Evaluating the Microbial Safety of Heat-Treated Fecal Sludge for Black Soldier Fly Larvae Production in South Africa

Abstract

Incorporation of black soldier fly larvae (BSFL) in fecal sludge management shows promise as a resource recovery strategy. BSFL efficiently convert organic waste into valuable lipids and protein, which can be further processed into commercial products. Ensuring the microbial safety of waste-derived products is critical to the success of resource-oriented sanitation and requires the development of effective sludge treatment. This study evaluates the microbial treatment efficacy of the viscous heater (VH) for fecal sludge management and potential application of the VH in BSFL production. The VH is a heat-based fecal sludge treatment technology that harnesses the viscosity of fecal sludge to achieve pasteurization temperatures. Inactivation of in situ Escherichia coli, total coliform, heterotrophic bacteria, and somatic coliphage was evaluated in fecal sludge that was treated for 1-6 min at VH temperature set-points of 60°C and 80°C. The VH inactivated in situ E. coli, total coliform, and somatic coliphage in fecal sludge to below the limits of detection (1- to 5-log₁₀ inactivation) when operated at the 80°C set-point with a 1-min residence time. Both temperature set-points achieved 1- to 3-log₁₀ inactivation of in situ heterotrophic bacteria. The VH was also evaluated as a potential pretreatment step in BSFL production. BSFL grown in untreated and VH-treated fecal sludge demonstrated similar results, indicating little impact on the BSFL growth potential by VH-treatment. However, BSFL bioconversion rates were low for both substrates (1.6% ± 0.6% for untreated sludge and 2.1 ± 0.4 VH-treated fecal sludge).

Keywords: Hermetia illucens; South Africa; fecal sludge management; heat treatment; microbial inactivation; sanitation.

FIG. 1.

Schematic of the VH housed at the BioCycle facility located at the Isipingo. VH, viscous heater.

FIG. 2.

Emptying and transporting of UDT sludge (Mercer and Buckley, 2019). UDT, urine-diversion toilet.

FIG. 3.

Log₁₀ concentration of in situ (a) Escherichia coli (CFU/g), (b) total coliform (CFU/g), (c) heterotrophic bacteria (CFU/g), and (d) somatic coliphage (PFU/g) in fecal sludge after heat treatment at 60°C (Δ) or 80°C (●). Four batches of sludge were treated (Trials 1 to 4). Results are shown as log₁₀ concentration plots with the nondetect results displayed at the limit of detection. Error bars represent the standard deviation of triplicate measurements (n = 3). CFU, colony forming unit; PFU, plaque forming unit.

FIG. 4.

(a) BSFL growth in VH-treated and untreated fecal sludge. The average of measurements from two growth experiments is shown with the range as error bars (n = 2). (b) Moisture content in the VH-treated (n = 6) and untreated fecal sludge (n = 6) with or without larvae. The average moisture content of two reactors for each condition is presented. BSFL, black soldier fly larvae.

FIG. 5.

Log₁₀ concentrations of (a) heterotrophic bacteria (CFU/g), (b) Escherichia coli (CFU/g), and (c) total coliform (CFU/g) in fecal sludge stored with BSFL or without BSFL over 13 days. Fecal sludge was obtained from UDTs, screened to remove large debris, and either heat-treated at 80°C for 5 min using the VH or left untreated before the experiment. Results from reactors with VH-treated fecal sludge (n = 4) are shown in black, and results from reactors with untreated fecal sludge are shown in gray (n = 4). Results from reactors that contained BSFL are shown with (□) square labels and solid lines. Results from reactors without BSFL are shown with (○) circle labels and dotted lines. Controls consisted of treated or untreated sludge stored without BSFL or additional feed. Only initial E. coli and total coliform concentrations in VH-treated sludge were below the limit of detection (shown at the limit of detection).

FIG. 6.

Somatic coliphage with the control (without BSFL) and experiment (with BSFL) in untreated fecal sludge (n = 4). Somatic coliphage in the VH-treated fecal sludge (n = 4) were below the detection limit in all samples (data not shown).