Water SOFTENING
GENERAL
Water may be generally classified as "soft" or "hard"
water. Hard water is characterized by the elevated concentration
of polyvalent metal ions (cations). Soft water is characterized
by a low concentration of these metal ions. The most common hard
water metal ions are those of calcium and magnesium, which are
divalent metal ions expressed as: Ca+2 and Mg+2 respectively.
While there are no hard and fast definitions of what soft water and hard water entail, we generally define soft water as having less than 75 mg/L calcium carbonate (CaCO3) and hard water as having greater than 150 mg/L calcium carbonate (CaCO3)(Sawyer and McCarty, 1967).
Typically, groundwaters are harder than surface waters, due to water dissolving and then carrying calcium and magnesium from the surrounding rocks.
Hard water, when it dries, creates water spots (white scale) on vehicle windows and finishes, household windows, shower doors, tile, etc. It also creates scale on the insides of water distribution pipes, boilers, and water heaters. Elevated water temperatures create the scale much faster than cold water temperatures, thus creating a problem for water heaters or boilers. The scale creates a barrier to the efficient transfer of heat, which requires a greater quantity of fuel to heat to the same temperature. Premature failure of boilers and hot water heaters is most often contributed to this scaling.
Treatment Process
There are three basic treatment processes to treat hard water:
1) Ion Exchange: this process involves the use of a high ion exchange
resin. An example of this resin is a polystyrene type. The resin
is specifically designed to hold sodium ions on its ion exchange
sites. As the hard water passes through the beads of resin, the
calcium and magnesium ions replace the sodium ions that are attached
to the resin. This process removes the calcium and magnesium from
water being treated and releases the sodium to the treated water.
The sodium does not form this scale, nor cause water spots. At
some point in time, all of the sodium has been replaced with calcium
magnesium and the resin can therefore no longer remove it from
the raw water. The resin is regenerated by stopping the flow water
through the resin bed, and then backwashing the resin bed with
a highly concentrated solution of rock salt (a sodium chloride
brine). This concentrated brine solution removes the calcium and
magnesium ions from resin and replaces it with sodium ions. The
resin bed is then rinsed gently with soft water, and made available
for use. In many communities utilizing groundwater sources, the
well sites possess ion exchange treatment units to soften groundwater
prior to the addition of a disinfectant or fluoridation. The ion
exchange process is more cost-effective when treating groundwaters,
as they are typically a non-carbonate form of water hardness.
That is, the hardness is specifically a predominance of the calcium
and magnesium ions by themselves, not attached to their anionic
components of carbonates, etc. Surface waters are typically the
carbonate form of water hardness, which is more cost effectively
treated by utilizing a lime or lime soda ash treatment of process.
Advantages & Disadvantages
a) An advantage of the ion exchange treatment process is that
it does not change the pH or alkalinity of the water. Other advantages
include excellent process reliability, process stability, and
chemical safety.
b) Disadvantages of the ion exchange process include:
1) First is the increase of total dissolved solids (TDS) in the
treated water due to the release of sodium.
2) Second is the cost of disposing of the regeneration cycle’s
salt brine. This regeneration backwash water contains sodium chloride,
calcium chloride, and magnesium chloride in a concentrated solution
that ranges from 30,000 to 50,000 mg/L TDS. The quantity water
may vary from 1.7 % to 7.5% of the softened water total depending
upon the raw water source, the type of hardness being removed,
and the quantity of water being treated. Proper disposal of this
regeneration backwash water is obviously difficult due to these
parameters.
3) Depending on the pH of the treated water, most waters treated
by the ion exchange process, are corrosive waters due to loss
of the calcium and magnesium. Blended water or the addition of
stabilizing chemicals will correct this.
4) Resin problems:
a) Iron in the ferrous state must not be allowed to enter the
ion exchange process, as it will oxidize to the iron oxide state
on the resin and become a permanent resident on the resin. If
the iron oxide state is achieved prior to entering the ion exchange
process, it will be removed from the process water during treatment
and is able to be removed from the resin during the normal backwash
cycle. Best practice is to remove all iron prior to the ion exchange
process.
b) Modern ion exchange resins are very resilient, with the life
expectancy in excess of 15 years, when the process is properly
operated. Excessive chlorine residuals will break down the resins,
and must therefore not be applied to the resin beds. Surface waters,
with accompanying biological growths, higher turbidities, and
color values must be treated prior to the ion exchange process
in order to prevent these materials coating the resin beads and
interfering with the softening process.
2) Membrane Filtration: this is a physical process whereby either
reverse osmosis or nanofiltration are utilized to physically remove
the calcium and magnesium ions from the raw water source. The
type of membrane will determine the degree of treatment.
Advantages and Disadvantages
c) Advantages include: hardness removal without large quantities
of chemicals involved such as lime and sodium chloride (rock salt),
simplicity of operation, increased operator safety due to lack
of potentially hazardous chemicals.
d) Disadvantages include: high cost of membranes, (which are experiencing
a trend of decreasing cost over the past several years), proper
disposal of concentrated rejection water, and a potential requirement
for pre-treatment of surface waters prior to membrane filtration.
