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- 17 December 2007 -

Energy efficiency: Wastewater treatment and energy production

Reducing the environmental impact of power generation is becoming imperative. In this article, Anthony Bennett looks at how energy production can be incorporated into wastewater treatment systems and describes its positive environmental effects.

Wastewater treatment aims to reduce the environmental impact of human activities. However, there is currently an incomplete understanding of the impact the wastewater treatment industry has in terms of carbon emissions and its climate change impacts. Furthermore, environmental policy does not necessarily balance the requirement for pollution emission control with the resultant carbon footprint. In this article I outline how energy generation, a proportion of which is classified as renewable energy, in tandem with technology and processes designed to improve energy use efficiency, can deliver overall reductions in the carbon footprint of wastewater treatment works (WWTW).

In this article I draw on examples of existing energy generation technologies incorporated into WWTW, and identify the scope for other feasible renewable energy sources. I particularly focus on the use of established combined heat and power (CHP) systems at WWTW and on the optimisation of sludge digestion conditions to maximise biogas generation rates. Current developments in sludge digestion technology are identified. Also outlined are approaches, both procedural and technological, that successfully reduce greenhouse gas emissions and improve upon energy efficiency, focussing particularly on conventional activated sludge (CAS) and membrane bioreactor (MBR) treatment systems.

Combined heat and power technology

The scope for generating energy whilw treating wastewater is increasing. Following experiences in landfill, incineration and other waste-to-energy schemes, CHP technology has already been successfully applied to treatment plants for several years. However, as more CHP experience is gained and specific implementation issues are addressed, there is much greater scope for using biogas generated at some plants.

For example, Severn Trent Water (STW) has expanded CHP at its WWTW from a base of 10 MW at five sites in 2002, now almost trebling the energy generated to 28.4 MW at 34 sites. This means that the company has managed to offset much of its increased energy requirement over this time, from approximately 800 to 950 GWh, by generating about 150 GWh renewable energy. Installation of CHP is not necessarily straightforward, however. In order to choose the appropriate CHP engine size, operators require an assessment of plant biogas yield and gas quality, including the degree of hydrogen sulphide and other trace contamination. STW's experience was that such data are difficult to accurately assess without the aid of monitoring; installation of the appropriate instrumentation has cost implications. Other site-specific practicalities can make CHP installation problematic, including planning issues and the specifics regarding electrical connection, which can affect investment payback time. Furthermore, regulatory issues with regard to atmospheric release permits under Integrated Pollution Prevention and Control (IPPC) legislation and the auditing necessary for Renewables Obligation (RO) accreditation, for example, also impact upon the decisions regarding the cost effectiveness of CHP power generation. These drawbacks notwithstanding, CHP is a proven technology with the potential to be adopted much more widely across the wastewater treatment industry to generate renewable energy, particularly at smaller WWTW.

Biogas generation

Given its potential for more widespread use, recent research has focused upon ways to maximise biogas generation rates, and on improving the quality of the gas through clean-up. Biogas can be efficiently generated from the anaerobic digestion (AD) of sewage sludge, otherwise considered a problematic waste product from the treatment process. The multi-stage AD process that converts complex organic matter ultimately to carbon dioxide (CO2, 35%) and methane (CH4 65%) has been extensively studied by Dr. Elise Cartmell at Cranfield University and reported on EnerNet, the Engineering and Process Sciences Research Council's network www.energy-network.net.

Loading can affect the often rate determining hydrolysis stage of AD. Although hydrolysing bacteria prefer a higher concentration feedstock, hydrolysis rates are inhibited by the accumulation of the amino acid and sugar hydrolysis products. Theoretically, at a steady state in a non-recycling system, the maximum bacterial growth occurs when cell residence time equals hydraulic retention time; overloading the hydraulic rate will cause cell flush-out and dilution. (For methanogenic bacteria, this critical rate is as low as 0.03 d-1.) The main operational reasons for hydraulic overload are over-pumping, a diluted sludge feed, excessive sludge production rates, poor mixing, grit deposition and scum formation. Therefore, a supply of well thickened sludge is ideal. Maximising sludge generation at the WWTW through settlement is desirable, since it is a source of volatile organics (and ultimately biogas) which otherwise requires chemical oxygen demand (COD) removal. In modern treatment plants, membrane separation technology is often used to further improve solids capture. Maintaining efficient grit removal systems and mixing helps maintain optimal hydraulic loadings.

A digestion temperature of between 30-35°C should be maintained avoiding fluctuations of more than 2 or 3°C. Influent sludge should be warmed to digestion temperature and heat losses from surfaces and pipework compensated, since biogas generation rates drop significantly below 30°C. The methanogenesis stage is strongly inhibited by a drop in pH and an accumulation of volatile fatty acids, toxins and other inhibitors. Careful control of organic loadings through often feeding smaller quantities of sludge is important and ensures sludge quality is as consistent as possible. Effective pH control is also vital. Mixing, either mechanical or by means of gas injection, is helpful in controlling a number of key AD processes and ideally results in a turnover time of 20-30 minutes. Heavy metals and hydrogen sulphide, to which methanogenic bacteria are sensitive, can be controlled by appropriate influent treatment and steady bacterial acclimation. Ammonia, whilst beneficial as a nutrient at lower concentrations, inhibits biogas generation above about 1500 mg/l and should likewise be controlled.

New developments

Research is being undertaken on several aspects of optimising biogas generation. The American Water Works Association (AWWA) Research Foundation has funded projects to develop continuous in-line thermal hydrolysis of biogas as a cheaper alternative to batch steam processing. High solids vertical plug flow AD systems will be able to generate biogas from unmixed or partially mixed sludge with 20-25% solids content (conventional digesters can cope with 4-6% solids sludge), while co-digestion with organic-rich wastewater potentially allows even higher biogas generation rates. Advanced clean-up technologies to decontaminate biogas are being developed, based on the oxidation catalysis of contaminants harmful for post-combustion release. Importantly, the carbon adsorption and gas chilling techniques employed to protect and prolong the catalyst are to be tested in small-scale units. Elsewhere, the Sustainable Environment Research Council (SERC) based at the University of Glamorgan , UK is investigating hydrogen generation from biosolid digestion by promoting the necessary conditions in the AD process to selectively produce H2. This group foresees hydrogen as an important potential source of renewable energy.

Renewable approaches

Although biogas combustion and CHP schemes have attracted much attention for generating energy from WWTW, other approaches have also been identified. Recognising that water utilities own substantial tracts of land with their reservoirs, offices and treatment works, Dr. Gareth Harrison at the University of Edinburgh suggests there is much scope for the generation of energy through hydropower, solar and wind energy. Although most hydropower potential derives from water stored for supply purposes, there are circumstances where water delivered directly to treatment plants can also be exploited. Using turbines as water energy reduction brakes, and utilising pressure reduction valves and brake pressure tanks, WWTW requirements of an overall low water pressure head can be achieved in combination with hydropower energy generation. Many WWTW are located where wind turbines can offer a readily adopted small or large-scale power source which does not interfere with ground activities, while the economics of solar power make its retrofitting or inclusion in the fabric of new-build infrastructure more attractive. Coastal WWTW can feasibly consider using wave or tidal power for renewable energy generation. Further details on these technologies can be found on www.energy-network.net.

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