Water quality improvement using a TWF/C-PAM treatment agent and reutilization potential of sewage sludge as an agricultural soil amendment

Yang, S., Lee, H., and Kang, S. (2026). "Water quality improvement using a TWF/C-PAM treatment agent and reutilization potential of sewage sludge as an agricultural soil amendment," BioResources 21(4), 9848–9860.

Abstract

The applicability of a torrefied wood flour/cationic polyacrylamide (TWF/C-PAM) treatment agent for pilot-scale wastewater treatment and the reutilization potential of the generated sewage sludge as an agricultural soil amendment were evaluated. The TWF/C-PAM treatment agent showed high removal efficiencies for turbidity 91.8%, suspended solids 96.4%, total phosphorus 90.0%, chemical oxygen demand 67.3%, and biochemical oxygen demand 61.2%. The treatment performance was comparable to that of conventional 10% polyaluminum chloride, although relatively low total nitrogen removal efficiency 10.3% was observed. Lettuce (Lactuca sativa L.) was cultivated in sewage sludge-amended soil presented with sludge addition ratios of 1%, 5%, 10%, and 30%. Chlorophyll and carotenoid contents increased up to the 5% treatment level, indicating the potential applicability of sewage sludge as an agricultural soil amendment. These findings demonstrate the potential of the TWF/C-PAM treatment process for simultaneous wastewater treatment and sewage sludge reutilization.

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Water Quality Improvement Using a TWF/C-PAM Treatment Agent and Reutilization Potential of Sewage Sludge as an Agricultural Soil Amendment

Seungmin Yang,^a Hyunjae Lee,^b and Seoggoo Kang^b,*

DOI: 10.15376/biores.21.4.9848-9860

Keywords: TWF/C-PAM treatment agent; Sewage sludge; Agricultural soil amendment; Soil nutrients; Total chlorophyll; Torrefied wood

Contact information: a: Institute of Agricultural Science, Chungnam National University, P.O. 34134, 99 Daehak-ro, Yuseong-gu, Daejeon, South Korea; b: Department of Biobased Materials, Chungnam National University, P.O. 34134, 99 Daehak-ro, Yuseong-gu, Daejeon, South Korea;

* Corresponding author: [email protected]

INTRODUCTION

Urban sewage and industrial wastewater generation have continuously increased because of improved living standards, urbanization, and population growth (Chen et al. 2007; Oh et al. 2011). Excessive influx of nutrients, such as phosphorus and nitrogen, can deteriorate water quality, resulting in eutrophication and algal blooms. Accordingly, polyaluminum chloride (PAC) coagulants have been widely used in sewage treatment plants to remove dissolved pollutants from water (Park and Kang 2020). However, excessive PAC application can increase the concentration of residual aluminum ions in treated water. Residual aluminum in water has been associated with environmental and health concerns, including neurological diseases, such as Alzheimer’s disease (Han and Kang 2016).

In addition to pollutant removal efficiency, floc formation and settling behavior are important factors in wastewater treatment processes. Small flocs generated during coagulation treatment can reduce settling efficiency and prolong sludge dewatering times, thereby increasing the operational burden of downstream filtration and sludge handling processes (Park and Kang 2020). Therefore, recent studies have focused not only on reducing the use of inorganic coagulants such as PAC, but also on improving floc growth and settling efficiency using environmentally friendly biomass-based materials.

Cationic polyacrylamide (C-PAM) has been widely used as a retention aid and polymer-assisted flocculation material in wastewater treatment and papermaking industries because of its particle bridging ability and flocculation performance (Shehram and Rani 2022). However, additional materials capable of improving floc growth, settling behavior, and dispersion stability are often required to further enhance treatment performance under practical wastewater treatment conditions. In particular, untreated wood flour (WF) has limited applicability in aqueous systems because of its poor dispersion behavior (Yang et al. 2016).

