Horizontal Flow Constructed Wetlands (HFCW)

Horizontal Flow Constructed Wetland

The water in constructed wetlands is treated by a combination of biological and physical processes such as adsorption, precipitation, filtration, nitrification, denitrification, decomposition, etc. (HOFFMANN et al. 2011). HFCWs, being water saturated filtering beds, are particularly efficient in suspended solids, carbon and pathogens removal, as well as for denitrification, whereas nitrification is limited (VYMAZAL & KROEPFELOVA 2008).

In horizontal flow systems the wastewater is fed at the inlet zone, usually by gravity, and flows horizontally through the porous filter medium (that is normally small, round, evenly sized gravel of 5−20 mm in Ø, while sand is more prone to clogging and should be avoided), remaining under the surface of the bed and without any contact with the atmosphere, until it reaches the outlet zone. To avoid clogging of the wetland, pre-treatment is necessary to separate solid materials, grease or oils from the liquid.

The basins are waterproofed by a plastic liner to avoid soil contamination and planted with aquatic plants (Phragmites is the most common). The depth of filter beds is normally 60-80 cm. The bottom slope should be 0.5-1% from inlet to outlet to achieve good drainage (MOREL & DIENER 2006) and the filter length no longer than 25-30 m. The hydraulic retention time and the specific surface area depend on the results to achieve, normally 2-5 days and about 2-5 m²/PEare enough for discharge in fresh water (the lowest values are applied in warm climates). The hydraulic loading should be 60-80 mm/d for greywater (RIDDERSTOLPE 2004; MOREL & DIENER 2006), 30-40 mm/d for mixed wastewater. The reduction of BOD is about 80-90 %, TSS is from 80 to 95 %, TN until 60 % and for Faecal Coliform is about 2 to 4 log.

O&M requirements for HFCWs are relatively simple and conducted by unskilled labour (no high-tech appliances or chemical additives), which may allow a community organisation or a private to manage the system. The maintenance includes a periodical sludge and scum control and emptying in primary treatment, plant harvesting, ensuring clogging does not occur in the bed (with time the gravel will become clogged, and may have to be replaced or regenerated every 10-20 or more years), sampling of the discharged water.

HFCWs construction costs are in the range of 40-100 €/m² depending by the design, the extension, the country, the availability of suitable material in the region and the labour cost. To this cost, the price of pre-treatment installation and pipe connection has to be added. Filling media constitutes the 30-50% of the total investment cost. WSP (2008) reports cost of 1,300 Rs/m² (30 USD/m²) for horizontal flow beds.In developed country 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).

This type of a constructed wetland was developed in the 1950s in Germany by Kaethe Seidel, who designed the HFCWs making use of coarse materials as rooting medium. In the ‘60s, Reinhold Kickuth experimented soil media with high clay content and called the system the “Root Zone Method”. In the early ‘80s, the HFCWs technology was introduced to Denmark and by 1987 nearly 100 soil-based systems were put in operation. During the late ‘80s, the HFCWs were also introduced to other countries, such as Austria and UK and then in the 1990s, this system spread into most European countries and also to North America, Australia, Asia and Africa. In this period, soil or sand was replaced by coarser material (VYMAZAL 2010).

Nowadays several thousands of HFCW systems are in operation, mainly applied to domestic/municipal wastewater as also for industrial effluents with high organic loads (wineries, dairy farms, landfill leachates, oil firms, etc.) and urban or agricultural runoffs. HFCWs are considered in most cases the most appropriate constructed wetland technology for greywater treatment (MASI et al. 2010). When compared to VFCWs, the main advantage appears to be the chance of feeding the system by gravity, with no needs of alternate dosing of the wastewater.

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. India's first constructed wetland (HF of 2,700 m²) was installed at Sainik School, Bhubaneshwar in the State of Orissa; planted with two types of macrophytes, viz. Typha latifolia and Phragmites karka. At present 180-200 m³ wastewater are being treated by the wetland. BOD and nitrogen removal were 67-90% and 58-63% respectively (JUWARKAR et al. 1995).

An HF demonstration unit was constructed by EPCO at Ekant Park in Bhopal to treat 70 m³/day: a septic tank of 35 m³ was installed before the HF system of 700 m² filled with gravel and planted with Phragmites karka. The monitoring results (April 2002-Sept 2003) show good removal for COD (77%), TSS (79%), Coliform bacteria (99%) (VIPAT et al. 2008). Another field scale HF case study was realised at the Ujjain Charitable Trust Hospital and Research Centre (Madhya Pradesh): the system, filled with gravel 10-25 mm and planted with Typha latifolia, treats 8 m³/day with a surface of 80 m² and showed during the monitoring good removal for BOD (75%), TSS (78%), NH4 (68%) (DIWAN et al. 2008), with a hydraulic retention time less than 2 days. In similar climatic conditions, at the Ravindra Nagar Township Ujjain (Madhya Pradesh), another HF system, filled with zeolite 3-9 mm, was monitored from 2006 to 2008 (BILLORE et al. 2008), showing ammonia removal of about 70%.

Few studies on pilot scale were carried on during the last 10-15 year: i.e. in Mahendragiri (Tamil Nadu) for domestic wastewater; and at Mother Dairy, Delhi, for dairy wastewater by CPCB and GTZ. Another interesting pilot study was carried on for a small community of residential areas in Ujjain, Central India, where the functioning of a horizontal flow system of 42 m² and planted with Phragmites karka was investigated: average treatment performance after five months from this HF system recorded removal efficiencies of 78% for NH4-N, TSS; 58-65% for P, BOD and TKN (BILLORE et al. 1999).

In 2000 in Ujjain, on the abandoned playground of the Education College, an HF system that receives the outfall of sewage from the Ravindra Nagar residential colony was built. The wastewater is pre-treated in sedimentation tank, and then it goes to the HF system, consisting on a rectangular bed with an effective surface area of 300 m² and hydraulic loading of 40 m³/d. The surface area of gravel bed was planted with Phragmites karka. The removal efficiency of organic nitrogen and of ammonium nitrogen were 86% and 40%, respectively (BILLORE et al. 2006).

The CDD Society is a non-governmental and non-profit organisation located in India, that promotes the use of DEWATS, and has so far implemented more than 350 projects in South Asia, including India (CDD 2013). The adopted treatment is a modular system with a simple design, non-dependent on energy, consisting in 4 treatment phases: a septic tank or UASB, an anaerobic filter or baffled reactor, a planted gravel filter (HF) and in some cases a polishing ponds (free water system). About thirty DEWATS system have been realised in India, most of these near Bangalore and in Karnataka regions, but also in Maharashtra, Kerala and Tamil Nadu. The volume of wastewater treated in these plants ranges from 1.5 to 615 m³/d and the size of the HF CWs range from 1.4 to 14.6 m²/m³ of wastewater, with an average of 5.7 m²/m³. The system allows in most cases a reduction of BOD and COD of 97/99%.

For further information please visit: Horizontal Subsurface Flow CW