Page 455 - Water and wastewater engineering

11-28 WATER AND WASTEWATER ENGINEERING

Screens are designed to be used without support gravel. Air scour may not be used with this
system.
The false bottom systems with nozzles are primarily used in filters using air/wash systems.
Fine openings in the nozzles eliminate the need for support gravel. This reduces the required fil-
ter box depth in new designs and allows for deeper media depth in retrofit designs.
Porous bottom underdrains are constructed of aluminum oxide plates with very small pore
sizes. The pores are susceptible to plugging and are not suitable for softening or iron/manganese
removal plants.
The underdrain is a crucial feature of the filter design, and, in turn, it is crucial for the suc-
cess of the plant in removing turbidity before disinfection. It needs to be physically strong as well
as easy to install and maintain. None are free from disadvantages, but many problems can be
avoided by selection of proven technology. Careful inspection during and after installation, but
also before placement of filter media, is essential.
The headloss during filtration or backwashing is on the order of 0.1 to 0.3 m for modern
underdrains. The pipe and lateral system headloss during backwash may be as large as 0.6 m
(MWH, 2005). During filtration the conduit losses will be negligible because the conduits are
sized to carry a backwash flow rate that is several times the filtration flow rate.

Backwashing
The need for backwash is indicated by one of three criteria:

• The headloss across the filter reaches the maximum design level or more customarily a
preset limit in the range of 2.4 to 3.0 m;
• The filtrate turbidity reaches a set upper limit;

• Or, some maximum time limit, usually in the range of three or four days, has been reached.
The traditional backwash system in the United States is an upflow water wash with full bed
fluidization. Backwash water is introduced into the bottom of the bed through the underdrain sys-
tem. As the flow rate is increased, the entire bed expands. The backwash flow is continued until

the wash water is reasonably clear. Figure 11-11 may be used to estimate an appropriate backwash
rate. Each degree Celsius increase in water temperature requires about a 2 percent increase in the
wash rate to prevent a reduction in bed expansion. The wash system should be designed for the
warmest wash water temperature that may be expected (Castro et al., 2005). Typical wash volumes
3
2
range from 4 to 8 m of wash water/m of surface area of the bed (Cleasby and Logsdon, 1999).
Storage capacity for two backwashes should be provided (Kawamura, 2000). GLUMRB (2003)
specifies that not less than 15 minutes of wash of one filter at the design wash rate be provided.
For sand filters, backwash flow rates range between 30 and 60 m/h for periods from 10 to
20 minutes (MWH, 2005). The limiting factor in choosing a backwash rate is the terminal set-
tling velocity of the smallest medium grains that are to be retained in the filter. Because the filter
backwashing process is effectively an upflow clarifier, the backwash rate becomes the overflow
rate that determines whether a particle is retained in the filter or is washed out through the back-
wash trough.
Another design criterion that is often used is to make sure that the largest, or the 90th percentile,
largest diameter particles are fluidized. In a dual-media filter, the d 90 of the anthracite is considered