Abstract
Disinfection by-products (DBPs) generated during the disinfection of drinking water have become an urgent problem. So, tea polyphenol, a natural green disinfectant, has attracted widespread attention in recent years. This review summarizes the antibacterial mechanism of tea polyphenols and the recent findings on tea polyphenols as disinfectants for drinking water. These studies show that tea polyphenol is an antibacterial agent that works through different mechanisms and can be used as a supplementary disinfectant because of its higher lasting effect and economical cost. The dosage of tea polyphenols as a disinfectant of ultrafiltration effluent is the lowest among all the tea polyphenols disinfection methods, which can ensure the microbial safety of drinking water. This application of tea polyphenols is deemed a practical solution to solving the issue of disinfecting drinking water and reducing DBPs. However, it is necessary to further explore the influence of factors such as pipeline materials on the disinfection process and efficacy to expand the application scope of tea polyphenols. The large-scale application of tea polyphenols still needs to be fine-tuned but with new developments in tea polyphenol purification technology and the long-term need for drinking water that is safe for human consumption, tea polyphenols have good prospects for further development.
HIGHLIGHTS
Tea polyphenols are potential disinfectants for drinking water.
The antibacterial mechanisms of tea polyphenols are introduced.
The research progress on tea polyphenols as a drinking water disinfectant is summarized.
Tea polyphenols as a disinfectant for ultrafiltration effluent have unique advantages in reducing DBPs and ensuring the microbiological safety of drinking water.
Graphical Abstract
INTRODUCTION
The disinfection process is essential to ensure safe drinking water and the maintenance of public health. Bacteria, viruses and protozoan parasites in drinking water treatment systems can be effectively destroyed in by disinfection as a means to effectively prevent the spread of human pathogens (Sigstam et al. 2013). Water produced by water treatment plants is channeled over long distances to the communities. During the transmission, there remain opportunities for bacteria to multiply resulting in secondary pollution of the water and presenting risks to human health. Therefore, in addition to the essential disinfection process, it is also necessary to maintain residual disinfectant concentration in the network of pipelines to inhibit the regeneration of bacteria and ensure that the water that comes out of household and community taps is safe for consumption. Chlorine is a commonly used disinfectant because of its high disinfection efficiency and low cost (Li et al. 2013). Sodium hypochlorite is a better substitute for liquid chlorine due to its low risk and convenient dosing. However, chlorine disinfection generates carcinogenic, teratogenic and mutagenic disinfection by-products (DBPs), such as trihalomethanes (THMs), haloacetic acids (HAAs), etc. (Lyon et al. 2012). Chlorine dioxide does not produce a large number of halogenated DBPs (Pereira et al. 2008; Han et al. 2017). However, it is costly and also generates inorganic DBPs such as chlorite and chlorate, which cause oxidative damage to human red blood cells (WHO 2011). Chloramine disinfection has a lasting disinfection effect. However, its effect is slow and it will produce a bad odor and nitrogenous DBPs (N-DBPs) (Wang et al. 2018; Sun et al. 2019). Ozone has been widely used in drinking water disinfection due to no halogenated DBPs as by-products (Nahim-Granados et al. 2020). However, ozone produces other DBPs such as aldehydes and bromates during the disinfection process, which may lead to kidney cancer (Wu et al. 2019). UV is widely used for drinking water disinfection because it does not generate DBPs and has a high sterilization ability (Werschkun et al. 2012). However, its disinfection efficiency will be affected by factors such as suspended particles and bacterial concentration (Liltved & Landfald 2000; Oguma et al. 2001). Since ozone and UV have no lasting bacteriostatic ability, they need to be combined with other disinfection methods or agents to ensure that water flowing through the pipe network is safe (Taghipour 2004; Fitzhenry et al. 2021). In general, among the currently applied drinking water disinfection methods, disinfection agents or methods produce toxic by-products and cannot ensure a safe supply of drinking water over an extended period of time.
Monomer structural formula of four major catechins (Yang & Zhang 2019).
