Filtration - Public Water Systems - page 390

water supplies that contain Giardia and Cryptosporidium cysts. During normal start-
up of a filter, turbidity passage is small when averaged with the effluent turbidity of
the other filters. However, Giardia and Cryptosporidium are infective even at low
levels, so pose a health risk when passed through only one filter. Filter-to-waste piping
typically discharges to the backwash waste water pipe header. An air gap is needed
on the filter-to-waste connection to the backwash waste water piping to avoid creating
a cross-connection.
The addition of coagulant or polymer to the backwash water as an alternative to
filter-to-waste has been used in some cases. The goal is to condition the filter before
filtration resumes. It is not always successful. Some instances of floc formation and
sludge deposition in clearwells have been reported. 56
Filter Backwash Concerns
Filters can be seriously damaged by slugs of air introduced during filter backwashing.
The supporting gravel can be overturned and mixed with the fine media, which requires
removal and replacement of all media for proper repair. Air can be unintentionally
introduced to the bottom of the filter in a number of ways. If a vertical pump is used
for the backwash supply, air may collect in the vertical pump column between back-
washings. The air can be eliminated without harm by starting the pump against a
closed discharge valve and bleeding the air out from behind the valve through a pres-
sure air release valve. The air release valve must have sufficient capacity to discharge
the accumulated air in a few seconds.
Also, air or dissolved oxygen, released from the water on standing and warming
in the washwater supply piping, may accumulate at high points in the piping and be
swept into the filter underdrains by the inrushing washwater. This can be avoided by
placing a air release valve at the high point in the line, and providing a 1 ⁄ 2 -inch (12.7-
mm) pressure water connection to the washwater supply header to keep the line full
of water and to expel the air.
The entry for washwater into the filter bottom must be designed to dissipate the
velocity head of the washwater in such a manner that uniform distribution of wash-
water is obtained. Lack of attention to this important design factor has often led to
difficult and expensive alterations and corrective repairs to filters.
The use of a high-pressure—above 15 psi (100 kPa)—source of filter backwash
water through a pressure-reducing valve is not advised. Numerous failures of systems
using pressure-reducing valves have so thoroughly upset and mixed the supporting
gravel and fine media that these materials have had to be completely removed from
the filter and replaced with new media.
Filter Agitators
Practice in the United States leans heavily toward installation of the essential, but
misnamed, ''surface wash'' on all new filters. ''Auxiliary scour'' or ''filter agitation''
better describes the function of this device, as it aids in cleaning much more than the
filter surface. Rotary surface washers are the most common, but fixed jets also have
been used successfully.
Adequate surface wash improves filter cleaning and prevents mudball formation
and filter cracking. Conventional rotary surface wash equipment consists of arms on
a fixed swivel supported from the washwater troughs about 2 inches (50 mm) above