Innovative porous mortar filters: wastewater purification for clean water

Water scarcity is a serious issue affecting both industrialized and developing countries worldwide. Various factors contribute to water shortage, including increased water demand owing to population growth, urbanization, industry, and inadequate water management techniques (Tzanakakis et al. 2020). Water scarcity has far-reaching repercussions, affecting various elements of human existence, agriculture, and the environment, including in Indonesia. The cause is a combination of geographical and climatic factors such as excess water during the rainy season and scarcity during the dry season (Fulazzaky 2014). Indonesia ranks as the 16th most water-stressed nation global. The issue is compounded by the over-extraction of groundwater in urban areas, where the availability of safe drinking water is crucial. A significant number of individuals rely on shallow wells and unregulated groundwater sources, leading to depletion and saltwater intrusion, while inadequate access to clean water and sanitation facilities in rural areas impacts public health and hygiene (Carrard et al. 2019). Furthermore, climate change intensifies Indonesia's water scarcity, with rising temperatures, altered rainfall patterns, and an uptick in extreme weather events such as droughts and floods straining water resources (Ebi & Nealon 2016; Pawitan 2018). Recognizing the gravity of the situation, the Indonesian government has taken action by implementing various measures, including improved water management, conservation efforts, and infrastructure development (Basuki et al. 2022).

Water treatment infrastructure is an important part of maintaining safe and potable water access, especially in locations where water quality is an issue (Wang et al. 2018). While membrane filtration systems are frequently employed for this purpose, they have several drawbacks. These systems frequently necessitate a large amount of energy to sustain the high hydraulic pressure required for successful filtration (Shahid et al. 2023), providing challenges in areas with intermittent or limited access to electricity. Furthermore, membrane materials, maintenance, and replacement costs might be prohibitively expensive in resource-limited places, particularly rural areas (Othman et al. 2022). Innovative techniques, such as porous concrete, have arisen to solve these difficulties, primarily for rainwater management and stormwater control (Lee et al. 2013; Xu et al. 2018; Faisal et al. 2020). By enabling rainwater to permeate the earth, minimizing surface runoff during floods, and replenishing groundwater supplies, porous concrete provides a long-term solution (Liu & Li 2020). Furthermore, several studies have shown that porous concrete increases the quantity and quality of surface runoff infiltrating into porous concrete, making it a vital tool for water purification initiatives targeted at providing clean domestic water access (Thives et al. 2017; Braswell et al. 2018; Liu & Li 2020; Teymouri et al. 2020; Alimohammadi et al. 2021; Singer et al. 2022). This approach has resulted in the creation of porous mortar filters (PMFs), which can play an important role in cleaning water for household consumption, particularly in locations where traditional water treatment infrastructure is limited.

PMF is a novel technique for water filtration, providing an effective and long-term solution to water quality issues. PMFs function by passing water through a porous material, thereby efficiently eliminating pollutants (Taghizadeh et al. 2007; Thives et al. 2017; Faisal et al. 2020; Teymouri et al. 2020; Alimohammadi et al. 2021; Harada 2023). The concept is that water is released through mortar pores, namely gel pores with a diameter of <10 nm and capillary pores with a diameter of >10 nm, while large capillary pores have a size 50 nm < d < 10 μm (Dong et al. 2017). The relationship between the capillary pores that form the filtration path can improve water's physical and biological quality because it is segmented by gel pores (Saleem et al. 2019). In this study, improving water quality with PMF can be enhanced further by integrating adsorbent materials into the PMF mixture. This method enables the removal of specific chemical compounds and pollutants, zeolite and activated charcoal (AC) as adsorbent materials are used in PMF as a partial replacement of fine aggregate. Zeolites are microporous minerals with a high surface area that can selectively adsorb various cations and molecules (Vasconcelos et al. 2023). AC, also known as activated charcoal, is a highly porous form of carbon that can adsorb a wide range of contaminants, including organic compounds, odors, and impurities (Saleem et al. 2019; Thinojah & Ketheesan 2022). Zeolite–AC is a promising method for reducing the levels of manganese (Mn) and iron (Fe) in water. This combination can effectively adsorb and remove these two metals from the water, enhancing its quality (Siabi et al. 2021). This research aims to develop PMF by integrating adsorbent materials into the PMF mixture to improve water quality in terms of physical, biological, and chemical parameters. The PMF can be manufactured locally using easily available materials, allowing them to be used in various places with distinct water quality challenges. Overall, the development and implementation of PMF offer great promise for improving access to clean water and alleviating water scarcity challenges in various global regions.

