This study evaluated the seasonal performance of the Koka water treatment plant in removing natural organic matter (NOM) and the implications for disinfection by-product (DBP) formation potential. Raw and treated water samples were collected during the dry and wet seasons and analyzed using physicochemical parameters and fluorescence spectroscopy. The results revealed significant seasonal variations in raw water (RW) quality, with higher turbidity, pH, temperature, conductivity, total organic carbon (TOC), dissolved organic carbon (DOC), and UV254 absorbance during both seasons and across treatment processes. The NOM removal efficiency of the treatment plant was poor, with mean TOC removal of 46 and 43% and DOC removal of 15.8 and 15.2% during dry and wet seasons, respectively. The sedimentation unit demonstrated negative TOC removal, indicating NOM accumulation likely due to biochemical reactions in the unit. Fluorescence analysis and the correlation between specific ultraviolet absorbance (SUVA) and DBP formation potential suggest a higher risk of DBP formation in chlorinated drinking water. These findings highlight the influence of seasonal variations, RW quality, and the treatment process dynamics on the plant's performance in removing NOM. There is a need to implement adaptable strategies to enhance NOM removal, accounting for seasonal fluctuations in RW quality.

  • Significant seasonal variations in raw water quality and NOM removal efficiency.

  • The sedimentation unit had a negative TOC removal, suggesting NOM accumulation was likely due to biochemical reactions within the unit.

  • Higher risk of disinfection by-product (DBP) formation potential in chlorinated drinking water.

  • Develop strategies to efficiently eliminate NOM taking into account seasonal variations in raw water quality.

Access to clean, safe drinking water is a fundamental human right that must be upheld as essential for public health and sustainable development (Spijkers 2020). Safe drinking water is not a commodity or privilege, but a vital resource that is fundamental to the health, well-being, and quality of life of all people. However, meeting this imperative is a persistent challenge faced by water utilities around the world, as they must contend with a range of natural and anthropogenic contaminants that can compromise the safety and quality of the final drinking water supply. The consequences of inadequate access to clean water can be severe, contributing to the spread of waterborne diseases, malnutrition, and other health problems, particularly in low-income countries (Wolf et al. 2023).

One of the key obstacles to consistently providing safe and clean water is the presence of natural organic matter (NOM) in raw water (RW) sources. NOM is an important concern in drinking water treatment due to its role as a precursor for the formation of potentially carcinogenic disinfection by-products (DBP) during chlorination. There are over 600 known DBP species that can form during water treatment processes. However, the most commonly regulated and monitored DBPs include trihalomethanes (THMs), haloacetic acids (HAAs), bromide, and chlorite (Gilca et al. 2020). These DBPs have been associated with increased cancer risk and other negative health outcomes in people exposed to them through drinking water (Choi et al. 2022). NOM is a complex mixture of organic substances that can pose operational challenges, such as increased dosages of coagulants and disinfectants, and membrane fouling, which can lead to the need for higher doses of disinfectants and potentially increase operational costs and the risk of DBP formation potential in water treatment processes (Peters et al. 2021; He et al. 2023).

The previous studies that have been conducted on evaluations of water treatment plant (WTP) performance have typically focused on the overall removal performance of NOM from RW to finished water, without a detailed examination of the dynamic behavior and transformations occurring within each treatment unit. This lack of stage-by-stage analysis in water treatment plants has constrained efforts to improve the efficiency and reliability of NOM removal in drinking water treatment (Wang et al. 2021). Furthermore, the impacts of seasonal variations in NOM characteristics on the performance of specific treatment stages, such as coagulation, sedimentation, and filtration to remove NOMs, are often not well documented (Vasyukova et al. 2013).

The lack of understanding regarding the complex dynamics of organic matter removal within the various treatment stages of the WTP has hindered researchers from optimizing the conventional treatment processes. This knowledge gap has also impeded the development of effective treatment strategies and the ability to address the operational challenges faced by the plant. Filling these critical knowledge gaps is essential for the Koka WTP to develop targeted strategies and operational changes that can improve its performance, particularly during seasonal variations in RW quality.

The Awash River, which supplies RW to the Koka WTP, is facing significant water quality challenges due to the impacts of rapid industrialization, urbanization, and intensive agricultural practices in the basin. Specifically, the discharge of untreated waste, chemical effluents, and excessive sediment loads into the Awash River has severely compromised the overall water quality (Assegide et al. 2022). This influx of pollutants and sediment from various anthropogenic activities in the basin is a major concern for the WTP, as it increases the complexity of the RW and the treatment requirements.

The Koka WTP, which serves the town of Adama and surrounding communities in Ethiopia, uses conventional treatment methods to treat water from the Awash River. However, the plant faces significant challenges in accurately predicting the efficiency of NOM removal due to the dynamic nature of RW quality and the temporal and spatial variations in NOM characteristics.

