Cost-effective and safe household-scale water disinfection using combined salt chlorination and residual chlorine removal filter Open Access

Water is one of the world's most precious and limited resources, essential for the functioning of the human body. Despite the United Nations recognizing water as a human right, 20–21% of the global population still lacked access to safe drinking water in 2024 (Osseiran & Lufadeju n.d.). Microbiological contamination of drinking water remains a prevalent issue worldwide, causing acute symptoms such as nausea, vomiting, diarrhea, and stomach cramps, as well as transmitting diseases such as cholera, typhoid fever, and bacillary dysentery (Cabral 2010; Chen 2022).

Waterborne diseases are caused by pathogens such as bacteria and viruses. Bacterial pathogens like Escherichia coli, Salmonella, Vibrio cholerae, and Shigella can cause severe illnesses. For instance, Vibrio cholerae is responsible for cholera, a disease characterized by severe diarrhea and dehydration, which can be fatal if untreated (Chen 2022). Diarrhea is a major cause of death and illness in developing countries and the fourth leading cause of death worldwide among children under five years old (Geremew et al. 2018). Viral pathogens such as norovirus, rotavirus, and hepatitis-A virus also pose significant health risks. Norovirus, for example, is a leading cause of gastroenteritis outbreaks, resulting in symptoms like vomiting, diarrhea, and stomach cramps (Chen 2022).

In communities without access to water treatment plants, such as energy-efficient conventional methods and land-efficient but high-cost pulsator technologies, it is essential to ensure the safety and health of residents by performing disinfection (Kawakami et al. 2016). Small and undeveloped communities often rely on untreated water sources that may harbor pathogens, chemicals, and other contaminants. Effective household water treatment systems are crucial to remove these hazards and provide clean, safe drinking water. Such systems must be affordable, easy to use, and capable of eliminating a wide range of impurities to protect against waterborne diseases and improve overall public health.

Although many water purification methods are available to safeguard against microbial contamination, chlorine disinfection remains the most economical and widely used method (Mazhar et al. 2020). However, chlorination can lead to the formation of disinfection by-products (DBPs) in the presence of natural organic matter (NOM), posing significant health risks. Common DBPs include trihalomethanes and haloacetic acids, both linked to adverse health effects such as cancer and reproductive issues (Gopal et al. 2007; Dayarathne et al. 2021). Minimizing the formation of DBPs while effectively using chlorine to kill microorganisms is thus a critical challenge in water treatment (Amarasooriya et al. 2018). Additionally, chlorine disinfection can alter the palatability of water, necessitating controlled chlorination and removal of chlorine residuals to ensure consumer acceptance (Li & Mitch 2018; Jayasinghe et al. 2023).

Water utilities and organizations equipped with advanced technologies and skilled personnel can effectively manage chlorine disinfection to reduce DBPs formation and preserve water palatability while maintaining adequate chlorine residuals. However, at the household level, chlorination requires careful consideration to minimize DBPs formation and ensure the water remains palatable. In such cases, activated carbon (AC) can be employed as a cost-effective method for removing DBPs and excessive chlorine levels (Nielsen et al. 2022).

Despite its potential, the application of controlled chlorination at the household or small community scale remains underexplored. Hence, this study proposes a low-cost system combining electrochemical chlorine generation and AC adsorption filtration in such a context. The proposed system uses salt electrolysis with graphite electrodes to generate chlorine and activated charcoal granules to create replaceable bricks for adsorbing DBPs and residual chlorine as AC proved effective in both chlorine and DBPs removal (Kim & Kang 2008; Tafvizi et al. 2021). This design ensures the system's operation by unskilled individuals with minimal costs, making it an accessible solution for safe drinking water at the household level. This method has the benefits of on-site generation of disinfectant and reduced chemical use while it eliminates the need for transporting and handling bulk chlorine or hazardous chemicals, clean by-products like hydrogen gas which can be used as a clean energy source, and oxygen.

