MASTER'S THESIS - SuSanA

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5 Learning from already implemented CWs (in Albania and the Balkan region) Figure 5.3: Horizontal filter bed (Bodenfilter 2012) 5.1.2 Parts of a constructed wetland A constructed wetland consists of following parts, from the bottom to the top ((Albold et al. 2011 ), (Heeb and Wafler 2011,a)): • Levelling layer • Impermeable liner (geo-textile layer or clay) • Drainage (drainage pipes in a gravel layer) • Biological treatment layer (gravel or sand) • Cover layer • Aquatic plants Pre-treatment is necessary to prevent clogging of the filter beds. An exception is the so called “French System” that works without pre-treatment (Albold et al. 2011 ). For pre-treatment purpose, following possibilities can be used: • Settling tanks (two or three-chamber settling tanks; settling of solids and organic matter) • Imhoff tanks (treatment and reduction of primary sludge by anaerobic stabilisation processes) • Ponds (need a large space away from the houses as they tend to smell; reduction of COD, BOD, SS and TSS (Regelsberger 2005)) • Raw wastewater filters (solids retain at surface and get mineralised (Nowak 2010,a)) It has to be considered that primary sludge has to be treated regularly. The pre-treatment tank has to be emptied from time to time. Sludge can be applied on a planted or unplanted drying bed or can be transported to another wastewater treatment plant for further treatment. The use in a digester to generate biogas for electricity production is another possibility. 53
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
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MASTER'S THESIS UIM/2011/7
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1 Problem Statement / Background At
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Bauhaus-University Weimar FACULTY O
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List of Abbreviations ADA ADF ALL A
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UNECE VF WB WFD WHO WRA WSP WSS WSS
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Table of content Preface...........
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5.3 Other constructed wetland plant
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Figure 5.15: Design of small size C
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MASTER'S THESIS - SuSanA

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