Dewatering Optimisation Tool (DOT) and Stackability Assessment

Efficient dewatering is key to the management of dewatered materials and whilst the interaction between different physical and chemical propoerties of these materials are well documented, this general knowledge doesn’t give an indication of how dewaterable any specific sludge might be – or how well any individual dewatering process is operating. Similarly, it is vital that dewatered material is stable and is able to be stored without it ‘spreading’. For example, when stored in fields, the Nitrate Vulnerable Zone regulations require that all organic materials “occupy as small a surface area as is practically required to support the heap”. Slumped and spreading field heaps can damage growing crops, result in fines and intervention from the environmental regulator.

In response to these issues Aqua Enviro has developed a range of essential tests to assess the physical properties of biosolids, digestates, sludges and other dewaterable materials.

These tests can be utilised to:

  • Assess ultimate dewaterability
  • Compare potential and actual dewatering performance
  • Identify optimum polymer type and dose rate
  • Robustly determine the potential success of new dewatering approaches
  • Investigate aspects of dewatering that effect cake/liquor quality and stability
  • Assess the effect of different sludge blends on dewatering and cake slumping
  • Assess the effects of chemical additives (e.g. ferric, alum, lime) on dewaterability and stackability of products
  • Quantify the stackability of the material
  • Compare stackability with benchmark materials
  • Implement improved control strategies

Aqua Enviro utilise two main techniques for assessing the dewaterability of sludges, with these being Capillary Suction Time (CST) test and the DOT test. These tests can be used independently or in combination. The CST test is utilised as a comparative tool to identify the effects of different polymer type/dose on dewaterability, whereas the DOT test can be used to select the optimum polymer conditions for a given sludge type and identify maximum achievable cake DS% and filtrate quality in a typical dewatering process.

As the dewaterability of a sludge/material has a direct effect on the stability (or stackability) of the product, Aqua Enviro also offer a stackability test. The stackability test quantifies how the physical characteristics of the material will affect its degree of slump when stockpiled. This test includes a comparative assessment of slump against a control sample of known ‘good’ stackable quality, along with dry solids and iron concentration measurements.

The DOT and stackability tests have been successfully used to determine current dewatering performance against the derived optimum. Any shortfalls can be subsequently addressed in the field following onsite investigation.


Case Study

Aqua Enviro were commissioned to identify any shortfalls in dewatering processes for a domestic WwTW. DOT testing was undertaken with a sample of dewatering centrifuge feed being processed through the DOT under the same polymer conditions as was specified for the site.

Table 1:      DOT Results

sample description DS VS TSS % difference between current and potential performance
Units % % mg/L %
Onsite dewatered cake 24.6 72.4 14
DOT Dewatered Cake 28.6 72.4
Onsite filtrate 1200 31
DOT filtrate 823


Results indicated that under optimised conditions, it would be possible to improve cake DS% by 14% and reduce filtrate TSS by 31%.

Based upon these findings the client was able to conduct further investigation into optimising polymer usage, polymer mixing, and centrifuge operation for enhanced cake dry solids, improved stackability and a reduction in cake transport costs.

For a quotation or general query please contact our Laboratory Manager Charlotte Elliott by tel 01924 242 255 or email

For more information about how Aqua Enviro
can help you, contact us...

Charlotte Elliott

Laboratory Manager

01924 242255

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