Performance evaluation of ICX reactor in treatment of paper mill wastewater: a case study in South Vietnam

Pulp and paper production, being one of the world's largest industries and the fifth most energy-consuming sector globally (Osaki 2019), confronts a critical juncture necessitating sustainable practices aligned with global initiatives such as the 28th UN Climate Change Conference of the Parties (COP28) in 2023. As the industry endeavors to align with sustainability goals, recycling paper using recovered materials emerges as an imperative option. Approximately 17% of the 285 million tons of global waste paper produced in 2019 were exported for recycling in 2019 (Provost-Savard et al. 2023). In the United States, over 70% of carton boxes produced in 2022 were successfully recovered for recycling (Ketkale & Simske 2023).

The packaging paper industry, a major consumer of recovered cardboard, faces the dual challenge of meeting growing demand while minimizing environmental impact. With the global packaging paper market projected to grow nearly 3% annually, reaching a value surpassing 1.2 trillion USD by 2028 (Keskin et al. 2020), it becomes imperative to explore environmentally conscious approaches. In Vietnam, packaging paper production was estimated to reach 4.3 million tons per year by the end of 2022, with exports totaling about 1.2 million tons per year. This figure is expected to rise to 1.3 million tons per year by the end of 2023 (Vietnam Pulp and Paper Association). However, this growth raises concerns about freshwater ecotoxicity and the carbon footprint associated with wastewater treatment processes, underscoring the need for sustainable practices in the industry (Bui et al. 2022).

The potential freshwater ecotoxicity linked with packaging paper ranges from 3.75 to 11.58 kg1,4-DCB-eq/ton of paper. As production increases, so does the pollution load on the environment due to heightened wastewater discharge demands. Wastewater from the packaging paper industry exhibits substantial organic pollution content, with total chemical oxygen demand (COD) over 20,000 mg/L (Harif et al. 2021), alongside significant calcium content ranging from 1,000 to 2,000 mg/L (Wang et al. 2023). These environmental challenges underscore the importance of adopting sustainable practices and technologies to reduce the carbon footprint of wastewater treatment processes.

Presently, upflow anaerobic sludge blanket (UASB) technologies constitute approximately 67% of global wastewater paper and pulp mill treatment methods, with other high-load treatment technologies such as anaerobic baffled reactor, expanded granular sludge bed (EGSB), fluidized bed reactor, accounting for around 33% (Zieliński et al. 2023). Despite their widespread use, UASB methods encounter challenges in granular sludge retention, leading to operational inefficiencies, elevated carbon emissions and reduced effectiveness under low organic load rates. In response to these challenges, the internal circulation (IC) reactor emerges as a noteworthy alternative within the UASB technology domain. IC anaerobic technology overcomes UASB limitations with a unique design featuring two vertically stacked cells and two three-phase separators. A proficient three-phase separator ensures substantial retention of sludge mass and extends the average solids retention time. This leads to efficient biogas generation at high hydraulic velocities, retaining 77.4–81.1% of biomass and enhancing the overall efficiency and effective volume of the anaerobic system by 16–25% (Pan et al. 2017; Guo et al. 2018; Hao & Shen 2021). Moreover, IC characterized by a negative carbon emissions index compared to traditional UASB technology (Bui et al. 2022), is specifically tailored for paper wastewater treatment, demonstrating robust and consistent treatment efficiency within a compact footprint. Comprehensive studies validate IC technology's capability to manage COD levels up to 10,000 mg/L and volumetric loading rate (VLR) reaching 20–25 kg/m³ day (Chen et al. 2019). However, the limited application of IC systems in practical settings is due to substantial space and material requirements, resulting in increased construction time and costs. The new generation of IC, Internal Circulation eXperience (ICX), could address this issue, particularly for small paper and pulp factories. Motivated by the promising capabilities of the IC reactor, ICX is designed to enhance the efficiency of paper wastewater treatment while reducing the system height. ICX has a single reaction compartment and two-phase separators to ensure stable granular sludge retention. Notably, ICX employs the down-flow technique, pumping wastewater from the bottom to create high-speed agitation of sludge particles. The treated water is then directed to the top-phase separator to reduce turbulent flow and separate biogas. Subsequently, sludge and treated wastewater move to the lower-phase separator, where the sludge layer acts as a filter, preventing sludge escape. The treated water exits the system through the riser, while the separated sludge is circulated back into the reaction sludge layer. This innovative design not only makes ICX accessible for small plants but also simplifies construction, shortening the startup time for wastewater treatment (Noordink et al. 2018; Hendrickx et al. 2019).

