Short Rotation Plantations

The reuse of pre-treated domestic wastewater or sewage sludge for irrigation and fertilisation of energy crops in short rotation plantations is a new approach, which aims at using the nutrients contained in waste residues for an enhanced biomass growth.

The name short rotation plantation (SRP) is used, since usually fast growing tree species such as willows and poplars are planted, which are harvested after relatively short periods. The harvesting periods usually take between 2 to 8 years, but in the case of herbaceous plants or grasses it can be also annually (SUSANA 2009). The cultivation intensity, the high nutrient uptake and the frequent harvests require constant irrigation and fertilisation. By irrigating with (pre-treated) nutrient-rich wastewater, fertiliser costs are reduced, plant growth is enhanced, and waste products are subjected to a more sustainable treatment.

While constructed wetlands focus mainly on wastewater treatment and are sealed at their base for groundwater protection, the advantage of SRPs over constructed wetlands lies in the combined wastewater treatment and the production of wooden biomass, which means an additional income for farmers. A SRP represents an open-bottom fixed-bed reactor of a construction height of between 1.0 and 1.5 m resulting in an effective reduction of pathogens (SUSANA 2009).

In order to avoid nutrient overload, the wastewater application has to follow a dosing recommendation depending on the site and plant species. In addition, the nitrate content has to be measured by soil samples or by sampling from drainage channels.

The following substrates can be applied on SRPs

• Domestic wastewater (which contains nutrients in ratios that are close to the nutrient needs of short-rotation-plantations),
• Sewage sludge originating from domestic wastewater,
• Industrial wastewater from food processing or beverage industries, and
• Diverted human urine

Wastewater is usually applied on SRPs by means of surface irrigation in order to avoid aerosol formation and spread of pathogens by air.

Besides the above-mentioned benefits, there are also a number of critical points to consider:

• Groundwater pollution (from nitrate, pathogens and toxic substances especially if industrial wastewater is applied) might occur and needs to be prevented
• The increase in soil salinity resulting from the irrigation with wastewater (containing salts like e.g. sodium chloride and hydrocarbonates) might become a problem

To ensure environmental and hygienic safety, wastewater and sewage sludge must be pre-treated before being used in SRPs.

In the EU the agricultural application is regulated through the Nitrates Directive (91/676/EEC), Sewage Sludge Directive (86/278/EEC), and other national specifications defining maximum application loads for nutrients and chemical pollutants.

Pre-treatment further reduces pathogenic risks and allows compliance with local legislation for soil and water quality.