Universal and equitable access to safe drinking water across the globe is an international priority and is part of Sustainable Development Goal number 6, as established by the United Nations (UNICEF, 2019). In different regions of the world, urban and rural populations lack access to drinking water. In Northern Brazil, home to the Amazon biome, 25% of households are not connected to a water supply network; this is well above the national average of 8% (IBGE, 2019). Rainwater and river waters are rural populations’ main drinking water sources (Cardoso, 2021; Gomes et al., 2022). The Amazon hosts high levels of biodiversity and provides ecosystem services globally, including carbon sequestration (Strand et al., 2018; Joly et al., 2019). Sustainable development is the most viable strategy for biodiversity conservation and maintaining ecosystem services. This development model concurrently seeks to improve the quality of life of local populations (Campos-silva et al., 2018; Franco et al., 2021).

The riverine people, recognized as traditional communities in Brazil, exist in a challenging environment for human development due to the annual flood pulses, waterway-only transport, long distances from urban centers, and a lack of infrastructure (de Andrade et al., 2021). Most of them live in wood-made stilt houses along the main rivers, with gain income from extractivism, fishing, agriculture, and services. Access to safe drinking water is fundamental to quality of life and is urgent for Amazonian populations.

Since the beginning of COVID-19, actions to prevent emerging diseases and new global pandemics have been urgent. Households in remote areas are in contact with wild animals, and populations are subject to zoonoses from the crossover of pathogenic organisms from wild and domestic non-human and human species (Ellwanger et al., 2020). In this context, high-quality water is a barrier to emerging diseases, as the pollution of waters, soils, and other objects also promotes widespread exposure to pathogenic microorganisms (O’Brien and Xagoraraki, 2019).

Household or point-of-use (POU) water treatment is encouraged in the absence of safe water supply systems to reduce diarrheal diseases (WHO, 2017). Nevertheless, positive effects depend on correct, effective, consistent, and continued use of these methods (Bivins et al., 2019). POU treatments are only effective when more than 90% of water consumed at home is being treated (Brown and Clasen, 2012) – or, in other words, great adherence to these practices is sustained (UNICEF, 2019). The quality of treated water is used to classify the risk level for health diseases (WHO, 2012). In the Central Amazon, many families use traditional POU practices, such as sedimentation, cloth filtrations, and hypochlorite. Sodium hypochlorite is employed by 43% of all households as the only treatment technique, and it is combined with other techniques at 25% of households (Gomes et al., 2022). Despite the widespread use of POU, its efficiency in improving water quality is unknown. Thus, this study aimed to evaluate the effectiveness of removing solids and microbial contamination of household water treatments in riverine communities from Central Amazon.

In this study we sampled water untreated and treated (through traditional household water treatment), from river and rainwater. Thus, we have identified cases where treatment techniques, and handling practices have not been effectively utilized to make the water potable and propose specific efforts that the government and institutions should undertake to enhance water quality.

We collected POU-treated water from 18 residences belonging to three communities (Caburini, Sítio Fortaleza, and Várzea Alegre) located in a floodplain area in the middle Solimões region, Amazonas State, Central Amazon, Brazil (Figure 1). Here, we also conducted observations to learn more about household treatments.

Rainwater samples were collected during the rainy season (Dec/2021 to Feb/2022). River water samples were collected during the dry season (July to October 2022) – when families rely more heavily on this water source.

For rainwater, we evaluated three treatment techniques: cloth filtration (C), chlorine disinfection (Cl), and cloth filtration + chlorine disinfection (C + Cl). For river water, five-technique combinations were tested (all of which included at least one sedimentation step): sedimentation (S); sedimentation + cloth filtration (S + C); sedimentation + cloth filtration + chlorine disinfection (S + C + Cl); sedimentation + chlorine disinfection (S + Cl); flocculant + chlorine disinfection (F + Cl). The commercial flocculant Water Purifier P&G® was used as a reference, as some families in the region have had access to this treatment type since 2017 (FAS, 2019). Sodium hypochlorite, distributed by the Brazilian government, has a concentration around 2.38% of active chlorine (Ministério da Saúde, 2018).

Residents were asked to carry out daily water treatment techniques as usual. Water samples were collected at each step of the treatment process, and details about the practice were described and recorded. Drinking water samples were collected from household containers readily available for drinking, and residents described the origin and type of water treatment conducted.

