Effectiveness of Rain Garden Facilities in Nitrogen and Phosphorus Removal through Bioretention Columns

Bioretention or rain gardens is widely used as stormwater best management practice (BMP)
and have been long implemented as a part of low impact development (LID) because of its
ability to remove nutrients from stormwater runoff. A major concern in water quality problem
is eutrophication which is caused by the nutrients, namely nitrogen and phosphorus. The
objective of this study is to examine and evaluate the efficiency of rain garden in nitrogen and
phosphorus removal by varying the types of the mulch layers in a bioretention column and
further select the best to be used in rain garden design. Three (3) bioretention columns with
size of 46mm in diameter and a height of 400mm were is used for this study where the inflow
and outflow runoff will be collected and analyzed to measure the nutrient concentration.
Filter media at the depth of 200mm consisted of river sand with soil mix of 80% fine sand
and 20% coarse sand were used. Three different types of mulch layer wood chip, tea waste
and coconut fibres were applied at the top of filter media at a height of 50mm. Phosphorus
concentration in the bioretention column was reduced by 73.9% using woodchip, 23.1%
using tea leaves and 50% using coconut fibres. Lower removal efficiency was seen for
nitrogen where 24.4% using woodchip, 0% for tea leaves and 4.4% using coconut fibre.
Woodchip was seen to be favourable compared to the other two mulch layer due to its
removal efficiency in removing both phosphorus and nitrogen from the incoming stormwater
runoff. The absorption capacity was seen as the main factor that affects the removal rate.
Further research can be conducted by adding a vegetative layer inside the bioretention
column or by changing the filter media depth and configuration to further enchance the
removal rate of pollutant from the stormwater runoff.