Horizontal Flow (HF) Wetland

HF wetlands typically consist of the following main components illustrated schematically in Fig.1 figure below:

• Plastic liner
• Filter layer: fine gravel (average diameter 10 - 20 mm)
• Vegetation (Phragmites)
• Drainage system
• Distribution system

Fig 1: Schematic illustration of a HF wetland. Source: Own illustration (2020)

The water in HF wetlands is treated by a combination of biological and physical processes such as adsorption, precipitation, filtration, nitrification, denitrification, decomposition, etc. (HOFFMANN et al. 2010). In HF systems, the wastewater is fed at the inlet zone, usually by gravity, and flows horizontally through the porous filter medium, 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 (see for example Settler), grease, or oils from the liquid.

A plastic liner waterproofs the basins and avoids soil contamination. The depth of filter beds, which are planted with aquatic plants, is normally 60 - 80 centimetre (cm). The bottom slope should be 0.5 - 1.0% from inlet to outlet to achieve good drainage (MOREL and DIENER 2006) and the filter length no longer than 25 - 30 metres. The hydraulic retention time and the specific surface area depend on the targeted treatment results with normally around 2 - 5 days and about 2 - 5 square metres per person equivalent (m²/PE) if the effluent is to be discharged in fresh water. The hydraulic loading should be 60 - 80 millimetre per day (mm/d) for greywater (RIDDERSTOLPE 2004, MOREL and DIENER 2006), and 30 - 60 mm/d for mixed wastewater. The reduction of BOD (Biological Oxygen Demand) is about 80 - 90%, for Total Suspended Solids (TSS) 80 - 95%, for Total Nitrogen (TN) up to 60% and for Faecal Coliforms (FC) about 2 - 4 log.

HF wetlands can adapt well to different climates. As warmer temperatures allow for faster biological activity, year-round warm climate (typically found in tropical or hot climates) can improve the overall efficiency of the system compared to cold climates. Nevertheless, if properly designed and operated, HF wetlands can also be used in mountainous regions with cold weather periods or even frost.

HFs are a suitable option for small and medium communities in peri-urban areas. Application in urban areas for various purposes - also in combination with other simple technologies such as anaerobic treatments - is also possible, i.e. greywater/mixed wastewater treatment and local reuse for groups of buildings, campuses, public areas, etc.

HFs can be set-up in a way that no pumping, respectively, electricity is required for operation, which can lower the operational costs, simplify operation and maintenance (O&M) and make this technology suitable for developing countries.

The higher space requirement (2 - 5 m2/PE) compared to other technologies limits the application of this technology mainly to areas with low population density or to areas where space availability is not a limiting factor. The use of anaerobic primary treatment plants with high efficiency in removing solids and organic matter (e.g. Anaerobic Baffled Reactors, Anaerobic Filters or similar anaerobic systems) as as pre-treatment step can significantly reduce the space requirement of HF wetlands; furthermore, anaerobic primary treatment options can work efficiently in colder climates if properly designed and operated.

Heavy rainfall like during monsoon periods must be taken into account for the hydraulic calculations; however, rainfall per se does not significantly affect the performance of HFs, because they are well adaptable to the oscillation of the hydraulic and organic flow.

O&M requirements for HF wetlands are relatively simple and can be conducted by unskilled labour after adequate training (no handling of high-tech appliances or chemical additives involved). This allows that also community organisations or even private individuals may manage such a system.

Maintenance includes periodic control and emptying of sludge (see also Sludge Drying Reed Beds for Faecal Sludge Treatment) and scum in the primary treatment system; plant harvesting (usable as biomass for energy production, or as building material for thermal and acoustic insulation); checking the perfect functioning of the distribution system, ensuring that no clogging occurs in the bed and sampling of the discharged water. With time the gravel will become clogged and may have to be replaced or regenerated every 10 - 20 or more years. The water level should be maintained 5 - 10 cm below the surface of the gravel to avoid odours and mosquito diffusion.

HF wetland systems, sometimes simply called planted gravel filter, are quite common in India as they are part of the so-called DEWATS (Decentralized Wastewater Treatment Systems) approach (https://cddindia.org; www.borda.org). Below are some examples of functional systems:

Ekant Park in Bhopal: a HF demonstration unit was constructed by the Environmental Planning and Coordination Organization (EPCO) to treat 70 m³/day: a septic tank of 35 m³ is installed before a 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).

Ujjain Charitable Trust Hospital and Research Centre (Madhya Pradesh): a HF 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%), Ammonia - NH₄ (68%) (DIWAN et al. 2008), with a hydraulic retention time of less than 2 days.

Ravindra Nagar Township Ujjain (Madhya Pradesh): a HF system filled with zeolite 3 - 9 mm was monitored from 2006 to 2008 (BILLORE et al. 2008) showing an ammonia removal of about 70%.

Ujjain, Central India, a small community of residential areas: pilot study where the performance of a HF system of 42 m² planted with Phragmites karka was investigated: in a period of five months from the implementation of this HF system average removal efficiencies of 78% for NH₄-N, TSS; 58 - 65% for Phosphorus (P), BOD and Total Kjeldahl Nitrogen (TKN) (BILLORE et al. 1999) were recorded.

The use of HF wetlands began started in Germany based on research by Käthe Seidel in the 1960s (and by Reinhold Kickuth in the 1970s). And for a long time, HF systems were the most widespread wetland technology worldwide, as reported by VYMAZAL and KRÖPFELOVÁ (2008).

Ondřejov, Czech Republic: the oldest HF wetland in the Czech Republic, built in 1991, is still in operation. The wetland treats sewage from 360 persons in a single HF bed planted with common reed (Phragmites australis). The system consists of a grit chamber, an Imhoff Tank and a single 806 m² HF bed filled with gravel (3 - 15 mm). The average measured flow was 56.3 m³/d resulting in a Hydraulic Loading Rate (HLR) of 70 mm/d. The average yearly removal efficiencies measured between 1991 and 2005 have been 93% for BOD5, 89% for COD (Chemical Oxyden Demand), 95% for TSS, 41% for NH₄-N, 37% for TN (data until 2004 only), and 46% for Total Phosphorus (TP) (DOTRO et al. 2017).

Village of Sarra, Palestine: in this case, two problems were addressed: the scarcity of water and the improvement of wastewater quality. Between 2012 and 2013 a treatment system designed with a capacity of 4.300 people equivalents was implemented by IRIDRA SRL. The secondary treatment system, a HF wetland in combination with a Vertical Flow (VF) wetland, has a total area of 4.500 m². The entire system includes pre-treatment, primary treatment (Imhoff Tanks), an accumulation pond and wetlands for the treatment of sludge from the Imhoff Tanks, the construction costs amounted to Euros 596.000.