«Dimambro ME, Lillywhite RD & Rahn CR Warwick HRI, University of Warwick, Wellesbourne, Warwick, CV35 9EF Corresponding author: ...»
Biodegradable municipal waste
composts: analysis and application to
Dimambro ME, Lillywhite RD & Rahn CR
Warwick HRI, University of Warwick,
Wellesbourne, Warwick, CV35 9EF
Corresponding author: email@example.com
Tel: 024 7657 5060
This project was primarily funded by The Onyx Environmental Trust under the Landfill Tax
Credit Scheme with additional contributions from the Department for Environment, Food and Rural Affairs (Defra).
Throughout the project, the authors were assisted by a number of individuals who raised issues and gave valuable feedback. In particular, the authors would like to thank the
Mary Turner (Warwick HRI) for help and advice with compost and soil analysis.
Rodney Edmondson (Warwick HRI) for undertaking statistical analysis Ralph Noble (Warwick HRI) for helpful discussion on composts.
Warwick HRI Horticultural Services for help with the field trial.
The compost suppliers for providing us with compost for the study.
Direct laboratories, Wolverhampton for the microbial and heavy metal analysis.
Warwick HRI analytical laboratory for nutrient analysis.
1.1 Composting of biodegradable municipal waste in the UK
1.3 The UK compost market
1.4 Compost characteristics
1.5 Project rationale
1.6 Project aims
2. MATERIALS AND METHODS
2.1 Compost acquisition
2.2 Compost analysis
2.3 Field trials
3. RESULTS AND DISCUSSION OF COMPOST ANALYSIS
3.1 Physical and chemical characterisation of batch 1 composts
3.1.1 pH and conductivity
3.1.2 Bulk density and moisture content
3.1.3 Particle size distribution
3.1.4 Physical contaminants
3.1.5 Water soluble carbohydrates, cellulose, lignin, organic matter and ash............. 22 3.1.6 Carbon and nitrogen content and the C:N ratio
3.1.8 Potentially toxic elements
3.2 Physical and chemical characterisation of batch 2 composts
3.2.1 pH & conductivity
3.2.2 Bulk density and moisture content
3.2.3 Particle size distribution
3.2.4 Physical contaminants
3.2.5 Water soluble carbohydrates, cellulose, lignin, organic matter and ash............. 32 3.2.6 Carbon and nitrogen content and the C:N ratio
3.2.8 Potentially toxic elements
3.3.1 Comparison of composts
3.4 Compost analysis summary
4 RESULTS AND DISCUSSION OF THE FIELD TRIAL
4.1 Soil analysis
4.1.1 Prior to compost application (9th March 2005)
4.1.2 Prior to top dressing (5th May 2005)
4.1.3 Post-harvest soil samples (8th August 2005)
4.2 Plant analysis
4.2.1 Plant samples prior to top dressing (10th May 2005)
4.2.2 Plant samples at harvest (4th August 2005)
4.3 Potentially toxic elements in soil and plant
4.4 Field trial summary
SUMMARYThe composting of organic wastes in the UK is driven by the need to reduce the quantity of waste entering landfill. Current UK production of compost is 2 million tonnes (Mt) per annum, which may need to increase to 15 Mt by 2020 in order for the UK to comply with the EU Landfill Directive. The majority of compost in the UK is produced using green waste. However, other forms of biodegradable municipal waste (BMW), including food waste, paper and card, must also be considered for composting, in order to meet the stringent targets of the Landfill Directive.
Compost is used in gardening, horticulture, landscaping, and land restoration. As compost production increases, these markets will become saturated. The addition of green waste composts to agricultural land as a soil conditioner has been emerging as a potential end use for composts.
The overall aim of the project is both to demonstrate agricultural benefit of spreading composted BMW to land, and to highlight potential problems.