The existing research on the impacts of seasonal variations in NOM levels, and how that affects the efficiency of NOM removal and DBP formation potential during water treatment processes, is limited in the Ethiopian context. This lack of localized, evidence-based knowledge hinders the efforts of researchers and WTP designers to develop effective, tailored solutions for improving the performance of conventional treatment facilities, such as the Koka WTP.

Therefore, the objective of this study is to evaluate the performance of the Koka WTP in the removal of NOM, with a focus on the seasonal variability of NOM concentrations and characteristics at each stage of the treatment processes and the implication of it on DBP formation.

The insights and knowledge gained from this study can be leveraged by other WTP designers and operators to develop tailored, evidence-based strategies for managing the challenges posed by seasonal variations in NOM and optimizing the overall performance of their treatment facilities.

Description of the study site and water sources

The Awash River, originating from the high plateau near Ginchi, west of Addis Ababa, serves as the primary drinking water source for the city of Adama and surrounding rural communities in the central Rift Valley of Ethiopia, as shown in Figure 1.
Figure 1

The study area map.

Figure 1

The study area map.

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However, the river is heavily polluted due to extensive industrial and agricultural activities along its basin, with studies showing that 90% of the waste generated by Upper Awash industries is directly discharged into the river without treatment (Assegide et al. 2022).

The Koka WTP, located at the coordinates 8°30′9″ N, 39°09′30″ E, treats the RW from the heavily polluted Awash River. The treatment plant is situated approximately 3 km downstream of the Koka Hydroelectric Power Station and 17 km east of the city of Adama. This water treatment facility plays a crucial role in providing clean drinking water to the residents of Adama and the surrounding rural communities, despite the significant challenges posed by the river's high pollution levels.

The treatment plant employs conventional treatment processes, including screening, coagulation, sedimentation, filtration, and disinfection, to produce 17,000 m3 of drinking water per day for Adama and surrounding communities. Currently, 285 L/s of RW from the river enters the screening of the Koka WTP.

During the coagulation and flocculation stage, the plant uses 30–40 mg/L of aluminum sulfate to facilitate the aggregation and settling of suspended particles. The sedimentation unit in the Koka conventional water treatment process is designed to have a detention time of 3–4 h. Pre-chlorination is applied at the inlet of the sedimentation unit, using 20–30 mg/L of chlorine at an average pH of 8.10. Despite these chlorination efforts, the plant has faced persistent challenges, such as algal blooms in the sedimentation treatment unit. Post-chlorination is also carried out after the sand filtration (SF) stage before the water is distributed to the public.

Water sample collection and storage

The water quality monitoring was conducted over 1 year, from July 2021 to August 2022, to capture the seasonal variations in the Koka WTP. Samples were collected monthly from specific locations within the Koka WTP, including the screening outlets (RW), coagulation and flocculation outlets (CF), sedimentation outlets (SD), sand filter outlets (SF), and the final treated water (TW).

The sampling schedule for this study was designed to capture the seasonal variations in water quality, aligning with the two main seasons of Koka, Ethiopia. Samples were collected during the rainy (wet) period from June to September, as well as the dry season from October to May. This allowed the researchers to evaluate how the water quality parameters may differ between the wet and dry seasons, as shown in the supplementary material (Table S1).

The sample collection and preservation followed standard methods (WHO 2022). The sample bottles were thoroughly washed with detergent, rinsed with deionized water, and oven-dried at 100°C for 1 h before use. Field blanks were collected at each sampling location to monitor possible contamination during sample handling (Van Winckel et al. 2021).

Sample preparation and analysis

The collected water samples were sonicated for 1 h and allowed to settle before being transferred to the analysis vials. For the analysis of dissolved organic carbon (DOC), samples were filtered through 0.45 μm membrane filters and acidified with hydrochloric acid (HCl) to adjust the pH to <2. Total organic carbon (TOC) analysis was performed on unfiltered, acidified samples to quantify TOC content, including both dissolved and particulate fractions.

TOC and DOC concentrations in the water treatment stages were determined using the Standard Methods for the Examination of Water and Wastewater. A standard stock solution of 1,000 mg C/L was prepared using potassium hydrogen phthalate (KHP) as the reference material solution (Potter & Wimsatt 2012; Cook et al. 2017). Based on the dilution method, nine standard working carbon solutions (0, 1, 2, 5, 10, 15, 20, 25, and 50 mg/L) were prepared and a highly accurate calibration curve was obtained, with a coefficient of determination (R2) of 0.998.

UV absorbance at 254 nm (UV254) was measured using an Agilent Technologies 60 UV-Vis spectrophotometer, with quartz cuvettes having a 1 cm optical path length and a wavelength range of 200–800 nm. Specific UV absorbance (SUVA) was calculated by dividing the UV254 absorbance by the concentration of DOC and multiplying the result by 100 cm/m, as an indicator of the aromatic carbon content of the NOM (Zhou et al. 2020).

