Process control spreads tentacles
July 1st 2004 New from Alfa Laval is the Octopus system of control and monitoring equipment. The system can increase process efficiency and enhance productivity by reducing downtime
One of major challenges facing any developed country in the 21st Century is the management and disposal of domestic and industrial waste. In fact, processing sewage sludge efficiently and with minimal environmental impact has become a highly specialised branch of the process industry.
Sludge can no longer be disposed of at sea. It cant simply be spread as slurry over agricultural land. It has to be pasteurised, dewatered and reduced to a dry cake that can be handled mechanically. In some areas, water companies sell the final product as a fertiliser to farmers and gardeners.
Around the world, water companies and others involved in sewage and waste treatment are opting for decanter centrifuges for the crucial job of dewatering sludge. Decanters are robust, reliable and produce a cake that is dry and easy to dispose of. Given the nature of the raw materials being processed and their origins most dewatering plants are designed to run 24 hours a day, 365 days per year. Consequently, anything that can optimise their performance has to be welcome news.
That was the thinking behind Octopus, developed by Alfa Laval as an autopilot for decanter centrifuges in sludge dewatering systems. Octopus is designed to provide cake of a uniform consistency and dryness; to save as much as 25% on polymer consumption and to enable dewatering plants to run, unsupervised, on a non-stop basis. It achieves this by monitoring and controlling the following key parameters:
The suspended solids content and flow rate of incoming sludge
The suspended solids content of the centrate (water removed from the sludge)
The rate at which polymer needs to be added to flocculate the sludge
The differential speed and conveyor torque of the decanter
Octopus uses infrared sensors to measure the solids content of both the ingoing and outgoing sludge. These sensors scatter infrared light into the target stream and then measure the reflected light via two light sensors; one of which measures density while both together measure solids content. The information they provide is then analysed by a bespoke software program that makes the appropriate changes in and around the decanter to optimise the process.
A Sewage Treatment Works (STW) or plant operator can set the desired minimum and maximum values for sludge flow rate, polymer dosing, dry-solids recovery, and other crucial operating parameters. In addition, cost factors, such as polymer costs, transport and energy can also be programmed in.
This then gives the STW a large number of options when it comes to programming Octopus. It can either opt for lower costs or to optimise different aspects of the sludge dewatering process, such as cake dryness or polymer consumption to meet disposal or environmental standards; or a combination of the two. Since the system automatically monitors as well as controls the process, it enables dewatering decanters to run unsupervised, thus saving labour costs.
Prior to its launch late last year, Octopus was put through extensive field trials at a STW in the Netherlands. The objective of the trials was to confirm the reliability of the sensor technology and the extent of the savings that could be achieved by the users.
The outcome was a reduction in polymer consumption from 12kg/tDS to 8kg/tDS and an increase in the average solids recovery from 94% to 98%. The improvements were especially noticeable during the night shift when the dewatering function was unmanned. The Dutch STW was so impressed with Octopuss performance and the scale of the savings it provided that it was the first to place an order for the system for its decanter centrifuge installations.
The results achieved at the test site were independently analysed and verified by STOWA, the Dutch foundation for water research. They estimated that Octopus had provided savings of 15Euros per ton dry solids. This figure is impressive, but becomes more so when you realize that it did not take into account labour savings, easier mechanical handling and simplified disposal of a drier cake. STOWA also confirmed the trouble-free operation of the sensors over a period of more than a year.
Since the Dutch trials, Octopus has been installed and is operating successfully at a Swedish STW while installations are also planned in Germany, Denmark, Italy and Spain and in the USA. At the time of writing, several of the largest UK water companies are in the process of testing Octopus and are achieving similar levels of success to those recorded in the Dutch trials. More articles from Alfa Laval Limited: | 
