Minimal depth of willow planted horizontal constructed wetlands?

  • BPopov
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Minimal depth of willow planted HCW?

Hello everyone!

I would be very much grateful for information what is the minimal depth of substrate in horizontal constructed wetalnd needed to support the willows. What about 1 meter or it is still too shalow? I am talking about Salix viminalis (shrub form)

Thanks ,
Bogdan

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  • arno
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Re: Minimal depth of willow planted HCW?

Hi Bogdan
I've been interested in willow shrubs to evapostranspire wet areas. Indeed these plants really can absorb water and if set up properly the constructed wetland will have no discharge.

Here is a reference from Denmark on the subject with some specifications.
pdfs.semanticscholar.org/9cf1/713e9ec80a...c0c22c719dc17433.pdf

Zero-discharge of nutrients and water in a willow dominated constructed wetland
P. Gregersen* and H. Brix**
* Centre of Recycling , Forsomho Skolevej 5, DK-6870 Ølgod, Denmark
** Department of Plant Ecology, University of Aarhus, Nordlandsvej 68, 8240 Risskov, Denmark
Abstract. A novel constructed wetland system has been developed to treat sewage, evaporate water and
recycle nutrients from single households at sites where effluent standards are stringent and soil infiltration is
not possible. Main attributes of the willow wastewater cleaning facilities are that the systems have zero
discharge, the willows evapotranspire the water, and nutrients can be recycled via the willow biomass
produced in the system. The willow wastewater cleaning facilities generally consist of c. 1.5 m deep highdensity
polyethylene-lined basins filled with soil and planted with clones of willow (Salix viminalis L.). The
surface area of the systems depends on the amount and quality of the sewage to be treated and the local
annual rainfall. For a single household the area needed typically is between 200–300 m2. Settled sewage is
dispersed underground into the bed under pressure. When correctly dimensioned, the willow will – on an
annual basis – evapotranspire all water from the sewage and rain falling onto the system, and take up all
nutrients and heavy metals from the sewage. The stems of the willows are harvested on a regular basis to
remove nutrients and heavy metals and to stimulate the growth of the willows. Initial experiences from fullscale
systems in Denmark show promising results.


Regards

Arno Rosemarin PhD
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  • goeco
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Re: Minimal depth of willow planted HCW?

Perhaps most importantly in designing such a system, "The seasonal variation in precipitation and evapotranspiration must also be considered as the system should have volume (depth) enough to be able to store the sewage and rain during winter. In addition, the amount of nutrients discharged into the system should balance the amount that can be removed by harvesting aboveground biomass."

There clearly needs to be additional capacity so that it doesn't overflow in winter when the willows do nothing. Wider and shallower might evapotranspire more water than deeper and narrower for the same volume, but not too wide/shallow because rainfall would need to be taken into account.

This paper was published in 2001, and at that time there were concerns about "the potential accumulation of salts and the sustained health of the willows." Research was being carried out "to evaluate these aspects and to further optimize the systems." Was anything published? How widely has this system been utilised in Denmark or elsewhere?

cheers
Dean

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  • BPopov
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Re: Minimal depth of willow planted HCW?

Hello Arno!

Thank you for the information!

I would like to use willows in constructed wetland to serve as a hedge between the road and property as well as material for basketry. I do not think I will be able to make basin more than 1 m deep in this location since it is on slope and plan the HCW as a rather narrow (4 metres) terrace on it.

1,5 m deep basin as said in the reference is quite a lot of substrate as well as a lot of expensive lining.

I wonder how willows can possibly survive in the shallower basin.

Regards,
Bogdan

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  • arno
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Re: Minimal depth of willow planted HCW?

Bogdan
Dean's point about winter conditions is important. The system needs to store the sewage and precipitation during that period if it is to have zero discharge. If there is limited storage capacity and discharge is part of the design then the treatment and nutrient trapping capacities need to be made optimal. That means starting with a septic tank and then aeration filter to reduce organics prior to discharge into the constructed wetland. Trapping nitrate and phosphate can then take place in the planted bed.

The Danes have taken this seriously with a map of the country specifying the size of the wetland for certain volumes of wastewater. Here attached is an article by Brix and Arias (2011). USE OF WILLOWS IN EVAPOTRANSPIRATIVE SYSTEMS FOR ONSITE WASTEWATER MANAGEMENT – THEORY AND EXPERIENCES FROM DENMARK.
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What is the depth of freezing in your part of the Carpathians?

Regards

Arno Rosemarin PhD
Stockholm Environment Institute
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  • BPopov
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Re: Minimal depth of willow planted HCW?

Dear Arno!

Thank you for the reference!
The freezing depth is varying in our region but I assume generally it is 60 cm deep. Usually less since the winters are mild lately.


Evapotranspiration and zero discharge is a very interesting however it seems like the bed area needed for it is huge (200 sqm per household means something like 30 sqm per person?). Also the evapotranspiration bed described in publication look like vertical pressure distributed filter which further complicates its building and operating.
Zero discharge and evapotranspiration are not objectives in my case as well as enhanced biomass production. What I want to built is a simplest possible relatively narrow (planted bed width is 4 m) Horizontal Gravel Filter ( according to DEWATS terminology) with the willows planted there instead of reeds. The main reason for it is having the permanent hedge as well as basketry material since this craft is widely practiced in the area. No pumps, simple distribution.
The influent will be pretreated with septic tank and aerobic biofilter. The substrate available is 10-20 mm crushed stone.
Regards,
Bogdan

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