Backwash water

Hydraulic classification The rearrangement of resin particles in an ion-ex- change unit. As the backwash water flows up through the resin bed, the particles are placed in a mobile condition wherein the larger particles settle and the smaller particles rise to the top of the bed. 

The chemical constituents of the backwash water (which may be acidified) and of the various eluants (recycling, new and special) are specific to the problem of displacing the captured ions from the resin and reactivating its sites. The final stage of IX, namely, the recovery in a solid form of the values from the eluting solution, is effected by chemical methods which yield the desired precipitate. This is settled out, filtered and dried for despatch. 

Its unique compact and efficient design is made possible by the use of the principle of zero velocity eliminating internal water turbulence (see below). The flocculated water thus stands still in the flotation tank for optimum clarification. The unit is complete with automatic backwash filter in which dirty backwash water is recycled back to the unit inlet for reprocessing. The average waste flow from the process is less than 1.0% of the incoming raw water. 

Wang, L.K. Recycling and reuse of filter backwash water containing alum sludge. Water Sewage... 

Wang, L.K. Continuous pilot plant study of direct recycling of filter backwash water. J. Am. Water Works Assoc. 1973, 65 (5), 355-358. 

Figure 10 Carbon adsorption flow diagram. The carbon columns are operated in series backwash water is provided by a pump (from Ref. 11).

The rating of this filter is 20-40 gpnv sq rt essentially twice mat ot tire downflow multimedia filter because of the dual flow. Backwash water enters at the bottom and expands the bed in the same fashion as the graded sand bed and multimedia filters In some cases, raw water may be used, it would be prudent to follow backwashing with a short rinse before returning to service. 

To reduce costs, the aerated lagoon effluent is used for the backwash water for the filter system. The same oil company reports similar results from an 8-ft-diameter, vertical pressure filter system in a refinery in Italy. 

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. 

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. 

For filtration to be effective and efficient, the suspended solids must be able to penetrate the filter bed to a sufficient depth without clogging the filter, so the choice of filter media, effective grain size, and bed depth is important. Equally, good distribution of raw water, efficient collection of filtered water, and distribution of backwash water are other vitally important design factors. 

Service flow rates for RO pretreatment should be about 5 gpm/ ft2 of media. Throughput can be estimated using a filter about 0.45 lb of suspended solids per square foot of filter loading of area. Backwash rates should be 15 gpm/ft2 at 60°F. Lower water temperatures require higher flow rates to adequately raise the bed for a complete backwash. A 30 - 50% bed expansion is necessary to achieve good backwashing of the media. Raw, unfiltered water can be used for backwash. Alternatively, a filtered product tank can be provided that also acts as a reservoir for backwash water. Some operators choose to recovery RO reject by using this water to backwash filters. However, for best results, use of RO reject water to backwash a multimedia filter is not recommended. In some applications where... 

Multimedia pressure filters can be vertically or horizontally oriented. Figure 8.7 shows a vertical filter while Figure 8.8 shows a horizontal multimedia pressure filter. Horizontal multimedia filters are separated internally into "tanks" or compartments. Each tank acts as an individual filter. When it is time to backwash one of the tanks, the effluent from the other tanks provides the backwash water. The key in selecting horizontal filters is that the filter should have enough internal tanks so that productivity (required effluent flow rate plus the... 

Backwashing Backwashing the resin removes suspended solids and resin fines that may have collected in the vessel. Typical flow rates for backwashing range from about 4 to 8 gpm/ft2 for a minimum of 10 minutes or until the backwash water runs clear. The target is to expand the bed by 50% for adequate backwash. Backwash water is usually sent to drain. 

Low suction pressure is typically a result of inadequate water supply to the RO feed pump caused by upstream demand starving the RO system. Upstream demands include filter backwash water and water diverted for other applications within the facility. Starving of the RO due to equipment backwashing upstream is a system design flaw. Diversion of feed water usually occurs during installation or... 

The annual energy consumption by microscreens is shown in Fig. 2 (9). Electrical energy requirements include backwash water pumping and screen drive (10). 

The chlorine concentration in stored water gradually decreases with time due to aeration, reaction with sunlight, and reaction with surfaces of holding tanks. A number of utilities store filter backwash water and main disinfection water in holding tanks to allow for residual chlorine decay. On some occasions the chlorine demand of the water is reduced by retention in a holding tank for some time, prior to dechlorination using chemicals. 

Sulfur dioxide is widely used in water and wastewater treatment plants for dechlori-nating backwash water and wastewaters containing chlorine. It is a colorless gas with a suffocating pungent odor. Sulfite ion, which is formed when SO2 gas dissolves in water, reacts instantaneously with free and combined chlorine according to the following stoichiometry (13) ... 

A similar but more improved evaporation lagoon system has been used at Feura Bush Filtration Plant of the City of Albany, New York, USA. The City of Albany s evaporation lagoon system involves the use of natural evaporation, freezing, thawing, and sedimentation processes for treatment of combined filter backwash water and sedimentation waste slndge. 

M. Krofta and L. K. Wang, Recychng of filter backwash water and alum sludge for reuse in water treatment Plants. Biennial Conference of the National Water Supply Improvement Association, July, 1988. 

Figure 8.8 Asahi moving packed bed. A, adsorption section B, elution section C, fluidized resin backwash D, resin collection hoppers with screen top and non-return valve outlet for resin E, transfer and backwash water overflow F, feed G, barren effluent H, eluant I, eluate product J, backwash supply K, resin flow [1]. Copyright 1987 John WUey Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

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