The use of plantations for wastewater treatment, is a technology that has been known
through several years. However, only few wastewater works exist in Denmark which have
planted willow (estimated 30-60 systems). The concepts vary, but they all have in common
that by normal operation they are free from drainage. In Tappernøje by
"Pilehuset", a shared living complex - there exists as far as is known, the
eldest willow bed plantation in Denmark. It was established during the winter of 1991/92.
The shared living complex is a part of the institution for handicapped,
"Marjatta", whose pedagogical fundament builds on the philosophy and principles
of Rudolf Steiner.
There is a necessity to establish a greater knowledge about how these plantations are
constructed, how they work, and not least on how they must be dimensioned. This is the
background for the project which the county of Storstrøm initiated, and which is one of
the projects under the Action Plan for promotion of ecological urban renewal and
wastewater treatment, supported by the National Agency of Environmental Protection.
The following will contain a summary of the project results. This includes a
description of the plantations’ construction and operation, its evaporating capacity
and capability. The investigation has furthermore involved the condition and growth of the
willow plants, as the application and the transformation of fertilisation, as well as
substances foreign to the environment, among others heavy metals.
On the background of the results from the investigation, it is accessed that the
plantation operates very well. The plantation has been dimensioned sufficiently large
enough to ensure that the applied amounts of wastewater from 11 PE as well as rain, has
been able to evaporate from it again. Overflow has therefore not been the case, and the
plantation is assessed to be leak-tight.
The projected area was 578 m2, but the area within the waterproof
geomembrane (from the middle of each bank) has been measured to be 707 m2. The
layers above the plastic membrane consists of 10 cm’s of sand, 10 cm’s of pebble
gravel, 70-85 cm’s of raw soil by the inlet well and the outlet well comparatively,
and topping this app. 30 cm’s of humus. The raw soil and the distributing layer (the
layer of pebble gravel) are separated by a fibre-textile-membrane. The inclination of
every side is about 45 degrees. The cubic content has been calculated to 690 m3
above the fibre textile, and 150 m3 concerning the sand- and pebble gravel
layer (the distributing layer). The plantation is enclosed against the surrounding soil by
a 0,75 mm membrane.
The wastewater is conducted to the plantation and distributed throughout the bottom of
the plantation by gravitation. It has thereby been possible to avoid establishing a pump
well and a pump.
Problems with freezing over of the distributing system do not occur by the deep
underground distribution. Another advantage is that one avoids an eventual risk of
contamination and possible obnoxious smells, which may occur by distribution at the
surface.
It has only once been necessary to flush the distribution pipe, and that was after the
plantation had been in operation for 7 years. The blockage is attributed operational
problems with the sand filter well, which was rebuilt to a final settling well by the
project start. No roots have been found in the distribution system. The final settling
well operates according to its purpose, as it catches up sludge discharged from the house
tank.
It has been ascertained that floating sludge is developed in the second chamber of the
settling tank already about ½ year after the usual draining of the tank. It is therefore
recommended to drain the settling tank twice every year. It has not been considered to be
a problem that the settling tank successive to draining is refilled with drainage water
(reject water) from the vacuum slurry tanker. If desired, the final settling well can be
filled with clean water, instead of the reject water, thereby reducing the risk of
alluvial deposit in the distribution pipe. In having this practise of draining the sludge,
there is nothing to hinder the tank from entering the municipal collection system.
One fourth of the willow area is harvested every year. Change of practise is
considered, and to harvest one third of the area every year. The tenants of the shared
living complex do this job during a weekend. What is harvested is used to make wickerwork,
or woodchips to be employed as corrective in the flowerbeds around "Pilehuset".
The current water level in the plantation is the result of the application of
wastewater to the bottom of the system, and the fall of rain on the surface as well as
evaporation from the ground. During the growth period however, the water level is
influenced especially as due to the transpiration of the willows. The chart development of
the water level follows the same pattern year after year. The water level raises
significantly and decreases just as significantly in the fall and in the spring
comparatively, but does not alter much during midwinter. Anneke Stubsgaard, DHI (2001),
has observed the same pattern in other willow plantations over the country.
