Cleaning the Water at Lakeside
In late 2001, a housing construction project called Lakeside in Redmond, WA, was experiencing problems meeting stormwater turbidity limits while discharging to Evans Creek, which is populated with endangered salmon. The project developer evaluated several innovative stormwater treatment technologies and selected a 500-gallon-per-minute chitosan-enhanced sand filtration system to purify turbid stormwater runoff from the site. After about a year of treatment under a temporary permit, with the system performing better than expected, it was decided to carefully monitor the performance of the system over a further one-year period to see how it performed over time. This article reviews the results of the study.
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| Sand filter with special sand media |
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| One-micron bag filter module |
The data presented here represent the treatment of sediment-contaminated stormwater at the Lakeside construction project from March 13, 2003, to March 9, 2004, with a total treatment volume of 17,295,000 gallons.
Site Description
The Lakeside Housing Project is an 80-acre private development located just east of NE 65th Street in southeast Redmond. Construction, which took place in three phases, included clearing, grading, installing utilities, and building single-family homes and town homes. During the test period the project progressed from approximately one-third completion to about two-thirds completion.
Stormwater from the site drains east to a natural wetland, which subsequently feeds Evans Creek. Because of the sensitive nature of Evans Creek, all treated stormwater was pumped to the existing SR 202 storm drain system, which discharged to Lake Sammamish.
Treatment System Description
All stormwater runoff from disturbed portions of the site was directed, via open channels and storm drains, to a dirty-water holding pond, which had a capacity of approximately 1 million gallons. Treated water from the chitosan-enhanced sand filtration system was directed to two treated-water holding tanks so that the water could be tested before discharge. Treated water that met the discharge requirements was pumped to the SR 202 storm drainage system, ultimately discharging to the north end of Lake Sammamish through a large wetland area.
There is also a permanent clean-water pond onsite with a capacity of approximately 2 million gallons. Clean offsite run-on stormwater (from a business next door, as well as runoff from clean surfaced areas of the site) is directed to this pond, which has an orifice-controlled discharge to the adjacent wetlands draining to Evans Creek. No treated water was discharged to the clean-water pond.
Water from the dirty-water pond was pumped, via two electric submersible pumps, to a Rain for Rent sand filter model 48-4 at a flow rate of approximately 500 gallons per minute and to the treated water holding tanks in a batch-treatment mode to start (Figure 1). Chitosan (Storm-Klear Liqui-Floc) was injected just upstream of the sand filter at a dose rate less than 1.0 milligrams per liter, depending on the turbidity of the water. Accurate dosing was achieved using a variable-output metering pump, and the dose rate of chitosan liquid was calibrated by timing the uptake of chitosan through the metering pump from a graduated cylinder and noting the flow rate of the sand filter.
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As a precaution against the discharge of turbid water in the event of a sand filter failure, a six-pod bag filter was installed on the output to the sand filter and equipped with 0.5-micron filter bags. All samples for effluent turbidity were collected after the sand filter and before the bag filter. During the project, the bag filters proved to be of little use, because the sand filter effluent was typically not dirty enough to blind the bags.
Performance Parameters
Over the one-year operating test period, a total volume of 17,295,000 gallons of stormwater was treated at an average flow rate of 500 gallons per minute. Field analyses were performed for turbidity, conductivity, temperature, and residual chitosan in both the influent and effluent stormwater. Laboratory tests were performed for phosphorus, toxicity to rainbow trout (acute 96-hour whole-effluent toxicity), and toxicity to Daphnia magna (acute 48-hour whole-effluent toxicity). An in situ flow-through rainbow trout test was also performed by a professional consultant. The frequency and number of samples taken are given in Table 1.
To meet the criteria of the quality assurance plan, there had to be a minimum of 480 influent and 480 effluent samples for turbidity and the same number for pH. This study represents nearly twice the number of samples required. These data were measured manually using a handheld portable Hach 2100P turbidimeter, a Hach handheld pH meter, a Hach handheld conductivity meter, and an environmental thermometer for temperature measurements. All instruments were calibrated daily when stormwater was treated.
