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Article

Comprehensive Solutions to Prevent Larvae Breakout in Water Filtration Systems

by
Hyuk Jun Kwon
1,
Haerul Hidayaturrahman
1,2,
Ravindranadh Koutavarapu
3,* and
Tae Gwan Lee
1,*
1
Department of Environmental Science, Keimyung University, Daegu 42601, Republic of Korea
2
National Research and Innovation Agency, Central Jakarta 10340, Indonesia
3
Department of Robotics Engineering, College of Mechanical and IT Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea
*
Authors to whom correspondence should be addressed.
Sustainability 2023, 15(20), 14881; https://doi.org/10.3390/su152014881
Submission received: 12 September 2023 / Revised: 10 October 2023 / Accepted: 12 October 2023 / Published: 15 October 2023
(This article belongs to the Section Sustainable Materials)
Browse Figures
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Abstract

:
The presence of invertebrates in drinking water distribution systems, particularly Chironomidae larvae, has raised concerns among the general public. This study aimed to comprehensively address the issue of larvae breakout in water filtration systems and provide potential solutions to prevent their escape into the water supply. The research investigated various factors contributing to larvae breakout, including the type of filtration column, sand depth, pretreatment methods, and the effective size and uniformity coefficient of sand media. Experimental results revealed that the GAC column, primarily utilized for adsorption, was ineffective in retaining Chironomidae larvae, leading to their escape within a short period. Similarly, the sand filter column, with a design that is currently widely used with sand specifications of an effective size of 0.7 mm and a uniformity coefficient of 1.7, failed to act as a barrier for larvae. Increasing the height of the sand media and applying a pretreatment method, which was expected to prevent larvae from entering the treated water, yielded unsatisfactory results. Our research results show that reducing the uniformity coefficient to 1.5 while maintaining an effective size of 0.7 mm proved to be important in preventing the release of larvae into treated water. The Sand/GAC and Sand/Anthracite systems, by maintaining adjusted media sand specifications, also succeeded in retaining larvae in the filtration system. Additionally, this study emphasized the importance of following the recommended backwash procedure, consisting of specific steps involving air flow, a combination of air and water flow, and final water flow. This sequence effectively removed contaminants, turbidity, and Chironomidae larvae from the filtration media, ensuring improved water quality and system performance. The findings of this study provide valuable insights and recommendations for water treatment plants to address the issue of larvae breakout and enhance water quality.

