Lime addition system for water treatment

A system for the preparation of a lime slurry and metering of the slurry, which prevents scaling and plugging of equipment and piping. The elimination of scaling and ease of metering are achieved by a combination of process steps in an automated operating sequence. The system is particularly adaptable to the addition of lime to municipal water systems for reduction of the acidity of the water.

BACKGROUND OF THE INVENTION 
Lime is often used as the preferred reagent for neutralization of acidic 
compounds or in other chemical applications. However, lime reacts with 
sulfates, carbonates and bicarbonates to form insoluble compounds which 
result in scaling of equipment and piping. It has a low solubility making 
solution storage impractical and requiring either weigh feeding of powder 
or slurry storage and metering of slurry. It is a finely divided powder 
which is difficult to handle and creates excessive dusting. 
It is known that when insoluble compounds are precipitated from solution 
they tend to crystallize on surrounding surfaces. Most compounds 
supersaturate and a finite time is required for precipitation to reduce 
the supersaturation. The phenomenon of precipitation on lime slurry 
particles has been referred to as lime stabilization. To date the 
potential of these effects has not been fully utilized. The invention 
provides am integrated process which is performed in a automated sequence 
in specialized equipment to mitigate the dusting problems, eliminate 
scaling and also provide for trouble free metering. It is best described 
with a comparison to the conventional lime addition systems employed in 
treating of municipal water supplies. 
The U.S. Environmental Protection Agency (EPA) has established maximum lead 
and copper contaminant levels in the National Primary Drinking Water 
Regulations. These regulations are expected to reduce the exposure of 
approximately 130 million people to lead in drinking water. In 1% of the 
municipal water systems there is excessive lead or copper in the source 
water. In the remainder of the municipal water systems the source of the 
excessive lead or copper contamination is the result of chemical solution 
of lead and copper components of the piping systems. In order to meet the 
Maximum Contaminant Level Goals most of the municipal water systems will 
be required to install some sort of corrosion control treatment. 
The copper and lead corrosion experienced in water systems is primarily 
caused by the acidity of the source water. This acidity results from 
mineral acids in acid rain and dissolved carbon dioxide which forms 
carbonic acid. Corrosion control can be accomplished by pH and alkalinity 
adjustment to reduce the acidity of the water, calcium adjustment to 
promote the formation of protective coatings inside pipes and plumbing or 
addition of a phosphate or silica-based corrosion inhibitor to form a 
protective coating inside of pipes and plumbing. 
Of the corrosion treatment options the most widely practiced is reduction 
of the acidity with an alkali. The three alkalies commonly used are 
calcium hydroxide, potassium hydroxide and sodium hydroxide. Selection of 
the alkali to be used involves consideration of the effectiveness of the 
reagent in reducing corrosion of piping components, ease of control of pH, 
the capital and operating costs of a treatment facility, the effect on the 
taste of the water, the possible health effects resulting from the 
introduction of the reagent into the water and the safety in handling the 
reagent. 
From a technical standpoint calcium hydroxide is the preferred reagent. In 
addition to reducing the acidity a protective coating is formed inside the 
piping system. Calcium neutralized waters have a greater buffering 
capacity, thereby providing a more stable pH and greater ease of control. 
Lime treated waters tend to taste better. Potassium hydroxide treatment 
tends to impart a bitter taste to the water. Sodium hydroxide treatment 
has the disadvantage of raising the sodium content of the water making it 
a health concern for certain individuals and is often dropped from 
consideration for this reason. Of the three reagents calcium hydroxide is 
the least hazardous to handle. Addition of lime does increase the hardness 
of the water, but since acid waters generally are low in hardness the 
effect of this increase is negligible. 
A financial comparison of acid water treatment costs given in Table 1 shows 
substantially less reagent cost for lime or calcium hydroxides. The 
difficulties in handling calcium hydroxide in the conventional systems 
increases the capital costs and the operating and maintenance costs to the 
extent that overall calcium hydroxide treatment is the most costly. While 
lime treatment is the preferred process many water suppliers are going to 
the use of other reagents because of the operating difficulties and high 
maintenance costs associated with lithe use in conventional systems. 
Conventional lime treating facilities usually consist of a covered storage 
area used for storing, for example, fifty pound bags of hydrated lime, a 
feed hopper into which the hydrated lime is manually transferred, a weigh 
feeder volumetric feeder which meters the hydrated lime into a dissolving 
tank and a pump for injecting the lime solution into the water main. The 
systems are designed to operate unattended. In large installations quick 
lime may be used instead of hydrated lime but this requires the addition 
of slaking equipment and continuous on site supervision. 
