Faculty of Science and Forestry
DEVELOPMENT OF SHORT ROTATION WILLOW PLANTATION FOR WATER WASTE PURIFICATION: CASE OUTOKUMPU
Nikolai Evstishenkov
MASTER’S THESIS CROSS BORDER UNIVERSITY JOENSUU 2016
purification. University of Eastern Finland, Faculty of Science and Forestry, School of Forest Sciences. Master’s thesis in Forest Science specialization in Cross-Border University, European Forestry, bioenergy, 33 p.
ABSTRACT
An area of 3000 m2 was established to investigate potentiality of fast-growing willow species (Salix schwerinii) for Nitrogen and Phosphorus retention in pre-treated municipal wastewater, coming from a treatment plant in order to prevent N and P leaching to the local waters and observe the behavior of SRW under northern conditions in Eastern Finland. Plantation was abundantly irrigated with pre-treated wastewater for 3 growing seasons after being cut down, 120 days annually. Presented data shows results for the last 2 seasons. Willows uptake adsorption rate was 60% for N and 50% for P in the year 2013 with 54 kg N and 0.33 kg P adsorbed, respectively. For the year 2014 ability of willows to uptake N and P dropped to 50%
and 41% with 52 kg N and 0.37 kg P. In addition, biomass increment for 3 years in average was evaluated to be 10 t DM. Various metals and nutrients presence in the soil was observed and recorded for further research in the area.
Key words: Wastewater treatment, willow, N, P
Acknowledgements
This thesis work is a part of a Cross-Border University Master degree program (2012 – 2016) which is funded by School of Forest Science, University of Eastern Finland. Therefore, I am highly grateful to the School of Forest Science to help me finish this thesis.
I would like to thank my mail supervisor, great researcher and a nice man Prof. Ari Pappinen for his continuous guidance and support, valuable comments, encouragement, constructive criticisms and giving suggestions and directions during the whole process of this thesis. I would also like to specially thanking him for giving me chance to work with a demandable thesis topic.
I would like to extend my thanks to my co-supervisor, senior researcher Erik Kaipiainen (School of Forest Science, University of Eastern Finland) for his support, patience and help with my work. His vast experience he shared with me about the topic and his guidance during all the writing process were very valuable. Without his help, this work would simply not be possible.
Finally, I would like to thank Harri Lasarov, who is the chief of wastewater purification plant and his daughter Olga Lasarov for their help and support of this work, as well as SYKE organization and Ilona Joensuu for their analyses and data provided in order to complete this work.
Table of Contests
1. INTRODUCTION ... 1
1.1 Introduction to SRC ... 1
1.2 Why to use SRC ... 1
1.3 Effect of Nitrogen and Phosphorus on waters ... 2
1.4 Willow vegetation filter, Swedish example ... 3
1.5 SRC in Finland ... 4
1.6 Economics of SRC ... 5
1.7 The aim of the study ... 5
2. MATERIALS AND METHODS ... 6
2.1 Plantation establishment ... 7
2.2 Planting ... 9
2.3 rrigation and water flow ... 10
2.4 Nitrogen and phosphorus concentrations analyses ... 10
2.5 Soil analyses ... 11
2.6 Stem biomass model ... 12
2.7 Leaves Biomass model ... 12
2.8 Precipitation ... 13
3. RESULTS ... 13
3.1 Amount of processed water pumped to the experimental site ... 13
3.2 Nutrient concentrations in the processed waters ... 14
3.3 Nutrient recovery by willows on the experimental plot ... 17
3.4 Nutrient concentrations in soil. ... 18
3.5 Biomass models. ... 18
3.6 Biomass growth rate. ... 19
4. DISCUSSION AND CONCLUSIONS ... 20
4.1 Irrigation ... 20
4.2 Comparison with other local plantations ... 21
4.3 Nutrient cycle... 22
4.4 Metals in soil ... 23
4.5 Biomass ... 24
4.6 Economics ... 26
4.7 Potential of using SRWC, irrigated with wastewater in the area ... 26
5. REFERENCES ... 28
1. INTRODUCTION 1.1 Introduction to SRC
Short-rotation plantations are intensively managed plantations represented with fast-growing tree species characterized by closely spaced tree plantings and short harvest rotations (1-5 years) with estimated economic life span of the plantation from 20 to 30 years. (Perlack et al.
1986; Nyland 1996; Steinbeck 1999; Christersson 2006; Dickmann 2006; McKay 2011). Mainly this kind of plantations are used for energy purposes and among various fast-growing hardwoods, willow has been widely introduced in the EU countries with a high interest of northern countries, as it presents a high productivity for Nordic conditions. (Mola-Yudego 2010).
Sweden is one of the pioneer countries, which started to implement Short-rotation willow coppice (SRWC) systems into practice and make a deep research with further knowledge expansion in the field of short-rotation forestry. SRWC plantations mainly used for bioenergy production in Sweden. Oil crises in 1970s forced to find alternative sources of energy, replacing fossil fuel. After an extensive research in plant biology and production systems among various fast-growing tree species (Alnus, Betula, Populus, Salix etc.), it has been found that willows are most performing for the Swedish conditions (Siren et al. 1987; Cristersson et al. 1997; Dimitrou & Aronsson 2005). Currently about 16 000 ha are planted with short rotation willow plantations in Sweden, which represents approximately 0.5% of the total arable land in the country (Mola-yudego 2010). Swedish concept of fast-growing willow cultivation for bioenergy purposes was caught up by other EU countries, mostly Poland and the UK, which are widely using large-scale willow plantation nowadays. Moreover, the development of similar systems swooped up by countries far away from Europe like USA and New Zealand (Volk et al. 2004).
1.2 Why to use SRC
One of the reasons why willows became so popular is because they perform a very high growth in their juvenile stage and high evapotranspiration rates. Many of the trees can be
propagated by means of cutting, and new shoots generate after old ones being cut down (Perttu 1999). The plants are usually cut back after the first growing season in winter in order to promote re-sprouting and reduce negative impact on the other agricultural operations and soils. Conventionally, harvesting occurs every 3 to 5 years, but harvesting period may vary depending on a tree growth and costs of harvesting operations, which are high. (Helby et al.
2006). There are two main methods of harvesting willow SRC – harvesting stems with further chipping and direct chipping of stems. Both methods have vantages and drawbacks in their technics, so owner of the plantations choses preferable method according to local conditions (Johansson 1994). SRC is considered as one of the promising means to meet EU targets to increase the amount of renewable energy, and has been identified as a very energy efficient carbon conversion technology to reduce greenhouse gas emissions (Styles et al. 2007).