Results
Turbidity. The treatment system reduced turbidity on average by 98.4% with a standard deviation of 1.29% and a coefficient of variation of 0.013. The average influent turbidity was 248 nephelometric turbidity units (NTU) with a high of 917 NTU and a low of 42.3 NTU (standard deviation = 145 NTU, coefficient of variation = 0.552). The average effluent turbidity was 2.98 NTU with a high of 5.18 NTU and a low of 0.58 NTU (standard deviation = 1.04 NTU, coefficient of variation = 0.348). Over the duration of the test, every effluent turbidity value was less than the turbidity limit set by the State of Washington (10 NTU).
Figure 2 shows graphically that effluent turbidity levels were extraordinarily consistent (0.58 to 5.18 NTU) over a broad range of influent turbidity levels (42.3 to 917 NTU).
Figure 3 shows the same data in a logarithmic scale to show the variations in the effluent turbidities. Recent work has shown that in-line static mixing devices deployed in advance of the sand filter improve the turbidity removal at influent turbidities less than 100 NTU.
The effluent turbidity levels remained consistent statistically from the beginning to the end of each operating day (Figure 4). This shows that system performance does not degrade throughout the operating day.
pH. The influent pH ranged from 6.0 to 8.4 with an average of 7.5 and a standard deviation of 0.31 with 794 samples. The effluent pH ranged from 6.2 to 7.9 with an average of 7.6 with 794 samples. The pH standard for discharge to fresh water is typically 6.5 to 8.0, and several low pH values were recorded below 6.5. This was transient, and there was no indication that the treatment process was responsible for creating the lower pH levels, because the influent as well as the effluent pH was low in these instances.
Residual Chitosan. Of the 253 samples of effluent that were field-tested for residual chitosan, all had less than 0.1 milligram per liter residual chitosan.
Rainbow Trout Toxicity. Nine samples of treated effluent were tested for aquatic toxicity using the EPA 96-hour acute whole-effluent rainbow trout test. In each test, all fish survived (100% survival). Rainbow trout have been determined to be the most sensitive species to chitosan.
Daphnia magna Toxicity. Ten samples of treated effluent were tested for aquatic toxicity using the EPA 48-hour acute whole-effluent Daphnia magna test. The average survival was 97.3%, ranging from 90% to 100% (no significant mortality).
Flow-Through Rainbow Trout Effluent Study. In addition to the above aquatic toxicity testing, a trout test was performed onsite in which treated effluent was continuously pumped into tanks containing juvenile rainbow trout. This was a whole-effluent toxicity study performed in the field with the fish subjected to treated water continuously rather than refreshed every 48 hours as in the standard EPA-821-R-02-012. The test duration was 14 days, and water was treated each day. After 14 days, all trout were alive and healthy.
Phosphorus. A total of eight influent and eight effluent samples were analyzed for total phosphorus. The average phosphorus reduction was 78% (standard deviation 8%), ranging from 64% to 90% reduction.
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Summary. This study demonstrates that the chitosan-enhanced sand filtration technology, developed by Natural Site Solutions, can reduce turbidity of construction-site stormwater by more than 95%. In addition, pH, conductivity, and temperature were not affected, and there was no sign of toxicity to rainbow trout or Daphnia magna .
One of the most important lessons learned during the testing was the importance of controlling periodic high-turbidity conditions of stormwater (typically during and directly after a major rain event). Chitosan-enhanced sand filtration can handle water with turbidity as high as 800 NTU, but the treatment efficiency is reduced significantly (backwash is continuous and the system is more at risk of malfunction). The most efficient operational turbidity level is less than 100 NTU, which points to the need for stormwater pretreatment. Experimentation has shown that adding chitosan to high-turbidity stormwater entering the site detention basin is capable of keeping the turbidity well below 100 NTU before sand filtration. Future projects will use pretreatment in conjunction with sand filtration to enhance the treatment efficiency.
