NATURAL SYSTEMS FOR WASTEWATER TREATMENT 743
Hydraulic loading rate for FWS system is closely tied
to the hydrologic factors and conditions specific to the
site. Typical hydraulic loading rate of 198 m^3 /d.ha (21,000
gpd/acre) is considered sufficient for optimum treatment
efficiency.BOD 5 loading rates There are two goals for organic load
control in the constructed wetlands. The first is the provision
of carbon source for denitrifying bacteria. The second goal
is to prevent overloading of the oxygen transfer ability of the
emergent plants. High organic loading, if not properly dis-
tributed, will cause anaerobic conditions, and plants die off.
The maximum organic loading rate for both type of systems
(FWS and SF) should not exceed 112 kg BOD 5 /ha.d. 2,9,10Performance expectations Constructed wetland systems
can significantly remove the biochemical oxygen demand
(BOD), total suspended solids (TSS), nitrogen and phos-
phorus, as well as metals, trace organics and pathogens The
basic treatment mechanisms include sedimentation, chemi-
cal precipitation, adsorption, microbial decomposition,
as well as uptake by vegetation. Removal of BOD 5 , TSS,
nitrogen, phosphorus, heavy metals and toxic organics have
been reported. 2,17 The performance of many well known con-
structed wetlands in terms of BOD 5 , TSS, ammonia nitrogen
and total phosphorus is summarized in Table 4.
Mosquito control and plant harvesting are the two opera-
tional considerations associated with constructed wetlands for
wastewater treatment. Mosquito problems may occur when
wetland treatment systems are overloaded organically and
anaerobic conditions develop. Biological control agents such
as mosquitofish ( Gambusia affins ) die either from oxygen
starvation or hydrogen sulfide toxicity, allowing mosquito
larvae to mature into adults. Strategies used to control mos-
quito populations include effective pretreatment to reduce
total organic loading; stepfeeding of the influent wastewater
stream with effective influent distribution and effluent recy-
cle; vegetation management; natural controls, principally
mosquitofish, in conjunction with the above techniques; and
application of approved and environmentally safe chemical
control agents.
The usefulness of plant harvesting in wetland treatment
systems depends on several factors, including climate, plant
species, and the specific wastewater objectives. Plant harvest-
ing can affect treatment performance of wetlands by altering
the effect that plants have on the aquatic environment.
Further, because harvesting reduces congestion at the water
surface, control of mosquito larvae using fish is enhanced. It
has been reported in the literature that a total of 29,000 kg/ha
dry weight of harvestable biomass of Phragmites shoots can
be harvested for single harvesting in a year. Higher yield is
achievable with multiple harvesting.
The BOD 5 , TSS, nitrogen and phosphorus removal effi-
ciencies of constructed wetlands are discussed below.BOD 5 Removal in FWS wetlands In the free water surface
constructed wetlands the soluble BOD 5 removal is due to
microbial growth attached to plant root, stems, and leaf litterthat has fallen into the water. BOD 5 removal is generally
expressed by a first order reaction kinetic (Eq. 2). 2,3,10[ C e / C o ] exp(− K T t ) (2)where,
C e effluent BOD 5 , mg/L
C o influent BOD 5 , mg/L
K T reaction rate constant, d −1
t hydraulic retention time, d.BOD 5 Removal in SF wetlands The major oxygen source
for the subsurface components (soil, gravel, rock, and other
media, in trenches or beds) is the oxygen transmitted by the
vegetation to the root zone. In most cases, there is very little
direct atmospheric reaeration as water surface remains below
the surface of the media.^17 Removal of BOD 5 is expressed by
Equation (3). This is also a first order equation and can be
rearranged to calculate the area required for the subsurface
flow system.log( C e / C o ) −[ A s K t d e]/ Q (3)where,
C e effluent BOD 5 , mg/L
C o influent BOD 5 , mg/L
K t reaction rate constant, d −1
Q flow rate through the system, m^3 /d
d depth of submergence, m
e porosity of the bed,
A s surface area of the system, m^2.Suspended solids removal Suspended solids removal is
very effective in both types of constructed wetlands. Most
of the removal occurs within few meters beyond the inlet.
Control dispersion of the inlet flow will enhance removal
near the inlet zone. Proper dispersion of solids can be
achieved by low inlet velocities, even cross sectional load-
ings, and uniform flow without stagnation.^10Nitrogen removal Nitrogen removal is very effective in
both the free water surface and submerged flow constructed
wetlands, and the nitrification/denitrification is the major
path of nitrogen removal. Total nitrogen removals of up to
79 percent are reported at nitrogen loading rates of (based
on elemental N) up to 44 kg/(ha.day) [39 lb/(acre.day)], in a
variety of constructed wetlands. If plant harvesting is prac-
ticed, a higher rate of nitrogen removal can be expected. 9,20Phosphorus removal Phosphorus removal in many wet-
lands is not very effective because of the limited contact
opportunities between the wastewater and the soil. The
exceptions are in the submerged bed design when proper
soils are selected as the medium for the system.^21 A signifi-
cant clay content and the presence of iron, and aluminum
will enhance the potential for phosphorus removal. 9,20,21C014_001_r03.indd 743C014_001_r03.indd 743 11/18/2005 10:44:18 AM11/18/2005 10:44:18 AM