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Review

Evaluating the Mechanisms and Efficiency of Johkasou Systems for Decentralized Domestic Effluent Treatment: A Review

by
Xu Wang
1,2,†,
Siyue Cheng
3,† and
Huilun Chen
3,*
1
National Engineering Laboratory for Advanced Municipal Wastewater Treatment and Reuse Technology, Beijing University of Technology, Beijing 100124, China
2
National Engineering Research Center for Urban Environmental Pollution Control, Beijing Municipal Research Institute of Eco-Environmental Protection, Beijing 100037, China
3
Beijing Key Laboratory of Resource-Oriented Treatment of Industrial Pollutants, School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Water 2024, 16(16), 2266; https://doi.org/10.3390/w16162266
Submission received: 3 July 2024 / Revised: 9 August 2024 / Accepted: 10 August 2024 / Published: 12 August 2024
(This article belongs to the Special Issue Wastewater Treatment and Reuse Advances Review)
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Abstract

:
Johkasou systems have emerged as quintessential examples of decentralized wastewater treatment technologies due to their compact design, easy operation, and robust resistance to mechanical impact attributes that are particularly effective in mitigating and treating rural domestic wastewater. Although the efficiency of the Johkasou process in removing nitrogen and phosphorus has been well-documented, a comprehensive synthesis of the underlying mechanisms and influencing factors is still elusive. This review seeks to elucidate these aspects by detailing the biogeochemical pathways involved in nitrogen and phosphorus removal, characterizing the key microbial consortia, and addressing the potential accumulation of nitrous oxide (N2O). Furthermore, the review critically examines the impact of various media used in Johkasou systems on nutrient removal efficacy, with a particular emphasis on nitrogen. It also proposes a range of practical adjustments to design parameters, including dissolved oxygen (DO), pH, temperature, and hydraulic retention time (HRT), to enhance process performance. Finally, the practical implementation of Johkasou systems and their integration with ancillary processes in actual domestic sewage treatment scenarios are synthesized, providing a theoretical foundation for advancing Johkasou methodologies in rural areas.

Graphical Abstract

1. Introduction

Rural domestic sewage is typically characterized by dispersed discharge points, seasonal and temporal variations in water consumption, and high concentrations of nitrogen and phosphorus nutrients [1,2]. These characteristics often necessitate the adoption of decentralized treatment technologies, which are generally categorized into biological and ecological treatment methods. Although biologically based treatments are environmentally friendly, they are prone to challenges such as sludge bulking in activated sludge systems, and anaerobic processes require long start-up periods [3]. Ecological treatment technologies such as artificial wetlands and oxidation ponds are simple to operate for treating rural domestic wastewater but have limitations, such as large land area requirements and the emission of foul odors [4,5], which hinders their popularization and application. Johkasou systems, also known as purification tanks, not only achieve effective pollutant removal effect but also save space, making them a representative solution for managing rural sewage and a promising option for comprehensive rural wastewater treatment [2].
The Johkasou originated in Japan, with its development rooted in the response to the 1919 Spanish influenza pandemic, when Japan launched a “national hygiene” initiative aimed at improving environmental sanitation [6]. Japan centralized the treatment of urban domestic wastewater using pipeline networks, oxidation ditches, biofilters, and other processes, resulting in significant improvements in public health. However, the drainage network could not reach remote rural areas, leaving rural domestic sewage untreated and causing severe pollution to rural rivers and farmland. In this circumstance, the Johkasou system was developed and manufactured in Japan in the 1970s [7].
The Johkasou is a compact domestic wastewater treatment unit that integrates the activated sludge method with the biofilm method in an anaerobic/anoxic/aerobic (A2/O) configuration. This system degrades domestic wastewater through a two-step anaerobic and a one-step aerobic biofilter bed [8], effectively treating all domestic wastewater, including wastewater from kitchens, baths, laundry, or similar wastewater. The main processes used in the Johkasou systems include anaerobic filtration, contact oxidation, activated sludge, membrane treatment, and disinfection, enabling the effective treatment of rural domestic wastewater and contributing to sustainable sanitation goals [7]. Their advantages in on-site treatment, reuse, and efficient wastewater discharge have led to their widespread adoption in rural areas of China and Japan [9].
Johkasou technology has evolved through three stages: separate treatment, combined treatment, and advanced treatment [10]. The use of separate treatment of Johkasou systems is prohibited in Japan. Japanese researchers have enhanced Johkasou systems by incorporating membrane devices [11], enabling the effluent to meet stringent quality standards: BOD5 < 10 mg/L, total phosphorus (TP) < 1 mg/L, and total nitrogen (TN) < 10 mg/L. Johkasou systems are classified into large, medium, and small categories based on their processing capacity, as detailed in Table 1. Small-scale Johkasou installations, which require low investment and cost, are the most prevalent in Japan [12], with over 8.6 million units installed [13]. Medium and large Johkasou systems are used to treat sewage from buildings, schools, hospitals, and supermarkets. A small-scale Johkasou with an area of approximately 2.8 m2 is often used for rural domestic sewage [2]. In accordance with the “Johkasou Law” and the “Structural Standards for Johkasou and their Explanations” enacted in Japan, the Johkasou is easy to operate and install in accordance with the topography of rural areas, facilitating their widespread use and effectively improving the living conditions of rural residents.
China began to adopt Johkasou technology in the 1990s, subsequently optimizing and improving it based on the small-scale combined Johkasou model [14]. These systems can treat rural sewage, blackened and odorous river water, and restaurant wastewater [15,16] (Figure 1 illustrates the structure of the Johkasou). Studies have demonstrated that by increasing the volume and number of anaerobic and aerobic tanks and optimizing aeration conditions, small-scale Johkasou systems can meet the Class I B standard of GB 18918-2002 [17] for effluent quality [18,19]. Research using tracer tests has also provided insights into the flow patterns within Johkasou systems [20], laying a scientific foundation for their broader adoption.
Currently, numerous studies focus on the nitrogen and phosphorus removal properties of Johkasou systems. For instance, Fajri et al. [21] found that higher aeration rates enhanced nitrogen removal, and Ebie et al. [22] demonstrated that incorporating zirconium adsorbent particles in Johkasou systems maintained TP levels below 1 mg/L for up to 90 days. However, a comprehensive and systematic summary of the nitrogen and phosphorus removal mechanisms in Johkasou systems is still lacking.
Therefore, this review aims to discuss the mechanisms of nitrogen and phosphorus removal in Johkasou systems, with a focus on microbiological processes. In addition, the issue of N2O accumulation is examined. The review also analyzes the effects of various influencing factors on removal efficiency and proposes reasonable adjustment ranges. Finally, the application of Johkasou in rural areas is summarized, and future research directions are proposed to guide further studies on the removal efficiency of the Johkasou process.

