A comparative study of faecal sludge stabilization with use of lime and black soldier fly larvae

Abstract

Faecal sludge contains nutrients such as phosphorus, nitrogen and potassium that are very beneficial to plant growth and agricultural reuse. It is for this reason that the demand for dried faecal sludge from farmers is very high at Lubigi sewage treatment plant. However, faecal sludge also contains disease-causing pathogens that can be transmitted to humans directly by coming into contact the faecal sludge, or indirectly by consuming drinking water or food contaminated by faecal sludge pathogens. Sludge is often sold before the end of the six-week storage period hence it is not effectively sanitized. It is on this basis that we decided to assess the possibility of enhancing sanitization of faecal sludge, with major focus on helminth die off, so as to have it in a period less than six weeks currently used, by comparing two methods, that is the use of quick lime and Black Soldier Fly larvae. With an optimal larvae density of 0.17 larvae/cm2 , we achieved a material reduction level of 81.01% on a wet weight basis and a 61.4% level of sanitization in relation to the initial concentration of viable helminths ova/g TS. With 15% optimal lime dosage, sanitization level of 88.5% was achieved while the control achieved 88.2%. This high level of sanitization in the Lime experiment was expected since high levels of pH, above 12, was maintained throughout the experiment. Unfortunately, none of the treatments was able to attain the requisite levels of sanitization by the end of the six-week period of storage, after which, the sludge is meant to be sold to farmers. The The lime experiment had a final Helminths ova content of 90 HO/g TS, the control had 92 HO/g TS and Black Soldier Fly larvae treatment had 302 HO/g TS out of the original 782 viable eggs. All these values are beyond the acceptable value for faecal sludge reuse, which is 1 HO/g TS. There is thus a need to extend the period of storage to achieve effective sanitization.