To overcome these limitations, torrefied wood flour (TWF) was investigated as a biomass-assisted water treatment material (Yang et al. 2016, 2019; Park and Kang 2020). Yang et al. (2016) studied the surface charge properties of WF under different thermal treatment conditions and reported that untreated WF showed a zeta potential of approximately -22.8 mV, whereas the zeta potential of TWF reached values of -52.5 mV depending on the thermal treatment conditions. The increased negative surface charge of TWF improved its dispersion behavior in water and promoted electrostatic interaction with cationic polymer systems. TWF could be effectively combined with C-PAM to produce a TWF-containing C-PAM water treatment agent (TWF/C-PAM treatment agent) (Yang et al. 2019).

Previous studies demonstrated that TWF/C-PAM treatment agent exhibited effective pollutant removal performance and improved flocculation behavior. Yang et al. (2017) modified the surface charge of TWF from negative to positive by mixing TWF with C-PAM solution and subsequently drying the mixture to produce surface-modified TWF particles with a maximum zeta potential of +31.2 mV. The surface-modified TWF showed rapid decreases in turbidity and optical density, with a chlorophyll-a removal efficiency greater than 90% for negatively charged algal bloom pollutants. Yang et al. (2019) developed a TWF/C-PAM treatment agent by mixing TWF with a C-PAM solution without a drying process and evaluated its applicability to algal bloom water treatment. Removal efficiencies of 86% to 92% for total organic carbon and 53% for ammonia were observed, while the treated water maintained a stable pH range of 6.5 to 7.7 and showed low ecotoxicity toward daphnia. Furthermore, Park and Kang (2020) developed a TWF-containing PAC-reduced water treatment agent (TWF/PAC treatment agent) using TWF and reported high removal efficiencies of turbidity (maximum 90%), T-P (maximum 91%), and T-N (32%) in sewage treatment applications. They also reported that the addition of TWF improved floc settling behavior, shortened sedimentation time, and reduced the residual aluminum concentration in treated water by approximately 70% compared with conventional 10% PAC coagulant. These results indicated that the interaction between TWF and cationic polymers contributed not only to pollutant removal efficiency, but also to improved floc growth and settling performance.

Recently, the reutilization of sewage sludge generated during wastewater treatment processes has become an important environmental issue in South Korea because ocean dumping of sewage sludge was prohibited in 2012, and the regulation was expanded to all waste types in 2014 (Lee et al. 2014). Prior to the ban, approximately 69% of the sewage sludge was dumped into the ocean, whereas only 18.5% was recycled, 10.9% was incinerated, and 2.1% was dumped in landfills (Lee et al. 2014). Sludge treatment can become a major source of greenhouse gas emissions under anaerobic treatment conditions (Préndez and Lara-González 2007). Therefore, considerable attention has been focused on developing sustainable sludge treatment and recycling methods (Oh et al. 2011; Tanifuji et al. 2022).

Previous studies reported that sewage sludge can potentially be utilized as fuel, landscaping materials, soil conditioners, and agricultural soil amendments because of sewage sludge generated after coagulation treatment contains various nutrients (Kwak et al. 2005; Bai et al. 2013; Moon et al. 2013; Um et al. 2015). Park and Kang (2020) evaluated the energy conversion potential of sewage sludge generated from a PAC-reduced TWF treatment system and reported a dry calorific value of 3,378 kcal/kg, which satisfied the auxiliary fuel requirements of thermoelectric power plants (Park and Kang 2020). In addition, land application of sewage sludge has been considered an economically and environmentally acceptable management strategy because sewage sludge can improve the physical, chemical, and microbiological properties of soil (Huang and Chen 2009; Lindsay and Logan 1998).