Besides being a very productive material for bioenergy purposes, SRC in a larger scale could help to meet social and economic targets of other European countries (EU Rural development, CAP reform). Early research shows that willow growth come along with high evapotranspiration rates (Guidi et al. 2007) and significant nitrogen and phosphorus retention rates (Aronsson & Perttu 2001). It has been recorded that willows have a proclivity to selective heavy metals uptake (Landberg & Greger 2002) during their vegetation period, which recognizes the potential of willows to be used for phytoremediation of heavy metals like Cd (Klang-Westin et al. 2003). Moreover, willows can be used as a biofilters for wastewater purification (Mirk et al. 2005), landfill leachate treatment (Duggan 2005), as an absorbent of nutrients from sewage sludge (Perttu 1999), as well as for cleaning polluted drainage waters from agricultural lands (Elowson 1999). Due to regular harvesting periods, pollutants are removed from the plant-soil system and enhancing new biomass growth occurs by addition of nutrients and waters. This multi-functional system aiming at biomass production for energy purposes, simultaneously benefiting environmental services by purifying waters, removing of potentially hazardous compounds and preventing eutrophication of water resources, which is a widespread issue of rivers, lakes, seas and ocean’s coastal lines.
1.3 Effect of Nitrogen and Phosphorus on waters
Negative effects of eutrophication represented with formation of dense bloom of noxious phytoplankton, which reduces water clarity and quality. The consequences affect marine
ecosystems causing limitations of light penetration, plant mortality and oxygen deficiency to support most of the organisms, impair quality of drinking and recreational waters. Over 245 000 sq. km in over 400 near-shore systems have been affected by these negative factors (Diaz
& Rosenberg 2009). One of the main reasons of eutrophication is surplus of nutrients input like Nitrogen and Phosphorus to the water resources. Studies in US show that excessive N and P losses from agricultural fields, suburban and urban settings negatively affecting local waters (Volk 2013). Moreover, agricultural lands are not the only sources of environmental problems concerning N and P release to the nature. Wastewaters from households and industries, as well as water leachate from landfills and other polluted sites have even greater impact.
However, these problems have been recognized earlier and have been partly solved by wastewater treatment plants and constructed wetlands; this is not enough to clean out environmental disturbance caused by human activity (Obarska-Pempkowiak & Gajewska 2003). In the Baltic region, for instance, improvement of wastewater treatment has been pointed at political level for more than a decade, by means of enhancing water quality of the Baltic Sea. In this context, nitrogen was the element of the most concern (Rolff et al. 2008).
Conventional way to achieve good result is to reduce amounts of nitrogen at the treatment plants, but it usually requires significant investments (Bresters et al. 1997).
The usage of wastewater for fertilization or irrigation of different crops has been applied since long ago. In Poland, for instance, large plantations of willows and poplars have been used as recipients for wastewater hundreds years ago (Kowalik & Randerson 1994). Wastewater irrigation for arable and forest land has been also practiced for years in USA (Crites et al. 2000), but during last decades focus has been put on a relatively new crop, which is well suited for removal and recycling of wastewater, i.e. short-rotation willow coppice.
1.4 Willow vegetation filter, Swedish example
Focusing on the wastewater purification ability of willows, there have been experiments done in many European countries, e.g. Estonia, Spain, Sweden, the UK (Kuusemets & Mauring 1995;
Dallemand et al. 2007). Sweden, being a pioneer country in research and application of SRWC plantations have good examples of introducing SRWC systems as a biofilter for pre-treated wastewater purification. In Kågeröt, a town in the southern part of Sweden, pre-treated water from conventional treatment plant is used for irrigation of 11 ha willow plantation. The plant
serves about 1500 people with combination of effluent from a milk powder industry. Total load of the biodegradable organic matter (BOD) on the plant is estimated to be equal to 5000 people. Another example is coming from central Sweden. Enköping, is a town with population about 20 000 people, where similar system has been introduced to reduce nitrogen outflow in an adjacent lake. The nitrogen-rich wastewater, which was pre-treated at the wastewater purification plant, is distributed to 75 hectares willow plantation located close by. The water, delivered to the plantation consists of about 25% of total nitrogen treated at the plant and is stored in the reservoirs during the winter season. In the summer time (May – September), the water is pumped to the SRWC plantation. Willows are irrigated about 120 days a year and watering automatically ceases on rainy days to exclude over flooding and leakage. The system treats nearly 11 tonnes of nitrogen and 0.2 tonnes of phosphorus annually with irrigation volume of 200 000 m3 of wastewater (Dimitriou et al. 2010).
1.5 SRC in Finland
In Finland, however, these methods have not been given much attention, as in Sweden, but interest in SRC plantations for different purposes has arisen in last years. At present, there are only 50 to 100 ha of willow plantations in Finland. To investigate potential of SRWC plantations under Finnish conditions, a three years project has been launched in 2011. The aim of the project was to promote utilization of willows for energy purposes and study positive effects on the environment through water and soil purification by SRWC plantations. During 2011 and 2012, there have been established four plantations, three in Central Finland and one in North Karelia region. Total area of all plantations is about 7.5 ha. One of the biggest plantation with 3.2 ha of planted willows located in Central Finland. It was established at cut-away peat area to investigate applicability of willow cultivation for after use of cut-away peat territory for drainage of waters and possibility for further afforestation of the land. Not far from that place, there is another plantation set on the peat production area of Vapo Oy (biggest peat producer in FInland). The aim of the 1.3 ha plantation is to study purification ability of willows on runoff waters after peat extraction. Total wetland area is about 63 ha. With positive results of willow performance, environmental situation on the land could improve significantly. Another plantation situated not far from the city called Pyhasalmi. Local waters and soils are polluted with processed water from the mining place, and experiment has shown that willows have a good ability for heavy metals uptake. Plantation in Outokumpu, North Karelia region, was
established in spring 2012 next to wastewater purification plant for water quality improvement by willows absorption of nutrients from pre-treated water (Leinonen et al.
2013).
1.6 Economics of SRC
The application of wastewater and sewage sludge has been identified as one of the most promising methods for the achievement of energy and environmental targets together with increasing the income of the SRC operators (Ericsson et al. 2006; Dimitriou and Rosenqvist 2011). The economic attractiveness of willow vegetation filters results from the reduced SRC cultivation costs and lower residue treatment costs comparing to conventional waste utilization technology (Rosenquist et al. 1997; Börjesson and Berndes 2006).
The premises for farmers to adopt perennial crops is only if these crops will have at least the same economical value as with traditional agricultural crops. The relative viability of traditional crops and energy crops depends not only on their production costs but also on crop market prices and general agricultural policy (Ericsson et al. 2009). Considering the possible effects of an increase in cereal prices in the future and of funding schemes for SRC cultivation (stable or missing) a remarkable increase in the area of SRC is probably not to be expected in Europe in the short term. Thus, the replacement of inorganic fertilizers by wastewater or sewage sludge can be critical to further indirect cost reductions in SRC cultivation and an increase in farmers` profit. The economic gains for SRC farmers would be made more attractive if these alternative fertilization applications would be accepted as legal residue treatment technologies and a subsidy system regulated accordingly (Dimitriou and Rosenqvist 2011).