September-October 2005
Cleaning the Water at Lakeside
In late 2001, a housing construction project called Lakeside in Redmond, WA, was experiencing problems meeting stormwater turbidity limits while discharging to Evans Creek, which is populated with endangered salmon. The project developer evaluated several innovative stormwater treatment technologies and selected a 500-gallon-per-minute chitosan-enhanced sand filtration system to purify turbid stormwater runoff from the site. After about a year of treatment under a temporary permit, with the system performing better than expected, it was decided to carefully monitor the performance of the system over a further one-year period to see how it performed over time. This article reviews the results of the study.
|
| Sand filter with special sand media |
|
| One-micron bag filter module |
The data presented here represent the treatment of sediment-contaminated stormwater at the Lakeside construction project from March 13, 2003, to March 9, 2004, with a total treatment volume of 17,295,000 gallons.
Site Description
The Lakeside Housing Project is an 80-acre private development located just east of NE 65th Street in southeast Redmond. Construction, which took place in three phases, included clearing, grading, installing utilities, and building single-family homes and town homes. During the test period the project progressed from approximately one-third completion to about two-thirds completion.
Stormwater from the site drains east to a natural wetland, which subsequently feeds Evans Creek. Because of the sensitive nature of Evans Creek, all treated stormwater was pumped to the existing SR 202 storm drain system, which discharged to Lake Sammamish.
Treatment System Description
All stormwater runoff from disturbed portions of the site was directed, via open channels and storm drains, to a dirty-water holding pond, which had a capacity of approximately 1 million gallons. Treated water from the chitosan-enhanced sand filtration system was directed to two treated-water holding tanks so that the water could be tested before discharge. Treated water that met the discharge requirements was pumped to the SR 202 storm drainage system, ultimately discharging to the north end of Lake Sammamish through a large wetland area.
There is also a permanent clean-water pond onsite with a capacity of approximately 2 million gallons. Clean offsite run-on stormwater (from a business next door, as well as runoff from clean surfaced areas of the site) is directed to this pond, which has an orifice-controlled discharge to the adjacent wetlands draining to Evans Creek. No treated water was discharged to the clean-water pond.
Water from the dirty-water pond was pumped, via two electric submersible pumps, to a Rain for Rent sand filter model 48-4 at a flow rate of approximately 500 gallons per minute and to the treated water holding tanks in a batch-treatment mode to start (Figure 1). Chitosan (Storm-Klear Liqui-Floc) was injected just upstream of the sand filter at a dose rate less than 1.0 milligrams per liter, depending on the turbidity of the water. Accurate dosing was achieved using a variable-output metering pump, and the dose rate of chitosan liquid was calibrated by timing the uptake of chitosan through the metering pump from a graduated cylinder and noting the flow rate of the sand filter.
|
|
As a precaution against the discharge of turbid water in the event of a sand filter failure, a six-pod bag filter was installed on the output to the sand filter and equipped with 0.5-micron filter bags. All samples for effluent turbidity were collected after the sand filter and before the bag filter. During the project, the bag filters proved to be of little use, because the sand filter effluent was typically not dirty enough to blind the bags.
Performance Parameters
Over the one-year operating test period, a total volume of 17,295,000 gallons of stormwater was treated at an average flow rate of 500 gallons per minute. Field analyses were performed for turbidity, conductivity, temperature, and residual chitosan in both the influent and effluent stormwater. Laboratory tests were performed for phosphorus, toxicity to rainbow trout (acute 96-hour whole-effluent toxicity), and toxicity to Daphnia magna (acute 48-hour whole-effluent toxicity). An in situ flow-through rainbow trout test was also performed by a professional consultant. The frequency and number of samples taken are given in Table 1.
To meet the criteria of the quality assurance plan, there had to be a minimum of 480 influent and 480 effluent samples for turbidity and the same number for pH. This study represents nearly twice the number of samples required. These data were measured manually using a handheld portable Hach 2100P turbidimeter, a Hach handheld pH meter, a Hach handheld conductivity meter, and an environmental thermometer for temperature measurements. All instruments were calibrated daily when stormwater was treated.
