1. 8th
International Perspective on Water Resources and the Environment
American Society of Civil Engineers /Environmental and Water Resources Institute
January 4-6, 2016, Colombo, Sri Lanka
IPWE2016/048
EFFECT OF HYDRAULIC RETENTION TIME ON ROOT OXYGEN
RELEASE AND POLLUTANT REMOVAL IN SUBSURFACE FLOW
CONSTRUCTED WETLANDS
Chamodh Wathugedara1
, Anupa Pathirana1
, Priyantha Bandara2
, Prabha Weerakoon1
1
Department of Civil Engineering, University of Peradeniya, 2
Central Environmental Authority
Abstract:
The amount of oxygen availability at the substrate media of a wetland system is a major factor
involved in efficient removal of several pollutants from wastewater. Oxygen can enter to the
substrate media by direct atmospheric diffusion or by releasing oxygen through root system of
wetland plants which is produced in leaves as a result of photosynthetic activity. However, root
zone oxygen release is predominant in subsurface flow wetland systems.
A laboratory scale experiment was conducted to quantify the root oxygen release by Narrow-leaf
Cattail (Typha Angustifolia), and to investigate the effects of hydraulic retention time (HRT) on
root oxygen release and pollutant removal at different stages of a wetland plant life. The
experimental setup consisted with 24 wetland microcosms of size 20 cm x 30 cm (diameter x
height) and operated as batch flow subsurface flow constructed wetland systems. The wetland
microcosms were prepared using 10 – 20 cm gravel media. Synthetic wastewater was supplied
to 12 microcosms at 7 days HRT and for other 12 microcosms at 3.5 days HRT. Wastewater
samples were collected from the influent and effluents of the microcosms at two weeks interval
and tested for five day biochemical oxygen demand (BOD5), pH, dissolved oxygen (DO),
ammonium (NH4
+
), nitrates (NO3
-
) and total nitrogen (TN). Root oxygen release (ROL), root
morphology and biomass production of wetland plants were estimated at four weeks interval in
two replicates of each 7 days and 3.5 days HRT microcosms. Average ROL values for 7 days
and 3.5 days HRT systems were obtained as 36.9 and 19.5 µmol/gDW/hr respectively. Average
removal efficiencies of BOD5, NH4
+
and TN for 7 days and 3.5 days HRT systems were not
considerably different in the two systems. However, the average NO3
-
removal efficiency in 7
days and 3.5 days HRT systems were 60.5% and 86% respectively. Results show that subsurface
flow constructed wetland systems with 3.5 days HRT are more efficient in NO3-
removal
compared to 7 days HRT.