MCH Konsulting Sweden
ROWAN HOUSE LTD
Keeping it clean . . .
by Clive de Salis and Essie Andersson
Ever since a well-known Swedish furniture store was associated with the work of the artist Carl Larsson, his home town of Falun became world famous. Yet the town of Falun has an interesting industrial history: It is the home of the world's oldest limited company, Stora Kopparberg AB.
Stora Kopparberg is a copper mining company. Copper has been mined in the area for over 600 years. The copper from Falun has typically been used as a wood preservative and it is the copper from the Falun region that gives Scandinavian wooden houses their distinctive colour. The copper mines of the Middle Ages were open cast but over the centuries mine shafts were dug and now the area is riddled with a mixture of active and disused copper mines.
Today's environmental requirements are much stricter than in the past when old mineshafts could be used for dumping chemical and any other types of waste. The result is that in the year 2000 the town's waste water as well as the industrial waste contains a significant cocktail of heavy metals and other contaminants. Sweden's laws dictate that all the local authority's actions are open to the public and so the performance of the water treatment works is automatically published on the internet every hour. Heavy metals are difficult to handle at any time but with the realisation that any mishap is public information within the hour, Falun Kommun authority didn't want to take any chances.
The process Falun Kommun chose was a High Density Sludge ( HDS ) process instead of the old-fashion method of just neutralizing the acid mine drainage water by addition of lime. The HDS-process can be designed in different ways, but the main point is that you recycle the sludge in the process, which will result in getting a very dense and easily dewatered sludge. The sludge is later dried on the site before disposal, thereby minimising the volume of sludge for disposal.
In the two first steps in the HDS process in Falun the acid mine drainage water is mixed in rapid mixing tanks with sludge to adjust the pH to pH 4.3 in tank one and then again to around pH 6.0 in tank two. The second, third and fourth tanks are all aerated by membrane aerators. In the third tank lime is added take the pH to pH 8.3. The fourth tank has been installed in order to ensure that you should get stable products (sludge and water), out of the process. The change in pH between tank three and tank four should be minimal to ensure complete oxidation of ferrous iron and stability of the sludge.
The quality of the contaminated acid mine drainage water in Falun varies not only according to the season but also from which depth in the mine you pump it. The main ingredients in the water are sulphur, ferrous iron and zinc. pH for this contaminated water varies between 2.5 and 4.0.
Due to the fact that almost all the iron exists in the ferrous form it is very important with efficient aeration to get all the iron oxidized into ferric iron. Otherwise the sludge and the water would not be in a stable condition when they leave the treatment plant.
From the treatment plant in Falun you achieve 99.9 % reduction of the content of iron and zinc in the acid mines drainage water and a stable sludge with 55-60 % solids.
The control of the process had to overcome significant problems. Each step of the process has to be adjusted to respond to changes in the dissolved heavy metal content and composition but, as with most water treatment processes, the time delays are significant. A truly distributed process control system (DCS) was used and configured to maximise the reliability and availability of the system for optimum safety. The selection of a DCS also facilitated optimum response to process changes. The use of Abacus4 enabled the alarm reporting and operator interfaces to also be distributed rather than needing to rely on a central computer. A central computer for the display of information and alarm reporting would have run the risk of failing to report process alarm conditions in the unusual event that it was off-line for any reason. Since the plant performance was public information every hour the operators need to know that they can always see what is happening even if one of the computers is off-line for maintenance or for software changes.
The water treatment site is often unmanned. Alongside distributed alarm handling the Abacus4 control system makes use of the telephones. SMS text messaging is used to allow critical alarms to be sent as text messages to the duty operator's mobile phone whilst the site is unmanned. If the duty operator fails to reply to the Abacus4 system the message is also sent to other operators on the duty list. The text messages are followed by a pre-recorded voice message being transmitted by the Abacus4 DCS by telephone ... until someone responds to the alarm. These back-up alarm systems are essential because the performance of the plant is being automatically displayed on a web-site by the Abacus4 DCS every hour for public information. Thus the result of failure to respond to an alarm would become well known throughout the town in a short space of time. Which is an excellent incentive to keep it clean!
Basic plant performance data is automatically posted on a website in compliance with Sweden's tough anti-secrecy laws. The performance can be seen by anyone logged onto the town's intranet. Falun Kommun's intranet is permanently linked to the internet allowing high speed internet access to everyone in the town. Thus the performance of the town's waste water treatment can also be seen from anywhere in the world on: http://www.users.wineasy.se/bbab/framby-reports/