Accordingly, the primary objective of this study is to investigate the efficiency of salt electrolysis chlorine generation and the effectiveness of AC bricks in removing excessive chlorine. The study aims to determine optimal operational parameters for chlorine generation and AC brick performance to ensure compliance with World Health Organization (WHO) guidelines for safe drinking water. By evaluating the performance of this integrated system, the study seeks to address the gap in low-cost, effective household water treatment solutions and improve access to safe drinking water globally.

Electrochemical electrolysis reactions occurred in solution anode, cathode, and overall reactions are stated in Equations (1)–(4).

Production of NaOH strong base could be hazardous when handling and vinegar as a readily available material for neutralizing NaOH was utilized.

The AC bricks served a dual purpose: removing excessive chlorine and minimizing the formation of DBPs, thereby enhancing the water's safety and taste.

Raw water samples were collected from six different low turbidity (0.1–1.5 NTU) rivers and streams in the Badula area of Sri Lanka. The collection sites included Demodara Black Bridge, Ancient Bridge, Mediriya Bunt, Malangaduva Bridge, Andeniya Bridge, and Thaldena Bridge. The bacteriological quality of the collected samples was assessed by determining the presence of Escherichia coli (E. coli) and coliforms. To perform this analysis, 100 mL of each water sample was filtered using a 0.45 μm membrane. The filters were then cultured on chromogenic coliform agar media (Himedia, India) and incubated at 45 °C. Colony forming units (CFU) of red and blue colonies were used to determine the presence and quantity of E. coli and coliforms, respectively, as CFU/100 mL. This comprehensive experimental setup aimed to assess the feasibility of a low-cost, efficient system for household water treatment, ensuring safe drinking water by effectively managing chlorine disinfection and minimizing DBP formation. Residual chlorine analysis was performed by standard method, 4500-Cl B (Apha et al. 1999).

Table 1 summarizes the experimental conditions used to perform the experiments.

These results underscore the importance of optimizing both the applied current and the mixing time to achieve the desired residual chlorine levels. For instance, if a lower residual chlorine concentration is required, reducing the mixing time or lowering the applied current can be effective strategies. Conversely, to achieve higher chlorine concentrations for effective disinfection, increasing the current or extending the mixing time would be appropriate. A statistical analysis was performed to understand the significance of increasing the current or extending the mixing time. The statistical analysis (Table 2) highlighted that all residual chlorine data groups (30, 15, 10, and 5 min) have no significant deviation from normality, as their p-values are greater than 0.05. This indicates that the data in each group is approximately normally distributed. Further significant effect of time on residual chlorine levels was observed and suggests that the residual chlorine concentration changes significantly over time across the different measurement points (30, 15, 10, and 5 min).

The relationship between current, mixing time, and residual chlorine concentration is crucial for the practical application of this disinfection method. By adjusting these parameters, the system can be tailored to meet specific disinfection requirements, ensuring both the efficacy of microbial inactivation and compliance with safety standards for residual chlorine levels. Finally, the ability to control residual chlorine levels through the adjustment of current and mixing time offers significant flexibility in the application of electrolysis-generated chlorine for water disinfection. This adaptability makes the system suitable for various household water treatment scenarios, allowing for effective disinfection while maintaining safe and acceptable chlorine levels in the treated water.

The WHO recommends a residual chlorine concentration of 0.2 mg/L for safe drinking water (Gordon et al. 2008). The same minimum guidelines were adopted by other countries such as Sri Lanka, Japan, and India as well (SLSI 1983; Gordon et al. 2008; Ministry of Water Resources 2012). To achieve this recommended concentration in our experimental setup, we varied the applied constant current to the reactor between 80 and 120 mA, while maintaining a raw water flow rate of 4 mL/min.

