Painting the right picture
The paint shop at BMW's Oxford plant has reduced water consumption and boosted its environmental performance
The announcement earlier this year that BMW Group is investing a further £100M to increase production at its Oxford plant is good news for buyers who have been waiting around three months to get their hands on the steering wheel of the new Mini. In the paint shop, this poses a special kind of challenge – how do you reconcile the demands of more throughput, less maintenance downtime and demands for better environmental performance?
The disposal of the liquid effluent by-products of the metal pre-treatment lines is not the most glamorous aspect of Mini production. Yet the design of the waste chemical treatment plant that does so is of vital importance to the company. For many reasons – not the least of which is to ensure the plant meets the environmental requirements of European and other directives – it has a role that combines production and maintenance with that of water conservation and the quality of discharge into rivers.
It was Gee & Company of Birmingham which designed and constructed the original effluent treatment plant at the Oxford site in 1997 and which has been closely involved with the ongoing programme of additions and improvements. These have been driven by BMW Group’s environmental objectives of setting best environmental practice.
Despite the fact that, for every improvement made, it becomes that much harder to make further improvements, the programme has delivered real results. It has, for example, succeeded in reducing total water consumption significantly, despite the increase in body production. A further reflection of this saving is in the fact that the volume of effluent generated per body produced has also fallen, by 25%.
The picture in the paint shop
It takes ten hours for a Mini body to travel through the Oxford paint shop. During this time it passes through more than 20 processes, many of which use water (this, incidentally, explains why the paint shop uses 70% of the total water consumed on the whole site). In just one hour, these processes produce 30 cleaned and painted Mini car bodies. But they also produce a cocktail of solids, oil or grease, sulphate, phosphate, copper, nickel and zinc, which is an accumulation from earlier stages of manufacture of the steel itself and of production of the body shells.
Before they get anywhere near the stage of receiving their final
showroom colour, bodies pass through a series of pre-preparation stages. These start with three cleaning processes and two rinses, designed to degrease and condition the steel. Each body is then dipped in a phosphate bath – a chemical etching process which
gives the body corrosion-resistance and is an to aid subsequent
paint adhesion. The phosphated body is then rinsed and passivated.
It then moves on to the electro-coat process, in which it is immersed in a tank where anodes “charge” the paint particles and cause them to
be attracted to every part of the car body surface.
It is interesting to note that, in line with European directives, the electro-coat stage is already a lead-free process. This is an environmental improvement designed to prevent soil contamination when any vehicle is finally disposed of at the end of its life. Chrome, too, has been removed from the pre-treatment process. In total, the preparation stages of the Mini car body occupy about half of the total paint shop process and involve total immersion in nine tank dips, which variously include cleaning agent, phosphate dip, passivation dip, deionised water, electro-coat dip and ultra-filtration.
Each of these processes generates liquid effluent, which is joined by further liquid and material waste produced in the ensuing underseal, sound-proofing, cleaning, painting and stoving stages. It is to treat these waste products that BMW Group operates what is now a state-of-the-art effluent chemical treatment plant. But with Mini production demands continuing to rise – from an initial plant capacity of 100,000 vehicles per year to nearly 190,000 last year – planned maintenance has had to be revised to allow greater production throughput.
With the potential loss of 30 vehicle bodies for every hour’s unplanned maintenance, downtime issues are clearly critical. Add to this BMW Group’s own policy on reducing water consumption, together with EU directives on wastewater treatment and the ultimate need for Integrated Pollution Prevention Control (IPPC) licensing, and the scope of the task facing the company and its quality and environmental engineers can clearly be seen.
So, how has BMW Group and its partner Gee approached the combination of these often competing issues? Since the original paint shop was designed in the mid-1990s by Rover (the Oxford site’s predecessor), the environmental goalposts have been moved. At that time, for example, the discharge consent limits meant phosphate removal from wastewater was not carried out. It is now, of course, reflecting the fact that as technology has advanced, so has the environmental responsibility placed on the company.
Commenting on the improvements made since that time, Rob Lissner of Gee said: “Because our relationship with BMW Group is a continuous one, we saw our role as more than just improving the efficiency of the effluent treatment plant. By comprehending how the whole paint process works, we were able to question schedules and seek to improve them. In this way, we were able to contribute to the design changes across the plant – with the aim of reducing the volume and the nature of its residual effluent. In other words, we approached the problem from both ends.”
Together with the paint plant supplier, Durr, Gee carried out
this review of the whole paint shop, which involved mapping not just normal operating criteria, but also the maintenance needs and the effects of discharges made during maintenance periods. New recommendations on the rinse configuration were just one area that enabled a reduction in usage of important water resources. Water sprays are now only activated when a body is passing through – a simple water-saving device yet one BMW Group was the first to exploit within
the UK automotive industry.