ANTIBACTERIAL MECHANISM OF TEA POLYPHENOLS
Destroying the bacterial cell wall
Tea polyphenols have damaging effects on bacterial cell walls. The cell wall of Gram-positive bacteria (GPB) is composed of peptidoglycan and teichoic acid whereas the cell wall of GNB is composed of an outer membrane (comprising of phospholipids and lipopolysaccharides) and a peptidoglycan layer (encapsulated by the outer membrane). The researchers noted that the inhibitory effect of tea polyphenols on Gram-negative bacteria (GNB) and GPB is very different due to the difference in cell wall structure. Yoda et al. (2004) found that the minimum inhibitory concentration (MIC) of EGCG on GPB such as Staphylococcus aureus is lower than that on GNB such as Escherichia coli. This suggests that GPBs are more sensitive to EGCG. The binding rate of EGCG to S. aureus was higher than that of E. coli. This suggests that the high sensitivity towards GPB may be because EGCG damages the GPB cell wall and binds to peptidoglycan to interfere with bacterial biosynthetic processes. The presence of the outer membrane and the low affinity between EGCG and lipopolysaccharide hinder the binding of EGCG to peptidoglycan, resulting in reduced GNB sensitivity to EGCG. To verify this suggestion, Cui et al. (2012) applied atomic force microscopy (AFM) to compare the morphological changes in GPB and GNB caused by EGCG and H2O2 at MICs. They showed that EGCG could cause S. aureus (GPB) to produce wrinkles and aggregates which led them to speculate that EGCG could directly bind to the peptidoglycan layer. The hydroxyl group or galloyl group in EGCG structure forms hydrogen bonds with proteins or polypeptides, thereby destroying the cross-linked bridge of the peptidoglycan layer, eventually degrading the cell wall which cannot be regenerated. Sub-MIC values EGCG can cause temporary pore-like lesions or collapse of the cell wall in E. coli (GNB), which may have resulted from H2O2 produced by EGCG degrading the outer membrane of GNB and causing temporary damage to the cell wall. However, H2O2 did not cause the surface of S. aureus to form aggregates. The extent to which this transient damage affects the GNB and whether EGCG itself had any effect on the GNB cell wall requires further investigation. The action mechanism of EGCG on bacterial cell wall is basically focused on its binding to the peptidoglycan layer and the degradation of the outer membrane. However, the extent to which cell wall damage affects bacterial survival requires further study. Since GPB are more sensitive to tea polyphenols than GNB, inactivation of GNB is more difficult. The inactivation of GNB reflects the inactivation of GPB to a certain extent.
Destroying the bacterial cell membrane
Under normal circumstances, a potential difference exists between the internal and external components of the bacterial cell membrane. The inner side of the membrane is electronegative while the outer side is electropositive. This polarity plays an important role in the bacterial physiological metabolic processes. The interaction between EGCG and the cell membrane can cause electrons to flow out of the membrane and cause the difference in potential between the inner and outer sides of the membrane to disappear. This depolarization of the cell membrane leads to irreversible cell decay (Cao et al. 2019). The binding mode of catechins to the lipid membrane is strongly affected by the presence of gallate groups. Sirk et al. (2008) performed molecular dynamics simulations using catechins and mixed phosphotidylethanolamine–phosphotidylcholine (POPE/POPC) bilayers. Their results showed that catechins formed hydrogen bonds with lipid headgroups and the movement of catechins on the cell membrane surface led to the formation and breakdown of hydrogen bonds. This ability to migrate and penetrate the bilayer may be related to apoptosis of the cell membrane. Existence of the gallate moiety and cis-configuration of the catechins B ring can promote the formation of multiple hydrogen bonds, which may lead to changes in the distribution of charges inside and outside the membrane and may explain the depolarization of cell membranes. Abram et al. (2013) reached the same conclusion using the phosphatidylcholine-sphingomyelin (2.4:1) model lipid membrane which showed that catechins containing gallate groups form hydrogen bonds with phosphate groups on the surface of lipid bilayers, and form hydrophobic interactions with the lipid alkyl chain. Hydrogen bonds formed between EGCG and lipid membranes reduce the fluidity of the membrane, leading to membrane rupture. To explore the key functional groups that EGCG acts on cell membranes, Nie et al. (2021) performed molecular dynamics simulations on phosphotidylglycerol (POPG) lipid bilayers. The results showed that EGCG could form hydrogen bonds with oxygen atoms deep in the POPG lipid bilayers. Up to 60% of the hydrogen bonds were from the gallate moiety suggesting that the gallate moiety is the key functional group for the formation of hydrogen bonds. Collectively, a consistent mechanism of action of EGCG was revealed by different research methods. These studies have relied mainly on the use of a model membrane or the bacterial cell membrane. Model membranes are more useful than cell membranes for exploring mechanisms at the molecular level. At the microscopic level, EGCG mainly destroys the cell membrane by depolarization and reducing fluidity. The mechanisms at the molecular level are mainly hydrogen bonding and hydrophobic interactions. However, there is disconnect between the research on model membranes and bacterial cell membranes. In the future, new research techniques are needed to confirm the molecular mechanism of EGCG on bacterial cell membranes. Furthermore, the extent to which this mechanism affects bacterial survival remains to be studied. At the same time, there is a need to establish whether the strong ability of the gallate group to form hydrogen bonds with the cell membrane can be utilized to create a disinfectant with stronger bactericidal ability.
