Evaluation of organics reduction performance of the GAC filtration with regenerated carbons using the long-term operational data of drinking water treatment facilities

Such successive incidents led to public distrust on drinking water quality. Public outcry promoted the Korean government to prepare countermeasures. One was the General Measures of Clean Water Supply (GMCWS) in 1989 and the other was the Water Quality Management Improvement Measures (WQMIM) in 1994. The GMCWS focused on an increase in wastewater treatment coverage (Joongangilbo 1989). It attempted to upgrade source water quality by building wastewater treatment facilities (Joongangilbo 1989). Nonetheless, raw water quality was not noticeably improved. The government, therefore, took another step. It was the WQMIM, at which the introduction of advanced water treatment was included (ME 1994). The government decided to introduce advanced water treatment into 21 drinking water treatment facilities (KMOE 2006). Detailed information on advanced water treatment introduced by the WQMIM is summarized in Table 1.

According to Table 1, advanced water treatment was mainly introduced into the facilities that relied upon the Nakdong river for their water sources (16 out of 21). In terms of the facility capacity, 94% was concentrated at the Nakdong river. The total facility capacity of advanced water treatment added was 5,446,500 m³/d, and the Nakdong river portion was 5,139,500 m³/d. Such concentration at the Nakdong river was caused by its water quality degradation. Increased discharge of untreated sewage and industrial wastewater into the Nakdong river in the 1990s resulted in serious water quality degradation of the Nakdong river, especially organic pollution. The biochemical oxygen demand (BOD) concentration of the Nakdong river exceeded 5 mg/L during those times (Kim & Yoon 2005). Subsequently, the government decided to introduce advanced water treatment since existing conventional treatments, such as coagulation, flocculation, sedimentation, and media filtration had limitations in coping with organic pollutants. According to Table 1, the main process of advanced water treatment was granular activated carbon (GAC) filtration and ozonation. GAC filtration combined with pre- and postozonation was the favorite choice of advanced water treatment in the WQMIM.

Almost 30 years have passed since advanced water treatment was introduced in 1994. The Busan Water Authority (BWA), which is responsible for drinking water treatment facilities in Busan, has been able to accumulate operational experience of GAC filtration including regeneration. It is, therefore, attempted in this study to evaluate the organics removal performance of both virgin and regenerated carbons using the long-term operational data of drinking water treatment facilities. Three Busan facilities were selected for this purpose. They were chosen because of their large production capacity and long operation period of GAC filtration. Their daily production capacity amounted to 2,432,000 m³. The Hwamyoung facility has been operating the GAC filtration since 1994, the Myoungjang facility since 1999, and the Deoksan facility since 2002.

The Busan drinking water treatment facilities possessed the same treatment processes (prechlorination, preozonation, coagulation, flocculation, sedimentation, media filtration, postozonation, GAC filtration, post chlorination). Information on advanced water treatment introduced at the Busan facilities is provided in Table 2. According to Table 2, the BWA installed the ozonation system even before the WQMIM announcement. The Hwamyoung facility has been operating the ozonation system since 1989, and the Deoksan #1 facility since 1995. The contact times of the ozonation system at these facilities were in the range of 2–12 min, and their ozone doses were in the range of 2.0–3.7 mg/L. These values generally corresponded to typical ranges reported in the literature. Kawamura (1991) recommended an ozone dosage of 1.5–3.0 mg/L, and a contact time of 5–15 min. The design and operation of GAC filtration were also in line with general practices. The GAC filter depth of the Busan facilities ranged from 3.0 to 3.5 m. The hydraulic loading ranged from 7 to 14 m/h, and the EBCT (empty bed contact time) ranged from 14 to 24 min. According to Roberts & Summers (1982), who surveyed design conditions for GAC filtration, the EBCT ranged from 5 to 24 min, the bed depth from 0.5 to 4 m, and the hydraulic loading from 2.6 to 17 m/h.

