Date of Award:

5-2-2014

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Civil and Environmental Engineering

Advisor/Chair:

Laurie McNeill

Abstract

In order to identify hexavalent chromium (Cr6) sources, behavior, and treatability, this thesis has profiled Cr6 in seven full-scale drinking water treatment plants and six distribution systems. Bench-scale jar tests assessed the treatment efficacy of coagulation and developed strategies to remove ultra-low level (0.01- 1.0 ~tg/L) Cr6.

All water sources measured in this project contained dissolved Cr6 greater than or equal to the California Public Health Goal (0.02 ~g/L Cr6). The investigated coagulation plants did not remove Cr6; in fact, four of the seven treatment plants inadvertently added Cr6 to the treated waters. Thirteen types of drinking water treatment chemicals were evaluated as a potential non-water source of chromium. Amongst these, only iron-based coagulants contained trace levels of chromium sufficient to account for the observed increases at the full-scale plants. Other discussed non-water sources include leaching of chromium-bearing infrastructure or oxidation of Cr3. One of the treatment systems showed chlorine oxidized Cr3 to Cr6 and raised the finished concentration, in less than seven hours.

One suggested improvement strategy was to use ferrous iron to reduce and remove Cr6 during coagulation. Bench-scale tests showed ferrous iron and a cationic polymer improved removal of both Cr6 and Total Cr. Chlorine interfered with that reduction. The full-scale test of this reduction-coupled coagulation treatment successfully decreased the finished Cr6 concentration when 40% ferrous iron was used and the point of chlorination was moved downstream from the coagulation process.