2. Mechanism of Pollutant Removal in Johkasou

This section discusses the related mechanisms and microbial communities involved in the removal of organic matter, nitrogen, and phosphorus. It provides a detailed understanding of the removal mechanisms.

2.1. Organic Matter Removal Mechanism

This section provides a comprehensive explanation involved in the pathways of organic matter removal in Johkasou systems, along with a summary of the microbial genera associated with this process.

2.1.1. Organic Matter Removal Pathway

The removal of organic matter in Johkasou systems occurs via two main pathways: anaerobic hydrolytic acidification and aerobic biodegradation.
In the anaerobic zone of the Johkasou, anaerobic biofilms attach and grow on the surface of the filler, which convert insoluble macromolecular organic pollutants into soluble small molecule organic matter through anaerobic hydrolysis and acidification, which improves the biodegradability of the wastewater [3].
In the hydrolysis stage, microorganisms convert macromolecular organic matter into small, water-soluble molecules. This is followed by the acid production stage, a rapid process in which small organic molecules are further converted into volatile fatty acids (VFAs) and CO2; and lastly, there is the methanogenic stage, where methanogens in a strictly anaerobic environment convert the VFAs into methane and CO2.
In the aerobic zone of the Johkasou, aerobic biofilm carries out the aerobic biodegradation of small molecule organic matter after anaerobic digestion. The filler in the aerobic zone achieves better fluidization due to comparable water velocity and airflow propulsion, increasing the contact area between the biofilm and organic matter, thereby improving the organic matter removal rate [23]. Part of the organic matter is consumed by endogenous microbial respiration in the presence of oxygen, while the remainder is synthesized into cellular material, which ultimately achieves the goal of organic pollutant removal [23].

2.1.2. Microbial Analysis in Organic Matter Removal

The microbial genera that play a dominant role in the removal of organic matter include Ferruginibacter, Terrimonas, Acidovorax, Sunxiuqinia, Thermomonas, and Zoogloea [3], of which Zoogloea is a common genus of organic matter removing organisms in wastewater treatment plants. Due to the presence of these genera, the COD removal rate in Johkasou systems exceeds 90%.

2.2. Nitrogen Removal Mechanism

The denitrification process in Johkasou systems is primarily carried out by microorganisms on the filler [24]. This section elaborates on the mechanisms and pathways of denitrification in Johkasou systems, with a focus on the associated microorganisms and the emission of N2O.

2.2.1. Nitrogen Removal Pathway

The denitrification pathways in Johkasou include traditional denitrification as well as newer pathways, mainly complete nitrification–denitrification, simultaneous nitrification–denitrification (SND) [25], and anaerobic ammonia oxidation (anammox) [26].
The primary nitrogen removal pathway is complete nitrification–denitrification, involving two-stage anaerobic filter beds and aerobic filter beds, ultimately removing nitrogenous pollutants from the water column (Figure 2a). The process stoichiometric equations are as follows [27]:
Nitrification: NH4+-N + 1.5 O2 → NO2−-N + H2O + 2 H+
NO2−-N + 0.5 O2 → NO3−-N
Denitrification: NO3−-N + 4 g COD + H+ → 0.5 N2 + 1.5 g biomass
Nitrogen is first converted from NH4+-N to NO2−-N, then oxidized to NO3−-N and finally reduced to N2 via the intermediate product NO2−-N [28]; this reaction chain is sustained by a significant source of oxygen and inorganic carbon. The autotrophic nitrification occurs in the aerobic reaction zone of the Johkasou, where NH4+-N is oxidized to NO2−-N and NO3−-N by nitrifying bacteria. The microorganisms involved in this process are primarily autotrophs, which consume inorganic carbon and oxygen to sustain growth and metabolism, with dissolved oxygen (DO) typically maintained above 1.0 mg/L [29]. Heterotrophic denitrification occurs in both the anaerobic and anoxic reaction zones of the Johkasou process, where NO3−-N and NO2−-N serve as electron acceptors for denitrification reactions, and nitrate reductase in the body reduces NO2−-N and NO3−-N to N2 [30].
SND requires the co-occurrence of nitrification and denitrification within the same tank [31]. Due to the limitation of oxygen diffusion, the concentration of DO decreases sequentially from the outside to the inside of the filler area in the Johkasou [32], thus forming an aerobic area and anoxic area (Figure 2b), which provide a suitable growth environment for autotrophic nitrifying bacteria and heterotrophic denitrifying bacteria, respectively, promoting the SND process in the aerobic section. The Johkasou design allows nitrifying and denitrifying bacteria to coexist in the same area [33]. In contrast, heterotrophic nitrifying bacteria and aerobic denitrifying bacteria are present [34]; Nitrosomonas europea and Nitrosomonas eutropica are capable of denitrification in the absence of sufficient DO, facilitating the SND process smoothly. Unlike conventional nitrogen removal pathways, SND saves reaction time and energy due to its lower aeration requirements [35].
Anammox is another important pathway for nitrogen removal in Johkasou systems. This process involves regulating conditions such as pH [36], DO [37], and temperature [38] to control nitrification at the nitrite stage, maintaining higher concentrations of nitrite for effective nitrogen removal. Anaerobic ammonia-oxidizing bacteria (AnAOB) are unique in their metabolism (Figure 2c); NO2−-N is used as an electron acceptor and eventually produces N2 and a small fraction of NO3−-N, neither requires a carbon source nor DO [39]. AnAOB are autotrophic, slow-growing, and easily constrained by environmental influences such as temperature [40], pH [41], and organic matter concentration [42]. Free nitrite is a primary inhibitor of the anammox process [43,44]; hence, maintaining the nitrite-to-ammonium ratio within the range of 1.1–1.3 is critical [45]. The stoichiometry of the reaction is considered as follows:
NH4+-N + 1.32 NO2−-N + 0.066 HCO3− + 0.13 H+
→ 1.02 N2 + 0.26 NO3−-N + 0.066 CH2O0.5N0.15 + 2.03 H2O
Anammox has the advantage of converting both NO2−-N and NH4+-N to N2 without adding carbon sources and DO while reducing CO2, N2O, and NO emissions [39], making it the most efficient and economical pathway known [46,47].

