To determine the difference between natural soil and sorption media in terms of particle size, Figure 2 shows the particle size distribution curves for the Hunter's Trace soil along with media mixes 1 and 2. Hunter's Trace soil is a well graded or evenly distributed soil and the effective size of the soil is 0.16 mm. The effective size is defined as 10% of the sample passing through that sieve size. According to Das , effective size is a good indication of intrinsic permeability (i.e., hydraulic conductivity). The selected media mixes are then packed into the other two columns. The particle size distributions and other characteristics for the two media mixes are shown in the following figures. Both the media mixes 1 and 2 are seen to be poorly graded with effective sizes ∼0.08 mm as shown in Figure 2.
The material characteristics of the natural Hunter's Trace soil and two sorption media amendments are shown in Table 2. When comparing the media mixes against the natural soil, media mixes have lower density, larger void ratio, larger porosity, smaller specific gravity, and higher permeability. Overall, the performance of the two media mixes appears to be similar. Both media mixes have specific gravities different to the control case (Hunter's Trace Soil) due to the increased amount of organic material. The porosity of the two media mixes is greater than that of the Hunter's Trace soil, thereby allowing for a more rapid flow and smaller HRT.
Table 2. Material characteristics .
| ||Hunter's trace (dry sample)||Hunter's trace (moist sample)||Media mix 1||Media mix 2|
|Density (g cm−3)||1.56||1.73||1.41||1.44|
|Specific gravity (Gs)||2.62||2.62||2.19||2.33|
|Surface area (m2 g−1)||—||—||0.13||0.24|
|Permeability (cm h−1)||62.48||4.47||11.12||9.19|
Results of Column Tests
The control column at 10°C had very low removal efficiencies for nitrogen species. This may be due to slower microbiological nitrification and denitrification resulting from the lower temperature and absence of an electron donor. However, the control column achieved high removal efficiencies for orthophosphate due to the adsorption/adsorption effect. The high dosage case outperformed the low dosage case. The two media mix columns showed similar responses to testing for both influent concentrations of the low and high dosage at 10°C. Both media mixes outperformed the control for the removal of nitrogen species; conversely, they were outperformed by the control case for orthophosphate removal. The media mixes had moderate removal efficiencies for nitrate and total nitrogen. Both media mixes were able to remove more of total nitrogen in the lower dosage case. Media mix 2 had higher removal efficiencies for nitrate in the lower dose case. Media mix 1 removes approximately the same amount of nitrate regardless of the initial nitrate dose. Media mixes 1 and 2 had higher removals of orthophosphate with higher initial phosphorus concentrations. When considering orthophosphate removal, media mix 2 removed twice as much orthophosphate as media mix 1 for the higher dose case, putting its removal capabilities around the average when compared to the control column. This is most likely due to the inclusion of 10% limestone in media mix 2.
The control column at 23°C showed moderate removal efficiencies for nitrogen species. When passing through the Hunter's Trace soil in the control case, nitrite and ammonia levels increased for both low and high dose concentrations. The control column did achieve moderate removals of nitrate when the initial dosage was low. For the case with the higher initial nitrate concentration the control column removed around 26% of the nitrate by the time it reached the second port. But the nitrate levels increased more than the initial dose by the time it reached the bottom causing the overall removal for the control column to be zero. Nitrite levels increased in the column, and the highest amount of nitrite was found at Port 2. These variations reflect the dynamics between nitrification and denitrification. Approximately 28% of the total nitrogen was removed for both dosing situations. The control column achieved higher removal efficiencies for orthophosphate as compared to the situations in these two media mix columns. Again, the high dosage case outperformed the low dosage case. At this temperature, dosing conditions had a greater impact on orthophosphate removals than that at the lower temperature comparatively. The final difference was 30% between the two initial dosing conditions in terms of the removal efficiencies. Besides, the two columns containing media mixes performed similarly with respect to the removal of nitrogen species and orthophosphate. The actual removal efficiencies of these columns improved around 15% for the removal of nitrate and 30% for total nitrogen as compared to the cases at 10°C. In terms of orthophosphate removal, media mix 1 had ∼63% removal efficiency for both dosages, while media mix 2 had 53% for lower dosage and 82% for higher dosages. In this respect, the two media mixes performed as well or better than the control case with lower dosages. The control column does outperform the media mixes overall with a 95% removal efficiency at high doses of orthophosphate.