Inhibiting the synthesis and expression of bacterial proteins and enzyme activity
Tea polyphenols can penetrate the damaged cell membranes and enter the interior of the bacteria, affecting the synthesis and activity of proteins and enzymes, which are essential to the bacteria. Cho et al. (2007) studied the soluble protein components of E. coli in an aqueous solution of tea polyphenols by two-dimensional polyacrylamine gel electrophoresis. They found that in the presence of tea polyphenols, bacterial cells over-expressed proteins related to defense mechanisms and also consumed more energy to down-regulate proteins related to metabolism and biosynthesis. Nonetheless, the extent to which changes in protein expression affected bacterial survival has not been thoroughly explained. Zhang (2020) found that the area of the hyaline circle produced by Klebsiella pneumoniae FK7 on nutrient broth medium in the presence of tea polyphenols, was significantly reduced, which indicated that tea polyphenols could inhibit the secretion or activity of K. pneumoniae protease, hindering the absorption of nutrients by bacteria. However, the specific inhibition mode and action site were not investigated. Yi et al. (2010) found that both dihdrollpoamide dehydrogenase (LPD) and succinyl Co-A synthetase beta subunit (enzymes involved in the tricarboxylic acid cycle (TCA)) of Pseudomonas aeruginosa were significantly downregulated after tea polyphenol treatment, which may lead to the disruption of TCA. In addition, many studies have shown that ATP synthase, DNA gyrase, catalase, dihydrofolate reductase, FabG and FabI reductase activities are inhibited by tea polyphenols. Decreased activity of these enzymes will interfere with energy metabolism processes in the bacteria and hinder the synthesis of biological macromolecules such as DNA, proteins, nucleic acids and fatty acids (Zhang & Rock 2004; Navarro-Martinez et al. 2005; Gradišar et al. 2007; Lan et al. 2021). Overall, tea polyphenols can inhibit bacterial growth by affecting protein synthesis and expression, and inhibiting enzymatic activities related to energy metabolism, material synthesis, nutrient absorption, respiration and other processes. However, the extent to which this process contributes to killing bacteria still needs further study. In addition, research on the inhibitory mechanism of tea polyphenols on enzyme activity lacks depth, including the specific binding mode and action site.
Damaging genetic material (DNA) and affecting gene expression
When tea polyphenols enter bacteria, they can damage DNA and hinder its synthesis. Fathima & Rao (2016) used agarose gel electrophoresis to study the binding activity of catechins to the DNA of E. coli and Bacillus subtilis. They found that linear or open circular DNA was present in the DNA samples from B. subtilis after catechin treatment, which indicated that catechin has a restriction endonuclease function. DNA damage may be due to hydrogen bonding between the catechin B ring and DNA bases. In addition to directly acting on DNA, Gradišar et al. (2007) used fluorescence spectroscopy and heteronuclear two-dimensional NMR spectroscopy to measure and confirm that EGCG binds to the ATP-binding site on the B subunit of DNA gyrase, inhibiting the activity of the enzyme and hindering DNA synthesis. At sub-lethal (SI) doses, some bacteria adapt to the bacteriostatic effect of tea polyphenols by regulating gene expression. Liu et al. (2013) used quantitative real-time PCR analysis to explore the effect of GTP on the expression of oxidative stress-related genes in P. aeruginosa. They reported that GTP exhibited pro-oxidant activity and inhibited the growth of P. aeruginosa under neutral or weak alkaline conditions. Degradation of P. aeruginosa DNA by H2O2 produced by GTP induced the expression of lexA (SOS regulon repressor gene) and recN (DNA repair gene), thereby enhancing P. aeruginosa tolerance to environmental stress. Differences in the effects of tea polyphenols on the DNA of different bacteria and the extent to which this mechanism affects bacterial survival remain to be elucidated. When disinfecting drinking water, the tea polyphenols dosage should not be too low to avoid the bacteria from acquiring enhanced tolerance. The effects of tea polyphenols on bacterial biofilms and virulence-related genes will be discussed in detail later.
Inducing bacterial oxidative stress and affecting bacterial metabolism
Although the oxidative stress response induced by EGCG is considered to be an important mechanism of its bacteriostatic effect, the source of the oxidative stress induced by EGCG remains controversial. In aqueous solutions, EGCG can react with dissolved oxygen to generate reactive oxygen species (ROS) such as H2O2 (Arakawa et al. 2004). Cui et al. (2012) found that EGCG was unable to cause severe damage to E. coli O157:H7 in the presence of peroxidase. This indicated that the bactericidal effect of EGCG on E. coli was mainly attributed to the H2O2 produced by EGCG. This process is known as EGCG-induced exogenous oxidative stress. However, Xiong et al. (2017) found that addition of EGCG did not generate H2O2 in E. coli OP50 or the generated H2O2 was completely eliminated. This suggests that the extracellular H2O2 produced by EGCG is not responsible for the inhibited growth of E. coli OP50. On the contrary, the bactericidal effect is attributed to the significant increase of ROS in bacteria, which can mutate or inactivate protein structure and accelerate bacterial senescence and death (Ran et al. 2013). This process is called endogenous oxidative stress in bacteria. The differences between the two studies may be due to differences in strains, media, or EGCG concentrations, and further research is needed. In order to further explore the mechanism of EGCG-induced endogenous oxidative stress, Nie et al. (2018) treated cpxR-deficient E. coli strains and common wild-type strains with EGCG and found that the cpxR-deficient strains had stable ROS and increased survival, while wild-type strains had elevated ROS levels and decreased survival. It is speculated that EGCG may interact with bacterial peptidoglycan and outer membrane proteins to induce cell envelope disorder. This leads to the activation of the bacterial Cpx two-component system and the formation and accumulation of intracellular ROS, ultimately causing bacterial oxidative damage. This may be one of the important targets of EGCG to induce endogenous oxidative stress in bacteria. Although the source of oxidative stress remains controversial, the oxidative stress response induced by EGCG has been shown to be an important part of its bacteriostatic mechanism. The extent to which this mechanism affects bacterial survival requires further study. In the process of drinking water disinfection, both endogenous and exogenous oxidative stresses may react under certain conditions.
