Assessing clogging of laminated hydrophobic membrane during fecal sludge drying

Conditioning fecal sludge of public toilets with coupled zero-valent iron and persulfate: Efficiency and mechanism

This study investigated the efficiency of fecal sludge conditioning using peroxydisulfate (PDS) activated by zero-valent iron (ZVI). For fecal sludge obtained from public toilets in a densely-populated rural area in China, the ZVI/PDS coupling greatly improved its dewaterability as well as the supernatant quality in terms of organic matter and nutrient contents. The capillary suction time (CST) and supernatant turbidity of fecal sludge can be reduced up to 97% and 73% respectively in 10 min by the combination of 0.15 g/g TS ZVI and 0.2 g/g TS PDS. Protein removal, especially for tightly and loosely bound extracellular-polymeric-substance (EPS), is more linearly correlated to CST reduction than polysaccharide removal. Fecal sludge dewatering was improved by the hybrid functions of radical oxidation and iron coagulation. The ZVI/PDS treatment produced larger and looser flocs, probably because 1) surface ionic and hydrophilic groups of fecal sludge were reduced, 2) surface charge was neutralized, and 3) secondary structures of EPS proteins were altered by the radicals. The excellent fecal sludge dewatering was related to strengthened particle hydrophobicity and reduced sludge viscosity and compressibility. The results highlight that the ZVI/PDS combination is potentially an effective conditioning approach for fecal sludge from public toilets.

An integrated investigation on anaerobic membrane-based thickening of fecal sludge and the role of extracellular polymeric substances (EPS) in solid-liquid separation

Pre-thickening or primary treatment of fecal sludge (FS) is a major bottleneck in designing fecal sludge treatment plants. This research demonstrates a practical analysis to improve the thickening efficiency of FS using woven fiber microfiltration membrane. A laboratory-scale anaerobic membrane-based thickening tank (MBTT) was investigated. Firstly, the system was operated with unconditioned FS at a flux range of 1-3 L/m²h. Secondly, the system was operated at an optimized flux of 2 L/m²h with conditioners, chitosan, and charcoal dust, at their optimized dosages reported in previous studies. It was observed that the solids accumulation in MBTT was linear and was specific to net solids accumulation. Feed FS was thickened to around 15% of total solids (TS) in 9-14 days. The overall solids accumulation rate was higher with conditioned FS. The less EPS accumulation and higher dewaterability in conditioned FS reduced the brushing frequency of the membrane and consequently, the average filtration duration per cycle was increased. Strong correlations of dewatering time with floc size, EPS, and electrical conductivity, indicated that higher EPS in FS tends to form flocs which can increase the dewatering rate in unstabilized FS. In permeate, the average TS and COD_t removal observed were 72-78% and 87-91%, respectively.

A critical review on sludge management from onsite sanitation systems: A knowledge to be revised in the current situation

Access to safe excreta disposal is a basic human right and an essential element of human development. Developing countries (like India) have constructed millions of toilets with on-site sanitation technologies (OSTs). However, these OSTs lack the required management system for collection, transportation and disposal of Faecal Sludge (FS, sludge getting accumulated in OST), leading to considerable water and soil pollution. The major challenge in FS Management is the collation of scattered data and concentrated experiences (i.e., existing knowledge remains with the practitioners in the field without a written record). Therefore, a critical review of existing knowledge on OST, FS collection, transportation, and its utilisation is essential. This study highlighted shortcomings associated with existing OST, FS collection, transportation, and treatment systems. Furthermore, this study provided a framework for appropriate selection of OST based on available sanitation chains, and a critical analysis of FS characteristics (i.e., total solids, pH, electrical conductivity, biological oxygen demand, chemical oxygen demand, and pathogens of FS ranged from 830 to 72000 mg/L, 6.8 to 7.8, 1.9 to 3.5 mS/cm, 500-5000 mg/L, 100 to 49,000 mg/L, and 10⁶ to 10⁷ E-coli and 10³ to 10⁴ parasitic worms per litre of FS, respectively) to design treatment systems for FS utilisation as a resource in agricultural, aqua-culture, and construction application.

Drying of fecal sludge in 3D laminate enclosures for urban waste management

Laminated hydrophobic membranes have been proposed as liners for container-based sanitation systems in developing countries. The laminate allows drying of fecal sludge, which might significantly reduce the frequency of container emptying, while containing liquids and solids. While previous laboratory tests demonstrated rapid drying of fecal sludge or water retained in laminates, experiments did not assess the effects of system dimension or scale on performance. In this study fecal sludge drying and water evaporation were evaluated in 3D laminate boxes (decimeter scale) or 3D laminate-lined 40 L and 55 gallon drums (meter scale) that are prototypes of toilet containers for field application. A stagnant film model described fecal sludge drying and water evaporation in the laminate boxes and laminate-lined drums well. The effective diffusion length (λ) for the laminate was fitted in all systems and increased with system dimension and scale: λ increased by a factor of 1.4 from 1D decimeter-scale envelopes to 3D decimeter-scale boxes, and by a factor of 1.3-1.7 from 3D decimeter-scale boxes to 3D meter-scale drums. The longer λ with increasing dimension and scale is likely due to nonuniform temperature and relative humidity in the air outside the laminate and nonuniform temperature within the laminate. Using best-fit λ for the laminate-lined 40 L and 55 gallon drum experiments conducted in a controlled laboratory, drying was predicted for an 11-day field experiment. Although the air temperature and relative humidity varied significantly in the field tests from -1 °C to 26 °C and 35% to 97%, respectively, the stagnant film model predicted drying over the 11-day period reasonably well with total error ≤ 13% using 24-h average air temperature and relative humidity. Drying of fecal sludge in laminate-lined drums in the field might be adequately described with a stagnant film model using daily-average weather conditions, if wind speeds are low.