However, it is important to consider that the chlorine demand of raw water can vary depending on its composition and quality. Factors such as the presence of NOM, turbidity, and other impurities can influence the amount of chlorine required to achieve effective disinfection (Kim & Yu 2007; Fan et al. 2014; Amarasooriya et al. 2018). As such, while 90 mA was sufficient to reach the desired residual chlorine level in our controlled conditions, higher applied currents may be necessary in practical applications to account for variations in raw water quality.

A statistical analysis (summarized in Table 3) performed for the data for the normality test using the Shapiro–Wilk test indicates that the residual chlorine data at each current level (80, 90, 100, 110, and 120 mA) does not follow a normal distribution, as all p-values are less than 0.05. This suggests that the data significantly deviates from normality, which violates one of the assumptions for parametric tests like ANOVA. Despite this, the repeated measures ANOVA results show a significant effect of the current levels on residual chlorine levels, with an F-value of 34.22 and a p-value < 0.001, indicating that the different current levels lead to significantly different residual chlorine concentrations at various time points.

This flexibility in adjusting the applied current ensures that the disinfection system can be adapted to different water sources and conditions, maintaining the effectiveness of microbial inactivation while adhering to safety standards for residual chlorine levels. The ability to manage the chlorine demand dynamically makes this system highly adaptable and suitable for a wide range of household water treatment scenarios. The experimental results demonstrate the capability of the electrolysis system to produce the recommended residual chlorine concentration for safe drinking water. By optimizing the applied current and mixing time, the system can effectively address the chlorine demand of different raw water sources, ensuring both the efficacy of disinfection and compliance with WHO guidelines.

To evaluate the chlorine removal capacity of AC bricks, an experimental setup was designed with continuous raw water flow rates of 4 and 10 mL/min. The water was mixed with chlorine generated at a constant current of 3 A and pumped through an AC brick containing a 200 mL (volume without AC brick (weight 10 g)) container. The objective of using a high current of 3 A was to produce a higher chlorine concentration, thereby allowing for a comprehensive assessment of the AC's adsorption capacity within the proposed system.

To quantify the total chlorine removed by the AC brick, the removal data was fitted to a polynomial function, which provided the best-fit curve (R² 0.989 and 0.980) as shown in Figure 6. By integrating this polynomial function, the total amount of chlorine adsorbed over the studied time was estimated using Equation (5). Furthermore, as this is only valid for the current system (not universal), simply for other studies the surface area under the data curve at each point shall be considered. The calculations revealed that the AC brick removed 250 mg of chlorine for the 4 mL/min flow rate and 291 mg for the 10 mL/min flow rate over the 105-min period. This translates to an effective removal capacity of 25 and 29 mg/g of chlorine, respectively.

To evaluate the efficiency of the proposed system, water samples were collected from natural streams and springs located in Badulla city, Sri Lanka with low turbidity ranging from 0.1 to 1 NTU. The collected water was first chlorinated and then de-chlorinated by passing through the AC brick to remove excess chlorine. The performance of the system was assessed by measuring both chlorine concentration and bacteriological parameters to ensure the system's efficacy in producing safe drinking water.

Table 4 provides a summary of the total coliform and E. coli counts before and after chlorination, as well as the residual chlorine levels before and after treatment with the AC brick. The results indicate that the proposed system effectively disinfects the water and produces safe drinking water.

The table shows that after chlorination, both total coliform and E. coli counts were significantly reduced to nil for most sample points, indicating effective disinfection. Only one sample point, Andeniya Bridge, had a remaining total coliform count of 1 after chlorination, but this was also eliminated after treatment with the AC brick.

In terms of chlorine concentration, the residual chlorine levels after chlorination were within the range of 0.21–0.33 mg/L. These levels were reduced to 0.00 mg/L after passing through the AC brick, demonstrating the AC's efficiency in removing excess chlorine and ensuring the water's palatability minimizing chlorine taste.