In this study, the applicability of a TWF/C-PAM treatment agent was evaluated using a pilot-scale wastewater treatment plant installed at a sewage treatment facility. In addition, sewage sludge discharged after treatment was evaluated for its potential application as an agricultural soil amendment by analyzing lettuce growth, chlorophyll content, soil nutrient characteristics, and residual heavy metal contents. Therefore, this study aimed to evaluate both the practical applicability of the biomass-assisted hybrid wastewater treatment system and the reutilization potential of generated sewage sludge through the effective utilization of unused forest biomass.

EXPERIMENTAL

The experimental procedure is presented in Fig. 1.

Fig. 1. Experimental procedure for preparation of the TWF/C-PAM treatment agent, pilot-scale wastewater treatment, and reutilization of sewage sludge

Materials

TWF was used to manufacture the TWF/C-PAM treatment agent using oak chips for pulp with a moisture content of 12% (± 0.5%) as the raw material. The supplied oak chips were prepared using a superheated steam treatment method under rapid thermal treatment (350 ℃, 20 min). After the thermal-treated, the wood chips were ground and sieved to obtain particles smaller than 106 µm (Yang et al. 2019). Cationic polyacrylamide (C-PAM) was supplied by Solenis (Gimcheon-si, South Korea). The C-PAM powder had a molecular weight of 8,000,000 g/mol and is commonly used as a retention aid and flocculant in the papermaking and wastewater treatment process. C-PAM was prepared as a 0.2% aqueous solution. The TWF/C-PAM treatment agent was prepared by adding 10 g of TWF to 1 L of the 0.2% C-PAM solution followed by mixing for 2 hours.

To evaluate the applicability of sewage sludge discharged after treatment, agricultural soil was used, and lettuce (Lactuca sativa L.), a representative leafy vegetable crop, was selected as the experimental plant species.

Preparation of the TWF/C-PAM Treatment Agent

To evaluate the water quality improvement performance of the TWF/C-PAM treatment agent, a pilot-scale wastewater treatment system was installed at a sewage treatment plant located in Daejeon, South Korea, where effluent from the primary sedimentation tank was continuously supplied as the influent wastewater (Fig. 2). The pilot-scale treatment system consisted of a flow control tank, fast mixing tank, slow mixing tank, quiescent settling tank, and treated water storage tank. The TWF/C-PAM treatment agent was injected at dosage of 10 mL/L of influent wastewater, and the supernatant collected from the settling tank was used for water quality analysis. The characteristics of the influent wastewater are listed in Table 1.

Fig. 2. Pilot-scale wastewater treatment system using the TWF/C-PAM treatment agent

Table 1. Water Quality Characteristics of Influent Wastewater from The Primary Sedimentation Tank

Turbidity, suspended solids (SS), total phosphorus (T-P), total nitrogen (T-N), biochemical oxygen demand (BOD), and chemical oxygen demand (COD) were analyzed to evaluate the water treatment performance. Turbidity was measured using a TB 210 IR turbidimeter (Lovibond, UK) with a measurement range of 0 to 1,100 NTU.

SS was measured by filtering a known volume of sample through a glass fiber filter (GF/C) attached to a filtration apparatus. After filtration, the filter paper was dried to a constant weight, and the SS content was calculated based on the weight difference before and after filtration. The SS concentration was calculated using Eq. (1),

(1)

where a, b, and V represent the weights of the glass fiber filter before filtration (mg), the weight of the glass fiber after filtration (mg), and the sample volume (mL), respectively.

T-N, T-P, BOD, and COD were analyzed using a water quality analyzer (HS-3300, HUMAS, South Korea) according to the standard method for water pollution analysis (No. 2017-4). The concentrations of raw and treated wastewater were determined based on absorbance measurements.

Preparation of Sewage Sludge-Amended Soil

Effluent from the primary sedimentation tank of the sewage treatment plant was used as the influent wastewater source for generating sewage sludge in this study. The collected sewage sludge was used after the dewatering process, and the moisture content of the dehydrated sewage sludge was 73.9% (± 0.6%). Commercial horticultural soil used for leafy vegetables cultivation showed a moisture content of 120% (± 3%).

