HOW TO INCREASE YOUR DRY SOLIDS BY 20% AND REDUCE POLYMER COSTS – A UK CASE HISTORY.
Author: RIDING, MICHAEL
Source: Proceedings of the Water Environment Federation, Residuals and Biosolids Management 2002 , pp. 324-332(9)
Publisher: Water Environment Federation
Abstract:Using a Streaming Current Monitor a trial was conducted at a UK Waste Water Treatment Plant. The instrument was installed on a Simon Hartley belt press. The objectives of the trial were to investigate the possibilities of a Streaming Current Monitor being used successfully to achieve one or more of the following goals;
Increase cake solids
Decrease Poly usage for the same dry solids
Provide an alarm for catastrophic failure of: belt, solids flow, poly flow etc.
Automatically varying the poly dosage to optimise 1 or 2 above.
In most WWTW the ideal system setup is left to an operator to establish using ‘eye’ and ‘experience’. The operator varies a number of factors to try and achieve an optimum setting. These are typically; sludge flow, poly flow and belt speed. The operator will generally be trying to balance high throughput with good cake solids while not spending too much on poly!
The operator also has to leave the system to attend to other duties around the plant so usual process variations cannot be allowed to upset the system catastrophically.
Typically this means that the operator will set the belt speed and the sludge flow and will vary the polymer feed pump throughout the day to manage the changes in sludge density, cationic demand etc.
What is needed is a device that can follow the process variations (i.e. polymer requirement), such as flow rates (both sludge and poly), and more importantly the polymer demand of the incoming sludge. Such a device could vary the poly dose effectively maintaining an ‘optimum’ setting, irrespective of normal process variations.
The streaming current monitor does this by measuring the filtrate ‘charge’. This is the amount of cationicity/anionicity left over after the sludge has been pressed or centrifuged.
Against our trial goals of:
1. Increase cake solids
The data clearly shows that there is scope for significantly increasing the cake solids (by 20%), by decreasing and controlling the poly dosage.
2. Decrease poly usage for the same dry solids.
To obtain the same dry solids the polymer can be reduced by 50% if it is controlled such that the set point can follow process variations.
3. Provide an alarm for catastrophic failure.
The data shows that any drastic change in the process produces a correspondingly drastic change in SCM reading (see graph below). Alarms can be set to warn an operator that such an event had occurred.
4. Automatically vary the poly dosage to optimise 1 or 2 above.
When the SCM was allowed to control the polymer addition it could be set to optimise either 1 or 2 above. Over the period of the trial the solids were improved by up to 20% and the polymer usage was reduced.
A case will be made, with supporting data, for using a Streaming Current Monitor to control the Polymer dosing on a belt press.
Document Type: Research Article
Publication date: January 1, 2002
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