Graduation Year

2015

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

James R. Mihelcic, Ph.D.

Co-Major Professor

Sarina J. Ergas, Ph.D.

Committee Member

Jeffrey A. Cunningham, Ph.D.

Committee Member

Ricardo Izurieta, M.D.

Committee Member

Norma Alcantar, Ph.D.

Keywords

Ascaris suum, Developing world, E. Coli, Pathogen removal, Resource recovery, Technology transfer

Abstract

Anaerobic digestion is a biochemical process in which organic carbon is biodegraded in an oxygen free environment through a microbial consortium. Engineered biological systems used for resource recovery often utilize anaerobic digestion to treat anthropogenic organic wastes by reclaiming the carbon as energy (methane gas) and a soil amendment (biosolids). Small-scale, or household, semi-continuous anaerobic digesters have been used in developed and developing countries for many decades to produce biogas from human and livestock waste, which is used for heating, lighting, and cooking. This application has been shown to improve the quality of life of the user. Although there is great potential for small-scale semi-continuous anaerobic digestion to provide much needed resource recovery functions and quality of life improvements in future development, the manner in which these systems are operated could lead to unintended consequences on human health because human waste often contains resistant pathogens. This paradigm is best demonstrated by soil-transmitted helminths that are known to be highly resilient in mesophilic anaerobic digestion environments and endemic to many developing countries. The idea that soil-transmitted helminths survive mesophilic anaerobic digestion is exacerbated when the biosolids from the digesters are land applied as a soil-amendment because this process fits perfectly into the lifecycle of soil-transmitted helminths that need soil environments to develop into infective larva.

This research was divided into three sections to investigate the fate of human pathogens during semi-continuous anaerobic digestion and investigate techniques to enhance their removal. The sections were: 1) an examination into the fate (embryonation, development, inactivation, destruction) of Ascaris suum ova during mesophilic semi-continuous anaerobic digestion, with an emphasis on increased inactivation, 2) an investigation into the performance (volatile solids (VS) removal, E. coli and Salmonella destruction, methane production) of semi-continuous mesophilic anaerobic digesters and the effect of variations to solids retention time (SRT) and feeding frequency, and 3) development and application of mathematical models for pathogen inactivation kinetics and typical semi-continuous reactor residence time distributions to predict the removal efficiency of Ascaris suum ova during semi-continuous anaerobic digestion under different operating conditions.

Results of these studies showed that during semi-continuous mesophilic anaerobic digestion variations in feeding frequency did not impact the fate of Ascaris suum ova or Salmonella; however it was observed that better removal of E. coli and higher methane production was achieved at the longer feeding interval (weekly). Additional results indicated that embryonated ova were destroyed faster than unembryonated ova under the experimental conditions, which suggests a potential mechanism to enhance removal of this common pathogen. Since an increased feeding interval proved to be beneficial for digester performance our findings suggest that wastes containing Ascaris suum ova could be stored in an aerated environment, for a period of time that does not negatively impact resource recovery, to lengthen the time between feedings and promote ova embryonation and ensuing destruction during digestion. Modeling results indicate that under mesophilic conditions (35oC) the ova of Ascaris suum could survive for 22 days and will not be completely removed from the effluent under typical feeding frequencies and average SRT were examined. Therefore, the use of anaerobic digestion as a resource recovery technology where soil-transmitted helminths proliferate should be applied with extra operational safeguards or be included as one step of several in a small-scale treatment train.

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