The dewatered sewage sludge was mixed with the horticultural soil at sludge addition ratios of 1%, 5%, 10%, and 30% based on dry solid content (Table 2). Before sowing lettuce seeds, water was added to adjust the moisture content of the mixed soil uniformly. Each treatment condition was prepared in triplicate, and the final weight of each soil mixture was 45 g.

Table 2. Mixing Ratios of Sewage Sludge and Horticultural Soil Based on Dry Solid Content

Growth Conditions and Characteristics

Five replicate pots were prepared for each treatment condition, and ten seeds were sown in each pot. The seeds were cultivated in a growth chamber under controlled environmental conditions (Fig. 3). The growth chamber was maintained at 21 ℃ (± 2 ℃) with a relative humidity of 50%. The light cycle was controlled at 10,000 lux for 16 h and 0 lux for 8 h per day (Lee et al. 2017). After 20 days of cultivation, the lettuce plants were harvested and separated into roots and leaves. The length and fresh weight of the roots and leaves were measured individually (Jeoung et al. 2013).

Fig. 3. Lettuce growth after 20 days of cultivation under different sewage sludge amendment conditions

Chlorophyll is an important photosynthetic pigment involved in light absorption and energy transfer in plants (No et al. 2016). Chlorophyll-a is the primary pigment response for light energy absorption during photosynthesis, whereas chlorophyll-b acts as an accessory pigment (Lee et al. 2005). In addition, carotenoids function as accessory pigments that participate in light absorption and protect plant cells from excessive light energy (Park et al. 2015).

To analyze total chlorophyll and total carotenoid content, 0.2 g of lettuce leaves was mixed with 8 mL of 80% acetone and extracted in a cool and dark environment for 7 days. After extraction, the supernatant was separated, and absorbance was measured at 663, 645, and 470 nm using a UV/VIS spectrometer (HS-3700, Humas, South Korea). Total chlorophyll and carotenoid contents were calculated using Eqs. 2, 3, 4, and 5 based on the measured absorbance values (Jeoung et al. 2013).

Soil Nutrients and Heavy Metal Content

After lettuce cultivation, the soil samples were air-dried and passed through a 100-mesh (≤ 254 µm) sieve prior to nutrient and heavy metal analysis (Elhindi et al. 2018). Moisture content was determined based on weight reduction after oven drying at 105 ℃ until a constant weight was reached.

Soil pH was measured after mixing soil and distilled water at a water-to-soil ratio of 5:1 and allowing the mixture to stand for 24 h. Soil chemical analyses were conducted at the Soil Environment Analysis Center, Chungnam National University. Available phosphorus was determined colorimetrically at 720 nm using the Lancaster method. Exchangeable cations were extracted using 1 M NH₄OAc buffer (pH 7.0) and analyzed using an inductively coupled plasma optical emission spectrometer (ICP-OES).

Statistical Analysis

Statistical analyses were performed using SPSS Statistics 26 software (IBM Corp., Armonk, NY, USA). One-way analysis of variance (ANOVA) followed by Duncan’s multiple range test was conducted to evaluate the effect of sewage sludge mixing ratio on lettuce growth characteristics. Statistical significance was determined at p < 0.05 (*), p < 0.01 (**), and p < 0.001 (***). According to Duncan’s multiple range test, mean values followed by the same superscript letter (i.e., a, b, or c) were not significantly different at the 0.05 significance level.

RESULTS AND DISCUSSION

Water Quality Improvement Effect of TWF/C-PAM Treatment Agent

The TWF/C-PAM treatment agent showed high removal efficiencies for turbidity, SS, and T-P, exceeding 90% and exhibiting performance comparable to those of 10% PAC and the TWF/PAC treatment agent (Table 3). Among the evaluated parameters, phosphorus removal is particularly important because phosphorus discharged from wastewater treatment plants is a major contributor to eutrophication in aquatic environments.

