Specialist Group on Use of Macrophytes in Water Pollution ... - IWA

The ‘French design’ system
The pulse fed two stage vertical flow constructed wetland (VFCW) for the treatment of raw
sewage is the most commonly design found in France for CWL (several hundreds of plants).
The first stage in the VFCW is divided into three filters (one active bed and two under rest
conditions receiving no flow), and the second stage is divided into two filters (one under
resting conditions). All 5 filters are of equal area. Oxygen transfer occurs passively. Average
values for the French small communities‘ wastewater are: COD= 733 mg/l; TSS=280 mg/l;
and TKN= 87 mg/l (Molle et al, 2005). The above configuration allows more than 90%
removal of COD and TSS, and about 85% nitrification, but only about 20% denitrification
(Molle et al., 2005).
The ‘tidal flow’ system
The ‗tidal flow‘ system, designed for pre-settled wastewater, is comprised of multiple flood
and drain (tidal) wetland cells where a flood and drain regime allows for efficient oxygen
transfer. The cells‘ aggregate is characterized by a high CEC (>4 meq/100g) to allow for
NH4 + adsorption. Nitrification occurs in drained wetland cells while denitrification occurs in
flooded wetland cells. A model system with a flow rate of 1000 m 3 /d was evaluated by
Austin and Nivala, (2009) with influent wastewater characteristics of 300 mg/L COD, 60
mg/L TN. The system was based on three paired CW cells with reciprocating pumped flow in
each cell pair and overflow drain to the next cell pair in series. The first stage rotates between
three paired cells in parallel to allow for a rest phase (two pairs under rest conditions). The
system achieves reliable tertiary treatment performance, with effluent BOD, TSS and TKN
all about 10 mg/L. Design effluent ammonia is less than or equal to 1.0 mg/L.
The CEPT–CWL system
In the CEPT–CWL system, raw wastewater is pretreated with CEPT using a low dose of
FeCl3 (10-13 mg/l) followed by a dual unit CWL. During the CEPT stage, removals of 69%,
79% and 60% for CODT, TSS and P, respectively, were obtained resulting in effluent with
average values of 230mg/L CODT; 73mg/L TSS; 2.3 mg/L PO4-P and 61 mg/L NH4-N. The
large reduction in COD and suspended solids from the wastewater by CEPT allows for a
much smaller CW footprint for the following treatment step. In addition, the phosphorus
concentration of the CEPT effluent using a low dose of FeCl3 was much lower than that
required by the Inbar regulations, leaving only residual COD and ammonia to be treated. The
first bed of the dual unit CWL treating CEPT effluent is an anoxic saturated subsurface
horizontal flow CW and the second bed is an unsaturated aerobic vertical flow CW similar to
the French design described above. The two beds are interconnected by recirculation and the
recycle to influent feed ratio is 2:1.
Comparison of the Area and Energy requirements
The comparison was based on a daily organic loading rate of 1 PE or 120 g COD/day and
only operational electrical energy requirements were considered.
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32 IWA <strong>Specialist</strong> <strong>Group</strong> on Use of Macrophytes in Water Pollution Control: Newsletter No. 37