Pär Aronsson

Economics of using municipal wastewater irrigation of willow coppice crops

Abstract Until now, treatment of municipal wastewater in Sweden has mainly consisted of removal of phosphorus and easily biodegradable organic substances. Most of the nitrogen has been released in the water phase to the recipients. The recently introduced legal requirements to remove also nitrogen have encouraged the recirculation approach, meaning that, before chemical phosphorus precipitation, municipal wastewater can be used as an adequate nutrient resource for biomass production. The general aim of this paper is to illustrate a realistic way of handling and utilising this resource in a recirculation system, applying a combination of waste water treatment and biomass production. The economic calculations (based on nitrogen, which is a topical problem at the treatment plants) show that the costs for a conventional technical system for removal of nitrogen and phosphorus in wastewater (70–180 SEK (kgN) −1 ) (1 USD = 6.70 SEK) can allow a good margin for investment, operation and management of an irrigation system for wastewater treatment in willow cultivation. The calculations show that treatment costs in willow cultivations vary depending on type of system and size. Nitrogen application rate also has a major impact on costs.

Nitrate leaching from lysimeter-grown short-rotation willow coppice in relation to N-application, irrigation and soil type

The use of willow vegetation /lters for wastewater treatment is gaining interest in Sweden. A study was conducted in order to determine reasonable rates of wastewater application to such vegetation /lters. NO 3-N leaching loads and N-uptake by the plants were quanti/ed in relation to nitrogen application rate, irrigation rate, shoot growth, and soil type during three years after planting of willow (Salixviminalis ) in sixteen 1200-l lysimeters /lled with clay or sand soil. Nitrogen was applied at rates corresponding to 110 –244 kg N=ha yr combined with irrigation of 3–6 mm=day during 100 –110 days each growing season. NO3-N leaching loads were very high the /rst year after plant establishment (on average 341 kg N =ha from clay and 140 kg N=ha from sand lysimeters). However, NO3-N leaching loads decreased and were low or negligible during the second (43 from clay and 17 kg N=ha from sand lysimeters) and third year (3 kg N=ha from clay and less than 1 kg N=ha from sand lysimeters). Harvest of the plants after the second growing season did not seemingly a8ect NO3-N leaching loads during the third year. Soil type and N application rate strongly in9uenced NO3-N leaching loads, whereas irrigation rate had only a slight e8ect, and thus, dosing of wastewater should be based on N-amounts. Assuming an annual shoot growth of 10 tonnes (dry weight)=ha in a willow vegetation /lter, wastewater could be applied at rates corresponding to 160 –190 kg N =ha without substantial NO3-N leaching losses. c 2001 Elsevier Science Ltd. All rights reserved.

Environmental assessment of energy production based on long term commercial willow plantations in Sweden.

The present paper analyzed the environmental assessment of short rotation willow plantations in Sweden based on the standard framework of Life Cycle Assessment (LCA) from the International Standards Organisation. The analysis is focused on two alternative management regimes for willow plantations dedicated to biomass production for energy purposes. The data used included the averages of a large sample of commercial plantations. One of the scenarios is carried out under nitrogen based fertilized conditions and the other under non-fertilized management with total biomass yields (dry weight) of 140t/ha and 86t/ha over a 21 and 22-year life time respectively. The environmental profile was analyzed in terms of the potentials for abiotic depletion, acidification, eutrophication, global warming, ozone layer depletion, photochemical oxidant formation, human toxicity, fresh water aquatic ecotoxicity, marine aquatic ecotoxicity and terrestrial ecotoxicity. In addition, an energy analysis was performed using the cumulative energy demand method (CED). The application of nitrogen based fertilizers allows an increase in the biomass yield per ha of up to 40% although the contributions to almost all impact categories, particularly the eutrophication potential and toxicity potential impact categories are also considerably higher. Conversely, due to the higher biomass yields achieved with fertilization of these willow plantations, that regime presents a better overall environmental profile in terms of energy yield and global warming potential.

Impact of Willow Short Rotation Coppice on Water Quality

Short rotation coppice (SRC) with willow has been grown in Sweden from the late 1980s to produce biomass for energy on agricultural land. This study evaluated the effects of SRC on water quality by determining differences in leaching of nitrogen and phosphorus to groundwater of a number of commercial “old” SRC willow stands in Sweden compared to adjacent arable fields grown with “ordinary” crops. The study was conducted in 16 locations under three vegetation seasons. NO3–N leaching from willow SRC fields was significantly lower than that from reference fields with cereals. The opposite was observed for PO4–P; concentrations in the groundwater of SRC were higher compared to reference fields. Sewage sludge applications were not responsible for the elevated PO4–P leaching under SRC compared to reference crops.

Transport and retention of bacteriophages in two types of willow-cropped lysimeters.

Abstract Irrigation and fertilization of short‐rotation willow coppice with wastewater is a new way of reusing wastewater in Sweden. To evaluate the possible impact of viruses on groundwater quality, the transport and retention of the bacteriophage Salmonella Typhimurium type 28B were studied in two types of willow‐cropped field lysimeters containing clay or sand soil. Phages were applied to the soil surface and moderate irrigation was done daily under field‐like conditions. In the clay, soil rapid transport of bacteriophages was recorded with breakthrough at 1,2‐m depth after 2–24 hours indicating macropore flow through the soil. Phage transport through the sand soil varied considerably, but was in general much slower and the phage retention much higher compared with the clay soil. The willow plants were not found to facilitate phage leaching. Instead, the results indicate the presence of phage retaining processes in the rhizosphere.

