Evaluating the effectiveness of pretreatment practices for rain gardens
Bioretention practices, commonly known as rain gardens, are a form of stormwater treatment where runoff is infiltrated through a media. An important job of bioretention media is to capture sediment and debris from stormwater; however, bioretention cells can quickly become clogged. Therefore, the Minnesota Stormwater Manual requires pretreatment practices prior to bioretention, to remove much of the sediment and debris. Pretreatment devices include settling devices, screens, and filterstrips.
The purpose of this project was to evaluate the effectiveness of different pretreatment devices in Minnesota. The objective was to gather quantitative data using a common method that will allow for comparisons across devices. Overall, project designers, government officials, and other practitioners can use data to make informed decisions about project design and maintenance plans.
Five pretreatment methods were tested at Minnesota field sites. The first site, in the city of Anoka, was suitable for testing a grass lined inlet, the Rain Guardian Turret, the Rain Guardian Bunker, and a rock lined inlet. A non-proprietary in-line shallow sump grit chamber was tested at a different site in Bloomington. The grass and rock lined inlets treat water by settling sediment and debris among the vegetation and rocks, respectively. The Rain Guardian Turret, the Rain Guardian Bunker, and the grit chamber treat water using a top grate as a screen for large items and a settling chamber for smaller particles and debris. The Rain Guardian products also use a debris wall to separate out floatables and a downstream screen to filter additional suspended particles.
Each pretreatment device was flooded with synthetic stormwater. A uniform mixture of sediment, artificial leaves, wooden dowels, and plastic forks was added to municipal water taken from an on site fire hydrant to represent the typical sediment and debris found in stormwater. Researchers collected and measured the sediment and gross solids retained on or within the pretreatment devices.
When tested from an initially clean state, all five pretreatment practices met the standards for sediment and debris removal set by the Minnesota Pollution Control Agency. During the high intensity tests, all five devices captured over 70% of the total sediment mass, and four of the devices captured over 55% of the gross solids. The grass lined inlet had poor gross solids retention: 20% for low intensity tests and 30% for high intensity tests.
The systems were also analyzed qualitatively regarding the ease of maintenance and inspection. The grass lined inlet and rock lined inlet were determined to be the most difficult and costly to maintain, because the inlet would need to be rebuilt and reinstalled after each cleaning. The Rain Guardian Turret, the Rain Guardian Bunker, and the sump grit chamber would all be easy to clean out using a shovel or vacuum. However, the Rain Guardian Turret and Bunker are both above ground and dry between rain events due to the permeable screen, which would make inspection much easier and more efficient.
Future work could include research on the maintenance required for different pretreatment practices, the optimal sizing of pretreatment practices, other pretreatment practices not analyzed in this study, and the cost-effectiveness of pretreatment practices.