Novel Circular Systems for Waste Water Treatment

Background
Reducing the requirements for importing energy is a key Scottish Water (SW) policy. While both wind developments and solar power are limited by intermittency of supply, anaerobic digestion (AD) has the potential to provide a steady and significant supply of energy in the form of biogas. AD is widely used as a technology for the treatment of sewage sludge from wastewater treatment plants (WWTPs) (in 2007, 66 % of all UK sewage sludge was treated by this method (DEFRA, 2011)), but AD of sewage sludge can be problematic due to variability of feedstock. Using silaged energy crops as feedstock for AD for dedicated biogas production can potentially solve this issue. However, growing energy crops on land which could otherwise be used for food production is controversial. One way to overcome this dilemma is by using marginal/derelict land (i.e. land that is unsuitable or unprofitable for agricultural production such as contaminated land, landfill and mine sites) for growing the energy crops. Cultivating marginal land can, however, be challenging due to several potential growth-limiting factors for the crops, such as a high phytotoxicity of the pollutants and poor fertility conditions, and questions have been raised about the feasibility of profitable energy crop production on marginal land without the substantial use of inputs such as fertilisers. Hence, soil amendments are often required to render the substrate suitable for plant establishment. Sewage sludge from WWTPs could act as such soil amendment and be included as part of an agricultural rotation.
In this project, we wish to investigate the environmental and economic feasibility of using marginal land treated with sewage sludge to grow energy crops for anaerobic digestion (Figure 1). A similar AD-energy crop-WWTP system has been adopted at Stoke Bardolph (owned and operated by Severn Trent Water). Such system has the potential to deliver multiple benefits, including:
• Provision of a flexible source of renewable energy that reduces reliance on imported energy (derived from fossil fuels) as well as reliance on subsidies associated with sale of energy. The biogas generated during the AD process can be burnt directly to produce electricity and heat, purified for injection into the gas network or used as a transport fuel. Energy from AD can potentially be recycled back into the AD process itself or to the WWTP. Because biogas can be stored, there is also an option for meeting peak energy demands with higher economic benefits in return.
• Recycling and reductions in the amount of sewage sludge being deposited in landfills thereby reducing the carbon footprint of WWT operations. This helps prevent emissions of methane being released directly to the atmosphere when biodegradable waste breaks down in landfills and when manures and slurries are stored.
• Production of a digestate rich in nutrients (e.g. nitrogen and phosphorus), which can be recycled to the land and used as an organic fertiliser, thereby returning valuable nutrients to the land and replacing the need for energy-intensive manufactured or mined fertilisers.
• Cultivation of energy crops can be used for phytoremediation of contaminated land and can thus improve the value of marginal and derelict land.
• Using marginal land for growing energy crops limits the diversion of crops from the food chain.

Furthermore, AD can potentially help the UK to meet several of its 2020 targets, as set out by the EU, e.g. the Renewable Energy Directive, the Waste Framework Directive, and the Landfill Directive (DEFRA, 2011).