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5.7 AERATED WETLANDS
Scott Wallace1, Dion van Oirschot2and Alexandros Stefanakis3
1
Naturally Wallace Consulting, P.O. Box 37, 126 2nd Street S, Stillwater, Minnesota 55082, USA
2
Rietland bvba, Van Aertselaerstraat 70, Minderhout 2322, Belgium
3
Bauer Nimr LLC, PO Box 1186, PC114 Al Mina, Muscat, Oman
5.7.1 Introduction
Aerated wetlands are saturated, HF or VF wetlands that rely on a mechanical system (an air pump connected
to a subsurface network of air distribution pipes) to introduce air bubbles into the water being treated
(Wallace, 2001). The use of an artificial aeration system dramatically increases the oxygen transfer rate
compared to passive wetlands (Table 5.9), enabling improved performance for treatment reactions that
require oxygen (such as nitrification) or occur more rapidly under aerobic conditions. The aeration
system can also be operated intermittently to promote nitrification/denitrification (van Oirschot &
Wallace 2014). A simple schematic and description of the process was covered recently by Dotro et al.
(2017) and the improved treatment performance through aeration of pilot scale systems fitted to the
first-order kinetic (P–k–C*) model by Nivala et al. (2019b).
5.7.2 Design considerations
Standard HF and VF wetland systems rely on passive diffusion of oxygen into the water column. This is a
very slow process in saturated-flow wetlands (HF and FWS) and passively improved upon in
unsaturated-flow wetlands (VF and French VF wetlands). Mechanically aerating the system allows the
amount of air introduced to be independent of the surface area of the wetland, allowing aerated systems
to be loaded up to maximum clogging limits, which greatly reduces the area required and associated
Table 5.9 Estimated oxygen consumption in g O
2
/m
2
/d for different TW types (adapted from Wallace, 2014)
TW Type Estimated
O
2
Consumption
Notes
HF wetland
1
6.3 50th percentile values from Kadlec and Wallace (2009)
assuming aerobic BOD removal and conventional
nitrification.
FWS wetland
1
1.47
VF wetland (unsaturated)
1
24.7
French VF wetland
(1st stage)
2
40–60 Data from France indicates that the first stage of a
French VF wetland can sustainable operate at roughly
1.5 m
2
/PE
Aerated (HF and VF) 250 Mechanically aerated wetlands can achieve higher
oxygen transfer rates, but 250 g/m
2
-d is considered an
upper CBOD
5
limit for clogging; most sustainable
designs operate at ,100 g/m
2
-d (Wallace, 2014).
Notes:
1
50th percentile values from Treatment Wetlands, Second Edition (Kadlec & Wallace, 2009); assuming aerobic BOD
removal and conventional nitrification.
2
Data from France indicates that the first stage “French VF”process can sustainably operate at roughly 1.5 m
2
/PE
(Molle et al., 2005).
Practical information on design of specific wetland types and typical pitfalls 105
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capital cost. Aerated wetlands are generally dimensioned based on clogging, hydraulics, uniform air
distribution, and first-order kinetics (Table 5.10).
Aeration of wetlands follows standard wastewater aeration design practices in terms of calculating
oxygen demands and air flows based on actual/standard oxygen transfer rates (AOTR/SOTR) protocols
(Metcalf and Eddy Inc., 2003). However, the hydrodynamic mixing of the water column induced by
aeration is greatly reduced in gravel-bed systems compared to ponds or tanks (Wallace, 2014). This
requires that the air distribution in wetland beds be very uniform (Wallace, 2014). Most air diffusers in
mechanical treatment systems are high-flow/small-area devices that are poorly suited to uniform
distribution, and successful wetland aeration designs have been based on alternative pipes or tubing that
can distribute air uniformly. This generally requires empirical testing to determine the air flow vs. air
pressure relationship for the product(s) under consideration.
Gravel media used in the system must have pore spaces large enough to allow the passage of air bubbles.
Sand is too fine for aerated systems as the air collects and “blows out”in just a few locations. Air bubbles
moving through the gravel media can combine and coalesce into larger bubbles (reducing oxygen transfer),
however air bubbles follow a tortuous path through the media, slowing their transit time (increasing oxygen
transfer). As a result, wetland aeration systems typically demonstrate an oxygen transfer efficiency
intermediate between fine-bubble and coarse-bubble diffusers (von Sperling & Chernicharo, 2005;
Wallace et al., 2007).
