Constructed Wetlands

Constructed Wetland TOKAI project
Pilot Farm, College of Agriculture, Hódmezövásárhely, Hungary

1. Introduction An important element of the TOKAI-project, was the construction of a constructed wetland, to demonstrate the possibilities of this type of wastewater treatment. The system was built on the site of the pilot farm of the College of Agriculture , Hódmezövásárhely. It treats the wastewater from the goat farm (wastewater from the milking parlour and sanitary wastewater). The effluent had to be discharged by means of a drainfield in the soil. This project offered the possibility to construct a system that consists of four different stages: subsurface vertical flow wetland, subsurface horizontal flow wetland, free water surface wetland, drainfield planted with trees.

2. Influent Data on the influent quantity and quality were scarce. Q 2 m³ pH 9.8 COD 640 mgO2/l Total KjN 28 mgN/l Total P 72 mgP/l The average daily amount of wastewater is approximately 2 m³. But this amount can vary a lot, from 0.5 m³ to 3 m³. One sample was taken from the septic tank and analysed. The results are presented in the table. Both the high pH and the very high phosphorus content are caused by the rinsing agents used to clean the milking parlour. 3. Effluent requirements The effluent that will be discharged through a drainfiled, may not edanger the quality of the groundwater. The quality standards that are put forward for the groundwater can be summarized as follows: pH 6.5-9 Total P <0.5 mgP/l NO3- <25 mgN/l NH4+ <0,5 mgN/l To monitor the quality of the groundwater, monitoring wells were built at different places in the neighbourhood of the constructed wetland. The phosphorus content could pose a problem. To deal with this problem, the use of the rinsing agents has to be reconsidered. The products that are used now, contain a lot of phosphorus (phosphoric acid, phosphonates). There are appropriate alternatives available. 4. Treatment Plant This constructed wetland was designed by Mr. Rob Van Deun and Mrs. Mia Van Dyck (Katholieke Hogeschool Kempen, Geel, Belgium) and Mr. Ernö Dittrich (Hidro Konstrukt Bt., Pécs, Hungary). 4.1. Pretreatment The wastewater is discharged in a existing septic tank with a volume of 7.5 m³. At the time of construction of the wetland, the effective volume of the septic tank was approximately 3m³ due to the accumulation of sludge. Therefore the tank has to be cleared out. One should aim at a residence time of 4 days in the septic tank. There is however no immediate danger for clogging of the constructed wetland, because the first stage is a subsurface vertical flow system, filled with gravel. This can also act as a pretreatment. 4.2. Vertical Subsurface Flow Wetland The wastewater is pumped from a pumping chamber two times a day onto the surface of the wetland. The water is evenly distributed over the surface. The wastewater drains vertically through the medium and takes up oxygen. Drainage pipes on the bottom, move the treated wastewater to the next stage. The wetland is planted with Phragmites australis, common reed, 9 plants per square meter. The wastewater is treated primarily by microbial degradation and physical processes. The organic compounds are mainly degraded aerobically and these conditions also favour nitrification (oxidation of ammonium to nitrate). Oxygen is provided by the percolation of the wastewater and the plants. Settleable organics are removed by deposition and filtration.


This wetland has a surface area of 16.5 m². Because of the steep sides, this value cannot be used for calculations concerning organic and hydraulic load. It is safer to use the surface area at half the height, appr. 9 m². From top to bottom the wetland is made of a 10 cm layer of gravel 4-8 (insulation), 15 cm gravel 4-16 (inlet pipes), appr. 60 cm gravel 4-8 (percolation column) and 20 cm gravel 4-16 (drainage pipes). A 1.5 mm thick HDPE liner was used.d by microbial denitrification.

4.3. Horizontal Subsurface Flow Wetland From the vertical flow wetland the wastewater flows by gravity in the subsurface horizontal flow wetland. The wastewater is distributed across the width of the wetland. It flows slowly through the porous medium under the surface of the bed in a more or less horizontal path until it reaches the outlet zone. A high water level, appr. 5 - 10 cm below the surface, is maintained. The wetland was planted with Carex elata, tufted-sedge, 5 plants er square meter. Organic compounds are degraded aerobically as well as anaerobically. During the flow through the wetland , the wastewater will come into contact with both aerobic and anaerobic zones. Because of the high water level the anaerobic zones will predominate. Aerobic zones will be found around the roots and rhizomes that leak oxygen into the substrate. If nitrification has occurred in the vertical stage, denitrification becomes possible in this stage. This way a better nitrogen removal is achieved. Sedges were chosen for this wetland because there is scientific evidence this plant species will give best results during a cold winter. Carex sp. provide a better insulation of the wetland during winter and they also have a better oxygen transport. Carex sp. also show a high evapotranspiration rate, 45 - 80 mm/day (=l/m².d). In some cases this can be an advantage. Common reed f.e. has evapotranspiration rates between 7,5 and 11 mm/day. Carex elata is probably not the best choice, but at the time of construction it seemed to be the only species available. Carex pendula or Carex acuta should be preferred.

The wetland has a surface area of 22.2 m². Aspect ratio (L/W) appr. 2/1. The average depth is 0.55 m. In the inlet and outlet zone gravel 16-32 is used. The wetland is filled with gravel 4-16. A 1.5 mm thick HDPE liner was used.


4.4. Free Water Surface Wetland with free-floating macrophytes The wastewater flows by gravity in a stabilization pond covered with Lemna minor or duckweed. Duckweed is a fast growing free-floating plant. Under ideal conditions it can double its biomass in 2 or 3 days. Some Lemna species are able to grow at temperatures as low as 1 to 3°C. The main use of duckweeds is in removing nitrogen ad phosphorus from secondary treated wastewater. A dense cover of duckweed on the surface of the pond inhibits both oxygen entering the water by diffusion and the photosynthetic production of oxygen by phytoplankton because of poor light penetration. The water becomes largely anaerobic, which favors denitrification. The light absorption of the duckweed cover restricts the growth of phytoplankton and therefore the production of suspended solids. Nitrogen and phosphorus may be removed by direct plant uptake with frequent biomass harvesting. The produced biomass has a high nutritive value or can be composted or used for biogas production. Nitrogen will also be removed by microbial denitrification.

The total surface area is appr. 25 m². The maximum depth is 0,6 m and maximum volume appr. 9 m³. A 1.5 mm thick HDPE liner was used.


4.5. Drainfield The wastewater is discharged through a drainfield. According to American standards, 20 m² is needed to discharge 1 m³ effluent from a septic system in coarse sand. This drainfield was designed according to Hungarian standards. The total surface arear is appr. 270 m² and this area is planted with 30 poplar trees.


5. Additional resources More pictures can be found on this page.