Requirements backwashing

After treatment has proceeded for a period of time, either the coating reaches its maximum capacity to remove metals or the filter requires backwashing. At this time, the column can be backwashed to recover particulate metals from the column, and an acidic solution can be used to recover the adsorbed metals, thereby regenerating the column. Because the ferrihydrite is trapped on the sand particles, only the contaminant metals and nonferrihydrite are released. Thus the need to dispose of large amounts of iron oxide with the metal sludge, one of the main drawbacks of a conventional treatment process, is eliminated. 

Once the bed reaches saturation, additional oil brought in by the inlet water will displace an equal amount of oil into the discharge water. The coalescer will then require backwashing if the oil content of the discharge... 

This type loalescer does not require backwashing when oil saturation breakthrough occurs. It can be operated until the pressure drop across the media reaches some predetermined value due to solids and oil accumulations within the bed. 

Filter runs will vary with influent load and filter rate. Generally, however, the filters are designed to operate in the range of IS to 30 hours so that backwash water percentages are usually 2 percent or less. This minimizes disposal problems of the backwash water. Automatic operation of the filters is also suggested to reduce operator attention requirements. Backwash is generally initiated on filter hcadloss or on time. 

Filtration is employed when the suspended soUds concentration is less than 100 mg/L and high effluent clarity is required. Finely dispersed suspended soUds require the addition of a coagulant prior to filtration. Filters most commonly used in wastewater treatment are a dual media (anthrafUt and sand) or a moving bed or continuous-backwash sand filter. Performance data for the tertiary filtration of municipal and industrial wastewater are shown in Table 10. 

Carbon should be prewetted prior to being placed in the test columns. Backwashing the carbon at low rates (2.5 m/hr) does not remove the air. Rates that would expand the bed 50 percent or 15-30 m/hr, are required. The liquid used for prewetting can either be water, if it is compatible with the liquid to be treated, or a batch of the liquid to be treated which has been purified previously. There are three types of carbon systems (1) fixed beds, (2) pulse beds, and (3) fluidized beds, and these can be used singly, in parallel, or in combination. The majority of systems are either fixed or pulse beds. The two basic types of adsorbers which can be designed to operate under pressure or at atmospheric pressure are the moving or pulse bed and the fixed bed. Either can be operated as packed or expanded beds. 

The filter application is typically applied to handling streams containing less than 100 to 200 mg/Liter suspended solids, depending on the required effluent level. Increased-suspended solids loading reduces the need for frequent backwashing. The suspended solids concentration of the filtered liquid depends on the particle size distribution, but typically, granular media filters are capable of producing a... 

A typical physical-chemical treatment system incorporates three "dual" medial (sand anthracite) filters connected in parallel in its treatment train. The major maintenance consideration with granular medial filtration is the handling of the backwash. The backwash will generally contain a high concentration of contaminants and require subsequent treatment. 

Backwashing requires locating a source that will supply the necessary flow and pressure of wash water. This water can be provided either by a reservoir at a higher location or by a pumping station that pumps treated water. Sometimes an automated... 

The proper measure of flocculation effectiveness is the performance of subsequent solids separation units in terms of both effluent quality and operating requirements, such as filter backwash frequency. Effluent quality depends greatly on the reduction of residual primary size particles during flocculation, while operating requirements relate more to the floe volume applied to separation units. 

Membrane systems consist of membrane elements or modules. For potable water treatment, NF and RO membrane modules are commonly fabricated in a spiral configuration. An important consideration of spiral elements is the design of the feed spacer, which promotes turbulence to reduce fouling. MF and UF membranes often use a hollow fiber geometry. This geometry does not require extensive pretreatment because the fibers can be periodically backwashed. Flow in these hollow fiber systems can be either from the inner lumen of the membrane fiber to the outside (inside-out flow) or from the outside to the inside of the fibers (outside-in flow). Tubular NF membranes are now just entering the marketplace. 

Improved results may be obtained by substituting the volcanic lava for BIRM at a bed depth of 30 to 36 inches (76-91 cm) and a flow rate of 4 to 5 gpm/sq ft of media bed surface area. BIRM acts as a catalyst and normally requires only a periodic backwash to remove surface debris (backwash rate in pressure filter tank is 10 gpm/sq ft). 

During filtration the suspended solids are trapped between the grains of filter media. Because there must always be space remaining for the water to percolate, there is a limit to the total volume of solid sludge that can be tolerated before backwashing is required. Designers typically set this limit at 25% of the total void volume and, irrespective of the media grain size, the void volume is approximately 45% of the total media volume. 

To backwash carbon requires a minimum of 30 to 35% bed expansion, with a flow rate of 8 to 10 gpm/sq ft for 15 minutes. Because of the presence of fines in new carbon, the initial backwash should be double this time period. 

Bed expansion (freeboard) is 50% minimum (thus, the resin tank must be at least double the volume of the resin requirement). Resin bed expansion is a function of backwash rates and temperature. 

Where an RO is designed for continuous operation, dual or triple MM filters are required (similar to ion-exchange softening) and, because an RO plant can operate only at a fairly constant RW supply rate (it is basically either on or off), additional water must be available for filter backwashing. 

Quench waste recirculation requires installation of a cooling tower to lower the temperature of the quench wastewater stream. The multimedia filter system for the final polishing of effluent includes a backwash mechanism, pumps, control media, and the filter structure. 

Filtration Test. The filtration test equipment consisted of a glass column, with a fritted disc 2 cm. diameter, fitted with a stopcock and a 25 ml graduated cylinder. After mixing the MCC sols with electrolyte solution, the mixture was immediately poured into the glass column with the stopcock closed. The time required to collect water (blank) and filtered sol between 5 to 25 mis in the graduated cylinder after 2 hrs. was determined. The fritted glass was backwashed with water thoroughly after each measurement and a new blank was established. The relative filtration time between sample and water was used to compare differences between samples. 

Similar to filter backwash, the concentrate from these membranes requires treatment before it can be disposed of with the membrane concentrate. However, the total amount of solids produced after the treatment of filter backwash can be 60-80% greater than MF and UF concentrate due to the addition of coagulants prior to the granular media filters (Bergman 2007). 

Pretreatment is required to remove suspended solids from the concentrate and to reduce the likelihood of precipitates forming in the injection zone. Solids may be present in the concentrate, especially when filter backwash is blended with the concentrate prior to injection. Pretreatment costs include filtration of the solids and chemicals for pH adjustment (Mickley 2006). 

Mouse bioassay. The digestive glands of shellfish were extracted thrice with acetone at room temperature. After removal of acetone by evaporation, the aqueous suspension was extracted thrice with diethyl ether, the combined ether solution was backwashed twice with small portions of water and evaporated. The residue was suspended in 1% Tween 60 solution and serially diluted suspensions were injected intraperitoneally into mice weighing 17-20 g each. The mice were observed for 24 hr and the minimum amount of toxin required to kill a mouse at 24 hr was defined as one mouse unit (1). 

Where very high-quality effluent is required, the effluent is used for the backwash supply. In other cases, the filter influent can be used. A rinse cycle may be required. It should be noted that mixed media can also be used with air-water wash. However, no advantages are seen with air wash other than a possible reduced quantity of backwash water at the sacrifice of the more complex backwash control system. A typical all-water backwash cycle is 5 to 8 minutes. An air-water wash cycle may require 20 to 30 minutes. 

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