Taking hold of sewage sludge

Pasteurising, dewatering and reducing sewage sludge to a cake for efficient handling and safe disposal has led to companies increasingly switching to decanter centrifuges. Pete Rose explains.

Thanks to modern environmental legislation, sewage sludge can no longer be disposed of at sea or spread as wet slurry over agricultural land. It has to be pasteurised, dewatered and reduced to a cake that is dry enough to handle efficiently and to dispose of safely.

This has led to a plethora of different separation techniques and equipment being used to dewater and thicken sludge. Presses and filters of all kinds have been and are still used but, increasingly, water companies and others involved in sewage and waste treatment are switching to decanter centrifuges for the crucial job of de-watering and thickening sludge.

A study carried out a few years ago at the Lower Potomac Pollution Control Plant in the US illustrates the reasons behind this trend. The plant had operated six A-Frame Plate Filter Presses constantly for about six years. The ravages of time and the nature of the process took their toll on the equipment, which was racking up downtime because of regular failures.

The study reviewed the available options and compared key parameters. Three different types of equipment were reviewed - belt filter press, vacuum filter and a decanter centrifuge - and parameters such as initial capital cost, operating costs, polymer costs and the level of cake dryness achieved were compared.

The decanter centrifuge proved more efficient, more cost-effective and more energy-efficient than the other two machines. As the author concluded, when making his purchase recommendation, decanters provided:
  • High solids content
  • Low maintenance
  • Low chemical usage
  • Minimal operator attention
  • Environmental advantage
He was right. Decanters are robust, reliable, compact and energy efficient and produce a cake that is dry and easy to dispose of. They are also very good at containing odours which, given the nature of the raw materials being processed, is a very important factor.

Given the increasing demands on the water and waste industry, virtually all treatment plants have to run 24/7, all year round. Consequently, anything that can optimise their performance has to be welcome news.

Data analysis
That was the thinking behind Octopus. Developed by Alfa Laval as an autopilot for decanter centrifuges in sludge dewatering systems, Octopus is designed to control crucial decanter operating parameters to ensure cake of a uniform consistency and dryness. It does this so effectively that it can save as much as 25% on polymer consumption and enable dewatering plants to run, unsupervised non-stop.

The key to its success is the number of parameters it is able automatically to monitor and control, such as:
  • 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 in-going and out-going 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. Cost factors, such as polymer costs, transport and energy can also be programmed in.

As a result, the STW enjoys a large number of options when it comes to programming Octopus. It can opt for lower costs or choose 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.

Cost savings
Before it was officially launched, 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 inevitable conclusion to be drawn was that Octopus was better than even an experienced operator at getting the most out of the decanters it was controlling.
The Dutch STW was so impressed with Octopus' 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. It estimated that Octopus had provided savings of ¬15 per ton dry solids.

This figure is impressive enough but becomes more so when you realise that it did not include any labour cost savings, easier mechanical handling or the simplified disposal of a drier cake. STOWA also confirmed the trouble-free operation of the sensors over a period in excess of a year.

Since the Dutch trials, Octopus has been installed and is operating successfully at a Swedish STW, while there are installations running in Germany, Denmark, Italy and Spain and the US. One of the UK's leading water companies has also just completed a two-year intensive trial, and has placed an order for its first Octopus system. All of the operators of the installed systems are reporting similar levels of success to those recorded in the Dutch trials.

Pete Rose is marketing manager at Alfa Laval.
T: 01276 413632.

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