Constructed wetlands

FAQ: Reed bed treatment systems / Constructed wetlands

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

Question 2:
Do the plants have to be replanted? See answer

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

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

Question 5:
What effect does the local temperature on the system have? See answer

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

Question 7:
Does the system need a constant inflow of sewage? See answer

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

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

Question 10:
What operation and maintenance is required? See answer

Question 11:

Can constructed wetlands (CW) be used for wastewaters contaminated by volatile organic compounds? See answer

Question 12:

What are the systematics of wastewater treatment and where are constructed systems located in this system? See answer

Question 13:

You argue that your spray nozzles allow a much better distribution of the wastewater on the filter substrate of a constructed wetland than the usual simple outlet holes in the HDPE distribution pipes. This is immediately obvious. Does this special form of wastewater distribution also have a significant effect on the purification performance? See answer

 

 

 


Answer to question 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:

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.  

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:

 

 

 


 

Answer to question 2

Do the plants have to be replanted?

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

 


 

Answer to question 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 question 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 question 5

What effect does the local temperature on the system have?

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 question 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 question 7

Does the system need 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 question 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 question 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 question 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.

 

­


 

 

Answer to Question 11

Can constructed wetlands (CW) be used for wastewaters contaminated by volatile organic compounds?

Yes, they can. In a pilot scale horizontal subsurface flow constructed wetland (CW) in Bitterfeld, Germany, both low-chlorinated hydrocarbons (monochlorobenzene) and high-chlorinated hydrocarbons (perchloroethylene) could be removed by CW. The removal rate for monochlorobenzene can reach up to 208 mg m -2 d -1, with the inflow load of 299 mg m -2 d -1. Perchloroethylene could completely be removed (>99%) with the inflow load of 49 mg m -2 d -1 (Chen et al., 2012a). On the other pilot scale plant in Leuna, Germany, both benzene and MTBE (methyl tert-butyl ether) could be removed by different CWs. The inflow load of benzene and MTBE has been 188–522 and 31–90 mg d−1 m−2, respectively. Higher removal efficiencies were obtained during summer. The benzene removal efficiencies were 24–100% and 22–100% in the horizontal subsurface flow constructed wetland and the floating plant root mat, respectively; the MTBE removal efficiencies amounted to 16–93% and 8–93% in the horizontal subsurface flow constructed wetland and the floating plant root mat, respectively. The volatilisation rates in the floating plant root mat amounted to 7.24 and 2.32 mg d−1 m−2 for benzene and MTBE, which is equivalent to 3.0% and 15.2% of the total removal. The volatilisation rates in the horizontal subsurface flow constructed wetland reached 2.59 and 1.07 mg d−1 m−2, corresponding to 1.1% and 6.1% of the total removal of benzene and MTBE, respectively. The results indicate that floating plant root mats are an interesting option for the treatment of waters polluted with benzene and MTBE under moderate temperature conditions (Chen et al., 2012b).


 

Answer to Question 12


 

 

 

Answer to question 13

You argue that your spray nozzles allow a much better distribution of the wastewater on the filter substrate of a constructed wetland than the usual simple outlet holes in the HDPE distribution pipes. This is immediately obvious. Does this special form of wastewater distribution also have a significant effect on the purification performance?

 

Yes, not only are the biofilms (communities of microorganisms) on the sand grains of the filter substrate much more evenly and extensively charged with the nutrient solution "wastewater" than with punctual hole outlet openings, but there is also a serious oxygen enrichment.  This is achieved by the distribution of the droplets generated by rotation in the spray nozzle in the atmosphere near the ground before they enter the actual matrix of the filter medium (usually sand of a special grain size). Ding, Y. et al 2014* have proven the highly significant increase in the cleaning performance of vertical flow constructed wetlands (vertical soil filters) by means of spray nozzles in two-year laboratory tests.

 

Four experimental plants were tested over a period of two years, two of which were unplanted as controls (one with hole distribution and one with spray nozzles) and two planted with Canna indica (one with spray nozzle distribution). The COD cleaning performance as well as nitrification and denitrification were significantly better than the three control systems in the version with planting and with spray nozzle distribution.

 

*Source: Ding, Y. et al 2014: Effect of spray aeration on organics and nitrogen removal in vertical subsurface flow constructed wetlands. In: Chemosphere 117 (2014) 502 – 505.