Pathogen flows from on-site sanitation systems in low-income urban neighborhoods, Dhaka: A quantitative environmental assessment

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Abstract

Background

Despite wide usage of on-site sanitation, there is limited field-based evidence on the removal or release of pathogens from septic tanks and other primary treatment systems, such as anaerobic baffled reactors (ABR). In two low-income areas in Dhaka, we conducted a cross-sectional study to explore pathogen loads discharged from commonly used on-site sanitation-systems and their transport in nearby drains and waterways.

Methods

We collected samples of drain water, drain sediment, canal water, and floodwater from April–October 2019. Sludge, supernatant, and effluent samples were also collected from septic tanks and ABRs. We investigated the presence and concentration of selected enteric pathogens (Shigella, Vibrio cholerae (V. cholerae), Salmonella Typhi (S. Typhi), Norovirus Genogroup-II (NoV-GII), and Giardia) and presence of Cryptosporidium in these samples using quantitative polymerase chain reaction (qPCR).The equivalent genome copies (EGC) of individual pathogens were estimated in each sample by interpolation of the mean Ct value to the corresponding standard curve and the dilution factor for each sample type. Absolute quantification was expressed as log10 EGC per 100 mL for the water samples and log10 EGC per gram for the sediment samples.

Results

Among all samples tested (N = 151), 89% were contaminated with Shigella, 68% with V. cholerae and NoV-GII, 32% with Giardia, 17% with S. Typhi and 6% with Cryptosporidium. A wide range of concentration of pathogens [range: mean log10 concentration of Giardia = 0.74 EGC/100 mL in drain ultrafiltration samples to mean log10 concentration of NoV-GII and Giardia = 7.11 EGC/100 mL in ABR sludge] was found in all environmental samples. The highest pathogen concentrations were detected in open drains [range: mean log10 concentration = 2.50–4.94 EGC/100 mL], septic tank effluent [range: mean log10 concentration = 3.32–4.65 EGC/100 mL], and ABR effluent [range: mean log10 concentration = 2.72–5.13 EGC/100 mL].

Conclusions

High concentrations of pathogens (particularly NoV-GII, V.cholerae and Shigella) were frequently detected in environmental samples from two low-income urban neighbourhoods of Dhaka city. The numerous environmental exposure pathways for children and adults make these findings of public health concern. These results should prompt rethinking of how to achieve safe sanitation solutions that protect public health in dense low-income areas. In particular, improved management and maintenance regimes, further treatment of liquid effluent from primary treatment processes, and appropriate application of onsite, decentralised and offsite sanitation systems given the local context.

Introduction

Globally, unsafe sanitation facilities contribute to an estimated 432,000 deaths from diarrhoea each year (WHO, 2019b). Poor sanitation is linked to transmission of infectious diseases such as cholera, diarrhoea, dysentery, hepatitis A, typhoid, and polio, and exacerbates stunting (WHO, 2019b). Chronic intestinal infections can also hinder child development by impeding the absorption of essential nutrients that are critical to the development of the mind, body, and immune system (Strande, L and Brdjanovic, 2014). It can also impede the absorption of life-saving vaccines (Strande; et al., 2014).

Bangladesh has had notable success reducing open defecation, and in 2007, UNICEF and the World Health Organization declared the country “open defecation free” (WHO, 2019a). Despite this success, environmental contamination with faecal pathogens still remains a widespread health and environmental hazard in Bangladesh and many low- and middle-income countries (World Bank, 2017). Recent evidence suggests that both urban and rural environments are affected by faecal contamination, identified through use of faecal indicator bacteria (Amin et al., 2019) and detection of specific pathogens (Ercumen et al., 2017; Harris et al., 2016). A recent study investigated the magnitude of environmental faecal contamination in 10 different environmental compartments in urban Dhaka and reported that open drains, surface water, floodwater, raw produce, soil, and street food were highly contaminated with E. coli, and some of this contamination was consistent across low-, middle-, and high-income communities (Amin et al., 2019). Similar results were observed for other exposure pathways in urban Bangladesh (Harris et al., 2016, Harris et al., 2018, Islam et al., 2010). These are in turn linked to adverse health outcomes such as diarrhoea, environmental enteric dysfunction, and stunting (Harper et al., 2018).

A common practice in low-, middle-, and high-income urban neighbourhoods in Bangladesh is to connect pour-flush toilets directly to open drains or canals, without any form of on-site containment or treatment (Ross et al., 2016). In Dhaka, more than 80% of pour-flush on-site and decentralised sanitation infrastructure does not properly confine faecal material, and sewage and faecal matter is commonly discharged to ground or surface water (Ross et al., 2016). The population in mega cities in Bangladesh relies heavily on shared sanitation facilities that often lack proper faecal sludge management (Gunawan et al., 2015; Joseph et al., 2018a; Opel and Bashar, 2013; SNV, 2014). A review of UN-Water Global Analysis and Assessment of Sanitation and Drinking-water (GLAAS) survey found that accountability was more developed for water as opposed to sanitation services, with little data provided on wastewater and faecal sludge management (Jiménez et al., 2018).