Fluorescence spectroscopy analysis was conducted using an RF-5301PC Spectro fluorophotometer. Fluorescence emission spectra were recorded from 200 to 600 nm with fixed excitation wavelengths of 280 and 320 nm. The scanning speed was set to 200 nm/min for emission and excitation measurements. A 5 cm quartz cuvette was used and the blank value was determined using deionized water as a reference to correct for any background fluorescence.

Evaluation of Koka WTP to remove NOMs

The effectiveness of the Koka WTP in removing NOM was evaluated by monitoring changes in TOC, DOC, and UV254 concentrations in different treatment stages and throughout seasons. The removal efficiency was calculated using the following equation 1:

where Ci is the input concentration (e.g., the RW from the inlet of each treatment unit) and Ce is the concentration at the outlet of each treatment unit of NOM parameters (TOC, DOC, and UV254).

Analytical methods

The physicochemical properties of the water samples were measured on site and, in the laboratory, using standard instruments and procedures. Turbidity was measured with a portable Hach 2100P turbidity meter, pH was determined using an AP115 portable pH meter, the water temperature was recorded with an FT708 digital thermometer, and electrical conductivity (EC) was measured using a Jenway Model 4510 conductivity meter.

The normality and homogeneity of variance assumption were checked for each parameter (TOC, UV254, DOC, and SUVA) at each treatment stage (RW, CF, SD, SF, TW).

A one-way analysis of variance (ANOVA) was applied for quantitative dependent variables using the season as the independent variable. Tukey Cramer test was used for mean separation for one-way ANOVA. Descriptive statistics such as means were applied for summarizing and presenting the findings. The analysis was done using R softer version 4.3.2.

The general linear model used for the analysis was:
where Yij denotes the response variables (RW, CF, SD, SF, TW); μ is the overall mean; Si is the effect of the season (dry season, wet season); eij is the random error.

Regression analyses were performed to evaluate the correlation between the treatment process in NOM concentration and the performance of the WTP. Descriptive statistics, such as mean and standard deviation, were calculated to identify any outliers that required further investigation. Field blanks, sample replicas, and certified reference materials were used to ensure the reliability and reproducibility of analytical procedures.

Seasonal variations in RW quality

Seasonal variations in RW quality at the Koka WTP revealed several significant differences between the dry and wet seasons (Table 1). The analysis revealed significant differences between the dry and wet season conditions for several key parameters.

Table 1

Seasonal variations of raw water quality at the Koka water treatment plant

ParametersDry season, n = 8
Wet seasons, n = 4
p-value
Mean ± SDRangeMean ± SDRange
pH 8.2 ± 0.5 7.7–8.7 7.8 ± 0.34 7.6–8.3 0.044 
Turbidity (NTU) 273.8 ± 32.34 219–321 177.25 ± 40.2 140–162 <0.001 
Temperature (°C) 23.3 ± 0.85 22.1–24.0 22.4 ± 0.71 21.6–23 0.017 
Conductivity (μS/cm) 392.5 ± 25 350–425 370 ± 20 345–395 0.035 
TOC (mg/L) 41.8 ± 2.39 36.0–43.1 30.24 ± 1.16 28.5–42.4 <0.001 
DOC (mg/L) 6.65 ± 0.85 5.77–8.5 5 ± 0.94 3.6–5.9 <0.002 
UV254 (cm−10.197 ± 0.023 0.499–0.596 0.1422 ± 0.016 0.08–0.191 <0.001 
ParametersDry season, n = 8
Wet seasons, n = 4
p-value
Mean ± SDRangeMean ± SDRange
pH 8.2 ± 0.5 7.7–8.7 7.8 ± 0.34 7.6–8.3 0.044 
Turbidity (NTU) 273.8 ± 32.34 219–321 177.25 ± 40.2 140–162 <0.001 
Temperature (°C) 23.3 ± 0.85 22.1–24.0 22.4 ± 0.71 21.6–23 0.017 
Conductivity (μS/cm) 392.5 ± 25 350–425 370 ± 20 345–395 0.035 
TOC (mg/L) 41.8 ± 2.39 36.0–43.1 30.24 ± 1.16 28.5–42.4 <0.001 
DOC (mg/L) 6.65 ± 0.85 5.77–8.5 5 ± 0.94 3.6–5.9 <0.002 
UV254 (cm−10.197 ± 0.023 0.499–0.596 0.1422 ± 0.016 0.08–0.191 <0.001 

During the dry season, RW had a significantly higher turbidity (273.8 ± 32.34 nephelometric turbidity unit (NTU)) than wet season (177.25 ± 40.2 NTU), with a p-value less than 0.001. This finding is consistent with previous studies that have reported the impact of seasonal hydrology on surface water turbidity (Bhurtun et al. 2019; Martins et al. 2019). The higher turbidity levels during the dry season are likely due to reduced water flow, increased microbiological activities, sediment transport from the catchment area, and the occurrence of algae blooms during the dry season (Freire et al. 2021; Zhang et al. 2021a).