2.2.2. Microbial Analysis in Denitrification

The microbial genera responsible for nitrogen removal in Johkasou systems include Nitrosomonas, Nitrospira, and Nitrosococcus; the main genera of nitrifying bacteria are Nitrococcus, Nitrobacter, and Comamonas [48]. Most of the bacteria that play a role in denitrification are parthenogenetic anaerobic bacteria, including Acidovorax, Sunxiuqinia, Pseudoxanthomonas, and Alicycliphilus [49].
Several studies have identified the presence of aerobic denitrifying and heterotrophic nitrifying bacteria in Johkasou systems, which promote the SND process. The genera of aerobic denitrifying bacteria include Terrimonas, Pseudoxanthomonas, Thermomonas, Dokdonella, and Bacillus [3,50,51], while the genera of heterotrophic nitrifying bacteria are Acinetobacter, Glutamicibacter, etc. [52,53].
Microbial diversity studies offer insights into nitrogen removal pathways from a biological perspective. By analyzing the relative abundance of species, changes in dominant species under different operating conditions can be elucidated. Zeng et al. [54] compared the relative abundance of microorganisms at different C/N ratios in Johkasou systems at both the phylum and genus levels. They found that the C/N ratio strongly influenced the microbial community structure, with a 30% higher relative abundance of Proteobacteria under a high C/N ratio compared to a low C/N ratio. At the genus level, a higher C/N ratio favored the proliferation of genera that promote the SND process.
Metabolomics analysis can quantify microbial community activities, particularly in soil environments, and reflect interactions between microorganisms and their environment [55], which helps to elucidate key functional microbial genes and communities in denitrification processes. Through metabolomics analysis, members of the genera Comamonas, Paracoccus, and Azoarcus were found to be the main denitrifying bacteria and key functional microorganisms in wastewater treatment plants [56,57,58]. However, metabolomics is less applied in the study of Johkasou and should be emphasized in the future.

2.2.3. N2O Accumulation Problem

The biological denitrification process in wastewater treatment produces N2O, a greenhouse gas with a global warming potential 300 times greater than CO2 [59]. The formation of N2O is associated with ammonia-oxidizing bacteria (AOB) [60] and heterotrophic denitrifying bacteria [61]. There are three main production pathways: NH2OH oxidation, nitrifier denitrification, and heterotrophic denitrification (Figure 3).
DO levels significantly impact the production of N2O; insufficient DO can lead to the conversion of NH2OH to N2O [62]. Nitrite reductase (NirK) and nitric oxide reductase (Nor) generate N2O during denitrification by nitrifying bacteria [63]. In heterotrophic denitrification, N2O serves as an intermediate product and is a key factor in the N2O consumption mechanism when the denitrification process is undisturbed [64].
Anammox is a potential N2O emission mitigation factor, as it consumes NO2−-N, which is the starting point for many N2O production mechanisms [65]. It has been shown that the emission factor for N2O from anammox is significantly lower compared to conventional denitrification, with an emission factor of 1.15% [66]. The denitrification mechanism of Johkasou involves anammox, which can effectively reduce the generation of N2O. However, research on N2O accumulation in Johkasou systems remains limited, indicating a need for future studies to focus on this area.

2.3. Phosphorus Removal Mechanism

Elemental phosphorus in rural domestic wastewater exists primarily in the forms of orthophosphate (PO43−-P) and polyphosphate (Poly-P). Polyphosphate-accumulating organisms (PAOs) are the microorganisms primarily responsible for phosphorus removal [67]. This section details the composition and metabolism of these microorganisms, focusing on the biological phosphorus removal mechanisms in Johkasou systems.

