DEVELOPMENT OF COMBINED FILTRATION AND BIORETENTION SYSTEM FOR THE TREATMENT OF URBAN STORMWATER RUNOFF

Abstract

Almost 97% of the world’s available water is present in oceans but high concentration of salts renders the oceanic water unusable for domestic, agricultural, and industrial activities. Desalination techniques have been incorporated but it is a cost and energy intensive process to produce huge volume of freshwater, although, few regions on earth rely on this technique. On the other hand, freshwater resources are scarce out of which most of the freshwater is stored as glacier, and a very few percent which accounts to be 0.0072 % of world’s total water budget is present in freshwater lakes and streams. As an alternative, the rainwater can be intercepted and utilised for non-potable application by providing minimal treatment since the rainwater is considered to be the purest form of water until it comes in contact with contaminants. These contaminants are the atmospheric impurities originating from natural and anthropogenic activities. Once the rainwater reaches earth’s surface, it becomes stormwater which first gets infiltered into the ground surface and few portion of it becomes surface stormwater runoff. This surface stormwater runoff when comes in contact with the surface of earth and traverses its path, takes along floating, suspended, and dissolved impurities with it. Thus, the source characterisation of the rainwater and stormwater runoff was performed through various locations which include commercial, industrial, institutional, residential, and road/ highway. For this aim, the rainwater and stormwater samples were collected during year 2021 and 2022. The physico-chemical analysis of rainwater reveals that most of the chemical species are present in trace amount suggesting that chemical impurities are the result of atmospheric washing during the down pour. Also, no event of acid rain (pH < 5.6) was witnessed in Delhi. The physico-chemical analysis of stormwater runoff in Delhi for year 2021 and 2022 suggests wide variation among the different chemical and physical parameters from different land uses. Also, it revealed that total suspended solids are the major contributor in surface stormwater runoff which originated from road side sweeping, and surface runoff coming from construction sites. Nutrients (PO4 3- and NO3 - ) are the second major contaminant found in the stormwater runoff samples which were found in higher concentration in the samples from residential areas. The heavy metals such as cobalt, chromium and lead were below the detection limit, but cadmium was v found in trace amount at a few locations along to road/highway. The contaminant in stormwater runoff were identified followed by suitable treatment to make it usable. Initially, the filter media was identified on the basis of its availability and ability to remove the suspended as well as dissolved impurities. The pollutants in stormwater runoff were treated in batch process to remove dissolved nutrients using various filter media. The study indicated that phosphate removal by different materials follows the order iron filing > calcite > limestone > brick > hematite > biochar. Biochar being one of the filter media, was prepared in the lab using used and refused bamboo at 500 °C through pyrolysis. Biochar was not able to remove the nutrient in batch process suggested repulsion between electro-negatively charged nutrient and surface charge of biochar. Thus, biochar was treated with FeCl3 to make its surface electro-positive to remove negatively charge nutrient (PO4 3- and NO3 - ) from synthetically prepared and real stormwater runoff. The use of biochar in batch process at a dose of 5 g/L can remove upto 61 % of the phosphates at the initial PO4 3- concentration of 5 mg/L when biochar treated with 10 % FeCl3 solution was used. On the other hand, when biochar treated with 5 % FeCl3 solution was used at a dose of 5 g/L, and the initial phosphate concentration was varied, suggested that removal efficiency increased when initial concentration of the phosphate was used to 2 mg/L. This underlines that removal of pollutant from is a function of pollutant initial concentration of contaminant and the dose of biochar. The adsorption capacity and performance of biochar are function of contact time. Followed by batch process, continuous flow filter was prepared using the identified filter media. The continuous flow study indicated that upto 97 % total suspended solids and phosphates upto 93 % can be removed from real stormwater runoff using filter media consisting of iron filing, calcite, limestone, brick, and hematite. Followed by identification of filter media for stormwater runoff treatment, bioretention system was designed consisting of different wetland macrophytes. In bioretention system, the available and total phosphate removal study was carried out which consisted of gap graded bed substrate for easy penetration of roots and easy movement of water. The available and total phosphate removal efficiency followed the order summer > spring > winter > autumn, for Phragmites, while it followed the order summer > spring > winter for available and total phosphate vi removal using Cyperus alternifolius, and Canna lily-based constructed wetland cell. Total phosphate removal efficiency using Phragmites based constructed wetland cell was of order 56.7 %, 80.6 %, 90.3 %, and 95.5 % for autumn, winter, spring, and summer season, respectively. Similarly, total phosphate removal efficiency of Canna lily based constructed wetland cell was of order 77.7 %, 82.5 %, and 88.7 %, respectively for winter, spring, and summer season. Cyperus alternifolius based constructed wetland cell indicated total phosphate removal efficiency of order 62.5 %, 74.0 %, and 81.7 %, during winter, spring, and summer season, respectively. Thus, the maximum total phosphate removal follows the order Phragmites > Canna lily > Cyperus alternifolius based constructed wetland during the respective season. In addition, similar trend was also shown by Phragmites > Canna lily > Cyperus alternifolius based constructed wetland during the respective season for available phosphate removal efficiency. The analysis of ferric to ferrous ratio in bioretention wetland cells was less than unity suggested that ferrous ions (Fe2+) was present in the cell in major concentration while ferric ions (Fe3+) were converted to ferrous by reduction. This rules out the removal and binding of phosphates with bed sediments and suggested that phosphate was removed by macrophytes. Lastly, a hybrid filter system was developed which consisted of filter media and the macrophytes in different cells. The selected filter media and the macrophytes are those which were suggested by previous experiments of same study. The hybrid filter was provided with vertical baffle walls such that polluted stormwater runoff flows above and below it while travelling from one chamber to another. The hybrid filter system reported almost 96 % removal of total suspended solids from stormwater runoff during continuous flow. The same filter was able to remove 55 % and 41 % of phosphates and nitrates, respectively, from synthetically prepared stormwater runoff when operated continuously. Thus, this is a promising design that can reduce the suspended solid load and dissolved nutrient load from stormwater runoff. The treated stormwater runoff from hybrid filter can be utilised for non-potable use which can reduce the dependence on freshwater and helps in bridging the demand and supply gap.