The experiments of this work were carried out in the Integrated Laboratory of Universitas Gorontalo and the Technical Implementation Unit of the Water Quality Laboratory Installation Service of the Gorontalo District Health Service. The experimental investigation comprised three stages. The first stage was the determination of PMF properties, which involved bulk density, porosity, and infiltration rate. The second stage includes making synthetic wastewater (SWW). The last stage involved SWW treatment, which is carried out to test total dissolved solids (TDS), turbidity, E. coli, total coliforms, Mn, and Fe, then the results were evaluated and analyzed.

The materials used to make PMF were cement, fine aggregate, zeolite, AC, and water. Portland cement and fine aggregate were the materials used to prepare all mixes of PMF. The cement used was Type I Portland Cement (OPC) based on SNI 15-2049-2004. The fine aggregate, zeolite, and AC had a maximum diameter of 0.60mm and minimum diameter of 0.30mm (size was retained on the #50 sieve and passed through the #30 sieve). The water to cement ratio (w/c) was essential to produce adequate strength. The w/c ratios were relied on 0.45. Some physical properties of materials were indicated in Table 1.

The ingredients prepared in this investigation for making SWW were 100 g of clay < passing through a 0.075mm sieve (No. 200)), 0.5 L of leachate from the Talumelito Gorontalo Leachate Processing Plant, and 200 L of clean water. All ingredients were mixed in a 600-L container and stirred using a stick for 10 min. Different SWWs were used for PMF with AC and zeolite. Three samples of each SWW were taken to be tested for concentration for TDS, turbidity, E. coli, total coliform Mn, and Fe.

The bulk density, porosity, and infiltration rate were studied in this work to designate PMF.

The porosity of PMF is closely related to its composition, as it measures the empty spaces or voids within the mortar structure (Lian et al. 2011). A higher porosity improves the mortar's ability to hold and move water, making it a useful material for water purification. As a general case for all experiments, the porosity decreased as the cement mass increased. For the cement content of AC-1:6 and Z-1:6, PMF exhibited the maximum value of porosity, while AC-1:4 and Z-1:4 showed the minimum value, as displayed in Figure 3(b), which indicates that more cement content gives lower porosity. This mainly occurs due to the increment of paste materials that fill the cavities between and around the aggregates. For example, using AZ/C-1:4 and AZ/C-1:6 with the alteration of cement content from 1:4 to 1:6, the obtained porosity was 25.46% (CA), 40.17% (CA), 29.29% (Z), and 41.52% (Z), respectively.

The infiltration rate, or the rate at which water can flow through the PMF, is positively related to porosity. Higher porosity indicates a higher volume of linked spaces, allowing water to flow more easily through the material (Lian et al. 2011). As a result, increasing porosity in the composition can improve PMF infiltration (Thives et al. 2017). PMF reached the peak infiltration rate in the AC-1:6 and Z-1:6 mixtures, showing an increase in the infiltration rate as the cement mass decreases. AC-1:4 and Z-1:4, on the other hand, have the lowest infiltration rates. The more the quantity of paste elements inside the PMF mixture, which tends to fill the vacuum spaces between and around the aggregates, the more inverse the relationship between cement content and infiltration rate. As the cement content increases, so do the paste ingredients, reducing the linked voids within the mortar structure (Lederle et al. 2020). As seen in Figure 3(c), this inhibits the passage of water through the material, resulting in a lower infiltration rate.

The composition of PMF's mixing can considerably impact its ability to reduce TDS and turbidity in water. TDS is the concentration of dissolved ions and minerals in water, whereas turbidity is the cloudiness or haziness caused by suspended particles in water (Adjovu et al. 2023). When employed in water quality management, the composition of PMF influences how successfully it can treat water quality in terms of lowering TDS (Yogafanny 2023) and turbidity (Taghizadeh et al. 2007). The kind and amounts of aggregates, cement, and adsorbent elements in PMF have an important role in defining its porosity. These elements are critical for the PMF's capacity to maintain and remediate water quality. The interconnected voids and pores within the mortar structure allow water to infiltrate, and various pollutants, such as TDS and suspended particles, can be trapped and filtered out during this process (Winston et al. 2020; Boulven et al. 2021). Fine-grained aggregates and a well-balanced mix design of PMF can result in a mortar matrix with smaller pores that are more effective at trapping and removing fine particles, thereby reducing turbidity (Boulven et al. 2021).

The differences in the types of pores formed in each PMF mixture composition, specifically gel pores (<10nm) and capillary pores (0.01–0.0005 μm) (Vernocchi et al. 2023), influence the reduction of TDS and turbidity levels, making the cement and fine aggregate composition less impactful on these parameters. Gel pores, which are finer and more interconnected, are effective at trapping and removing dissolved fine particles that cause turbidity, thereby contributing to TDS and turbidity reduction in SWW. Capillary pores, which transport water through capillary forces, can have varying effects on TDS and turbidity reduction depending on their size and connectivity. When gel pores dominate the PMF structure, they can significantly contribute to the reduction of TDS and turbidity, compensating for the potential impact of cement and fine aggregate composition.

