the filter medium under gravity or under pumped pressure and the flocculated material is trapped in the sand matrix.
Mixing, flocculation, and sedimentation processes are typical treatment stages that precede filtration. Chemical additives, such as coagulants, are often used in conjunction with the filtration system. Filters must be cleaned frequently, often several times a day, by backwashing, which involves reversing the direction of the water and adding compressed air if required.
The effective pre-treatment of source water and adequate and regular cleaning of filter media should maintain filter media in a satisfactory and clean condition. Proper cleaning would remove suspended matter that has been retained on the filter medium from the clarified water.
Problems with the filter bed and filtration media.
Calcium carbonate precipitation
It is often found that deposits, that are difficult to remove, accumulate in the filter bed. This deposit may consist of residues of the coagulants, inorganic compounds, or biological residues. Wastewater treatment plants may also experience an increase in the grain size of the filter media. This may be due to inorganic chemical precipitates, most often calcium carbonate.
Another important factor is the abundance of calcium in almost all waters. Calcium readily reacts with available alkalinity to form calcium carbonate. At elevated pH, common to many of these reactions, calcium carbonate is readily formed and, because of its low solubility in water, It tends to precipitate and crystallize onto any available surface area, specifically filter media that offers a large reactive surface area. Filter media provide a site for the accumulation of the precipitated particles, which is the start of the crystal growing cycle.
Although each layer may be microscopic, over a period of time, just like a pearl develops, the grains of the filter media gradually increase in size (Underwood, 1993). Cleaning the filter can be effective in reversing this growth by providing an abrasive action of one particle against another. However, for a variety of reasons, not all of the accumulated materials may be removed. Progressively the filter media grow over a period of time, usually too gradually to be noticed immediately, until some of the particles may be several times their original size. This often goes unnoticed because as the bed size gradually increases, small
Amounts of the filter media may be lost through the backwash so that the freeboard in the filter appears to remain the same. As the size of the particles increases so does the size of the space between the particles. This allows larger particulate matter to flow into and through the filter with potentially serious consequences. This may allow suspended matter and pathogenic organisms to pass through the filter and into the distribution system.
Metal hydroxide precipitation
Hydrolyzed metals that precipitate as hydroxides are often used as coagulants in conventional drinking water treatment. These processes require addition of chemicals such as hydrated lime to the water to attain the optimum pH for the minimum solubility of the target contaminants and optimal formation of floc particles. The hydroxide compounds formed are generally sticky, gelatinous particles that are easily trapped on the filter media and once there, may not always be easily removed by normal cleaning procedures.
Organic fouling
Filter bad media being in continuous contact with flowing water is always a heaven for the microbial growth. In plants where water is no disinfected in the post treatment stage, filtration media are the most likely place to have a contamination. Organic fouling grows and prospers in the filter bed sand, resulting in clogs or mud ball formation. This in turn reduces the filterability of the sand filter, as due to grown up size of the media on account of biofilm and organics formation gives more space for the water to pass and clogging of the turbidity in the media is stopped. And water passes through the sand media without any significant filtration.
In case of plant where presanitation of the water is done (Chlorine In most cases) residual chlorine at the inlet of filter bed is very small and insufficient to take care of the filter media contamination. Secondly even if there is presence of Biocide in water due to phenomenon called – Immunity development. The bacteria or microbes develop immunity/resistance to the biocide. Which in turn make the disinfection capacity of the biocide redundant. Due to above phenomenon development of biofilm starts getting developed inside the sand media. As the bacteria’s and microbes – excrete a glue like structure which works as an adhesive for the sand particle joining them together. This reduces the filtration efficiency to a very large extent.
By the time the increase in filter media size is noticed, it is usually too late to correct bynormal operational procedures, except by applying more drastic measures.
Three possible drastic measures can be used to rehabilitate filter media.
- Removing the fouled media and replacing with new, properly sized media would solve the problem. This is time consuming and expensive.
- Not only is the new media costly, but removing and disposing of the old media is not always a simple task.
- Another alternative would be to remove the media from the filter and clean the particles, and reuse the original media. This choice of renovating the media is time consuming and expensive. Moreover this method is not very successful as it is difficult to remove inorganic chemical precipitates or crystallized deposit.
- A third alternative is in situ cleaning, which is not effective.
Clarifications
Three possible drastic measures can be used to rehabilitate filter media.
By the time the increase in filter media size is noticed, it is usually too late to correct bynormal operational procedures, except by applying more drastic measures.
Performance of the sand stays consistent till the life of the filter.
There is a gradual drop in the filtration performance of the sand. As per the passage of time and due to biocoagulation and the polysaccharide glue secretion on the surface of the sand particle there is a gradual reduction in the coarseness of the sand and also due to abrasion. The filtration performance is inversely proportional to the size of the sand particle.
The size of the sand particle stays the same there is no change in the dimensions.
In addition to the biological aspect there is also scaling on the surface of the sand which is explained above in the discussion which would increase the size of the sand particle and also reduce the filtration ability of the media.
Quality of sand has no impact on the consumption of coagulants & Biocides.
Biocides – In most cases filter media are the most common place for the growth of any bacteria, fungus, molds, etc… The temperature and the humidity are ideal for any bacterial growth in the sand media and bed. Only the topmost layers of the sand are in continuous contact with water which is actually a filter media. All the remaining are the support media and they are the most common zones for the bacterial loads.
If the filter beds are not uniformly backwashed and cleaned than the residual biocide in the filter inlet water would be utilized and also the bed will exert its own demand.
Coagulants
The doses/quantity of coagulants are determined on the basis of the following parameters.
- Raw water turbidity.
- Turbidity at clarifier outlet
- Turbidity at filter bed outlet.
If the filtered are not functioning than there is a proportionate increase in the dose of coagulants in the pretreatment. And more particles would be required to settled down. Also filterbed are responsible for color & odour removal which will not be possible.
Particle size sand can filter stays same for the entire life span of sand.
Due to gradual increase in the size of the media the particle size sand can increase filter will increase gradually.
Eg – A system designed for less than 5 NTU filterbed discharge turbidity will be funcition at 7-8 NTU after 6-9 months. Hence the proportionate burden to remove the turbidity would be on the additional units.