Systematic characterization of faecal sludge from various sources for its use as a solid fuel

Faecal sludge (FS) is not extensively evaluated for its potential as a solid fuel mainly due to the general conception of its "highly variable characteristics" in relation to the wide range of on-site sanitation systems. An extensive and systematic FS characterization was therefore conducted on twenty-four samples collected directly from pit latrines, ventilated improved pit latrines (VIPs) and urine-diverting dehydrating toilets (UDDTs) at two depths to understand the impact on properties relevant for combustion. The higher heating value (HHV) for these samples lies between 13 to 22 MJ/kg DM (dry matter). However, such high values should be taken with caution since the measurement guidelines recommended the removal of the large inert pieces found in FS. Besides this potential bias of procedure, differences could be observed between containments, where pit latrines showed the largest variability as compared to VIP and UDDT. These differences are mainly correlated with the ash content, ranging from 15 to 50% DM, while the organic elements concentrations were similar for all samples. Interestingly, the same major inorganic elements could be measured in all samples, namely Si, P, Ca and K followed by Mg and Na. Such similar composition is probably due to similar sanitation practices and staple diet. However, the overall concentration of minor elements was below 1000 ppm DM for most samples. The N concentrations were quite high, between 2.5 to 4.5% DM. Abovementioned results may be problematic for process and environmental aspects if FS is combusted alone. FS can therefore be suitable preferably for co-combustion in blend with lignocellulosic biomass waste.