In urban areas, faecal sludge often looks like this. It is extremely liquid and contains few solids. Often, more than 95% of faecal sludge is water. Dewatering of this sludge usually requires a lot of space that is commonly not available in urban areas. In this module we introduce you to settling-thickening tanks that can reduce the volume that requires dewatering by more than 50%. No, it's not by magic. It is by gravity. Following this module, you will be able to explain the treatment process and operation of settling-thickening tanks. Discuss operation and maintenance requirements, and name design parameters specific to settling-thickening tanks. This slide shows a cross-section of a settling-thickening tank. The treatment of settling-thickening tank is based on sedimentation. Faecal sludge enters the tank through inlet at the front. As an outlet for the sludge is only provided at the back end, the sludge has to travel through the tank. During its way through the tank dense solids settle out at the bottom. These solids are further compressed by the weight of the settled sludge layer. As a result of these two processes called settling and thickening, the sludge that settle at the bottom of the tank has a higher solid content, and the effluent leaving the tank, a lower solid content than the thicker sludge discharged into the settling-thickening tank. So all settling-thickening tanks really do is to separate liquids and solids. Usually between 50 to 80% of suspended solids that are discharged into the settling-thickening tank stay in the tank, where as the remaining solids stay in the effluent. This means that both liquids effluent and the solids require further treatment. So what is the benefit of a settling-thickening tank? Let's consider a hypothetical example. This slide shows the change of sludge volume as a function of the solids content. As you can see from the slope of the curve the reduction in sludge volume is highest at low solid contents. Let's assume we have 1000 liters of sludge with a solid content of 1% which is typical for septic tank sludge. How will this volume change when we increase the solid content from 1% to 5%? Or when we increase it from 1% to 10%? These are typical solids contents from settling-thickening tanks in Accra and Dakar. Increased solid content would reduce the sludge volume to 200 liters for 5% solids and 100 liters for 10% solids. This is a reduction of the initial volume of 80 and 90%. As the required space for dewatering technologies, such as drying beds uses the function of the volume of sludge that needs to be dewatered, settling-thickening tanks can reduce the required footprint of sludge dewatering technologies. Settling-thickening tanks are usually loaded with sludge for between 1 week to 1 month. To increase the solid content in a settled sludge the sludge can be left for thickening beyond this period. This is usually done for several days or weeks, as shown here for anaerobic ponds another type of settling-thickening technology in Dakar. Following this period, the sludge that accumulates at the bottom requires removal before another cycle of loading and thickening can be started. This sludge removal should be done by pumps to ensure protection of workers. backhoes, tractors, or front-loaders or a combination of those may be applied when sludge is too thick to be pumped. This slide here shows a backhoes in the background removing sludge from an anaerobic pond in Dar es Salaam. As you can see in this picture, removal of sludge from settling-thickening tanks can be a challenge. But is an absolute must to ensure that they operate as designed. This will be covered in more detail in another module with a design example of a settling-thickening tank. Maintenance of pumps and tractors, as shown in Kampala, is the most important maintenance activity for settling-thickening tanks. In order to ensure that faecal sludge can be treated during thickening, sludge removal and also maintenance activities, two settling-thickening tanks are usually operating in parallel or alternately. In this picture from Dakar, one settling-thickening tank is being loaded, while the other one is left for settling and thickening. What you also notice on this picture is that this settling-thickening tank is covered with a layer. We call this layer the scum layer. This layer also requires removal and management after settling-thickening period of a settling-thickening tank. The settling velocity and the solid liquid separation efficiency are design parameters that are specific to settling-thickening tanks. The settling velocity is an estimate of the speed with which solids settle to the bottom of a settling-thickening tank. If the speed with which the solids travel through the settling-thickening tank is faster than the settling velocity the sludge will leave the tank with the effluent. For example, with this pathway here. If the settling velocity is faster than the velocity with which the solids travel through the tank, the solids will settle out at the bottom. Based on experience from Accra, Kampala, and Dakar, a settling velocity of equal or smaller than 0.5 meters/second seems to be appropriate for faecal sludge in settling-thickening tanks. But in contrast to waste water sludge treatment experience is lacking on the appropriate design values for settling velocity of faecal sludge. Due to the wide range of faecal sludge characteristics, from different on-site sanitation technologies or cities. In general, unstabilized faecal sludge for example, from public toilets as shown here appear to have poorer solid liquid separation in settling-thickening tanks, for example as compared to septic tank sludge. Due to this high variability in characteristics, the design of settling-thickening tanks requires bench scale testing with Imhoff cones, with the actual sludge that would be treated in the settling-thickening tanks. This slide shows four Imhoff cones. To estimate the sludge volume index, 1 liter of sludge is settled in these Imhoff cones for around 30 to 60 minutes. The sludge volume index is then calculated from the volume that settles at the bottom of these Imhoff cones divided by the initial suspended solids concentration. Based on waste water sludge treatment, a sludge with a sludge volume index of equal or smaller than 100 milliliters per gram TS, good solid liquid separation in settling-thickening tank can be expected. During such laboratory bench scale test, the solid liquid separation efficiency can also be calculated. It is an estimate of how many solids settle at the bottom and would stay in the tank, and how much of the solids would leave the tank with the effluent. A solid liquid separation efficiency of smaller than 80% could indicate that the sludge in your city has poor solid liquid separation in settling-thickening tanks. In addition, the solid liquid separation efficiency is important for the design of settling-thickening tanks and effluent treatment. This picture shows the effluent of a settling-thickening tank in Dar es Salaam. This effluent is further treated in waste stabilization ponds. It is important to consider that the solid liquid separation efficiency that you will have in full-scale settling-thickening tanks will be lower than the ones you've obtained in controlled laboratory or bench scale testing. A discharge area for sludge, as shown here before a settling-thickening tank, can be a good way for mitigating this turbulence. In addition, baffles at the inflow and at the outflow of settling-thickening tanks can be a good way of reducing turbulences and ensure that you have good solid liquid separation efficiencies in settling-thickening tanks. In this module we learned that the treatment process and operation of settling-thickening tanks is based on sedimentation. Frequent removal of sludge is important for settling-thickenings so they operate as designed. This requires maintenance of mechanical equipment, such as pumps and front-loaders. Design parameters specific to settling-thickening tanks are related to the sludge properties. And include the settling velocity, and solid liquid separation efficiency.
