(1)
Acceptability.
Slow sand filtration means a process involving passage of raw
water through a bed of sand at low velocity resulting in substantial particle
removal by physical and biological mechanisms. The acceptability of slow sand
filters as a substitute for "conventional surface water treatment" facilities
(detailed in R309-525) shall be determined by the Director based on suitability
of the source water and demand characteristics of the system.
(2) Source Water Quality.
The Director may impose design requirements in addition to
those listed herein, in allowing this process. The following shall be
considered, among other factors, in determining whether slow sand filtration
will be acceptable:
(a) Source water
turbidity must be low and consistent. Slow Sand Filtration shall be utilized
only when the source waters have turbidity less than 50 NTU and color less than
30 units (see
R309-515-5(2)(a)
).
(b) The nature of the turbidity
particles shall be considered. Turbidity must not be attributable to colloidal
clay.
(c) The nature and extent of
algae growths in the raw water shall be considered. Algae must not be a species
considered as filter and screen-clogging algae as indicated in "Standard
Methods for the Examination of Water and Wastewater" prepared and published
jointly by American Public Health Association, American Water Works
Association, and Water Environment Federation. High concentrations of algae in
the raw water can cause short filter runs; the amount of algae, expressed as
the concentration of chlorophyll "a" in the raw water shall not exceed 0.005
mg/l.
(3) Pilot Plant
Studies.
The Director shall allow the use of Slow Sand Filtration only
when the supplier's engineering studies show that the slow sand facility can
consistently produce an effluent meeting the quality requirements of rule
R309-200. The Director should be consulted prior to the detailed design of a
slow sand facility.
(4)
Operation.
Effluent from a Slow Sand Filtration facility shall not be
introduced into a public water supply until an active biological mat has been
created on the filter.
(5)
Design requirements.
The following design parameters shall apply to each Slow Sand
Filtration plant:
(a) At least three
filter units shall be provided. Where only three units are provided, any two
shall be capable of meeting the plant's design capacity (normally the projected
"peak daily flow") at the approved filtration rate. Where more than three
filter units are provided, the filters shall be capable of meeting the plant
design capacity at the approved filtration rate with any one filter removed
from service.
(b) All filters shall
be protected to prevent freezing. If covered by a structure, enough headroom
shall exist to permit normal movement by operating personnel for scraping and
sand removal operations. There shall be adequate manholes and access ports for
the handling of sand. An overflow at the maximum filter water level shall be
provided.
(c) The permissible rates
of filtration shall be determined by the quality of the source water and shall
be determined by experimental data derived during pilot studies conducted on
the source water. Filtration rates of 0.03 gpm/sf to 0.1 gpm/sf shall be
acceptable (equivalent to two to six million gallons per day per acre).
Somewhat higher rates may be acceptable when demonstrated to the satisfaction
of the Director.
(d) Each filter
unit shall be equipped with a main drain and an adequate number of lateral
underdrains to collect the filtered water. The underdrains shall be so spaced
that the maximum velocity of the water flow in the underdrain will not exceed
0.75 fps. The maximum spacing of the laterals shall not exceed three feet if
pipe laterals are used.
(e) Filter
sand shall be placed on graded gravel layers for an initial filter sand depth
of 30 inches. A minimum of 24 inches of filter sand shall be present, even
after scraping. The effective size of the filter sand shall be between 0.30 mm
and 0.45 mm in diameter. The filter sand uniformity coefficient shall not
exceed 2.5. Further, the sand shall throughly washed and found to be clean and
free from foreign matter.
(f) A
three-inch layer of well rounded sand shall be used as a supporting media for
filter sand. It shall have an effective size of 0.8 mm to 2.0 mm in diameter
and the uniformity coefficient shall not be greater than 1.7.
(g) A supporting gravel media shall be
provided. It shall consist of hard, durable, rounded silica particles and shall
not include flat or elongated particles. The coarsest gravel shall be 2.5
inches in size when the gravel rests directly on the strainer system, and must
extend above the top of the perforated laterals. Not less than four layers of
gravel shall be provided in accordance with the following size and depth
distribution when used with perforated laterals:
|
TABLE 530-1 |
|
Size |
Depth |
|
2 1/2 to 1 1/2 inches |
5 to 8 inches |
|
1 1/2 to 3/4 inches |
3 to 5 inches |
|
3/4 to 1/2 inches |
3 to 5 inches |
|
1/2 to 3/16 inches |
2 to 3 inches |
|
3/16 to 3/32 inches |
2 to 3 inches |
Reduction of gravel depths may be considered upon
justification to the Director when proprietary filter bottoms are
specified.
(h) Slow sand
filters shall be designed to provide a depth of at least three to five feet of
water over the sand.
(i) Each
filter shall be equipped with: a loss of head gauge; an orifice, venturi meter,
or other suitable metering device installed on each filter to control the rate
of filtration; and an effluent pipe designed to maintain the water level above
the top of the filter sand.
(j)
Disinfection of the effluent of Slow Sand Filtration plants will be
required.
(k) A filter-to-waste
provision shall be included.
(l)
Electrical power shall be available at the plant site.
