Non-sewered sanitation (NSS) has been adopted by a significant population, especially in developing countries. For the effective operation of NSS, it is important to ensure optimal treatment of the collected faecal sludge (FS) in on-site sanitation systems. Solid-liquid separation is among the most important steps in the treatment of FS in NSS. The ability of the FS for solid-liquid separation has assumed greater significance in the design and operation of sludge treatment process. Although dewatering indices such as specific resistance to filtration (SRF) and capillary suction time (CST) have been extensively used to determine the limits of sludge dewaterability, these parameters do not provide information on the limits of unbound moisture. The unbound and bound moisture fractions of the FS samples were determined using different methods, and each method was evaluated for reliability. Different FS samples were collected from ventilated improved pit (VIP) latrines, urine diversion dehydrating toilets (UDDT) and septic tanks (ST). Batch settling tests (BST), centrifugation, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and water activity (WA) measurements were performed on the samples. While the BST provided the settleability of FS at gravitational force, and centrifugation estimated the relative limit of solid-liquid separability upon application of a force, the boundary of unbound moisture or the interface of unbound and bound moisture fractions as determined by TGA, DSC and WA. Evidently there was a fine overlap of the unbound and bound moisture at the interface due to presence of some tightly held unbound moisture and loosely held bound moisture. The unbound-bound moisture interface, thus, can be experimentally determined to be between 51.07 and 60.35 % for VIP sludge, 51.48-64.38 % for UDDT sludge, 62.29-66.71 % for ST-wGW sludge and 59.11-60.37 % for ST-GW sludge. Although hypothesis testing revealed no statistically significant difference between the methods and sample types, it can be concluded that WA demonstrated the highest reliability in terms of accuracy, ease of measurement, rapidity and repeatability.

The mechanical properties of faecal sludge (FS) influence its moisture retention characteristics to a greater extent than other properties. A comprehensive fundamental characterisation of the mechanical properties is scarcely discussed in the literature. This research focused on bulk and true densities, porosity, particle size distribution and zeta-potential, extracellular polymeric substances, rheology and dilatancy, microstructure analysis, and compactibility in the context of using the FS as a substitute for soil in land reclamation and bioremediation processes. FSs from different on-site sanitation systems were collected from around Durban, South Africa. The porosity of the FSs varied between 42% and 63%, with the zeta-potential being negative, below 10 mV. Over 95% of the particles were <1000 µm. With its presence in the inner part of the solid particles, tightly bound extra-cellular polymeric substances (TB-EPSs) influenced the stability of the sludge by tightly attaching to the cell walls, with the highest being in the septic tank with the greywater sample. More proteins than carbohydrates also confirmed characterised the anaerobic nature of the sludge. The results of the textural properties using a penetrometer showed that the initial slope of the positive part of the penetration curve was related to the stiffness of the sludge sample and similar to that of sewage sludge. The dynamic oscillatory measurements exhibited a firm gel-like behaviour with a linear viscoelastic behaviour of the sludges due to the change in EPSs because of anaerobicity. The high-TS samples exhibited the role of moisture as a lubricating agent on the motion of solid particles, leading to dilatancy with reduced moisture, where the yield stress was no longer associated with the viscous forces but with the frictional contacts of solid–solid particle interactions. The filtration–compression cell test showed good compactibility, but the presence of unbound moisture even at a high pressure of 300 kPa meant that not all unbound moisture was easily removable. The moisture retention behaviour of FS was influenced by its mechanical properties, and any interventional changes to these properties can result in the release of the bound moisture of FS.