When the temperature was raised to 28°C, the control column performed quite differently than at the other two lower temperatures. The control column reached its highest nitrate removal at low dosage at Port 2 (77%) instead of the bottom of the column (48%). This is the phenomena of leaching due to the over-consumption of adsorption/absorption capacity. The nitrate removal for high dosage was reached at the bottom of the control column (44%), but was lower than the removal efficiency for low dosage. The highest total nitrogen removal occurred at Port 2 in the control column, and was higher for the low dosage case. Orthophosphate removal followed the same trend as at the other two temperatures; however, it had higher removal efficiency at Port 2 for higher dosages and higher removal efficiency at the bottom of the control column for lower doses. Besides, the two media mixes columns also followed this new trend of achieving higher removal efficiency at Port 2 instead of at the bottom of the column for some instances. Media mixes 1 and 2 had the highest removal efficiency of nitrate at the second port for the low dosage case. But the highest removal efficiency for the high dose continued to be at the bottom of the column. Media mix 1 had approximately the same removal efficiency of around 95% at the bottom of the column. Media mix 2 had a 40% difference in removal efficiency at the bottom, achieving the highest removal efficiency of 90% at high dosage case. Nitrite addition continued to be a problem for both media mixes, with nitrite levels at their highest at ports 1 and 2. Total nitrogen removals were 73.6 and 85.0% at the bottom of the columns containing media mixes 1 and 2, respectively. The only exception is for media mix 1, which showed a 2% greater removal of total nitrogen at Port 2. Orthophosphate removal efficiencies mostly decreased as the water passed through both of the media columns. However media mix 1 has its highest removal efficiency at Port 1 for the low dosage concentrations. This difference, however, was within 1% of the removal efficiency at the bottom of this media column.
Figures 3–5 summarize the nitrate removal through the columns at each temperature. The values for removal efficiency at each port were calculated by averaging the removal efficiency collected at each port combining high and low dosage cases. In the case of 10°C the removal efficiencies seem to follow an increasing trend as the water travels through the column. Media mix 1 presents the highest removal efficiency at each port. Nitrate removals at 23°C appear to follow a similar pattern. However, the columns responded unusually at Port 3 and the removal efficiency of the control case and media mix 1 decreased slightly. In fact, the decrease for media mix 1 of only 0.3% could be considered negligible. Lastly, at 28°C the removal efficiencies increased as the water flowed down the column and media mix 1 always achieved the highest removals. The removal efficiency in the control column decreased slightly between the second and third ports.
Orthophosphate removal through the columns at each temperature is shown in Figures 6–8. Once again, the values for removal efficiency at each port were calculated by averaging the removals at each port combining high and low dosage cases. The data does not appear to follow the same trends as those in nitrate removals with respect to port and temperature. At the lowest temperature, the removal efficiency increased as the water passed through the columns. The control column achieved the highest removal efficiency for most cases. Media mix 1 operated in a similar manner to media mix 2 having just slightly higher removals. At 23°C, all of the columns showed different reactions as the water flowed downward. The control column was outperformed by media mix 2 at the second port, and highest removal of each column was achieved at the bottom port. Media mixes 1 and 2 have similar removals at Port 1 and 3, but media mix 2 has the highest removal efficiency of all the columns at Port 2. Lastly, the 28°C case was considered. The control column achieved the highest removal of all three ports by Port 2 and then removal efficiency was decreased at Port 3. The average removal efficiency of media mix 1 was around 56%. For media mix 2, the removal efficiency amplifies with each port, causing it to go from last place to first.
A comparison of the nitrate removal for the columns at a specific temperature was carried out utilizing an average of the removals at the bottom of the columns for all dosages as shown in Table 3. From this table, it is easy to distinguish how the media mixtures reacted to the change in temperature in terms of nitrate removal. All of the columns achieved their highest removal efficiency at 28°C. Obviously, nitrate removal increased with temperature. Media mixes 1 and 2 had similar increases; with ∼10% increase from 10 to 23°C and then a 15% increase from 23 to 28°C. The control column experienced a 12% increase for the first temperature gap and 25% increase between the higher temperatures. Media mix 1 has the best removal efficiencies for 23, 25, and 28°C with 69.7, 79.7, and 95.3%, respectively. It had the highest nitrate removal for all of the experiments with 95.3% nitrate removal. Overall, media mix 2 is the best on average, and both improvements could be derived from having a better microbiological effect of denitrification.
