Designing Limestone Contactors for Small Public Water Systems: An

Overview

Prof. Dr. Marwan Haddad

PurposeA study was conducted to investigate the use of limestone contactorsas a technique for mitigating corrosion in small water supply systems that use dilute acidic water. As water is transported through a packed

bed of crushed limestone, calcium carbonate dissolves and the pH, calcium ion concentration, and alkalinity increase.

Mathematical ModelingA mathematical model was derived for use in contactor design. The model is based on the interfacial transport of calcium ion and relates the depth of limestone required in the contactor to the desired effluent water chemistry, influent water chemistry, limestone particle size and shape, limestone bed porosity, and water temperature, and superficial velocity. The model was calibrated and tested using laboratory column experiments.

Calcium Carbonate Dissolution Rate Equation

Partial Differential – Continuity Equation

Solution

Computer ModelingA computer model to simulate the dissolution process was developed using APL program including several subroutines. The model would lead to the best/suitable:• Contactor size• Limestone particle size to be used• Needed replenishment• Effluent water quality• Contactor cost

Long Term Validation of the Model

Long term field contactor (reactor) and lab-column experiments were conducted to validate the computer model

Field Experiments

Laboratory Column Experiments

X-Ray Electron Microscopy of Limestone Particle Surfaces

X-Ray Electron Microscopy of Limestone Particle Surfaces

X-Ray Electron Microscopy of Limestone Particle Surfaces

Tracer Experiments Results for Residence Time Estimation

Limestone Dissolution Theory