3) Chemical Precipitation: this process is characteristically
both chemical and physical in nature. This type of softening process
does not completely remove all hardness from the treated water
(such as an ion exchange process), and therefore requires less
of a requirement for a downstream water stabilization process.
Approximately 45 to 90 mg/L of hardness, expressed as calcium
carbonate (CaCO3 ), will exist in the treated water which will
provide an adequate corrosion protection value for the water distribution
system and consumer plumbing fixtures. This may be accomplished
by either of two manners:
a) Sodium hydroxide (NaOH)(caustic soda) may be utilized for chemical
precipitation. Advantages: produces less sludge than lime, or
lime-soda ash processes. Disadvantage: higher total chemical cost.
b) Utilizing lime, or lime and soda-ash. This is the preferred
method of most facilities, and therefore the one we will detail.
Lime-soda ash Treatment Process
The raw water is brought to a rapid mixer where calcium hydroxide
Ca(OH)2 is added to it. In the majority of cases soda ash is also
added to it, for what is termed a "lime-soda ash" softening
process. (On a side stream, dry lime (CaO) is "slaked"
by adding water to it to create the Ca(OH)2.) This slurry is added
to the water being treated to increase the pH to 10 for calcium
removal or to a pH of 11 for magnesium removal. The addition of
the lime and soda ash to the hard water creates a precipitate
consisting of calcium carbonate and magnesium hydroxide. The water
then flows into flocculation basins for a detention time for approximately
15 to 20 minutes. The lime utilized in the slaking process generates
approximately 8 to 10% silica grit by weight. Much of this grit
is removed in the lime slaker, but a fair quantity usually finds
its way into the rapid mix and flocculation basins. The grit must
be removed periodically from the flocculation basins to minimize
damage to the flocculation equipment. The water then flows into
sedimentation basins specifically designed for capture of the
calcium and magnesium precipitates. Up-flow or solids contact
clarifiers are usually utilized for combining the flocculation
and sedimentation process into one unit. We discussed these clarifiers
in the sedimentation chapter previously. Solids contact clarifiers
are especially suited for this type of treatment. Rectangular
sedimentation basins are usually avoided due to the excessive
wear that can occur on the chains and flights due to the abrasive
qualities of the grit and precipitates.
After the sedimentation process, the pH of the water must be properly adjusted prior to further treatment. This is usually accomplished by the addition of an acid or carbon dioxide (CO2 ). "Recarbonation" is the term utilized to describe the addition of carbon dioxide to the water being treated. For smaller facilities, carbon dioxide is usually purchased and delivered in liquid or gas cylinders, or in dry ice containers. Larger facilities have found it more cost-effective to produce carbon dioxide onsite in submerged combustion burners where natural gas is burned to create the carbon dioxide or by utilizing cleaned and scrubbed exhaust gases from furnaces, lime regeneration or carbon regeneration equipment process units. Generally speaking re-carbonation by itself can only reduce the pH to approximately 8.3. Additional reductions in the pH value will require the addition of an acid.
Advantages and Disadvantages
Advantages of this type of process include: the ability to soften
water yet maintain an adequate water stability for corrosion protection;
and it’s cost-effectiveness in treating large quantities
of surface water.
Disadvantages include: proper disposal of the large quantity of
high pH sludge; constant removal of the calcium carbonate scale
on slaking equipment, rapid mixers, and flocculation basin equipment;
safety issues regarding dosing sodium hydroxide or soda ash, and
the slaking and feeding of lime. Extreme care must be taken when
working around dry lime (CaO). The addition of water generates
a lot of heat. One must never apply water from a hose directly
to a dry lime spill or accumulation in a slaker, as there exists
the potential of explosion resulting from escaping gases in heat
from the interior of the lime pile.
Operational & Design
Notes
Impacts of calcium carbonate scale and particles on downstream
piping and filtration processes are as follows:
a) Slaked lime creates calcium carbonate scale on anything it
comes in contact with. This includes pump impeller's, pipes, pipe
fittings, and all assorted equipment and containment vessels.
The best installations utilize rubber hoses, with a minimal number
of fittings, to convey the lime slurry. This is to reduce the
number of man hours required to remove calcium carbonate from
quick couplings, etc. The constant flexing of the rubber hose
reduces scale buildup. It is highly recommended that the lime
slaker be located within the shortest distance possible to the
rapid mixer to minimize the length of rubber hose/couplings used
in conveying the slaked lime. Sample lines and the sludge piping
must be cleaned on a routine basis. Scale must be removed periodically
to maintain pipe flows. This may be accomplished by the use of
high-pressure water blaster's (up to 10,000 psi, although experience
has shown that a cost effective range may be in the order of 4,000
psi), high concentrations of chlorine, and alum solutions as appropriate.
b) Provisions must be made for the removal of grit from the flocculation
basins on a routine basis. The frequency will be determined by
the quantity of grit in the dry lime, the lime slaker's grit capture
rate, and the number of pounds of lime fed him each day.
In summary, some facilities utilize both lime and soda ash in combination to better control the process and chemical costs. Generally speaking, chemical precipitation is utilized only when large quantities of water need to be softened. Obviously, chemical precipitation is followed by a stabilization process be designed and operated to control the pH and alkalinity of the processed water.