The aim of the European Water Framework Directive (WFD), due to be fully implemented by 2015, is to integrate every aspect of the control of water quantity and quality in the water cycle across Europe, so that all parties work to one timetable and objective.
A key part of the WFD looks at how water used for domestic, commercial and industrial purposes is treated and returned to the water courses. As such, upgrading the effectiveness of treatment processes has been a priority in the current five year spending round. To continue this progress, the forthcoming AMP (Asset Management Plan) 5 round (from 2010) will most likely focus on improving the lifetime cost-effectiveness of the treatment processes.
As part of this, treatment works management will be encouraged to focus on lifetime costs and their improvement through better specification and assessment of equipment.
The big screen
As key elements of the treatment process, the performance of pumps, screens and grinders has a considerable bearing on how energy and cost efficient a treatment works is, as well as its quality of discharge.
Traditionally, in a large number of existing UK treatment plants, coarse materials were removed from incoming waters with simple screens, accompanied by manual cleaning and removal of the accumulated debris for further treatment.
The size of the apertures in the screen is important. Up until the early 1990s, the standard plant inlet screens had 20mm apertures which resulted in a relatively coarse screening process. Pollution prevention and clean water legislation led to this being reduced to 5mm in each direction, improving screening effectiveness but adding a considerable load to the screening process with a larger volume of screened out material.
As well as imposing higher capital costs to deal with this volume, 5mm screens are much more liable to blinding and rapidly become less effective, unless cleaning equipment, like revolving brushes, scrapers, or jetting with compressed air or pressurised water, is included. These measures not only add to the cost of the system, but also require high levels of energy to run, as well as on-going maintenance.
High performance, self-cleaning disc-based screens have emerged to provide a more efficient alternative to this method. They provide excellent open channel inlet screening at lower energy and operating costs than traditional static screens with powered cleaning equipment. Typical aperture sizes can be specified between 2.5mm and 18mm and capacities extend to over 13000m3/hr.
The latest grinders incorporate cutting discs on contra-rotating shafts, which enables them to deliver highly effective size reduction of solid inclusions in incoming waste water. They can be deployed in-channel, on-line in pipework, or used to grind the dry screenings from inlet screens for subsequent treatment.
These high performance grinders are energy efficient and have low maintenance and running costs. Market leading systems are extremely durable and offer reliable long life under harsh conditions with minimal downtime, plus they are backed by intelligent control systems with long warranties.
Grinders are also effective as protection for pumps and other equipment within the waste treatment plant, especially from solid inclusions like rag, string or plastics which are often flushed into the sewers. These inclusions are particularly dangerous to centrifugal pumps, as the propeller vanes and shaft can be totally entangled.
With their high rotational speeds, solid inclusions can also cause excessive wear to centrifugal pumps, which results in excessive downtime and on-going high maintenance and replacement costs.
Progressive cavity pumps are increasingly used throughout treatment plants for media more viscous than water, as well as where there are solid inclusions. Typical media include primary, secondary and dewatered sludges, as well as supernatant liquids and even filter cake.
The combined benefits of the discrete cavities within the pump, the interface between hardened steel rotor and resilient stator and an optimum rotational speed ensures far more efficient movement of difficult to handle media than centrifugal pumps.
A specific benefit of progressive cavity pump designs is that flow volume is effectively linear to running speed, so it is easy to respond to changes in flow demand (for example with a variable speed drive), ensuring more energy efficient pumping and processing versatility.
They are also extremely robust and leading progressive cavity pump manufacturers offer warranties up to 10 years on special drive shaft connections, emphasising their reliability and low costs of ownership over a long working life.
This linear characteristic of progressing cavity pumps is also of great benefit when combining equipment such as screens, grinders and pumps into packaged solutions.
When working with specialist designers and manufacturers of this equipment, the treatment plant design engineers and the installation contractors have their specifications met exactly with equipment packages ‘tuned’ to their site conditions and process requirements.
Leading manufacturers have the detailed knowledge and practical experience to devise a solution that will provide optimum efficiency and low lifetime costs, a valuable service that cannot be offered by supply-only sources.
Importantly, this approach delivers an optimum packaged solution in every application, which, in many cases, incorporates powered disc screens, grinders and progressive cavity pumps that are specifically designed to work together to maximise efficiency and performance.
With this approach, the right pump, screen and grinder package has proved to play a key role in treatment works considerably improving their efficiency in terms of energy, cost and performance, enabling them to not only meet the WFD, but also the requirements of the forthcoming AMP5 spending round.
Gerald Muldoon is with Mono NOV, Manchester, UK. www.mono-pumps.com