(1) Chemical
Addition and Mixing.
Direct Filtration is conventional surface water treatment
without the sedimentation process. Rules for Chemical Addition and Mixing shall
be the same as found in sections
R309-525-11 and
R309-525-12.
(2) Source Water Quality.
Direct Filtration applies the destabilized colloids to the
filter rather than removing the majority of the load through sedimentation.
While this process represents considerable construction cost savings, the
source water must have low average turbidity in order to provide reliable
service without excessive backwash requirements. Source water with low average
turbidity is generally only obtained from large capacity reservoirs.
(3) Design Requirements.
The following requirements shall apply to Direct Filtration
plants:
(a) At least one year's record
of source water turbidity, sampled at least once per week, shall be presented
to the Director. A Direct Filtration facility will only be permitted if the
data shows that 75% of the measurements are below five (5) NTU. The Director
shall judge whether Direct Filtration is suitable given the quality of the
proposed source water (see
R309-515-5(2)(a)(ii)).
(b) Pilot plant studies,
acceptable to the Director, shall be conducted prior to the preparation of
final engineering plans.
(c)
Requirements for flash mix and flocculation basin design are given in
sub-sections
R309-525-12(1)
and
R309-525-12(2).
(d) Chemical addition and mixing equipment
shall be designed to be capable of providing a visible, but not necessarily
settleable, floc.
(e) Surface wash,
subsurface wash, or air scour shall be provided for the filters in accordance
with sub-section
R309-525-15(7).
(f) A continuous monitoring turbidimeter
shall be installed on each filter effluent line and shall be of a type with at
least two alarm conditions capable of meeting the requirements of subsections
R309-525-15(4)(b)(vi)
or
R309-525-15(4)(c)(vii).
The combined plant effluent shall be equipped with a continuous turbidimeter
having a chart recorder. Additional monitoring equipment to assist in control
of the coagulant dose may be required (i.e. streaming current gauges, particle
counters, etc.) if the plant cannot consistently meet the requirements of rule
R309-200.
(g) In addition to the
alarm conditions required above, the plant shall be designed and operated so
that the plant will automatically shut down when a source water turbidity of 20
NTU lasts longer than three hours, or when the source water turbidity exceeds
30 NTU at any time.
(h) The plant
design and land ownership surrounding the plant shall allow for the
installation of conventional sedimentation basins. Sedimentation basins may be
required if the Director determines the plant is failing to meet minimum water
quality or performance standards.
