Human excreta use in agriculture in Vietnam : health, economic and environmental aspects

The use of excreta in agriculture has a long tradition in Vietnam. While this practice has advantages in terms of environmental and economic impacts, it represents potential health risks if excreta are not properly managed. International and national organizations have established guidelines for safe use of excreta in agriculture to ensure the safety of health for users. However, information on excreta treatment and health risk is still missing to refine the guidelines and policies on excreta handling and health protection. The goal of this PhD work is to identify proper options for storing and using human excreta in agriculture in a perspective of minimizing the health safety and maximizing the environmental and economic benefit.
The objective of this study was to assess the health, economic and environmental impact of human excreta use in agriculture in Vietnam. The specific objectives were as follows: i) to define the main environmental factors that influence the die-off of Ascaris lumbricoides egg as well as nutrient values of excreta along the process of storage and use in agriculture, ii) to assess human exposure to excreta handling along the process of storage and use in agriculture focusing on the pathways, frequency, and intensity of exposure, and then to estimate the health risk by handling human and animal waste, iii) to assess the environmental and economic impact of excreta use.
The study used an integrated approach of experiments in the field and laboratory, and coupled with health, environmental and economic analyses. First, human excreta were collected and stored in an experimental station that is built under semi-field conditions. Main environmental factors chosen for the study were temperature and pH, which are controlled by the addition of rice husk, lime and ash – locally available materials – to study the influence of these factors on the die-off of Ascaris lumbricoides eggs and nutrient values of stored excreta. Human exposure to excreta, main pathways of exposure as well as intensity of exposure in agricultural activities and environmental sanitation systems were assessed by participatory observation, surveys and experiments. Quantitative data on the transmission of Ascaris lumbricoides eggs in excreta to humans through involuntary ingestion from “soil to hand” and “hand to mouth” was collected. Health risk related to waste reuse and waste handling from biogas systems were evaluated by quantitative microbial risk assessment (QMRA) which includes the assessment of the risk of Ascaris lumbricoides infection during excreta reuse and biogas handling. Material Flow Analysis was used to assess the environmental impact of sanitation systems. Finally, a Systematic Literature Review (SLR) combined with field data were used to study the profitability of human excreta reuse in agriculture. The study was conducted in Kim Bang district, Ha Nam province, Vietnam.
The results showed that the number of Ascaris lumbricoides eggs in all vault options stored excreta at was less than 1 egg per gram of total solid excreta at the 181st storage day, and thus the stored excreta met the World Health Organization (WHO) safety limit for reuse in agriculture. A mixture of high pH and the addition of 10% lime already reached the WHO standard for safe reuse of excreta at 111 storage days. With regard to human exposure, we estimated that a farmer may ingest 91 mg of excreta per year (95% confidence interval: 73-110; range: 5-1082 mg). QMRA simulation models predicted annual risks of helminth infection for farmer used excreta stored from 3.7 months to 6 months and used excreta stored below 3 months were 1.69±5.88% and 9.67±16.37% respectively. The annual diarrhea risk caused by exposure to biogas effluent through irrigation activities ranged from 17.4 to 21.1% (E. coli O157:H7), 1.0 to 2.3% (G. lamblia), and 0.2 to 0.5% (C. parvum), while those caused through unblocking drains connected to biogas effluent tanks were 22.0% (E. coli), 0.7% (G. lamblia), and 0.5% (C. parvum). The Material Flow Analysis (MFA) of excreta utilization showed that half of the pig manure was collected for biogas production, and the remainders were freely discharged to the commune’s drainage system (25%) or directly reused in the paddy fields (25%). While wastewater in the drainage system was the biggest source of nitrogen (contributed 46%), paddy fields were the biggest source of phosphorous (contributed 55%) discharged to the Nhue River, totaling 57 ± 9 ton N and 29 ± 6 ton P, annually. Consequently, mitigation measures for nutrient resource management were proposed. The most effective option was to to reuse all excreta and manure in the paddy fields which reduces the use of chemical fertilizers by 50%. The economic analysis identified average cost-savings were highest for farmers using fresh excreta (847’000 VND) followed by those who stored over 183 days (312’000 VND) and 153 days with 10% lime (37’000 VND/yr), without considering the health care costs of treating acute or chronic soil transmitted helminth infection.
In conclusions this PhD study could generate new knowledge on the behaviour of pathogens in human excreta in Vietnam under different handing conditions, the health, environmental and economic impact of human excreta management and its use in agriculture in Vietnam. It showed when and how excreta are safe enough to use in agriculture and how much we save lives, environmental quality and economic cost be doing so. These results can help plan the management and intervention to minimize the human health risks and maximize the benefits of human excreta and animal manure in agricultural areas of Vietnam.