Automatic Polymer Control

Automatic polymer control on centrifuge dewatering for sludge, centrifuges and filters

• reduces polymer usage by up to 40% (25% typically)
• reduces under-dosing ‘events’
• gives drier, more consistent cake solids

The Need for Automatic Polymer Control

The consequences of overdosing polymer to a de-watering device such as a sludge centrifuge are much less than the consequences of under-dosing and with time pressures on operators this leads to overdosing on most centrifuges and belt filters. This overdosing of polymer gives the operator a ‘buffer’ against process variations.

Lack of good automatic polymer control of the polymer can lead to:-

• increased polymer costs
• increased solids in the centrate
• increased foaming in the centrate
• decreased solids in the cake
• increased transport costs per tonne of waste
• blinding of the belt
• increased load on the works
• reduced efficiency of driers and digesters

The Solution

The CRIUS^® PolySense (Patent Pending) is a simple, but effective and reliable method of controlling the polymer dosing via the polymer dosing pump and in some cases the feed rate to the centrifuge or belt filter. It does this by making reliable online measurements of the dry solids of the cake, and the suspended solids in the centrate/filtrate, and in some cases the feed solids. These measurements are the key to the success of the CRIUS^® PolySense and the suspended solids instrument includes de-bubbling technology and self-cleaning where required.

Essentially the CRIUS^® PolySense measures the suspended solids in the centrate (or sludge feed) and the dry solids in the cake and adjusts the polymer dosing pump, (and in some cases the feed pump), to maintain those ranges of cake solids and centrate suspended solids that produce the best outcomes for the site.

Case History

In late 2008 a compliance engineer for one of the UK Waste Water Utilities contacted Pi in order to start the process of determining the best instrumentation to control his Alfa Laval dewatering centrifuge. He had determined that by manually controlling the centrifuge and constantly making small adjustments to the polymer dose he could significantly reduce his poly usage while maintaining the Cake Solids. Having determined this manually he approached Pi seeking to automate the process. This was a perfect application for the CRIUS^® PolySense. In consultation with the engineer, it was determined that he could achieve his goals using a two point measurement system (online cake solids and online suspended solids in the centrate), with the appropriate automatic polymer control algorithm feeding back to the DCS that would in turn control the polymer feed pump. His total cost including installation and commissioning was £26,000 and his average reduction in poly usage was 25% giving a payback inside 18 months. “Although the financial justification for the project was based on polymer savings the additional benefits were significant. The reduction in Carbon Footprint is as yet unquantified but significant, and the improved consistency of cake solids has resulted in higher average cake solids and a reduction in transportation costs. The operators have all bought in to the system and are happy that they are freed up to carry out their other duties secure in the knowledge that if anything should go wrong they will be informed via the alarms generated from the control signals. The company are now beginning the process of rolling the system out across other sites.”