2.3.1. Phosphorus Removal Pathway

Biological phosphorus removal in Johkasou systems is achieved through a cyclical process of phosphorus release in anaerobic zones and phosphorus accumulation in aerobic zones, ultimately removing phosphorus from the wastewater [68]. As illustrated in Figure 4a, biological phosphorus removal occurs in two main steps. First, under anaerobic conditions, PAOs convert intracellular Poly-P into PO43−-P and release it into the surrounding environment. During this phase, PAOs absorb volatile fatty acids (VFAs) and store them as PHAs through secondary transport [69]. In the second step, in an aerobic environment, the accumulated PHAs are oxidized by available electron acceptors, and the released ATP is used to take up excess extracellular PO43−-P and synthesize glycogen, so the PAOs complete the process of aerobic phosphorus uptake to achieve phosphorus removal [70].
Recent research has identified new pathways for biological phosphorus removal, including denitrifying phosphorus removal within Johkasou systems [74]. As shown in Figure 4b, this denitrifying phosphorus removal process is mediated by PAOs and glycogen-accumulating organisms (GAOs). GAOs reduce NO3−-N to NO2−-N and PAOs convert NO2−-N to N2 and store inorganic phosphorus as intracellular Poly-P, thereby facilitating the simultaneous removal of both nitrogen and phosphorus [75].
In addition to biological processes, the Johkasou system also employs physical processes for phosphorus removal. The primary physical removal mechanisms include adsorption, filtration, and interception by carrier fillers [76]. Nevertheless, when the filler adsorption reaches saturation, the phosphorus release phenomenon occurs, resulting in increased phosphorus content in the effluent water and secondary pollution. Therefore, selecting a biofilm carrier filler with high adsorption capacity is crucial for maintaining effective phosphorus removal.
Given the limited and sometimes unstable effectiveness of biological and physical phosphorus removal in Johkasou systems, chemical dosing and electrocoagulation have been employed to enhance phosphorus removal efficiency [77,78]. Chemicals are mainly coagulants such as calcium, iron, and aluminum salts [79], which can react with PO43−-P in water to form water-insoluble phosphate precipitates and complete chemical adsorption. Electrocoagulation combines coagulation, flotation, adsorption, and electrochemistry, providing advantages over chemicals that can affect effluent quality and are costly [80]. Electrocoagulation is an electrochemically based technique for the in situ generation of coagulant substances by electrolysis at a sacrificial anode, whereby electric current destabilizes suspended, dissolved, or emulsified contaminants [81]. Figure 4c shows a schematic of a typical electrocoagulation process for contaminant removal and phosphorus removal from wastewater using aluminum, iron, and magnesium electrodes by electrocoagulation [82]. The metal ions generated on the anode flocculate with PO43−-P to produce a precipitate. In addition, metal ions can also be hydrolyzed to generate a high-valent polymeric ion floc network, effective adsorption of other pollutants in water, and the role of suspended solids. Electrocoagulation has the advantage of reducing energy consumption and operational costs, and its application in Johkasou systems is increasingly being studied.

2.3.2. Microbial Analysis in Phosphorus Removal

PAOs represent a complex group of microorganisms classified into several genera, including Fusobacterium, Shewanella, Aeromonas, Corynebacterium, Enterococcus, Tetrasphaera, and Staphylococcus [83,84]. Candidatus Accumulibacter in the γ-proteobacteria was the first microorganism discovered to remove phosphorus, referred to as Accumulibacter, and accounted for 40–69% of PAOs [85]. Accumulibacter I and Accumulibacter II can perform denitrification, belonging to denitrifying phosphate accumulating organisms (DPAOs) [86]. In addition, Johkasou exists for Dechloromonas, Tetrasphaera, and other DPAOs [87]. Tetrasphaera bacteria, discovered later than Accumulibacter bacteria, also have anaerobic phosphorus uptake and denitrification. These two groups of bacteria contributed the most strongly to phosphorus removal [88].
Compared to denitrifying bacteria, the diversity of PAOs in Johkasou systems has been less extensively studied. Therefore, future research should focus on PAO diversity analysis to further elucidate the mechanisms underlying phosphorus removal.

2.4. Comparison of Johkasou with Other Wastewater Treatment Processes

The Johkasou system is anticipated to gain widespread adoption in rural areas due to its superior nitrogen and phosphorus removal capabilities. Table 2 demonstrates the advantages and disadvantages of Johkasou with constructed wetlands, oxidation ponds, and membrane bio-reactors (MBRs), and it also compares the pollutant removal effects of these processes. Johkasou systems demonstrate significantly higher nitrogen and phosphorus removal efficiency compared to constructed wetlands and oxidation ponds, which are ecological treatment technologies. The Johkasou operates in the A2/O mode, which helps to convert NH4+-N to N2, and the PAOs effectively enrich PO43−-P, thus improving the nitrogen and phosphorus removal effect. Although the removal efficiency of Johkasou and MBR processes is comparable, MBR systems are prone to membrane fouling and clogging, in addition to being cost-prohibitive [89]. Overall, the Johkasou system not only provides effective pollutant removal but also offers convenient post-maintenance, making it a promising option for wastewater treatment in rural areas.

3. Influencing Factors of Nitrogen and Phosphorus Removal in the Johkasou Process

This section analyzes the impact of fillers and operating parameters on nitrogen and phosphorus removal efficiency in the Johkasou process.