DOC (dissolved organic carbon) concentration was measured by the TOC analyzer (Sievers, USA), and THM concentration was measured by the gas chromatography (Agilent, USA) with the electron capture detector. A predetermined amount of NaOCl was added into a sample of 70 mL so as to maintain a chlorine residual of 1 ppm for the trihalomethane formation potential (THMFP) measurement. After 24 h of reaction at 20 °C, THM concentration was measured. The KMnO₄ consumption was measured in accordance with the Korean drinking water quality analytical methods (KMOE 2018). The concentration of 1,4-dioxane, which was purchased from Sigma-Aldrich, was measured by gas chromatography with mass spectrometry (Agilent, USA) in accordance with the Korean drinking water quality analytical methods (KMOE 2018).

Physical characteristics of GAC were measured in accordance with the Korean standards M (KMOE 2018), and the Korean drinking water quality analytical methods (KATS 2021). The pore volume of GAC was measured by the gas sorption analyzer (model iQ/MP-Xr, Quantachrome, USA). The 1,4-dioxane removal was examined by pilot-scale experiments. A pilot plant with a capacity of 100 m³ per day was used for this purpose. The pilot plant processes consist of preozonation, rapid mixing, coagulation, sedimentation, media filtration, postozonation, and GAC filtration. The 1,4-dioxane at the predetermined concentration was fed into the preozonation.

The organics reduction performance was compared between the conventional and the advanced treatment at the Deoksan drinking water treatment facility using the operational data from 1996 to 2002. Such organics parameters as DOC, KMnO₄ consumption, THMFP, and total trihalomethane (TTHM) were used for the comparison. The comparison results are summarized in Table 4. The Deoksan facility has three units. The introduction timing of the advanced water treatment at these units was different. Unit #1 and #2 started operation of the advanced water treatment in March 2000, while unit #3 from April 2003. According to Table 4, the reduction efficiencies of KMnO₄ consumption and THMFP were greater than the DOC reduction efficiency. The KMnO₄ consumption level was reduced by 67% through the conventional treatment, and by 86% through the advanced treatment. On the other hand, the DOC reduction efficiencies were 50, and 65% after the conventional and the advanced treatment, respectively. THM formed at the presedimentation where prechlorination was provided. The TTHM concentration increased through the conventional treatment and then decreased as THM was adsorbed during the GAC filtration.

The advanced water treatment also helped reduce the treated level of microorganic pollutants such as 1,4-doixane. It was first detected at the Hwamyoung and the Deoksan facilities in 2004. Its influent concentrations ranged in 2.5–5.1 μg/L at these facilities. Although these values were substantially lower than the World Health Organization drinking water quality guideline of 50 μg/L (WHO 2011), the BWA has decided to monitor this microorganic pollutant. They found that their facilities equipped with the advanced treatment successfully handled 1,4-dioxane. They were able to suppress the treated concentration of 1,4-dioxane as low as 1.0–2.4 μg/L.

The BWA also investigated the reduction performance of 1,4-dioxane by conventional treatment and advanced treatment using pilot plant experiments. They spiked 1,4-dioxane into their pilot plant so as to make its influent concentrations in the range of 2.6–36 μg/L. According to their pilot plant experimental results, the advanced treatment of ozonation and GAC filtration was effective for 1,4-dioxane reduction, but the conventional treatment was not. The preozonation reduced the 1,4-dioxane level by 49–70%, which remained unchanged through the media filtration. However, the GAC filtration following the postozonation further reduced the treated level of 1,4-dioxane as low as 0.5–1.3 μg/L. Overall reduction of 1,4-dioxane ranged from 83 to 97% at the pilot plant. This result indicated that the ozonation and GAC filtration was effective for 1,4-dioxane reduction.

The Busan drinking water treatment facilities currently use only coal-based carbons although they initially selected GACs made of various materials of coal, coconut, and wood. The Myoungjang facility filled its GAC filters with wood-based carbons in 1998, which were replaced by coal-based carbons in 2015. Two different materials were introduced into the Hwamyoung facility, at which 14 GAC filters were filled with coconut-based carbons, and 4 filters with coal-based carbons in 1994. Coconut-based carbons were replaced by coal-based carbons in 2007. The Deoksan facility has been using coal-based carbons since 2001.