A further change saw wash water being cascaded back from the later, cleaner stages of pre-treatment for reuse at the earlier, dirtier stages. De-mineralised water is also recirculated by being pumped back, checked for clarity, then passed through a UV sensor before being reused. To comply with pending changes, such as the European IPPC legislation, tighter controls were introduced to the production process to reduce waste generation – and therefore to minimise its treatment needs.
Design of the effluent plant was changed to make allowance for increased stream segregation. In simple terms, this allows high- and low-concentrate waste solutions to be segregated. High-concentration solutions can then stored and slowly bled into the rinse water stream – a means of balancing out the effluent treatment chemistry and enhancing both performance and cost-effectiveness.
From 2002 onwards, BMW Group implemented a programme of environmental improvements, many of them made through the working partnership with Gee. These included small but important infrastructure changes such as the chemical sealing of the floor and bunds in the effluent treatment plant – to ensure no ground pollution can occur. Oil removal from effluent water was also addressed at this stage.
Once again, enhancements began on the process line, with additional centrifuge separation equipment installed as an integral part of the cleaning systems. This technique – extracting much of the oil at the front-end of the process – means, of course, that the effluent treatment plant is exposed to much lower concentrations of oil than is the case with other car plants. Any not extracted in this way is further treated by new equipment installed in the treatment plant. This includes plate separator modules to remove free oil, together with membrane filters to extract the soluble content.
These are examples of significant results being achieved by combining efforts at each end of the cycle – namely pre-treatment issues and water discharge quality. Further improvement saw the addition of electric mixers in three of the holding tanks, which can contain up to 300,000 litres of effluent waste. These mixers serve to agitate the contents so homogenised, or consistent, effluent enters the treatment plant, to improve its working efficiency. The installation of further new equipment, together with the introduction of a new treatment chemistry, addressed the key question of the extraction of phosphates from the discharge water. The new treatment includes the use of kalic lime, a safer and more cost-effective alternative to the previously-used sodium hydroxide, which is corrosive and difficult to handle.
Produced by Buxton Lime Industries, kalic has been introduced in line with environmental and safety best practice to meet new regulations on phosphate discharge from Thames Water, and BMW Group’s own standards. Gee believes kalic is significantly safer than sodium hydroxide and results in cleaner effluent and a better-quality waste product. The kalic dosing unit was designed and built by Gee. Following a period of evaluation after the change to kalic, the results achieved with both nickel and zinc removal have proved to be better than they were with sodium hydroxide. Nickel is the more toxic of the two metals and nickel reduction levels are now well within the prescribed discharge limits.
With even greater car production output at BMW’s plant in Oxford, further demands have been placed not only on the performance of the effluent treatment plant, but also on its ability to handle more rapid receipt and turn-around of waste processes. Because the paint process line cannot operate without the effluent plant, a key objective has been to reduce cleaning and maintenance downtime to allow greater production availability. To meet these objectives, Gee has just completed a further contract, part of which has been to manufacture a new thermoplastic holding tank for demineralised water and install it at the end of the pre-treatment line.
Following planned maintenance work, its purpose is to enable make-up to be completed much more rapidly – in 2h, compared with the previous 4.5h. As a result, downtime has been halved – without prejudice to routine cleaning work. Although Gee is primarily a chemical engineering company, it has long had its own facility for manufacturing large-capacity thermoplastic storage tanks.
This policy decision was taken, says Gee’s Rob Lissner, “because we are very conscious of the responsibility for safe containment of liquids that are often corrosive or toxic. I don’t feel these criteria can satisfactorily be met from external sources, so we produce all tanks ourselves, within a controlled regime that meets all international quality requirements. Designing such tanks according to their purpose and manufacturing them in our own factory is also a logical part of our policy of supplying total systems.” Gee is contracted to provide BMW Group with ongoing technical support within the effluent treatment plant, a programme that includes training of all relevant personnel and preparation of training manuals.
The multiplicity of issues and objectives at the Mini plant are clearly led by environmental best practice. But what are the objectives and the guidelines, and how are they monitored? The governing IPPC legislation means the whole environmental issue is conducted in a transparent way. As a result, the company is accountable as it strives to match or exceed best practice. The base parameters are documented for all industry and they are getting tougher and more controlled all the time.
As a benchmark, the regulators look at Europe as a whole and then select the best. In part through the intensity of its investment programme, BMW Group is expecting the Oxford plant to gain full IPPC approval later this year.
Despite the increased production demands made upon it, the environmental performance of the Oxford paint shop and its effluent treatment plant is creditable. In a period of just two years, the measures taken have reduced water consumption by a third. Over the last year alone, the average oil and grease concentration in effluent has been virtually halved. And the quality of discharge water at Oxford meets or exceeds the requirements of
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