Reed bed research offers wastewater solution

The Water Framework Directive (WFD) came into force on 22 December 2000 with the aim of promoting the sustainable use of water while progressively reducing or eliminating pollutants for the long-term protection and enhancement of the aquatic environment. The major implication of implementing the WFD, for the water industry, is the likelihood of more stringent discharge constraints which will, in certain circumstances, require tertiary treatment systems (wastewater polishing) to be implemented to enable more effective removal of nitrogen and phosphorus from wastewater.

The Urban Wastewater Directive will be a less significant driver within the UK but it is interesting to note that the majority of catchments in the UK have been classified as eutrophic, that is, having a large or excessive supply of nitrates and phosphates. In situations where increased nutrient removal from sewage is required to meet these more stringent discharge consents under the WFD, there are a number of tertiary treatment options available, including denitrification and reed bed systems.

Energy from crops

With the entry into force of the Kyoto Protocol on 16 February 2005, the UK government is now legally obliged to reduce CO2 emissions to 12.5% below the 1990 levels by 2010. By 2020, 20% of the UK’s electricity supplies must come from renewable sources. In addition, the current renewable obligation arrangements state that existing power stations powered by fossil fuel and biomass co-firing must source 75% of their biomass from dedicated energy crops from 1 April 2006.

At present, there are 167 such stations in the UK, generating 877,670kW from the biomass fraction. If 75% of this capacity were to be generated from energy crops, approximately 1000,000 oven-dry tonnes of wood chips would be required per annum. As a result of these and other drivers, markets for renewable energy crops are developing rapidly in the UK.

Dual benefit

There is the potential to combine the requirements of both the WFD and the Kyoto Agreement in a system whereby tertiary treatment is achieved through application of wastewater to energy crops. Such integrated systems, known as water-renew systems, offer the dual benefits of cost-effective wastewater tertiary treatment, and a renewable bioenergy (or energy crop) source.

The improved yields in crops and the reuse and recovery of nutrients from wastewater can help to reduce greenhouse gas emissions, the requirements for inorganic fertilisers, nutrient enrichment of surface waters, and the costs associated with these activities. By comparison to other commercially available systems, water-renew systems have several advantages:

· They do not require chemical
additions

· They do not generate a sludge,
requiring disposal

· They have lower operational
energy requirements

· They generate a commodity
crop for renewable energy
production.

However, water renew systems may not be suitable where baseline river flows are heavily dependant on treatment works discharges.

Optimisation model

The EU-Life project aims to investigate methods to optimise this system, using different species, application rates, soils, climates and compare greenhouse gas emissions and energy use to conventional wastewater treatment. A model will be developed to improve the operational performance of such systems using detailed results from a controlled environment and field trials established for this project to develop an operational demonstration system and provide industry focused operational guidelines and investigate concerns over potential long-term environmental effects.

The application of wastewater to renewable-energy crops may provide a practical solution to tertiary wastewater treatment and provide an economically viable biomass crop for the renewable-energy industry. This differs from other land-based water-polishing systems such as rapid infiltration basins or constructed wetlands as it relies on the export of nutrients from the site in the harvested crop.

Bioenergy or renewable energy crops include trees such as willow, poplar and eucalyptus, grown under short rotation coppice (SRC), and fast growing grass such as miscanthus. The potential for an increase in the supply of energy crops in the UK is due to several factors:

· Large areas of suitable land

· The diversification of farming
away from food production

· Government and EU incentives
for renewable energy crops.

The energy white paper issued by the UK Department of Trade and Industry (DTI) in 2002 set out the government’s implementation strategy and detailed several financial incentives for renewable energy in general and, more specifically, for bioenergy production. It was acknowledged that conversion of 15% of land currently under agricultural regime to biomass crops would be needed to meet the target of 20% renewable energy by 2020.

While bioenergy crops have relatively low nutritional requirements compared with arable crops, the removal of large amounts of biomass from the land, results over time, in the depletion of soil nutrients, reducing yields, in the absence of fertiliser. In addition, their high demand for water might deplete potable groundwater resources or reduce yields when the crop is unable to access groundwater.

Pilot trials at the Swedish municipality of Kågeröd concluded that wastewater irrigation increased the growth of willows by up to five times and that the removal rate of nitrogen and phosphorus in the willow-soil system was higher than conventional nitrification/denitrification and phosphate chemical precipitation treatment processes.

The pilot trials were so successful that the municipality decided to scale-up the trials to develop full-scale application. A 13ha SRC willow plantation is currently being used to treat 40,000m3 year secondary effluent, 12% of the wastewater produced by the 1500 occupants of the town. It is anticipated that the system will eventually accept the entire 135,000 m3 year flow from the town’s sewage treatment works.

In New Zealand, research has focussed on the radiata pine although other species, including eucalyptus have been evaluated. Several large trials have been conducted in Australia including at Wagga Wagga and Shepparton. The International Energy Agency (IEA) Task 30 report suggested that despite the very high growth rates observed (with crops growing at approximately twice the UK rate), bioenergy production from wastewater-irrigated plantations can at most contribute only a very small proportion of total energy production in Australia simply because of the limited volume of the wastewater resource. The reports did state, though, that the development of such plantations could make a relatively large contribution to solving a wastewater disposal problem, which was particularly applicable to rural Australia.

The results of previous studies suggest that the use of renewable energy crops for sewage effluent treatment, offers a cost-effective alternative to conventional methods of tertiary sewerage effluent treatment, along with the added advantage of generating a commodity crop. This has important benefits and applications within the UK water industry as well as in the EU at smaller, often more rural, treatment works where tertiary treatment installations maybe costly.

Water-Renew is a collaborative research project led by WRc Group in partnership with Queen’s University Belfast and Cranfield University. The results of the project will bring this technology closer to commercial implementation within the UK and provide another alternative to traditional tertiary wastewater treatment and a renewable energy source for future generations.

Project CP231 is part of WRc’s £1.5M programme of collaborative research called Portfolio. The Portfolio research programme addresses topical problems in the water industry through promoting innovation, sharing research costs and leveraging budgets/experience.

The projects in Portfolio are focused on meeting the needs of business and the environment, including:

· Reducing risk

· Anticipating and influenc-
ing legislation/regulation

· Improving capital and
operating efficiency and
reducing costs

· Strategic asset management and investment planning.

Contact: WRc Group

Tel: +44 1793 865000

Email: hutchinson_a@wrcplc.co.uk