The carbon replacement at the Myoungjang facility was related to the GAC standards, shown in Table 5. The GAC standards of Korea (KMOE 2014) and Busan (Busan 2019) were shown in Table 5 together with the AWWA (American Water Works Association) standards (AWWA 2012). Wood-based carbons could not satisfy the apparent density requirement of the Busan standards. The apparent density of wood-based carbon (AWWA 2012) was reported in the range of 0.21–0.35 kg/m³, which could not satisfy the Busan standards requirement of greater than 0.4 kg/m³. Consequently, the Myoungjang facility replaced wood-based carbons by coal-based carbons.

The Busan standards were stricter than the Korean standards for the adsorption characteristics such as the Iodine number and the methylene blue (MB) number. According to Table 5, the Korean standards require greater than 950 mg/g for the Iodine number, which can characterize the adsorptive capacity of GAC of smaller molecules (Alaya et al. 2000), while the Busan standards require 1,000 mg/g. Similarly, the Korean standards require the MB number, which can indicate the adsorption capacity of GAC of larger molecules (Alaya et al. 2000; Baçaoui et al. 2001; Nunes & Guerreiro 2011), greater than 150 mL/g. The Busan standards requires 200 mL/g. It is also noted in Table 5 that different parameters are used to characterize similar carbon properties. The Busan standards used the hardness number to characterize the carbon durability, while the AWWA standards used the abrasion number instead.

The carbon replacement at the Hwamyoung facility was related to the GAC adsorption capacity. The replacement was caused by the superior DOC reduction capacity of coal-based carbons. According to their operational data, coal-based carbons were more effective for DOC reduction than coconut-based carbons. The GAC filtration with coal-based carbons removed 5 kg of DOC per m³ of carbons more than the filtration with coconut-based carbons, based on the bed volume of 19,000 (Ryu et al. 1995). However, both carbons showed similar adsorption capacity of chloroform. According to their operational data (Son et al. 2005), the chloroform breakthrough occurred at a bed volume of 10,929 with coal-based carbons. The corresponding bed volume was 10,210 with coconut-based carbons. However, the chloroform breakthrough occurred at the bed volume of 1,726 when wood-based carbons were used. Consequently, the Hwamyoung facility replaced coconut-based carbons by coal-based carbons.

Characteristics of GAC changed with use. Changes in GAC characteristics such as the Iodine number, the MB number, the apparent density, and the ash content in accordance with the period of carbon use are summarized in Table 6. The Iodine number and the MB number of used carbons were smaller than those of virgin carbons, while the apparent density and ash content of used carbons were greater than those of virgin carbons. The Iodine number of used carbons decreased to 65% (687 mg/g) after use of 2 years, compared to the virgin carbon value of 1,050 mg/g, and to 55% (578 mg/g) after the use of 40 months, respectively. The decreasing extent of the MB number was less than that of the Iodine number. The MB number of used carbons decreased to 72% (152 mL/g) after use of 2 years, compared to the virgin carbon value of 212 mL/g and to 64% (135 mL/g) after 40 months. Unlike these adsorption characteristics, the apparent density and the ash content increased with an increasing period of carbon use. The apparent density and the ash content of used carbons increased to 110% and to 137% after use of 2 years, compared to the virgin carbon values, respectively. They increased further to 113 and 151% after the use of 40 months. An increase in the ash content also contributed to a decrease in the adsorption capacity of used carbons.

The DOC reduction efficiency of GAC filtration deteriorated with time. Compared to the initial efficiency, the reduction efficiency deteriorated to 40–90% after the use of 10 months, to 12–25% after the use of 2 years, and to 4–10% after the use of 3 years. The BWA subsequently installed two multiple earth furnaces to regenerate used carbons in 2001. The capacity of one furnace was 12 m³/d. The furnace capacity was determined, considering the GAC amount of 26,205 m³ owned by the BWA, and the regeneration period of 3 years. The BWA conducted plant-scale experiments to determine the optimum regeneration conditions. They evaluated changes in the adsorption capacity of GAC, and its carbon recovery while monitoring the operational stability. The furnace temperature varied from 800 to 900 °C, the residence time from 25 to 90 min, and the injected steam quantity from 0.5 to 1.5 kg per kg of GAC during the experiments. Based on the experimental results, they determined the furnace temperature of 850 °C, the residence time of 35–40 min and the steam quantity of 0.75–1.0 kg per kg of GAC as the optimum regeneration conditions. Detailed information on the determination of the optimum regeneration conditions can be found elsewhere (Kim et al. 2005, 2015).