Choi et al. (2015) investigated the phosphate adsorption characteristics of oak biochar and reported phosphate phosphorus removal efficiencies ranging from 2.7% to 31%, depending on the biochar dosage (4.0 to 16 g/L) (Choi et al. 2015). The TWF/C-PAM treatment agent achieved a T-P removal efficiency of 90%, which was only slightly lower than 10% PAC (92%), These results suggest that the combination of TWF and C-PAM contributed to effective phosphorus removal through combined adsorption and coagulation effects.

Table 3. Water Contaminant Removal Efficiencies of the TWF/C-PAM Treatment Agent

For T-N removal, 10% PAC and the TWF/PAC treatment agent showed removal efficiencies of 35.6% and 22%, respectively, but the TWF/C-PAM treatment agent showed a relatively low removal efficiency of 10.3%. The lower T-N removal efficiency may be attributed to the nitrogen-containing components of both TWF and C-PAM. Lee et al. (2012) reported that the oak biomass contained approximately 0.19% nitrogen based on elemental analysis. In addition, C-PAM consists of nitrogen-containing repeating units with the chemical formula (C₃H₅NO)_n (Wang et al. 2023). Therefore, the nitrogen containing characteristics of the raw materials may have affected the T-N removal performance.

The COD removal efficiencies of 10% PAC and the TWF/C-PAM treatment agent were 59.8% and 67.3%, respectively. In contrast, the BOD removal efficiencies were 68.0% and 61.2%, respectively. The relatively lower BOD removal efficiency of the TWF/C-PAM treatment agent may be associated with the organic characteristics of TWF and C-PAM.

Overall, the TWF/C-PAM treatment agent exhibited water pollutant removal efficiencies comparable to those of 10% PAC, except for T-N removal. These results indicate the potential applicability of the TWF/C-PAM treatment agent as an eco-friendly alternative for wastewater treatment while improving the utilization of unused forest biomass resources.

Lettuce Growth in Sewage Sludge-Amended Soil

The growth results of lettuce cultivated for 20 days in sewage sludge-amended soil are presented in Table 4. Increasing the sewage sludge mixing ratio did not significantly affect the number of leaves or root weight. However, leaf length and leaf weight increased with increasing sewage sludge content. In particular, leaf length increased from 30.3 mm in the control to 44.6 mm in the T30 treatment, whereas leaf weight increased from 289.0 mg to 522.2 mg. In contrast, root length and fresh weight showed relatively small differences among treatment conditions. Root length ranged from 84.7 to 95.4 mm, whereas root weight ranged from 67.7 to 79.9 mg. These results suggest that sewage sludge amendment had a greater influence on above-ground lettuce growth than on root development during the early cultivation stage.

Table 4. Growth Characteristics of Lettuce Cultivated in Sewage Sludge-Amended Soil

The chlorophyll content of lettuce cultivated in sewage sludge-amended soil are presented in Fig. 4. Total chlorophyll content generally increased with increasing sewage sludge addition. However, chlorophyll-a content increased up to the T5 treatment (24.6 mg/g) and remained relatively similar at higher sludge addition levels (24.9- 25.8 mg/g). Chlorophyll-b showed a similar trend, with values of 13.1 and 13.9 mg/g in the T5 and T10 treatments, respectively.

Statistical analysis of total chlorophyll content showed significant differences among treatment conditions (F = 25.679***). The total chlorophyll content increased in the order of T30 > T10 > T5 > T1 > Control. However, no significant difference was observed between T5 and T10 treatments, suggesting that increasing sewage sludge addition beyond 5% had a limited effect on chlorophyll accumulation.

Total carotenoid content increased from the control to the T5 treatment and then slightly decreased at higher sludge addition levels (Table 5). Carotenoids function as accessory pigments involved in light absorption and protection against excessive light energy (Park et al. 2015).