1. Introduction

Chironomidae larvae serve as valuable indicators of water quality and are extensively studied in aquatic ecosystems. They thrive in various aquatic environments with abundant phytoplankton and zooplankton [1]. Their life cycle comprises four stages: egg, larva, pupa, and adult, with four larval instars before adulthood [2]. Invertebrates in drinking water systems have been a historical concern, with attention drawn since 1827 due to health risks [3,4,5]. Efforts to improve water quality included filtration methods, yet challenges persisted [6,7]. In 2000, the emergence of small larvae midges in water treatment plants and tap water worldwide became a growing concern, continuing into 2020 in Korean facilities [8]. The presence of Chironomidae larvae in tap water results from multiple factors, including accidental entry through intake pipes [9].
Water treatment plants often rely on natural water sources like rivers and reservoirs, but when these sources are near areas with a high Chironomidae larvae population, the risk of larvae entering the water supply increases [10]. Chironomidae larvae have unique adaptations that enable them to survive water treatment processes even resisting disinfection methods like chlorination [11]. Despite undergoing standard disinfection procedures, some Chironomidae larvae may remain viable and persist in the water supply [8]. Understanding the factors contributing to the presence of Chironomidae larvae in tap water is crucial for ensuring the safety and quality of drinking water. Ongoing efforts are dedicated to improving water treatment methods, refining filtration techniques, and implementing advanced monitoring systems to minimize the presence of various organisms, including Chironomidae larvae, in treated water.
The removal of Chironomidae larvae from drinking water systems has been a subject of ongoing efforts and research. Various attempts have been made over time to remove Chironomidae larvae from drinking water systems. Coagulation is a water treatment process that plays a crucial role in the aggregation of small particles into larger flocs and the adsorption of dissolved organic matter onto these aggregates, enabling their subsequent removal during solid/liquid separation processes [12]. Controlling Chironomidae larvae in water treatment with the implementation of the coagulation process has emerged as a promising strategy in recent research and practice [13]. Rapid mixing ensures dispersion and contact between the coagulant and water, forming small, stable flocs that enhance the likelihood of larvae collision and aggregation. Subsequent slow mixing allows floc growth and stabilization, facilitating larval entrapment and settling. Optimization of coagulation considers factors like coagulant type, dosage, pH levels, and other water constituents affecting efficiency [14]. Sand filtration is a widely utilized and crucial component of water treatment plants, effectively removing suspended solids, organic matter, and pathogens from raw water sources by passing water through a sand filter bed [15]. The effective size (ES) and uniformity coefficient (UC) of sand filters are critical parameters in sand filtration systems used for water treatment. ES represents the median size of sand particles, with 10% of particles smaller and 90% larger than this size [16]. The uniformity coefficient indicates the spread of sand particle sizes, with a lower UC reflecting a more uniform size distribution [17]. In sand filtration, a smaller ES can better capture finer particles but may reduce flow rates due to increased friction, while a larger ES allows for higher flow rates but may not effectively capture smaller particles. A lower UC ensures a more uniform distribution of sand sizes, leading to improved filtration performance and reduced channeling. In the context of controlling larvae in water treatment, sand filters have shown promise as a potential barrier to minimize the presence of these larvae in treated water. The physical characteristics of sand, such as its grain size, shape, and porosity, contribute to trapping the particulate matter, including larvae. The sand filter acts barrier, intercepting larvae directly through void spaces between grains or straining them out if they exceed sand pore sizes, significantly reducing larval concentrations in treated water [18]. Granular activated carbon (GAC) is primarily used to remove various contaminants from water sources by passing water through a bed of activated carbon particles with a high surface area and porosity. These filters are particularly effective in removing organic compounds, taste and odor compounds, and specific disinfection by-products, enhancing water quality by reducing undesirable compounds [19]. While GAC filters are not designed for controlling microorganisms like larvae, they can offer some degree of control over certain microbial populations [20]. The porous structure of activated carbon allows microbial colonization, leading to the entrapment and removal of microorganisms from water. Chlorination is a widely used disinfection method in water treatment plants to control microorganisms and ensure the delivery of safe drinking water. It involves adding chlorine-based compounds like chlorine gas or sodium hypochlorite to water sources to inactivate or destroy pathogenic microorganisms such as bacteria, viruses, and protozoa. Chlorination works by releasing free chlorine species, primarily hypochlorous acid and hypochlorite ion, which disrupt essential cellular processes and structures within microorganisms, rendering them unable to reproduce or cause infection [21]. While its primary target is pathogenic microorganisms, chlorination can also provide some control over organisms like larvae by inactivating or killing a wide range of microbial species [22]. The effectiveness of chlorination depends on factors such as chlorine concentration, contact time, water temperature, pH, and the presence of organic matter or other substances that may react with chlorine. When applied correctly and at appropriate concentrations, chlorination can be an effective method for controlling microorganisms, including larvae [1].
The aim of this study was to comprehensively understand and tackle the issue of larvae breakout in water filtration systems. The specific objectives included investigating various factors that contribute to larvae breakout, such as the type of filtration column, sand depth, pretreatment methods, and the effective size and uniformity coefficient of sand media. Furthermore, this study aimed to provide a comprehensive solution to prevent larvae from escaping into the water supply. This objective was particularly important considering the limited research on larvae occurrence in tap water and the lack of information on the causes of larvae escape during the filtration process. This study’s findings present a potential solution to prevent larvae from entering tap water, making a significant contribution to the field and offering valuable insights for water treatment plants.

2. Materials and Methods

2.1. Larvae Sampling Site

The presence of chironomid larvae in the study area, particularly near the Nakdong River and the drainage system where domestic wastewater accumulates, was observed (Figure 1). To maintain a controlled environment for the larvae, they were housed in a plastic container covered with mesh fabric and filled with fine sand. Continuous aeration was provided with an aerator to ensure sufficient oxygen levels, and the water temperature was maintained at an average of 28 ± 1 °C, representing the typical summer season temperature in Daegu, Republic of Korea.
To sustain the larvae population within the container, regular maintenance activities were conducted. Floating dead adults and exuviae were removed from the surface water, and any decaying material was carefully brushed every 2 days. In addition, the larvae were provided with a daily suspension of tetramine fish food to ensure an adequate food supply. The growth of larvae was monitored by observing morphological changes related to body length and head width. Prior to conducting the experiment, each larvae stage was manually selected using plastic Pasteur pipettes and placed in separate beakers. It should be noted that under resource-limited conditions, such as scarcity of food, the growth of larvae may be prolonged, and there may be no significant changes in the larval stage development within the first 3 to 5 days of the experiment [2,23].

2.2. Filter Media Preparation

The sand, granular activated carbon (GAC), and anthracite used in this study were sourced from KSYS Corporation, located in Ansan, Republic of Korea, an authorized distributor that supplies these materials to various water treatment plants. Furthermore, sieve analysis was carried out to determine the effective size (ES) and uniformity coefficient (UC) of the sand prior to its usage. The result of the sieve analysis was depicted in a distribution accumulation curve to find the ES and UC values. Then, the effective size (ES = D10) and uniformity coefficient (UC = D60/D10) value of sand media were obtained from a distribution accumulation graph of sand analysis (Figure S1). In this study, two different-sized packages of granular activated carbon (GAC) were selected as the filter and sorbent material, considering their typical usage in water purification plants. GAC is widely recognized for its ability to adsorb contaminants and is commonly used in water treatment processes. Anthracite filter coal is widely used in water filtration due to its exceptional filtration properties. This high-quality coal possesses desirable characteristics, including hardness and durability, and is available in various sizes. In this study, the anthracite coal was carefully sieved to obtain the required size for the experiment. Subsequently, it was thoroughly washed with tap water and dried under sunlight until all moisture was evaporated, ensuring optimal condition, and removing any potential impurities. The detailed properties of the filter media are outlined in Table S2.