Despite these advances, there is a scarcity of studies investigating the application of ICX in wastewater, particularly in paper and pulp wastewater. This study presents the operation of one ICX in a paper and pulp mill in Vietnam, examining treatment capacities, and the effect of various factors such as VLR, volatile fatty acids (VFA), alkalinity, calcium ion, and biogas production yield. This nuanced examination aligns with the overarching objectives of COP28 and broader sustainable development initiatives, advancing the discourse on environmentally responsible practices in industrial processes.

The experiment spanned 90 days, comprising 7 days for load and equipment testing, 1 day for O₂ removal in the ICX reactor (using N₂), and 12 days for sludge loading. The ICX reactor was inoculated with sludge to handle the theoretical pollutant load, estimated at approximately 120 tons of sludge with with a ratio of mixed liquor volatile suspended solids to mixed liquor suspended solids (MLVSS/MLSS) content of 0.87 (Tian et al. 2023). The sludge for system inoculation was sourced from an IC reactor at a different paper mill during wastewater treatment. After sludge inoculation, the VSS concentration was verified and the IC system was activated. As the ICX began biogas production, the ICX reactor was initiated. Since the sludge had already adapted from the operating IC reactor, it took only about 2 h for transportation and 3 h for sludge loading, making the ICX system operational after 8 h of IC. The design parameters of the ICX are presented in Table 1.

During operation, nutrient concentrations were maintained at a COD:N:P ratio of 650:5:1. Due to the high alkalinity in the wastewater, no NaOH supplementation was required; however, HCl acid (32%, Dong A Company, Vietnam) was added to the treatment process to maintain stable pH. The characteristics of paper production wastewater are described in Table 2.

Table 2

Composition of the raw wastewater and ICX influent

Type wastewater .	Q (m3/h) .	tCOD (mg/L) .	sCOD (mg/L) .	pH .	TSS (mg/L) .	Ca2+ (mg/L) .	(mg/L) .	TN (mg/L) .	TP (mg/L) .	VFA (mg/L) .
Raw	<500	4,500–14,000	3,000–11,800	6.8–7.1	2,000–3,000	400–2,200	50–150	30–55	10–17	1,450–4,960
Dissolved air flotation effluent	<500	3,100–12,200	2,800–11,800	6.5–6.8	200–230	400–22,00	50–148	28–30	9–12	1,650–6,500
aVietnam Standard	–	<100	–	6–9	<50	–	–	–	–	–

Type wastewater .	Q (m3/h) .	tCOD (mg/L) .	sCOD (mg/L) .	pH .	TSS (mg/L) .	Ca2+ (mg/L) .	(mg/L) .	TN (mg/L) .	TP (mg/L) .	VFA (mg/L) .
Raw	<500	4,500–14,000	3,000–11,800	6.8–7.1	2,000–3,000	400–2,200	50–150	30–55	10–17	1,450–4,960
Dissolved air flotation effluent	<500	3,100–12,200	2,800–11,800	6.5–6.8	200–230	400–22,00	50–148	28–30	9–12	1,650–6,500
aVietnam Standard	–	<100	–	6–9	<50	–	–	–	–	–

^aNational technical regulation on the effluent of pulp and paper mills.

View Large

The study employs a comprehensive approach by integrating the Stover–Kincannon and Monod models in the investigation of anaerobic systems. The Stover–Kincannon model focuses on substrate removal rates, providing a crucial perspective on reactor performance. In contrast, the Monod model emphasizes the relationship between microbial growth and substrate concentration.

The water analysis was collected at the effluent of each tank, which covered diverse water quality parameters, encompassing COD or total COD (tCOD) total nitrogen (TN), total phosphorus (TP), total solids (TS), volatile solids (VS), total suspended solids (TSS), volatile suspended solids (VSS), alkalinity, VFA, calcium, sulfate ion, and pH. The VFA analysis was conducted based on insights from our previous study (Nhat-Ha & Manh-Ha 2019), while other parameter analyses strictly followed the standard methods of water and wastewater by the American Public Health Association (Rice et al. 2012). While anaerobic granulars were taken at the valves at 1, 3, 5, 7, 9, 11, and 13 m according to the height of the ICX reactor.

Gas production was systematically measured daily using a thermal flow meter (Combimass ECO Binder Group, Germany) and the outcomes were verified through validation using a gas chromatograph (GC-8A Shimadzu) with a flame ionization detector.

