Proceedings

Abstract: 
The new series of cationic water-soluble flocculants for sludge dewatering, XD-series, has been
developed by Kemira, a global chemicals company serving customers at municipalities and in water
intensive industries. Steps to develop the new products in the laboratory are described including
performance comparison to conventional cationic flocculants in biosolids conditioning. New testing
methods to validate the performance of the flocculant in laboratory scale in terms of free-drainage time
and sludge cake dry solids content have been developed and optimized in order to imitate the full-scale
sludge dewatering processes as close as possible. First successful full-scale results show improved
tolerance for shear forces in solid-liquid separation by centrifugation with up to 2-3% improved cake
dry solids after dewatering with even 20% lower polymer dosage, still achieving an improved more clean
centrate quality.

Introduction:
Wastewater residues, known as either sludges or biosolids, are generated through efficient wastewater
treatment processes. Sewage sludge is formed as a by-product of the different treatment stages of raw
sewage from domestic households, but may also include industrial and commercial effluent. Mechanical
dewatering takes place to reduce the water content of the sludge product resulting in drier sludge cake
and filtrate of water fraction. The dry solids content of a dewatered sludge cake plays an essential role
when determining the efficiency of mechanical dewatering process. Higher dry solids content means
lower overall volume of sludge leading to lower disposal costs. Centrifuges are commonly used as
dewatering device. In centrifuge the sludge flocs are exposed to high shear forces. In order to improve
mechanical dewatering and get a cleaner centrate, cationic flocculating agents are commonly used; this
process step is also called biosolids or sludge conditioning. When using centrifugation, the flocs formed
in conditioning need to obtain a specifically good shear resistance (Gillberg 2003, Williams 2005,
Dieudé-Fauvel et al. 2011).
There are many studies about the different parameters affecting the efficiency of flocculation as a
conditioning step prior to dewatering; one of the factors influencing is the flocculant structure.
Polyacrylamides and different co-polymers of polyacrylamides as well as different polymer blends and
different structures – linear, cross-linked, branched – have been studied in flocculation of different
processes. Charge density has an effect on how the branching is going to effect the flocculation. In one
of the flocculation studies performed for a papermaking pulp suspension it has been found out that floc
stability and reflocculation ability increased when highly charged and branched cationic polyacrylamides
were used (Blanco, A. et al. 2009). It has been also suggested that flocculant dosage is the main
parameter influencing sludge rheological and structural properties (Dieudé-Fauvel et al.2011).