Inhibiting the formation of bacterial biofilms and toxins
Biofilm leads to a significant increase in the resistance of pathogenic bacteria to fungicides, which weakens the effect of disinfectants (Srey et al. 2013). Studies have confirmed that tea polyphenols can inhibit the formation of bacterial biofilms and the secretion of toxins by interrupting QS which is the process by which bacteria communicate with each other by secreting autoinducers (AIS) to the external milieu (Abisado et al. 2018). Lee et al. (2009) found that the AI-2 concentration and biofilm formation by E. coli O157:H7 decreased to 13.2 and 11.8%, respectively, when the bacteria were treated with 25 μg/mL of EGCG. Real-time PCR analysis showed that EGCG reduced the transcription of virulence factor genes related to QS regulation by interfering with AI-2 signaling, ultimately reducing the pathogenicity of E. coli. Further research showed that EGCG inhibited biofilm formation mainly by inhibiting the expression of related genes. Through real-time PCR analysis, Xu et al. (2012) demonstrated that 15.6 mg/mL EGCG could significantly inhibit the glycosyltransferase (Gtfs) genes gtf B, C, and D of Streptococcus mutans UA159, thereby inhibiting the formation of biofilm. In addition, Liu (2019) proposed that tea polyphenols inhibited mRNA expression of planktonic drug-resistant Acinetobacter baumannii biofilm formation-related genes abaI, Bap, CsuE and OmpA. The biofilm formation-related genes in different bacterial species respond differently to EGCG. Nonetheless, if common targets can be identified, they could be exploited to improve the biofilm inhibitory property of tea polyphenols. In addition to biofilms, tea polyphenols also have inhibitory effects on bacterial capsule formation. Zhang (2020) investigated inhibitory effect of tea polyphenols on uronic acid in K. pneumoniae and found that tea polyphenols could significantly inhibit the secretion of mucus-like substances and drastically reduced capsular polysaccharide (the main virulence factor) content, thereby attenuating K. pneumoniae virulence. Nevertheless, there is a dearth of information on the inhibitory mechanism used by tea polyphenols on bacterial capsules and warrants future research. The inhibitory effect of tea polyphenols on the formation of bacterial biofilms and capsules is beneficial in reducing bacterial pathogenicity and ensuring microbial safety of the pipeline network.
Proposed bacteriostatic mechanisms of tea polyphenols as outlined in the summary of antibacterial mechanisms presented above.
Proposed bacteriostatic mechanisms of tea polyphenols as outlined in the summary of antibacterial mechanisms presented above.
TEA POLYPHENOLS AS A DRINKING WATER DISINFECTANT: CURRENT REASEARCH PROGRESS
Feng et al. (2016a) showed that for routine drinking water disinfection, the tea polyphenols dosage should be 0.1 g/L. This dose is able to maintain the total number of bacteria at below 80 CFU/mL following the addition into water for 20 min, and retained the good disinfection feature continuously over 2 days. Further research has shown that although the sterilization efficiency increases in tandem with the increase in tea polyphenol dosage, chromaticity also increases. For routine treatment of drinking water, when tea polyphenols are the sole disinfectant used, a large dose is required for effective and sustained sterilization and this contributes to increased economic costs and substandard chromaticity (the standard limit for chromaticity is 15°), which limits its potential application (Feng et al. 2016b). Dosage, disinfection period, pH and light all influence water chromaticity. Nonetheless, the adjustment range and degree of influence based on disinfection time, pH and light are very limited, hence, reducing the disinfectant dosage, is effective and economical (Feng et al. 2017). To assess the effect of a reduced dose while increasing the disinfection durability within the pipe network, the researchers explored its use as a supplementary disinfectant coupled to ozone and UV-based disinfection and as the sole disinfectant for ultrafiltration (UF)-based sterilization. A series of studies have also been carried out on the chemical and biological stability of tea polyphenols.
