(130d) Innovation in Solving Total Suspended Solids (TDS) Problem in a Groundwater Chromium Treatment Facility

1- Abstract The groundwater at a site is contaminated with Hexavalent Chromium. A treatment plant facility was constructed to treat the groundwater using electrochemical treatment. Metal reduction is the first treatment step after collection of the groundwater and lowering of the pH to 3 in an on-line acid tank. Soluble ferrous ions are produced in an off-line electrochemical cell using sacrificial electrodes. Using a slight excess of the stoichiometric 3.2 pounds of iron per pound of chromium, the ferrous ions are mixed with the contaminated groundwater. In the resulting redox reaction, Hexavalent Chromium is reduced to its insoluble trivalent state and divalent iron is oxidized to its insoluble trivalent state. After the reaction is completed, the pH is adjusted to a slightly alkaline state (pH = 8) to allow for precipitation of the metal ions in a pH adjustment tank. The stream is advanced to a clarifier where a polyelectrolyte polymer is added to promote coagulation of the reduced metal and consequently, settlement at the bottom of the clarifier. Solids are drawn from the bottom of the clarifier and allowed to settle in a gravity thickener followed by a plate and frame filter press. The treated ground water is pH adjusted to below 8 and polished with multimedia filters and carbon, prior to re-injection to the aquifer.

The water is continually checked for total dissolved solids, total suspended solids, and total chromium concentration. The chrome concentration is continuously kept below 30 ppb; however, the total dissolved solids oscillate between 1200 to 2400 ppm, depending on the plant influent concentration, which ranges from 400-700 ppm. The total suspended solids range from 10 to 15 ppm.

Shear membrane VSEP was tested in the water treatment plant to reduce total suspended solids as well total dissolved solids. The unit was installed at different points along the process including: before the acid tank, after the acid tank, after the pH adjustment tank, after the clarifier, after the multi-media filters, and after the carbon filters. During the membrane experiments, a chemical substitution innovative idea was discovered and tested. The patented process used phosphoric acid/calcium hydroxide system to replace Sulfuric acid/ Sodium hydroxide system.

When membrane was used, it was clear that none of the chemical operations could be replaced by the membranes, the experiments showed that the objectives were satisfied when the unit was placed before or after the carbon filters, i.e., it succeeded in reducing the total dissolved and suspended solids to acceptable values however the reject stream remained to be a problem. When the innovative chemical substitution method was used, at no extra cost to the plant, all objectives were easily met. Met in this sense mean that not only we achieved a below 400 ppm TDS but also, we achieved low chrome.