Filter backwashing

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. 

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. 

Over 22.7 million kg (50 million lb) of zinc sulfate are used annually in the U.S. for the manufacture of approximately 454 million kg (one billion lb) of viscose rayon. Zinc is used as a regeneration retardant in the acid spinning bath. Because it is not consumed in any of the viscose reactions, these 22.7 million kg (50 million lb) of zinc represent process losses, through dragout by the filaments to the subsequent wash streams, filter backwashing, splashes, leaks, and the washing of equipment.14... 

The flotation-filtration unit can be either manually operated or completely automated with a level control [17] that operates the inlet flow valve [18]. Filter backwashing can also be automated by a timer or head loss control [19]. 

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). 

However, because both acids and bases are used in cleaning, it may be possible to store these chemicals in a stirred buffer tank to allow the solution to neutralise. This solution may then may be slowly added to the concentrate waste to dilute it and allow for a safe disposal (Mauguin and Corsin 2005). Note that the volume of this discharge is much lower than both the concentrate and filter backwash discharges. 

Coagulants, which are present in filter backwash, are of low toxicity and are not considered a major environmental concern. One of the greatest effects of coagulants comes through the use of ferric salts, which are likely to cause colouration and increase the turbidity of the backwash (Lattemann and Hopner 2008). The quantity of coagulants in the concentrate can in part be regulated by the slow release of filter backwash from a buffer tank into the concentrate flow. 

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). 

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. 

The major waste streams in bar soap manufacture are the filter backwash, scrubber waters, or condensate from a vacuum drier, and water from equipment washdown. The main contaminant of all these streams is soap that will contribute primarily BOD and COD to the wastewater. 

The process centers on a fixed-bed downflow reactor that allows catalyst replacement without causing any interruption in the operation of the unit (Figure 9-28). Feedstock is introduced to the process via a filter (backwash, automatic) after which hydrogen and recycle gas are added to the feedstock stream which is then heated to reactor temperature by means of feed-effluent heat exchangers whereupon the feed stream passes down through the reactor in trickle flow. Sulfur removal is excellent (Table 9-18), and substantial reductions in the vanadium content and asphaltenes content are also noted. In addition, a marked increase occurs in the API gravity, and the viscosity is reduced considerably. 

There are advantages to hard piping most of the RO system, but some piping needs to remain open so that flows can be observed and measured. In particular, waste flows to drain, such as filter backwash waste, RO reject, and RO permeate divert streams, should be accessible before they enter the drain so that they can be easily observed and sampled if needed waste-flow piping should end 6 to 10 inches above the 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... 

Hall, D. and C. S. B. Fitzpatrick (1997). Mathematical filter backwash model. Water Science and Technology Proc. 7th Int. Workshop on Instrumentation, Control and Automation of Water and Wastewater Treatment and Transport Syst, July 6-9, Brighton, England, 37, 12, 371-379. Elsevier Science Ltd., Exeter, England. 

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. 

An engineer s rule of thumb for sludge production rate of a comparable conventional water purification plant is usually set at 75 dry Ib/d/MGD. It is believed that the low sludge production rate of a conventional plant is caused by discharge of filter backwash wastewater, without recycle. Thus, the sludges in the discharged wastewater are not included in sludge quantity estimation (30,31,44) ... 

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. 

Non-process Non-contact cooling (recycle to clear well) Facility, equipment and implement wash-up Filter backwash Environmental (humidity) control in processing/storage areas... 

According to the TCLP Method 1311 (EPA SW 846) (40), for a liquid waste containing less than 0.5% solids, the liquid portion of the waste after filtration is defined as the TCLP extract. For a microfiltration system operated at a 2.5 mg Fe(III)/L dose, 2.8 gpm flow rate (1.4 gpm/m ), and 29-min backwash interval, and assuming that all the solids are removed from the filter upon backwash, the backwash water (assuming a backwash volume of 1.8 gal/m ) will have a solids content (calculated) of 0.01% (by wt). Arsenic concentration in such a filtered backwash water (average of 20 filtered samples) was determined to be 2.6 2.4 ug/L. The backwash water can thus be directly disposed as a nonhazardous waste assuming that the arsenic TCLP limit stays at its current value of 5000 fig/L. 

Pilot-scale study on treatment of spent filter backwash water by ultrafiltration... 

ABSTRACT Ultrafiltration membrane was used to treat Spent Filter Backwash Water (SFBW) of carbon filter and sand filter in a certain Water Treatment Plant (WTP). The results showed that the removal rate of turbidity by ultrafiltration was above 98%, the total bacteria count was less than 3 CFU/mL and the CODMn was about 1 mg/L. Ultrafiltration membrane ran stably when inflow flux was 60 L/(m h) and the cumulative water yield was about 1135 m after continuously running of 26 d, the reclamation rate was higher than 95% and the water producing cost was about 0.15 CNY/m. ... 

Arora, H. Lechevallier, M. 2001. Spent filter backwash water contaminants and treatment stratgies. AWWA (American Water Works Association) 93(5) 100-111. 

Reissmann, F. G. Uhl, W. 2006. Ultrafiltration for the reuse of spent filter backwash water from drinking water treatment. Desalination 198 225-235. 

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