Vertical Flow Constructed Wetland (VFCW)

Vertical Flow Constructed Wetlands

The water in CWs is treated by a combination of biological and physical processes such as adsorption, precipitation, filtration, etc. (HOFFMANN et al. 2011). VFCWs are particularly efficient in suspended solids and carbon removal, as well as for nitrification, while denitrification is limited. Pre-treated wastewater is distributed above the entire surface of the bed by a system of pipes, fed by a pump or energy-free siphon devices, with an intermittent short-term loading intervals (4 to 12 doses/day), and long resting periods (2-6 h); the wastewater flows through the porous medium in a vertical path until it reaches the drainage system on the bottom connected to an outlet manhole.

Normally the filling media consists of a sand layer of at least 50 cm, with an additional 15-20 cm of drainage layer with coarse gravel on the bottom, a transitional layer between sand and coarse gravel of 10-15 cm, and 10-20 cm of fine gravel on the top of the bed for ensuring a proper even distribution over the sand layer. The total height is generally around 0.8-1.2 m.To avoid clogging of the wetland, pre-treatment is necessary, except in “French System” vertical flow beds (FRB) that are used for treating raw wastewater without any primary treatment and any production of primary sludge. French systems are composed by two stages for a total of 2-2.5 m²/PE (MOLLE et al. 2005). Here the sludge is accumulated on the top of the 1st stage and the filling material has a different layers and granulometries distribution.

In VFCWs as well as FRB the basins are waterproofed by a plastic liner and planted with suitable plants. In Austria a two-stage VF system that reduces the footprint to 2 m²/PE is developed (LANGERGRABER et al. 2010). The hydraulic loading rate in cold climates should not exceed 100-120 mm/d (DWA 2006). The reduction of BOD₅ is about 90-99 %, for TSS is from 90 to 99 %, for TN about 30 % (RIDDERSTOLPE 2004) and for FC is about 1-2 logs (MOREL & DIENER 2006).

O&M requirements for VFCWs are relatively simple and can be conducted by unskilled labour (there are not high-tech components or chemicals to be added). The maintenance includes a periodical sludge and scum control and emptying in the primary treatment unit (not for FRBs), plant harvesting (usable as biomass for energy production, or as building material for thermal and acoustic insulation), ensuring clogging does not occur in the bed (a VF bed can recover well after a resting period of two-three weeks in sunny and dry season) and sampling of the discharged water. For FRBs, the organic layer which is developing and increasing in thickness on the surface of the first stage has to be removed with a frequency of about 10 years (this organic matter is not similar to a primary sludge, but instead to a composted and dehydrated sludge that can be used as soil conditioner if not polluted by heavy metals or persistent organics).

VFCWs realisation costs are in the range of 50-110 €/m² depending by the design, the country, the availability of suitable material (i.e. filling media and waterproofing liner) in the region and the labour cost. The price of pre-treatment units and pipe connections has to be added to the above mentioned costs. Filling media constitutes 30-50% of the total investment cost. WSP (2008) reports cost of 2,100 Rs/m² (50 USD/m²) for vertical flow beds.In developed countries the maintenance cost is in the range of 10-15 €/PE, in developing countries this cost is in the range of 2-8 €/PE. The average O&M cost in Nepal is about 0.5-2 USD/m² (UN-HABITAT 2008).

VFCWs are less affected by the low temperatures compared to the other CWs and also require less land, so they are mostly used in the northern European countries (Austria, Denmark, France, Germany, and UK) (KADLEC & WALLACE 2009). Moreover, the vertical flow systems are better suited to manage fluctuations in the hydraulic and the organic load. In Orhei, Moldova, the largest CW in the world for secondary treatment is under realisation: construction began in early 2012, founded by World Bank. The urban wastewater will be pre-treated, and then treated by a “French CW system” formed by four parallel lines, each one composed by two stages in series, the first being a RBF and the second a normal VF bed;at the end the water will have the option of an emergency disinfection treatment, and then it will be discharged to the Raut river.

The plant has been designed for an average flow of 4,600 m³/d estimated for 2020 (in 2010 the measured flow was 2,100 m³/d) and occupies an area of 4 ha (1.8 ha for VFB and 1.7 ha for VF bed). The French System has been used since over 20 years, and approximately 1,000 CWs of this type are in operation in France (MOLLE et al. 2005). Several thousand of conventional VF plants are nowadays in use in Germany (more than 100,000 units, mainly for households), Portugal, Spain (HOFFMANN et al. 2011) and more than 3,000 in Austria (LANGERGRABER & HABERL 2012).

Despite CWs have a strong potential for application in developing countries, particularly by small rural communities, due to their low cost and easy maintenance, these systems have not found widespread use in India, due to lack of awareness, and local expertise in developing the technology on a local basis.

BORKAR & MAHATME (2011) studied a VF CW with raw wastewater collected from Amba Nala. The experiment was conducted with a constructed wetland pilot model made of plastic, and consisting in a cylinder with diameter 23 cm and height 45 cm filled from the top with 15 cm of soil media (black cotton soil and sandy soil), 5 cm of sand layer, a geotextile layer and two layers of gravel, 10 cm deep (16 mm Ø) and 15 cm deep (31.5 mm Ø). The experiment compared a system without plants and a system with Typha orientalis: the planted system with a black cotton soil showed the better results in terms of BOD and COD reduction (respectively 59 % and 53 % for unplanted CW and 86% and 63% for the planted one).

A pilot scale CW unit was realised in the at Anna University, Chennai, to study the capacity of a VF bed to remove heavy metal (nickel); the cells were filled with gravel and sand and planted with Arundo donax, showing removal of 70-75% of nickel from an initial concentration of 7-8 mg/l (SIVARAMAN et al. 2011).

In Nepal, thanks to the efforts of various NGOs and UN cooperation services, several CWs were realised for various type of wastewater (as reported in UN-HABITAT 2008). Examples of VF CWs reported are:

• Combined laboratory and domestic wastewater treatment (ENPHO), a little VF bed of 15 m² for 7 PE running since 2002;
• Grey water treatment (Private residence), a little 6 m² VF bed for 8 PE planted with Phragmites Karka and Canna latifolia that permits the reuse for gardening.

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