Handling the finely divided lime powder creates a severe dust problem and 
generally necessitates a separate building for the process equipment. 
Filling the feed hopper and metering the powder into the dissolving system 
are dusty and labor intensive operations. The feed hopper and feeder are 
subject to frequent plugging. Scale formation in the dissolving tank and 
associated piping is a continuing occurrence causing line plugging and 
injection pump malfunction. These problems result in excessive operating 
and maintenance costs. In municipal water systems having multiple, widely 
separated well sites requiring on site treatment the addition of lime 
requires substantially more capital for buildings and the maintenance and 
operating problems are increased. The operating and maintenance problems 
affect the reliability of these systems to the extent that State 
regulatory agencies are reluctant to issue permits for new calcium 
hydroxide treatment systems. 
Use of the present invention eliminates all of the aforementioned 
disadvantages of lime handling and results in being the most economic 
means of water neutralization. 
TABLE 
______________________________________ 
FINANCIAL COMISON OF pH TREATMENT 
SYSTEMS FOR A 272,000,000 GALLON PER YEAR 
MUNICI WATER SYSTEM SUPPLIED BY 
FIVE WIDELY SEATED WATER WELLS. 
IM- 
CONVENTIONAL PROVED 
TREATMENT SYSTEMS SYSTEM 
Calcium Potassium Sodium Calcium 
Hydroxide 
Hydroxide Hydroxide Hydroxide 
______________________________________ 
Reagent Cost 
3,000 19,800 11,600 
3,000 
Operating 
25,500 5,000 5,000 5,000 
and Mainte- 
nance Labor 
Debt Service 
125,100 112,200 112,200 
48,800 
on Capital 
Facility (8% 
for 15 Years) 
Total Annual 
$153,600 $137,000 $128,800 
$56,800 
Operating 
Costs 
______________________________________

SUMMARY OF THE INVENTION 
The lime treating system of the invention is an integrated neutralization 
facility using lime as the neutralizing agent, which while it may have 
other applications, is particularly adapted to water systems having 
multiple widely separated water wells. For adjusting the pH in a municipal 
water system with multiple wells, the system consists of a central mix 
location, truck transport of slurry to each well location and equipment 
modules at each of the well stations for preparation and injection of 
dilute lime slurry into the water mains. The entire system is highly 
automated and runs without operator supervision. The invention comprises 
the unique combination of operational steps and equipment to overcome the 
shortcomings of the conventional lime treating systems. 
The dust problem at the stations has been completely eliminated by delivery 
of the lime as a slurry. The process equipment required at each station 
consists of two small tanks, three pumps and a control panel. Elimination 
of the dust problem and the need for bag storage and solids feeding 
equipment permits the installation of all of the process equipment in 
existing pumping stations. 
The central mix location provides for storage of lime in large containers, 
such as "Super Sacks" and also for the dust free preparation of the 
slurry. 
DESCRIPTION OF THE INVENTION 
The invention as practiced in municipal water systems is described with 
reference to the figure. 
Super Sacks, containing about 1500 pounds of hydrated lime each, are 
received at the central mix location which serves as a lime storage 
facility and a slurry preparation area. To prepare a batch, Super Sack 1 
is emptied into agitated mix tank 2 and sufficient water 3 is added to 
prepare an 18% slurry. The lime and water are added, with agitation, to a 
heel of at least 800 gallons of 18% slurry 4 remaining in the tank from 
the previous batch. During the solids addition a heavy spray of incoming 
water continuously washes dust particles from the tank roof and walls. A 
portion of the incoming water is diverted to a venturi scrubber 5 mounted 
on the tank vent to maintain a slight negative pressure within the mix 
tank. After addition of the hydrate and water has been completed the 
slurry is agitated for thirty minutes. 
Loads of the 18% slurry are transferred, for example by truck, 6 to each of 
the water well locations. The slurry is pumped to the truck by a diaphragm 
pump 7. The pump and lines are mounted above the tank and provision is 
made to drain and flush the pump and lines back into the tank. The entire 
transfer is automated. 
Each of the well pump stations is equipped with three equipment modules. 
