Inactivation of Ascaris Eggs in Human Fecal Material Through In Situ Production of Carboxylic Acids

Abstract

Discovering new ways to inactivate pathogens in human waste is critical for the improvement of worldwide access to sanitation and for the reduction of the environmental impact of conventional waste treatment processes. Here, we utilized the carboxylate platform and chain elongation to produce n-butyric acid and n-caproic acid via the anaerobic fermentation of human fecal material. Then, we inactivated Ascaris eggs through exposure to these carboxylic acids. Using batch experiments with human fecal material as substrate, we accumulated n-butyric acid and n-caproic acid at total concentrations (uncharged acid plus conjugate base) of 257 and 27.1 mM, respectively. We then showed that carboxylic acids at these concentrations inactivated Ascaris eggs when the pH was below the pK for the acids, causing them to exist primarily in the uncharged forms. We observed that uncharged carboxylic acids affected viability rather than the pH itself or conjugate bases. In addition, we modeled the viability of Ascaris eggs as a function of uncharged carboxylic acid concentration for n-butyric acid and n-caproic acid at exposure times of 2, 6, 12, and 20 days. The results presented here indicate that in situ biological production of carboxylic acids in HFM provides a promising method of pathogen inactivation and may lead to new developments in sanitation technology and treatment of fecal sludge.

Temperature-Phased Conversion of Acid Whey Waste Into Medium-Chain Carboxylic Acids via Lactic Acid: No External e-Donor

“…Beyond n-butyric acid (C4), the undissociated carboxylic acid is no longer miscible, with the maximum solubility concentration in water decreasing by more than 10-fold for each elongation step (Table S1). Many studies have also shown a trend of increasing toxicity with carbon-chain length [13][14][15][16][17] ; however, some Gram-negative bacteria show resistance to MCCAs longer than C8. 13,14 As a result, the MCCAs could be continuously extracted with a membrane-based liquid-liquid extraction (i.e., pertraction) or other extraction systems to prevent product inhibition.…”

Field-Scale Co-fermentation of Solid Waste From Urine-Diverting Dry Toilets (UDDT-SW) and Banana Waste to Produce Undissociated Carboxylic Acids to Inactivate Ascaris Eggs

“…One method under investigation uses anaerobic fermentation of human fecal material (HFM) to produce carboxylic acids (Harroff et al, 2017). Complex organic molecules are first broken down to short-chain carboxylic acids (e.g., acetic acid, propionic acid, and n-butyric acid) through hydrolysis and primary fermentation.…”

“…Therefore, molecules with even-numbered hydrocarbon chains are more common than those with odd-numbered chains, but both are produced. Previous work has shown that HFM can be used as substrate to produce up to 257 mM n-butyric acid, 11.3 mM n-valeric acid, and 27.1 mM n-caproic acid (Harroff et al, 2017).…”

“…Carboxylic acids have been used as food preservatives for decades due to their antimicrobial properties (Freese et al, 1973), and past work has demonstrated that for short-chain (one to five carbons) and medium-chain (six to eight carbons) carboxylic acids, antimicrobial activity increases as the length of the hydrocarbon chain increases (Abdul and Lloyd, 1985;Salsali et al, 2006;Royce et al, 2013). More recent work has demonstrated that carboxylic acids are also toxic to Ascaris eggs, which are commonly used as indicators for pathogen inactivation in waste treatment processes due to their high resistance to inactivation (Butkus et al, 2011;Harroff et al, 2017;Riungu et al, 2018a). The importance of hydrocarbon chain length has also been demonstrated for Ascaris inactivation.…”

“…The importance of hydrocarbon chain length has also been demonstrated for Ascaris inactivation. When exposure time, pH, and temperature were constant, ncaproic acid (C6) concentrations required for a 3-log reduction in Ascaris viability were more than an order of magnitude smaller than concentrations required for n-butyric acid (C4) (Harroff et al, 2017).…”