2.3. Pilot-Scale System

The experiment was performed on a pilot scale located in Korea Water Cluster, Daegu, Republic of Korea. Granular activated carbon (GAC), sand filter, and a double-layer column were installed and designed with continuous flow to observe larvae escape along with treated water. The column was cylindrical and made of opaque plexiglass (Figure S2). A schematic diagram of the treatment process is shown in Figure 2. Each column had a 30 cm internal diameter and 3 m height. A screening box was placed on the outlet of each column to observe larvae breakout. Furthermore, all pipes for water flow were made of PVC in the pilot-scale system.

2.4. Experiment Protocols

The system was operated in a down-flow mode, which maintained a standard filtration of 240 m/day for the GAC filter and 150 m/day for the rapid sand filter. The main physico-chemical characteristics of the water source are summarized in Table S1. The pH of the water source was found to range from 7.09 to 7.29, indicating a slightly alkaline to neutral nature. During this study, the average turbidity of the water source was recorded as 0.13 ± 0.04 mg/L, indicating relatively clear water. The dissolved oxygen (DO) levels in the water source were observed to average 2.17 ± 0.68 mg/L, indicating sufficient oxygen availability for the larvae and other aquatic organisms. All filtration systems were operated continuously for a week, with regular monitoring intervals. Observations were made every hour on the first day and every three hours on the following days to determine if the larvae were successfully removed from each filtration system. The performances of the filtration systems, including GAC, sand filter, sand/GAC, and sand/anthracite, are comprehensively discussed in the Section 3.

2.5. Quality Control

Initially, all filters flowed with water from coagulation, which had low turbidity for clogging testing, which further reduced the effectiveness of the filter. To confirm that there were no larvae in the water source, all filters were operated for 3 days and checked the presence of larvae in effluent water. Furthermore, a backwash protocol was carried out to remove sediment. In addition, the backwash water was filtered and checked for larvae present in the water.