1.7 The aim of the study
The aim of the study is to investigate ability of SRWC plantation to purify pre-treated wastewater coming from the treatment plant and answer the question: can short rotation willow plantations be an effective solution for environmental improvement under Finnish conditions? Observe the amounts of various metals presented in the soils during experimental
time. Evaluate biomass increment of the willow under irrigation with processed wastewater from a treatment plant. Assess the potential for further study.
2. MATERIALS AND METHODS
2.1 Outokumpu wastewater treatment plant description
Outokumpu wastewater treatment plant (in Jokipohja 62.712257, 29.049321) is two lined parallel coagulation plant equipped with powerful pretreatment. The wastewater from the residential and industrial area of the city and runoff waters from landfill (in Jyri) are processed in the plant. The purified water flows through the river Lahdenjoki to Sysmäjärvi lake (Figure 1).
Figure 1. Area of Outokumpu and wastewater treatment plant location.
The wastewater treatment plant has capacity for an organic load from about 10 000 inhabitants and the maximum flow of 3750 m3/d. The plant has been established in 1991 and the last renovation was in 2006. According to valid load monitoring program, that supervises
operation on the plant, 12 samples has to be taken each year. Center for Economic Development, Transport and Environment of North Karelia lead their own investigation and samples are analyzed and reported by Ympäristötutkimus Oy of Savo-Karjala.
2.1Plantation establishment
In 2012 the experimental plot was established next to the plant, close to Lahdenjoki River, and the area of the future plantation was calculated as 3000 m2 (Figure 2). Land was organized by the municipality of Outokumpu and local farmer prepared the area for planting. The ground was plowed and leveled with different machinery equipment (Figure 3). Plastic rows were also stacked with the help of the tractor for weed growth prevention. The pipes were installed after surface has been leveled out and the process water from the plant was pumped to the experimental field, where the water was then distributed evenly with thinner pipe branches.
The overflow water that was not absorbed by the field drifted on the tilted surface of the field towards Lahenjoki to collector ditches made by excavator (Figure 4). These ditches converged into a basin. A V-dam was constructed in the lower corner of that basin (Figure 5). The amount of pumped water was read from a water strider installed to the plant. The amount of the water outflow from the field was measured from the V-dam, at the same time the water samples were taken. The runoff waters around the field were not able to reach the field, but were gathered to collector ditches. These ditches drained from the eastside of the field to Lahenjoki.
Salix schwerinii was chosen as the best clone to establish plantation. It is an easy accessible clone of Salix genotype that was taken from the other experimental plantation of University of Eastern Finland located close by. Samples were cut and prepared at the University’s technical laboratory.
Figure 2. The location of the willow field next to the treatment plant (purple building). The field is marked red. From the lower corner of the field, as marked in the picture, the overflow waters run into Lahenjoki that flows next to the plant. The basins marked in the picture are no longer in use at the treatment plant.
(Photo by Ilona Joensuu)
Figure 3. Picture of the field on 4th June 2012 next to the waste water treatment plant before the willow plantation was established 12th June 2012
(Photo by Ilona Joensuu) Figure 4. Collector ditches next to Lahenjoki river.
(Photo by Ilona Joensuu)
Figure 5. Making of the measuring V-dam. On the picture on the left is Harri Lasarov and on the right Aki Villa. The final assembly of the panels was done with an excavator.
2.2Planting
12th of June cuttings of Salix Schewerinii were planted on the field. Planting was carried out following system similar to Swedish, except that it was done by pressing the cuttings to the soil manually. Seedlings of Salix Schwerinii were planted in pair rows where the row spacing was 70 cm and the spacing between the pair rows was 150 cm. The distance between the
cuttings in the rows was 40 cm. Rows were covered with black plastic to prevent weeds from growing. Planting density of the cuttings was approximately 24 000 seedlings per hectare and length of one seedling was about 20 cm and estimated weight 10 g (Figure 6).
Figure 6. Cutting of Salix Shwerinii 2.3irrigation and water flow
The flow of treated water from the plant was carried out by using automatic systems. Water supplement to the plantation was done with a help of specially adjusted pipe connected to the main stream pipe. Encounter was installed to monitor the water load to the experimental field. At the peak of the growing season (for local conditions is July-August), the water load amounted to 40 m3 per day. At the beginning of the season (for local conditions is May), less water supply occurred, and at the end of season (September) irrigation was as abundant as in the middle of the season. Water flow regulation in the rainy days was carried out manually by workers from the plant, and with a large amount of precipitation, water flow has been ceased.
2.4Nitrogen and phosphorus concentrations analyses
The water samples were taken from the basin below the willow field and from the collector sample bucket once a month. The samples were taken to laboratory of SYKE in Joensuu in a cool box and the experimental methods have been validated to natural, waste and purified waste waters.
PTO-315 stands for the determination of total nitrogen, where the serial number 315 is a reference to a LIMS (Laboratory Information Management System) code. The method is
suitable for the determination of total nitrogen in water, after a pretreatment of potassium peroxide sulfate decomposition (with K2S2O8). The orthophosphate ions together with molybdate and antimony ions form an antimony-phosphomolybdate complex in an acid solution. The ascorbic acid reduces the complex that is formed into a strongly colored molybdenum blue complex. The absorbance of the complex is measured at a wavelength of 800 nm. FIA is the abbreviation for automatic flow injection analyzer.
The PTOT-639 determination refers to the analyzation of total filtered phosphorous that is done before analysis with a (for example Gelman) membrane filter (0,45 μm). Otherwise the determination is done as previously described. The SFS-EN ISO 6878:2004-standard is followed in both analyses.
In the NTOT-323 determination the ammonia, nitrite and other compounds that contain organic nitrogen in the sample are oxidized into nitrate in alkaline conditions buffered by potassium peroxide sulfate, by boiling the sample in pressurized conditions in a closed vessel.
The nitrate is reduced into nitrite by directing the boiled sample through a mixture that contains copperized cadmium. The nitrite that is obtained reacts with 4-amino benzene sulfonamide and n-(1-naphthyl)-1 2-diaminoethane dihydrochloride, which transforms the color of the liquid into pink. Photometric measurement is done at a 520 nm wavelength using the flow injection analyzer. During analysis the SFS-EN ISO 11905-1:1998 standard is followed.
In the NO23N-405G determination the FIA (Flow Injection Analysis) technique is used. The sample is fed through an injection valve into a buffer solution that is in constant flow. The nitrate in the sample is reduced with metallic cadmium into nitrite. Afterwards a constantly flowing phosphoric acid reagent solution is added into the sample. The samples nitrite together with sulphanilamide in an acid solution form a diazonium salt, which reacts with the n-(1-naphthyl) ethylenediamine forming a red pigment. The color is measured spectrometrically at a wavelength of 520 nm.
