MASTER'S THESIS - SuSanA

5 Learning from already implemented CWs (in Albania and the Balkan region)
5.1.3 Design Criteria for constructed wetlands
The guidelines for constructed wetlands differ from country to country, depending e.g. on the experiences
and the treatment requirements of the country (Regelsberger 2005). To equalise fluctuations, a storage tank
(buffer tank) should be used. This will lower the costs if big fluctuations of the produced wastewater
amount occur. Usually, constructed wetlands can cope very well with quantitative fluctuations.
In 2010, GIZ hired consultant Jens Nowak wrote a design manual for the construction and use of
constructed wetlands in Albania called ”Albanian guidelines for the design and implementation of
wastewater treatment plants using constructed wetlands”. This document describes vertical and horizontal
flow filter beds with sand as filter medium. According to (Nowak 2010,a), the biggest problem for CWs is the
clogging of the filter. For the described types, “soil clogging problems can be avoided by correct calculation,
design and construction”, considering the following design criteria (Nowak 2010,a):
• Corresponding design of pre-treatment
• Selection of suitable filter substrates
• Sufficient filter area
• Uniform distribution of sewage
• Vertical flow planted soil filters supplied intermittently and by a hydraulic bore
• Regular rest phases without feeding in vertical flow planted soil filters
As Albania has two very different climate areas - the plain lowland and coastal area and the high mountain
regions - design values have to be determined for both areas. While the lowland and coastal areas show
high temperatures in summer, the winter is mild. In the mountain regions, summer is mild and winter can
bring very low temperatures below the freezing point what requires larger filter areas.
(Nowak 2010,a) adapted the specific values for the necessary filter area for CWs to Albanian climate. He
gives design values that are presented in Table 5.1 (p. 55). German design criteria is given as a comparison
(Heise and Nowak 2007). The required minimum surface area has to be available in parallel use and not inline.
If several beds are linked in a row, the first filter has to have the required minimum surface (Heise and
Nowak 2007).
The filter material can be sand or gravel. The whole filter layer should be constructed with the same
material. Filter with decreasing permeability in flow direction did not proof to have advantages but tend to
clog easily. Horizontal filter have different material at the inlet and outlet that needs to have higher
permeability for drainage purpose. The permeability of the filter material is given by the k f -value, that
should be in the range of 10 -4 m/s ≤ k f ≤ 5·10 -3 m/s. VF have the drainage layer below the filter layer. HF beds
should be constructed with filter material with higher k f -value because of hydraulic losses while filter
material for VF beds should have lower k f -value to support the even water distribution. Also a cover layer
for frost protection or better water distribution can be applied on top of the filter layer (DWA 2007).
Most common vegetation that is planted on the filter bed is “Phragmites australis” (reed), “Typha ssp.”
(cattail) and “Scirpus ssc.” (bulrush) (Regelsberger 2005).
Bottom and sides of the filter bed have to be waterproof in order to prevent contamination of the
underground and the groundwater. Natural sealing should have a k f -value < 10 -7 m/s and be at least 0.3 m
thick. Artificial sealing must be “acid-resistant and alkali-proof, frost-persistant, roots and rodent resistant,
non toxic, easy to carry and move, and made of recyclable materials” (Regelsberger 2005).
54