The studies conducted on the Awash River have consistently demonstrated the degradation of the physicochemical and biological water quality due to the presence of various pollutants (Eliku & Leta 2018; Yimer & Geberkidan 2020). These findings highlight the broader water quality challenges faced in the Awash River Basin, which serves as the primary source for the Koka WTP.

Building upon this understanding, the on-site observations at the Koka WTP have revealed a persistent challenge with algae blooms in the sedimentation treatment unit, even though chlorine is being applied at the inlet of the sedimentation stage. This suggests that the existing treatment processes may not be adequate to effectively manage the algal growth and the associated water quality issues.

The RW is expected to have high levels of organic matter, which can react with and consume chlorine, reducing its availability and disinfection capacity for effectively controlling the algae growth. The presence of persistent algae blooms in the sedimentation unit is a significant concern, as it can contribute to the increased levels of TOC observed in this stage of the treatment process (Park et al. 2018).

The pH of RW also showed a statistically significant difference between the dry and wet seasons, with the dry season having a higher mean pH of 8.2 ± 0.5 compared to 7.8 ± 0.34 in the wet season (p-value = 0.044). This variation in pH may be influenced by factors such as the concentration of dissolved ions, biological activity, and the weathering of geological formations in the catchment area (Freire et al. 2021). Similarly, the RW temperature was significantly higher in the dry season (23.3 ± 0.85 °C) compared to the wet season (22.4 ± 0.71 °C), with a p-value of 0.017. Seasonal variations in RW temperature are commonly observed and can be attributed to factors such as solar radiation, air temperature, and precipitation patterns (Álvarez-Cabria et al. 2016).

The EC of the RW also exhibited a statistically significant difference, with the dry season having a higher mean value of 392.5 ± 25 μS/cm compared to 370 ± 20 μS/cm in the wet season (p-value = 0.035). This variation may be related to the effects of dilution, evapotranspiration, and biological activity that probably contribute to the seasonal variations observed in the EC of RW in the Awash River. Evidence from the table, such as differences in conductivity, TOC, and DOC between the dry and wet seasons, supports the influence of these factors on the water's ionic composition and, consequently, its EC (Unrine et al. 2024).

The RW at the Koka WTP exhibited significantly higher specific conductance during the dry season (403.1 ± 25.0 μS/cm) compared to the wet season (384.6 ± 20.0 μS/cm), with a statistically significant difference (p = 0.035). This seasonal variation in specific conductance can be attributed to the concentration of dissolved ions in the water during the drier period, which is likely due to reduced dilution from rainfall.

The higher water temperatures observed in the dry season (23.3 ± 0.85°C) compared to the wet season (22.4 ± 0.71°C) contribute to the increased concentration of dissolved minerals, salts, and other ions in the RW. In contrast, the wet season is characterized by lower water temperatures and higher river flow, which dilutes the water and reduces the concentration of dissolved ions, resulting in lower specific conductance values.

TOC and DOC levels were significantly higher in the dry season compared to the wet season, with p values less than 0.001 and 0.002, respectively. This may be attributed to reduced dilution, increased microbial activity, and the release of organic matter from the catchment areas (Zhang et al. 2021a). The UV absorbance at 254 nm (UV254) was also significantly higher in the dry season (0.197 ± 0.023 cm−1) than in the wet season (0.142 ± 0.016 cm−1), with a p-value less than 0.001. These findings highlight significant seasonal variations in RW quality, which have important implications for treatment processes and the quality of the final drinking water supply in the Awash River Basin (Álvarez-Cabria et al. 2016).

In general, there were significant seasonal variations in the RW quality at the Koka WTP. The dry season showed higher turbidity, pH, temperature, conductivity, TOC, DOC, and UV254 compared to the wet season. These variations are linked to factors such as reduced flow, increased microbial activity, and concentration of dissolved ions during the drier period.

Characteristics of NOM in water treatment processes

The DOC concentration provides an initial measure of the overall organic carbon content, serving as a broad indicator of the potential for DBP precursors. However, the DOC alone does not fully characterize the nature and reactivity of the organic matter. Complementing the DOC analysis, the fluorescence properties of the humic substances can offer additional insights that help distinguish the presence of more reactive, humic-like compounds from less reactive, fulvic-like compounds (Hua et al. 2020; Fernández-Pascual et al. 2023).

The fluorescence excitation-emission matrix (EEM) analysis was performed on water samples collected at multiple points throughout the treatment train, including RW, coagulation and flocculation (CF), sedimentation (SD), and SF. The fluorescence analysis of the Koka water treatment process revealed a complex composition of organic matter as shown in Table 2. At an excitation wavelength of 280 nm, two distinct emission peaks between 300 and 350 nm, corresponding to protein-like substances, and another between 400 and 460 nm, indicating the presence of humic-like materials. When the samples were excited at 320 nm, a single emission peak was identified in the 400–460 nm range, suggesting the existence of fulvic-like substances (Yusup Rosadi et al. 2023).

