Improving faecal sludge dewatering efficiency of unplanted drying bed

This thesis examines how to improve on faecal sludge dewatering efficiency of unplanted filter beds to produce biosolids and less concentrated filtrate. The experiment was conducted on bench scale drying beds constructed at the Kwame Nkrumah University of Science and Technology campus. Local sand was graded into particle size ranges of 0.1- 0.5 mm, 0.5 - 1.0 mm and 1.0 - 1.7 mm, with uniformity coefficients of 2.422, 1.727 and 2.029, respectively. They were represented as filter media, FM1, FM2 and FM3 respectively. Faecal sludge, consisting of public toilet sludge and septage collected from cesspit emptiers discharging at Dompoase treatment ponds in Kumasi, Ghana, were mixed in the ratio of 1:1 by volume, and was dewatered using the three different filter media, FM1, FM2 and FM3 to determine which one was most efficient. The Solid Loading Rate (SLR) of the faecal sludge was varied in the phase two whereby public toilet sludge and septage, mixed in the ratio of 1:1, 1:2 and 1:3 by volume representing SLR1, SLR2 and SLR3 respectively. These were dewatered on filter medium one (FM1) which was selected in phase one. Six cycles of dewatering were run for each of the phases. Percolate volume was measured every 24 hours. The total solids (TS) of the faecal sludge used for dewatering varied in every cycle so the TS was kept constant at 36.64 g/l as SLR1 and 26.93 g/l as SLR3 in all the cycles in the phase three to determine the effect of constant TS on the dewatering process. Further improvement of the dewatering was investigated by mixing sawdust with faecal sludge in phase four. Different percentages of sawdust, 50%, 100%, 150% and 0% (control) by weight of the TS of the faecal sludge, with TS of 26.93 g/l which was selected in phase two, were mixed with the faecal sludge to determine the effect of different quantities of sawdust as physical conditioner on the dewatering of faecal sludge. The dried biosolids obtained from the different phases of dewatering were analysed for plant nutrients and heavy metals to determine their agricultural potential. The dewatering on different filter media, FM1, FM2, and FM3 showed average dewatering times of 9.8, 9.9 and 9.1 days respectively without significant differences (p=0.212). However the percolate quality showed significant differences between the different filter media in the removal of TS, TVS, SS, COD, DCOD and NH3-N with FM1 having the highest removal efficiency for each parameter. Accumulation of organic matter in the top 10 cm of the filter bed indicated that FM1 was least likely to clog since it had the least quantity of organic matter in the sand. It also produced the largest quantity of organic matter and thus had the potential to generate the most biosolids. The faecal sludge of SLR1, SLR2 and SLR3 dewatered significantly at different average dewatering times of 7.2, 4.8 and 3.8 days respectively. Removal efficiencies at the different solid loading rates, though very high for TS, SS, TVS, COD, DCOD, NH3-N, did not show any significant difference. Organic matter build up in the top 10 cm of the filter bed was least in SLR3, hence least likely to clog the filter bed. Again, SLR3, SLR2 and SLR1 showed the potential for annual generation of biosolids at 438, 421 and 379 (kg/m2/year) respectively. Therefore SLR3 of faecal sludge was recommended for dewatering on the selected filter bed. When TS of the faecal sludge for dewatering was kept constant and the number of cycles was increased to eight, FM1, FM2 and FM3, dewatering FS of SLR1 at constant TS of 36.64 g/l improved their dewatering time to 8.9, 8.8 and 8.7 days respectively while SLR3 with constant TS of 26.93 g/l, dewatering on FM1 had the shortest dewatering time of 4.4 days. SLR3 was significantly most efficient in removing TS, SS, TVS, COD and EC. Organic matter accumulation rate in the top 10cm of filter bed was least for SLR3 followed by FM1, FM2 and FM3. The percentage organic matter of the biosolid were 69, 68, 62 and 59, leading to estimation of annual organic matter production of 334, 193, 202 and 177 kg TVS/m2/year for SLR3, FM1, FM2 and FM3 respectively. The sawdust-faecal sludge mixture of 50%, 100%, 150% and 0% TS of faecal sludge dewatered at 5.3, 4.9, 3.9 and 5.6 days respectively with 150% being fastest to dewater. The 150% sawdust-faecal sludge treatment was most efficient with respect to removal of contaminant loads like TVS, SS, COD and NH3-N, but was least in TS and EC removal. The 150% sawdust-faecal sludge mixture showed the least organic matter accumulation rate in the top 10cm of the filter media, followed by the 100%, 50% and the 0% treatments. The percentage TVS of the biosolids produced were 70.1, 77.3, 80.6 and 66.3 for 50%, 100%, 150% and 0% sawdust-faecal sludge mixtures, leading to annual organic matter estimation of 359, 567, 987 and 225 (kg TVS/m2/year) respectively. The agricultural potential of the biosolids analysed showed that, dried biosolids from the filter beds had percentage carbon (C) ranging between 28% and 43.5% while percentage nitrogen (N) in the same samples ranged between 1.82% and 3.53% leading to C/N ratio, ranging between 8.7 and 23.9. The percentage phosphorus (P) in the same samples ranged between 1.73% and 3.69% while the percentage potassium (K) values were within the range of 0.66% and 1.67%. The maximum concentration of heavy metals recorded in the dried biosolids were 0.225, 4.38, 0.024, 0.89, 0.55 and 0.208 mg/kg for Cu, Fe, Pb, Cd, Zn and Mn, respectively which were all far below the respective standards permissible in biosolids worth for use in agriculture. Summary of criteria needed to improve dewatering efficiency of faecal sludge using unplanted filter beds The faecal sludge generated in most cities in Ghana consists of mostly septage and public toilet sludge which contain high contaminant load. This study has shown that it is possible to dewater it using unplanted sand filter beds. Sand particle sizes ranging between (0.1 - 0.5, 0.5 - 1.0 and 1.0 - 1.7) mm with uniformity coefficients of 2.422, 1.727 and 2.029, respectively are effective, with particle size of (0.1 - 0.5) mm being most effective. Using these particle size ranges, faecal sludge of 217 – 360 kgTS/m2 could be dewatered on the filter bed in one year which is equivalent to faecal sludge of 22 – 27 persons. Public toilet sludge (PTS) and septage ratio of 1:3 by volume is most effective in improving the dewatering time as well as generating largest volume of biosolids though other ratios like 1:1 and 1:2 are also effective. These ratios have the capacity of reducing dewatering time per cycle from 9 - 12 days to 4 – 7 days. This can dewater faecal sludge of between 379 – 532 kgTS/m2 of filter bed. Addition of sawdust of between 50% - 150% TS of faecal sludge by weight reduced the dewatering time to less than 4 days. Addition of sawdust of 150% TS is most effective in reducing the dewatering time and contaminant load removal. About 575 kg TS/m2 of faecal sludge can be dewatered on a filter bed of particle size range of 0.1 - 0.5 mm and a solid loading rate of 1:3 for PTS and septage in a year if sawdust of 150% TS of the faecal sludge is added. This generates almost three times the quantity of biosolids that could have been generated with very good organic bulking quality.

Source: KNUST