Quantitative Microbial Risk Assessment for faecal management – health consequences in the Mekong Delta, Vietnam

Abstract

Vietnam’s Mekong Delta (MD) is known as the rice bowl of the country. Rapid development and population growth there have lead to an increasing demand for water use and wastewater treatment. Yet there are no central wastewater treatment plants in the region and water supply systems are generally lacking in rural areas. Only septic tanks (STs) have been introduced to treat human effluent. Small-scale biogas plants, mostly plastic bio-digesters (PBDs) have been promoted to treat animal slurries. However, the operation and maintenance of both systems are unregulated and their microbial treatment efficacy has not been a priority. Poor sanitary practices of local people add to this creating a potentially serious health hazard. This study aims to analyse the microbial risk associated with faecal management in MD as it impacts on public health. <br /> The topic is explored by three vehicles: pilot study, field study and quantitative microbial risk assessment (QMRA). The pilot study replicates tropical conditions to determine microbial reduction and related factors of anaerobic treatment. Reduction rates of phages and bacteria in river water and on terrestrial spinach were determined. The field study was conducted in MD to verify the microbial make-up of faecal substrates, surface water and aquatic spinach. Pathogen treatment efficacy of PBDs was considered. A survey was also conducted to find out human exposure to contaminated sources. All data were used for the QMRA study, which calculates the probability and annual risk of infection via @Risk 5.5 (Palisade Corporation).<br /> The pilot study showed there was a hygienic effect in the anaerobic treatment of excreta but microbe reduction rates were low. The reduction of phages (somatic coliphage, male-specific bacteriophage) and bacteria (Escherichia coli, Salmonella Senftenberg, Enterococus faecalis) in lab-scale PBDs increased with longer hydraulic retention time (HRT). Longer HRT played a vital role in yielding more gas. Besides HRT pathogen reduction also depended on initial concentration, species tested and substrate type. High levels of volatile fatty acids (VFAs) had no effect on microbial reduction at neutral pH. Moreover phage and bacteria reduction also depend on operation conditions – batch-wise or continuous digestion. Microbial reduction in STs was not significant even at maximum HRT (3 days). <br /> Anaerobic digestion in tropical PBDs had little effect on the inactivation of Ascaris suum ova. Yet helminth ova do settle in the sludge at the reactor’s base if HRT is long and the relationship between an ova’s viability rate and sludge retention time was established by exponential equation. A few Ascaris suum ova survived in sludge for up to one year. There was no difference between the viability of Ascaris suum ova in biogas or septage sludge.<br /> Faecal substrates sampled during the field study contained high levels of microbial indicators and pathogens. E. coli and Enterococcus spp. were detected in all pig slurry and septage samples. Salmonella spp. were detected in over 60% and coliphages in over 50% of samples. Helminth ova were present in 80% of pig slurry samples, 95% of untreated septage samples, and in all septage sludge samples in high concentrations. Ten ova varieties were found in pig slurries and twelve in septage. <br /> Field study results suggest that the functionality of PBDs and STs is not optimal to inactivate microbial indicators and pathogens. Volume of PBD is not compatible to the amount of pig slurry. PBDs are rarely desludged and STs are emptied only when blockages occur. Thus reduction of bacteria was < 1 log10 and phages < 1.5 log10 while their influent concentrations were high (up to 6.2 log10 CFU ml-1). Salmonella spp. were detected more frequently in effluents than in influents. In most PBDs helminth ova did not settle but were released to surface water via effluents, the highest concentration being 175,000 no. l-1. Most PBD effluents and overflows from full STs flow directly and contaminate the surface water, which is used by many people every day. <br /> Surface water and aquatic spinach samples were contaminated. The average E. coli level in canal water was over the total coliform limit set by the Vietnamese Surface Water Quality Standard (TCVN 5942-1995). Salmonella spp. were routinely detected. Decimal reduction time (T90) of phages and bacteria in Mekong river water was over 2 days. Aquatic spinach was contaminated much like its habitat. Enterococcus spp., E. coli, somatic coliphage and Salmonella spp. were all found in samples, though average E. coli concentrations on spinach grown in urban canals was twice that of those grown in fishponds receiving PBD effluent. On terrestrial spinach microbial reduction was 0.2 – 0.4 log10/day.<br /> By QMRA infection risk was high, ranging in descending order from helminth to rotavirus to Salmonella. The probability of salmonellosis and helminthiasis was higher per exposure to PBD effluent than with pig slurry. MD sewage workers were most at risk due to constant exposure to faecal matter. Incidental ingestion of pig slurries, bathing/swimming in canals, drinking untreated surface water and eating raw spinach constituted chronic exposure scenarios for MD people generally. All mentioned scenarios were found to exceed acceptable risk levels.<br /> Besides health programs and personal hygiene routines, barriers reducing risk of infection include wastewater treatment (e.g. PBDs), due time between last crop irrigation and harvest, treating water before consumption and food preparation. Risks were reduced when PBDs ran at HRTs of 15 and 30 days as effluent was assumed to be free of helminth ova. The high pathogen load of surface water means this is only potable when boiled. Aquatic spinach is not safe to eat unless cooked. Spinach irrigated with improved PBD effluent (HRT ≥ 15 days) can be eaten raw, but then only when the time between final irrigation and harvest is long enough. As a rule spinach should be washed prior to consumption. <br /> Current faecal management practices in MD equate to high infection risks for its population. The microbial treatment efficacy of anaerobic digestion there can be improved by relatively simple changes to operations and maintenance. To reduce infection rates a campaign that integrates faecal management, water supply and behavioural change is recommended. While QMRA data collation and modelling requires much effort communicating health risks to the government and public is challenging. Thereby lasting technical, legislative and cultural can be changed so as to improve the public health effectively.