Waste heat recovery from cement production for faecal sludge drying

In Africa and Asia, 65–100 % of urban residents are served by on-site sanitation facilities, such as septic tanks or latrines. These systems are typically emptied with suction trucks or manual labour. The contents are most commonly dumped directly into the environment, or disposed of in a treatment plant if one is available and affordable. This practice has its origin in the common perception that FS is a waste product without any value. However, this is a misperception, as faecal sludge not only contains nutrients for use as agricultural fertilisers but also could replace fossil fuel sources in industrial processes, such as boilers and kilns. Use of alternative fuel sources (e.g. tires, animal meal, sewage sludge, waste oil) in industrial kilns and boilers is a recent trend driven by increasing fossil fuel costs. However, before using FS which was passively dried on sand and gravel filter beds, it would need to undergo an additional drying process before being blown into a burner. A possible option for an energy efficient approach to eliminate excess moisture is the use of waste heat from the clinker production process in cement factories. The objective of this study was to assess the technical and economic feasibility of using waste heat from a cement factory in Rufisque, Senegal, to enhance the dryness of faecal sludge. Two sources of waste heat are generally available in a clinker production line: i) recovery of hot gases and ii) radiant heat loss from the kiln’s surface. In this case, the flue gas from the kiln was already used directly in the preheater and for drying the raw material, leaving the flue gas from the chimney as the only hot gas stream available for recovery. Its heat transfer rate is 2.8 MW, which would be sufficient to evaporate 2.5–3.1 tons of water per hour. If the total FS production of Dakar was dewatered on filter beds to 80% solids content, the recovered waste heat would be sufficient to achieve a 90% dry solids content. The resulting 26.7 tons of dried faecal sludge would cover ~2% of the daily energy requirements of the factory. [authors abstract]