3. Results and Discussion

3.1. Larvae Breakouts from Various Types of Filtration Columns

Chironomidae larvae are known for their highly active behavior, characterized by their lopping swimming movements [8]. These larvae exhibit undulating movements, which further enhance their oxygen-capturing capabilities [24]. Despite the lower dissolved oxygen (DO) levels in the source water, which were below the recommended value of 3 mg/L [25] due to the water being sourced from the effluent of the coagulation process, the larvae remained consistently active. Throughout the filtration processes conducted, it was evident that the larvae successfully broke out from all types of filtration systems after one week. The first instar larvae were observed to break out within a short period, for instance, after approximately three hours in the GAC column. Subsequently, the second instar larvae emerged within 24 h, followed by continuous emergence in the subsequent days. Interestingly, the third and fourth instar larvae only managed to break out from the GAC and sand columns, while they failed to break out from the sand/GAC and sand/anthracite columns. This indicates variations in the larvae’s ability to pass through different filtration media compositions. At the end of the experimental period, the total number of larvae that emerged during the filtration process was found to be higher in the GAC column compared with the sand filter and the combinations of sand/GAC and sand/anthracite. According to the findings, it can be observed that the GAC filtration system exhibits a higher incidence of larvae breakout compared with the other filtration systems (Figure 3).
The ability of the larvae to break out of the filtration column was observed to occur within a relatively short period of time. It was particularly notable that the larvae breakout was in their early developmental stages, specifically instar 1 and instar 2. This finding aligns with previous studies that have shown limited effectiveness of GAC columns in removing larvae from treated water [26]. In the case of the sand filter, the effective size (ES) of the sand particles and the uniformity coefficient (UC) played a crucial role. The results indicated that the ES and UC values currently used in water treatment were relatively large, which allowed the larvae to pass through without being trapped. These findings corroborate the findings of Baek et al., 2022, who reported larvae escaping from a sand filter with a larger effective size [27]. Similarly, Adam et al., 1998, conducted experiments using a sand filter with an effective size of 0.75 mm and a uniformity coefficient of 1.2, but the larvae still broke out from the column [28]. These findings underscore the importance of selecting appropriate sand specifications to effectively retain larvae and achieve successful filtration. Regarding the sand/GAC and sand/anthracite filtration systems, the media specifications were relatively similar to those of the sand and GAC media individually. Consequently, the larvae were also able to escape from these filtration columns.
During the experiment, the observation of larvae breakout from the filtration column provided insightful findings. Notably, it was observed that the larvae remained active and alive upon exiting the column, as depicted in Figure 4. This indicates that the larvae were able to traverse through the column’s pores with ease. The relatively larger size of the media’s pores, in comparison with the Chironomidae larvae’s body size, allowed the larvae to maneuver through the pores effortlessly (Figure 2). Moreover, the direction of water flow played a significant role in facilitating the breakout larvae. The downward flow direction provided assistance to the larvae in their effortless exit from the column. The combination of larger pores and the flow direction enabled the larvae to bypass effective retention within the filtration column. The larvae’s ability to exit the column can be attributed to their flexible bodies, which allow them to navigate through the media’s pores without impediment [29].
Furthermore, it is worth noting that previous studies have also demonstrated the limited effectiveness of filtration columns in removing bacteria and viruses. Despite the smaller size of bacteria and viruses when compared with larvae, GAC columns have shown limited capacity in capturing and effectively removing these microorganisms from treated water [30]. These findings highlight the challenges associated with utilizing filtration columns as comprehensive barriers for the removal of various contaminants, including larvae, bacteria, and viruses. The primary objective of this study was to prevent larvae from entering tap water. However, the results indicate that none of the filtration methods used successfully contained the larvae. This finding underscores the need for optimization of each filtration type to enhance their efficacy in larvae removal. By improving the filtration processes, it becomes possible to mitigate the risk of larvae entering tap water, thereby ensuring the delivery of safe and larvae-free water to consumers.
The ability of larvae to escape filtration in GAC (granular activated carbon) and sand filter columns primarily depends on the size and behavior of the larvae, as well as the mechanisms involved in the filtration process. Insights into the mechanisms that enable larvae to escape filtration are as follows: (1) Larvae come in various sizes and shapes, especially instar 1 larvae with the smallest size tend to pass through the gaps between filter media particles. On the other hand, larger larvae may get trapped more easily [31]. (2) Some larvae have adaptations that make them less dense than water, allowing them to float or stay suspended in the water column. This makes it harder for them to settle and be captured by the filter media. According to our experiments, instar 3 and 4 larvae are more likely to float actively, while instar 1 and 2 larvae tend to become trapped on the surface of the filter media and attempt to move through it, thereby increasing their chances of avoiding capture [25]. (3) Over time, the filter media in GAC and sand filters can become clogged with particles and biological matter. Larvae may find refuge within these layers, avoiding the direct flow of water through the filter. However, it is important to note that when this occurs, the filters should be washed [32]. (4) Some larvae may form aggregates or clusters that can be larger than individual larvae. These clusters may be more challenging to capture due to their size and irregular shape [33]. Understanding the behavior and filtration mechanisms is essential for finding solutions to prevent larvae from escaping into tap water.

3.2. Larvae Breakouts in Relation to Sand Depth in Filtration Columns

We attempted to optimize a sand filter column commonly used in every WTP by increasing the thickness of the sand media to 100 cm. Increasing the depth of sand filter media aims to maintain water quality stability, adhering to practical guidelines that specify an allowable ratio between media depth and particle size, typically ranging from 1000 to 1200 in various water treatment plants [34]. It is expected that by increasing the thickness of the sand media, no larvae will break out of the sand filter column. Figure 5 presents the results of the distribution of Chironomidae larvae at various instar stages in two sand filtration columns with depths of 60 cm and 100 cm. In the sand filtration column with a depth of 60 cm, a higher number of larvae were observed compared with the 100 cm column. Specifically, six larvae were detected at the first instar stage, while five larvae were observed at the second instar stage. Additionally, one larva was found at the third instar stage, but no larvae were observed at the fourth instar stage. These findings indicate that the sand filter with a depth of 60 cm was less effective in capturing and retaining larvae, particularly as the instar stage advanced. The presence of larvae at the third instar stage suggests that the filtration process in the 60 cm column allowed some larvae to progress to later stages before being captured or removed. In contrast, the sand filtration column with a depth of 100 cm exhibited lower larval numbers at each instar stage. Two larvae were observed at both the first and second instar stages, while no larvae were detected at the third or fourth instar stages. These results suggest that the sand filter with a depth of 100 cm was more effective in capturing and retaining larvae at the early instar stages but had limited success in preventing larvae from advancing to later stages.
The findings of this study demonstrate that using sand media with a thickness of 100 cm effectively reduces the number of larvae escaping from the filtration system [35]. This indicates that the increased depth of the sand media provides a physical barrier that hinders the movement and escape of larvae. While this reduction in larvae breakout is promising, it is important to note that solely relying on this parameter may not be sufficient to fully prevent larvae from entering tap water. The increased depth of the sand media resulted in enhanced filtration efficiency, leading to a notable reduction in the levels of total coliform bacteria. Turbidity, which is a measure of the clarity of water, also exhibited a substantial decrease. Furthermore, the presence of TSS, BOD, and COD, which are indicators of organic and inorganic pollutants, significantly declined in the treated water [36,37].
The differences in larval distribution between the two sand filtration columns may be influenced by various factors. These include the physical properties of the sand media, such as the effective size and uniformity coefficient, which can affect the filtration efficiency and the ability to retain different sizes of larvae. It should be noted that the absence of larvae at the third and fourth instar stages in both sand columns may be due to the larvae being retained by the sand screening process. It is also possible that a small number of first- and second-stage instar larvae break out of the column due to cannibalism. Cannibalism among larvae was attributed to a scarcity of food in the experimental setting, leading to a decrease in larval population. As organisms typically demand more energy as they mature, fourth-stage instar larvae likely had higher energy needs compared with earlier instar larvae. Consequently, a greater occurrence of cannibalism was observed during the fourth instar stage [26,38].