Only a limited number of studies have been conducted to detect and quantify the levels of enteric pathogens in low-income urban sanitation compartments (Berendes et al., 2017, 2018). This is not surprising since detecting human-specific pathogens in the environment is challenging and expensive (Ramírez-Castillo et al., 2015). The existing studies explore the presence of faecal pathogens in the urban living environment, but consider only to a limited extent where and how pathogens are released into the environment from sanitation systems and with what frequency and magnitude (Berendes et al., 2017). The overall goal of this cross-sectional study was to explore pathogen loads associated with two common on-site sanitation systems designed to contain and treat household faecal waste from toilets (i.e., septic tanks and anaerobic baffled reactors (ABR)) and associated wastewater and drainage infrastructure (i.e. open drains and canals) in two low-income urban neighbourhoods of Dhaka. Specifically, the study addressed three objectives: (i) to identify the frequency and concentration of target pathogens in a range of environmental sample types and across wet and dry seasons by using two different sampling methods (small-volume grab samples and large-volume ultrafiltered samples); (ii) to investigate the influence of coverage of septic tanks on pathogen concentration in drains; and (iii) to examine the relationship between the level of general faecal contamination in each sample type (measured by E. coli) and the likelihood of pathogen detection in the sample. This study was part of a broader study comprising systems modelling to examine the impact of different sanitation options on relative health risk (Foster et al., 2020 forthcoming).

Section snippets

Enrolment of study neighborhoods

This study was conducted from April to October 2019 in Mirpur, Dhaka, a densely populated, low-income area where people live in compounds containing multiple households and where the incidence of cholera is high (1.64 cases per 1000 person-years) (Chowdhury et al., 2011; Paul et al., 2016) (see map in Figure S2).

The first study site (Site-A) was selected based on five key criteria: (i) high population density, (ii) low-income status, (iii) variation in sanitation technologies, (iv) high

Site characteristics

In study Site-A, a total of 4792 people lived in 1493 household compounds in the four selected roads with a mean of 3.2 members per household. Road A had the highest proportion (79%) of latrines with septic tanks compared to 11% in Road B, 3% in Road C, and 15% in Road D. Drinking water for almost all households on Roads A, C, and D was supplied through the municipal piped water system into the compounds. More than 90% of the compounds on Road A, 85% on Road C, and more than 60% on Roads B and

Discussion

On-site sanitation is a common sanitation strategy in many cities in low- and middle-income countries and has been embraced as part of the pathway towards achieving citywide inclusive sanitation (Schrecongost et al., 2020). The primary objective of this study was to: 1) examine the impact of two common types of on-site sanitation systems, community-scale ABRs and household/compound septic tanks, on the surrounding residential environment by measuring the frequency and concentration of five

Conclusions

In Dhaka, Bangladesh, this study detected high frequency and concentrations of V. cholerae, S. Typhi and NoV-GII in effluent from septic tanks and ABRs which subsequently entered open drains and waterways and may present a risk of exposure to children and adults in this community and downstream communities. Given what is known about the infectivity of the five target pathogens we examined, the concentrations of these pathogens detected in the environmental samples are of significant concern and

Funding statement

The study was financially supported by the Water and Sanitation for Urban Poor (WSUP) (grant no. WSP0056) as part of the Urban Sanitation Research Initiative (USRI) (www.wsup.com/research) supported by the UK Department for International Development through the Institute for Sustainable Futures (ISF) at University of Technology Sydney (UTS). Icddr,b acknowledges with gratitude the commitment of the WSUP and ISF-UTS to its research efforts.

Declaration of competing interest

The authors alone are responsible for the views expressed in this article and they do not necessarily represent the views, decisions or policies of the institutions with which they are affiliated. NA, MR, MRM, GBA and MK work at icddr,b Dhaka and implemented the research activities. PL, SR and CLM work at the Center for Global Safe Water, Sanitation, and Hygiene at Emory University, Atlanta, Georgia, USA and have a long-term history of collaboration with icddr,b. TF and JW work at Institute for

Acknowledgements

WSUP staff Dr. Guy Norman from United Kingdom and Dr. Farzana Begum in Dhaka, Bangladesh are sincerely thanked for their support with background information, networks and inputs to the research. We are grateful to Dr. Mami Taniuchi from University of Virginia, USA, for sharing the TaqMan card during the pilot phase and Dr. Simon Fane from UTS, Australia for his support at the beginning of the project. We also acknowledge the efforts of the SaniPath team who supported during different stages of

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