Samples were placed inside sterile flasks and bags containing sodium thiosulphate when containing chlorine, transported at 4°C to the laboratory at the Mamirauá Institute (Tefé/AM) in thermal boxes, and packed and processed within 48 h after collection.

The initial results comprise a description of the household water treatments based on the observations, followed by an assessment of their effectiveness in enhancing the quality of river and rainwater.

Fecal contamination of untreated rainwater (median 3.46 log CFU/100 mL E. coli) can occur during water collection and storage. Rainwater can become contaminated when running off roofs and into gutters (or uncovered reservoirs) by animals (bats, rats, possums, vultures) or through atmospheric pollution (Novaes and Cintra, 2013; Ahmed et al., 2014; Hamilton et al., 2019). E. coli bacteria deposited on rooftops in animal feces only remain active for 2 h on sunny days; however, they survive from 9 to 53 h in cloudy conditions (Ahmed et al., 2014) and can be carried to household water reserves when it rains. In water reservoirs (tanks), 90% of bacteria will become inactive (Ahmed et al., 2014) after 38–72 h, and recontamination may also occur during this period.

Water straining (C), a common POU, was ineffective and eventually damaging (Table 1) for microbiological water quality. The absence of larger suspended solids in rainwater, such as plankton (Colwell et al., 2003) for adhesion of microorganisms (Ali et al., 2011) potentially retained in the straining process, may have negatively affected the effectiveness of this method. Kotlarz et al. (2009) also reported that straining water with a single layer of fabric showed low effectiveness in reducing the turbidity of slightly turbid water. Enterobacteria are typically 2–3 μm in length and 0.6 μm in thickness (Rogers et al., 2016) – and are not removed during cloth filtration.

Positive results in straining efficacy could be obtained in the field for surface waters (rivers and lakes), with a 50% removal of turbidity using 16 layers of fabric or four folds (Ali et al., 2011). Old fabrics, which have been washed several times, are more effective in treating water due to the reduction in pore size (Huq et al., 2010). In this study, since fabric wear does not negatively impact the straining process of river water, the use of a single layer of fabric is likely to explain the results found. Nonetheless, as total coliform and E. coli quality indicators could be abundant on surfaces, objects, and fabrics under inadequate hygienic conditions, the water handling potentially altered the results of water straining (Zimmer and Dorea, 2023). For example, better fabric hygiene could be achieved by rinsing with chlorinated water or heating.

Natural sedimentation (S), for 16–18 h (overnight), reduced the apparent color (11%) and turbidity of the river water (p < 0.05) but had only 16% efficacy and a final value of 36 NTU above the 0.5 NTU portability limit (Ministério da Saúde, 2021). Longer settling times could change the results. With controlled values of 10–300 NTU for untreated water, sedimentation for 24 h had a 79–87% efficacy rate in reducing turbidity (Kotlarz et al., 2009).

The increased number of microorganisms after the sedimentation processes was also observed in previous studies about water storage for domestic use (Mintz et al., 1995; Yi et al., 2019). Despite the coverage of storage containers, contact with unhygienic hands and objects can lead to contamination of treated and untreated water. Microorganisms in biofilm covering the surfaces of containers, such as buckets and bottles, can survive for more than 48 h. They can also detach, becoming a continuous source of water contamination (Momba and Kaleni, 2002). Other microorganisms such as E. coli, Salmonella sp. and coliphages persist during water storage and display little growth (Momba and Kaleni, 2002). E. coli is a commensal bacterium that lives in the intestinal tract of humans and vertebrate animals (Berthe et al., 2013). Researchers are studying the possibility of the multiplication of these bacteria outside the intestinal tract in the case of phylogenetic groups that contain strains adapted to the conditions of the aquatic environment (Nowicki et al., 2021).

Water turbidity provides suitable conditions for bacterial multiplication through the association of nutrients with organic carbon (Momba and Kaleni, 2002; Yi et al., 2019). It is likely that, after the proliferation of bacteria immediately following water straining (with contaminated cloth), a decay process would begin due to nutrient and oxygen reduction, in addition to changes in temperature, and other factors (Allwood et al., 2003; Blaustein et al., 2013).

Removing turbidity from water before disinfection influences its efficacy, as it reduces the demand for chlorine by reducing oxidizable components in the water (Kotlarz et al., 2009). Removing turbidity through sedimentation processes and other simplified techniques can guarantee the maintenance of residual free chlorine for up to 24 h after chlorination (Kotlarz et al., 2009). Free chlorine, in turn, prevents recontamination of water during storage (WHO, 2017).