The addition of AC and zeolite to the PMF mix can significantly reduce the amount of E. coli and total Coliform bacteria in water (Buang et al. 2018; Asghari et al. 2022; Alvarino et al. 2023). AC and zeolite are highly porous materials with strong adsorption properties that can capture and immobilize microorganisms and bacteria. The porous structure of AC and zeolite allows for a large surface area for the adsorption of organic compounds and microorganisms (Burchacka et al. 2021). When E. coli and total coliform bacteria come into contact with AC and zeolite in the PMF, they can become trapped and adsorbed on the surface of the charcoal and zeolite. This method physically removes bacteria from SWW, lowering its concentration as it passes through the PMF. Furthermore, the chemical properties of AC and zeolite can help to reduce microbial growth by promoting chemical interactions that inhibit bacterial growth (Alvarino et al. 2023). The use of AC and zeolite in PMF creates a powerful synergistic effect, leveraging the superior adsorption and ion exchange properties of both materials. This method significantly improves the PMF's ability to reduce microbial contaminants, resulting in better water quality and environmental protection. In addition, gel pores of mortar or concrete with a diameter of <10nm, which are mostly of 0.01–0.0005 μm (Kumar & Bhattacharjee 2003). They can function to filter E coli and total coliform bacteria measuring (1.0–1.5 × 2.0–6.0 μm) (Vernocchi et al. 2023), where the size is larger than the size of gel pores. Bacteria and other smaller particles may pass through the pores on the surface of PMF. However, the pores on the PMF may trap them due to the wide range of pore sizes from nanoscale pores to microscale pores and pore structure (Boulven et al. 2021). Table 5 shows that all PMF variations can remove 100% of E. coli and total coliform from SWW. In line with the findings of Taghizadeh et al. (2007), it has been discovered that PMF has the ability to significantly reduce bacterial concentrations. Standard Quality for water hygiene and sanitation purposes based on Regulation of the Minister of Health of the Republic of Indonesia Number 2 of 2023 for E. coli and total coliform are 0 and 0 CFU/100mL, respectively. Therefore, E. coli and total coliform were appropriate based on the standards above.

The PMF with AC and zeolite emerges as an admirable solution for improving water quality, demonstrating notable success across multiple dimensions – physical, biological, and chemical parameters. Its effectiveness is particularly evident in the reduction of biological and chemical elements reaching 80–100%, which aligns with Standard Quality for water hygiene and sanitation purposes based on Regulation of the Minister of Health of the Republic of Indonesia Number 2 of 2023. While the impact on physical parameters varies, the overall positive results position PMF as a versatile technology capable of addressing a wide range of water quality issues. What distinguishes PMF is its usability and low cost, making it not only a powerful tool for improving water quality but also a practical solution in areas with limited water resources or dealing with low-quality water sources. The adaptability of PMF to such diverse conditions emphasizes its practicality and potential to play a pivotal role in ensuring access to clean and safe water, particularly in areas where more complex water purification methods may be logistically or economically challenging to implement.

The limitation of this study is the interconnectivity of pores which is crucial for ensuring a continuous flow of water through the PMF, allowing for filtration and potential purification. Difficulties in establishing proper interconnections may result in uneven water distribution, reducing the overall efficiency of the PMF in water purification. Future research may explore the integration of PMF with incorporating innovative materials with superior filtration properties or engineering the PMF at the nanoscale to enhance its ability to capture and remove contaminants.

We extend our deepest appreciation to the individuals and organizations who played a crucial role in the completion of this paper. Our profound gratitude to our research collaborators for their invaluable insights and expertise. We are also grateful to the reviewers for their constructive feedback. Lastly, we acknowledge the financial support provided by the Ministry of Education, Culture, Research and Technology of the Republic of Indonesia.

A grant from the Ministry of Education, Culture, Research and Technology of the Republic of Indonesia was provided for domestic cooperation research schemes in 2023.

MRO was responsible for the conceptualization and design of the experiment, data collection, analysis, and manuscript writing. II actively participated in the experimental procedures, performed data analysis, and contributed to the interpretation of results. RH critically reviewed the manuscript and offered valuable insights. LD contributed to the experimental design data collection, and provided valuable feedback during the analysis and interpretation stages. The combined efforts of all authors resulted in a comprehensive and impactful laboratory experiment.

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

The authors declare there is no conflict.