Assessment of membranes used in drying faecal sludge to minimise the sand take-away in the sludge drying beds

Abstract

Sludge drying beds provide sludge dewatering by allowing the liquid to both drain under gravity through a permeable medium on which the sludge sits, and to evaporate under ambient conditions. Lubigi plant with 19 sludge drying beds handles faecal sludge from pit latrines and septic tanks separately for sludge dewatering. Clogging in filter beds and sand membrane material is challenging as it continues to get off with dry sludge cakes. The study's main aim was to assess different membranes used in faecal sludge drying to minimise the sand layer takeaway in the sludge drying beds at Lubigi Faecal Sludge and Wastewater Treatment Plant. This study analysed the chemical and physical characteristics of faecal sludge in the existing and unplanted drying beds at Lubigi. A pilot-scale dewatering facility was designed and constructed with four unplanted sludge drying beds (A, B, C, and D) with a one m2 effective drying area. The cumulative percolate was measured following a non-uniform time interval of 5, 10, 30, 60, 180, 360, 720, and 1440 minutes. This lasted for ten days until the faecal sludge was dewatered entirely. Five raw faecal sludge samples collected from different points of the existing and operating beds were analysed using the Makerere University laboratory. The cake samples had average weights ranging from 0.48 to 46.98% of the respective chemical elements. Carbon and oxygen had the highest weight ratios of 46.96% and 36.73%, whereas sodium had the lowest, with 0.48% in all elements, respectively. Low sodium content implies low alkaline indices in the faecal sludge, which is desirable. Additionally, moisture content (ranging from 73.68% to 75.91 % by mass, mean= 74.81±1.11 %), sand content (between 17.43% (89.62 g) and 19.66% (118.08 g), mean = 18.344±0.86%) and calorific value (between 2846.38 kcal/kg to 3185.46 kcal/kg. Sand (Bed A) exhibited the highest mean dewatering rates with a peak of 502 mm after 4.5 hrs, followed by pavers with 416 mm after 7.5 hrs, nylon with 384 mm in 4.5 hrs and lastly, hessian with 59mm after 20hrs. The efficiency shows that hessian, nylon and pavers were efficient (R2 = 0.632, 0.592, and 0.540), followed by sand (R2 = 0.491). All membranes had a very significant correlation between dewatering time (t min) and percolate (Q ml), p < 0.05 based on a> 95% confidence level. Nevertheless, nylon expedited average filtration rates at all times compared to the rest of the media. This is because its cake yielded moderate moisture, sand, and calorific values compared to the rest of the media hence optimised. Nylon filter media was optimised based on different gauge sizes of 1.0 mm (reference size) (Bed A), 0.5 mm (Bed B), and 2.00 mm (Bed C). The average dewatering times of the filter media in Beds A, B, and C were 5, 60, 180, and 360 minutes, respectively. Bed C (2.0 mm) had the highest flow rate (156.61 ml) but the lowest sand content (82.14%, 85.16%), Bed B (0.5mm) had the lowest flow rate (49.09 ml) and the highest levels of moisture content (86.91 %, 90.92%) and Bed A (1.00mm) had the intermediate flowrate (83.37 ml), moisture (83.28%, 87.28%), sand (13.24%, 14.8%, and calorific value (2745.12 Kcal/kg, 2600.12 Kcal/kg). These were determined based on the size of holes in the filter media, hence recommending the 1.00 mm nylon gauge. Keywords: Calorific value, Dewatering, Faecal Sludge, Lubigi, Moisture Content, Optimisation, Sludge Drying Beds and Sand Content.