2.1. Experimental set up and operating conditions of CW
To avoid odor problems, CW units were operated in
subsurface-flow mode in which all the influent wastewater
was made to flow through the CW beds and no wastewater
flowing above the CW beds. This type of CW is called subsurface-
flow CW or SFCW.
Two pilot-scale SFCW units, made of reinforce concrete,
were built at the Asian Institute of Technology, Thailand,
each with a dimension of 0.5 · 4.0 · 0.5 m (width ·
length · depth), and a bed slop of 1%. The support media
of these units consisted of large gravel (2–3 cm in diameter),