A.J. Erickson, J.S. Gulliver, R.M. Hozalski, P.T. Weiss
Maintenance of filtration practices can range from the relatively easy task of trash removal to the much more expensive and time consuming complete removal and replacement of the filter media and underlying system. This section is intended to provide information on the frequency, complexity, and required time of maintenance efforts related to filtration practices as well as typical factors that commonly inhibit the performance of filtration treatment practices. Much of the information was gathered through a survey (Erickson et al. 2009) designed to collect information related to stormwater treatment practice use in Minnesota and Wisconsin.
The two primary failure mechanisms for filtration practices are clogging and the presence of macropores. Clogging can result in long periods of standing water, flooding of surrounding areas, bypassing of the filter by untreated stormwater, lack of measurable effluent, or any combination thereof.
Maintenance actions for all sand and soil filters, whether under or above ground or the filtration portion of hybrid systems, is generally the same. Thus, maintenance actions for all sand and soil filters is discussed together in this section.
Surface and underground sand filters and hybrid systems are typically implemented to reduce suspended solids in stormwater runoff. Pretreatment with a separate stormwater treatment practice just upstream of the filtration practice is required (Minnesota Stormwater Manual) and must be maintained, as well. Like all stormwater treatment practices, filters need regular inspection and maintenance to ensure proper operation.
If any level of assessment indicates that the filter is not draining or will not drain the design runoff volume within the required time, the following steps can be taken to address the problem.
- Inspection of outlet structures and underdrain system with removal of any objects obstructing flow and/or replacement of structural components, if necessary.
- Performing field capacity or synthetic runoff tests to determine filtration rates and hydraulic conductivity values of the filter media. If filtration rates are low, the following steps may correct the problem.
- Roto-tilling of the top six inches of filter media
- Removal and replacement of a sediment layer if it exists and the top six to eight inches of filter media
- Removal and replacement of the entire media bed
If, at any time, it is determined that filtration rates are too large or the total suspended solids retention rate is too low, it is likely that there is a short-circuit in the filter media. The following steps can be taken to confirm and address the problem.
- Visual inspection of the filter media to ensure there are no large holes, ruts, or other openings in the media that would allow runoff to pass without being sufficiently filtered. If any such areas are present the media in the suspect area should be replaced only after any underlying causes (e.g. insufficient gravel subbase, tear in geosynthetic fabric separating filter media and gravel subbase) are found and corrected.
- Capacity testing to determine filtration rates at various locations on the filter surface. Any locations where the hydraulic conductivity is determined to be larger than 280 ft/day should be corrected by removing and replacing the filter media in that area.
- Synthetic runoff tests to determine an overall average hydraulic conductivity value for the filter. A disadvantage of this test is that it will not identify specific locations in the media where short-circuiting is likely occurring. As a result, if synthetic runoff tests identify short-circuiting and no additional testing is performed (i.e., capacity tests), the entire media bed must be replaced.
While frequency of filter media replacement will vary depending on watershed and filter size, land use, rainfall amounts and intensities, etc., Wossink and Hunt (2003) report that removal of the top layer of filter media typically is required from once per year to once every three years. Landphair et al. (2000) report that surface sand filter media typically needs to be replaced every three to five years.
Frequency, effort, and cost
The frequency of inspection required to maintain a filtration practice is dependent on many variables including rainfall amounts and intensity, watershed use, other stormwater treatment practices within the watershed, and many other factors. Survey results (Erickson et al. 2009) indicate that the frequency of inspection of filtration practices of the responding municipalities is once a year or less (see table 8.2). In addition to the frequency of inspection, the median annual staff-hours devoted to the inspection and maintenance of filtration devices is also given in table 8.2. It must be noted, however, that these results are estimates of the hours spent and do not indicate that this level of maintenance was sufficient to keep the stormwater treatment practice performing properly.
In the survey (Erickson et al. 2009), municipalities were also asked to gage the level of complexity of their maintenance efforts as minimal, simple, moderate, or complicated. These results are also given in table 8.2 as the percentage of respondents selecting each complexity level.
The U.S. EPA (1999) estimated that typical annual operating and maintenance (O & M) costs for sand filters, as based on expected maintenance schedules, range from 11%-13% of the total construction cost. Weiss et al. (2007) reviewed the literature and found that, based on expected maintenance schedules and estimated prices, the expected annual O & M costs ranged from 0.9%-9.5% of the total construction cost. It must be noted that these cost estimates are not based on actual O & M cost data but rather on typical maintenance schedules and activities combined with the estimated costs to perform these activities.
Factors affecting performance
Maintenance efforts for filtration practices are typically focused on the removal of filtered sediment buildup and litter/debris. Survey (Erickson et al. 2009) results indicate that, at times, maintenance efforts were also needed to address the issues of groundwater and oil spills. Table 8.3 lists the percentage of respondents that indicated the corresponding factor most frequently caused deterioration of the stormwater treatment practice performance. Other factors not listed in Table 8.3, such as invasive vegetation and bank erosion, were listed as possible survey choices but received zero responses and are, therefore, not listed.
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