Even after settling-thickening technologies, faecal sludge is typically mostly water. So how can you convert it into a solid product like as shown here? It could be used, for example, as a soil conditioner or a solid fuel. In this module we will cover how to achieve this with unplanted drying beds. Following this module, you will be able to explain the treatment process and operation of unplanted drying beds. You will be able to discuss operation and maintenance requirements and name design parameters that are specific to unplanted drying beds. Welcome to the Niayes Waste Water and Faecal Sludge Treatment Plant in Dakar. Here, faecal sludge and waste water sludge are dewatered and dried separately on unplanted drying beds. You can see sludge on the surface of the drying bed. But the heart of a drying bed really lies below the surface in the filter media. Let's have a closer look. This slide shows a cross-section of an unplanted drying bed. The filter media in a drying bed typically consists of different layers of gravel and sand in decreasing diameter. The bottom layer is typically made of different layers of gravel as shown here as a layer of coarse gravel and a layer of fine gravel. Based on designs from Kampala, Dakar and Accra the coarse gravel layer typically has a depth of 15 to 20 centimeters and the fine gravel of 10 centimeters. The size of the gravel is commonly 10 to 40 millimeters for the coarse gravel and 5 to 40 millimeters for the fine gravel. Below the bottom layer there is a perforated pipe for the collection of the effluent. This picture shows the perforated pipe and the coarse gravel layer during filling in at our pilot-scale research facility in Dar es Salaam. The top layer of the filter media is typically sand with a depth of 10 to 20 centimeters. Sludge is loaded on the surface of this sand filter layer for solid-liquid separation. This picture shows the surface of an unplanted drying bed next to a drying bed that has been loaded with faecal sludge. The treatment process on drying beds can be separated into two stages. Dewatering and drying. During dewatering, liquids and solids are separated by filtration. The free water and sludge percolates through the sand and the gravel filter layers, and is collected at the bottom with a drainage pipe for further treatment. The sludge accumulates on top of the sand filter layer. Following dewatering during drying, liquids and solids are further separated by evaporation. Dewatering usually takes several hours to days. Whereas the duration of drying is usually longer and takes several weeks to months. Similar to settling-thickening tanks, drying beds are designed for solid-liquid separation, and not sludge stabilization, nutrient management or pathogen inactivation. From a mass balance perspective, solids, organics, nutrients, pathogens, for the most part either stay in the liquids and leave with the drainage pipe or the solids on the top of the sand filter layer. For example, typically 95% of the solids remain on the surface of the sand filter layer as shown here in Kampala, where as the remaining solids stay in the liquid effluent. These effluents also called leachate or percolate. Depending on intended use, for example in agriculture both liquids and solids usually require further treatment. Solid-liquid separation on drying beds is a batch process. This means that beds are loaded once with sludge and once the desired solid cone has been obtained, for example for use in agriculture or for co-composting, the sludge has to be removed from the surface of the sand filter layer. This can either be done manually with shovels and wheelbarrows or mechanically with front-loaders. During unloading of sludge, care should be taken to not remove sand with the sludge. This increase the frequency of the replacement of the sand filter layer, and also reduce the volume of the sludge for resource recovery, for example, as a solid fuel. Drying beds have relatively low maintenance requirements. But they are equally important to ensure that they operate as designed. During each drying cycle, some sand sticks to the faecal sludge and is removed with it. This means that the sand filter layer needs to be replaced regularly as shown here at our facility in Dar es Salaam. A replacement of the sand filter layer may also be required when solids clog the filter layer. The design and operation of drying bed is typically based on the hydraulic and solid loading rate. Hydraulic loading rate is the height of sludge that is loading on a drying bed in one cycle. It is usually between 25 and 30 cm. The solid loading rate is the amount of solids that can be loaded on one square meter of drying bed per year without risking clogging. It is usually between 50 and 300 kg total solids per square meter and year. In tropical countries 100 to 200 kg total solids per square meter and year are appropriate. We'll use these design parameters in another module for the design of unplanted drying beds. So what is the dewatering time with unplanted drying bed in your city? Here are some important considerations to determine these times for your city which are important for the design of unplanted drying beds. It is important to consider the dewatering properties of faecal sludge in your city as they can be very variable. In our research, we saw significant difference in the dewatering properties of sludge from pit latrines in comparison to sludge from septic tanks. In addition to these sludge properties, the local climate also has an influence on the actual drying time. A high ambient temperature, low relative humidity so a dry climate, are optimal for drying. In climates with frequent rainfall, dewatering and drying times can be prolonged. In these climates with frequent rainfall a cover over the drying beds should be considered as here shown in Lusaka, to ensure year round operation. Research from Ghana identified that the dewatering time on a drying bed will also depend on the sand's diameter. A coarser sand, say for example, with a diameter of 1 to 1.5 millimeters means faster filtration, but an increased risk of clogging due to the accumulation of solids in the sand filter layer. In contrast, a finer sand, for example 0.1 to 0.5 millimeters means slower filtration, but also less risk of clogging. This diameter is often preferred over coarse sand. And ultimately, the drying time will also depend on the desired dryness you want to achieve. If you would like to use as fuel, 90% dryness is usually required whereas a much lower dryness is required if you want to do pelletizing or co-composting or you want to produce the feedstock for fly larvae composting. All these factors and different drying bed designs, operations and conditions, explain the large variability in dewatering and drying times reported in literature. Literature values can provide general references. However, characterization of the specific faecal sludge and bench scale testing are always required for the design of large-scale urban treatment plants. Based on operating experience from unplanted drying beds in various countries, some defined element are specially important to consider for reliable and long-term operation. Here they are. As you can see here, pumps transferring sludge often have a high pressure. This could disrupt the sand filter layer. This can be mitigated by a splash plate or separate inflow chambers. Next to the particle size of the sand filter layer the uniformity of particles is important. Fine particles need to be washed out before they are used. You can see this process at our pilot-scale research facility in Dar es Salaam. Over here there were still a lot of fine particles which were then continuously remove by washing. This is important to prevent clogging of the sand filter layer. It is worth to invest in the development of a local supply for the sand filter layer, considering that it has to be replaced regularly. The design also has to ensure the side walls are high enough and correspond to the hydraulic loading rate used in design plus a freeboard. In addition, a ramp can be helpful when removing the sludge from the drying beds. Unplanted drying beds require a lot of space. For example, in comparison to mechanical dewatering equipment such as screw press or filter press. Modification of drying beds have the potential to reduce the treatment times, and so also reduce the treatment footprint that is required. This for example, includes turning of the sludge from the surface of the drying beds. During our research in Dakar, we could reduce with these drying times by 20%. In this module, we learned that the treatment process of unplanted drying beds is based on filtration and evaporation. Unplanted drying beds are operated in batch mode and require removal of sludge from the surface of the drying beds. In addition, the replacement of the sand filter layer is required for reliable operation. Drying beds need to be designed based on the local climate and sludge characteristics. The most important design parameters include the hydraulic and the solid loading rate.