Treatment of landfill leachate by irrigation of willow coppice – Plant response and treatment efficiency

Landfill leachates usually need to be treated before discharged, and using soil-plant systems for this has gained substantial interest in Sweden and in the UK. A three-year field study was conducted in central Sweden to quantify plant response, treatment efficiency and impact on groundwater quality of landfill leachate irrigation of short-rotation willow coppice (Salix). Two willow varieties were tested and four irrigation regimes in sixteen 400-m2 plots. The willow plants did not react negatively, despite very high annual loads of nitrogen (<or=2160 kg N/ha), chloride (<or=8600 kg Cl/ha) and other elements. Mean annual growth was 1.5, 9.8 and 12.6 tonnes DM/ha during years 1-3. For one of two willow varieties tested, relative leaf length accurately predicted growth rate. Irrigation resulted in elevated groundwater concentrations of all elements applied. Treatment efficiency varied considerably for different elements, but was adequate when moderate loads were applied.

Spatial variation in above-ground growth in unevenly wastewater-irrigated willow Salix viminalis plantations

Abstract In several Swedish wastewater treatment plants, an improved level of water treatment has been achieved through irrigation of short-rotation willow ( Salix viminalis ) coppice (SRWC) with secondary or tertiary treated wastewater. The spatial variation in above-ground growth in two unevenly wastewater-irrigated SRWC fields in southernmost Sweden was measured during 3 consecutive years. Wastewater was distributed on the soil surface through pipes with emitters spaced at distances varying from 10 to 21 m. During the first year of irrigation, a very marked gradient in growth developed, with stools (i.e. resprouted stubs) standing within 2 m of wastewater emitters growing several times faster than those standing 5 m or more away. This situation prevailed after three growing seasons with wastewater irrigation, indicating that any redistribution of plant roots towards the emitters was insufficient for a spatial levelling of the growth. We conclude that, in order to promote growth and fully utilize the treatment potential of SRWC, wastewater should be applied fairly evenly, i.e. through emitters at spacings of less than 5 m.

Large carbon-sink potential by Kyoto forests in Sweden – a case study on willow plantations.

Fluxes of CO2 were measured in a 75-ha short-rotation willow plantation at Enkôping, central Sweden. The plantation was irrigated with wastewater for fertilization and water-filtering purposes. The harvested biomass was used locally for combined heat and power production. The plantation was a sink of ca. 8 tonnes C ha-1 during 2003, of which ca. 50% was estimated to be attributed to fertilization. Biomass increment by shoot growth was 5 tonnes C ha-1 during the same year. Belowground carbon allocation was estimated to 3 tonnes C ha-1 yr-1 by a model that relates carbon allocation to shoot growth. Thus, the ecosystem carbon balance was closed by these estimations. The carbon uptake by the willow plantation was 5.5 times as high compared to a normally managed spruce forest, but only half as high as from an experimental, well-managed willow plantation in the same region. This illustrates the vast potential of short-rotation willow plantations for CO2 uptake from the atmosphere.

Dynamics of nitrate leaching and 15N turnover in intensively fertilized and irrigated basket willow grown in lysimeters

Abstract A 13-month experiment was conducted to study the dynamics of nitrate leaching from basket willow ( Salix viminalis ), and the fate of fertilizer-N applied after the growing season. Lysimeters with or without plants were supplied daily with water and liquid fertilizer. Nitrate leaching and transpiration were quantified, as well as leaf-, stem- and root growth and N-uptake. 15 N-labelled fertilizer was applied in autumn after leaf fall for the determination of the fate of fertilizer-N. The N-uptake by the willow plants was high during the summer and was correlated with transpiration. It decreased drastically in autumn in the absence of transpiration, but was still significant, which was also the case in early spring, the following year. A leaf rust infection during autumn led to a 9-times increase in nitrate leaching, whereas transpiration was reduced by 26%. After a fertilizer application for 10 days after leaf fall in autumn, an average of 8% of the fertilizer-N was taken up by the plants, and 32% was unaccounted for, of which a large portion was likely to be lost through denitrification. Recalculated on a hectare basis, the whole-season nitrogen budget shows that 191 kg N / ha was applied as liquid fertilizer during the first season, of which 98 kg N / ha was taken up by plants, 69 kg N / ha was leached, and 24 kg N / ha was unaccounted for (i.e. possibly denitrified). The amount of nitrogen taken up by plants or lost from the system (possibly through denitrification) corresponded to 36 kg N / tonnes of dry stem wood produced.

Soil resistivity monitoring of an irrigation experiment

Resistivity measurements were used for tracing water transport during a three-year irrigation study. Three different rates of landfill leachate irrigation and one control treatment were applied to two cultivars of short-rotation willow coppice. Groundwater level measurements and water sampling were carried out in pipes installed in the centre of each plot. Resistivity was measured with permanently installed electrodes along six lines running through the centre of the plots. The resistivity results were inverted to produce vertical sections of ground resistivity at different time steps and as change in resistivity relative to the start of the experiment. Changes in resistivity linked to differences in irrigation quantities and plant growth were observed. The results showed that a repeated soil resistivity measurement has the potential as a tool to monitor changes in soil water and ion contents. Furthermore, expanding zones of increasing soil resistivity immediately under and around the plants indicate that the method may be useful for imaging plant root development.