PACKAGE : I Bid Document VOLUME : II - Public Health Engineering

21.12 Filtration Works.
The works to be included by the contractor in this section of the treatment process
shall comprise of atleast but not limited to the following : The design and necessary
arrangements for conventional downward rapid gravity filtration of the works,
flows in a filter battery, operating at variable head and declining rate of flow. The
design and necessary arrangement to distribute the flows between the filters, cleanse
the filters, drain out the back wash water through drains, also the design of back
wash arrangement inside the filter units with adequate provisions of piping and
pumping machinery required and collect the filtered water for final disinfecting.
The design and necessary arrangement to construct the filtration works. The design
and layouts for administrative purposes for the filter house and machinery's hall
attached with the filter battery and back wash works. The supply of filtering media
and all the necessary machinery and equipment required for the filters and the
construction of pump house, for necessary pumping machinery and arrangements
for collection of filter back wash water and to provide measuring devices of head
losses and flow etc.
The filter bed shall be constructed in reinforced concrete boxes, shall be covered,
total no. of filter beds 2 Nos. made out of 2 sections with common in and out
channel and a common filtered water channel and pipe gallery on the side of filter
controls. The filter to back wash waste water channels, shall be arranged below
filter influent channel, and connected to drain pipe. The filters shall be arranged for
convenience of operation and maintenance, minimization of hydraulic losses and
the optimization of performance in terms of quantity and quality of treated water.
The filtration works shall be designed to receive water from sedimentation tanks.
The average filtration rate shall be ranging from 80 to 100 Lpm/m² Each filter unit
shall contain a single sized silica sand bed (effective size in the region of 0.45 mm
to 0.70 mm, uniformity coefficient not greater than 1.70 and not less than 1.30 &
depth not less than 0.75 m) carried on a gravel media of 0.60 m layer thickness, for
a piped under-drain system. The filters shall be designed for cleaning with reverse
flow of air followed by a water wash at sufficient rate to fluidize and expand the
bed, detach and remove the filter burden, without assistance of surface wash.
21.13 The backwash rates for air and water shall be.
Air Water
0.35 Kg/cm² (min) pressure at orifice 600 Lpm/m 2 for a total duration of
for a duration of 5 minutes 10 minutes
DTS : Detailed Tender Specification. Page 249