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It was important to ensure that the samples were collected when the system was operating at steady state and that the injected chemicals (i.e., 1,4-dioxane & H 2 O2 ) were completely mixed. There are two alternatives to ensure that the system is operating under steady state conditions prior to sample collection. The simplest is to wait for at least five hydraulic retention times (HRTs) after a process change before collecting samples. One HRT is defined as the time required for one system volume to pass through the system assuming plug flow conditions. In this case, the system volume is defined as the total water volume between the injection ports and the effluent sample port. Thus, the HRT is calculated by dividing that volume by the flow rate. A conservative approach to allow for deviations from plug flow is to allow five HRTs to pass before assuming that the system is at steady state. 4 UV-AOP SYSTEM TEST PROCEDURES 4.1 UV-AOP MIXING TEST PROCEDURE The mixing test was performed with hydrogen peroxide as the tracer chemical. The hydraulic residence time distribution within the system, from the H 2 O 2 /1,4-dioxane injection ports to the final effluent sample port, was assessed, which allowed the equilibration time to be calculated and used for subsequent tests. This tracer study involved initiating the injection of a known concentration of tracer compound (e.g., 6 ppm H 2 O 2 ) into the influent stream at time zero with the UV lamps off and collecting a series of samples at the influent and effluent sample ports. It was recommended that samples be collected as frequently as necessary to adequately define the tracer curve (i.e., concentration vs. time curve). By monitoring the H 2 O 2 level in the effluent samples, the time required for the system (between injection and effluent ports) to reach steady state was determined. The time required to reach steady state determined from this test was used to determine run times for the subsequent performance tests. 4.2 UV-AOP PERFORMANCE TEST PROCEDURE The UV -oxidation system operating parameters to be investigated during this study included flow and hydrogen peroxide dose. The flow range for the pilot tests was between 0.5 and 2.0 gpm (0.5, 1.0 & 2.0 gpm). Hydrogen peroxide was dosed into the influent stream at concentrations between 0 and 20 ppm (i.e., 0, 5, 10, & 20 ppm). That provided a test matrix of 3 x 4 totaling 12 unique test runs. In addition, a run with the UV power off provided a control condition that allowed the sample collection, handling and analytical procedures to be validated. Other test conditions with 0 ppm H 2 O 2 and especially with no UV did not need to be performed with all of these conditions. The recommended test matrix is provided in Table 1. For each run influent and effluent sample pairs were collected and analyzed for 1,4-dioxane and H 2 O 2 . 8