The modules are designed to be located independently in the pump stations 
and then interconnected. This design permits making the installation in 
the tight confines of existing pump stations and eliminates the need for 
costly structural additions. A single wall mounted control module 8 
services the other two modules. The second module 9 consists of a 
diaphragm pump 10 to receive truck shipments of slurry, an agitated slurry 
stock 11 tank and a positive displacement slurry pump 12 for slurry 
recirculation and metering. The third module 13 consists of an agitated 
injection tank 14 containing 2% lime slurry and a lime injection pump 15 
to meter and inject the dilute slurry into the water main. 
The entire operation at each of the stations is automated. The unit is 
completely shut down when the water well is not operating. When a signal 
is received to start water well operation both tank agitators are started. 
After the contents of both tanks are thoroughly mixed, the water well pump 
is turned on, the lime injection pump begins injecting the 2% lime slurry 
into the water main at a constant preset rate and the positive 
displacement slurry pump begins recirculating the 18% lime slurry at a 
rate which achieves the optimum line velocity for slurry pumping. Two 
minutes after circulation of the 18% slurry has been established the 2% 
slurry make up controls are activated. 
When the level of the injection tank drops below a preset value a level 
sensor begins the makeup cycle. During makeup the speed of the positive 
displacement lime slurry pump is temporarily adjusted for optimum metering 
precision and the three-way valve 16 diverts flow of 18% slurry to the 
injection tank for a preset time interval. Simultaneously a timed flow of 
treated water 17 is delivered to the injection tank to maintain the 
desired concentration. 
Control of the system is based on an essentially constant flow and acidity 
of the well water 18 and on an essentially constant volumetric output 19 
of the slurry injection pump. The concentration of the slurry in the 
injection tank is maintained in a range which provides sufficient 
precipitation nuclei and yet is dilute enough to be easily metered into 
the water main. The concentration range is predetermined by manual 
adjustment of the injection pump output which remains constant after the 
initial adjustment, A pH sensor 20 located in the water main downstream of 
the injection point is equipped with two relays. When the pH exceeds the 
set value, the 18% slurry is diverted to the injection tank for a preset 
shorter time interval during the next makeup cycle. Since each makeup 
batch is only a fraction of the tank's contents the changes in slurry 
concentration and the resultant pH correction are minor. All of the 
controls are simple relays and timers and require no calibration. The only 
operator attention is a once a day check, during which he may adjust the 
water input timer to maintain uniform cycling above and below the pH set 
point. 
When the water well is turned off the agitators and the positive 
displacement slurry pump are turned off. The lime injection pump is 
flushed for three minutes prior to shutdown. 
If it is necessary to adjust the pH of the water supply and also to 
chlorinate it, both of these operations can be carried out simultaneously 
by the addition of a chlorinating agent to the 18% slurry. The output from 
a water well has an essentially constant flow and the ratio of the lime 
requirements for pH adjustment to the chlorine demand will be essentially 
constant, varying only over an extended period of time. The hydrated lime 
and chlorinating agent are admixed in the ratio of their requirements. If 
the ratio of the lime and chlorinating agents is the same for all of the 
wells the chlorinating agent can be added during the preparation of the 
18% slurry in the mix tank. If the ratio varies from well to well the 
chlorinating agent can be added to the truck shipments. 
EXAMPLE 
A lime neutralization system was installed for a municipal water supply 
consisting of five widely separated water wells providing 272,000,000 
gallons of water per year. On average once every 15 days an 18% lime 
slurry batch was prepared at a central mix station in a tank containing a 
heel of at least 800 gallons of 18% lime slurry from the previous batch. 
Water at the rate of 25 gallons per minute was directed into four spray 
nozzles and a venturi scrubber on the tank vent. The spray nozzles washed 
down the exposed surfaces of the tank and provided a curtain of water 
droplets to wet the incoming lime hydrate. The venturi scrubber provides a 
negative tank pressure and vent gas scrubbing to eliminate dust release. 
After water flow was started the contents of a 1500 pound Super Sack of 
hydrated lime was emptied into the tank. When 890 gallons of water were 
added the water flow was automatically shut off and the tank contents were 
agitated for an additional 30 minutes. 
On average once every four days a 200 gallon load of the 18% slurry was 
transferred to a stock tank in one of the pump stations. 
At a given station when a signal was received to operate the water well the 
agitators on the stock tank and the injection tank water were turned on. 
