Wastewater and excreta use in agriculture in northern Vietnam : health risks and environmental impacts

Wastewater is commonly used in agriculture and aquaculture in developing countries but also in developed countries due to the growing water scarcity. In Vietnam, the use wastewater and excreta in agriculture has a long tradition. While this practice has clear advantages (fertiliser, economic impacts etc.), it harbours potential risks for health and environment. The aim of the thesis was to understand the health and environmental risks related to wastewater and excreta reuse in an agricultural community in Northern Vietnam, with a focus on intestinal parasitic infections and diarrhoeal diseases and nutrients flow. This work is to optimizing the benefit of wastewater and excreta reuse in agriculture in from health and environmental perspective, which allows proposing potential interventions for health and environmental improvement.
Methods
In the study area - 2 communes of Hoang Tay and Nhat Tan in Hanam province, Northern Vietnam - wastewater (i.e. Nhue River and local pond), human and animal excreta are commonly used as water irrigation and fertilisers: (i) Cross-sectional, cohort and nested case-control studies were conducted to assess the relative importance of exposure to wastewater and excreta for parasitic infection and diarrhoeal episodes. Exposure data were obtained from household and individual interviews. Stool examinations were used to assess infection status. (ii) Quantitative microbial risk assessment (QMRA) of Escherichia coli, Giardia lamblia and Cryptosporidium parvum infection due to the exposure to wastewater and excreta was conducted using multi-trial Monte Carlo simulations to estimate diarrhoeal risks. (iii) Material flow analysis (MFA) was used to analyse nitrogen (N) and phosphorus (P) flows in the environmental sanitation and agricultural systems.
Results
Helminth infections were prevalent (e.g. Ascaris lumbricoides 24%, Trichuris trichiura 40%, and any helminth infections 47%). Risk of infection increased for people having direct contact with Nhue River water (OR = 2.1, 95% CI 1.4-3.2), and using human excreta as fertiliser (OR = 1.5, 95% CI 1.0-2.3). Tap water use in household was a protective factor against T. trichiura infection (OR = 0.6, 95% CI 0.4-0.9). Entamoeba histolytica infection was not associated with contact with Nhue River and pond water, and human and animal excreta, but with close contact with domestic animals (OR = 5.9, 95% CI 1.9-18.9), never or rarely washed hands with soap (OR = 3.4, 95% CI 1.1-10.0) and average socioeconomic ststus (OR = 4.3, 95% CI 1.3-14.0). Diarrhoeal incidence in adults was 0.28 episodes per person per year (pppy). The direct contact with water from the Nhue River (OR = 2.4, 95% CI 1.2-4.7) and local ponds (OR = 2.3, 95% CI 1.3-4.3), handling practices of human excreta (OR = 5.4, 95% CI 1.4-21.1), and animal excreta (OR = 3.3, 95% CI 1.8-6.0) as fertilisers were important risk factors for diarrhoeal diseases. Furthermore, inadequate use of protective measures (OR = 6.9, 95% CI 3.5-13.9), close contact with people having diarrhoea (OR = 3.7, 95% CI 1.4-10.3), never or rarely washed hands with soap (OR = 3.3, 95% CI 1.8-6.3), eating raw vegetables the day before (OR = 2.4, 95% CI 1.2-4-6), and rainwater use in household for drinking (OR = 5.4, 95% CI 2.4-12.1) were also associated with increased the risks of diarrhoeal diseases.
QMRA revealed that the most hazardous exposures included direct contact with Nhue River, local pond and field water, household sewage, and composted excreta. The annual infection risks due to exposure to wastewater exceeded the WHO reference level (10-4, i.e. ? 1 infection per 10,000 individuals), e.g. in scenario of growing rice, G. lamblia caused an infection risk of 0.75, C. parvum (0.39), and E. coli (0.96). The annual diarrhoeal risks were much greater than the WHO threshold values of 10-3 (i.e. 0.001 pppy), e.g. due to G. lamblia (0.50), C. parvum (0.15) and DEC (0.24) in scenario of growing rice.
MFA simulations highlighted that the sanitation system is an important source of nutrients entering the surface water. Every year, 109 tonnes of N and 35 tonnes of P (75% N and 65% P from on-site sanitation system effluents) are discharged into the drainage system; and 118 tonnes of N and 25 tonnes of P released into surface water. Furthermore, simulations revealed that if nutrient management is not improved, levels of nutrients due to wastewater, faecal sludge, and organic solid waste will double until 2020.
Conclusions
In the agricultural settings, where wastewater and excreta are commonly used, important health and environmental impacts were documented. For mitigation purposes, personal hygiene practices and safe water and food consumption must be further addressed. Adequate on-site sanitation system technologies are warranted to assure waste treament and reduce nutrients discharge to the environment. Further investments in this direction are warranted to improve benefit-risk ratio for the agricultural community and increase sustainability of this agricultural system.