Constructed wetlands


Question 1: Did marsh plants develop in your artificial wetlands for watewater treatment  in an unexpected manner?

Question 2: Must the plants be replanted?

Question 3: What are the area requirements of artificial wetlands for wastewater treatment?

Question 4: Do reed plants match with the local flora and fauna?

Question 5: What effect has the local temperature on the system?

Question 6: Is water  ”lost” by the reed plants?

Question 7:
Do the system needs a constant inflow of sewage?

Question 8: Will the reed plants invade other landscaping areas?

Question 9: What is the life time of the system?

Question 10: What operation and maintenance is required?


 



Answer to 1: Did marsh plants develop in your artificial wetlands for watewater treatment  in an unexpected manner?

Yes, in a large scale project we designed in China.
Because nobody could know, which marsh plants would thrive best under the very special industrial wastewater conditions at Changshu (refractory chemical compounds, high salinity , low nutrients concentration and so on) I recommended to test a wide variety of helophytes, to see which species are best suited under the given circumstances. Now (2015) we can see the very surprising results of that strategy. The worldwide mostly used wetland species for constructed wetlands do not perform best, but others (seldom used) tolerate the wastewater or develop very well.

Vertical flow filter system:


Arundo donax is clearly the winner of the competition in the vertical flow wetlands.


Iris pseudacorus is the second best.


Miscanthus sacchariflorus gets the bronze medal.


Typha angustifolia is not good developed, but maybe on the way to a higher biomass in the next vegetation period.

All others are not developing in a vital manner, obviously they are suffering from lack of nutrients, as can be seen from the following pictures.  


Cyperus ssp.  (It is not papyrus !)


Phragmites communis


Canna indica


Horizontal filter system:

Concerning the horizontal filter system also very surprisingly Typha minima and Thalia dealbata thrives best and should be prefered for possible replanting:


Typha minima


Thalia dealbata


Answer to 2: Must the plants be replanted?

No, once the marsh plants are established they will regenerate each year for decades.


Answer to 3:  What are the area requirements of artificial wetlands for wastewater treatment?

About 2 to 5 m² per person, 5 -25 m² per m³,if artificially aerated less than 1m²/person equivalent.


Answer to 4: Do reed plants match with the local flora and fauna?

Yes, Phragmites australis and most of the other useful helophytes are cosmopolitan plants. Wherever brackish or freshwater moisturise the soil or is appearing at the surface (wadis, oasis, sewage and irrigation water spills), reeds will grow naturally. Reed plants could also be used to link fresh water wetlands with salt water wetlands including sabkhas, mangroves and salt marshes.   (sea grasses are under  seawater!!)


Answer to 5: What effect has the local temperature on the system?

The higher the better, in contrast to technical sewage treatment plants, the reed bed technology has its optimum performance in hot climates as the physiology of Phragmites and other marsh plants are boosted to photosynthesis operating at these temperatures.



Answer to 6: Is water  ”lost” by the reed plants?

Water is not lost, as it is used to create greenery with the reed plants. If the reed bed is integrated into the landscaping, it often replaces dry areas, which would otherwise consume water for irrigation. Water is removed from the system by transpiration, a process by which the plant loses water through its stomata (breathing pores). Under arid climatic conditions and during hot summer days in under moderate climates these pores are closed by terrestrial plants to reduce transpiration.  Marsh plants however are lacking this regulation and the total water loss of evaporation and transpiration maybe up to 10 mm/d (= 10 l /m² x d).



Answer to 7: Do the system needs a constant inflow of sewage?

No, only during establishment is  it  important to have a constant inflow to prevent the young plants from drying out (the first half year after planting).


Answer to 8: Will the reed plants invade other landscaping areas?

There is no danger of seeds from  reed plants establishing in nearby areas, unless there is regular inundation by water. Precautions can be taken against the spread of the root system of the established reed stock in a reed bed, but this is already done by the PVC or HDPE liner, which divides the root system from the surrounding soils. Only horizontally growing  arial shoots can invade surrounding areas, but will survive only under wet soil conditions. They can easily be cut and removed during gardening and maintenance.


Answer to 9: What is the life time of the system?

As the reed stock is a regenerating system and the sewage solids will be decomposed by microorganisms, there is nearly no sludge accumulation in the system. The lifetime expectancy is at least 40 years. The first large scale constructed wetland in Germany (5.000 person equivalents) is now running since 1974 in the municipality at Liebenburg-Othfresen.  The only mechanical part of the system is a pumping station, which is assumed to have  a lifespan of at least 7-10 years.  


Answer to 10: What operation and maintenance is required?

Depending on the size of the system a weekly visual check and a monthly trimming of the reed shoots is enough.