The effect of tea polyphenols on human health
Tea polyphenols have been shown to have many beneficial biological activities, such as anti-cancer (Jilani et al. 2020), antitumor (Mei et al. 2011), anti-inflammatory (Li et al. 2019), antioxidant (Yan et al. 2020) and anti-allergy (Fujimura et al. 2007) etc. As a natural anti-allergic substance, the application of tea polyphenols is not only harmless to people who are prone to allergies, but may also play a potential role in preventing and treating allergies (Zhang et al. 2022). Animal chromosome aberration experiments showed that green tea polyphenols are not genotoxic. However, chronic toxicology studies in mice, rats and dogs have shown that high doses of EGCG are associated with liver toxicity (Kapetanovic 2013). A review of 38 green tea product intervention studies by the European Food Safety Authority Panel on Food Additives and Nutrient Sources added to Food (ANS) found that EGCG above 800 mg/day was associated with hepatotoxicity. The study by Hu et al. (2018) showed that both the intake pattern and EGCG content of green tea extract were associated with liver toxicity. For adults, the safe dose of EGCG ingested in pill form is 338 mg/day, while the safe dose of EGCG ingested in beverage form is 704 mg/day (Hu et al. 2018). Most studies limit the safe intake of EGCG (in solution form) to 700–800 mg/day. Adults generally consume between 2.5 and 3.0 L of water per day. Assuming a maximum tea polyphenol dose of 0.1 g/L (if taking into account its attenuation in the pipe network, the value will be lower) and drinking water volume of 3.0 L/day, the average intake of tea polyphenols for an adult is 300 mg/day (about 144 mg EGCG/day), which is safe for the human body. The average green tea consumption among tea drinkers is reported to be three cups per day, while in some countries it may be as high as ten cups per day. Calculated at 1 g leaf/100 mL, the EGCG content of brewed green tea is 77.8 mg/100 mL. Therefore, each cup (8 oz.) of green tea beverages contains 184 mg of EGCG. For people who drink an average of three cups of green tea per day, their daily intake of EGCG is about 560 mg/day (Kuriyama et al. 2006). Combined with the maximum EGCG intake of 144 mg/day in drinking water, tea drinkers intake an average of 704 mg of EGCG per day, which is safe. The daily intake of four or more cups of tea (≥740 mg EGCG/day) exceeded the safe range (704 mg EGCG/day). Therefore, tea polyphenols as disinfectants are harmless to ordinary tea drinkers (three cups and less per day). It is recommended to drink no more than three cups of tea per day. The dosage of tea polyphenols proposed in this article will not affect human health.
Advantages of tea polyphenols in reducing DBPs
Concentration of DBPs in drinking water disinfected by different disinfection processes (modified from Du et al. 2021). Note: All disinfection processes use the optimal dosage to ensure the disinfection effect. Negative numbers on the axis indicate that the effluent has less DBPs than the influent.
Concentration of DBPs in drinking water disinfected by different disinfection processes (modified from Du et al. 2021). Note: All disinfection processes use the optimal dosage to ensure the disinfection effect. Negative numbers on the axis indicate that the effluent has less DBPs than the influent.
Tea polyphenols as a supplementary disinfectant for ozone and UV disinfection approaches
The total number of bacteria and log10 reduction value in the water in the simulated pipe network within 48 h under different disinfection processes. Note: The red line represents the standard limit for total number of bacteria (≤100 CFU/mL). UV and ozone doses were 40 mJ/cm2 and 2.5 mg/L, respectively. The dosage of tea polyphenols after UV, ozone and UF was 75, 20 and 5 mg/L, respectively. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wh.2022.062.
The total number of bacteria and log10 reduction value in the water in the simulated pipe network within 48 h under different disinfection processes. Note: The red line represents the standard limit for total number of bacteria (≤100 CFU/mL). UV and ozone doses were 40 mJ/cm2 and 2.5 mg/L, respectively. The dosage of tea polyphenols after UV, ozone and UF was 75, 20 and 5 mg/L, respectively. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wh.2022.062.
To explore the effect of pipeline materials and disinfection methods on the sterilization efficiency, Guo (2019) compared the disinfection processes of ozone–tea polyphenols and ozone–sodium hypochlorite in stainless steel and Polypropylene-Random (PPR)-simulated pipe networks (water residence time 10 h), respectively. As can be seen in Table 1, water in the stainless steel pipe network subjected to the ozone–sodium hypochlorite disinfection method had a risk of exceeding the standards for microorganism counts, however, the number of bacteria in the water under other working conditions meets the requirements for microbial-safe water. The disinfection effect of tea polyphenol as a supplementary disinfectant in the two pipe networks is better than that of sodium hypochlorite, and its disinfection