2.5Soil analyses
Soil samples were taken in the end of each vegetation season. About 30 cm of soil in depth was dug from two sides of the plantation. Thereafter soil was packed and transported to the University’s cooling room. Soil analyses for metal containment were carried out by SYKE organization.
2.6Stem biomass model
There were special stools, selected for biomass model creation on the plantation. Willows with various diameters from smallest to largest (about 30 samples) were cut and de-leaved.
Willow stems were cut into suitable pieces, put in the paper bags and transported to the University laboratory. Cut stems were then put into the oven for drying. For three days samples stayed in the oven with the temperature of 105 degrees to release moist and get absolute dry weight. Thereafter, the dried material has been separately weighed on the laboratory scales and the data for biomass assembly was ready. Formula for DW determination was obtained through SPSS Statistic program and components variables of the formula are presented in Table 1.
DW = a * D^b,
where D – diameter; a and b –parameters of model obtained through statistic program (IBM SPSS statistics ver. 21); R2 – coef. of determination.
Table 1. Components of the calculation of shoots biomass model
Year Parameters of the model Determination
a b R2
2012 0.042 2.846 0.986
2013 0.105 2.595 0.983
2014 0.059 2.861 0.981
2.7Leaves Biomass model
Leaves were collected at the same time from the stems that have been cut, put into the bags and moved to the laboratory. Drying occurred with the same technology as with shoots of the examined trees. After three days drying in the oven, data for the leaves biomass has been ready. Data for leaves biomass was gained only for the year 2014 (Table 2).
Table 2. Components of the calculation of leaves biomass model
Model Parameters of the model Determination
a b c R2
Polynomial 0,17 0,465 0,429 0.963
Power 0,097 0,465 0,941
Two different models functions were used in order to obtain most precise coefficient of determination R2. Polynomal function was used in the final model construction.
2.8Precipitation
Table 3. Amount of precipitation
Month 2013 2014
mm/m2 m3 mm/m2 m3
May 37.3 112 27.7 80
June 70.9 213 96.64 290
July 96.2 289 128.78 386
August 128.5 386 77.31 232
September 84.7 254 22.52 68
Total 380.3 1254 325.25 1056
Detailed daily precipitation data was collected with the help of Viuruniemi meteorological station (Table 3). Final monthly precipitation was obtained by summarizing daily precipitation from the meteorological station report. Amount of rain water was transferred to m3 lately.
3. RESULTS
3.1Amount of processed water pumped to the experimental site
The amount of processed water pumped to the experimental plantation in the year 2013 was 4920 m3. (Table 1). Maximum water load occurred in August and September, which was 1240 m3 and 1200 m3. Considering the amount of precipitation 1254 m3 this year, total amount of water supplement combining processed and rain water was 6174 m3. The water outflow from the field was 2649 m3 with described above water load amounts.
Processed water supply was increased from 4920 m3 to 5400 m3 in 2014. From June to September the water added to the field was 1200 m3 and above (Table 1). Precipitation amount was less than in 2013 and accounted 1056 m3. However, total load of processed and rain waters led to water supplement increase up to 6456 m3 in 2014. Magnification of the amount of water on the experimental site enhanced water outflow from the field and accounted to 2931 m3 correspondingly. As most of the literature presents precipitation data in mm, there are transformed figures in mm in Table 2.
Table 1. Water flow and evaporation for two years from treatment plant, precipitation and water outflow from the plantation in m3.
Year Month PW, m3 RW, m3 W+RW, m3 Outflow m3 Evaporation, m3
2013
May 600 112 712 247 465
June 800 213 1013 413 600
July 1080 289 1369 439 930
August 1240 386 1626 696 930
September 1200 254 1454 854 600
Sum 2013 4920 1254 6174 2649
2014
May 520 80 600 135 465
June 1200 290 1490 890 600
July 1240 386 1626 696 930
August 1240 232 1472 542 930
September 1200 68 1268 668 600
Sum 2014 5400 1056 6456 2931
*PW – processed water, RW – rain water, W+RW – waste + rain water.
Table 2. Water flow and evaporation for two years from treatment plant, precipitation and water outflow from the plantation in mm.
Year Month PW, m3 RW, m3 W+RW, m3 Outflow m3 Evaporation, m3
2013
May 200 37 237 82 155
June 267 71 338 138 200
July 360 96 456 146 310
August 413 129 542 232 310
September 400 87 487 287 200
Sum 2013 1640 420 2060 885 1175
2014
May 173 27 200 45 155
June 400 97 497 297 200
July 413 129 542 232 310
August 413 77 490 180 310
September 400 23 423 223 200
Sum 2014 1799 353 2152 977 1175
*PW – processed water, RW – rain water, W+RW – waste + rain water.
3.2Nutrient concentrations in the processed waters
Nitrogen uptake by willows was varying during the whole year 2013, there was 4.77 kg of N absorbed by the plantation in May and double in June with 9.55 kg (Table 3). Within July and August, a slight decline in uptake of N appeared, but by the end of the year in September, plantation consumed 23.94 kg of N.
Phosphorus had a lower uptake ratio compare to Nitrogen with 0.01 kg in May 2013 (Table 3).
Maximum absorption appeared in June with 0.13 kg of P consumption, but after that it started to decline and in September dropped down back to 0.01 kg.
In 2014 uptake ability of the trees varied more than in 2013. The year started with more Nitrogen uptake, and in May it was 11.6 kg adsorbed, but in June consumed N dropped to 5.12 kg (Table 3). July and August were the most N consuming months, but in September Nitrogen appeared to be released and not absorbed.
For Phosphorus, uptake ratio in 2014 stayed positive with the highest amount of P absorbed in July, which was record for both year with 0.24 kg consumed (Table 3).
Table 3. Monthly N and P uptake by willows.
Nutrient 2013 2014
May June July Aug Sep May June July Aug Sep
N, kg 4.77 9.55 9.41 6.46 23.94 11.6 5.12 16.43 20.64 -1.12
P, kg 0.01 0.13 0.11 0.07 0.01 0.06 0.04 0.24 0.01 0.03
Nitrogen load with processed water varied from month to month and for 2013. The minimum amount of added Nitrogen was 6 kg in May and 1.24 kg left out the field. The maximum of N received by the experimental plantation was 30 kg in September. In spite of big load of Nitrogen, trees absorbed most of it and amount of outflowed Nitrogen was only 6.06 kg this month (Figure 1).
In 2014 the amount of Nitrogen pumped to the experimental plantation was higher than in 2013. In May and June it was about 12 kg a month (Figure 1). However, the trees uptaking of N was high and only 0.36 kg outflowed in May. June was less positive with 7.48 kg outflow.