This lines up with EGSB technology, data presented in Table 3, which typically achieves less than 77% removal of sCOD in paper mill wastewater (Liang et al. 2021). Yet, ICX is 10 times faster than EGSB, operating in just 2 h vs. 20. The VLR, evaluating the system's capability with flow rate and sCOD, gives a clearer picture of the overall sCOD removal efficiency in 24 h. Practical observations show VLR for packaging paper ranging from 2 to 26.8 kg/m³ × day (see Figure 2(b)). Results indicate removal efficiency below 70% for VLR under 5 kg/m³ × day, but it jumps over 80% for VLR between 5 and 26.8 kg/m³ × day. The top efficiency is 89% at VLR of 24.2 kg/m³ × day, slightly dropping to 81% at VLR of 26.8 kg/m³ × day. This confirms ICX's suitability for treating organic loads when sCOD exceeds 5 kg/m³ × day. Extended retention times in UASB reactors for paper wastewater start impacting COD treatment efficiency at over 96 h (Bakraoui et al. 2020). In contrast, ICX manages this in just 12 h. ICX has a higher VLR than other anaerobic technologies like EGSB. For paper mill wastewater, IC has a VLR range of 2.5–18.94 kg/m³ × day, treating leachate from a molasses wastewater with up to 96% efficiency (Luo et al. 2016), with IC's retention time at 15.84 h. So, ICX might be around 41% more effective. During tests, ICX's VLR can reach up to 50 kg/m³ × day with efficiency up to 72% (Hendrickx et al. 2019). Keeping VLR between 25 and 35 kg/m³ × day could hit up to 80% efficiency for the paper mill wastewater in Allard Emballages, France (Noordink et al. 2018). However, for paper mill wastewater in Vietnam, the suitable VLR should stay under 26.8 kg/m³ × day due to falling treatment efficiency, linked to the high organic sCOD concentration in the paper industry (sCOD up to 11,800 mg/L) and the high calcium concentration (Nhat-Ha & Manh-Ha 2019).

The role of VLR within the ICX system is evaluated through the fluctuations in VFA, specifically propionic and butyric acids. VFAs play a pivotal role in anaerobic digestion, significantly impacting the performance of acetogenic and methanogenic bacteria. The accumulation of VFAs results from the acidogenic phase during the conversion of organic compounds. Consequently, monitoring VFA dynamics in the ICX reactor becomes a crucial tool for assessing system stability.

As VFA concentrations reach 350 mg/L, sludge disintegration occurs, resulting in a decrease in COD treatment efficiency to approximately 50–60% (Figure 3(b)). This finding is consistent with previous studies by Batstone & Steyer (2007) using batch UASB, where VFA concentrations were maintained within the 300–600 mg/L range for wine production wastewater and below 500 mg/L. Another study by Heydari et al. (2019) on a continuous UASB system indicates that maintaining VFA concentrations below 180 mg/L during the startup phase contributes to sustaining stable treatment capabilities. However, when VFA concentrations exceed 450 mg/L with an organic loading rate of 2.69 kg COD/m³ × day, treatment efficiency decreases to approximately 60%.

In the case of EGSB treating simulated dairy wastewater, when VFA < 200 mg/L with VLR < 14 kg/m³ × day, the system remains stable. However, when the VLR increases to 28 kg/m³ × day, the system experiences shock loading, with VFA levels ranging from 500 to 600 mg/L (Mills et al. 2023). Another study utilizing EGSB for Cassava alcohol wastewater treatment showed that during the startup phase with VLR ranging from 1.55 to 5.37 kgCOD/m³ × day, stable VFA concentrations were maintained below 300 mg/L. However, when the VLR increased to 16.14 kgCOD/m³ × day, methane production efficiency decreased by 60%, with VFA levels ranging from 450 to 500 mg/L (Xu et al. 2023). These findings indicate that high-rate anaerobic technologies such as EGSB and ICX need to maintain VFA concentrations below 300 mg/L for stable operation. ICX demonstrates superior capability in handling higher loads and maintaining stability compared to UASB and EGSB, as it can sustain stable VFA levels even at higher loads. Table 4 summarizes the stable VFA concentrations observed in various anaerobic treatment systems.

The concentration of VFA, calcium, and alkaline of the input to the ICX reactor were varied according to the dilution between the influent (after the conditioning tank) and the effluent (after the secondary clarifier), please see Figure 1.