3.1. Filler

The Johkasou process, a typical biological contact oxidation method, relies heavily on the filler, a core component that serves as a carrier for microbial growth and provides a stable environment for microbial habitat and reproduction [94]. The filler significantly impacts oxygenation performance [95], providing conditions for SND and having adsorption and immobilization, which is a critical factor for nitrogen and phosphorus removal in the Johkasou. Commonly used fillers include fixed, suspended, and dispersed types, each with distinct advantages and disadvantages, as summarized in Table 3.
The role of filler in nitrogen removal within the Johkasou has been extensively studied. For instance, Wang et al. used porous corrugated plate filler as a biofilm carrier to treat domestic wastewater and found that the NH4+-N average effluent concentration was 9.64 mg/L [98]. Yang et al. [96] compared combined filler, elastic filler, and homemade fiber filler and found that homemade fiber filler satisfied the primary A discharge standard for TN effluent and enriched DPAOs during the process’s stable operation. In order to improve the performance of the filler for nitrogen removal, researchers have improved the hydrophilicity and biocompatibility of the filler to meet the water treatment requirements [99]. For example, Liu et al. performed surface modifications on fillers using polymer blending, achieving TN removal rates of 62.0–75.9% under low dissolved oxygen and low C/N with more biomass SND accelerated initiation on the carrier [50]. Cai et al. utilized emergent basalt fibers as biofilm fillers in the Johkasou, achieving TN removal rates of up to 82.22% [3].
Although filler can adsorb and immobilize some PO43−-P, it has a certain promotional effect on phosphorus removal. Xu et al. added two kinds of filler: ecological fibers and ceramic particles; each season, the average TP effluent concentration was less than 2 mg/L [100]. However, since the adsorption effect of the filler is quickly saturated and the filler needs to be replaced periodically, there is less related research on Johkasou. Although DPAOs existed on the filler, the effect of phosphorus removal was limited. Johkasou is generally coupled with electrolysis to achieve phosphorus removal. Therefore, the effectiveness of fillers on phosphorus removal needs to be investigated in order to develop fillers with high adsorption capacity [100].
In conclusion, filler selection for the Johkasou should prioritize structural stability, impact resistance, and excellent adsorption capabilities to provide a stable environment for nitrification–denitrification and enhance TP adsorption capacity.

3.2. Effects of Operation Parameters

At present, numerous studies have focused on factors such as temperature, pH, dissolved oxygen (DO), hydraulic retention time (HRT), and carbon sources to optimize nitrogen and phosphorus removal performance. This section summarizes the effects of these operating parameters on nitrogen and phosphorus removal to provide a suitable adjustment range for future studies.

3.2.1. pH

The pH is a critical factor influencing nitrogen and phosphorus removal in the Johkasou process [101], so excessive high or low pH will inhibit the efficiency of nitrogen and phosphorus removal [4]. Extreme pH levels can inhibit the efficiency of these processes.
Nitrification requires a certain level of alkalinity. Ruiz et al. found that nitrification occurs completely when pH is varied between 6.45 and 8.95, and ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) are completely inhibited at pH values below 6.45 or above 8.95 [102]. In contrast, denitrifying bacteria grow best when the pH is 7–8 [103]. Zhang et al. [104] found that nitrate removal could reach 98.03% at pH 7. Hu et al. [105] found that denitrification of low C/N wastewater was most effective at pH 7, with 97.86% TN removal. Therefore, nitrogen removal is more stable under neutral pH conditions.
The pH also strongly influences the kinetics of phosphorus release and uptake by PAOs. It was shown that anaerobic phosphorus release increased with increasing pH when pH was varied from 6 to 8, and phosphorus removal was positively correlated with pH [106]. Nguyen et al. [107] found that at pH 8, phosphorus was released twice as fast as at pH 6, with a more than twofold increase in uptake rate, while high pH promoted both Tetrasphaera and Accumulibacter activity. Chen et al. [108] showed that near-alkaline conditions were favorable for phosphate reduction, with TP removal reaching 70% when the pH was 8.
Therefore, it is recommended that the pH in the Johkasou be maintained between 7 and 8 to support optimal growth conditions for nitrifying bacteria and PAOs.

3.2.2. Temperature

Temperature directly impacts the growth rate and metabolic activity of microorganisms, thus affecting the performance of nitrogen and phosphorus removal [109]. Higher temperatures destroy enzymes associated with activity and inhibit microbial metabolic activity; with lower temperatures, the α-diversity of microbial communities decreases [110].
It has been shown that the activity of nitrifying microorganisms is inhibited as the temperature decreases [111]. Most nitrification and denitrification efficiencies were inhibited at temperatures below 10 °C [112]. Moreover, denitrifying bacteria were much more sensitive to low temperatures than nitrifying bacteria [113]. Wang et al. [114] found severe inhibition of NO2−-N reduction to N2 at low temperatures of 10–15 °C. Jin et al. [90] treated domestic wastewater at 25–30 °C; the removal of NH4+-N was 97.12%, and the TN removal was 52.64%, consistent with the optimal temperature range for biological denitrification. Gong et al. [115] found that the removal of NH4+-N decreased from 90% to 20% after the temperature was lowered from 30 °C to 10 °C. The removal of NH4+-N was reduced because of the decrease in microbial activity due to the decrease in temperature.
Temperature affects phosphorus removal mainly by influencing the amount and activity of PAOs. Low-temperature conditions severely inhibit PAO activity [116]. Xu et al. [117] found that the TP removal rate at 10 °C was reduced by 26.1% compared with that at 24 °C. Fang et al. [118] found that the temperature range varied from 20 °C to 35 °C, where PAOs grew and removed phosphorus optimally, with a maximum phosphorus removal rate of 80%.
Since winter’s low temperature, the Johkasou process’s performance in removing nitrogen and phosphorus is seriously constrained. Therefore, high microbial activity needs to be maintained by increasing the HRT [119], DO [120], and bioaugmentation [121]. In conjunction with many studies, it is recommended that the operating temperature of the Johkasou be maintained in the range of 20–35 °C to maintain the activity of the nitrifying microorganisms and PAOs, thus ensuring stable denitrification and phosphorus removal.

