By Charles R. O’Melia and William C. Becker
The general goal of the following project was to develop an improved understanding of the interactions between natural organic matter (NOM), calcium and particles (turbidity). Some specific objectives were to:
- Determine the effect of calcium addition on coagulant demand,
- Determine the effect of calcium addition on removal of disinfection by-product (DBP) precursors, and
- If beneficial effects occurred, determine conditions under which these effects were produced and, if beneficial effects were observed, develop recommendations for applications in practice.
It was hypothesized that interactions of calcium ions with NOM may lower the demand for conventional metal coagulants (alum, ferric salts), thereby reducing sludge production and perhaps increasing precursor removal. Stated another way, it was proposed that the addition of calcium to a surface water supply using coagulation may have beneficial effects on plant operation and performance.
Beneficial effects of calcium addition, where observed, related to a reduction in coagulant dosage that can be achieved without loss in process performance. These effects were manifested for waters that contained moderate to high concentrations of dissolved organic carbon (DOC) and low to moderate hardness levels. Calcium addition to waters low in DOC didn’t alter coagulant requirements.
The research plan involved three phases—batch laboratory jar tests using synthetic model waters and surface water supplies, pilot-scale experiments, and addition of calcium in a full-scale trial at one water treatment facility. The synthetic waters were prepared using inorganic particles and NOM isolated from natural waters. Following the studies with the synthetic waters, batch experiments were made with 10 surface water supplies. Based on the results of the batch jar tests, three supplies were selected for on-site pilot plant testing for possible beneficial effects of calcium addition. Finally, a full-scale treatment plant was operated with and without added calcium. This plant was selected from among the three systems tested in the pilot-scale studies.
EXTRA—Calcium, coagulation and water treatment
- A.I. Schäfer, A.G. Fane and T.D. Waite, “Direct coagulation pretreatment in nanofiltration of waters rich in organic matter and calcium,” IWA Publishing, Water Science & Technology: Water Supply Vol 1 No 4 pp 25–33, 2001.
- Fields, K.A., A. Chen and L. Wang, “Arsenic Removal from Drinking Water by Coagulation/Filtration and Lime Softening Plants,” Battelle, Columbus, Ohio, for USEPA, EPA/600/
R-00/063, June 2000.
- “Lime Softening,” Tech Brief, a National Drinking Water Clearinghouse Fact Sheet, West Virginia University, website: www.nesc.wvu.edu/ndwc/pdf/OT/TB/TB8_lime_softening.pdf
- Chemical of the Week, “LIME: Calcium Oxide — CaO,” website: http://scifun.chem.wisc.edu/chemweek/lime/lime.html
Reacting with coagulants
The addition of calcium to water allowed a reduction in coagulant dose in many, but not all cases. Calcium addition usually allowed a reduction in coagulant dose for waters that contained moderate to high concentrations of DOC and low to moderate hardness levels. Calcium addition to waters low in DOC didn’t alter coagulant requirements. The beneficial effects of calcium addition, when observed, can increase as the pH of coagulation increases. Based on limited testing, it appears that when the addition of calcium to water allows a dose reduction with one coagulant, it can do so with other coagulants.
The magnitude of the calcium effects appear to be in the order of alum ~ ferric chloride > Cat-Floc TL. Reducing coagulant dose is normally expected to reduce DOC removal and, in turn, increase the formation of DBPs. This was observed for waters in which calcium addition didn’t have a beneficial effect; however, for waters in which beneficial effects of calcium addition in lowering coagulant requirements were observed on the basis of settled or filtered water turbidities, there was little or no change in DOC removal or DBP formation potential.
Bench or pilot-scale tests should be made to determine if calcium addition can enhance coagulation (reduce coagulant dose). Target raw waters include those with low to moderate hardness and moderate to high levels of DOC. For plants where a base is added, consideration should be given to using lime since it adds calcium while increasing pH. Where calcium addition doesn’t demonstrate a favorable effect on coagulant dosage, reducing coagulant dosage may lead to higher DOC concentrations. Finished water DOC concentrations should be monitored whenever a change in the coagulation process is made.
The authors would like to thank the 32 surface water treatment plants of the Pennsylvania-American Water Company (Hershey, Pa.).
About the authors
Charles R. O’Melia and William C. Becker are principals in Becker and O’Melia LLC, a small, independent research and consulting firm focusing on drinking water treatment processes and technologies based in Severna Park, Md. Both are also environmental engineering professors at Johns Hopkins University. The above study was conducted for the AWWA Research Foundation of Denver. Becker and O’Melia can be reached at (410) 421-5276, (410) 421-5476 (fax) or email: email@example.com