Results
Turbidity. The treatment system reduced turbidity on average by 98.4% with a standard deviation of 1.29% and a coefficient of variation of 0.013. The average influent turbidity was 248 nephelometric turbidity units (NTU) with a high of 917 NTU and a low of 42.3 NTU (standard deviation = 145 NTU, coefficient of variation = 0.552). The average effluent turbidity was 2.98 NTU with a high of 5.18 NTU and a low of 0.58 NTU (standard deviation = 1.04 NTU, coefficient of variation = 0.348). Over the duration of the test, every effluent turbidity value was less than the turbidity limit set by the State of Washington (10 NTU).
Figure 2 shows graphically that effluent turbidity levels were extraordinarily consistent (0.58 to 5.18 NTU) over a broad range of influent turbidity levels (42.3 to 917 NTU).
Figure 3 shows the same data in a logarithmic scale to show the variations in the effluent turbidities. Recent work has shown that in-line static mixing devices deployed in advance of the sand filter improve the turbidity removal at influent turbidities less than 100 NTU.
The effluent turbidity levels remained consistent statistically from the beginning to the end of each operating day (Figure 4). This shows that system performance does not degrade throughout the operating day.
pH. The influent pH ranged from 6.0 to 8.4 with an average of 7.5 and a standard deviation of 0.31 with 794 samples. The effluent pH ranged from 6.2 to 7.9 with an average of 7.6 with 794 samples. The pH standard for discharge to fresh water is typically 6.5 to 8.0, and several low pH values were recorded below 6.5. This was transient, and there was no indication that the treatment process was responsible for creating the lower pH levels, because the influent as well as the effluent pH was low in these instances.
Residual Chitosan. Of the 253 samples of effluent that were field-tested for residual chitosan, all had less than 0.1 milligram per liter residual chitosan.
Rainbow Trout Toxicity. Nine samples of treated effluent were tested for aquatic toxicity using the EPA 96-hour acute whole-effluent rainbow trout test. In each test, all fish survived (100% survival). Rainbow trout have been determined to be the most sensitive species to chitosan.
Daphnia magna Toxicity. Ten samples of treated effluent were tested for aquatic toxicity using the EPA 48-hour acute whole-effluent Daphnia magna test. The average survival was 97.3%, ranging from 90% to 100% (no significant mortality).
Flow-Through Rainbow Trout Effluent Study. In addition to the above aquatic toxicity testing, a trout test was performed onsite in which treated effluent was continuously pumped into tanks containing juvenile rainbow trout. This was a whole-effluent toxicity study performed in the field with the fish subjected to treated water continuously rather than refreshed every 48 hours as in the standard EPA-821-R-02-012. The test duration was 14 days, and water was treated each day. After 14 days, all trout were alive and healthy.
Phosphorus. A total of eight influent and eight effluent samples were analyzed for total phosphorus. The average phosphorus reduction was 78% (standard deviation 8%), ranging from 64% to 90% reduction.
Summary. This study demonstrates that the chitosan-enhanced sand filtration technology, developed by Natural Site Solutions, can reduce turbidity of construction-site stormwater by more than 95%. In addition, pH, conductivity, and temperature were not affected, and there was no sign of toxicity to rainbow trout or Daphnia magna .
One of the most important lessons learned during the testing was the importance of controlling periodic high-turbidity conditions of stormwater (typically during and directly after a major rain event). Chitosan-enhanced sand filtration can handle water with turbidity as high as 800 NTU, but the treatment efficiency is reduced significantly (backwash is continuous and the system is more at risk of malfunction). The most efficient operational turbidity level is less than 100 NTU, which points to the need for stormwater pretreatment. Experimentation has shown that adding chitosan to high-turbidity stormwater entering the site detention basin is capable of keeping the turbidity well below 100 NTU before sand filtration. Future projects will use pretreatment in conjunction with sand filtration to enhance the treatment efficiency.