This study compared and contrasted 12 composts produced from components of BMW. Ten composts contained 100% source segregated BMW and were produced from a variety of wastes including green, fruit, vegetable and kitchen waste, paper and cardboard. One compost was 72% mixed municipal solid waste (MSW) plus 18% source segregated BMW, and one compost was 100% mixed MSW. Composts were analysed for pH, electrical conductivity, carbohydrates, nutrients and contaminants. In general, compost pH was above neutral. Composts containing kitchen or meat wastes had higher nitrogen contents than composts produced from purely green and vegetable wastes. The two MSW composts contained high levels of physical contaminants (glass, plastic and metal) compared to the 10 source segregated BMW composts. The 100% MSW compost had significantly higher levels of potentially toxic elements (PTEs or heavy metals) than the other composts.
In order to ascertain the efficacy of composts as agricultural soil conditioners, a fully replicated field trial was established at Warwick HRI. Four source segregated BMW composts and one mixed MSW compost were incorporated into agricultural soil to examine the effect, either beneficial or harmful, on a response crop of spring barley. The composts were applied according to their nitrogen content at two rates of 250 and 500 kg N ha-1. The third treatment was 250 kg N ha-1 compost plus 125 kg N ha-1 inorganic nitrogen fertiliser. The control was no compost or fertiliser. The nutrient supply, organic matter and PTE content of the soils were assessed before and after compost application. The harvested grain was analysed to investigate any effect of the composts on yield, nutrients or PTEs.
In general, yields were lowest in the 250 kg N ha-1 treatment and highest in the 250 kg N ha-1 compost + 125 kg N ha-1 treatment. Three of the source segregated BMW composts increased yield compared to the control in all three treatments, and could be recommended for use in agriculture as a soil conditioner. The mixed waste compost reduced barley yield by up to 33% compared to the control. This reduced growth could be attributed to the mixed MSW compost having higher PTEs and sodium contents than the four source segregated composts. Levels of copper and zinc were highest in grain from the mixed MSW compost treatment. In all treatments, levels of lead and cadmium in the barley grains were below European Commission limits for PTEs in cereal grains (CEC 2001). Soil PTEs were not significantly increased on application of the source segregated composts. The mixed MSW compost increased soil lead concentration.
However, if these composts were applied annually for 10 years, soil PTE levels would still be well below the recommended UK limits.
The total municipal waste produced in the UK in 2003/04 was estimated to be 29.1 million tonnes per year (Defra 2005). Approximately 60% of MSW is organic in origin and can be termed biodegradable municipal waste (BMW). In order to comply with the EU Landfill Directive, levels of BMW going to landfill must be reduced. Composting of BMW is one option. To date, the majority of composting in the UK has used green waste. However, in order to meet recycling targets, other forms of BMW disposal and recycling must be considered. Increasing the quantity and capacity of composting facilities will result in an increased quantity of compost being produced, which will require a disposal route.
This section summarises BMW composting in the UK, and both EU and UK legislation which influences the UK composting industry. Following this, compost characteristics and effects on plants and soil are discussed.
1.1 Composting of biodegradable municipal waste in the UK BMW contains a wide variety of materials which vary substantially in particle size, moisture, chemical and nutrient content. BMW includes food, green waste, paper and cardboard. Green waste can be defined as post-consumer waste material of botanical origin from gardens, parks and other horticultural activities.
In 2003/04 in the UK, 325 composting facilities were in operation; with 84% of the waste composted being green waste and less than 3% kitchen waste (The Composting Association 2005). Uses for compost in 2003/04 included horticulture, gardening and land maintenance (36%), and land restoration, landfill cover and landfill engineering (24%). In recent years, there has been a rise in compost use by the agricultural sector, with 0.29Mt compost used in 2001/02, increasing to 0.48Mt in 2003/04. Indeed, agriculture accounted for 40% of compost used in 2003/04 (The Composting Association 2005). Returning composted material to agricultural land is the government’s preferred option.
