Environmental Engineering Reference
In-Depth Information
Flood
Drain
Flood
N
2
+
NH
4
O
2
O
2
-
NO
3
-
+
NO
3
NH
4
-
O
2
NO
3
O
2
FIGURE 19.11
Nitrification and denitrification schematic for a tidal flow wetland. In the
flooded phase, ammonium cations (NH
4
+
) adsorb to negatively charged surfaces of aggregate
and organic material. In the drained phase, air enters pore spaces and oxygen saturates the
thin films of water remaining in the wetland bed. Bacteria rapidly nitrify adsorbed ammo-
nium anions to nitrate (NO
3
-
). In the next flooded phase, negatively charged nitrate ions des-
orb into bulk water, where bacteria convert them to atmospheric nitrogen (N
2
).
is a 6,000 m
3
/d system in operation since 2003 for groundwater remediation (Wallace
and Kadlec 2005).
Flood and drain wetlands, known as tidal flow systems, also use limited power
to improve treatment. Oxygen transfer to the wetland in flood and drain cycles takes
advantage of the mechanical efficiency of pumps and cation exchange chemistry to
remove nitrogen (Figure 19.11).
Several types of wetland systems use some kind of mechanical power inputs to
boost treatment performance. All of them are semipassive technologies, using limited
power inputs to improve treatment. Introduction of oxygen into the wetland system
boosts already effective, passive, anaerobic treatment mechanisms to a more energetic
state. Anaerobic treatment still occurs, but is combined with aerobic treatment.
For domestic wastewater, the advantage of this thermodynamic boost is
manifested in the removal of excess nitrogen. Urine quickly breaks down into
ammonia, the principle form of nitrogen in wastewater. Ammonia is stable under
anaerobic conditions. Passive subsurface-flow wetlands are incapable of nitri-
fication (bacterial conversion of ammonia to nitrate) because they are purely
anaerobic. Nitrification requires oxygen. If nitrification occurs, bacteria can then
convert nitrate into nitrogen gas under subsequent anaerobic conditions (denitrifi-
cation). Removal of nitrogen from wastewater can be desirable for reuse applica-
tions. Although nitrogen is a vital plant nutrient, excess nitrogen can harm plants.
Moreover, nitrate contamination of groundwater is not desirable. Aeration of the
wetland provides a means to nitrify wastewater ammonia which, when combined
with subsequent denitrification, is the crucial step for nitrogen removal in waste-
water destined for reuse.
Conventional wastewater treatment consumes a lot of energy. Purely passive treat-
ment systems consume no process energy. The semipassive nature of certain advanced
ecotechnologies occupies a middle ground of energy use (Table 19.2). Semipassive
systems can be designed to treat wastewater when power is turned off, albeit to a
lesser standard, provided that sewage still flows. Most conventional technologies,
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