We have by now introduced you to several treatment technologies for faecal sludge. Treatment that provides an adequate level of environmental and public health protection requires the combination of several of these treatment technologies which have different treatment objectives. In this module, we walk you through process flow of this treatment plant here in Kampala, Uganda. And have a closer look at the design of settling-thickening tanks and unplanted drying beds. Following this module, you'll be able to draw a process flow of the faecal sludge treatment in Kampala and design settling-thickening tanks and unplanted drying beds. Welcome to the Lubigi waste water faecal sludge treatment plant in Kampala, which is being operated by the National Water and Sewerage Corporation since 2013. Thanks to Fishnam Water and Transportation that designed this treatment plant, we are able to share the design example of this treatment plant with you, which is slightly modified for training purposes. Lubigi is designed for the treatment of 400 cubic of faecal sludge that are mostly delivered by vacuum trucks that collect faecal sludge from septic tanks and pit latrines. A characterization study conducted before the treatment design estimated average concentration of solids, organics, nutrients and pathogens. For example, 26.4 kg total solids per cubic meter of faecal sludge. Following discharge at Lubigi. faecal sludge passes through screens for removal of solid waste and falls by gravity into settling-thickening tanks for solid and liquid separation. As discussed in another module, the design of settling-thickening tanks is based on the settling velocity and a solid liquid separation efficiency. For Lubigi a settling velocity of 0.125 meters per hour and a solid liquid separation efficiency of 80% were used in the design. These values were selected from preliminary guidelines developed at Sandec and its research partners in Ghana. The first step in the design of the settling-thickening tank at Lubigi was the calculation of the surface area based on the maximum amount of faecal sludge per hour, which is called the peak flow. In the design of Lubigi it was assumed that at some days a maximum of 600 cubic of faecal sludge will be delivered per day, within ten hours of operation. We can calculate the peak flow by dividing the maximum daily discharge volume by the operating hours. With this information we calculate the peak flow to 60 cubic meters per hour. We can then calculate the required surface area by dividing the peak flow by the settling velocity. This results in an area requirement of the settling-thickening tank of 480 square meters. The second step in the design of the settling-thickening tank at Lubigi was to determine the tank depth. This slide shows a cross section of a settling-thickening tank. Settling thickening tanks are commonly separated in the different layers. These are called the scum layer, the clear water of supernatant layer the separation layer and a thickening layer. The scum layer is the layer where the scum accumulates. The clear water layer includes the liquid effluent and the separation thickening layer is where settling and thickening occurs. Based on the preliminary guidelines developed in Ghana, for Lubigi a scum layer of 0.8 meter, a clear water layer of 0.5 meter, and a separation layer of 0.5 meter was considered for the design. The depth of the thickening layer needs to be calculated based on the volume of sludge that needs to be stored in this layer. The settling-thickening tank in Kampala was designed based on the loading period of 30 days. With this information we can calculate the mass of total solids that will accumulate in the settling thickening tanks during this period by multiplying the average daily discharge volume by the total solids concentration of the sludge and the solid liquid separation efficiency. with 30 days. This results in a mass of 253 tons of total solids. The design of Lubigi considered that 50% of this sludge is pumped once a week onto unplanted drying beds. And only the remaining 127 tons total solids require storage capacity in the four settling-thickening tank layers. According to the design, the sludge that would not be pumped would be removed following the 30 day settling and thickening period with front loaders and transported to covered storage areas as shown here. To calculate how much of this 127 tons accumulate in the thickening layer, the design needs to estimate the amount of total solids in the four layers of a settling-thickening tank. Lubigi assumed similar total solids concentration than those included in the preliminary guidelines from Ghana of 160 kg per cubic in a scum layer, 5 kg per cubic in a clear water layer, 60 kg per cubic in the separation layer and 140 kg per cubic meter in a thickening layer. With this information and the surface area that we calculated before, we can calculate how much total solids are included in the scum, clear water and separation layer by multiplying the area with the depth and a total solids concentration of the layer. As the total amount was 127 tons TS this means that 77 tons total solids will be stored in these layers whereas 15 tons total solids will require storage in the thickening layer. We can then calculate the volume of the thickening layer by dividing this mass by a total solids concentration in the thickening layer. This results in 357 cubic. By dividing this volume by the surface area of the tank we calculate the depth of the thickening layer to around 0.7 meters. This means that the settling thickening tank has a total depth of 2.5 meters and a total volume of 1200 cubic. The third step in the design of the settling-thickening tank in Lubigi was to select the width and the length. This slide shows the top view of the two settling-thickening tanks With sludge flowing from the right to the left side. Settling thickening tanks are usually 5 to 10 meters longer than they are wide, to allow settling and thickening of particles. At Lubigi, settling-thickening tanks were sized 10 meters wide and 50 meters long. This picture shows the settling thickening tanks in Lubigi as they were constructed. Two tanks were constructed to allow reliable operation during sludge removal and maintenance. Faecal sludge pumped from the settling thickening tanks during the 30-day period requires further dewatering and drying. At Lubigi, sludge from the settling thickening tanks is dewatered and dried on unplanted drying beds. As discussed in another module, the hydraulic loading rate and the solid loading rate are design parameters that are specific to unplanted drying beds. For the design of Lubigi hydraulic load of 0.3 meters and a solid loading rate of 300 kg total solids per square meter in year was used for the design. The first step in the design of the drying beds in Lubigi was the calculation of the drying bed area based on hydraulic loading rate. We calculated before that 127 tons of total solids will require treatment on unplanted drying beds within the 30-day period. The design assumes that sludge pumped from settling-thickening tanks has a total solids concentration of 80 kg per cubic meter. Which means that 12.5 cubic meter of sludge are around 1 ton of total solids. With this information, we can calculate the volume of sludge that will be pumped on drying beds within 30 days. By multiplying the mass of sludge to be pumped by 12.5 cubic meter per ton total solids. These are 1588 cubic meter. Based on the local climate, design assumes that a loading, dewatering, drying and unloading cycle on these drying beds, which are covered, will take around 30 days. With this information we can calculate the required drying bed area by dividing the volume discharge onto drying beds by the hydraulic loading rate. This means that 5,292 square meters are required. In the second, step the required drying bed area was validated with the maximum solid loading rate. By dividing the number of operation days per year, by the days of one drying cycle, we calculate the drying beds will be loaded around 12.2 times per year. By multiplying the number of cycles with the mass of sludge loaded onto one drying bed per cycle divided by the drying bed area, we can calculate the solid loading rate as 293 kg total solids per square meter and year. This is in reason to the solid loading rate considered for the design. This slide shows the drying beds in Lubigi. As you can see they were covered and the drying bed area was distributed to 18 equally sized drying beds. As shown in this picture, at Lubigi effluents from settling-thickening tanks and drying beds and waste water are co-treated in parallel in 3 anaerobic waste stabilization ponds. Followed by 2 facultative waste stabilization ponds. Sludge from this waste stabilization ponds is then being dewatered and dried on these uncovered drying beds shown here. In this module, we walked you through the process flow of the Lubigi waste water and faecal sludge treatment plant in Kampala. Where settling-thickening tanks, drying beds, waste stabilization ponds and storage are combined for treatment. We also introduced you to the design of settling-thickening tanks and drying beds. These designs were based on preliminary guidelines developed in Ghana. In the absence of clear design standards for faecal sludge treatment technologies, monitoring treatment plants like Lubigi could be really important to improve the designs for future implementations.