effect is the best in the PPR-simulated pipe network. Regardless of the disinfection method, the biomass of the stainless steel tube wall is larger than that of the PPR tube. This may be because the PPR pipe has a smoother wall than the stainless steel pipe and is not easy for microbes to attach. The biomass of the tube wall under ozone–tea polyphenol disinfection was smaller than that for ozone and ozone–sodium hypochlorite disinfection. After comparing the simulated PPR pipeline coupon material by scanning electron microscopy, it was noted that during the ozone-alone disinfection process, the microorganisms quickly aggregated into clusters and formed spatial structures on the PPR pipeline coupons. The biofilm appeared to fall off after 35 days, which affected the safety of the drinking water. When sodium hypochlorite was used as a supplementary disinfectant for ozone, the strong penetrability of sodium hypochlorite resulted in large porous erosions within the biofilm on the coupons, and the microbial community structure was loose and easily dislodged. When tea polyphenols were used as the supplementary disinfectant for ozone, the biofilm in the pipe network was slow to form. Adsorption of tea polyphenols on the inner wall of the pipe network reduced biofilm formation on the pipe wall and inhibited any formation of layered spatial structure. The enrichment and propagation of microorganisms on the tube wall were relatively reduced. This may be due to the fact that tea polyphenols are more easily adsorbed onto the tube wall and were available to interact with the microorganisms. This suggests that tea polyphenols are able to reduce the bacterial load in the water as well as the tube wall. Feng et al. (2021a) used a stainless steel pipe network simulation system to explore the disinfection effects of ozone, ozone–tea polyphenols, and ozone–sodium hypochlorite disinfection methods and the microbial characteristics in the pipe network. When the initial total number of bacteria in the raw water is 540 CFU/mL, the disinfection effect of the ozone–tea polyphenol disinfection process over 35 days in the pipe network was better than other disinfection methods and also very stable. The high-throughput sequencing analysis identified that the ozone–tea polyphenol disinfection process had a stronger inhibitory ability on Actinobacteria (part of Actinomycetes are chlorine-resistant bacteria), Gemmatimonas, Porphyrobacter, Pseudomonas (including opportunistic pathogenic bacteria and biofilm-forming Pseudomonads), Methylobacterium (a key biofilm former) compared to sodium hypochlorite, which indicated a higher and more effective ability to kill pathogens and chlorine-resistant bacteria for the ozone–tea polyphenol disinfection process. This approach would be beneficial to ensure that the microbial safety of the water in the stainless steel pipe network. The combined disinfection technology has good potential for application purposes, but comprehensive factors such as pipe material, hydraulic condition and water quality need to be considered when developing a practical application. Although tea polyphenols are good disinfectants in a stainless steel pipe network and PPR pipe network, their performance in commonly used ductile iron pipes needs to be further explored. At the same time, if tea polyphenols can be added directly to the secondary pump station, the construction of disinfection structures can be avoided which, in turn, would reduce the cost of disinfection.
The bacterial concentration in water and the amount of bacteria on the pipe wall of the stainless steel pipe network and PPR pipe network under ozone, ozone–tea polyphenol, ozone–sodium hypochlorite disinfection processes within 35 days (Guo 2019)
Disinfection methods . | Bacterial concentration in water within 35 days of stainless steel pipe network operation (CFU/mL) . | Bacterial concentration in water within 35 days of PPR pipe network operation (CFU/mL) . | The amount of bacteria on the pipe wall after 35 days of operation in the stainless steel pipe network (log CFU/cm2) . | The amount of bacteria on the pipe wall after 35 days of operation in the PPR pipe network (log CFU/cm2) . |
---|---|---|---|---|
Ozone | 250–550 | 100–450 | 4.14 | 4.01 |
Ozone–tea polyphenols | 52–95 | 19–73 | 3.04 | 2.69 |
Ozone–sodium hypochlorite | 75–131 | 43–84 | 3.05 | 2.98 |
Disinfection methods . | Bacterial concentration in water within 35 days of stainless steel pipe network operation (CFU/mL) . | Bacterial concentration in water within 35 days of PPR pipe network operation (CFU/mL) . | The amount of bacteria on the pipe wall after 35 days of operation in the stainless steel pipe network (log CFU/cm2) . | The amount of bacteria on the pipe wall after 35 days of operation in the PPR pipe network (log CFU/cm2) . |
---|---|---|---|---|
Ozone | 250–550 | 100–450 | 4.14 | 4.01 |
Ozone–tea polyphenols | 52–95 | 19–73 | 3.04 | 2.69 |
Ozone–sodium hypochlorite | 75–131 | 43–84 | 3.05 | 2.98 |
Note: The initial bacterial concentration of raw water is 540 CFU/mL. The doses of ozone, tea polyphenols and sodium hypochlorite were 2.5, 20 and 2 mg/L, respectively.
