WILLOW SYSTEMS

WILLOW SYSTEMS

AUTHORS

Darja Istenič, University of Ljubljana, Faculty of Health Sciences,

Zdravstvena pot 5, 1000 Ljubljana, Slovenia
Contact: darja.istenic@zf.uni-lj.si
Carlos A. Arias, Aarhus University, Department of Biology –

FA C T S H E E T
Aquatic Biology, Ole Worms Alle 1, 8000 Aarhus C, Denmark
Contact: carlos.arias@bios.au.dk

1 - Inlet
2 - Feeding system
3 - Soil
4 - Drainage system
5 - Original soil
6 - Trees
7 - Maintenance pipe
8 - Waterproof liner
9 - Inspection manhole

Description
Willow systems are treatment wetlands (TW) dominated by willows. They are used for on-site
wastewater treatment and reuse by production of woody biomass. They are designed to treat all inflow
water through evapotranspiration and thus there is no outflow from the system. Zero-discharge willow
systems are most appropriate for the sites with strict wastewater discharge standards or where soil
infiltration is not possible; however, systems with outflow or percolation are also in use. Zero-discharge
willow systems produce a significant amount of biomass that can be used for energy purposes, as well
as soil amendment, etc.
 Advantages Disadvantages

WILLOW SYSTEMS
● No specific hazard with mosquito breeding ● Has to be coupled with biomass harvesting and use
● Robust against load fluctuations
● Zero emissions of pollutants to the environment
● No recipient or infiltration needed
● Woodchip production

Co-benefits
Biomass Carbon

FA C T S H E E T
High Pollination
production sequestration

Biodiversity Biodiversity Flood Aesthetic

Medium Recreation
(fauna) (flora) mitigation value

Compatibilities with Case Studies

Other NBSs In publication

Can be combined with horizontal flow and vertical flow ● Zero-discharge wastewater facilities: willow systems
wetlands as well as with free water surface wetlands and
ponds for evapotranspiration to take place at the outflow
and produce biomass or to contribute to treatment when
operating as a flow-through system.
Operation and NBS Technical Details

WILLOW SYSTEMS
Maintenance
Regular Type of influent
● Control of primary treatment and plant health ● Primary treated wastewater
inspection (visual) ● Secondary treated wastewater
● 12 hours for regular maintenance per year; additional ● Greywater
15 minutes per 100 m2 for machine harvesting of
willows during the harvesting year Treatment efficiency
● Sludge removal from pretreatment. The emptying
Zero discharge systems have no outflow, resulting in
interval depends on the volume of the tank
overall 100% treatment efficiency. Pollutants such
● Harvesting (half or one-third of system every second
as heavy metals can be stored in the system. The
or third year, respectively)
systems with percolation have the following treatment

FA C T S H E E T
efficiency:
Extraordinary
● Water level inspection in the case of extraordinarily ● COD 92–100%
high precipitation ● BOD5 98–100%
● TN 85–100%
Troubleshooting ● NH4-N 90–100%
● TP ~100%
● Salinity increase after 20 years’ or more operation: ● TSS ~100%
necessary to flush the system through maintenance ● Escherichia coli < 1,000 CFU/100 mL
pipe

Requirements
● Net area requirements: based on water production
use rather than on a pollutant load and is 68-171 m2
for 100 m3 water per year or 30–75 m2 per capita (if
water production is 120 L per capita and day)

Literature ● Electrical consumption: intermittent pumping of

inflow water: 7–10 kWh per capita and year

Brix, H., Arias, C. A. (2011). Use of willows in

Design criteria
evapotranspirative systems for onsite wastewater
management – theory and experiences from Denmark. ● COD and TSS (pollutant load g/m2/day): due to zero
“STREPOW” International Workshop, Novi Sad, discharge willow systems are designed according to
Serbia, February 2011, pp. 15-29. the volume of water to be used (see requirements);
the COD and TSS are not design criteria
Curneen, S. J., Gill, L. W. (2014). A comparison of the ● HLR: depends on willow evapotranspiration rate at
suitability of different willow varieties to treat on-site specific location
wastewater effluent in an Irish climate. Journal of
Environmental Management, 133, 153-161.
Commonly implemented
configurations
● Individual system (most common)
● HF/VF/FWS - willow system
NBS Technical Details

WILLOW SYSTEMS
Climatic conditions
● Suitable for both warm and cold climates; however,
local species and clones of willow must be selected
● In areas with higher evapotranspiration, the surface
area needed can be smaller and vice versa

FA C T S H E E T