Microbes, filtering

The filter media must be a source of inorganic nutrients for the microbes. In cases of long-term operation, inorganic nutrients can be periodically added to the bed. 

Similar to other types of water treatment, AC filtration is effective for some contaminants and not effective for others. AC filtration does not remove microbes, sodium, nitrates, fluoride, and hardness. Lead and other heavy metals are removed only by a very specific type of AC filter. Unless the manufacturer states that its product will remove heavy metals, one should assume that the AC filter is not effective in removing them. 

Biofilters—Processes in which contaminants are biodegraded in a fixed-film bioreactor, typically consisting of a bed of high surface area filter media, such as GAC, that acts as a support matrix for a thin film consisting of microbes that are acclimated to the biodegradation of MTBE or other contaminants. 

The first level of treatment, with sand filters and chlorination to remove suspended matters and disinfection of pathogens, may be good enough for the low-cost water. The removal of discoloration and bad smell is accomplished by activated charcoal absorption. Ozone and ultraviolet treatments are much more expensive for the removal of microbes and organic matter, and should be considered only when necessary to solve a technical problem, or to satisfy an advertisement need. Reverse osmosis is the most effective method used to recover clean water from brackish water, and to remove inorganic minerals such as sodium, copper, iron, and zinc. The removal of calcium and magnesium ions can be accomplished by the method of ion exchange with sodium, which would also increase the sodium concentration, and could cause objections. Different levels of treatment require a variety of costs, and can produce different levels of customer satisfaction. 

T0826 U.S. Filter/Zimpro Products, Powdered Activated Carbon Treatment (PACT) T0829 U.S. Microbics, Inc., Bio-Raptor... 

T0812 Trans Coastal Marine Services, Bioplug/Bioconduit T0823 U.S. Filter Corporation, PO WW ER Wastewater Treatment System T0826 U.S. Filter/Zimpro Products, Powdered Activated Carbon Treatment (PACT) T0827 U.S. Filter/Zimpro Products, Wet Air Oxidation T0829 U.S. Microbics, Inc., Bio-Raptor... 

The technology is a combination of biodegradation, adsorption onto activated carbon, and microfiltration. It uses microbes grown on active powdered carbon as a prefilter and to degrade organics and a cross-flow membrane filter to remove biomass, viruses, and suspended impurities. 

FDA Guidance The specific bulk product solution filtration processes, including tandem filter units, prefilters, and bacterial retentive filters, should be described. A summary should be provided containing information and data concerning the validation of the retention of microbes and compatibility of the filter used for the specific product. Any effects of the filter on the product formulation should be described (e.g., adsorption of preservatives or active drug substance, or extractable). 

Naturally, the size ofthe membrane pores should depend on the size of microbes to be filtered. For example, membranes with 0.2 pm pores are often used to sterilize culture media, while membranes with 0.45 pm pores are often used for the removal of microbes from culture products. It should be noted here that the so-called pore size of a membrane is the size of the largest pores on the membrane surface. The sizes of the pores on the surface are not uniform moreover, the pore sizes usually decrease with increasing depth from the membrane surface. Such a pore size gradient will increase the total filter capacity, as smaller microbes and particles are captured within the inner pores of the membrane. 

The rates of filtration of microbes (particles) at a constant pressure difference decrease with time due to an accumulation of filtered microbes on the surface and inside the pores of the membrane. Hence, in order to maintain a constant filtration rate, the pressure difference across the membrane should be increased with time due to the increasing filtration resistance. Such data as are required for practical operation can be obtained with fluids containing microbes with the use of real filter units. 

The impure water is then removed and sent to a trickling filter, where it is allowed to drain through gravel on which microbes have been deposited. These kill off any remaining bacteria in the water by aerobic processes. This stage is known as biological filtration. 

Microbes are not destroyed by bacteria-retentive filters but instead become concentrated in and on them. Certain bacteria have the capability of growing through a membrane fdter. Also, filters can become damaged by frequent or sudden changes in water pressure (water hammer). 

Most faucet units and pitchers use carbon filters that absorb lead, chlorine by-products, and some organic chemicals, as well as odors and tastes. They won t remove heavy metals, pesticides, nitrites, bacteria, or microbes, but they are the least-expensive filter type and are sufficient for most needs. 

Distillers boil water into steam, then condense it back into water in a separate chamber, leaving behind particles and dissolved solids. Since the water is heated, distillers kill microbes as well as eliminate other pollutants, including arsenic, but not volatile organic compounds and chlorine, which are usually removed by an accompanying carbon filter. 

Chlorine can be removed from RO feed water using sodium bisulfite or carbon filtration (see Chapters 8.2.4 and 8.1.4, respectively). As discussed in Chapter 8.1.4, carbon in carbon filters can aide the growth of microbes so carbon filtration is typically not recommended for dechlorination of RO feed water unless the concentrations of organics is high enough to warrant its use, or if the dosage of sodium bisulfite is too low for accurate control. 

Cartridge filter replacement frequency, high replacement rates (every 2 weeks or less) could indicate a fouling problem. Low replacement rates (every 1 month or more) could lead to microbial fouling as microbes grow in the "old" cartridges. 

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