Adding sodium hypochlorite did not effectively disinfect river water due to its high turbidity (more than 31 NTU). In decanted river water (S + Cl treatment), chlorination substantially reduced E. coli by 95% (Table 1), but only 5.5% of the samples measured <1 CFU/100 mL (Figure 4). However, the correct chlorination dose for this turbidity level is 3.75 mg/L of chlorine (Lantagne, 2008), representing 4 drops/L addition. The Brazilian Ministry of Health recommends a standard dose of 2 drops/L – and in practice, smaller doses are used (median 1.2 mg/L). Considering that the local population rejects the taste of chlorine, it is likely that residents would disregard the recommendation to use even higher doses of chlorine.

The household treatment of river water with the best result of the risk of water consumption was sedimentation, followed by sodium hypochlorite (S + Cl) (Figure 4). This technique had the highest percentage of samples (33%) in the No risk to Medium risk classes (WHO, 2012).

The Water Purifier (F + Cl) method combines coagulation of sediments and water disinfection by the simultaneous use of ferrous sulfate and calcium hypochlorite (Souter et al., 2003), thus having advantages over the simple and natural techniques used in this study. Chemical coagulants and combination products are challenging to purchase compared to sodium hypochlorite, which is distributed free of charge by the Brazilian government. Its relative inaccessibility makes it less sustainable to use in the long term. Studies have reported a 44% reduction in per capita consumption of treated water 2 months after the delivery of flocculant sachets despite intensive follow-up (Shaheed et al., 2018). Using devices like filters results in greater adherence and prolonged use (Sobsey et al., 2008). Promoting a technique that depends on the continuous distribution of the product is not a viable medium/long-term solution, being more suitable for emergencies (Ariel Branz et al., 2017).

While in the region, 67% of families had reported using chlorine for water treatment (Gomes et al., 2022), this study found that only 18% of families reported using chlorine for this purpose; we detected the presence of residual chlorine in a mere 8% of samples. Brown and Sobsey (2012) also found a difference between families who stated that they boiled water (>90%) and those who boiled it (31%). In rural Peru, only 23% of the families who had reported POU water treatment consistently did so, which may indicate a failure to collect “good” data (Rosa et al., 2014), but also poor adherence to treatment or its inappropriate use.

The sodium hypochlorite distribution program (around 2.5% active chlorine solution) in Brazil is widespread and involves Community Health Agents (CHAs). Considering its wide reach and impact, we recommend further training of CHAs to raise public awareness on the proper use of chlorine. This would enhance the effectiveness of the investment and promote public health by reducing diseases and health consequences like malnutrition and child mortality. Improving and monitoring the quality of drinking water in riverine communities in Amazonia is urgent to promote the well-being of the local population. Additionally, it would improve preparedness for water access during emergencies, such as extreme droughts and heavy rainfall events, which are increasingly common in the Amazon (Flores et al., 2024).

The riverine population lives in scarcity of drinking water despite individual efforts to treat water at home. Considering the analyzed parameters, the water available could not be classified as safe for human consumption even after treatment. Therefore, emergency investment in the topic is needed, thus contributing to quality of life and health barriers for unknown emerging pathogens.

Improving the techniques used, particularly the use of hypochlorite and the straining of water (cloth filter), could potentially promote greater water treatment efficacy. Treating rainwater with sodium hypochlorite had the best results despite observing that residents were using doses below the recommended level. This thus represents the most recommended treatment of all those evaluated. No treatment effectively removed turbidity or disinfected river water. Therefore, investments are needed to improve the efficacy of removing solids from water; improvements to filters or coagulants may show promise.

Investing in education and outreach initiatives to teach proper cleaning of utensils and containers used in water treatment and storage is imperative. These measures are recommended to enhance treatment efficacy for local families, along with investments in adequate water supply infrastructure delivering potable water.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving humans were approved by Comitê de Ética em Pesquisa Instituto de Desenvolvimento Sustentável Mamirauá Plataforma Brasil/CONEP. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

MG: Writing – original draft, Writing – review & editing. LA: Writing – review & editing. MB: Writing – review & editing. BL: Writing – review & editing. CM: Supervision, Writing – review & editing.