Table 2

Fluorescent peaks found in the Koka water treatment process

Excitation (nm)Emission (nm)Components
280 300–350 Protein-like 
280 400–460 Humic-like 
320 400–460 Fulvic-like 
Excitation (nm)Emission (nm)Components
280 300–350 Protein-like 
280 400–460 Humic-like 
320 400–460 Fulvic-like 

This predominance of humic-like substances is a cause for concern, as these materials are known to be the primary precursors for the formation of DBPs during the chlorination process (Young et al. 2018; Nguyen et al. 2021). The consistent and high SUVA values observed across the treatment plant processes and throughout different seasons confirm the persistent presence of these hydrophobic, aromatic NOM fractions, which are generally more difficult to remove compared to hydrophilic organic matter. The EEM analysis revealed distinct fluorescent peak regions associated with different NOM components (Table 2).

In contrast, the presence of non-humic substances, like protein-like and fulvic-like materials, with relatively low UV254 absorption and SUVA values, indicates a reduced probability of DBP formation in the treated drinking water (Song et al. 2018).

The EEM analysis revealed distinct fluorescence peak regions associated with different NOM components (Table 2).

The prevalence of humic-like substances, as indicated by intense fluorescent peaks in the 400–460 nm range, points to a higher overall NOM content in the Koka water source. This is significant because humic substances are known to be precursors for the formation of DBPs, such as THMs and HAAs, during the chlorination process (Nguyen et al. 2021). In contrast, the presence of non-humic substances, such as protein-like and fulvic-like materials, with relatively low UV254 absorption and SUVA values, indicates a reduced probability of DBP formation in the treated drinking water (Song et al. 2018). This observation is particularly relevant, as protein-like NOM substances can play a role in fouling during the treatment process (Sillanpää et al. 2018; Peters et al. 2021). In addition, the characterization of NOM in the Koka WTP in different seasons has shown the impact of seasonal variations on NOM composition and treatability (Assegide et al. 2022).

The findings of this study offer valuable insight that can be used to optimize the performance of the Koka WTP in removing NOM fractions that are more prone to DBP formation.

Seasonal performance of Koka WTP in removing NOM

As shown in Figure 2, the RW exhibited higher TOC concentrations during the dry season compared to the wet season, contrary to the expected increase in organic matter from rain runoff during the rainy season.
Figure 2

The effects of seasonal variability on TOC concentration. Note: Raw water in intake (RW), coagulation and flocculation (CF), sedimentation (SD), sand filter (SF), and distribution system (DS).

Figure 2

The effects of seasonal variability on TOC concentration. Note: Raw water in intake (RW), coagulation and flocculation (CF), sedimentation (SD), sand filter (SF), and distribution system (DS).

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The evaluation of the performance of the Koka WTP focused on its ability to remove different components of NOM during the dry and wet seasons and across the treatment processes.

The results of this study revealed a concerning trend in the performance of the Koka WTP during the dry season. This disparity can be attributed to the elevated temperatures experienced during the dry season. The higher temperatures can stimulate increased bacterial activity and promote the growth of algal biomass within the sedimentation unit. This can lead to longer residence times in the sedimentation unit, providing more opportunity for the biological conversion of organic matter into additional biomass, including increased algal growth (Xu et al. 2021).

Figure 3 demonstrates the TOC removal efficiency in each treatment stage. The study found that the TOC concentration gradually decreased throughout the treatment process, except for the sedimentation (SD) stage. During the coagulation (CF) and flocculation stages, the TOC removal efficiency was 58 and 59% in the dry and wet seasons, respectively. However, as shown in Figure 3, the sedimentation process exhibited a negative TOC removal efficiency, with values of −26 and −1.9% during dry periods.
Figure 3

The effects of seasonal variation on TOC removal efficiency of the plant.

Figure 3

The effects of seasonal variation on TOC removal efficiency of the plant.

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This unexpected decrease in TOC removal efficiency during sedimentation could be attributed to the activities of microorganisms involved in the formation and transformation of organic matter, which are more prominent during the high temperatures of the dry season (Abdelrady et al. 2019). The presence of algal blooms and sludge formation during the sedimentation stage can also contribute to increased TOC levels due to nutrient loading and sufficient residence time (Volk 2001; Adams et al. 2018; Wurtsbaugh et al. 2019). Additionally, the resuspension of settled organic matter and the desorption of organic compounds from suspended solids in the sedimentation stage can lead to a net increase in the TOC concentration in the effluent from this treatment stage.

In contrast to the sedimentation stage, the SF process demonstrated a higher TOC removal effectiveness, with an average removal of 52% during the dry season and 55% during the wet season. This indicates that the SF stage is more efficient in reducing the organic matter content compared to the sedimentation process (Marais et al. 2018).

