A major concern in the water quality problem is eutrophication caused by nutrients, namely nitrogen and phosphorus. The objective of this study is to examine and evaluate the efficiency of rain garden in removing nitrogen and phosphorus by varying the types of mulch layers in the bioretention column and further to select the best one for use in rain garden design. Three different types of mulch layers of wood chips, tea waste and coconut fibers were applied on top of the filter medium at a height of 50 mm.
The phosphorus concentration in the bioretention column was reduced by 73.9% using wood chips, 23.1% using tea leaves and 50% using coconut fibres. Lower removal efficiencies were seen for nitrogen with 24.4% using wood chips, 0% for tea leaves and 4.4% using coir. Wood chips were considered to compare favorably with the other two mulch layers due to its removal efficiency in removing both phosphorus and nitrogen from the incoming stormwater runoff.
Further research can be done by adding a vegetative layer inside the bioretention column or by changing the depth and configuration of the filter media to further increase the pollutant removal rate from the stormwater runoff. In addition, I would like to thank my fellow friends, especially Aishah, Shahraziman, Izatul, Amynurashid, and Yu Yang Long, who were helpful and cooperative whenever I needed ideas, opinions, and help throughout the process of completing this project.
INTRODUCTION
- Project Background
- Problem Statement
- Objectives
- Scope of Study
- Significance of Study
Nitrogen and phosphorus, the primary nutrients involved in eutrophication, enter water bodies through different pathways (Davis et al., 2006). The presence of nitrogen in stormwater runoff can come from fertilizers, atmospheric deposits, and nutrient cycling, while the presence of phosphorus can also come from fertilizers, atmospheric deposits, and other sources such as soil erosion, animal waste, and detergents. Bioretention has been shown to be promisingly effective in removing phosphorus and organic nitrogen from infiltrated runoff (Davis et al., 2006).
Another concern regarding the effectiveness of bioretention gardens according to Hatt et al. 2009) was the lack of field-scale performance data when conducting the system performance investigation. According to Hsieh and Davis (2005), media configuration in bioretention media can also affect bioretention performance as it depends on the rate of runoff infiltration through the media. To examine and evaluate the effectiveness of the mulch layer in removing nutrient pollutants based on the current bioretention model in a laboratory setting ii.
Results obtained from this study will prove that bioretention media in rain gardens are capable of removing nutrients from the stormwater runoff. The present study conducted is to provide nutrient removal of nitrogen and phosphorus information for bioretention under laboratory environment.
LITERATURE REVIEW
Introduction
Low Impact Development (LID) Stormwater Management 4
- Bioretention Garden Design
- Pollutant Removal
- Mulch Layer
In a typical configuration, bioretention includes a layer of approximately 75 to 100 cm of an engineered soil/sand/organic media, supporting a mixed vegetative layer (Hsieh et al., 2007b). The soil usually has a high sand content to ensure rapid infiltration, but low levels of silt and clay to promote attenuation of contaminants during infiltration (Davis et al., 2001). A 5- to 8-cm layer of shredded wood mulch is added to the surface to conserve soil moisture and filter incoming sediment (Davis et al., 2006).
Several investigations have been conducted to measure the pollutant removal capacity of a bioretention cell in a laboratory setting and at the field scale. 2010) was able to demonstrate the ability of bioretention filter media in removing nutrient loads from stormwater runoff in a laboratory setting. From a study conducted by Hsieh et al. 2007a) for phosphorus removal, high hydraulic conductivity over low hydraulic conductivity column media was more efficient in total phosphorus removal ranging from 67% to >98% compared to low hydraulic conductivity over conductivity column media high hydraulic as the less permeable soil layer at the bottom of the column increases the retention time between dissolved phosphorus and the environment. The mulch layer is one of the main means of filtration in bioretention as it has its own advantages that lead to its evaluation in this project.
Pine bark, tree ferns, rice husks, and wood fibers are a number of natural materials mentioned by Ray et al. 2006) used to remove dissolved pollutants from aqueous media as they are inexpensive compared to activated carbon and synthetic resin. From the research conducted by Ray et al. 2006), conventional hardwood mulch can be used to remove water-soluble pollutants such as heavy metals and toxic organic compounds commonly found in stormwater runoff.
METHODOLOGY
Research Methodology
General Experimental Procedure and Materials
Pump Flow Rate
Collecting Stormwater Sample
This test was carried out to determine the distribution of the coarse and fine sand in the soil mixture. Figure 3.6 shows the cross-section of the bioretention column design used for the laboratory study. The bioretention column consists of the mulch layer, the soil mix layer and the crushed gravel layer.
Before the mulch layer was inserted into the bioretention column, all materials were soaked in distilled water and air-dried in an oven for 12 h to remove any contaminants present. The complete bioretention column setup for each mulch layer can be found in APPENDIX C. According to the Facility of Advancing Water Biofiltration (2009) guidelines for biofiltration media, the particle size distribution of the soil mixture should be between 0.075 mm and 4.75 mm. mm sieve to ensure that the filter medium is well sorted.
Although the sand particle size distribution is not the main focus of the project, it is nevertheless important in terms of the hydraulic performance of the bioretention column. Therefore, sieve analysis was performed to determine the particle size distribution of the selected coarse and fine sand. A key factor is that the surface area of the wood chip is greater compared to tea leaves and coconut fibers, which enables it to better absorb storm runoff.
To support the justification made above regarding the correlation between absorption and retention capacity of the filtration media and the removal efficiency, the hydraulic performance for each column with the different mulch layer was tested. According to Table 9, the time required for water to settle in the bioretention column with wood chips when the mulch layer is longer compared to the tea leaves and coconut fibers. This shows that the large surface area of the wood chips absorbs the water from the incoming runoff and when all the pores are filled, the water flows to the underlying soil layer, which takes some time before the bioretention column is filled with water and reaches the ponds. a depth of 50 mm.
Therefore, the absorption and retention capacity affect the pollutant removal efficiency in the bioretention column, which supports the rationale. Compared to previous studies that have been conducted, the size of the bioretention used for this project was small. Therefore, the absence of a vegetative layer may have affected the efficiency, especially the ability to remove nitrogen.
During the period in which the experiment was conducted, there were not many rainy days in the university area. Of the three organic mulches, wood chips have the highest nitrogen and phosphorus removal efficiency with an average removal efficiency of 24.4% for nitrogen removal and 73.2% for phosphorus removal. In addition, the type of soil used in the mixture should be varied not only between coarse and fine sand, but also loam, silt and clay, according to the composition established in Guidelines for Soil Filter Media Bioretention Systems by Facility of Advancing Water Biofiltration. (2008), to improve the permeability of the bioretention column.
Since the nitrogen removal rate was very low, further research work can be continued by applying a vegetation layer in the bioretention column.