After four minutes of agitation a progressive cavity pump mounted on top 
of the stock tank began recirculating 18% slurry from the stock tank at a 
flow rate of two gallons per minute, the water well began pumping at 500 
gallons per minute and injection of dilute lime slurry at 0.75 gallons per 
minute commenced. Two minutes later the makeup controls were activated. 
When the level of the injection tank dropped to 90% of the operating level 
the speed of the progressive cavity pump was temporarily reduced to 
provide a flow rate of one gallon per minute. The three-way valve diverted 
the recirculating flow to the injection tank for 40 seconds and 
simultaneously water was added to the injection tank for 75 seconds. This 
action restored the operating level and maintained the desired lime slurry 
concentration of about 2%. 
The set point for the pH of the treated water was set at 7.0. When the pH 
exceeded 7.0 a relay reset the time period for the next transfer of 18% 
slurry to 20 seconds. Reducing the amount of slurry transferred lowered 
the pH back to the set point. A control range of + or -0.2 of a pH unit 
was achieved. Initial manual adjustment of the injection pump rate 
established the dilute slurry concentration and once set this adjustment 
was not changed. Over time any slight tendency for the pH to drift from 
the operating range was corrected by increasing or decreasing the water 
addition by one or two seconds. 
When the water well was shut down both agitators and the progressive cavity 
pump were turned off. Residual slurry in the pump and recirculation loop 
drained back into the stock tank. The injection pump was flushed with 
water for three minutes before shut down. 
The prototype unit operated for an extended period with no maintenance 
required. At the end of the period all lines were clear and there was no 
scale forming on the tank walls. 
As shown in the above example, the basis of the invention is the unique 
combination of equipment, operating parameters and controls that have 
eliminated the scaling and plugging problems associated with the 
conventional lime addition systems. The 18% slurry in the mix tank 
contains highly active crystalline nuclei precipitated during the 
preparation of the slurry. In the makeup of subsequent batches of 18% 
slurry starting with a heel from the previous batch assures that at all 
times there are sufficient active crystalline nuclei in suspension to act 
as precipitation sites to avoid the formation of scale on the tank walls 
and other surfaces in contact with the slurry. Washing of the tank roof 
and walls during batch preparation avoids dust collection above the slurry 
level which may initiate future scaling at these sites. Agitation for 
thirty minutes after batch preparation assumes adequate dispersion of the 
active nuclei until all supersaturation has been eliminated and avoids 
scaling of lines and equipment downstream of the tank. 
Plugging of the 18% slurry transfer lines is eliminated by assuring that 
the slurry is uniformly dispersed before pumping, by avoiding flow 
throttling, maintaining high flow rates and automatically draining or 
flushing lines and equipment on shutdown. In the automated start-up 
sequence at the well stations the slurry is uniformly dispersed prior to 
operation of the pumps. The 18% slurry metering loop has a positive 
displacement pump which has no dead pockets to accumulate solids. The pump 
is operated by a DC motor at a speed which provides the optimum velocity 
for slurry transport in the recirculation mode. In the transfer mode the 
pump speed is reduced to lengthen the transfer time for optimum metering 
precision. The transitions between the recirculation and transfer modes 
are accomplished without throttling by use of a three-way valve. The pump 
is mounted above the stock tank and the dead portions of lines created by 
the valve switching automatically drain to the tanks. On shutdown all of 
the lines in the transfer loop drain to the tanks. 
Scaling of the injection tank, injection pump and injection line is avoided 
by adjusting the injection pump rate to provide a high enough velocity for 
ease of metering and transport, and at the same time maintain a high 
enough concentration of active nuclei for precipitation within the slurry. 
Addition of the 18% slurry and dilution water at frequent intervals and 
the almost instantaneous mixing with a substantially larger volume reduces 
the magnitude of supersaturation. There is adequate time between addition 
cycles for reduction of the supersaturation to a harmless level. On 
shutdown the injection pump and the suction and discharge lines are 
flushed with water. 
Dust problems at the well stations were completely eliminated by having no 
dry hydrate solids present. Dust release at the central mix location was 
eliminated by having the Super Sack connected firmly to the tank prior to 
emptying of the contents of the sack and maintaining a negative pressure 
with a venturi scrubber on the tank vent line. 
A prototype unit operating on a 400 gallon per minute water well was 
operated for an extended period with no maintenance being required. At the 
end of the period the was flow scale buildup on the tank walls and in the 
piping system. The progressive cavity pump and the lime injection pumps 
required no servicing during the entire period.