UV is a physical disinfection method, which has broad-spectrum sterilization capabilities and does not produce DBPs. It not only kills bacteria, fungi, viruses and spores effectively, but also removes Cryptosporidium and Giardia which are resistant to killing by chlorine (Lui et al. 2016). However, UV does not disinfect continuously (Wen et al. 2019), and some bacteria are able to repair the UV-induced damage through photoreactivation and dark repair (Wen et al. 2019). Therefore, some water plants will also add chlorine or sodium hypochlorite as a supplementary disinfectant after the UV process, to prevent microorganism recovery (Zhu & Chen 2013). Liu et al. (2020) used a simulated water supply pipe network system to explore the effect of different dosages of tea polyphenols on the total number of bacteria when the UV dose was 40 mJ/cm2. Tea polyphenols at 25 and 50 mg/L could not ensure that the bacterial concentration in water was lower than 100 CFU/mL within 48 h. The optimal dosage of tea polyphenols was 75 mg/L after comprehensively considering factors such as sterilization effect, sustainability and economy. The disinfection effect of UV–tea polyphenol process is shown in Figure 4. The UV disinfection process is highly efficient but the effectiveness does not increase at UV doses greater than 40 mJ/cm2 (Shi et al. 2020). When the initial bacterial concentration is too high, increasing the UV dose may not be effective if the lower dosage of tea polyphenols is maintained. As such, increasing the dosage of tea polyphenols to complement the higher UV dose required when initial bacterial concentrations are high would add to the treatment costs. Liu et al. (2019a) compared the characteristics of microorganisms in different pipe networks under the disinfection methods of UV, UV–tea polyphenols and UV–sodium hypochlorite. It can be seen in Table 2 that the biomass of ductile iron pipe wall is larger than that of unplasticized polyvinyl chloride (UPVC) pipe wall. This may be due to the fact that the corroded ductile iron pipes provide a place for bacteria to attach, which is conducive to the growth of biofilms, while UPVC pipe is not conducive to bacterial adhesion due to its smoothness. No matter which type of pipe material, the ability of tea polyphenols to inhibit the biomass on pipe walls is higher than that of sodium hypochlorite. This may be due to the strong inhibitory ability of tea polyphenols on bacterial biofilm formation. After UV–tea polyphenol and UV–sodium hypochlorite disinfection, the bacterial concentration in the water of the UPVC pipe network was lower than 100 CFU/mL within 30 days of operation. However, ductile iron pipes have the risk of bacterial overload, making them less conducive to the long-lasting disinfection effect. The UV–tea polyphenol disinfection process strongly inhibits Pseudomonas and Mycobacterium (both containing a variety of opportunistic pathogenic bacteria and chlorine-resistant bacteria), which is conducive to ensuring the safety of drinking water in the pipe network.
The bacterial concentration in water and the amount of bacteria on the pipe wall of the ductile iron pipe network and UPVC pipe network under UV, UV–tea polyphenol, UV–sodium hypochlorite disinfection process within 30 days (Liu et al. 2019a)
Disinfection methods . | Bacterial concentration in water within 30 days of ductile iron pipe network operation (CFU/mL) . | Bacterial concentration in water within 30 days of UPVC pipe network operation (CFU/mL) . | The amount of bacteria on the pipe wall after 30 days of operation of ductile iron pipe network (log CFU/cm2) . | The amount of bacteria on the pipe wall after 30 days of operation of UPVC pipe network (log CFU/cm2) . |
---|---|---|---|---|
UV | 510–730 | 390–580 | 4.25 | 3.81 |
UV–tea polyphenols | 80–210 | 9–88 | 3.28 | 3.07 |
UV–sodium hypochlorite | 44–340 | 2–17 | 3.77 | 3.77 |
Disinfection methods . | Bacterial concentration in water within 30 days of ductile iron pipe network operation (CFU/mL) . | Bacterial concentration in water within 30 days of UPVC pipe network operation (CFU/mL) . | The amount of bacteria on the pipe wall after 30 days of operation of ductile iron pipe network (log CFU/cm2) . | The amount of bacteria on the pipe wall after 30 days of operation of UPVC pipe network (log CFU/cm2) . |
---|---|---|---|---|
UV | 510–730 | 390–580 | 4.25 | 3.81 |
UV–tea polyphenols | 80–210 | 9–88 | 3.28 | 3.07 |
UV–sodium hypochlorite | 44–340 | 2–17 | 3.77 | 3.77 |
Note: The initial bacterial concentration of the raw water is 600 CFU/mL. The doses of UV, tea polyphenols and sodium hypochlorite were 40 mJ/cm2, 75 mg/L and 2 mg/L, respectively.
In further research, Liu et al. (2020) found that when the bacterial concentration in the raw water was 600 CFU/mL, the UV–tea polyphenol disinfection process could keep the bacterial concentration in the water below 100 CFU/mL for 30 days. The sterilization effect of tea polyphenols in the water within the UPVC pipe network was better than that for ductile iron pipes, which may be due to the complex reaction between tea polyphenols and ferric ions released from ductile iron pipes, which increases the attenuation rate of tea polyphenols and decreased antibacterial activity. However, it is also possible that ductile iron pipes are more conducive to bacterial growth, and some biofilm bacteria are transformed into planktonic bacteria, resulting in an increase in the number of bacteria in the water. The scanning electron microscopy images of the biofilm on the UPVC pipe wall under different disinfection methods show that under UV disinfection alone, a relatively dense biofilm is formed on the coupons after the system was operated for 30 days; under UV–tea polyphenol disinfection, the biofilms on the coupons were thinner and formed obvious network structures after the system ran for 30 days. The UV–tea polyphenol disinfection process has a stronger destructive effect on the biofilm found on the UPVC pipe wall, which can effectively ensure good water quality as it flows through the pipe network. This may be due to the strong inhibitory effect of tea polyphenols on bacterial biofilms. Compared with the single application of tea polyphenols, the UV–tea polyphenol disinfection process not only reduces the dosage, but also makes up for the shortcomings of UV disinfection poor durability. If the initial bacterial concentration is high, there may be the risk of too high dosage of tea polyphenols to be used and further experiments need to be conducted on this issue. The disinfection effect is greatly affected by the pipe material, and the specific reasons need to be further explored in terms of the chemical mechanism and reaction kinetics to expand the application range of this process.
