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Short-rotation Willow Biomass Plantations Irrigated and Fertilised with Wastewaters

Conclusions and recommendations

The project has resulted in a quantity of information on wastewater use and treatment in willow biomass plantations. The following conclusions and recommendations have been extracted from our findings.
Wastewater, urine mixture or sludge could stimulate wood biomass production (willow stem growth) substantially and could replace conventional fertilisers to a large extent. In general, growth levels after a first 3-year rotation were higher compared with non-irrigated and pure water irrigated plants, as well as those reported from commercial willow plantations in Sweden.

The biomass production increased to some extent with increased wastewater application rates, especially at Larissa. This was likely a result of increased fertilisation rather than increased water supply. Plants supplied with the human urine mixture developed best and showed the highest growth. Although attacks from leaf beetles and rust were observed on some of the plantations, the effects on the biomass production seemed relatively limited.

The water requirement of willow plantations was assessed as being close to the theoretical potential evapotranspiration (PE) rate, independent of site location. Wastewater application up to three times the PE was proven to increase biomass growth to some extent, especially at Larissa, where the climate normally supports high evapotranspiration. However, with regard to economising with water and nutrient resources as well as minimisation of the operational costs, the total water application rates should not exceed the expected evapotranspiration rate to a considerable extent.
The nutrient content of the wastewaters (N, P and K) varied due to their origin and pre-treatment level. The primary municipal effluent at Larissa and the stored industrial effluent at Orchies were fairly well balanced but resulted in higher applications of nutrients than the total requirement. The primary municipal effluent at Culmore contained N and K amounts similar to the plant requirement but almost 10 times the level of P required. At Roma the biologically treated and stored municipal effluent had N and P contents similar to the stem uptake, while K was in excess. The urine mixture contained a fairly balanced nutrient solution although the P content was low compared with the long-term plant requirement.
The annual uptake of nutrients in willow stems at the different sites varied within 18-73 kg N/ha, 3-9 kg P/ha and 6-27 kg K/ha. Average P/N- and K/N-ratios of analysed stems were low at Larissa indicating that some disturbances in plant nutrition may have occurred. At Roma, N concentrations of stems were low, probably as a consequence of the markedly low N content in the applied wastewater. At Culmore, the plants seemed fairly well supplied with nutrients.
Application of heavy metals (Cu, Zn, Cd and Pb) with the wastewaters exceeded, in general, plant uptake rates except for Zn at the sites at Roma and Culmore. No obvious correlations were found between plant uptake of metals and soil or wastewater content of metals. In a long-term perspective, added metals should balance with metals removed by the crop (harvested stem wood after defoliation) in purpose to avoid accumulation of metals in the soil. Source control of applied wastewater and selection of willow clones with abilities to assimilate specific metals are possible methods of meeting the "balance criteria".
A general pattern was that the groundwater quality was not specifically affected by the various treatments compared with the controls. At Larissa and Culmore, the concentrations of total N and NO3 in superficial groundwater appeared higher after low than high application rates. This was likely caused by better prerequisites for denitrification in treatments with higher hydraulic loads resulting in temporary anoxic conditions.

Metal concentrations in groundwater beneath the urine treatment were lower compared to other treatments at Roma including the control treatment. This could probably be explained by the relatively low concentrations in the applied urine mixture.

Calculations of wastewater treatment effects using a mass balance technique over the willow-soil systems at Culmore resulted in removal rates of BOD, total N and total P within 67-74 %, 52-75 % and 90-98 %, respectively. In general, the highest removal rates were found in the most loaded systems. The results clearly indicate that wastewater purification of primary effluent in willow plantations could be substantial and fully comparable to tertiary effluent qualities.

A hydraulic load up to three times the evapotranspiration rate from the system did not influence wastewater treatment capacities as exemplified with data from Culmore. Thus, managing a system with wastewater irrigation according to water and nutrient requirements of a willow biomass plantation seems possible without negative environmental impacts with regard to oxygen demanding substances and eutrophying components.

The potential transmission routes for pathogens to animals and humans after wastewater irrigation are via aerosols (depending on the type of irrigation equipment installed), transportation via the groundwater zone to production wells or transportation via surface run off to surface waters. A general recommendation concerning elimination of the aerosol problem is to install equipment based on solid-set sprinklers operating at low pressures with a short throwing distance or pipes laid on the ground and equipped with drip emitters. The risk of contamination of surface waters increases with high hydraulic loads and low soil permeability in combination with the actual slope of the terrain.

The risk of groundwater contamination primarily depends on the soil permeability and/or the distance to the saturated groundwater zone or actual production wells. In this study was found that the number of indicator microorganisms in superficial groundwater indicated a contamination risk to the groundwater at the Culmore site. Probable explanations could be that the unsaturated zone was shallow (varied between 0.5 m and 2.5 m) and that the soil profile was relatively permeable (sandy loam). The concentration of indicators increased with the wastewater application rates.
Economic calculations regarding willow growing in Northern Ireland show that 9 tonnes of biomass (dry matter basis) per hectare is required to achieve a positive income from the land. The outcome from a willow plantation could be competitive with grassland-based enterprises. In connection with wastewater treatment, the possibilities of reducing costs, compared to conventional nutrient removal from wastewater, is more important than the reduced costs for the farmer in savings from purchasing fertiliser. Factors for keeping costs low in a willow bio-filter system are high concentrations and applications rates of nutrients, an extended irrigation season (since fixed costs are much higher than running costs), short pumping distances and large areas of willow plantations to be irrigated.
Recycling of resources and production of environmentally sound biofuels are integral parts of sustainable development and thus are promoted by the political society. Nevertheless, the public acceptance of the use of wastewater, urine or sludge in willow biomass systems, as in other projects with possible environmental impact, is most important and must be treated with the same consideration as anything else in the realisation phase of a project. Any fear the public may feel has to be met with openness and confidence.

In most European countries the environmental laws include obligatory parts where any plaintiff or citizen with appropriate authority has the right of objecting before any permission or licence can be accepted and approved. In Sweden, some facilities with wastewater irrigation of willow plantations have been in operation since 1997. Before approval, the Swedish environmental authorities required the municipalities to carry out risk assessments on the possibility of infectious diseases spread, to inform the public via posters at the site, and to use ground based irrigation equipment for elimination of aerosols. No complaints from the citizens or others have been reported from the Swedish sites.

An irrigation pipe lateral placed in a double-row of a willow plantation. (Photo: Stig Larsson)

 

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