3.2.3. Dissolved Oxygen

DO is a crucial parameter in denitrification and phosphorus removal in the Johkasou. If DO is too low, it results in poor nitrogen and phosphorus removal, and if it is too high, it leads to excess aeration and increased costs.
The Johkasou usually provides sufficient DO for nitrification through an aeration device. The higher the aeration, the more favorable the growth of nitrifying bacteria [122]. It was shown that DO must be above 2.0 mg/L to maintain nitrification, Wang et al. [123] achieved about 89.9% TN removal efficiency under this condition. As long as DO is lower than 0.5 mg/L [124], nitrite is easy to accumulate, which is favorable for denitrification. Li et al. found that DO can be completely nitrified after 5 days when DO concentration decreased from 2 mg/L to 0.5 mg/L, which provided new insights for low DO treatment of low C/N domestic wastewater [125].
DO mainly affects the phosphorus removal rate by influencing the microenvironment distribution within the biofilm. High DO favors aerobic phosphorus uptake. Ju et al. found that when DO was 1.0 mg/L, TN removal was 66.45%; increasing to 3.0 mg/L increased TN removal to 73.97% [126]. However, excessive DO can detract from the biofilm [108]. Maintaining a low-oxygen environment is conducive to simultaneous nitrogen and phosphorus removal [127]. Zaman et al. [128] found that DO in the range of 0.25 mg/L to 0.35 mg/L favored the enrichment of various DPAOs.
In conclusion, it is recommended that DO levels in the anaerobic anoxic section of the Johkasou be kept below 0.5 mg/L to promote denitrification and anaerobic phosphorus release. Meanwhile, the aerobic section should maintain DO levels between 2.0 mg/L and 3.0 mg/L to ensure continuous and stable nitrification and aerobic phosphorus absorption.

3.2.4. Hydraulic Retention Time

HRT directly affects the rate of denitrification and phosphorus release in Johkasou, so choosing a suitable HRT helps improve nitrogen and phosphorus removal efficiency.
Too short HRT increases hydraulic loading and affects nitrification; longer HRT helps enrich more versatile microorganisms for complete nitrification and denitrification [129]. Nevertheless, if the HRT is too long, the thickness of the biofilm will be reduced, which is not conducive to the site where the biofilm provides SND [130]. Liu et al. [131] increased the HRT from 8.8 h to 11.3 h, which increased the average TN removal rate; while the HRT continued to increase from 11.3h to 18.6 h, the average TN removal decreased. This indicates that a moderate amount of HRT can increase the contact time of influent water with microorganisms and improve denitrification performance. Guan et al. [132] extended the HRT from 3 to 9 hours to increase the TN removal efficiency in Johkasou, and the TN effluent concentration reached 16.3 mg/L. Hou et al. [133] treated domestic wastewater at an HRT of 8h, and an average TN effluent of 7.2 mg/L was achieved.
The effect of HRT on phosphorus removal was more significant than nitrogen removal because PAOs are limited by the carbon source [134]. The phosphorus release directly affects the subsequent aerobic phosphorus uptake, and the prolongation of HRT under anaerobic conditions helps enhance PO43−-P release. Brown et al. extended the HRT from 0.5 to 2 h under anaerobic conditions, and phosphorus removal increased from 40% to 82%, which had little effect on TN removal [135]. However, prolonging the HRT in the anoxic zone only increased the TN removal, but the TP removal decreased. Therefore, balancing the effect of denitrification and phosphorus removal in an anoxic zone is necessary.
In summary, it is recommended that the HRT in Johkasou be maintained at 8–12 h, ensuring that the HRT in the anaerobic zone is more than 2 h. The anoxic zone must balance the nitrogen and the phosphorus removal efficiency to achieve a better nitrogen and phosphorus removal target.

3.2.5. Carbon Sources

The abundant carbon source in the Johkasou process provides electron acceptors for denitrification and is stored by PAOs as intracellular polymers for excess phosphorus uptake [136].
The denitrification process is generally enhanced by increasing the carbon source content [137], utilizing different types of carbon sources [138], and refluxing the digestate [139]. Different external carbon sources denitrify at different rates. Due to the highest denitrification rate, acetate and propionate are the preferred external carbon sources [140]. Agricultural waste is an environmentally friendly alternative carbon source. Li et al. [141] used corn cobs as an external carbon source; when the C/N ratio is 4.2:1, it achieves complete denitrification. Nitrogen removal efficiency is better at higher C/N; when the C/N ratio is increased from 3:1 to 10:1, the TN effluent concentration is reduced from 16 mg/L to 12 mg/L [142].
Carbon sources have a limiting effect on PAOS. PAOs compete with denitrifying bacteria; thus, a suitable C/P ratio is the key to phosphorus removal capacity. However, fewer studies have been carried out on the contribution of the C/P ratio to nutrient removal in the Johkasou process. Iannacone et al. investigated the effect of the C/P ratio on a moving bed biofilm reactor; when the C/P ratio is 11:1, the removal rate of PO43−-P reaches 83.86%, which provides a reference for the Johkasou [143].
In order to ensure that Johkasou can be denitrified entirely, keep the C/N ratio around 10. In contrast, the C/P ratio has been studied less in previous studies, which should attract the attention of researchers to achieve the goal of biologically efficient phosphorus removal.

4. Application of Johkasou in Domestic Sewage

Johkasou can be divided into conventional and innovative Johkasou. This section explains the structure and actual treatment of domestic wastewater of these two types of Johkasou to support the promotion of Johkasou in rural areas.

4.1. Application of Conventional Johkasou

Conventional Johkasou mainly consists of an anaerobic tank, an anoxic tank, an aerobic tank, a sedimentation tank, and a disinfection tank concentrated in a small integrated unit [144]. By adding fillers to anaerobic filters and contact oxidation tanks, a stable biofilm is formed on the surface of the fillers so that microorganisms are fully attached to the fillers, thus improving the efficiency of wastewater treatment [145].
Wang et al. treated domestic sewage by gradually adding biofilter beds inside the Johkasou; NH4+-N average effluent was 9.64 mg/L, whereas the average effluent of NH4+-N without the addition of biofilter beds was 29.6 mg/L [98]. The addition of a biofilter bed significantly improves the nitrogen removal capacity of Johkasou systems. Liu et al. used the Johkasou to treat black and odorous rivers; the average effluent of NH4+-N was 5 mg/L. It has a good effect on improving river water quality and enhancing river landscape [146].
Most application studies of Johkasou have shown excellent and stable nitrogen removal performance, but the phosphorus removal effect is unstable. Liu et al. increased the return flow in the Johkasou; COD, NH4+-N, and TN met the Class I B standard of GB 18918-2002, but TP did not meet the standard [147]. Therefore, the Johkasou must be combined with other advanced treatment processes to achieve better phosphorus removal.

