Stickiness of faecal sludge for drying applications.

Abstract

Faecal sludge management (FSM) seeks to achieve safe disposal of faecal sludge with minimum risks to the environment and overall human population with the possibility of attaining resource recovery. Thermal drying is a solution in FSM; however, the stickiness of the faecal sludge during the drying process can lead to clogging, and sludge build-up on dryer surfaces potentially causing equipment fouling. Stickiness is a physical property of faecal sludge that influences the drying processes, and thus an understanding of the phenomena allows for better design of drying systems. Faecal sludge samples from ventilated improved pit (VIP) latrines and urine diversion dry toilets (UDDT) were analysed to characterize their stickiness. The stickiness was studied through their cohesive and adhesive forces components in a texture analyser. The temperature and moisture content of the faecal sludge was varied within the experiments from 25 to 80oC and moisture content from 20 to 90%wt to find limits of the sticky region and the sticky peak where the stickiness of faecal sludge is highest. The stickiness was correlated to the drying kinetics, water activity, rheological properties and consistency (Atterberg) limits. The sticky region was quantified by the use of the Stable microsystems TA. XT express texture analyser. The stickiness (cohesive and adhesive forces) of faecal sludge increased with increasing temperature and a decrease in moisture content. The increase in stickiness reached a peak for all temperatures investigated at a moisture content of 50%wt for UDDT sludge and a sticky peak of 60%wt for VIP sludge for all temperatures investigated. The maximum adhesive force required for the separation of the samples from the probe was lower than the force needed for the compression of the sample (cohesive force), implying that the faecal sludge has a greater cohesive force than adhesive force. The faecal sludge exhibited shear-thinning behaviour. The stickiness and viscosity at a shear rate of 1 s-1, increased by a factor of approximately 1.5 for every 10% decrease in moisture content for the sticky region. Therefore, this implies that the rheology and stickiness of the sludge are directly proportional within the limits of the sticky region. The effect of moisture content was determined to influence the stickiness more significantly than temperature. As such, the drying rate in dryers is significantly decreased during the sticky region. The drying rate reduced significantly during the sticky region. The stickiness of the sludge was seen to start after free moisture was removed during drying (water activity less than 1), the moisture removed during the sticky phase was interstitial. The plastic and liquid limits gave the region in which the plastic behaviour is observed corresponding to the sticky phase of sludge.