Effects of regeneration on organics reduction performance of the GAC filtration was different, depending upon the organics type. The calculated amounts of DOC and TTHM removed by one GAC filter (300 m³) at the Hwamyoung drinking water treatment facility until its breakthrough was reached are summarized in Table 7. The calculation was performed based on the influent flowrates, influent and effluent concentrations, and time to breakthrough. Different breakthrough conditions were used for the calculation. The DOC breakthrough was considered to be reached when the DOC concentration of the GAC filter effluent reached 90% of its influent concentration. The TTHM breakthrough was considered to be reached when the filter effluent concentration of TTHM became identical as the influent concentration. It was noted in Table 7 that the GAC filtration with regenerated carbons generally removed more DOCs, but less TTHMs than the filtration with virgin carbons. on average, the GAC filtration with virgin carbons removed 2,654 kg of DOC, and 52 kg of TTHM until the breakthrough. The amount of DOC removed was increased to 3,076 kg, and the amount of TTHM was decreased to 45 kg for the GAC filtration with RC_1. The GAC filtration with repeatedly regenerated carbons generally extended the changes of the increase or the decrease. The GAC filtration with RC-2 removed 3,771 kg of DOCs, and 22 kg of TTHM. It was evident that the TTHM reduction performance of the GAC filtration deteriorated as the number of the regeneration was increased. The amount of TTHM removed was decreased to 6 kg when the GAC filter operation was conducted with RC-6. Unlike the TTHM reduction, the increasing trend of the DOC reduction was not evident as the number of regeneration was increased. The amounts of DOC removed by one GAC filter fluctuated after three regenerations.

The organics reduction performance of the GAC filtration was related to the pore volume distribution of GAC and the organics type. Large and small molecular organics are more easily adsorbed onto meso pores and micro pores of GAC, respectively. The Nakdong River, which the BWA relied on as their water source, was mostly composed of large molecular organics. It consisted of 42% of humic substances (HS), 26% of building blocks, and 27% of low molecular weight organics (Son et al. 2021). A relationship between HS removal and meso pore volume was reported previously. According to Gui et al. (2018), HS removal was closely related to the meso pore volume (3–24 nm). Golea et al. (2020) also reported similar results that the adsorption of DOC and disinfection byproduct precursor was closely related to the meso pores (5–10 nm). Therefore, the GAC filtration with regenerated carbons, which possess more meso pores and of less micro pores, was effective for DOC reduction, but less effective for THM reduction.

Organics reduction performance of the GAC filtration was evaluated using the long-term operational data in this study. This study focused on the GAC filtration with regenerated carbons. According to the operation data, GAC filtration together with ozonation helped reduce the treated levels of DOC as well as of 1,4-dioxane. The regeneration changed the pore size of GAC. The regeneration increased the meso pore volume, but decreased the micro pore volume. The regeneration cycle was important for the pore size change of GAC. The decrease in the micro pore volume was insignificant after the first regeneration, but was significant after the second regeneration. The GAC filtration with regenerated carbons was found more effective for DOC reduction, and less effective for THM reduction, compared to the filtration with virgin carbons. These results can provide insight into the GAC regeneration strategy. The purpose of the GAC filtration should be considered when to determine the regeneration strategy. When the main purpose of the GAC filtration is DOC reduction, the regeneration can be repeated as many as possible. The regeneration cycle may be extended to more than three or four times. On the other hand, when the GAC filtration is mainly used for the reduction of microorganic pollutants, the regeneration cycle should be minimized.

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

The authors declare there is no conflict.