Ascaris viability and assessment of risk for a vermicomposting ecological sanitation system in El Alto, Bolivia Público

Collender, Philip Andrew (2013)

Permanent URL: https://etd.library.emory.edu/concern/etds/3r074v60w?locale=es
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Abstract

An estimated 64 million Disability Adjusted Life-Years (DALYs) are lost to diseases caused by unsafe water, poor sanitation and poor hygiene every year, mostly in developing countries. Ecological sanitation (Ecosan) provides low-cost sanitation appropriate for many developing regions, funded through the sale and reuse of excreta for horticultural applications. Reuse of incompletely sanitized excreta may result in transmission of infectious agents, especially the ova of helminths such as Ascaris lumbricoides. Composting with earthworms, or vermicomposting, is a potential sanitization technique, but has received little formal study. To address this knowledge gap and provide guidance for a Bolivian NGO managing an Ecosan system in El Alto, Bolivia, Ascaris ova were quantified in samples of vermicomposts after 3, 6, 8, 13, or 18 months of composting. Bayesian models of inactivation of Ascaris in the vermicomposts estimated 97.5th percentile times for 90% inactivation of thousands of months, and did not indicate statistically significant decay over time. Best-case estimates of the median annual burden of disease for consumers of raw produce fertilized with vermicompost ranged from 1.64*10-5 to 8.25*10-2 DALYs/person/year, depending on produce type and the dose-response model used. Best-case estimates of the median annual burden of disease for agricultural workers laboring on plots fertilized with vermicompost ranged from 6.05*10-7 to 1.98*10-2 DALYs/person/year. Best-case estimates of the median annual burden of disease for children playing in parks fertilized with vermicomposts ranged from 9.05*10-4 to 6.84*10-2 DALYs/person/year. Estimated burdens of disease for most scenarios did not fall below 10-4 DALYs/person/year unless the concentration of viable Ascaris ova in the vermicompost was at levels undetectable (≤0.25 ova/g total solids) by the USEPA method for Ascaris detection in biosolids. Due to the unacceptably high burdens of disease estimated for children and consumers of produce, even when ova concentrations are below the limit of detection, a conservative recommendation to protect public health is to restrict application of biosolids from public spaces and crops that might be eaten raw unless helminth infections are known to be extremely rare in the population. Future research is recommended to address assumptions made in the risk models constructed for the present study.


Table of Contents

Role ........................................................................................................................................1
Abstract ...................................................................................................................................2
Literature Review ......................................................................................................................3
Health Risks and Standards for Safe Re-utilization of Ecosan Compost ..............................................5
Survival and Inactivation of Ascaris Ova in Soils and Compost .........................................................8
The Suitability of Vermicomposting for Sanitization ......................................................................16
Quantitative Microbial Risk Assessment of Ecological Sanitation Systems .........................................27
Study Description and Context ..................................................................................................34
Methods..................................................................................................................................39
Sampling ................................................................................................................................39
Recovery and Quantification of Ascaris Ova .................................................................................39
Measurement of Physical Characteristics of the Compost Samples ..................................................41
Data Management ....................................................................................................................41
Bayesian Statistical Model for Ova Concentrations ........................................................................41
Bayesian Models for Ascaris Inactivation Rate in the Vermicomposts ...............................................43
Microbial Risk Assessment Models ..............................................................................................46
Sensitivity Analyses ..................................................................................................................55
Modeled Accumulation of Ascaris Ova in Soil with Repeated Applications ..........................................59
Results.....................................................................................................................................60
Laboratory Measurements ..........................................................................................................60
Modeled Distributions of Viable Ova Concentrations .......................................................................62
Modeled Decay of Ascaris Ova in Vermicomposts ...........................................................................66
Risk Scenario 1: Consumption of Raw Produce Fertilized with Vermicompost .....................................67
Risk Scenario 2: Soil Ingestion by Agricultural Workers at Sites Fertilized with Vermicompost ..............84
Risk Scenario 3: Soil Ingestion by Children at Parks with Groundcover Fertilized with Vermicompost .....93
Modeled Accumulation of Ascaris Ova in Soil with Repeated Application of Vermicompost ...................105
Discussion................................................................................................................................108
Conclusion ...............................................................................................................................119
Bibliography .............................................................................................................................121

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