The softening process selected shall be based upon the
chemical qualities of the raw water, the desired finished water quality, the
requirements for disposal of sludge or brine waste, the cost of plant and
chemicals, and plant location. The applicability of the process chosen shall be
demonstrated and discussed in detail in an engineering report. For very hard
water, the sodium levels in cation exchange softened water shall be considered
in selecting the treatment process. Following are requirements for specific
processes:
(1) LIME-SODA PROCESS. The
applicable design standards for lime-soda softening of groundwater are the same
as those for conventional clarification-filtration surface water treatment
plants, except that the minimum settling time may be reduced to 2 hours. Where
softening is included in the surface water treatment process, the clarification
criteria shall govern. In addition:
Note: See s.
NR 811.47 for criteria pertaining to softening with
solids contact units and s.
NR 811.49 for filtration requirements.
(a) Mechanical sludge removal equipment shall
be provided in the sedimentation basin.
(b) Determinations shall be made for the
carbon dioxide content of the raw water.
Note: When concentrations exceed 10 mg/l, the
economics of removal by aeration as opposed to removal with lime should be
considered. See s.
NR 811.45 for aeration requirements.
(c) Equipment for stabilization of water
softened by the lime-soda process is required.
Note: See s.
NR 811.58 for stabilization
requirements.
(d) Provisions
shall be included for proper disposal of softening sludges.
Note: See s.
NR 811.858 for design requirements.
(e) The use of excess lime may not be
substituted for chlorination or any other approved method of disinfection.
Note: See s.
NR 811.48.
(2) CATION EXCHANGE PROCESS. Iron, manganese
or a combination of both in the oxidized state or unoxidized state may cause
resin fouling in the ion exchange process. Pretreatment shall be required
whenever the content of iron, manganese, or a combination of both is one
milligram per liter or more. In specific instances, the department may also
require pretreatment when lesser amounts exist. All of the following
requirements shall be satisfied unless the department determines sufficient
justification for alternative operational parameters is provided by the vessel
manufacturer or demonstrated through pilot testing under s.
NR 811.44:
(a) The
units shall be of pressure or gravity type, of either an upflow or downflow
design, using automatic or manual regeneration. Automatic regeneration is
suggested for small plants. A manual override shall be provided for all
automatic controls.
(b) The design
capacity for hardness removal may not exceed 20,000 grains per cubic foot when
resin is regenerated with 0.3 pounds of salt per kilograin of hardness
removed.
(c) The depth of the
exchange material may not be less than 3 feet.
(d) The rate of softening may not exceed 7
gallons per square foot per minute, and the backwash rate shall be 6 to 8
gallons per square foot per minute.
(e) The freeboard design shall be based upon
the specific gravity of the media and the direction of water flow.
(f) The bottoms, strainer systems, and
support for the exchange materials shall conform to criteria provided for rapid
rate gravity filters in s.
NR 811.49.
(g) Facilities shall be included for even
distribution of the brine over the entire surface of both upflow or downflow
units. Backwash, rinse, and air relief discharge pipes shall be installed in
such a manner as to prevent back-siphonage.
(h) A bypass shall be provided around
softening units to produce a blended water of desirable hardness. Meters shall
be installed on the bypass line and on each softener unit. An automatic
proportioning or regulating device and shut-off valve shall be provided on the
bypass line. The department may require treatment of the bypassed water to
obtain acceptable levels of iron or manganese in the finished water.
(i) Waters having 5 units or more of
turbidity may not be applied directly to the cation exchange softener. Silica
gel materials may not be used for waters having a pH above 8.4 or when iron is
present. When the applied water contains a chlorine residual, the cation
exchange material shall be a type that is not damaged by residual chlorine.
Phenolic resin may not be used.
(j)
Brine storage tanks shall conform to the following requirements:
1. The wet storage tank shall be designed to
hold at least 1.5 times the volume of salt delivered to permit refill before
the tank is completely empty. The volume of both salt and brine storage to be
provided depends upon the size of the plant, the proximity and assuredness of
the salt source, and the method of delivery.
2. It shall be isolated from possible sources
of contamination.
3. It shall be
properly covered and equipped with manholes having overlapping watertight
covers to prevent entry of surface runoff where applicable.
4. Overflows and vents shall be designed in
accordance with s.
NR 811.64 (4) and
(8), respectively, where
applicable.
5. The water for
filling the tank shall be distributed over the entire surface of the tank by
pipes at least 2 pipe diameters above the maximum liquid level in the tank or
be protected from back-siphonage.
6. The underdrain collection system shall be
covered with a screen or perforated plate to allow brine but not salt to pass
through.
7. A sampling tap shall be
provided on the brine discharge line in order that the concentration of brine
can be determined. A suitable means for measuring the volume of brine used for
regeneration shall be provided.
8.
Pipes and contact materials shall be resistant to the aggressiveness of salt.
Steel and concrete shall be coated with a non-leaching protective coating that
is compatible with salt and brine and is certified by ANSI/NSF standard 61,
dated March 15, 2022, which is incorporated by reference.
Note: A copy of ANSI/NSF standard 61 dated
March 15, 2022 is available for inspection at the Legislative Reference Bureau
and may be obtained for personal use from NSF International,
https://www.nsf.org.
9. Bagged salt and dry bulk salt storage
shall be enclosed and separated from other operating areas to prevent damage to
equipment.
(k) The
requirements for brine wastes are found in s.
NR 811.854.
(l) Smooth-end sampling taps shall be
provided for control purposes. Taps shall be located on each raw water source,
each treatment unit influent and each treatment unit effluent. Testing
equipment shall be provided to adequately control the treatment process at all
plants.
(m) Water from ion exchange
treatment plants shall be stabilized as required in s.
NR 811.58 (4), except where it can be shown that the
treated water will be non-corrosive.