Table 3. Nitrate removal comparison between temperature and column (final port comparison)
|Column|| || || |
|Media mix 1||69.70%||79.70%||95.30%|
|Media mix 2||63.20%||77.90%||93.60%|
A comparison of the orthophosphate removal for the columns at a specific temperature was also performed utilizing an average of the removals at the bottom of the columns for all doses Table 4. From this table it is easy to distinguish how the media mixtures reacted to the change in temperature in terms of orthophosphate removal. All columns achieved very high orthophosphate removal at different temperatures; namely the control, media mix 1, and media mix 2 at 10, 23, and 28°C, respectively. However, the difference between the highest and second highest removals for the control was only 1%. The highest overall orthophosphate removal was achieved by media mix 2 at 28°C. The control column outperformed the media mixes for 10 and 23°C and was a fairly close second for 28°C. Overall, media mix 2 and control are the best.
Table 4. Orthophosphate removal comparison between temperature and column (final port comparison)
|Column|| || || |
|Media mix 1||38.50%||63.30%||59.90%|
|Media mix 2||58.80%||67.40%||85.50%|
Two additional parameters were tested along with the nutrients for each column. The pH levels through the column were taken for use in consideration of real life application of the media. The pH of the stormwater should not be significantly increased or decreased. pH levels around 7 are preferable. The DO levels were also read at each port as the water traveled through the columns. The DO is of special importance when considering the denitrification process. In order for denitrification to occur, the DO levels should be less than 1.0 mg L−1. This information provides insight as to whether the nitrate removals achieved throughout the column could be the result of denitrification. The average pH and DO values can be found in Table 5.
Table 5. Average pH and dissolved oxygen levels in the columns
|Port||Control||Media mix 1||Media mix 2|
The control column experienced a slight drop in pH for all of the temperatures tested, except for the 23°C. The DO in the control column decreased as the water traveled downward. The DO levels of less than or equal to 1.0 were achieved for the 23°C and 28 cases. However, the DO levels reported for the 10°C case are most likely higher than actual levels in the column due to the reduced flow and increased exposure to the atmosphere during the 10°C scenario. Media mix 1 achieved similar results to the control in terms of pH reduction through the column. The dissolved oxygen levels in the media mix 1 column were almost always around the desired level. The media mix 2 column was the only one able to maintain the pH level for 10°C, increase the pH for 23°C and very slightly decrease the pH for 28°C case. The DO levels reported are similar to the control column.
To discern what effect temperature, dose, and media mixture have on the removal efficiency in our study, the two-way ANOVA analysis was conducted for each dosage case. The removal efficiencies measured at the bottom of each column were used to standardize the data for final comparison. With the existing module in SAS®, a two-way ANOVA table was executed in which removal efficiencies and media were designated as the main effects. The assumptions associated with performance of an ANOVA test, namely independence, normal distributions, and equal variance, were considered across all cases. The assumed alpha value for all of the tests was 0.1, providing 90% level of confidence.
There is a considerable difference between column 1 (media mix 1) and 2 (control), as well as column 2 (control) and 3 (media mix 2). Thus, there is a noteworthy difference between the media mix columns and the control case, but not between each other. However there was sufficient evidence to prove that there is a difference between the temperatures for the low dosage nitrate case. However, there is a considerable difference between the media mix columns and the control, but not between each other. Sufficient evidence is available that there is a difference between 10 vs. 28°C and 23 vs. 28°C for the high dosage nitrate case. Statistically, there is insufficient evidence to suggest a difference in the removal efficiencies between 10 and 23°C. Besides, there is no significant evidence indicating an interaction between column and temperature.
On the other hand, a similar two-way ANOVA table was constructed for orthophosphate in the low dosage case. For all of the effects tested during the orthophosphate low dosage case there is no significant evidence that a difference among column, temperature or interaction between column and temperature exists. The final Two-Way ANOVA table was constructed for Orthophosphate at high dosage. Under 90% level of confidence, there is a considerable difference between column 1 (media mix 1) and 2 (control), as well as column 2 (control) and 3 (media mix 2). There was insufficient evidence to unequivocally show that there is interaction between column and temperature.