During July and August amounts of added Nitrogen were several times higher than in first months with 33.48 and 43.40 kg respectively. Notwithstanding high load of N, plantation adsorbed half of the added amounts in July and August (Figure 1).
September appeared to be negative in uptake rate and amount of outflowed Nitrogen exceeded the load with processed water.
Figure 1. Nitrogen concentration in the processed water, coming from the treatment plant (blue) and from the plantation (red).
As with Nitrogen, Phosphorus amounts added to the field varied from month to month. The year 2013 started with lower loads of P in May and accounted 0.06 kg (Figure 2). However, the outflow amount of P was close to added one with 0.05 kg. In the middle of the season P loads increased, but the nutrient outflow stayed more or less the same as in May. In the end of the year, in August and September Phosphorus outflow started to rise up and accounted 0.09 kg and 0.1 kg correspondingly.
In 2014 levels of P loads and outflows were varying much more than in 2013. Amounts of receiving Phosphorus by the experimental plantation in the beginning of the year were similar to the year 2013 with 0.07 kg and 0.11 kg in May and June. Peak of P concentration occurred in July with 0.55 kg this month. However, the uptake of P by willows increased as well and only 0.31 kg of P outflowed that month (Figure 2). In August received and outflowed amounts of Phosphorus were much lower compare to July with 0.09 kg and 0.08 kg respectively, but September was rich for P and 0.34 kg were added to the plantation. This time the uptake ability of the trees was not so high and 0.31 kg leaked from the field.
6.00
17.60 15.12 21.08
30.00
11.96 12.60
33.48
43.40
27.60
1.24
8.05
5.71
14.62 6.06
0.36 7.48
17.05
22.76 28.72
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00
May 2013 June July August September May 2014 June July August September
Nitrogen, kg
Waste water Exit water
Figure 2. Phosphorus concentration in the processed water, coming from the treatment plant (blue) and from the plantation (orange).
3.3Nutrient recovery by willows on the experimental plot
In 2013 there was 89.80 kg of Nitrogen released with the processed water to the experimental plantation (Table 4). Absorbed N accounting 54.13 kg which is 60% of total N load. The amount of Phosphorus received by plantation was 0.66 kg with 0.33 kg absorbed. Uptake percentage was 50% from total P supplement.
In 2014 amount of N load increased by 30% from 89.80 kg to 129.04 kg respectively. The ability of the plants to absorb N dropped down to 52.66 kg. With the amount of added N it resulted in 41% of total N uptake. Quantity of Phosphorus was raised almost twice as well and accounted to 1.15 kg against 0.66 kg in 2013 (Table 4). There was more P absorbed by the trees in 2014, but the total uptake percentage dropped down to 32% due to high amounts of P added to the experimental plot.
Table 4. Nitrogen and Phosphorus retention for two growing seasons
Nitrogen kg Phosphorus kg
year 2013 2014 2013 2014
Pumped 89.80 129.04 0.66 1.15
Outflowed 35.67 76.38 0.33 0.78
Absorbed 54.13 52.66 0.33 0.37
Uptake % 60 41 50 32
0.06
0.17 0.16 0.16
0.11 0.07 0.11
0.55
0.09
0.34
0.05 0.04
0.05 0.09 0.10 0.01
0.07 0.31
0.08 0.31
0.00 0.10 0.20 0.30 0.40 0.50 0.60
2013May June July August September May
2014 June July August September
Phosphorus, kg
Waste water Exit water
3.4Nutrient concentrations in soil.
Concentration of Nitrogen were taken in percentage rate, compare to other nutrients, which were taken in g/kg (Table 5). N concentration varied from year to year, but it is clearly seen that the percentage drops down from 0.31% in 2012 to 0.25% in 2014. Phosphorus concentration appeared to be very unstable varying from 1.95 g/kg in 2012 to 0.61g/kg in 2013 and raising up to 2.39 g/kg in 2014 (Table 5). K and Na concentrations are also behaving differently. Gaining concentrations after 2012, they meet a decline in the year 2014. Ca and Mg concentrations constantly declined after the establishment year in 2012 (Table 5).
Table 5. Metal concentrations in soils.
Element/year 2012 2013 2014
N % 0.31 0.23 0.25
g/kg g/kg g/kg
Ca 2.52 2.34 2.11
K 2.07 2.71 2.23
Mg 5.02 5.52 4.42
Na 0.12 0.31 0.26
P 1.95 0.61 2.39
3.5Biomass models.
Trend line of the calculation has been chosen according to higher R2 value. In case of stem biomass power trend line was used for model creation (Figure 3).
Figure 3. Stem biomass model created through SPSS Statistic program.
R² = 0.9808
0 500 1000 1500 2000 2500 3000 3500 4000
0 10 20 30 40 50
Dry Weight, g
Diameter, D
Biomass model 2014
DW = 0,286*D^2.396
For leaves biomass calculations, polynomial trend line has been used due to higher R2 value and as following more precise model data obtainment (Figure 4).
Figure 4. Leaves biomass model created through SPSS Statistic program.
3.6Biomass growth rate.
Each year trees are gaining more biomass and growth increment is rising rapidly. The establishment year 2012 shows that the average mass of one tree was 41.2 g and in the year 2014 the mass gained 1447.7 g (Table 6).
Table 6. Experimental plantation characteristics.
Year 2012 2013 2014
Average mass of one tree (shoots), g 41.2 368.2 1447.5
Average mass of one tree (leaves), g 14.21 75.2 211.17
Biomass in t DM/ha 0.98 8.47 30.4
Number of shoots per plant 2.1 1.6 1.27
Mortality, % 3.2 4.1 11.5
Density alive plants/Ha 24000 23000 21400
The same with the leaves mass correspondingly, from 14.21 g in the beginning of experiment to 211.17 g in the end. Total biomass on the plantation was estimated as almost 1 t DM/ha in 2012 and raised up to 30.4 t DM/ha in 2014.
The highest mortality rate occurred in 2014 and reached 11.5% from total density on the plantation after 3.2% in 2012 and 4.1% in 2013. That resulted in reduction of plants density from 24000 plants/ha to 21400 plants/ha (Table 6).
y = 0.17x2- 0.465x + 0.4285 R² = 0.9631
0 20 40 60 80 100 120 140 160 180
0 5 10 15 20 25 30 35
Dry weight, g
Diameter, cm
From the growth increment graph (Figure 5) average biomass, that can be collected after one generation (3 years old) is calculated to be 10 t DM/ha.
Figure 5. Willow growth increment for 3 years from 2012 to 2014.
4. DISCUSSION AND CONCLUSIONS
This thesis offers the research in evaluation of effectiveness of willow short rotation plantation to purify pre-treated wastewater from a treatment plant in Outokumpu, Eastern Finland, in combination of various metals presence analyzation in the soils and biomass increment estimation of the experimental plantation. The topic is not new and there have been research made in Poland, Sweden, the UK and other countries (Mola-Yudego et al. 2015). However, for Finland it is one of the first steps to investigate the potential of short-rotation willow plantations for bioenergy purposes in combination of natural conditions improvement.
