Microbial deodourisation and neutralisation of pit latrine odour causing compounds

Abstract

In this study, the degradation of one of the identified odour causing compounds from pit latrine feacal sludge, butyric acid, was determined to follow modified logistic, Gompertz and Richards models. The results revealed that the modified logistic model could be applied to adequately describe and predict Bacillus cereus and Serattia.marcessens growth under isothermal conditions from 25 to 45 oC. The maximum growth rates (µmax) and lag times (ʎ) derived from the modified logistic model were fitted to Ratkowsky model and inverse Ratkowsky model to determine the effect of temperature on µmax and ʎ, respectively. The equations to describe this relationship have been developed which may be valid to predict µmax and ʎ at sub-optimal temperatures. In order to realistically model the system, the activity was limited to the performance of key players in the degradation processes in pits, i.e., B.cereus and S.marcessens, which were identified earlier in feacal sludge using 16S rRNA genotype fingerprinting. Prior to the optimisation and modelling processes, the volatile organic compounds (VOCs) emitted from pit feacal sludge samples were identified using Gas Chromatography coupled with Time of Flight Mass Spectrometry (GC-ToF-MS) system and characterised. Nineteen VOCs including; alpha-pinene, butyric acid, dimethyldisulfide (DMDS), dimethyltrisulfide (DMTS), ethylacetate, ethylformate, indole, isobutyric acid, limonene, methyl thioacetate, methyl thiophene, p-Cresol, phenol, toluene, 1-propanol, 2-butanone and 2-methylbutyric acid were the most-frequently occurring compounds in the sampled pit latrines. The full 16S rRNA gene analysis yielded nine genotype homologies in the range 93-100% probability, i.e., Alcaligenes sp. strain SY1, Achromobacter animicus, Pseudomonas aeruginosa, S.marcescens, Achromobacter xylosoxidans, B.cereus, Lysinibacillus fusiformis, Bacillus methylotrophicus and Bacillus subtilis. An elimination matrix was designed to select most influential parameters of model compounds for the experiment and representative cultures for evaluation of degradation and deodourisation of pit latrines. Finally, based on degradation potential of butyric acid by different strains, six bacterial strains were selected for construction of bacterial consortia, which could be utilised in the formulation of bacterial cultures to be used in actual pit latrine biodeodourisation processes. Nineteen bacterial consortia were artificially prepared and consortium C3 that was composed of B.cereus and S.marcescens, resulted in remarkably higher butyric acid degradation efficiency. High Performance Liquid Chromatographic (HPLC) analysis showed 100% degradation of 1000 mg/L butyric acid after 16 h. The results indicated that some bacterial consortia may effectively degrade butyric acid, even though other bacterial consortia showed non-synergetic degradation of the compound despite high degradation activity in pure cultures. Clearly, the environmental conditions such as temperature, pH and inoculation size showed that they have an influence on butyric acid degradation of each of the members of the consortium C3.