Research project: Technologies for bioprocessing organic urban wastes

The home composting project was led by Ian Williams. The purpose of the study was to investigate the environmental impact of home composting and how it compares with large scale centralised composting. The primary objectives of the project were to:

• Identify and compare the available techniques for measurement and analysis of the emissions from home composting in order to find the most accurate and reliable methodology.
• Assess the potential for environmentally harmful emissions from home composting.
• Add to the body of knowledge within composting science regarding the relationship between key factors, including temperature, CO2 emission, pH, moisture content and feed properties.
• Compare the environmental impacts of unmonitored and possibly poorly managed home composting with well monitored and controlled centralised composting and its associated transport and processing emissions, in order to recommend which disposal route local authorities should emphasize.

The study produced many interesting findings:

• Home produced composts can be used as a safe and beneficial soil improver
• Highest composting activity is typically in the first 2-3 days
• Air exchange mechanism in home compost bins shown to be primarily diffusion not bulk convective flow as previously assumed
• Trace gas emissions in home composting were found to be low
• Combined with low gas flow rates from diffusion this leads to extremely low environmental impacts

Home composting should continue to be encouraged as a waste management tool. It fully adheres to the proximity principle and diverts waste from costly and energy consuming collection, handling and processing steps. Very little effort and only basic knowledge is required by the public to produce safe composts with negligible environmental impacts compared to the alternative treatment methods.

In addition, a number of lessons were learnt in the course of this study that will be of value to future studies and in interpreting existing data, particularly the following:

• With regard to temperature and gas composition measurements the monitoring frequency and timing relative to feeding and turning has a large impact on the results. In the first few days following a feed addition, daily monitoring is essential, and hourly monitoring would be beneficial.
• Ambient temperature fluctuations over the course of a single day have a significant influence on compost temperature which could have an impact on the results from studies using the public where the timing and geographical location of measurements could be different.
• Measurements of headspace gas composition were shown to be highly dependent on the headspace volume, making this an important additional factor when headspace gases are to be used as a measured parameter.
• Evidence from gas concentration data and rates of emission calculated from the reactor experiments indicated that the primary gas exchange mechanism in home compost bins was diffusion rather than bulk convective flow. This is important in determining appropriate methods to quantify gaseous emissions from headspace gas concentrations.
• Based on the assumption that the rates of CO2 emission were not significantly different between the reactor and H.C. bin systems, a model of gas diffusion through a stagnant layer of air was applied to H.C. bins to produce quantitative estimates of the upper ranges of emission rates of trace gases such as methane.