4.1 Irrigation
Water supply on the plantation was extremely high. Daily irrigation with treated water was approximately 40 m3/ha day during peak of growing season July – August 2013. With the presented value of water supply N and P adsorption was 60% and 50% respectively. During the year 2014 daily water irrigation increased in June – July compare to the previous year, which did not result in significant adsorption decrees for June. However, starting from July 2014 N adsorption by willows drops down to 41% for N and 32% for P, when compare to the
0.98
8.47
30.4
0 5 10 15 20 25 30 35 40
2012 2013 2014
tDM/Ha
Willow growth increment
year 2013. Dimitrtiou 2005 concluded several factors that should be carefully considered when SRWC is irrigated with wastewater to achieve sustainability and prevent leaching of N.
Planting material plays a key role and has to be tolerant to the local condition in order to produce high yields of biomass and resist pest attacks. Moreover, species irrigated with high loads of wastewater should have high evapotranspiration rates. The more evapotranspiration rate is – the more water is filtered by the trees and the less risk to hazardous leaching occurs.
According to Hasselgren 1998, irrigation of SRWC plantation with pre-treated wastewater from a treatment plant can reduce amounts of N by 75-95% and P by 95-96%. Possible reason for lower uptake by willows was high amount of treated water added to the plantation.
Conventionally, in such kind of systems, water supply to the SRWC fields ceases at rainy days (Dimitriou et. al 2010), but in Outokumpu treatment plant water addition has not been stopped.
Dimitriou 2005 mentions a big importance of precipitation when considering levels of wastewater irrigation. Precipitation amounts for both years were high and total water load exceeded the physical ability of willow to uptake nutrients. Moreover, amounts of N and other nutrients in the rainwaters were not calculated and considered as nutrient-free. Water supply of the plantation was carried out manually. Due to this, human factor plays key role in nutrient delivery to the plantation. It is not possible to trace if the irrigation has been done daily with the same amount of water.
Even if Swedish conditions supposed to be closest to Finnish due to geographical proximity, plantation in Outokumpu is the most northern SRWC plantation with severe natural conditions compare to other plantations. The growing period for willows at the experimental plot is presumably shorter than other plantations have. However, water supply was provided according to standard Nordic conditions with approximately 120 days. That explains high N losses in outflow waters in the end of the vegetation period in September.
4.2 Comparison with other local plantations
Five plantations under different conditions are located around Joensuu, North Karelia region.
Presented values of growth increment are valid for Salix schwerinii clones (Figure 1).
Plantations in Viinijarvi and Eno are established to prevent waterlogging and swamping of agricultural fields and transport systems. Papelonsaari SRWC plantation aiming to stop water runoff with fertilizers from local agricultural fields. Siikasalmi plantation is a genetic reserve
and was established for breeding planting material for future plantation. It is considered as a control plot.
In compare to other plantations, the one in Outokumpu has much more favorable conditions for biomass formation and enhancement. That means irrigation with wastewater is the best for willows growth increment under local conditions.
Figure 1. Willow growth increment for the first 3-year-old generation plantations in North Karelia (unpublished data).
4.3 Nutrient cycle
Many factors are playing an important role in the development of willow plantation. Some authors consider temperature and precipitation as the most important factors (Pertuu 1983, 1999), when other studies put emphasis on soil type as an addition to climate conditions (Aylott et al. 2008). Fertilization is one of the main steps to achieve high yields in SRWC growing cycle. It has been mentioned before by Dimitriou and Rutz 2015, that in a 3-years rotation, willow plantation requires 150 – 400 kg of N and 24 – 40 kg of P per hectare. For Irish conditions, it is recommended to fertilize SRWC with 120 – 150 kg of N (Caslin et al. 2010). In the UK, recent recommendations suggest to use 3 kg of N per 1 t DM, which is rather low parallel to Swedish and Irish conditions. Average amount of N added in Outokumpu was 354 kg/ha (Table 1), which is rather high, but still under the rage of requirement for SRC (Dimitriou and Rutz 2015). Amounts of P, however, were much lower with only 3.6 kg of N per ha.
0 5 10 15 20 25 30 35
Outokumpu Papelonsaari Viinijarvi Siikasalmi Eno 2012 2013 2014
Table 1. Amounts of N and P added to the experimental plot.
Year Nutrients added, kg/ha
N P
2013 286 2,8
2014 422 4,4
Average 354 3,6
The fertilization response of willows was studied before and results vary from case to case.
Investigation of various clones in different environments is needed for better understanding of nutrient cycle and exclude nature pollution. Labrecque and Teodorescu 2001 presented increased yields of Salix viminalis, (which is very similar to Salix schwerinii, used in Outokumpu) of 65–77 kg biomass kg N−1 applied with 150 kg available N ha−1 in a sewage sludge. In the other study, the same authors Labrecque and Teodorescu 2003 reported 185 kg biomass kg N−1 of fertilization response on a sandy soils after N application of 100 kg ha−1. The same trial on a clay soils showed 84 kg biomass kg N−1 applied. Adegbidi et al. 2003 found much higher fertilization response – about 250 kg biomass kg N−1 when applying just 42 kg N ha−1 in form of composted poultry manure to Salix dasyclados. Studying in Estonia, Heinsoo et al. 2002 found a fertilization response of on average 52 kg biomass kg N−1 following a aggregated N load of 450 kg N ha−1 over 4 years. Alriksson et al. 1997 found a wide variation in fertilization responses depending on the year in which the fertilizer was applied. Nitrogen applied during year 2 after harvest increased the yield by 51 kg biomass kg N−1, whereas nitrogen added during year 3 showed only 28 kg biomass kg N−1. No any attempts to predict the fertilization response of willow for a certain site based on soil properties have been seen so far.
In several studies (Aronsson et al. 2000, 2010), it has been reported that applying 160 kg N ha−1 may have a negative consequences when treating wastewater or landfill leachate by irrigation of SRWC. Those studies report SRWC, being very efficient in retaining N, but continuous application of N doses should be followed by monitoring of groundwater or drainage water in order to assess impact on water quality.
4.4 Metals in soil
Recent study about phytoremediation of heavy metals and other nutrients by Salix express a high potential of Salix schwerinii species to uptake metals from landfill soils in southern
Finland. On 16 different experimental plots, willow species were analyzed and soil inspection concluded that with addition of 40 – 70% of peat soil at pH 6 to polluted soils willow perform higher root system development and enhance of growth. Besides promising expectations, the study was made during one growing season and for more precise data some extra research is needed. (Mir Md Abdus 2016).
