Floating constructed wetlands
Question 1: Which pollutant and hydraulic load can be used for your aerated floating constructed wetlands?
Question 2: What is the total weight of the water saturated filter substrates including the biomass of marsh plants per m³ ?
Question 3: Is there any smell nuisance?
Question 4: What about mosquitoes?
Question 5: Are their nuisances by rats?
Question 6: Is a pretreatment of sewage required?
Question 7: How to treat the sewage sludge?
Question 8: Must the reed plants be harvested?
Question 9: Can other plants be used?
Anwer to 1: Which pollutant and hydraulic load can be used for your aerated floating constructed wetlands?
Up to 60g BOD5 per m² and day with a hydraulic loading of about 300-400 l/m²x d depending on the organic load of the water to be treated (domestic sewage, greywater, river or lake water).
Answer to 2: What is the total weight of the water saturated filter substrates including the biomass of marsh plants per m³ ?
The total weight is 1.050 kg/m³ based on a filter depth of 1.1 m .
Answer to 3: Is there any smell nuisance?
The treatment process in the upper layers is an aerobic process. Oxygen is supplied by the reed plants through their aerenchyma tissues. Noxious smells are only created by anaerobic processes, therefore smells are absent near the reed beds.
Answer to 4:What about mosquitoes?
In contrast to water lagoons, reed beds do not have constant open water areas. Therefore mosquitoes are unable to breed. Other insects (e.g. flies) are reduced by the high biodiversity of the system (predators).
Answer to 5: Are their nuisances by rats?
Rats do not find any food in the pretreated or totally shredded sewage which is fed into the reed beds (depending on the design). Therefore they are not attracted by the wetland searching for food.
But as any other plantation reed beds can give shelter for birds and small mammals, including rats. But because the filter bed is charged with water several times a day, and there is always water close to the surface, rats and mice are expelled. Rabbits cannot burrow in the reed bed subsoil. Any infestations for small mammals can be controlled in the usual way for plantation areas in urban landscaping.
Answer to 6: Is a pretreatment of sewage required?
That depends on the design; there are reed bed systems which require a pre-treatment in form of a screen, septic tank or even an aerobic biological process (SBR, trickling filter, fixed bed reactor). But there are also systems which can be charged with raw sewage. These systems have a special reed bed filtration stage (stage A) which removes and converts the suspended solids prior to a second biological reed bed treatment stage. Which reed bed technology should be chosen depends on the specific project needs and client wishes.
Answer to 7: How to treat the sewage sludge?
If a reed bed system with a pre-treatment in the form of sedimentation is chosen, the accumulated sludge in the sedimentation stage can either be discharged by tankers or converted into humus in a special sludge composting reed bed. The produced volume of sewage sludge, if any, depending on the chosen reed bed system, is always less than in any other conventional sewage treatment plant.
In a raw sewage reed bed (stage A) the mineralized sludge in the first filtration stage should be removed after 15 – 20 years depending on the design.
Answer to 8: Must the reed plants be harvested?
No, but for visual appearance the plants can be mowed at the banks of the system. However, if required the plants can frequently be harvested (up to 2 times per year) and used as biomass for burning, biogas production or eco-friendly construction materials, without any negative effects on the treatment process.
Answer to 9: Can other plants be used?
Because of its physiology, Phragmites australis (common reed) is the best marsh plant for reed beds. But there are several other native wetland plants, which can be used in constructed wetlands, including species of Typha, Cyperus and Schoenoplectus (=Scirpus). These other species are also effective in processing the wastewater, but less effective than Phragmites in removing heavy metals, organic pollutants and higher levels of phosphates. We propose to use these other native plants as well in the system to increase the biodiversity of the site with respect to nature conservation goals.