Storm Water Treatment
A city of Western Canada has a series of storm water retention ponds used to improve storm water quality, preserve the natural hydrology and mitigate the impacts of urban development. These ponds also function as the centerpieces to the development of the area providing aesthetic qualities to the town as well as irrigation services.
Biological Treatment of Storm Water Retention Ponds (1 page version for print – PDF)
Due to the physiochemical properties of storm water, it is common to have water quality issues when this water is put into a basin with a large retention time such as in this case. These water quality issues include:
- These organisms can produce potentially harmful neurotoxins causing harm to animals when consumed.
- Aesthetically unpleasing
- Due to high amounts of nutrient runoff
- If the oxygen needed to degrade organic matter is not available then there will be a build-up of sediments of the retention ponds reducing its efficacy over time.
- Low dissolved oxygen makes ponds unsuitable for aquatic life such as fish.
- Low dissolved oxygen can cause hydrogen sulfide production by bacteria.
- Suspended Solids Build-up:
- Erosion, especially from surrounding construction, may reduce the clarity of the water and increase oxygen demand.
- Causes sediment build-up reducing the efficacy of the retention ponds over time
- Aesthetically unpleasing
Seven storm water retention ponds were treated using Bacterius C and Bacterius N. Each retention pond received four doses of the bacteria from August 11 to September 24. Bacteria dosage depended on pond size and volume. Water samples were collected before and after treatment August 7 and October 7 and tested using an independent lab.
The city targeted eight different criteria to measure the effectiveness of the storm water treatment. The primary goal of the project was to reduce the algae cover to under 5% of the pond surface. Also, included in the criteria is pH, Total Phosphorous (TP), Total Nitrogen (TN), Chlorophyll a (Chl-a), Total suspended solids (TSS), Dissolved Oxygen (DO) and Turbidity. In order to determine success, the reduction of the above-mentioned parameters was calculated from the inlet of the ponds to the outlet of the ponds. This was done before the treatment and after. Effective treatment of the water includes the minimization of TP, TN, Chl-a, TSS and Turbidity while maximizing DO and maintaining a pH near to 7.
Results and Discussion
Figure 1. Pre and Post Treatment changes in water parameters from inlet to outlet. Values are the average of all treated storm retention ponds of this case study.
The results of this treatment are summarized in figure 1 and 2. The results for each individual parameter are explained below.
Algae cover of less than 5% was observed in six of the seven ponds after the one-and-a-half-month treatment. Pond #7 which did not meet the quality requirement is much smaller than the others and had much more algae at the start of the treatment. This pond, however, has shown improvement in terms of chlorophyll-a as will be explained below.
pH measures the balance of acids and bases in the water. It is important for many biological processes that the pH of fresh water remains slightly above 7. There was an overall acidification that occurred in all ponds from inlet to outlet except for Pond #7. These values, Pond #7 not included, averaged 7.65 after treatment compared to 9.42 before. Pond #7 showed an increase from 7.25 to 11.26. pH is highly variable so it is possible that this higher value was an artifact from the sampling. The average reduction in pH was 0.56% after treatment whereas there was an increase of 1.46% before treatment.
Total phosphorus is an important limiting nutrient in freshwater. Lower nutrient concentrations give lower the potential for algae growth. There was an overall increase in TP values after the treatment except in Pond #1 which experienced a massive decrease of over 300% over the treatment time. It is unlikely that this was caused by the bacteria treatment so there must have been an external factor affecting the result. One potential cause of the drop is that the bacteria successfully competed with the algae for nutrients which limited aquatic respiration and therefore increased dissolved oxygen on the pond floor preventing internal phosphorus loading. The percent change from inlet to outlet went from 1.55% reduction of TP before treatment to an increase of 10.68% after treatment.
Figure 2 – Percent removal efficiency of the treatment compared to pretreatment. These values represent the negative change between treatments where positive values indicate greater removal and negative values indicate release.
Total nitrogen is also an important limiting factor in freshwater lakes. In all cases, there was a decrease in TN from inlet to outlet for both before and after the treatment. This means that this system is naturally denitrifying the water. There was a reduction in TN of 29.12% before treatment and 32.44% afterwards. This was fairly consistent between all ponds. The denitrification signal given by the treatment of bacteria may have been muffled by the colder October water temperature since temperature is a limiting factor of denitrification rates.
Chlorophyll-a is a good indicator of algae growth and turbidity with higher values increasing both of these parameters. From inlet to outlet there was an increase in chlorophyll-a of 39.13% before treatment and only 11.86% after treatment. This means that there is a net reduction of 27.27% due to the bacteria treatment. This was especially true of Pond #7 which did not pass the 5% algae cover test set by the city. This pond showed a 155.02% increase in Chl-a from inlet to outlet before the treatment but showed a decrease of 42.12% after the treatment. This means that the bacteria treatment is extremely effective in this pond even though it is not visible to the naked eye yet.
Total Suspended Solids
Total Suspended Solids is an indicator of turbidity and erosion. From inlet to outlet there was an increase of TSS by 10.09% before treatment and a reduction of TSS by 12.96% after the treatment. Pond #2 was taken out of this calculation because it saw an increase of TSS by 1366.67%. There is nothing to suggest that this could be caused by the bacteria treatment so it is assumed that a large amount of erosion occurred after treatment began; probably due to construction.
Dissolved Oxygen is a driver of water redox potential, that is, it drives the balance of soluble and insoluble compounds within the water and between the water and soil. In the case of low DO production of hydrogen cyanide may occur and buried phosphate may leach from the soil to the water. In all cases the measured DO was well above the threshold of 1.5 mg/L. The DO of the entire water column was not taken therefore it is impossible to say whether or not the DO at the bottom of the water column actually decreases passed 1.5 mg/L. Results showed a decrease in DO from before and after treatment. The change in DO went from positive 12.46% to a decrease in 6.75%. This could be due to an increase in oxygen use by the added bacteria as well as a decrease in photosynthesis efficiency.
Turbidity is another aesthetic parameter used to determine water clarity. Higher turbidity is related to lower water clarity. Turbidity is directly related to chlorophyll-a and TSS as these parameters obscure sunlight. Turbidity raised 12.46% from inlet to outlet before treatment and decreased by 6.75% afterwards.
Recommendations for the Future
In order to maximize storm water retention ponds treatment efficiency and ensure that the designated targets are met there are a series of additional methods that may be used. One method that could increase nitrification, bacterial production and to increase dissolved oxygen is to install bottom aeration systems into the retention ponds. The net effect of this treatment would be to increase turbidity and total suspended solids. The issue of high turbidity and TSS may be fixed through the application of flocculating material such as calcium or buffered alum application. The addition of these products would clarify the water and improve the aesthetic quality of the water.