Efficiency harnessed at Esholt
Energy efficiency was at the heart of the design of Esholt Wastewater Treatment Works in Yorkshire. Project manager, Adrian Marsden of Arup, explains how the scheme was delivered.
The site dates back to the early 1900s and at the outset of the scheme there was a need to replace a number of ageing assets with new, state of the art process units to meet a tightened ammonia consent, of 3mg/l, applied to meet the requirements of the EU Freshwater Fish Directive.
Arup has provided a full range of inputs on the £75M scheme to deliver a sustainable and innovative final solution. The new works can generate up to 45% of the power demand from the on-site sources of the new combined heat and power (CHP) engines and hydro-turbine.
This comprises over 20MW of installed capacity. It is estimated that these measures will reduce emissions by 760 tonnes carbon dioxide equivalent per year, with associated operating cost reductions of over £700,000 per annum.
The Esholt WwTW project was completed in December 2009, comfortably before the March 2010 regulatory date, and within the authorised budget. It is now delivering effluent qualities significantly better than the consent requirements with operating costs below forecast levels.
Initially Arup co-ordinated a strategic study, looking at all of the sites needing substantial investment and the best way of delivering the business requirements. Esholt was used to test the ideas being considered.
The result of the study was a reference design for activated sludge plants, alongside typical operating criteria, that took account of operational costs associated with energy use. The approach also looked at the ability to undertake future expansion or modification.
Inherent in the design were means of making future modifications to accommodate potential process changes. As a result, U-shaped aeration lanes were specified by Arup, to reduce internal pumping distances. Structural modifications will be minimised as a result of this approach, therefore providing the most sustainable solution which maximises the operating lifetime for the new assets.
Underlying the physical structures, Arup worked closely with Yorkshire Water to develop a generic control philosophy which has a number of different options allowing control to be tailored to process conditions and hence save operational costs and energy use. These modes of operation have been seen to reduce operating costs.
Associated with the design of the aeration lanes, Arup identified the opportunities to use physical and numeric modelling to deliver an efficient plant. CFD modelling was undertaken on the aeration lanes to optimise retention times and flow paths.
Working with specialist consultants Hydrotec and contractor Morgan Est, capital savings of over £90,000 were identified, whilst increasing retention times to give a more stable plant performance. Physical modelling was also used to ensure good flow splits and therefore stable plant performance.
These optimisations lead to even plant loading and therefore more easily controlled aeration performance. As power supply for the aeration system represents the single largest energy use these focussed inputs provide maximum returns.
On a region-wide basis, Arup undertook a project to identify ways to reduce the operational costs on aeration plants. This work involved a collaborative approach with Yorkshire Water to develop a dynamic computer model able to replicate various process control strategies.
Using this model, it was possible to predict the performance of a WwTW system under a range of different energy saving scenarios. This approach allowed for rapid testing of a large number of energy saving strategies under a range of conditions. The project identified savings of 13% of total process power usage for the wastewater treatment plant.
When applied to other Yorkshire Water sites using similar technologies, this equates to savings in excess of £1.2M per annum.
From a carbon footprint perspective, it is estimated that over 16,500 tonnes of CO2 will be saved each year, once the activated sludge process strategy is integrated across the large Yorkshire Water activated sludge treatment schemes. If rolled out to all similar treatment sites across the UK the strategies have the potential to save over £14.8M and 203,000 tonnes of CO2 each year.
In early 2009, Yorkshire Water also recognised the impact of the study, awarding Arup a Partner 2008/2009 award under the innovation category. Arup is the first consultant to have won this. Also the IChemE awarded the project the 2009 Innovation & Excellence Award in Water Management & Supply for its impact on reducing environmental impact.
At project level, at Esholt WwTW Arup has supported Yorkshire Water with energy saving features of the new works. Arup undertook sewer modelling to optimise storm tank return times.
This work prevents storm flows being prematurely pumped back up to the inlet works, a distance of over 20m, where they were then overflowing again. Now returned flow is only transported back to the inlet works once the plant has capacity to treat it.
A hydro-turbine has also been constructed, an application which the project team believe is the first application of its type in the world. Arup worked with Yorkshire Water, and contractor MMB, to identify the opportunity to capitalise on a 10m fall between the inlet works and new primary settlement tanks.
Following investigation and design work, the preferred solution developed was a system of Archimedean screw generators installed in series and working in reverse to generate renewable energy. Archimedean screws have been used as a means of lifting flows within a sewage environment for many years, however, this is the application of the same technology working in reverse to generate electricity and minimise operating costs/carbon emissions associated with wastewater treatment.
Investigations were carried out and the project team believes this is the first use of a low head hydro-generating system operating on untreated sewage flows anywhere in the country and probably the world. The system is designed to generate power over a range of flows. The design was optimised to achieve maximum output at 80% (2.6m/s) of the flow to full treatment such that it benefits from all flows, not just peak flows.
It is estimated that the energy generated will be the equivalent of £110,000 per year with a peak generating capacity of 180kW. The unit was sized such that capital costs would be repaid within five years.
The capital costs and embodied carbon were minimised by installing the two Archimedean screws in pre-fabricated steel troughs, which avoided the need for extensive concrete structures. Therefore the capital cost of the installation was allowable within the AMP4 rules.
In addition to the hydro-turbine, Esholt also incorporates energy generation from CHP engines. The engines use gas given off from the anaerobic digestion process to produce both heat, which is reused to heat sludge entering the digesters, and electricity.
The project team identified the opportunity to invest in larger engines, with waste heat recovery technology, to maximise the energy generated. Arup worked with Morgan Est to assess alterations which resulted in an additional £500,000 of expenditure. However this additional investment will be recovered within three years due to the increased energy production.
Arup also developed an optimised control philosophy to achieve the best blend of sludge for energy production rather than just sludge processing. This control philosophy will result in an additional 3MWh per day of energy generation, or around £120,000 of additional revenue per year. Arup provided a commissioning engineer to work with the operators, contractor and systems integrator, to thoroughly test and refine the control system.
The Esholt WwTW scheme represents an excellent example of the delivery of a highly complex works. Opportunities to save energy and minimise future work have been incorporated throughout the design and construction.
Additionally, energy generation from the CHPs and hydro turbine, form an integral part of the project. A high level of consideration has been given to the control systems to ensure optimum process management.
All these features have delivered a highly efficient wastewater treatment which is recognised by Yorkshire Water's environmental business unit director, Graham Dixon, who commented "Following this investment Esholt now stands as one of the most efficient and technologically advanced sewage works in the UK."