Undergraduate Thesis Defense || Tahsina Alam || CE, BUET

Thesis Topic: Effectiveness of Zinc and Phosphate as Scale Inhibitors in Delaying Calcium Carbonate Precipitation for Water Treatment/Distribution Systems.
Authors: Tahsina Alam and Tasnim Ahmed
Supervisor: Dr Muhammad Ashraf Ali, Professor, Department of Civil Engineering, Bangladesh University of Engineering and Technology (BUET).
Abstract:
Precipitation of CaCO3 has long been a concern in water treatment/distribution systems; however, this problem has been identified only recently in some areas (e.g., Meherpur) of Bangladesh. Scaling is the most obvious manifestation of CaCO3 precipitation in water distribution systems, resulting in flow restrictions and head loss; it also damages plumbing fixtures and causes clogging of membranes/ filters in treatment systems. Globally, the scaling issue is reemerging due to climate change. The potential methods for control of CaCO3 precipitation include lowering of pH, removal of Ca2+, and delaying/preventing precipitation using scale inhibitor. In Bangladesh, groundwater often contains high concentration of Fe (and in some places As), and lowering of pH adversely affects removal of iron (and also As). Removal of Ca2+ by raising pH (lime softening) has a number of major operational difficulties, while removal of Ca by other means (e.g., ion exchange resins) at municipal/city scale often becomes prohibitively expensive. This study presents an assessment of the potential of Zinc and Phosphate as scale inhibitors in delaying precipitation of CaCO3 in water treatment and distribution system. In batch experiments carried out in 0.1 M NaCl solution (in distilled water), concentration of Ca2+ and carbonate have been varied to achieve the desired degree of supersaturation. Effect of the inhibitors (at concentrations varying from zero to 5 mg/l) on precipitation of CaCO3 was evaluated as a function of inhibitor concentration, mixing time (varying from 30 minutes to 5 hours), and the degree of saturation (varying from about 3.10 to 10.06 for Zn2+ and 22.19 to 56.45 for PO43-). For a particular degree of saturation and mixing time, precipitation of CaCO3 in electrolyte solution decreased as inhibitor concentration increased. In general, Zn2+ was found more effective in reducing calcium precipitation at lower degree of saturation for all inhibitor (Zn2+) dosages. One interesting observation from the experiments was the positive effect of increased mixing time on Zn2+ inhibitor performance. In electrolyte solution, the inhibition effect of phosphate appears to diminish to some extent with time, while in case of Zn, the inhibition effect appears to become stronger with time. Results for this study suggests that Zn2+ or PO43- applied at low doses (complying with drinking water standards) could effectively prevent CaCO3 precipitation/scaling in electrolyte solution. Studies need to be carried out to assess the effect of the inhibitors in natural groundwater in order to assess potential of these inhibitors in preventing precipitation of CaCO3 in water supply and distribution systems.