3.3. Larvae Breakouts in Different Pretreatment Methods Prior to Sand Filtration

Subsequent experiments were conducted in an effort to prevent larvae breakout from the sand filter column. These experiments involved pretreatment methods aimed at rendering the larvae inactive and thus unable to escape from the sand filter column. The pretreatment methods include coagulation, chlorination, coagulation followed by chlorination, and chlorination followed by coagulation. During the pretreatment stage, a coagulant dosage of 2 mg/L as Al3+ was used in conjunction with a turbidity level of approximately 150 mg/L. Additionally, a chlorine dosage of 1–1.5 mg/L as Cl− was used. These dosage parameters were consistently maintained when combining coagulation and chlorination for pretreatment. Following the pre-treatment process, the treated larvae were introduced into the sand filter column. This introduction occurred 30 min after the completion of the chlorination process and 10 min after the sedimentation process resulting from coagulation.
During the coagulation process, the coagulant neutralizes the negative charge on suspended particles, destabilizing them and causing them to aggregate into flocs. Once flocs are formed, Chironomidae larvae tend to adhere to their surfaces and settle alongside them. Eventually, the larvae become covered by the flocs, rendering them inactive. On the other hand, chlorination serves as a potent oxidizing agent capable of penetrating the cell walls of Chironomidae larvae. It then oxidizes crucial cell components, including enzymes and genetic material, disrupting the larvae’s vital functions and rendering them inactive [8,20].
The experimental results revealed the breakout of Chironomidae larvae under different treatment conditions (Table 1). In the blank treatment, only one larva in instar 1 (i-1) managed to break out after approximately 11 h. Similarly, the coagulation treatment resulted in the escape of two larvae in instar 2 (i-2) after 15 h. The chlorination treatment also proved ineffective, with one larva in instar 1 (i-1) and one larva in instar 2 (i-2) escaping after 12 h. In the coagulation followed by chlorination treatment, two larvae in instar 2 (i-2) escaped after 17 h. Lastly, in the chlorination followed by coagulation treatment, three larvae in instar 2 (i-2) managed to escape after 19 h. All experiments were conducted for three days of the filtration process.
Interestingly, the results indicated that all treatments failed to prevent the breakout of Chironomidae larvae regardless of the method implemented. Although larvae inactivation could be achieved, the larvae remained active and were able to reappear within the filtration column [8]. An alternative approach explored in previous studies involved the combination of coagulation and chlorination. This approach demonstrated a reduction of 24% in total organic carbon (TOC) and the inhibition of microbial activity by continuously adding a hypochlorite solution to maintain a residual chlorine concentration of 8.9 (STD 6.4) mg/L [39]. However, it should be noted that while this approach proved effective in decreasing TOC and inhibiting microbial activity, it did not effectively control the presence of larvae [40]. Considering these findings, modifying the sand filter media appears to be a promising method for preventing larvae from entering tap water. Further details regarding the specific modifications and their effectiveness can be found in Section 3.4 of this study.

3.4. Impact of Effective Size and Uniformity Coefficient on Larvae Removal

The variation in effective size (ES) and uniformity coefficient (UC) was investigated. The purpose of this experiment was to assess the impact of different combinations of effective size and uniformity coefficient on the ability of the filtration system to restrain the larvae from breaking out of the sand filter column. Table 2 showed that with an effective size of 0.7 mm and a uniformity coefficient of 1.7, two larvae in instar 2 managed to break out after approximately 15 h. During the screening process, which had a consistent duration of 3 days for all observations, the status column provided the outcomes of each observation. When the effective size was 0.6 mm and the uniformity coefficients were 1.7 and 1.5, and when the effective size was 0.7 mm with a uniformity coefficient of 1.5, no larvae were able to break out of the sand filter column (SUCCEED). However, in the observations where the effective size was 0.7 mm and the uniformity coefficient was 1.7, the larvae successfully broke out from the sand filter column (FAILED).
We recommend that the sand media parameters be modified to effectively restrain larvae from escaping the sand filter column, particularly by using an effective size (ES) of 0.7 mm and a uniformity coefficient (UC) of 1.5. Although there is no other research available to support this recommendation, a study conducted by [41] demonstrated that a sand layer with an effective size (ES) of 0.7 mm and uniformity coefficient (UC) of 1.6 showed high efficiency in controlling organic matter in drinking water treatment. Similarly, [42] tested a sand filter with an ES of 0.6 mm and UC of 1.6 for reducing common pollutants in water treatment plants. The results showed a turbidity removal efficiency of 97%, which can also be applied to prevent larvae breakout from the sand filter column. Our findings suggest that the combination of an appropriate ES and UC in sand media can contribute to efficient filtration and removal of impurities and larvae in water treatment processes.