4.2. Application of Innovative Johkasou

The innovative Johkasou, based on the traditional Johkasou, combined with ecological treatment technology and electrolytic flocculation process, significantly improves the phosphorus removal effect.

4.2.1. Combination of Johkasou and Ecological Treatment

The Johkasou is often combined with ecological floating beds and constructed wetlands to form a bio-ecological combination technology. The coupling process has a stable effluent effect, low cost, and simple management, which is suitable for promotion in rural areas.
The combined Johkasou and ecological floating bed process has been shown to have a stable phosphorus removal effect. The combined process removed nutrients in rural domestic sewage on Chongming Island [148], with an average effluent TP < 0.75 mg/L. Meanwhile, the TP removal rate was as high as 79.4%. This relies on the adsorption of Ceramic grains in the Johkasou, the interception of fiber filler in the floating bed, and plants’ respiration to remove TP. However, when the adsorption and interception reach saturation, the phosphorus removal effect will be significantly reduced, which is a problem that needs to be studied in the future. Huang et al. designed the plant Johkasou, the plant placed in the Johkasou to grow, COD, TN, and TP effluent concentrations of 17 mg/L, 1.43 mg/L, and 0.13 mg/L, respectively, significantly improving the removal of pollutants to meet the ‘Environmental Quality Standards for Surface Waters’ (GB 3838-2002) Class III standards [149,150].
The combination process of a Johkasou and constructed wetland purified the black and smelly river water in the center of Shanghai [151], and the TN average effluent concentration was less than 0.20 mg/L. Because the plant roots have an adsorption and filtration effect on granular phosphorus [149], the biological Johkasou combined with plants and fillers strengthens the phosphorus removal effect. Aiming at the high content of organic pollutants and animal and vegetable oils in Sichuan farmhouse’s sewage, a combined process of Johkasou-constructed wetland was proposed [152], which uses a constructed wetland for advanced treatment; the removal rate of TP reaches 63%. The phosphorus removal effect is good and stable, and it provides ideas for the sewage treatment of the farmhouse.

4.2.2. Combination of Johkasou and Electrolysis

Iron electrolysis is an effective method for chemical phosphorus removal in Johkasou. By adjusting the amount of iron, efficient phosphorus removal can be achieved in the long term.
Mishima et al. conducted a long-term study on phosphorus removal by iron electrolysis and found that the release of phosphorus into the bulk phase is prevented by the accumulated iron provided by iron electrolysis, resulting in stable phosphorus removal [153]. Augmenting the phosphorus removal effect of iron electrolysis with the addition of Ca [154]. Jiang et al. [155] used industrial steel as an electrode for small-scale rural domestic wastewater treatment and achieved 93.91% phosphorus removal. Mores et al. [156] investigated the influence of operation time, current density, and initial pH on phosphorus removal from swine farm wastewater using an iron electrode electrocoagulation process, which resulted in 96% TP removal under optimal conditions.
In a word, the combination process of Johkasou and electrolytes can not only significantly improve the effect of phosphorus removal but also have low cost, which is vital for promoting the Johkasou process.

5. Conclusions

This paper reviews the latest advancements in Johkasou systems, with a particular emphasis on nitrogen and phosphorus removal. While traditional denitrification has been well-studied, newer pathways such as SND and anammox have also been identified as important mechanisms in Johkasou systems. The paper provides a comprehensive overview of phosphorus removal from biological, physical, and chemical perspectives, highlighting the central role of microorganisms in these processes. However, the diversity of microorganisms and the application of metabolomics in Johkasou systems remain underexplored. Extensive research has established optimal ranges for key operational parameters, such as DO, pH, temperature, and HRT, which are crucial for system design. Moreover, the application of Johkasou systems in domestic sewage treatment offers valuable insights into their potential for widespread use, particularly in rural areas where centralized sewage treatment facilities are often lacking. As government interest in rural wastewater management grows, Johkasou systems are well-positioned for broader application.

6. Prospects

Despite significant progress, there remain several areas where further research on Johkasou systems is needed, as follows:
(1)
Metabolomics analysis. This approach could provide detailed insights into the denitrification and phosphorus removal processes at the molecular level, elucidating microbial growth and metabolic pathways. Currently, metabolomics analysis in Johkasou systems is limited, and expanding this research could reveal critical functional genes and pathways involved in nutrient removal;
(2)
Microbial diversity analysis. Understanding the diversity of microbial communities, particularly phosphorus-accumulating organisms (PAOs), is crucial for optimizing phosphorus removal. While there is extensive research on the diversity of denitrifying bacteria, the diversity of PAOs in Johkasou systems remains underexplored. Future studies should prioritize identifying key microbial species involved in phosphorus removal;
(3)
N2O accumulation. While many studies report N2O accumulation, Johkasou systems effectively reduce N2O production through the anammox pathway. However, N2O emissions have been rarely addressed in Johkasou’s research, highlighting a need for future studies to investigate this issue;
(4)
Influencing actors. Carbon source availability is a primary constraint on nitrogen and phosphorus removal efficacy in Johkasou systems. While there is extensive research on the C/N ratio, studies on the C/P ratio are relatively lacking. Future research should focus more on the C/P ratio’s impact on removal processes;
(5)
Phosphorus Removal Enhancement. The limited and unstable phosphorus removal efficiency in Johkasou systems often necessitates combining them with advanced treatment processes. Combining Johkasou systems with electrolysis has proven to be an effective and cost-efficient method for enhancing phosphorus removal, particularly in real-world domestic wastewater treatment. This approach represents a promising area for future research;
(6)
The removal of sulfate from domestic wastewater is an important but under-addressed issue in Johkasou research. Future studies should focus on developing strategies to enhance sulfate removal in Johkasou systems, further improving their ability to meet environmental health standards.