Overall, the results confirm that the proposed chlorination and AC dechlorination system is effective in disinfecting water from natural sources with low turbidity, ensuring compliance with safe drinking water standards. This system offers a promising low-cost solution for household-scale water treatment, capable of providing microbiologically safe and palatable drinking water.

The cost estimation for the entire household chlorination system prototype is approximately $421.50 USD for a system with a capacity of 5,000 L/day, which is significantly lower, by at least a factor of three, compared to commercially available water treatment systems. The detailed cost breakdown includes various essential components: sodium chloride (NaCl) required for the electrolysis process costs $2.00 for a 1-month supply, while carbon electrodes, crucial for producing chlorine gas, have an annual cost of $25.00 for 12 electrodes. The electrolysis cell, where chlorine gas is generated, costs $20.00, and the power supply with a capacity of 1.0 A is estimated at $25.00. The system also includes a pump, ensuring continuous water flow, which costs $50.00. AC, used for de-chlorination, costs $1.50 for a 1-month supply of 1 kg, demonstrating its cost-effectiveness. The tubing arrangement necessary for proper flow and connection between components costs $50.00, and the storage tank and casing, which house and protect the system, are estimated at $250.00. This cost-efficient system, totaling 421.50 US$/5000L/day for the treatment system, and 19 US$/month operational cost for the treatment of 150,000 L/month (5,000 L/day) as considering a cost of 0.3 US$/kWh, provides an economical solution for household/small community water treatment, ensuring microbiologically safe drinking water and supporting its potential for widespread implementation, particularly in low-resource settings. All the cost calculations discussed above are summarized in Table 5.

Beyond the financial advantages, the system has the potential to deliver significant health benefits by reducing waterborne diseases such as cholera, typhoid and diarrhea, particularly in low-resource settings. Further consideration of these health improvements, alongside their cost-effectiveness, is essential to fully realize their value and justify their widespread implementation.

The experimental results robustly demonstrate the effectiveness and feasibility of the proposed household-scale chlorination system. By varying reactor currents and mixing times, the system's chlorine production can be precisely adjusted to achieve specific disinfection concentrations. To comply with the WHO standard of 0.2 mg/L residual chlorine as well as the Sri Lankan Water Quality Guideline of the same 0.2 mg/L, an optimal current of 90 mA should be applied, with the system optimized for a 10-min contact time at a flow rate of 4 mL/min.

Additionally, the AC brick exhibited high chlorine adsorption capacities even at elevated chlorine doses (595 and 294 mg/L at 3 A current), effectively removing residual chlorine from treated water over varying flow rates and operational durations. This capability, along with the system's proven ability to disinfect water from low turbidity sources, underscores its practical utility for household-scale applications.

The cost-effectiveness of the system is another significant advantage. With the prototype system's components totaling just $27.50 USD, it presents a viable alternative to more expensive and complex commercial systems. This affordability enhances its potential for widespread adoption, especially in regions where access to safe drinking water remains a critical concern. However, there are limitations to this study that need to be addressed. The long-term durability of the AC bricks and the system's performance with highly turbid water were not extensively tested. Future research should focus on these aspects, as well as exploring the scalability of the system for larger communities or different water qualities. The practical implications of this study are substantial. Implementing this cost-effective chlorination system could significantly improve water quality and public health, particularly in low-resource settings. Furthermore, the system's environmental benefits, such as reducing the need for chemical disinfectants, and its potential to reduce waterborne diseases, make it a promising solution.

In conclusion, the studied electrolysis salt-generating chlorination system, coupled with an AC brick, confirms its robust performance in disinfection, water treatment, and cost efficiency. These findings support its potential as a practical solution for improving water quality and public health in diverse settings. Future research should continue to refine and expand upon this system to maximize its benefits and applications.

The authors gratefully acknowledge Uva Wellassa University of Sri Lanka for providing laboratory, chemical, and analytical facilities for the undergraduate final year research projects, the data from which were used in the preparation of this manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.