Predicting sludge dewaterability: A reverse-run mass balance approach to close the expectation gap

Smyth, M. and Minall, R.,

Aqua Enviro, UK



Sludge dewatering is integral to the cost effective recycling of Municipal Mesophilic Anaerobic Digestate to land.  Any mass balance or cost model for a plant in the municipal MAD market will make reasonable assumptions based upon a historical database for: i) the dry solids content (%DS) of the cake; ii) the mass of polymer or conditioning agent and dry solids (kg poly/tds); and iii) the quality of the return liquors.

A dewatering asset is designed to achieve performance guarantees for these measures and this is critical to the long term financial and operational success of any given project.  For new build assets the process guarantees are held by the principle contractor, often with a back-to-back agreement with the equipment supplier, and the contract will only be awarded to those willing to sign up to the guarantees.   It is imperative therefore that all parties have a good understanding of the feed sludge characteristics and how changes in these may impact upon performance.  Asset standards can be used to provide an indication of how the sludge and performance may vary, but they are not a guarantee that the feedstock to be processed will be within the limits defined.  Where there is a difference in what is achieved tough dewatering and what is anticipated (Expectation Gap) the mass balance can become a source of debate and controversy.  Quantifying why there is an Expectation Gap is therefore essential to achieving long term project success.

Depending upon the contract, the specified limits will be ‘easier’ or ‘harder’ to meet.  The volatile solids content of the sludge is increasingly understood as being critical to both polymer consumption and cake dry solids, whilst it has been suggested that orthophosphate-P can also adversely affect the outputs.

This paper reviews the factors known to influence sludge dewaterability; it details the investigative analysis required to input into a ‘reverse-run’ mass balance to provide a realistic output product quality for a range of different feedstocks.

Key Words

Keywords: cake dry solids, dewaterability optimisation tool, dewatering, polymer consumption


There is often a gap in what is thought to be achievable from a belt press, centrifuge, Bucher press or other dewatering asset and what is actually achieved.  It is not unusual for the final product quality to be lower and the chemical consumption higher than expected, and when this is the case operational budgets will be stretched for the life of the assets.

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