The improved TOC removal at the SF stage can be attributed to the combined effects of coagulation, oxidation, and pre-chlorination treatments applied at this stage of the treatment process.

These findings suggest that the SF stage plays a crucial role in enhancing the overall TOC removal performance of the Koka WTP, particularly in comparison to the sedimentation unit where the organic matter levels may increase due to the resuspension and desorption of organic compounds.

Interestingly, TOC levels in the TW were observed to be very low after chlorination, and the highest TOC removal efficiencies of 87.5 and 79.7% were recorded during the dry and wet seasons, respectively. This suggests that additional TOC removal occurs within the distribution networks, even after the water has left the treatment plant. The low levels of TOC in the TW can be attributed to the further oxidation of organic matter by the residual chlorine present in the water, as the chlorine continues to break down and transform the organic compounds as well and the presence of chlorine inhibits microbial activity within the DS, preventing microorganisms from transforming organic compounds into biomass (Sun et al. 2019; Song et al. 2021).

The overall TOC removal efficiencies of the Koka WTP were found to be only 28% during the dry and 34% during the wet seasons. These relatively low TOC removal rates across the entire treatment process suggest that the performance of the Koka WTP in removing TOC is poor.

Previous studies on WTPs have reported TOC removal efficiencies by solely considering the difference between the inlet and outlet TOC levels, with reported removal rates reaching up to 75% (Awad et al. 2017; Ghernaout & Ghernaout 2020). However, the current study suggests that such simplistic methods of determining removal efficiency may have been inaccurate or misleading. Therefore, when evaluating the performance of a WTP, it is crucial to go beyond simply considering the difference between the inlet and outlet TOC levels. Instead, it is important to examine the complex processes of TOC removal, formation, and transformation that occur within the various treatment stages of the plant.

Performance of treatment plant to reduce DOC, UV absorbance, and SUVA

The study examined the effects of seasonal variations in DOC, UV absorbance, and SUVA across the various treatment units of the Koka WTP, as shown in Figures 46.
Figure 4

The effect of seasonal variation on the concentration of DOC across treatment processes.

Figure 4

The effect of seasonal variation on the concentration of DOC across treatment processes.

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Figure 5

Effects of seasonal variation on UV absorbance across treatment processes.

Figure 5

Effects of seasonal variation on UV absorbance across treatment processes.

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Figure 6

The effects of seasonal variation on SUVA across treatment processes.

Figure 6

The effects of seasonal variation on SUVA across treatment processes.

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The results reveal that the concentration of DOC in the RW varied significantly between the dry and wet seasons, as well as across the different treatment processes as shown in Figure 4. This can be attributed to the reduced dilution effects of rainfall and the increased microbial activity during the dry, which leads to the transformation and release of more organic matter (So et al. 2017; Andersson et al. 2024).

As the water progressed through the treatment stages, the DOC, UV254 absorbance, and SUVA values generally decreased. During the coagulation stage (CF), DOC was reduced from 6.5 to 5.9 mg/L in the dry season and from 5.0 to 4.3 mg/L in the wet season.

The UV254 absorbance also decreased from 0.198 to 0.181 cm−1 in the dry season and from 0.143 to 0.117 cm−1 in the wet season (Figure 5). The results showed that the UV254 absorbance, a useful indicator of the presence and removal of aromatic and unsaturated organic compounds, exhibited distinct seasonal variations across the treatment processes.

The observed decreases in UV254 absorbance during both seasons suggest that the treatment processes are removing a portion of the organic matter with these characteristics, which are often associated with DBP formation potential. However, the magnitude of the UV254 absorbance reduction varied between the dry and wet seasons, indicating that the source water quality and the treatment plant's performance may be influenced by seasonal factors.

The SUVA values throughout the treatment process, as shown in the supplementary material (Table S2), were 3.0 to 2.3 L/mg m across the water treatment process during the dry season, and 2.9 to 2.3 L/mg m during the wet season across the treatment process (Figure 6). This indicates there is a non-significant difference between treatment plant processes and seasons. This indicates that the treatment processes employed at the Koka WTP are not able to effectively remove the aromatic and humus-like organic matter from the RW, as evidenced by consistently high SUVA values >2 L/mg.m throughout the treatment processes and seasons (Young et al. 2018; Beauchamp et al. 2020).

The seasonal variations in RW quality, as reflected by the SUVA values, do not significantly impact the treatment plant's ability to remove these organic matter fractions, which are known to be precursors for the formation of DBPs. This means that the aromatic and humic-like organic matter persists throughout the entire treatment process, from the RW intake to the final TW delivered to the DS (Li et al. 2023). This suggests that the potential for DBP formation after chlorination in this water remains high. As a result, there is a significant likelihood of adverse health impacts associated with exposure to these DBPs among the population served by this water supply (Krasner et al. 2022).