In present study, metals were observed in order to obtain data for further research in that direction. So far it is just the numbers stated and for now analyzes show the differ of metals presence when comparing between the years. For instance, amounts of N, Ca and Mg are decreasing with the years and, on the other hand, K, Na and P concentrations in the soil appeared to be rising.
Several studies concluding that willows have high phytoremediation potential for various heavy metals like Pb (Zhivotovsky et al.2011).and Cd (Klang-Westin et al. 2003). Also Jensen 2008 reports that on moderately polluted soils, Salix spp. show promising results for Cd and Zn uptake in Denmark.
There are premises to expand the research and investigate metals remediation by willows in Outokumpu.
4.5 Biomass
Under presented conditions, willows at the experimental plot performed 10 t DM/ha in average for the first 3 years growing cycle, which is above average results in Europe. (Mola- Yudego et al. 2015). However, these promising results are not always the same when it comes to large-scale commercial plantations. Zavitkovsky 1981 discovered overestimations due to the edge effects of small plots and Hansen 1991 found that yields could be 4 to 7 times higher at small plots compare to average biomass production from large plantations. Recent research of Searle and Malins 2014, reaffirm previous concerns, although overestimations are more conservative when it comes to a yields result from large measured areas.
The general estimations for the Eastern Europe and Baltic countries according to Fischer et al.
(2005) vary between 13.8 to 18.1 t DM/ha in the most suitable conditions, and from 7.3–8.4 t DM/ha in the moderately suitable conditions. Dam et al. (2008), predicted yields estimations around 9.05, 10.13 and 9.71 t DM/ha for Estonia, Lithuania and Latvia, respectively, in very suitable conditions, and a bit less than 5 t DM/ha in all three countries in moderately suitable land. Heinsoo et al. 2002 reports that the results from experimental plantations were up to 10
t DM/ha per yr, on the high quality soils with proper management practices, and 6 t DM/ha per yr, in the medium quality soils in Estonia. In Eastern Finland, the production ranges were estimated from 6 to 9 t DM/ha yr (Regional Energy Agency of Eastern Finland 2004).
Nevertheless, the estimates produced by the models in the different scenarios mentioned above are similar to the average measures in commercially managed plantations. For instance, the measured productivity from plantations managed by local farmers in Finland vary between 0.37 and 8.35 t DM/ha yr (Tahvanainen and Rytkönen 1999) for the first harvest. However, productivity of commercial plantations in Finland and Sweden with less than 2 t DM/ha yr were 56% and 44% respectively, for the first cutting cycle and Finnish sample consisted of only 16 plantations (Tahvanainen and Rytkönen 1999, Mola-Yudego and Aronsson 2008). On the other hand, the average productivity for the first cutting cycle at commercial level has been estimated to be around 7–8 DM/ha yr in Denmark (Venendaal et al. 1997).
Conventionally, first harvest of SRW occurs in winter after establishment year in order to gain more shoot from re-sprouting and to fortify the root system. However, in Outokumpu first cut carried out only after 3 years. As a result, some of the trees feel down, due to weak root system which could not support rapid growth of willows.
Figure 2. Willow SRC plantation in Outokumpu, August 2014. (Photo by Erik Kaipiainen)
Productivity varying widely from country to country and from site to site. It is difficult to say that the same numbers, which other authors reported, will occur at Outokumpu area in commercial scale. Further research is need in order to obtain more reliable data and premises for future development of the study.
4.6 Economics
In this study, costs of establishment and cultivation of the willow plantation are not calculated.
However, Rosenquist et al. 1997, 2007 and Volk et al. 2006 concluded that usage of municipal wastewaters can benefit economics compare to conventional wastewater purification by chemicals at treatment plants. In Sweden, the nitrogen treatment cost could be 3-6 euros lower per kg N in vegetation filters, compared to conventional wastewater treatment, which is approximately 10 euros per kg N. Cultivation costs could be reduced by 1.2-1.8 euros per GJ biomass, according to cost reduction for fertilization and biomass yields enhance. This is equivalent to 30-50 % of cultivation costs in conventional plantations. Despite many benefits of using willow vegetation filters, potential barriers against their large-scale implementation still exist. Some of these are, for instance, due to lack of knowledge about pathogen spreading risks. (Petersen J.E. 2007).
4.7 Potential of using SRWC, irrigated with wastewater in the area
Average calculations for both years show that for utilizing all the wastewater from the treatment plant in Outokumpu, the area of 43 ha will be needed (Table 2). For utilization of all year water load, a Swedish model with storage ponds could be made as Dimitriou 2011 shows. Treated water after purification at the plant will be stored at the ponds off growing season and at rainy days during vegetation period. Construction of these extra facilities will increase overall costs for plantation establishment. Nevertheless, there are some abandoned facilities for storing wastewater located next to the treatment plant. It can be beneficial to use these trenches and reduce constructing cost. Utilization of all nitrogen coming from the plant will require a twice bigger area of 78 ha in average under presented adsorption rates and water supply, Moreover, even larger area of 116 ha will be needed to employ all the phosphorus (Table 2).
Table 2. Area required for utilization all wastewater, N and P, under presented values.
Year Total water,
m3 Area needed for all treated water utilization, N and P purification, ha
water N P
2013 738350 45 76 91
2014 723269 41 101 127
Average 730810 43 78 116
Presented area is rather big in comparison to Swedish example in Enkoping, where 75 ha plantation is used for utilizing municipal wastewater from a double number of inhabitants.
Possible solution for narrowing the area is increasing the amounts of N and P delivered to the plantation. Average amount of P adsorbed by experimental plot was 1.8 kg with biomass increment of 10 t DM (Table 2). Theoretical P applications to SRC by Dimirtiou 2005 are based on the assumption that 0.8 kg of P comprises in 1 t DM in order to be equal with crop demand.
Following the Swedish concept, P value can be increased up to 8 kg for the experimental plot.
Conclusively, plantation was highly oversaturated with pre-treated water, but even with such abundant irrigation, willows perform high potential to be used as a vegetation filter for N and P retention. More detailed strategy has to be planned in future research of the topic. Based on the obtained data there should be a very clear plan developed with implementation of strategies used in similar researches earlier in other countries.
Various metals concentrations were observed in soils after application of treated wastewater, but no much investigation has been done in that direction. Presence of nutrients like N, Ca, K, and Mg declined with years, but concentrations of Na and P increased in numbers by the year 2014. Gained data can be used as premises for further phytoremediation of soil research in combinations with wastewater irrigation of SRWC in Outokumpu.
There is no doubt that irrigation of SRWC with pre-treated wastewater have a positive effect on biomass production increment. Comparably to the other plantations in the area, utilization of wastewater is the best way to maximize biomass growth.
There were no calculations of economical aspect of SRWC application in such kind of systems for Finland, but based on the Swedish experience it is reasonable to assume that with more detailed research it is possible to achieve benefits from using SRWC simultaneously for wastewater treatment and biomass production.