On sounding the water level in the inspection wells connected to the distribution pipe,
one must be attentive of the characteristics, which follows the course of the year. The
soundings are most suitably used to compare the maximum water level in the work from year
to year - not for telling the variations over a shorter period of time.
A willow plantation must be dimensioned in a way, where the area consisting of willow
is adequately large enough to help the applied quantities of water to evaporate.
Furthermore, the plantation must also be able to hold those quantities of water applied
outside the growth season, where the evaporation is minimal.
The evaporation capacity of the plantation has been calculated on basis of the amount
of applied wastewater and rain per m², and corresponds to the actual evaporation from the
plantation.
Having the period 1992-1996 as the basis of calculation, the actual evaporation from
the plantation can be calculated to 1,31 m³/m² or 1310 mm, as the annual amount of
wastewater amounted to 0,52 m³/m² (365 m³) and rain amounted to 0,79 m³/m² (555 m³).
Having the period of 1999 as the basis of calculation, the actual evaporation has been
calculated to 1370 mm, and meanwhile, when looking at a 30-year average of the
precipitation and the present application of wastewater, the evaporation can only be
measures to 1030 mm per m². The plantation has therefore plenty of evaporation capacity.
The evaporation from willow plantations used for wastewater treatment is considerably
higher than what one would usually expect from ordinary willow plantations. This concerns
the circumstance that the willow coppice in the plantations always have plenty of access
to water, and that the plantations generally are small and oblong. This means that the
situation is a very large border effect, which also increases evaporation due to the large
influence from the sun and the wind, creating a kind of "oasis-effect".
The plantation’s theoretical capacity as calculated on the basis of volume and
pore space, constitutes 245 m³ per year. This number corresponds to the amount of water,
which the plantation must be able to hold during the wintertime. The application of
wastewater and rain during the period November - Marts, subtracting the potential
evaporation, constitutes 253 m³ (30-year average). The maximum height of the water level
in the winters of 1998 and 1999 though, was 40 cm under the terrain. This indicates that
the period where no evaporation takes place (transpires) via the willows, in reality is
more brief.
Under all circumstances it important that a plantation is dimensioned on the background
of the factual circumstances, concerning the number of people and the actual amount of
produced wastewater in the household, as well as up to date data concerning the
precipitation of the area.
By inspection of the plantation, the willows have generally come through as being
healthy and growing well. The leaves are fresh and green and show no sign of disease. Firm
weed control during the years of establishing the plants has had the consequence that the
willow has been able to establish itself very well.
The life span of the willow can be expected to be about 25-30 years, as seen from the
experiences with pruning of the willow and from plantations with short-rotation willow
coppice (high-yielding). After this there must be replanted. Of consideration to the
evaporation capacity of the plantation, the replanting must take place over a span of
several years.
After the harvest in January 2000, it was found that some of the willow shoots were
attacked by mould, and some had perished (about 5-10% of the willow). This is attributed
to the harvest procedure, which will therefore be altered. On the harvested area weed
control should take place, especially in order to secure better growth for the replanted
small willow cuttings which have been replaced instead of those that have perished.
The root growths of the willow seem, from a qualitative assessment, to follow the
change of the water level during the year.
As the willow only sets very fine roots in the plantation, and as the cambium is
separated from the distributing layer by fibre textile, it is assessed that there is no
risk that the roots will penetrate the plastic membrane or go out into the distribution
layer.
The production of 1999 (4-year old shoots on 8-year old roots) was determined to 8,5
tonnes/hectare/year. The production resembles the average production of 7,5
tonnes/hectare/year, as Morsing et. Al. (1995) has found to be the cases in a number of
Danish willow plantations, though variation has been great (from 1,8-22,6
tonnes/hectare/year).
The production, however, is below the average production of 13 tonnes/hectare/year,
which has been measured on willow lots in the FAIR project (Larsson 2001). On this basis
it is not possible to ascertain if a decrease has happened in the productivity on the
Marjatta plantation since its establishment.
Far greater amounts of nutrients are applied to the willow plantation, than is
withdrawn. In the project only the applied amounts of compounds and that which is in the
soil, has been measured. The standards of the metabolic process are theoretical.
