| Proposed stormwater regulations within the Lake Tahoe Basin provide the backdrop and motivation for the study of advanced stormwater treatment techniques including chemical coagulation, flocculation and sedimentation. In this investigation, jar and settling column experiments were performed with stormwater runoff to identify (1) the best chemical (or formulation) for treating stormwater, (2) mixing requirements, and (3) the relationships between stormwater quality and chemical treatment.; In one investigation, 20 different commercial formulations were jar tested using four stormwaters with diverse properties. Results showed that as a class of treatment chemicals, the polyaluminum chloride (PACl) products containing sulfate in their formulation (polyaluminum hydroxychlorosulfates) were particularly effective in reducing turbidity. The second best performing class of chemicals in treating the stormwaters were the PACl/polymer blends.; Mixing energy and contact times are critical in chemical treatment. An extensive battery of jar tests showed that under less than optimal mixing conditions (i.e., short times and low intensities), poor initial turbidity removal can be partially overcome with extended settling times. In the tests, a rapid mix contact time of 60 s. at G = 475 s-1 provided optimal turbidity removal after 15 minutes of settling; however after 30 minutes of settling, there was no significant difference (alpha = 0.05) in settled turbidity for jars mixed at G = 165 s-1 for 30 s. At the optimal chemical dose, a rapid mix Gt value between 10,000 and 15,000 may be adequate for treating stormwater if longer settling times are possible (3 to 24 hours).; Polynomial regression relationships with R-square values >0.6 were found that describe the expected turbidity removal (via chemical treatment) based on initial stormwater turbidity for four products (JC-1720, SC-50, PASS-C and Liqui-Floc). For one of the chemicals tested (SFA-100), a regression model was found that describes the required treatment dose based on initial stormwater turbidity. Other regression relationships were noted that relate stormwater turbidity, TSS, alkalinity and electrical conductivity (EC) and the expected width of the treatment range. Experiments with salt and bicarbonate addition verified the observation that improved treatment sometimes occurs with higher stormwater EC and alkalinity levels. |
