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The Chemical Process: When carbon dioxide (CO2) is added to water (H2O), carbonic acid (H2CO3) is formed which can further dissociate into a proton (H+) and a bicarbonate anion (HCO3-) as shown below: CO2 + H2O H H2CO3 H H+ + HCO3- The free proton is a weak acid that can lower the pH. Water temperature has an effect on the reaction as well. The colder the water temperature is the slower the reaction occurs and the warmer the water temperature is the quicker the reaction occurs. Most con- struction applications in Washington State have water temperatures in the 50°F or higher range so the reaction is almost simultaneous. Design and Installation Specifications Treatment Process: High pH water may be treated using continuous treatment, continuous discharge sys- tems. These manufactured systems continuously monitor influent and effluent pH to ensure that pH values are within an acceptable range before being discharged. All sys- tems must have fail safe automatic shut off switches in the event that pH is not within the acceptable discharge range. Only trained operators may operate manufactured systems. System manufacturers often provide trained operators or training on their devices. The following procedure may be used when not using a continuous discharge system: 1. Prior to treatment, the appropriate jurisdiction should be notified in accordance with the regulations set by the jurisdiction. 2. Every effort should be made to isolate the potential high pH water in order to treat it separately from other stormwater on -site. 3. Water should be stored in an acceptable storage facility, detention pond, or con- tainment cell prior to treatment. 4. Transfer water to be treated to the treatment structure. Ensure that treatment struc- ture size is sufficient to hold the amount of water that is to be treated. Do not fill tank completely, allow at least 2 feet of freeboard. 5. The operator samples the water for pH and notes the clarity of the water. As a rule of thumb, less CO2 is necessary for clearer water. This information should be recor- ded. 6. In the pH adjustment structure, add CO2 until the pH falls in the range of 6.9-7.1. Remember that pH water quality standards apply so adjusting pH to within 0.2 pH units of receiving water (background pH) is recommended. It is unlikely that pH can be adjusted to within 0.2 pH units using dry ice. Compressed carbon dioxide gas should be introduced to the water using a carbon dioxide diffuser located near the 2014 Stormwater Management Manual for Western Washington 1 1 I 1 1 1 1 1 1 I 1 1 L� 1 1 Volume 11 - Chapter 4 - Page 410 1