Backwash water reuse

The trend in the use of deep bed filters in water treatment is to eliminate conventional flocculators and sedimentation tanks, and to employ the filter as a flocculation reactor for direct filtration of low turbidity waters. The constraints of batch operation can be removed by using one of the available continuous filters which provide continuous backwashing of a portion of the medium. Such systems include moving bed filters, radial flow filters, or traveling backwash filters. Further development of continuous deep bed filters is likely. Besides clarification of Hquids, which is the most frequent use, deep bed filters can also be used to concentrate soflds into a much smaller volume of backwash, or even to wash the soflds by using a different Hquid for the backwash. Deep bed filtration has a much more limited use in the chemical industry than cake filtration (see Water, Industrial water treatment Water, Municipal WATERTREATiffiNT Water Water, pollution and Water, reuse). 

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

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. 

During the backwash phase, water is transferred from the water reuse tank to the GMF by the backwash pump at 170 m /h and 3 bar g. Water flows from the bottom upwards through the bed. A high superficial velocity of25—35 m/h is required to fluidise the media bed and remove dirt from the bed. The backwash phase lasts 15 min and is followed by a 2-min bed settle phase when no water flows to the filter. This allows the bed media to recover or rest before it is rinsed. During the rinse phase, the standby feed water pump transfers water to the media filter at 70 m /h and 3 bar g. Water flows downwards through the media bed and out from the bottom to drain. The rinse phase lasts 5 min. Once the GMF has been washed, it is placed in standby mode until the online unit needs to be washed. 

Semi-dead end UF/MF membranes (effective pore size of the membrane is <0.1 pm) with intermittent backwash are being increasingly used for surface water and wastewater treatment for re-use, e.g. secondary or tertiary effluent is treated for industrial, non-potable and, in some cases, potable water reuse using UF/RO (or MF/RO) plus advanced oxidation techniques such as UV disinfection and hydrogen peroxide. The process is described in detail in Chapters 2 and 4 and several examples discussed in Chapter 3. Prominent examples of advanced reclamation plants include Water Factory 21 in Cahfornia, NEWater Factory in Singapore and the Goreangab Reclamation plant in Namibia [2]. 

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

It is understandable that the potable water flotation plant recycles its backwash wastewater for reuse, and in turn, has higher sludge production rate (150.8 instead of 75 dry Ib/d/MGD) because almost all sludges are captured by dissolved air flotation. Table 5 further confirms the plant s sludge flow rate is about 0.7% of influent flow rate. The raw sludge concentration of TSS is about 2600 mg/L. 

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