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.

designReactor 1

•Diameter ≈ 7 cm

•Bottom was too flat, not ideal cone shape

•Difficult to keep particles in suspension

Reactor 2

•Diameter ≈ 3 cm

•Closer to ideal cone shape

•Easier to maintain lime “blanket”

D

design

effluent

Tap Water

Lab Bench

CaCO3 Waste

To Plant

CollectionTank

Metal Influent Tube

Solenoid Valve

Pressure Sensor

Flow AccumulatorpH

Probe

design

resultsExperiment 1• Q = 350 mL/min• CaO dose = 178 g

pH as a function of time

8

9

10

11

12

13

0 3 6 9 12

Time (hr)

pH

resultspH and ANC as a function of time

0.00000

0.01000

0.02000

0.03000

0.04000

0.05000

0.06000

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00

Time (hr)

AN

C (

eq

/L)

12.15

12.20

12.25

12.30

12.35

12.40

12.45

12.50

12.55

12.60

pH

ANC pH

Experiment 1• Q = 350 mL/min• CaO dose = 178 g

results

pH as a function of time

8

9

10

11

12

13

0 3 6 9 12Time (Hr)

pH Series1

Experiment 2• Q = 200 mL/min• CaO dose = 178 g

resultspH and ANC as a function of time

0.00000

0.00500

0.01000

0.01500

0.02000

0.02500

0.03000

0.03500

0.04000

0.04500

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (hr)

AN

C (

eq/L

)

10.00

10.50

11.00

11.50

12.00

12.50

13.00

pH

Initial ANC After Mixing ANC pH

Experiment 2• Q = 200 mL/min• CaO dose = 178 g

results

ANC as a function of time

0.00000

0.01000

0.02000

0.03000

0.04000

0.05000

0.06000

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00

Time (hr)

AN

C (

eq

/L)

Q = 350 mL/min Q = 200 mL/min

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

analysisANC Calculations

HOHCOHCOANC 233 2

)(34.6)(1

)(2

1

)(1

56.1gCao

1CaO178

2

2

OHmolesOHmoleCa

OHmoles

moleCaO

OHmoleCamoleCaOg

Experiment OH- Initial

OH- after 7 hours

OH- after 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

analysis

FeasibilityAssuming: 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