Sludge bomb: The impending sludge emptying and treatment crisis in Blantyre, Malawi.

Blantyre, Malawi has approximately one million people who rely on pit latrines for sanitation and yet there is a limited pit-emptying sector to empty, transport and safely discharge the faecal sludge. Over three years, we monitored the volume of sludge that was safely discharged at the authorized Wastewater Treatment Plants (WWTP) and in parallel, estimated the volume of sludge that is currently buried within pit latrines of Blantyre, based on measurements at 321 households. Comparing the volumes of sludge emptied to the volumes of sludge buried, we highlight the fact that a) there is a suspiciously low volume of pit-derived faecal sludge being safely disposed; b) most safely discharged sludge is taken from the small proportion of septic tanks in the city, and not from pit latrines; and c) that the cost of safely discharging does not seem to correlate with the volumes delivered (despite an option to discharge for free). There is a large and growing amount of sludge buried beneath the city which will need to be safely emptied and treated in coming years. The city and the WWTPs must move quickly to ensure that the businesses and infrastructure are prepared for the future faecal sludge management challenges that lay ahead, especially if they are to achieve SDG 6 by 2030.