Development of a Depth-Integrated Sample Arm to Reduce Solids ...

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Selbig and Bannerman 1.25 x 0.375 Inch elliptical Stainless steel shaft \ Mounting bracket/ Polycarbonate end cap with 0.25 Inch I.D. Intake orifices Figure 1-Schematic drawing of the DISA. et al. (2005) suggested designing a new autosampler system that uses a float system to place the intake at the midpoint of a flow path. DeGroot et al. (2009) designed an intake manifold that adjusts itself to the depth of flow in the pipe by use of a fin. Although this device showed promising results for accurately collecting sand-sized particles in a small-diameter pipe, an alternate mechanism would be necessary for pipes larger than 61 cm (24 in.) in diameter (DeGroot et al., 2009). These ideas mark the genesis of new technologies to improve the way a stormwater quality sample is acquired by use of autosamplers. This study discusses the development of a new prototype depthintegrated sample ann (DISA) designed to integrate with existing autosampler configurations for collection of stormwater quality samples of urban runoff in a storm sewer. Use of the DISA facilitates collection of stormwater quality samples from a single or multiple point(s) in the water column. Integrating samples from the entire water column, rather than from a single, fixed point, can result in a more accurate representation of stormwater-borne solids. In this study, the DISA and a fixed-point sampling method were used to collect samples of urban runoff. Results from the two methods are compared on the basis of concentrations of suspended sediment, organic content, and PSDs using field-collected samples of urban runoff. Deficiencies of the DISA and suggested modifications to improve the device also are discussed. 348 Ram piston Rear mounting hole 12v wMre 'F -12 VDC Motorized: ram Aluminum support frame Materials and Methods The general field of application of the DISA described herein is intended to be in closed conveyances, such as storm sewers, that are used to route stormwater runoff in an urban environment. However, the DISA also may be used in any water quality sampling environment where the distribution of sediment in flow can be shown to be heterogeneous and not easily corrected using established manual sampling techniques, such as equal-width or equal-depth increment sampling. Use of the Depth-Integrated Sample Arm to Collect a Water Quality Sample. The DISA has a support frame, a motorized piston (also referred to as a linear actuator), a rotary potentiometer, and a sample arm assembly (Figure 1). As the piston extends or retracts, it pivots the sample arm assembly around the potentiometer axle. The rotation of the DISA, and thus the position of the intake orifice, is controlled by an external data logger or other programmable logic control (PLC) device that activates a relay to energize the piston. The PLC may be programmed to set the intake orifice to a percentage of the water depth. The depth of water is measured by an acoustic-velocity sensor or similar device. For example, if the target sample position is set to be 50% of a water depth of approximately 0.2 m (0.5 ft), then the PLC will activate the motorized ram until the potentiometer reads the voltage representing 0.08 m (0.25 ft). Water Environment Research, Volume 83, Number 4
Thus, the motorized ram rotates the sample arm assembly to any vertical depth within the storm sewer. Once the PLC determines that the potentiometer has reached the target voltage, it triggers the autosampler to operate with normal purge/withdraw cycles to collect a stormwater quality sample and deposit it in one or more storage containers. Once the sample has been acquired, the DISA either moves to a new position for collection of another sample or fully retracts to the horizontal position, whi6h removes the DISA from the flow path with any debris that may have accumulated on the sample arm assembly while acquiring a sample. Method to Compare the Depth-Integrated Sample Arm with a Fixed-Point Autosampler Configuration. This study involved collection and chaiacterization of data derived from stormwater quality samples collected in two urban drainage areas located in Madison, Wisconsin. The first was a 2.4-ha (6-ac) commercial parking area adjacent to a shopping center complex; the second was a 21-ha (53-ac) single-family residential area.. Runoff was collected in multiple storm sewer inlets at each site and then was conveyed through a single, circular, concrete pipe with diameters of approximately 91 and 107 cm (36 and 42 in.) at the parking lot and residential drainage areas, respectively. Each monitoring station was equipped with two automated water quality samplers and instruments to measure water level and velocity. Measurement, control, and storage of data were performed by electronic data loggers. Sample collection was activated by a rise in water level in the pipe during'a storm. Once the water level threshold was exceeded, typically a depth of approximately 0.06 m (0.2 ft) from the pipe floor, each automated water quality sampler collected a discrete water sample into I-L plastic containers. This process was repeated for every approximately 0.03-m (0.1-ft) increase and approximately 0.06-m (0.2-ft) ,decrease in water level over the duration of the storm hydrograph until the water in the pipe receded below the water level threshold. Each autosampler was located side-by-side in an aluminum shelter directly over the storm sewer. ISCO 3700 pump heads with a 24-bottle configuration were used to provide consistent intake velocities. Pump heads were programmed with similar purge/ withdraw cycles to flush the sample tubing before sample collection. One autosampler collected a water quality sample from a fixed point located approximately 2.5 cm (1 in.) off the pipe invert (herein called the fixed-point sampler), and the other collected a sample from multiple points using the DISA. Upon sample initiation, each sampler would collect a series of three successive sub-samples separated 'by 1-minute increments. Because the fixed-point sampler's intake nozzle was secured to the pipe floor, each sub-sample was taken from the same location. The DISA was programmed to collect each sub-sample from different vertical locations in the water column. The first subsample was collected approximately 2.5 cm (I in.) from the.pipe invert to coincide with the intake location of the fixed-point sampler. The DISA then would relocate the intake nozzle to a vertical point approximating 30 and 60% of the water level for the second and third sub-samples, respectively. In doing so, subsamples were collected from the lower, middle, and upper onethird of the full water column. For example, if the water level was 0.3 m.(1.0 ft), the first, second, and third sub-samples (herein called the lower, middle, and upper sub-samples) would be collected at a depth of 0.02, 0.09, and 0.2 m (0.08, 0.3, and 0.6 ft) "from the pipe invert, respectively. Although the vertical spacing of each sub-sample remained consistent for this study, the location of April 2011 Selbig and Bannerman the nozzle intake can be programmed to accommodate any vertical position in the pipe, from completely vertical to completely horizontal. Upon runoff cessation, water quality samples were retrieved and transported to the U.S Geological Survey Middleton Field Office in Middleton, Wisconsin. Each discrete sample was split equally into two laboratory-prepared containers by passing the 1 L sample through a cone splitter. Approximately 180 mL also was transferred from the cone splitter into a separatory funnel for determination of PSD. Except for the separatory funnel, processed samples were chilled on ice and delivered to the Wisconsin State Laboratory of Hygiene (Madison, Wisconsin) for analysis of total suspended solids (TSS), total volatile suspended solids (TVSS), and suspended sediment. Particle Size Distribution Measurement. The PSD.of runoff samples was measured using a Laser In-Situ Scattering and Transmissometry (LISST) particle size analyzer (Sequoia Scientific, Bellevue, Washington). The LISST-Portable uses laser diffraction to measure PSD as particle volume concentration (microliters per liter) in 32 logarithmically spaced bins ranging in size from approximately 2 to 350 gtm. Additional details are described in Agrawal and Pottsmith (2000). Contents of the separatory funnel were transferred into the holding chamber of the LISST-Portable. Use of a separatory funnel allowed for complete transfer of the sample without the potential to bias coarse particle concentration by pouring or pipetting. The upper particle size fraction reported by the LISST-Portable was approximately 350 gim. Although the particle size fraction in the majority of samples collected as part of this study was below this reporting limit, ,some samples contained particles exceeding that size (
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