5. Biofiltration Media Evaluation; Geosyntec Consultants and Boeing Co., 2008 – 2012
This study investigated a variety of media types that can be used singly and in combination for use in stormwater treatment facilities, including media filters, biofilters, bioretention, rain gardens, etc. These tests were conducted specifically to determine the different media performance options for use in advanced biofiltration systems at an industrial site having very stringent numeric effluent limits. These stormwater treatment systems were designed to treat 90% of the long-term runoff volume from drainage areas ranging from 5 to 60 acres at the site. The main pollutants of interest for the project are cadmium, copper, lead, and dioxins, with other constituents being of secondary interest, based on historic stormwater quality monitoring results at the site. One primary project objective is for treated effluent concentrations to meet the low numeric effluent limits that have been applied to stormwater discharges through the site’s NPDES permit. These numeric effluent limits are based on water quality standards. A challenge to the project design is that current site runoff concentrations for the pollutants of interest are generally below levels typically seen in urban and industrial stormwater runoff. A review of the literature on filtration media and onsite monitoring data (including existing treatment system performance results and previous media pilot testing studies) indicated that several promising media exist for consistently treating the pollutants of interest to the required effluent concentrations. However, many of these materials are very expensive; with potential construction costs being significant given the large volumes required for the systems based on early designs (estimated media volumes for the project have ranged from 5,000 – 12,000 cubic yards). There are newly available materials that are promising, but little, if any, data are available to quantify their performance. These tests therefore evaluated these candidate materials under procedures that have proven successful during past media investigations for stormwater treatment effectiveness.
Observed infiltration and clogging characteristics for tested media.
Media mixtures performed more consistently under a broader range of conditions than individual components used separately. The mixtures capitalize on the pollutant removal strengths of their components, while providing other components that may address the weaknesses (such as the release of cations in large concentrations during ion exchange).The media mixtures that are most robust (longest run times before clogging, with moderate flow rates and suitable contact times for pollutant removal) are:
Rhyolite sand, SMZ, and GAC mixture (blended mixture) and the Rhyolite sand, SMZ, GAC-PM mixture (blended mixture). They had very similar performance attributes. The added peat provided some additional benefits for metal reductions at high flow rates. The GAC in these mixtures (when mixed with the other components) also provided better control for a number of other constituents, including nitrates.
Site filter sand-GAC-site Zeolite (layered) clogged earlier, but possibly would have fewer exceedences overall. The drawback to the layering of the filter components is the change in flow rate and contact time.
In terms of statistically significant removals, both R-SMZ-GAC and S-Z-GAC (layered) media combinations performed similarly, although the current site layered media combination did not demonstrate statistically significant removals for lead.Any media combination that included GAC was effective for TCDD removal. All of the media tested had very high levels (approaching 90%) of removals of particulates, even down to very small particle sizes (as small as 3 µm), with concurrent good removals of pollutants strongly associated with the particulates (such as for total aluminum, iron, and lead).
Media performance plots for copper from long-term, full-depth column tests.
Some constituents and some media required a certain contact time before retention, while others were more capable of pollutant retention more rapidly and at lower influent concentrations. For example, when the contact time was less than 10 minutes, the metal removals were much less than for the longer contact times. Also, greater contact with GAC resulted in slightly better nitrate removals, while the greater contact time for phosphate resulted in greater losses of the phosphate from the media. This type of trade-off between improved removal and increased leaching was seen for several media-constituent combinations.
Longer retention times can be achieved through deeper media beds or slower flow rates and larger surface areas. The column tests confirmed generally the results of the laboratory studies that showed that good removals could be achieved with relatively slow to moderate flow rates (5 to 60 meters/day) and moderate contact times of the water with the media (10 to 40 minutes).
Media performance plots for lead from varying-depth column tests.
The GAC was the most important component in these mixtures, while the addition of either of the zeolites was also needed. The specific choice of which would be dependent on costs and specific ion exchange issues. The sand is critical to moderate the flow rates and to increase the contact times with the coarser media, unless other flow controls were used in the filter designs. The Rhyolite sand added some removal benefits compared to the site sand. As noted, a small amount of peat added to the mixture increases metal removals during high flow rates. Therefore, the best mixture for removal of pollutants to levels that met the effluent discharge limits was the combination of Rhyolite sand (30%), surface modified zeolite (30%), GAC (30%), and approximately 10% peat. To minimize the leaching of constituents from the GAC, its concentration could be reduced, but then nitrate removals would be limited.
5.1 Enhanced Underdrain Systems for Biofilters
The treatment of stormwater by biofilters is dependent on the hydraulic residence time in the device for some critical pollutants. The effective use of biofilters for the control of stormwater in combined sewered areas is also related to residence time, as it is desired to retain the water before discharge to the drainage system in order to reduce the peak flows to the treatment plant. This research is conducting a series of tests to determine the hydraulic characteristics of sand-based filter media (having a variety of particles sizes representing a range of median particle sizes and uniformity coefficients) during pilot-scale trench tests. The drainage rate in biofiltration devices is usually controlled using an underdrain that is restricted with a small orifice or other flow-moderating component. These frequently fail as the orifices are usually very small (<10 mm) and are prone to clogging. A series of tests were also conducted using a newly developed foundation drain material (SmartDrainTM) that offers promise as a low flow control device with minimal clogging potential. A pilot-scale biofilter using a trough 3m long and 0.6 x 0.6m in cross section is being used to test the variables affecting the drainage characteristics of the underdrain material (such as length, slope, hydraulic head, and type of sand media). Current tests are also being conducted to test the clogging potential of this drainage material.
SmartDrainTM installed in pilot-scale biofilter.
The results from the experiments conducted to test the variables affecting the drainage characteristics of the filter media indicate that slope of the SmartDrainTM material had no significant effect on the stage-discharge relationship whereas effect of length of the SmartDrainTM material had a very small effect on the discharge. Research is ongoing to investigate the clogging potential of the SmartDrainTM material. Only about 20% reductions in the outflow rate of the filter media have been observed during the clogging tests having a total load of about 40 kg/m2 onto the filter area, about twice the load that can usually cause clogging of biofilter media.
Reduction in flow with increased sediment loading.
SmartDrainTM flow rates compared to very small orifices.
Turbidity (NTU) measurements showed that the effluent NTU decreased rapidly with time, indicating significant retention of silt in the test biofilter during the clogging tests. These preliminary tests indicate that the SmartDrainTM material provides an additional option for biofilters, having minimal clogging potential while also providing very low discharge rates.