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Short-rotation Willow Biomass Plantations Irrigated and Fertilised with Wastewaters

Summary

This report summarises results and experiences gathered from field trials with recycling of pre-treated wastewater, diverted human urine mixed with water, and municipal sludge, within plantations of willow species specifically selected for biomass production. Experimental sites were established in Sweden (Roma), France (Orchies), Northern Ireland (Culmore) and Greece (Larissa). The project was carried out during a 4-year period with financial support from the EU FAIR Programme.

The experimental sites were supplied with primary effluent from municipal treatment plants (Culmore and Larissa), stored industrial effluent from a chicory processing plant (Orchies), biologically treated and stored municipal wastewater (Roma) and human urine mixture from diverting low-flush toilets mixed with water (Roma). Application rates of the wastewaters or the urine mixture were equivalent to the calculated evapotranspiration rate at each site. Wastewaters were also applied up to three times this value to evaluate any possible negative effects.

Estimations and evaluations were carried out mainly concerning: biomass growth, potential biological attacks of the plantations, plant water requirements, fertilisation effects of the wastewater, plant uptake of nutrients and heavy metals from applied wastewater, possible soil or groundwater impact, sanitary aspects, and potentials for removal in the soil-plant filter of nutrients and biodegradable organic material from applied wastewater.

The results clearly indicated that biomass production in young willow plantations could be enhanced substantially after recycling of wastewater resources. The impact on soil and groundwater of nutrients (nitrogen and phosphorus) and heavy metals (copper, zinc, lead and cadmium) was limited, even when the application of water and nutrients exceeded the plant requirements. Also, the soil-plant system seemed to function as a natural treatment filter for pre-treated (primary settled) wastewater, with a treatment rate fully comparable to a tertiary effluent quality with regard to biodegradable organic material and eutrophying nutrients (nitrogen and phosphorus).

Introductory analyses of the costs of a wastewater irrigated willow plantation for bio-fuel production indicate that the benefits of the wastewater treatment per se appear to be greater than the benefits from the increased production of wood chips. The risks of contamination via faecal micro-organisms of animals and humans seem possible to reduce or eliminate if proper precautions are taken. The awareness of the hygienic aspects is among the most important issues to deal with concerning the public acceptance.

The gathered opinion from the members of the multidisciplinary project team is that the concept of recycling wastewater or fractions of wastewater within willow plantations for combined energy production and wastewater treatment would be worth developing on a wider scale. Experiences from a few full-scale facilities in Sweden are well in accordance with the findings outlined here. The fact that wastewater could be treated at reasonable costs might encourage the municipal sector as well as the energy and agricultural industry in Europe to further expand the concept with increased willow plantation areas as a consequence. This would increase the opportunities for an over all better environment for generations to come.