From analysis of the applied wastewater, the application of nitrogen to the plantation
has been calculated to 512 kg N/hectare. Withdrawal matters (leafs and branches) have been
calculated to 171,5 tonnes N per hectare.
Concerning a surplus of nitrogen and organic matter, the microbiological processes in
the plantation will cause the compounds to decompose.
The results of the soil constituent analysis indicate that an increase of phosphor has
been the case since the establishment of the plantation in 1991/92. This is without
problems to the growth of the willow, and gives the soil greater nutritional value, if the
plantation one day is terminated, and the soil eventually spread out on agricultural land.
There is no sign that the content of potassium in the ground has been increased. An
increase hereof would nevertheless have no immediate consequences
to the growth of the willow.
As water is never discharged from the willow plantation, the conveyed heavy metals
would either be absorbed by the willow or accumulated in the willow plantation. On the
other hand, the organic environmental hazardous matter is assessed to decompose in the
plantation by microbiological processes in an environment that changes between the aerobic
and the anaerobic.
There is a difference concerning the extent to which the different clones absorb heavy
metals. This depends partly of the concentration of heavy metals in the vegetation.
Measurement results from Peder Gregersen (personal announcement) and Anneke Stubsgaard
(2001), indicate that there is no immediate risk that the amount of heavy metals in the
foliage is as much as to be hazardous to prevent the use of the harvested wood chips for
soil cover in the garden, or to burn in the furnace.
The theoretically applied heavy metals to the willow plantation have been calculated on
the grounds of the content of heavy metals in wastewater from the household. On the
background of these calculations, an accumulation of heavy metals is assessed not to
shorten the life span of the willow plantation. In this context, the relatively large size
of the willow plantation is important, compared to other types of wastewater works, as
root zone systems and sand filter systems. Even though there are outlets from these
systems, one can imagine that the accumulation of heavy metals of these are greater than
in willow plantations, simply because their volume are considerably smaller.
All in all, it is assessed that there is a need for further investigations concerning
the destiny of heavy metals in the willow plantations.
The concentration of salt in the plantation can become the limitation factor, which
determines the life span of the plantation. It is therefore of great importance to limit
the application of salt to the system. The source is the use of salt in the household.
The plantation should be projected in a way to have a limited amount of water
containing concentrated amounts of sodium chloride drained from the system. This must
naturally be done during late summer, where the content of water is at its lowest, and the
concentration of salt at its highest.
In cultivation programmes today, the focus is especially on the cultivation of clones
for ordinary willow coppice plantations, where the purpose is to achieve as large a
production as possible, compared to accessible resources of nutrients and water.
Concerning the cultivation of clones destined for wastewater treatment plantations,
high-demanding varieties can be a good solution, as there is access to plenty of water and
nutrients. Furthermore, it should be considered if it really is an advantage that the
chosen clones don’t process a high quantum of heavy metals. All this depending on
whether one prefers to accumulate these in the ground or in the foliage. Finally there is
the advantage of developing clones containing a high tolerance of salt.
It is open to discussion, how often the plantation users must control the wells/water
level and so on. During the first couple of years after the establishment of the
plantation, it is a good idea with frequent control, ex. every month. In older plantations
it should be sufficient with a sounding 4 times a year.
The soundings should then be repeated with a day or two in-between, as there appears to
be some flux of the water level in the wells on the distribution pipe, because of ex.
temporary fluctuations in the application of wastewater, and temporary stoppages in the
system.
It is primarily the maximum water level of winter, which is interesting. Partly as
control for overflow, or any risk hereof, and partly to assess whether the plantation is
leak-tight, or if the evaporation capacity changes over the years.
It is sufficient to read the water consumption 1-2 times a year.
The price of the wastewater treatment plantation in 1992 was about 92.300 Kr. + taxes.
The Entrepreneur considers the price to be 20-25% higher today. The clones were at that
time purchased for about 2 Kr. per stk. + taxes. Today they are considerably cheaper.
The prices concerning the remaining plantations in the country vary a lot, among other
reasons because of price differences in excavation work.
Working on developing the construction of the plantations, with the intention of
reducing their size, and thereby also their costs, is important. In this context it is
also important to have a closer look at the possibilities of reducing precipitation on the
plantation.
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