The study also evaluated the removal efficiency of the Koka WTP units for the absorbance of DOC and UV254 during the dry and wet seasons, as shown in Figure 5. The results revealed that the coagulation (CF) treatment stage achieved a DOC removal efficiency of around 9.23 and 14% during the dry and wet seasons (Figure 7). The UV254 reduction for this treatment process was 8.6% in the dry season and 18% in the wet season.
Figure 7

The effect of seasonal variation on DOC removal efficiency of treatment processes.

Figure 7

The effect of seasonal variation on DOC removal efficiency of treatment processes.

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On the contrary, the sedimentation stage (SD) showed unexpected performance, with a negative DOC removal efficiency of −5.1% during the dry season and 12.56% during the wet season (Figure 8). Similarly, the UV absorption reduction efficiency at the sedimentation stage was also unexpected. During the dry season, the reduction was 2.2%, which means that the absorbance of UV254 increased.
Figure 8

The effects of seasonal variation on UV254 absorbance reduction of the treatment plant.

Figure 8

The effects of seasonal variation on UV254 absorbance reduction of the treatment plant.

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In the wet season, the UV absorption reduction was more substantial at 16.24%, but still lower than the performance of other treatment stages. These unexpected results at the sedimentation stage can be attributed to the formation of NOM through the biochemical transformation of plant and animal residues from sources, facilitated by the production of DOC during this treatment stage (Krasner et al. 2022).

The dry season conditions, such as a higher water temperature and a reduced dilution effect from rainfall, promote the activity of algae and microorganisms. The longer residence time in the sedimentation treatment stage also creates favorable conditions for the proliferation of microbes and algae, leading to an increase in DOC content (Beauchamp et al. 2020; Yao et al. 2020).

The SF stage of the Koka WTP demonstrated superior performance in removing organic matter from the water samples compared to the other treatment processes. During the dry season, the efficiency of DOC removal at the SF stage was 40.32%, which is significantly higher than the DOC removal achieved at the other treatment stages. Even during the wet season, when organic matter levels are typically higher, the SF stage maintained a respectable DOC removal efficiency of 24.47%, outperforming the other treatment processes.

Furthermore, UV254 absorbance was significantly reduced by 45.7% in the dry season and 48.8% in the wet season in the SF stage. This highlights the importance of the SF process in effectively removing organic matter and reducing UV-absorbing compounds. The higher organic content in the sedimentation treatment stage during the dry season may contribute to the formation of DBPs during primary chlorination (Du et al. 2022).

The observed differences in the removal efficiencies of DOC and UV254 absorbance between the dry and wet seasons suggest that the dilution effect of higher water flows during the wet season may have had a positive impact on the performance of certain treatment stages. Specifically, at the coagulation–flocculation (CF) and sedimentation (SD) stages, the higher water flows and increased dilution during the wet season appear to have contributed to improved DOC and UV254 removal efficiencies compared to the dry season.

However, the impact of dilution did not seem to significantly influence the performance of the SF stage. Instead, the higher temperatures observed during the dry season may have been a more dominant factor impacting the SF's organic matter removal efficiency. This suggests that the sand filters may be less dependent on the dilution of organic matter and more responsive to other factors, such as temperature, that can influence the adsorption, filtration, and/or biological processes within the sand filter media (Zhang et al. 2021b).

This finding indicates that the SF process may be more sensitive to thermal influences than the initial concentration of organic matter in the water. This has important implications for understanding the performance limitations and optimization opportunities for this key treatment step at the Koka WTP.

The DOC concentration in the water DS was reduced by 18.9% during the dry season but increased by 9.86% during the rainy season. This difference can be attributed to the effects of temperature and the reaction of residual chlorine with the organic matter.

Similarly, UV254 absorbance was reduced by 14.6% during the dry season and increased by 15.36% during the wet season in the DS. These findings suggest that water quality may be more susceptible to changes during the dry season, possibly due to the higher organic content and the potential for the formation of DBPs (Beauchamp et al. 2020).

The decreasing SUVA values throughout the treatment process, from 3.0 in RW to 2.3 in the DS during the dry season, and from 2.9 to 2.3 during the wet season, indicate a change in the composition and character of organic matter. The SUVA values suggest that the remaining organic matter has aromaticity and is susceptible to the formation of DBPs (Mutemi et al. 2020; Andersson et al. 2024).

Implications for the Koka WTP and DBP formation

The results of this study reveal significant challenges with the overall performance of the Koka WTP in removing NOM from the RW.

The conventional WTP used at the Koka facility may be insufficient compared to advanced water treatment systems such as membrane filtration or advanced oxidation processes to remove NOM. While conventional treatment is generally more cost-effective, advanced systems can provide better organic matter removal, but may face challenges related to energy consumption and fouling making it not economically feasible (Chaukura et al. 2021).