Typically, the alkalinity increases with the rise in sCOD due to the formation of CO₂, which can exist in the solution as ions ion ⁠, (Chen et al. 2023). However, in the ICX reactor, the role of alkalinity in relation to the concentration of calcium ions and the stability of sludge is more apparent. As shown in Figure 5, when the effluent alkalinity in the ICX system is maintained below 19 meq/L, the accumulation of calcium in the biomass remains in the range of 70–160 mg/L. At this concentration, it aligns with the development of granular sludge, proven to enhance the production of protein extracellular polymeric substances and maintains a MLVSS/MLSS ratio in the sludge of 0.85 with influent sCOD less than 5,000 mg/L (Wang et al. 2023). Nevertheless, when the effluent alkalinity in the ICX exceeds 19 meq/L, the accumulation of calcium in the biomass fluctuates between 200 and 400 mg/L. At this point, the MLVSS/MLSS ratio decreases to about 0.72 (reaching 0.69 at the bottom of the 1 m depth after 1 month of operation). Despite this, the sCOD removal efficiency is still maintained above 70%, a result consistent with the findings of Zhou et al. (2019). However, the prolonged variations in VSS over time led to an increase in sludge density, necessitating additional energy for sludge recirculation, consequently diminishing the effective treatment volume of the reactor. This issue has also been addressed in the study by Diamantis & Aivasidis (2018) involving an EGSB reactor. In their case, maintaining a velocity of > 5 m/h through an increased VLR > 15 kg/m³ × day successfully induced calcium precipitation in the range of 15–33%. However, this maintenance strategy did not yield the expected results for the ICX system in this study. With a water flow maintained at 6.5–8.5 m/h, calcium precipitation reached 40–60%, leading to the excessive accumulation of calcium in the sludge. This not only increased the energy requirement for recirculation but also reduced the sludge's lifespan. Practical operation of the ICX reactor has demonstrated the need for additional acid supplementation to maintain alkalinity below 19 meq/L for sludge stability. However, this approach incurs significant costs and environmental and health risks for operators. In addition, the primary influencing factor is the direct relationship between influent sCOD and calcium. The use of recirculated water after treatment in the production process dilutes both sCOD and calcium in the wastewater.

Monitoring the graph depicted in Figure 6(b) reveals a notably higher biogas production efficiency/sCOD compared to previous anaerobic digestion studies, reaching 0.85 on day 16. Remarkably, the system demonstrates resilience in sustaining biogas production even when subjected to high calcium levels. This suggests that biogas generation is influenced by the nature of the sludge, with floc sludge yielding higher biogas output compared to granular sludge (Wang et al. 2018). Furthermore, biogas production results in the formation of large-sized sludge particles (3–3.5 mm), medium-sized particles (1.5–2 mm), and small-sized particles (0.5–1 mm) with correspondingly highest specific biogas production rates of 0.031, 0.016, and 0.006 m³ /kgVSS day, respectively (Wu et al. 2016). In addition, due to the higher VLR in the ICX reactor compared to UASB technology, there is more effective transport of COD into the sludge bed, leading to improved biogas production rates (Afridi et al. 2019). Moreover, the predominant Methanosarcina and Methanosaeta bacterial strains play a crucial role in the conversion of organic matter into biogas. These strains demonstrate a higher density within the ICX reactor compared to other anaerobic digestion systems, even under conditions of high sludge retention times and turbulent flow (Andrén 2018). The empirical data obtained from this ICX system, featuring a sludge retention time exceeding 80 days and an average hydraulic velocity of 8.5 m/h, may provide conducive conditions for the maintenance and proliferation of both Methanosarcina and Methanosaeta strains, which were mentioned in the investigation of Owusu-Agyeman et al. (2019).

The Monod kinetic model serves to elucidate the interdependency between two sets of variables: those related to substrate concentration (sCOD) and the microbial growth rate, typically measured through MLVSS (Jin et al. 2022).

The evaluation of the relationship between substrate and biomass typically relies on K_s and μ_max. Calculations based on the Monod model suggest that the μ_max value is notably lower than K_s (K_s/μ_max = 152) in this context. This implies that the growth rate of biomass is stable under high organic loads in the ICX, thereby offering an advantage for biological technologies by mitigating excess biomass treatment costs.

In contrast, for UASB technology treating seafood wastewater, with K_s = 1.079 kg/m³ (K_s/μ_max = 37.2), a low COD load and high biomass growth rate are apparent compared to the ICX (Jijai et al. 2016). Similarly, the Sonic Anaerobic-moving bed biofilm reactor (MBBR) technology exhibits a K_s/μ_max ratio of 1, indicating a mismatch between COD load and biomass growth rates for industrial applications (Som & Yahya 2021). The K_s coefficient of the ICX surpasses that of other anaerobic digestion technologies and is comparable to the hybrid Sonic Anaerobic-MBBR technology.