Sludge amendment promoted chlorophyll and carotenoid accumulation in lettuce, particularly at sludge addition levels of 5% or higher. Previous studies reported that biochar can improve soil quality and promote nutrient accumulation in plants (Gałązka et al. 2019; Torabian et al. 2021; Cai et al. 2023). Therefore, the organic compounds derived from sewage sludge and torrefied wood have contributed to the enhance plant growth and pigment accumulation observed.

Fig. 4. Chlorophyll-a, Chlorophyll-b, and total chlorophyll contents of lettuce cultivated in sewage sludge-amended soil

Table 5. Total Carotenoid Content in Lettuce Cultivated in Sewage Sludge-Amended Soil Added

The soil chemical properties after lettuce cultivation are presented in Table 6. Soil pH slightly increased with increasing sewage sludge addition, ranging from 4.9 in the control to 5.5 in the T30 treatment. Total nitrogen (T-N) and available phosphorus (P₂O₅) contents generally increased as the sewage sludge mixing ratio increased. In contrast, exchangeable sodium and potassium contents tended to decrease with increasing sludge addition.

Table 6. Soil Chemical Properties after Lettuce Cultivation

The heavy metal contents of the soil are presented in Table 7. Heavy metal concentrations generally increased sludge addition. However, all measured heavy metal contents remained below the soil contamination standards for region 1 defined by the Korean Soil Environment Conservation Act, which includes agricultural and residential land uses such as dry paddy fields, rice paddies, orchards, school grounds, and children’s playgrounds. The sewage sludge-amended soil satisfied the regulatory criteria for agricultural soil applications.

Table 7. Heavy Metal Contents of Soil after Lettuce Cultivation

Lee et al. (1996) studied the effects of municipal sewage sludge on lettuce cultivation and reported that sludge application promoted lettuce growth while maintaining heavy metal accumulation within acceptable levels. Lettuce growth increased with sewage sludge addition while heavy metal concentrations remained below the regulatory limits.

Therefore, the results indicate that sewage sludge generated from the TWF/C-PAM wastewater treatment process has potential applicability as an agricultural soil amendment. In addition, the utilization of TWF in the treatment agent may contribute to the effective utilization of unused forest biomass resources.

CONCLUSIONS

The study evaluated the applicability of the TWF/C-PAM treatment agent for pilot-scale wastewater treatment and investigated the reutilization potential of the generated sewage sludge as an agricultural soil amendment.

The TWF/C-PAM treatment agent showed high removal efficiencies for turbidity (91.8%), SS (96.4%), and T-P (90%), with treatment performance comparable to that of conventional 10% PAC coagulants. However, relatively low T-N removal efficiency (10.3%) was observed because of the nitrogen-containing characteristics of TWF and C-PAM.
Sewage sludge amendment promoted lettuce growth characteristics, chlorophyll accumulation, carotenoid content, particularly at sludge addition levels of 5% or higher. Soil analysis showed that the sewage sludge-amended soil remained with the slightly acidic range and contained abundant nitrogen, calcium, and phosphate contents. In addition, all measured heavy metal concentrations satisfied the Korean Soil Environment Conservation Act standards for Region 1 soils.
Sewage sludge generated from the TWF/C-PAM treatment process has potential applicability as an agricultural soil amendment while contributing to the effective utilization of unused forest biomass resources. Additional long-term cultivation and field-scale studies are needed to evaluate long-term nutrient balance, soil stability, and heavy metal accumulation under practical agricultural conditions.

ACKNOWLEDGMENTS

This study was carried out with the support of “R&D Program for Forest Science Technology (Project No. 2020213B10-2222-AC01)” provided by the Korea Forest Service (Korea Forestry Promotion Institute).

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Article submitted: September 13, 2023; Peer review completed: December 9, 2023; Revised version received: June 9, 2026; Accepted: July 20, 2026; Published: August 17, 2026.

DOI: 10.15376/biores.21.4.9848-9860