Plantation in Outokumpu is one of the most northern short rotation willow plantations in the world. Even with all the provided data within this topic in the past by other countries e.g.
Sweden and Canada, it is still hard to obtain precise data for the specific region like North Karelia, Finland. The pioneer research has started with this work, hopefully further investigation of the potentials and abilities of SRWC will help to achieve knowledge for environmental improvement in severe north conditions.
5. REFERENCES
Adegbidi, H.G., Briggs, R.D., Volk, T.A., White, E.H., Abrahamson, L.P. (2003). Effect of organic amendments and slow-release nitrogen fertilizer on willow biomass production and soil chemical characteristics. Biomass Bioenergy 25:389–398.
Alriksson, B., Ledin, S., Seeger, P. (1997). Effect of nitrogen fertilization on growth in a Salix viminalis stand using a response surface experimental design. Scand J For Res 12(4):321–327.
Aronsson P, Dahlin T, Dimitriou I (2010) Treatment of landfill leachate by irrigation of willow coppice — plant response and treatment efficiency. Environmental Pollution 158:795–804 Aronsson, P., Perttu, K. (2001). Willow Vegetation Filters for Wastewater Treatment and Soil remediation combined with Biomass Production. The forestry chronicle, 77: 293-299.
Aronsson, P.G., Bergström, L.F., Elowsson, S.N.E. (2000). Long-term influence of intensively cultured short-rotation willow coppice on nitrogen concentrations in groundwater.
Environmental Pollution 58:135–145.
Aylott, M.J., Casella, E., Tubby, I., Street, N.R., Smith, P., Taylor, G. (2008). Yield and spatial supply of bioenergy poplar and willow short rotatuion coppice in the UK. New Phytologyst, 178, 358-370.
Börjesson, P., Berndes, G. 2006. The prospects for willow plantations for wastewater treatment in Sweden. Biomass and Bioenergy 30, 428-438.
Bresters, A.R., Coulomb, I., Matter, B. (1997). Sludge treatment and disposal. Environmental Issues Series no. 7.
Caslin B, Finnan J, Mc Cracken A (2010) Short rotation willow coppice guidelines. Teagasc ISBN 1-84170-568-3.
Christersson, L. (2006). Short-rotation forestry—a complement to “conventional” forestry.
Unasylva (0041–6436), 57 (223), p. 34.
Christersson, L., Sennerby-Forsse, L. 1994. The Swedish programme for intensive short- rotation forests. Biomass and Energy, Vol. 6, No. 1/2, pp. 145-149.
Crites, R.W., Reed, S.C., Bastian R.K (2000). Land treatment systems for Municipal and Industrial wastes. McGraw-Hill, New York.
Dallemand, J.F., Petersen, J.E., Karp, A. (2007). Short rotation forestry, short rotation coppice and perennial grasses in the European Union: Agro-environmental aspects, present use and perspectives.
Diaz, R.J., Rosenberg, R. (2009). Spreading dead zones and consequences for marine ecosystems. Science 321:926-928.
Dickmann, D.I. (2006). Silviculture and biology of short-rotation woody crops in temperate regions: then and now. Biomass Bioenergy 30(8):696–705.
Dimitriou, I. (2005). Performance and sustainability of short rotation energy crops treated with municipal and industrial residuces. Doctoral thesis. Uppsala, Sweden.
Dimitriou, I., Aronsson, P. (2005). Willows for energy and phytoremediation in Sweden.
Unasylva 221(56); 46-50.
Dimitriou, I., Rosenqvist, H. (2011). Sewage sludge and wastewater fertilization of Short Rotation Coppice (SRC) for increased bioenergy production – Biological and economic potential. Biomass and bioenergy 35. 835-842.
Dimitriou, I., Rutz, D. (2015). Sustainable Short Rotation Coppice. A Handbook. Pp 29-41.
Dimitrou, I., Aronsson, P., Mola-Yuego, B., Weih, M. (2011). Willow short-rotation coppice for energy and environmental benefits in Sweden. Swedish University of Agricultural Sciences.
Uppsala, Sweden.
Duggan, j. (2005). The potential for landfill leachate treatment using willows in the UK – A critical review. Resources, Conservation and Recycling 45: 97-113.
Elowson, S. (1999). Willow as a Vegetation Filter for Cleaning of Polluted Drainage Water from Agricultural Land. Biomass & bioenergy 16: 281-290.
Ericsson, K., Rosenqvist, H., Ganko, E., Pisarek, M., Nilsson, L. 2006. Anagro-economic analysis of willow cultivation in Poland. Biomass and Bioenergy 60, 16-27.
Ericsson, K., Rosenqvist, H., Nilsson, L.J. 2009. Energy crop production costs in the EU. Biomass and Bioenergy 33 (11), 1577-1586.
Fischer, G., Prieler, S., Velthuizen, H. (2005). Biomass potentials of miscanthus, willow and poplar: results and policy implications for Eastern Europe, Northern and Central Asia. Biomass and Bioenergy 28: 119–132.
Guidi, W., Piccioni, E., Bonari, E. (2007). Evapotranspiration and crop coefficient of poplar and willow short-rotation coppice used as a vegetation filter. Bioresource Technology (2008) 4832- 4840.
Hansen, E.A. (1991). Poplar woody biomass yields. A look to the future. Biomass and Bioenergy 1-7.
Hasselgren, K. (1998). Use of Municipal Waste Products In Energy Forestry: Highlights From 15 Years of Experience. Biomass and Bioenergy. Vol. 15, No. 1, pp. 71-74.
Heinsoo, K., Sild, E., Koppel, A. (2002). Estimation of shoot biomass productivity in Estonian Salix plantations. For Ecological Management 170(1–3):67–74.
Helby, P., Rosenquist, H., Roos, A. (2006). Retreat from Salix – Swedish experience with evergy crops in the 1990s. Biomass and Bioenergy 30:422-427
Johansson, H. (1994). Economic calculations on harvesting capacity and costs. In: Harvesting technics for energy forestry. Swedish University of Agriculture, pp 27-28.
Klang-Westin, E., Eriksson, J. (2003). Potential of Salix as a Phytoextractor for Cd and Moderately Contaminated Soils. Plant and soil 249: 127-137.
Kowalik, P.L., Randerson, P.F. (1994). Nitrogen and Phosphorus removal by willow stands irrigated with municipal wastewater – a review of the Polish experience. Biomass and bioenergy, 6, 133-139.
Kuusemets, V., Mauring, T. (1995). Wastewater purification in willow plantation. The case study at Aarike. Centre of Ecological Engeneering. Tartu. Estonia.
Labrecque, M., Teodorescu, T. (2001). Influence of plantation site and wastewater sludge fertilization on the performance and foliar nutrient status of two willow species grown under SRIC in southern Quebec (Canada). For Ecol Manag 150(3):223–239.
