Abstract
This paper presents initial results of an investigation of the potential of using PAS 100 graded compost and Compost-Like Output (CLO) as a nutrient source for short rotation coppice energy crops. Initial analysis of the PAS 100 compost and CLO measured; electrical conductivity, pH, total nitrogen and total heavy metal content. Pot trials using both PAS 100 compost and CLO were then used to assess the growth rates of Willow (Salix viminalis) at application rates equivalent to 1500 and 3000 kg N/ ha mixed with an inert growing medium (perlite). A plant image analysis program was designed as a quick, simple and non-destructive plant growth estimation method to be compared to manual measurements.
Initial results showed higher total metal content in the CLO than PAS 100 compost, with both showing electrical conductivity levels of over 2.0 mS/cm. Excess soluble salts thought to be the cause of reduced initial growth of Salix viminalis. There are however indications of re-growth after leaching of the excess salts from the main body of compost.
Keywords
Recycled organic waste, PAS 100 compost, compost-like outputs, soil amendments, short rotation coppice.
Introduction
The European Landfill Directive (1999/31/EC) was introduced to reduce the dependence on landfills for the disposal of waste and to drive waste up the waste hierarchy by reducing, re-using and recycling materials. In particular as part of the Landfill Directive, strict limits have been introduced to control the amount of biodegradable municipal waste reaching landfills and by 2020 quantities of biodegradable municipal waste must be reduced to 35% of the levels reaching landfill in 1995 (Stretton-Maycock and Merrington, 2009).
The diversion of biodegradable waste from landfills results in the beneficial reduction in generation of methane from the decomposing organic material in anaerobic conditions. To enable this diversion local waste disposal authorities encourage source segregation of recyclable waste by households as well as biodegradable waste suitable for anaerobic digestion and energy production (CIWM, 2010; Environment Agency, 2010a). As an additional measure mixed Municipal Solid Waste (MSW) is sent to Mechanical Biological Treatment (MBT) plants to mechanically sort and segregate waste before biologically treating biodegradable material (Merrington et al. 2010).
The source and pre-treatment of waste streams determines the characteristics and future potential uses of the Recycled Organic Wastes (ROW). PAS 100 compost and Compost-Like Output (CLO) have received increased commercial interest as MBT by-products (WRAP, 2008; Stretton-Maycock and Merrington, 2009). Compost created from source segregated organic waste streams can be commercially sold as PAS 100 compost under a British Standards certification scheme to demonstrate a minimum quality. In contrast, CLO is the biodegradable fraction from mixed MSW and has the potential to contain heavy metals, glass and plastics (Merrington et al. 2010); currently there is no standard specification for CLO so its characteristics can vary widely. CLO use is currently limited to a temporary landfill cover, or habitat restoration of landfill sites if it can be demonstrated that it brings ecological benefit to the restoration.
The uses of ROWs as soil amendments has two main benefits; 1) Use as a plant nutrient source, 2) as a soil conditioner to improve the soil structure (Britt et al. 2002). Source segregated ROWs which pass the PAS 100 Standard can be applied to land as an agricultural or horticultural nutrient source. The use of CLO however is restricted to brownfield sites only by England and Wales Environment Agency because of the potential heavy metal content from the MSW (Environment Agency, 2011). It has been suggested that CLO can be used on brownfield sites as a fertile soil amendment to aid the growth of short rotation coppice crops (Bardos et al. 2007; Chapman 2007).
Short rotation coppice has been identified as a suitable renewable biomass for use as a low carbon fuel to produce renewable heat and /or electricity. This is current of high relevance as the UK is committed to producing 20% of electricity from renewable fuel sources under the Climate Change Act 2008 (Environment Agency, 2010b). To be economically effective short rotation coppice crops require fast growing species to produce high yields every 2 to 4 years (Tubby and Armstrong, 2002).
The plantation of short rotation coppice crops on brownfield sites with ROW amendments to improve the growing medium and ensure sufficient nutrients are available for rapid growth has been identified as being potentially beneficial (Bardos et al. 2007). In particular ROWs are a significant source of nitrogen which is one of the most common limiting factors to the growth of plants (Madrid et al. 2007; Fircks et al. 2001). It should be noted that fertiliser application to agricultural land is restricted to 250 kg N/ ha due to the potential leaching risks associated with soluble nitrates and this restricts the usage of ROW on a site-by-site basis. Furthermore whilst ROW total nitrogen content typically ranges between 1.1% and 2.1% (Merrington et al. 2009ab) it is present mainly in organic forms which are less soluble because organic nitrogen must be mineralised by bacteria to become available to plants as nitrate or ammonium (Amlinger et al. 2003).
Currently the Environment Agency grant bespoke permits for the application of CLO to land for scientific trials (Environment Agency, 2011). In such cases the heavy metal, nitrogen and electrical conductivity content of the CLO is often the main limiting factor in restricting the use of CLO to brownfield sites because of evidence from previous studies that repeated CLO application to soil results in the build up of heavy metals in soil (Ayari et al. 2010; Madrid et al. 2007; Smith, 2009; Lee et al. 2004).
This paper presents an investigation into the potential use of two ROW products as nutrient sources for Willow (Salix viminalis) to be grown as energy crops in the UK. PAS 100 compost and CLO was analysed to compare the material characteristics. Pot trials are used to assess the ability for S. viminalis to be grown at the equivalent of 1500 and 3000 kg N/ ha of CLO and 3000 kg N/ ha of PAS 100 compost to control pots.