5.7.3 Potential design and operational issues
Since aerated wetlands are high-rate treatment processes, they are sometimes designed very close to
clogging limits, especially for HF; if overloaded, they can clog and require resting or refurbishment like
other types of treatment wetlands.
During construction, testing of the aeration system to verify proper air delivery is essential. Since the air
distribution lines are buried at the bottom of the wetland bed, replacing/repairing air lines after construction
is difficult.
Table 5.10 Typical design parameters for aerated wetlands.
Design Parameter Recommendation References
Pre-treatment Primary treatment common (CSO systems
typically do not have pre-treatment)
DWA-A262E (2017)
Influent loading (inlet
cross-sectional area)
,250 g CBOD
5
/m
2
/d (maximum)*
≤100 g CBOD
5
/m
2
/d (recommended)
Wallace (2014)
Specific area ≥0.5 m
2
/PE
≤80 g/m
2
/d CBOD
5
Stefanakis and Prigent
(2018)
Influent distribution ≤50 m
2
per feed point (unless bed is
permanently flooded)
Dotro et al. (2017)
Air flow rate ≥0.6 m
3
/m
2
/h DWA-A262E (2017)
Air distribution 30 cm ×30 cm DWA-A262E (2017)
Media size 8–16 mm DWA-A262E (2017)
Treatment kinetics pilot testing Nivala et al. (2019b)
*Mechanically aerated wetlands can achieve higher oxygen transfer rates, but 250 g/m
2
-d is considered an upper CBOD
5
limit for clogging (Wallace and Knight, 2006); most sustainable designs operate at ,100 g/m
2
-d (Wallace, 2014).
Wetland Technology106
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Fouling of the air distribution lines has been reported in isolated cases due to iron precipitates forming at
the air distribution orifices. Using acid (HCl) to clean fouled air lines has been reported to be a successful
quick and low-cost method (van Oirschot & Wallace, 2014).
Although the selection of the appropriate blower for the air distribution network should be based on air
requirements, they can sometimes be limited by the smallest available size that a client can accept (based on
rigorous health and safety requirements). To illustrate, four systems in the UK used the same size of blower
to provide aeration to different size tertiary and secondary systems, resulting in a specific power allocation
ranging from 4 W/m
3
of wetland to 26 W/m
3
wetland (Butterworth et al., 2016a). In systems that are over
aerated, venting of the air has been necessary resulting in wasted energy and noise complaints from adjacent
residents. To minimise this, the selection of the correct aeration equipment should be emphasized to
the client.
Stress of plants in both passive and artificially aerated wetlands has been reported in the literature, with
chlorosis (yellowing of the leaves) being most predominant (Weedon, 2014) and a downward gradient
observed in plant height from inlet to outlet in highly aerobic systems. In an assessment of four full-scale
systems, one of the systems struggled to establish the common reed (Phragmites australis) whilst its
twin bed under equal conditions but without aeration thrived with the same plants (Butterworth et al.,
2016a). The other three artificially aerated systems reported normal plant growth. The difficulty
experienced with plant establishment in some UK systems did not affect treatment performance. A
side-by-side full-scale trial comparing reeds (P. australis) to reedmace (Typha latifolia) plantings
showed both plant species exhibited signs of stress (chlorosis and stunted growth) when grown with
artificial aeration. Further controlled trials proved reedmace is proportionally more affected by aeration
than the common reed but its higher natural growth rate can offset the true impact of aeration on biomass
production (Butterworth et al., 2016b). Plant stress has been attributed to iron deficiency and/or toxicity
in aerobic systems. The fact it happens on some systems but not all suggests complex interactions
between the biogeochemical conditions in the wetland subsurface and the plants. To illustrate, from 27
aerated wetlands built with expanded clay aggregates as their main media (instead of gravel), there have
been no reports of plant stress to date. Recent research suggests observed iron-induced stress in reeds
could be related to the plant’s genetic code, with an iron foliar spray currently being assessed as
mitigation strategy (Ren et al., 2018). In practice, plant species selection for artificially aerated wetlands
is typically done by the designer based on previous experience, and a variety of native wetland plants
have been used to date.
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