In this study, the μ value in the ICX reactor is calculated as 0.3478. This value tends to be higher compared to other anaerobic digestion reactors such as UASB (Campos et al. (2014)), which reported μ values of 0.1925 for coffee production wastewater and 0.162 for urban wastewater (Singh & Vaishya 2017). Faekah et al. (2020) employed anaerobic fixed film reactor up-flow anaerobic filter (UAF) granular sludge systems to eliminate contaminants from synthetic rubber wastewater. The authors presented a higher endogenous decay coefficient K_d compared to conventional technologies due to longer growth and attachment times on the media. However, this technology exhibited a lower μ_max = 0.011 1/day and biomass yield Y = 0.027 kgVSS/kgCOD, resulting in extended adaptation periods.

In another study utilizing an EGSB reactor to remove organic contamination from simulated wastewater (Yoochatchaval et al. 2008), a Y coefficient of 0.121 kg VSS/kgsCOD was determined, indicating biomass yield efficiency comparable to ideal laboratory growth conditions. These results highlight the effectiveness of the ICX system not only in robust microbial growth but also in maintaining efficient granular sludge with a high specific growth rate (μ) compared to other anaerobic granular sludge technologies.

Through dynamic kinetic modeling and comparison with established models like the Monod and Kincannon–Stover, the ICX demonstrates several advantages. Notably, it effectively retains sludge and boasts a high sludge yield coefficient. This characteristic proves particularly beneficial in treating recycled paper mill wastewater, which often contains high levels of calcium, leading to potential inorganic accumulation within the system. The elevated sludge yield coefficient and specific growth rate of the ICX facilitate the accumulation of organic matter, resulting in higher MLVSS/MLSS ratios compared to alternative technologies when addressing paper wastewater.

In addition, the ICX showcases resilience against shock loading and exhibits superior processing efficiency when faced with high organic loads. Its reaction rate surpasses that of many other anaerobic wastewater technologies, further enhancing its appeal as a viable treatment option.

While the ICX proves effective in treating high pollutant loads, such as the VLR of 26.8 kg/m³/day for pulp and paper mill wastewater, and demonstrates the capability to reduce area and volume requirements compared to traditional anaerobic technologies, it is not exempt from encountering challenges and limitations. The compact design of the ICX results in a lower dilution factor, making it highly susceptible to factors that impede microbial activity. Empirical observations highlight that exposure to VFAs exceeding approximately 2 h can impede microbial activity within the ICX. Therefore, it is crucial to maintain influencing factors within permissible thresholds.

Moreover, the ICX's compact design presents challenges concerning the accumulation of calcium and carbonate. The reduced accumulation time per unit volume exacerbates this issue and the compact separator unit is prone to clogging due to calcium buildup, posing a threat to the system's long-term stability.

In addition, the ICX's reliance on IC technology necessitates investment in circulation pumps, leading to increased energy consumption. Improper pump selection may result in the fragmentation of sludge particles, further complicating operational efficiency. Consequently, ensuring the stable operation of the ICX relies heavily on managing wastewater composition and the operational expertise of personnel overseeing the system.

The ICX reactor exhibits exceptional efficiency in treating high-strength paper mill wastewater, managing organic concentrations (sCOD) up to 11,800 mg/L. Operating stably at a VLR of 26.8 kg/m³ × day, the system achieves processing efficiency exceeding 81%, while consistently maintaining VFA below 300 mg/L. Employing Monod and Stover–Kincannon kinetic modeling, dynamic parameters, including K_s = 56.809 kg/m³, Y = 0.121 kgVSS/kgsCOD, K_d = 0.0242 1/day, μ_max = 0.372 1/day, U_max = 151 kg/m³ × day and K_B = 175.92 kg/m³ × day, underscore the ICX reactor's superior efficiency compared to alternative technologies. The ICX reactor's heightened sensitivity to VFA levels necessitates maintaining influent concentrations below 1,400 mg/L for effective sludge treatment. In addition, calcium significantly influences treatment efficiency, requiring post-treatment alkalinity maintenance below 19 meq/L to stabilize MLVSS/MLSS concentration. While the observed biogas production rate ranges from 0.6 to 0.7 Nm³ biogas/kg sCOD, the impact of calcium in paper mill wastewater diminishes this ratio, reducing overall treatment efficiency and biogas production. This research significantly contributes to the body of knowledge in wastewater treatment, offering valuable insights for addressing challenges related to complex industrial wastewater.

There is no funding for this research.

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

The authors declare there is no conflict.