3.5. Application of Optimum ES and UC of Sand Media in Sand/GAC and Sand/Anthracite Filters

The optimal operating factors for controlling Chironomidae larvae in a multi-layer filtration process were identified. The filtration process involved using a combination of granular activated carbon (GAC) with sand and anthracite with sand as filter media. The granular activated carbon (GAC) used has an effective size of 0.7 mm, a uniformity coefficient of 1.7, and a bed height of 110 cm. The anthracite used in the experiments has an effective size of 1.0 mm, a uniformity coefficient of 1.6, and a media depth of 20 cm. Meanwhile, the sand used in the experiment has an effective size of 0.7 mm and a uniformity coefficient of 1.5. Additionally, the bed depth of the sand was 30 cm in each column.
The experimental results showed that in both the sand/GAC and sand/anthracite filter column, no Chironomidae larvae escaped during the 3-day operation (Table 3). This result suggests that the sand layer with ES: 0.7 mm and UC: 1.5, along with the GAC and anthracite layer, acted as a barrier that trapped and retained the larvae within the filtration system. The absence of larvae breaking out in this experiment shows that using sand with optimum ES and UC effectively controlled Chironomidae larvae. This is different from the previous study by Baek et al., 2022 [27], where larvae continued to escape even after 15 days. The difference in larvae escape between the two studies can be attributed to the characteristics of the sand used in the filtration system. In this experiment, the sand had an effective size (ES) of 0.7 mm, which is smaller compared with the sand with an ES of 1.0 mm used in the study by Baek et al., 2022 [27]. The larger grain size of the sand used in their study might have provided more opportunities for larvae to pass through the filter media and escape from the system. This is important for water treatment processes where the presence of larvae can impact water quality and aesthetics. The use of appropriate filter media and their proper arrangement in the filtration system, as demonstrated in this experiment, can help ensure the removal and retention of larvae, leading to cleaner and safer water. Similar to the sand filter column, the effective size of the sand is an important parameter that determines the ability of the filter media to retain Chironomidae larvae.

3.6. Impact of Water Quality on Larvae Breakout

The impact of water quality on larvae breakout in a filtration system is a critical consideration for ensuring the effectiveness of water treatment processes. In the provided water quality dataset, several key parameters such as pH, dissolved oxygen (DO), conductivity, turbidity, and temperature are measured (Table S1). These parameters collectively influence the ability of the filtration system to retain larvae, thereby affecting the quality of treated water.
The pH levels in the range of 7.09 to 7.29 suggest a slightly acidic to neutral environment. The pH of water can influence the surface charge of larvae and filter media and can impact the adhesion of larvae to sand particles in the filtration system [27]. The dissolved oxygen (DO) levels range from 1.11 to 2.86 mg/L, indicating adequate oxygen levels for the survival of various aquatic organisms, including larvae. Higher DO concentrations, as observed here, may favor larval survival and mobility within the filtration system, potentially increasing their chances of escaping the filter bed. The turbidity values range from 0.10 to 0.16 NTU, showing that the filtration system has effectively removed most of the particles, allowing fewer opportunities for larvae to be trapped within the filter media. The conductivity values, ranging from 27.06 to 31.00 ms/m, suggest variations in ion concentrations that could affect water chemistry but may not significantly influence larval behavior within the filter. Temperature, which was relatively stable at around 30 degrees Celsius, falls within a range typical for many water treatment processes and is unlikely to have a pronounced impact on larvae retention.
The observed water quality parameters, including pH, DO, and turbidity, can collectively influence the likelihood of larvae breakout in the filtration system. The neutral pH, higher DO concentrations, and relatively low turbidity may contribute to greater larval mobility and potential escape from the filter media.