Author Contributions

Conceptualization, X.W. and S.C.; investigation, X.W. and S.C.; data curation, X.W. and S.C.; writing—original draft preparation, X.W. and S.C.; writing—review and editing, H.C.; supervision, H.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the National Key Research and Development Program of China (2019YFD1100200).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Structure of small Johkasou.
Figure 1. Structure of small Johkasou.
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Figure 2. The main nitrogen removal pathways. (a) Complete nitrification and denitrification; (b) simultaneous nitrification and denitrification; (c) anaerobic ammonia oxidation. Note: AOB: ammonia-oxidizing bacteria; NOB: nitrite-oxidizing bacteria; DNB: denitrifying bacteria; AnAOB: anaerobic ammonia-oxidizing bacteria.
Figure 2. The main nitrogen removal pathways. (a) Complete nitrification and denitrification; (b) simultaneous nitrification and denitrification; (c) anaerobic ammonia oxidation. Note: AOB: ammonia-oxidizing bacteria; NOB: nitrite-oxidizing bacteria; DNB: denitrifying bacteria; AnAOB: anaerobic ammonia-oxidizing bacteria.
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Figure 3. Simplified introduction of the three N2O production pathways [62] (reprinted with permission).
Figure 3. Simplified introduction of the three N2O production pathways [62] (reprinted with permission).
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Figure 4. The main phosphorus removal pathways. (a) Anaerobic phosphorus release and aerobic phosphorus absorption [71]; (b) simultaneous nitrogen and phosphorus removal [72]; (c) electrocoagulation for phosphorus removal [73]. Note: PAOs: polyphosphate accumulating organisms; GAOs: glycogen accumulating organisms; AnAOB: anaerobic ammonia-oxidizing bacteria; CAOB: Comammox Nitrospira (reprinted with permission).
Figure 4. The main phosphorus removal pathways. (a) Anaerobic phosphorus release and aerobic phosphorus absorption [71]; (b) simultaneous nitrogen and phosphorus removal [72]; (c) electrocoagulation for phosphorus removal [73]. Note: PAOs: polyphosphate accumulating organisms; GAOs: glycogen accumulating organisms; AnAOB: anaerobic ammonia-oxidizing bacteria; CAOB: Comammox Nitrospira (reprinted with permission).
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Table 1. Types of Johkasou process [10].
Table 1. Types of Johkasou process [10].
TypeProcessing ScaleManufacturing Method
Small size5–50 people
The average sewage volume is 10 m3/d		Mass production in the factory
Medium size51–500 people
The average sewage volume is 10–100 m3/d		Installation of FRP on-site
Large sizeMore than 500 people
The lowest sewage volume is 100 m3/d		On-site construction of reinforced concrete structures
Table 2. Characteristics of common treatment processes for domestic wastewater.
Table 2. Characteristics of common treatment processes for domestic wastewater.
ProcessAdvantagesDisadvantagesRemoval Percentage of
Parameters (%)		Reference
CODTNTP
Johkasoucompact design, relatively low maintenance, Effective nitrogen removallimited phosphorus removal, emission standards, and maintenance requirements are influenced by the region80–9560–9560–95[90]
Constructed wetlandsLow cost, easy to operate, with economic, ecological, and aesthetic valueLarge land area, low loading capacity, aquatic plant vulnerability to pests and diseases60–8560–8040–90[91]
Oxidation pondsSimple construction, low investment, landscaping functionSmall treatment load, long hydraulic retention time, large floor space, odor, and mosquito breeding50–7070–9030–50[92]
MBRSimpler process, more efficient treatmentEasy to cause membrane contamination, membrane clogging80–9060–8080–98[93]
Table 3. Advantages and disadvantages of different types of filler.
Table 3. Advantages and disadvantages of different types of filler.
TypesRepresentativeAdvantagesDisadvantagesReference
Fixed filler
Suspended filler		Honeycomb filler, Spherical fillerHigher volume loading,
higher biological activity		Uneven water and air distribution[96]
Fixed filler
Suspended filler		Soft filler, Semi-soft filler, Combined fillerLong service life,
low cost		Easily accumulates sludge[94]
Dispersed fillerStacked filler, Suspension fillerSimple operation, high oxygen transfer efficiencyHigher cost[97]
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Wang, X.; Cheng, S.; Chen, H. Evaluating the Mechanisms and Efficiency of Johkasou Systems for Decentralized Domestic Effluent Treatment: A Review. Water 2024, 16, 2266. https://doi.org/10.3390/w16162266

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Wang X, Cheng S, Chen H. Evaluating the Mechanisms and Efficiency of Johkasou Systems for Decentralized Domestic Effluent Treatment: A Review. Water. 2024; 16(16):2266. https://doi.org/10.3390/w16162266

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Wang, Xu, Siyue Cheng, and Huilun Chen. 2024. "Evaluating the Mechanisms and Efficiency of Johkasou Systems for Decentralized Domestic Effluent Treatment: A Review" Water 16, no. 16: 2266. https://doi.org/10.3390/w16162266

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Wang, X., Cheng, S., & Chen, H. (2024). Evaluating the Mechanisms and Efficiency of Johkasou Systems for Decentralized Domestic Effluent Treatment: A Review. Water, 16(16), 2266. https://doi.org/10.3390/w16162266

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