(1) General.
Solids contact units are generally acceptable for combined
softening and clarification where water characteristics, especially
temperature, do not fluctuate rapidly, flow rates are uniform and operation is
continuous. Before such units are considered as clarifiers without softening,
specific approval of the Director shall be obtained. A minimum of two units are
required for surface water treatment.
(2) Installation of Equipment
The design engineer shall see that a representative of the
manufacturer is present at the time of initial start-up operation to assure
that the units are operating properly.
(3) Operation of Equipment.
The following shall be provided for plant operation:
(a) a complete outfit of tools and
accessories,
(b) necessary
laboratory equipment, and
(c)
adequate piping with suitable sampling taps so located as to permit the
collection of samples of water from critical portions of the units.
(4) Chemical feed.
Chemicals shall be applied at such points and by such means
as to insure satisfactory mixing of the chemicals with the water.
(5) Mixing.
A flash mix device or chamber ahead of solids contact units
may be required to assure proper mixing of the chemicals applied. Mixing
devices employed shall be so constructed as to:
(a) provide good mixing of the raw water with
previously formed sludge particles, and
(b) prevent deposition of solids in the
mixing zone.
(6)
Flocculation.
Flocculation equipment:
(a) shall be adjustable (speed and/or
pitch),
(b) shall provide for
coagulation in a separate chamber or baffled zone within the unit,
and
(c) shall provide the
flocculation and mixing period to be not less than 30 minutes.
(7) Sludge concentrators.
(a) The equipment shall provide either
internal or external concentrators in order to obtain a concentrated sludge
with a minimum of waste water.
(b)
Large basins shall have at least two sumps for collecting sludge with one sump
located in the central flocculation zone.
(8) Sludge removal.
Sludge removal design shall provide that:
(a) sludge pipes shall be not less than three
inches in diameter and so arranged as to facilitate cleaning,
(b) the entrance to the sludge withdrawal
piping shall prevent clogging,
(c)
valves shall be located outside the tank for accessibility, and
(d) the operator may observe and sample
sludge being withdrawn from the unit.
(9) Cross-connections.
(a) Blow-off outlets and drains shall
terminate and discharge at places satisfactory to the Director.
(b) Cross-connection control must be included
for the finished drinking water lines used to back flush the sludge
lines.
(10) Detention
period.
The detention time shall be established on the basis of the
raw water characteristics and other local conditions that affect the operation
of the unit. Based on design flow rates, the detention time shall be:
(a) two to four hours for suspended solids
contact clarifiers and softeners treating surface water, and
(b) one to two hours for suspended solids
contact softeners treating only ground water.
(11) Suspended slurry concentrate.
Softening units shall be designed so that continuous slurry
concentrates of one percent or more, by weight, can be satisfactorily
maintained.
(12) Water
losses.
(a) Units shall be provided with
suitable controls for sludge withdrawal.
(b) Total water losses shall not exceed:
(i) five percent for clarifiers,
(ii) three percent for softening
units.
(c) Solids
concentration of sludge bled to waste shall be:
(i) three percent by weight for
clarifiers,
(ii) five percent by
weight for softeners.
(13) Weirs or orifices.
The units shall be equipped with either overflow weirs or
orifices constructed so that water at the surface of the unit does not travel
over 10 feet horizontally to the collection trough.
(a) Weirs shall be adjustable, and at least
equivalent in length to the perimeter of the basin.
(b) Weir loading shall not exceed:
(i) 10 gpm per foot of weir length for units
used for clarifiers
(ii) 20 gpm per
foot of weir length for units used for softeners.
(c) Where orifices are used the loading rates
per foot of launderer shall be equivalent to weir loadings. Either shall
produce uniform rising rates over the entire area of the tank.
(14) Upflow rates.
Upflow rates shall not exceed:
(a) 1.0 gpm/sf at the sludge separation line
for units used for clarifiers,
(b)
1.75 gpm/sf at the slurry separation line for units used as
softeners.