3.7. Evaluation of Larvae Removal Using Backwashing

The backwash operation conditions for the sand filtration process were determined in accordance with the Korean waterworks facility standards. The backwash process is an essential part of maintaining the efficiency of filtration columns, such as sand filters, sand/GAC, and sand/anthracite layer filters. It involves reversing the flow of water through the column to clean and remove accumulated particles, debris, and contaminants that have been captured by the filtration media. The aim was to evaluate the effectiveness of each method in removing larvae from the filtration column.
Figure 6 shows the number of Chironomidae larvae that can be removed during the backwash process in different filtration systems and at different instar stages. In the sand filtration system, no larvae were removed at the first instar stage. However, at the second instar stage, five larvae were successfully removed during the backwash process. This number increased to four larvae at the third instar stage and eight larvae at the fourth instar stage. In the sand/GAC filtration system, similar to the sand filtration system, no larvae were removed at the first instar stage. However, only one larva was successfully removed during the backwash process at the second instar stage. The number of larvae removed increased to four at the third instar stage and six at the fourth instar stage. For the sand/anthracite filtration system, again, no larvae were removed at the first instar stage. However, at the second instar stage, four larvae were successfully removed during the backwash process. This number increased to six larvae at both the third and fourth instar stages. The high number of third- and fourth-stage instar larvae observed during the backwash process can be attributed to the behavior of these stages, as they tend to dig and occupy only the top 10 cm of the filter media. This shallow position makes them more susceptible to being flushed out during the backwash [26,43].
The proper backwash procedure consists of multiple steps, each serving a specific purpose in the cleaning and maintenance of the filtration system. The sequence of these steps is designed to ensure the effective removal of particles, contaminants, and turbidity from the filtration media [44]. The first step is air flow only for 15 min. This step involves introducing a flow of air into the filtration system. This helps create turbulence and agitation within the media, dislodging accumulated debris and particles [45]. The air flow helps break down any solids, biofilms, or other contaminants that may have adhered to the filtration media. This step prepares the media for further cleaning and enhances the effectiveness of subsequent steps. The second step is combining air and water flow until water reaches the overflow tank. After the air flow step, the combination of air and water flow is initiated. The purpose of this step is to flush out the dislodged particles and contaminants from the media [46]. The flow of water, along with the remaining air bubbles, helps carry away the debris and suspended particles, ensuring they are directed toward the overflow tank. The combination of air and water flow enhances the flushing action, facilitating a more thorough cleaning of the filtration media. The final step is water flow only for approximately 15 min until the outflow turbidity reaches 10 NTU. Once the water has reached the overflow tank, the next step involves continuing the backwash process with water flow only. The purpose of this step is to further rinse the filtration media and remove any remaining particles or turbidity. The duration of approximately 15 min is determined based on reaching a specific target turbidity level (in this case, 10 NTU) in the outflow. This step ensures that the filtration system is thoroughly cleaned and ready for normal operation [44].

4. Conclusions

The findings from our experiments highlight that the current water treatment technology primarily focuses on maintaining overall water quality, which is a crucial objective but may not address other pollutants like preventing Chironomidae larvae from entering treated water. We observed this limitation in several methods we examined, including the GAC column, designed primarily for adsorption, and the commonly used sand filter column in WTPs. The sand filter, characterized by an effective size (ES) of 0.7 mm and a uniformity coefficient (UC) of 1.7, proved ineffective in preventing larvae from entering the treated water, even with adjustments like increased sand media height and pretreatment methods. However, our research yielded promising results in larvae prevention by modifying the sand media, specifically by reducing the UC to 1.5 while maintaining an ES of 0.7 mm. Additionally, combining various filtration media, such as sand/GAC and sand/anthracite, while maintaining this sand media modification, proved successful in addressing the larvae issue. Moreover, our experiments underscore the significance of the backwash process in upholding filtration system efficiency. The backwash procedure involves a series of steps, including an initial air flow, a combination of air and water flows, and a final water flow, with the goal of effectively removing particles, contaminants, and turbidity from the filtration media. Adhering to recommended backwash procedures, including the correct sequence and duration of steps, is essential for ensuring a thorough cleaning and maintenance of the filtration system, ultimately leading to enhanced water quality and optimal system performance. Looking beyond our experiments, we recommend that water treatment plants (WTPs) implement a comprehensive set of long-term strategies to sustainably manage Chironomidae larvae and maintain consistent water quality. These strategies encompass regular monitoring of larval populations near WTPs, improved water resource management practices, exploration of advanced filtration technologies, biofilm control measures, investigation of alternative larval control chemicals, routine equipment maintenance, public awareness campaigns, collaboration with research institutions, adherence to regulatory guidelines, and the adoption of adaptive management approaches. The combined application of these strategies offers a proactive and holistic approach to address concerns related to Chironomidae larvae and ensure the long-term quality and safety of drinking water treatment processes.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su152014881/s1, Figure S1: Sieve analysis carried out to determine the effective size (ES) and uniformity coefficient (UC) of sand; Figure S2: Filter column in the pilot-scale system; Table S1: The main physico-chemical characteristics of the water source; Table S2: Filter media properties.

