Sewage sludge disposal is an increasing issue in all the European countries. The European commission encourages recycling of these sludges in agricultural applications, provided that sewage sludge is hygienised. In fact, this disposal route leads to the recycling of valuable plants nutrients (N and P), which contributes to the maintenance of the organic matter level in European soils.
Liming is recognized as one of the most effective, economical and widely used treatments to hygienize sewage sludge. As liming does not negatively affect and in some cases improves the bioavailability of sludge nitrogen and phosphorous, it can be considered as an advanced treatment, transforming sludge into a safe organo-mineral fertilizer. Additionally, liming affects the rheological properties of sludge.
These properties influence the farmer’s perception of limed sludge. Furthermore rheological properties of sewage sludge are of prime importance in designing parameters in transportation, storage, and ultimately, landspreading operations. They are also useful in controlling sludge treatment processes such as liming. The present study proposes a simple parameter to characterize sludge rheology, ‘the sludge’s cohesion’ and describes a reliable, quick and easy method to determine this parameter. This method has been applied on a series of dewatered sludges and on a series of limed dewatered sludges. It has been observed that an exponential relationship can describe the sludge’s cohesion as a function of the dry solids contents of sludges. Furthermore, the effect of the mixing time on sludge cohesion has been studied at the laboratory scale.
An example of monitoring of the physical stability of stockpiles of limed sludges is also given.
KEY WORDS Biosolids, Liming, Rheological properties, Structuration , Stockpiling ability, Cohesion, Texturometry.
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