1.2 Legislation The European Commission Directive on the Landfill of Waste (1999/31/EC), (commonly referred to as the Landfill Directive), requires that the volume of BMW sent to landfill in the
UK is reduced to:
• 75% of the amount produced in 1995 by 2010 • 50% of the amount produced in 1995 by 2013 • 35% of the amount produced in 1995 by 2020 In order to meet these requirements, up to one-third of the UK’s BMW must be diverted away from landfill (Strategy Unit 2002). Although this diversion will be gradual as facilities become available, by 2010 the UK may need to compost and find alternative methods of disposal for between 4.9 and 7.7Mt of BMW per annum, increasing to between 10.6 and
15.5Mt by 2020 (The Composting Association 2003).
This huge amount of composted BMW may swamp the future compost market. Moreover, high quality soil conditioners and composts are principally produced from source separated green waste only. The other major components of BMW including kitchen waste, paper and cardboard must also be diverted from landfill, preferably via composting.
A number of UK composting companies are using source segregated kitchen waste, fruit and vegetable waste and cardboard. Source segregated BMW which does not contain animal wastes can be composted outside (e.g. in open air windrows or aerated piles).
In contrast, some companies utilise mixed MSW. The BMW fraction is separated out mechanically from the other materials either before or after composting. This process is known as mechanical biological treatment (MBT).
The handling, processing and disposal of animal wastes are regulated under the EC Animal By-Products Regulation 1774/2002 and implemented in the four home countries under the following regulations: Animal By-Products Regulations (England) 2003, (Scotland) 2003, (Northern Ireland) 2002 and (Wales) 2003. Current regulations require that the composting of food wastes which contain or may have been in contact with meat or fish must include a period of time in-vessel at a minimum temperature: either a period of one hour at 70oC or two days at 60oC (barrier one). This must then be repeated in a second vessel (barrier two).
The Code of Agricultural Practice for the Protection of Soil (MAFF 1998) states that the spreading of specific wastes onto agricultural land does not require a licence subject to certain conditions. These materials include compost, waste plant matter, waste food and paper. A principal condition is that no more than 250 tonnes/ha waste are spread on the land in any 12 month period.
The EC Nitrates Directive (91/676/EEC) has led to the introduction of nitrate vulnerable zones (NVZs); areas where there is a risk that the nitrate (NO3) content of surface freshwater and groundwater may exceed 50mg l-1. In 2002 NVZ status was applied to 55% of arable land in the UK.
In NVZs, the maximum amount of fertiliser applied in any 12 month period to grassland is 250 kg N ha-1, and to non-grassland 170 kg N ha-1 (http://www.defra.gov.uk). As the N content of compost varies so too will the total volume that can be spread.
1.3 The UK compost market There are a number of established outlets for composts. Lower quality composts are used for landfill engineering and restoration. Higher quality composts with low levels of contaminants and PTEs are sold for the amateur gardening, grounds maintenance, horticulture and land restoration markets (The Composting Association 2003). The total amount of composted products in 2001/02 in the UK was only 1.0Mt, increasing to 1.2Mt in 2003/04, which is small compared to the several million tonnes of compost, albeit of lower quality, which will be produced to meet the requirements of the Landfill Directive over the next few decades.
There are a number of standards in the UK for composts produced from green waste and used as growing media. These include the BSI PAS 100, produced by the Composting Association and the Waste Resources Action Programme (WRAP 2005), which sets upper limits for concentrations of physical, chemical and microbial contaminants. The APEX specification (www.Apexcompost.co.uk/standards.asp) has guidelines for a number of physical and chemical characteristics, in addition to contaminants. APEX was formed by three large waste management companies in the UK (Cleanaway, Onyx and SITA).
Although the composts in this study are being investigated to ascertain their suitability for application to agricultural land, the PAS 100 and APEX standards will be referred to as they are a good bench mark for quality compost.
1.4 Compost characteristics A number of parameters can significantly affect compost properties. They include the source and nature of the raw materials or feedstock, pre-treatment (particularly source separation), and the composting method (He et al. 1992). Moreover, the chemical composition of composted BMW will vary widely with seasonal variation in raw input (Ward et al. 2005).
For example, green waste will contain a greater proportion of dead leaves and woody material in the winter months, and more grass cuttings and other green plant matter in the summer months.
1.4.1 Compost pH and use as a fertiliser