Author Contributions

Conceptualization, methodology, and writing—original draft preparation: H.J.K.; writing, validation, and formal analysis: H.H.; validation and formal analysis, and writing—review and editing: R.K.; supervision, project administration, and funding acquisition: T.G.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Korean Ministry of Environment in 2022 (Project No. 2022040D7F5-00).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are grateful to the Korean Ministry of Environment for funding this research (Project No. 2022040D7F5-00) and for providing facilities support.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Larvae sampling point (A) near river and (B) domestic drainage system.
Figure 1. Larvae sampling point (A) near river and (B) domestic drainage system.
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Figure 2. Filter column and schematic diagram of the pilot-scale system. (A) GAC column, (B) Sand filter column with a 60 cm depth, (C) Sand filter column with a 100 cm depth, (D) Sand/GAC column, (E) Sand/Anthracite column.
Figure 2. Filter column and schematic diagram of the pilot-scale system. (A) GAC column, (B) Sand filter column with a 60 cm depth, (C) Sand filter column with a 100 cm depth, (D) Sand/GAC column, (E) Sand/Anthracite column.
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Figure 3. Total Chironomidae larvae breakout from 4 different filtration systems.
Figure 3. Total Chironomidae larvae breakout from 4 different filtration systems.
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Figure 4. (A) Chironomidae larvae escape from the filtration column and are still alive. (B) Larvae move through the media pores. The arrow symbol indicates that the individual larvae are alive and still actively moving. The circle indicates the larvae move through the sand pores.
Figure 4. (A) Chironomidae larvae escape from the filtration column and are still alive. (B) Larvae move through the media pores. The arrow symbol indicates that the individual larvae are alive and still actively moving. The circle indicates the larvae move through the sand pores.
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Figure 5. Comparison of Chironomidae larvae breakout from 60 cm and 100 cm sand filters.
Figure 5. Comparison of Chironomidae larvae breakout from 60 cm and 100 cm sand filters.
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Figure 6. Comparison of larvae removal during the backwash process in different filtration systems and instar stages.
Figure 6. Comparison of larvae removal during the backwash process in different filtration systems and instar stages.
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Table 1. Breakout of Chironomidae larvae under different treatment conditions.
Table 1. Breakout of Chironomidae larvae under different treatment conditions.
ObservationBlankCoag. 1Chlo. 2Coag. → Chlo. 3Chlo. → Coag. 4
Number of escaped larvae (stage)1 (i-1) 52 (i-2) 61 (i-1)
1 (i-2)		2 (i-2)3 (i-2)
Escape after (hour)11 h15 h12 h17 h19 h
Filtration (day)3 d3 d3 d3 d3 d
STATUSFAILEDFAILEDFAILEDFAILEDFAILED
1 Coag.: coagulation; 2 Chlo.: coagulation; 3 Coag. → Chlo.: coagulation followed by chlorination; 4 Chlo. → Coag.: chlorination followed by coagulation; 5 (i-1): instar 1; 6 (i-2): instar 2.
Table 2. Effect of effective size and uniformity coefficient on breakout larvae from the sand filter column.
Table 2. Effect of effective size and uniformity coefficient on breakout larvae from the sand filter column.
ObservationES 1: 0.6 mm
UC 2: 1.7		ES: 0.6 mm
UC: 1.5		ES: 0.7 mm
UC: 1.7		ES: 0.7 mm
UC: 1.5
Number of escaped larvae (stage)--2 (i-2) 3-
Escape after (hour)--15 h-
Filtration
(day)		3 d3 d3 d3 d
STATUSSUCCEEDSUCCEEDFAILEDSUCCEED
1 ES: effective size; 2 UC: uniformity coefficient; 3 (i-2): instar 2.
Table 3. Comparison of larvae that escaped in the sand/GAC and sand/anthracite filtration systems.
Table 3. Comparison of larvae that escaped in the sand/GAC and sand/anthracite filtration systems.
ObservationSand/GACSand/Anthracite
Number of escaped larvae (stage)--
Escape after
(hour)		--
Filtration
(day)		3 d3 d
STATUSSUCCEEDSUCCEED
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Kwon, H.J.; Hidayaturrahman, H.; Koutavarapu, R.; Lee, T.G. Comprehensive Solutions to Prevent Larvae Breakout in Water Filtration Systems. Sustainability 2023, 15, 14881. https://doi.org/10.3390/su152014881

AMA Style

Kwon HJ, Hidayaturrahman H, Koutavarapu R, Lee TG. Comprehensive Solutions to Prevent Larvae Breakout in Water Filtration Systems. Sustainability. 2023; 15(20):14881. https://doi.org/10.3390/su152014881

Chicago/Turabian Style

Kwon, Hyuk Jun, Haerul Hidayaturrahman, Ravindranadh Koutavarapu, and Tae Gwan Lee. 2023. "Comprehensive Solutions to Prevent Larvae Breakout in Water Filtration Systems" Sustainability 15, no. 20: 14881. https://doi.org/10.3390/su152014881

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