The results of this study reveal significant challenges with the overall performance of the Koka WTP in removing NOM. The average TOC removal efficiency of the treatment plant processes is only 43% during the dry season and 45.9% during the wet season, while the average DOC removal across the treatment processes is 15.84 and 15.22% during the dry and wet seasons, respectively. The UV absorbance reductions of 17.8 and 20.87% during the dry and wet seasons further indicate the limited effectiveness of the treatment plant in removing NOM. The SUVA values across treatment processes and seasons are also >2 L/mg m indicating a significant proportion of humic and aromatic organic compounds, which are known to be highly reactive with chlorine and contribute to DBP formation (Young et al. 2018).

The treatment plant is demonstrating low efficiencies for TOC and DOC across treatment processes (coagulation/flocculation and sedimentation). This indicates that the plant struggles to effectively remove NOM from the water, which is a critical step in minimizing DBP formation potential.

Moreover, as shown in Table 2, the fluorescence characteristics of the organic matter in the treatment plant, particularly the presence of humic-like and fulvic-like compounds, suggest a high potential for the formation of DBPs during chlorination (Nguyen et al. 2021). Similarly, the correlation between SUVA and DBP formation potential, as well as the characterization of NOM, indicate that the RW quality at the Koka WTP is a key factor contributing to the risk of DBP formation in the final TW (Tak & Vellanki 2018).

The predictive model developed by Assefa et al. (2024) on the Koka WTP also provides quantitative evidence that the organic matter composition, metallic concentration, and the prevailing environmental conditions, such as water temperature which impact the performance of the treatment plant, are key factors driving the formation of DBPs during the chlorination process treatability (Assefa et al. 2024).

Similar studies have shown that seasonal differences in NOM quality and quantity can significantly impact the performance of water treatment plants to remove NOM. The cumulative evidence from this study strongly implies the high potential for the formation of DBPs in the Koka WTP's final TW after the chlorination process (Nguyen et al. 2021; Andersson et al. 2024).

The findings of this study reveal significant challenges with the overall performance of the Koka WTP in removing NOM from the water samples. Fluorescence analysis of RW indicated the dominance of humic-like substances, suggesting a higher potential for the formation of DBPs during chlorination. The plant's overall organic matter removal efficiency of the plant was suboptimal, with TOC and DOC removals ranging from 36.1 to 42.3% and from 16.3 to 24.9% in the wet and dry seasons, respectively. The limited reduction in UV absorption (around 16%) and elevated SUVA values in TW further indicated the potential for DBP formation, 16% in both seasons, and the SUVA values in TW indicate the potential for DBP formation. The overall NOM removal efficiency of the Koka WTP during both seasons is poor, and the SUVA at all treatment processes and seasons showed no significant differences, being consistently >2 L/mg m. This indicates the presence of hydrophobic, aromatic NOM compounds that are typically more challenging to remove compared to hydrophilic NOM. This indicates that the treatment processes, such as coagulation, sedimentation, and filtration, are not effectively removing the hydrophobic, aromatic NOM fractions that are characteristic of the source water.

The sedimentation treatment stage of the Koka water treatment process exhibited negative removal efficiency for TOC and DOC, indicating that the concentration of organic matter increased after this treatment stage. This unexpected result could be attributed to the conversion of particulate organic matter into dissolved forms or the release of organic compounds from the settled sludge back into the water. Surprisingly, the concentrations of TOC and DOC were highly reduced in the DS after the SF stage. Furthermore, the prolonged exposure to chlorine during the distribution process resulted in the transformation of organic matter into various DBP compounds, rather than physically removing the organic matter and reducing the TOC levels in the TW. This study recommends implementing continuous water quality monitoring programs and developing cost-effective methods to remove DOC and decrease DBP formation. Further investigation is necessary to explore the health impacts of exposure to DBP on the community in the studied area.

The authors express their sincere gratitude to Addis Ababa University, Haramaya University, and Adama Science and Technology University for providing access to their laboratory facilities, which played a vital role in the successful completion of this study. Furthermore, the authors extend their appreciation to the Chemistry Department of Haramaya University for access to their laboratory equipment. The authors also acknowledge and thank Ephriem Tadesse for his invaluable guidance and advice throughout the laboratory work. The authors are very grateful for the support and resources provided by these universities and individuals, which were instrumental in enabling the successful implementation and completion of this research.

E.A. and S.A.J. conceptualized the whole article, E.A. developed the methodology, E.A. arranged the software, E.A., M.D., A.M.T., and E.M. validated the data, E.M. rendered support in formal analysis, E.A. investigated the data, E.T. arranged the resources, E.M. rendered support in data curation, E.A. wrote the original draft, A.M.T. wrote the review and edited it, M.D. visualized the data, and S.A.J. supervised the work. All authors have read and agreed to the published version of the manuscript.

This research received no external funding.

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

The authors declare there is no conflict.

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