The mechanical properties of faecal sludge (FS) influence its moisture retention characteristics to a greater extent, besides other properties. A comprehensive fundamental characterization of the mechanical properties is scarce in literature. This research focused on bulk and true densities, porosity, particle size distribution and zeta-potential, extracellular polymeric substances, rheology and dilatancy, microstructure analysis and compactibility, in the context of using the FS as a substitute for soil in land reclamation and bioremediation processes. FS from different on-site sanitation systems were collected from around Durban, South Africa. The porosity of the FS varied between 42% to 63%, with the zeta-potential in negative, below 10 mV. Over 95% of the particles were <1000 µm. The TB-EPS influenced the stability of the sludge, highest being in the septic tank with greywater sample. More proteins than carbohydrates also ascertained the anaerobic nature of the sludge. The results of the textural properties using penetrometer showed similar behaviour as that reported for sewage sludges. The dynamic oscillatory measurements exhibited a firm gel-like behaviour with a linear viscoelastic behaviour of the sludges due to the change in EPS because of anaerobicity. The high TS samples exhibited the role of moisture as a lubricating agent on the motion of solid particles, leading to dilatancy with reduced moisture, where the yield stress was no longer associated with the viscous forces but with the frictional contacts of solid-solid particle interactions. The filtration-compression cell test showed good compactibility, but presence of unbound moisture even at high pressure of 300 kPa.

In faecal sludges (FS) from non-sewered sanitation systems, bound moisture consti-tuted 46-67% of total moisture across all sanitation types investigated, yet the energet-ic basis for its resistance to removal has not previously been characterized. Existing classifications of moisture fractions lack quantitative binding energy data, leaving the thermodynamic limits of solid–liquid separation undefined for FS. This study investi-gates the distribution and binding energies of bound moisture fractions in FS obtained from ventilated pit latrines, urine-diverting dehydrating toilets, and septic tank sys-tems. Bound moisture fractions were determined using moisture sorption isotherms, low-temperature convective drying, nuclear magnetic resonance, and thermogravi-metric–differential scanning calorimetry analyses. Results show that interstitial mois-ture constituted 37–50% of total moisture, followed by vicinal (6–14%) and intracellu-lar (3–9%) fractions, with net isosteric heat rising sharply below 20–30% moisture content (w.b.). Evaporation enthalpy exceeded that of bulk water at moisture contents below ~30% (w.b.), consistent with EPS-mediated adsorption and capillary confine-ment contributing to increased energy requirements for moisture removal and indi-cating a transition from capillary-controlled to structure-influenced retention. These findings provide a thermodynamic basis for interpreting why conventional mechani-cal dewatering stalls at a residual moisture content that differs systematically between VIP, UDDT, and septic tank sludges. These insights are relevant for improving FS treatment strategies, particularly in selecting appropriate combinations of dewatering, drying, and pre-treatment processes.

In fecal sludges (FSs) from non-sewered sanitation systems, bound moisture constituted 46–67% of total moisture across all sanitation types investigated, yet the energetic basis for its resistance to removal has not previously been characterized. Existing classifications of moisture fractions lack quantitative binding energy data, leaving the thermodynamic limits of solid–liquid separation undefined for FS. This study investigates the distribution and binding energies of bound moisture fractions in FS obtained from ventilated pit latrines, urine-diverting dehydrating toilets, and septic tank systems. Bound moisture fractions were determined using moisture sorption isotherms, low-temperature convective drying, nuclear magnetic resonance, and thermogravimetric–differential scanning calorimetry analyses. Results show that interstitial moisture constituted 37–50% of total moisture, followed by vicinal (6–14%) and intracellular (3–9%) fractions, with net isosteric heat rising sharply below 20–30% moisture content (w.b.). Evaporation enthalpy exceeded that of bulk water at moisture contents below ~30% (w.b.), consistent with EPS-mediated adsorption and capillary confinement contributing to increased energy requirements for moisture removal and indicating a transition from capillary-controlled to structure-influenced retention. These findings provide a thermodynamic basis for interpreting why conventional mechanical dewatering stalls at a residual moisture content that differs systematically between VIP, UDDT, and septic tank sludges. These insights are relevant for improving FS treatment strategies, particularly in selecting appropriate combinations of dewatering, drying, and pre-treatment processes.