The use of a conical lime
reactor to control the pH of
drinking water in small scale
water treatment systems
Kavita Mahulikar and Taylor Reiss
Cornell University
10 December 2005
objective
Counteract the pH lowering effects of alum
through the addition of lime in a small
scale water treatment plant
objective
Why?
 Honduras Water Supply Project uses alum
during flocculation
 Alum lowers the pH of the water
 Want to deliver clean, safe water of pH > 6
objective
How?
 Calcium Oxide (CaO) “lime”
Saturation pH ≈ 12.4
• High acid neutralizing capacity (ANC)
• Readily available in Honduras
•

Conical Reactor
•
Direct part of plant flow into reactor
• Maintain consistent effluent pH around 12
design
From Schult, Christopher R., and Okun, Daniel A. Surface Water
Treatment for Communities in Developing Countries.” Great Britain:
Intermediate Technology Publications, 1984.
design
Reactor 1
Reactor 2
•Diameter ≈ 7 cm
•Diameter ≈ 3 cm
•Bottom was too
flat, not ideal cone
shape
•Closer to ideal
cone shape
•Difficult to keep
particles in
suspension
D
•Easier to maintain
lime “blanket”
design
pH
Probe
Pressure
Sensor
Flow
Accumulator Solenoid
Valve
effluent
Metal
Influent
Tube
B
e
n
c
h
Collection
Tank
CaCO3 Waste
To Plant
L
a
b
Tap Water
design
Experiment 1
• Q = 350 mL/min
• CaO dose = 178 g
results
pH as a function of time
13
pH
12
11
10
9
8
0
3
6
Time (hr)
9
12
Experiment 1
• Q = 350 mL/min
• CaO dose = 178 g
results
pH and ANC as a function of time
0.06000
12.60
12.55
0.05000
12.50
12.45
12.40
0.03000
12.35
0.02000
12.30
12.25
0.01000
12.20
0.00000
0.00
12.15
1.00
2.00
3.00
4.00
Time (hr)
ANC
pH
5.00
6.00
7.00
pH
ANC (eq/L)
0.04000
Experiment 2
• Q = 200 mL/min
• CaO dose = 178 g
results
pH as a function of time
13
12
pH
11
Series1
10
9
8
0
3
6
Time (Hr)
9
12
Experiment 2
• Q = 200 mL/min
• CaO dose = 178 g
results
pH and ANC as a function of time
0.04500
13.00
0.04000
12.50
0.03000
12.00
0.02500
11.50
0.02000
0.01500
11.00
0.01000
10.50
0.00500
0.00000
10.00
0
2
4
6
8
10
12
14
16
18
Time (hr)
Initial ANC
After Mixing ANC
pH
20
22
24
pH
ANC (eq/L)
0.03500
results
ANC as a function of time
0.06000
ANC (eq/L)
0.05000
0.04000
0.03000
0.02000
0.01000
0.00000
0.00
1.00
2.00
3.00
4.00
5.00
Time (hr)
Q = 350 mL/min
Q = 200 mL/min
6.00
7.00
results
Why?





↓ flow rate = ↓ exit velocity
Difficult to keep particles in
suspension over time
Lime has low solubility, thus,
ANC is directly related to
concentration of particles
As particles settle out, ANC
goes down
At 200 mL/min, there was a
lot of lime that never left the
reactor
analysis
ANC Calculations

 
 
  
ANC  HCO3  2 CO32  OH   H 
1moleCaO 1moleCa(OH ) 2 2moles(OH  )
178 gCaO 


 6.34moles(OH  )
56.1gCao
1moleCaO
1moleCa(OH ) 2
OHInitial
OHafter 7 hours
OHafter 4 days
1 Q = 350 mL/min
6.34 moles
2.78 moles
N/A
2 Q = 200 mL/min
6.34 moles
4.4 moles
2.54 moles
Experiment
analysis
Feasibility
Assuming:
Alum dose = 40 mg/L
 Average reactor effluent ANC = 0.035 eq/L
 Q = 350 mL/min
Then:
 Max. Plant Flow Rate = 30.5 L/min

analysis
Feasibility
Potential Problems:
 Unstable System
 Calcium Carbonate
(CaCO3) Removal
conclusion
Our experimental design is not feasible for
small scale water supply in Honduras
But maybe someday…
conclusion
Possible Alternatives
Use a base with a higher solubility
 Larger Reactor, higher residence time