As the main disinfectant of UF effluent
UF is effective at removing bacteria, viruses, Giardia and organic matter (such as humus) present in water (Gao et al. 2011). Although the concentration of microorganisms in the UF effluent is low, it is still necessary to add disinfectants to avoid secondary contamination of drinking water in the pipe network. Wei et al. (2021) used an UF membrane with a pore size of 0.01 μm to explore the sterilizing effect of tea polyphenols at different concentrations on UF effluent. The results showed that tea polyphenols below 5.0 mg/L could not contain the bacterial concentration within the UF effluent at below 100 CFU/mL over 48 h. Therefore, in order to retain the lower dosage as far as possible, the optimal dosage for tea polyphenols as a disinfectant for UF effluent is 5.0 mg/L and the chromaticity of the effluent is 5. All indicators meet the requirements of the Standards for Drinking Water Quality (MOH/SAC 2007) (Figure 4). To explore changes in the microbial community structure in water after UF–tea polyphenol disinfection, Wei et al. (2021) used high-throughput sequencing to perform metagenomics analysis of disinfected water. The results showed that the proportion of Janthinobacterium that produce metabolites such as Violacein with antibacterial and antiviral activity increased after tea polyphenol disinfection. The proportions of Acinetobacter, Flavobacterium, Mycobacterium (including potential pathogenic bacteria and drug-resistant bacteria) and Pseudomonas (including opportunistic pathogenic bacteria) decreased significantly. This indicates that tea polyphenols are advantageous in inhibiting chlorine-resistant bacteria and pathogenic bacteria. Feng et al. (2021b) used the BLAST similarity comparison search method to establish the MvirDB protein database. Their analysis found that the types of water-based toxicity factors decreased by 56 following tea polyphenol treatment, and the total absolute abundance decreased by 51%. This indicates that tea polyphenols have a strong ability to inhibit the virulence of pathogenic bacteria. Among the three tea polyphenols combined disinfection processes, the UF–tea polyphenol disinfection process had the best disinfection effect (2.0–2.78 log), which was achieved with the lowest dosage of tea polyphenols at 5 mg/L (Figure 4). Supplementing the disinfection of UF effluent with tea polyphenols confers an advantage to the UF process achieves a better continuous disinfection effect and also greatly reduced the dosage of tea polyphenols, chroma and pharmaceutical costs. The sterilization effect of the UF–tea polyphenol disinfection procedure in pipe networks of different materials needs to be further studied, but the better disinfection outcome and lower dosage currently established, are satisfactory. Therefore, UF in combination with tea polyphenols as the sole disinfectant is currently the preferred solution in the field of drinking water disinfection.
Chemical and biological stability
Simulated decay curves of different disinfectants (Ammar et al. 2014; Guo et al. 2018; Feng et al. 2020, 2021b; García-Ávila et al. 2020; Curling et al. 2022; Li et al. 2022).
Simulated decay curves of different disinfectants (Ammar et al. 2014; Guo et al. 2018; Feng et al. 2020, 2021b; García-Ávila et al. 2020; Curling et al. 2022; Li et al. 2022).
Feng et al. (2020) reported that when tea polyphenols acted as a supplementary disinfectant for ozone, its decay rate was significantly slowed down (Figure 5), which indicated that the killing of microorganisms by ozone could reduce the consumption of tea polyphenols during the disinfection process. Tea polyphenols can exist in water for a long time and therefore can complement the short duration of ozone disinfection. Guo et al. (2018) found that when tea polyphenols were used as supplementary disinfectants for UV, the decay of tea polyphenols still conformed to the first-order kinetic equation, and showed a slower decay rate (Figure 5). The killing effect of UV rays on microorganisms reduces the loss of tea polyphenols during the disinfection process, which can keep tea polyphenols at a certain concentration in water for a long time, and contribute to the long-lasting disinfection effect. With the ozone–tea polyphenol and UV–tea polyphenol disinfection methods, attenuation coefficient values for tea polyphenols were very similar, but UV rays require a higher tea polyphenol dose at similar attenuation rates. This indicates that the long-lasting tea polyphenol disinfecting property is more pronounced for the ozone–tea polyphenol disinfection strategy. Feng et al. (2021b) found that the decay characteristics of tea polyphenols when used as the only disinfectant together with ultrafiltrated effluent fit the second-order kinetics equation. The attenuation of tea polyphenols was greatly affected by the initial concentration and reaction temperature. The lower the initial concentration and the higher the reaction temperature, the faster the tea polyphenol decays. When tea polyphenols are used as the only disinfectant on UF effluent, the dosage is the lowest, and the decay rate is also the slowest. The residual concentration of 5 mg/L of tea polyphenols after disinfection for 48 h was 2.11 mg/L, and the attenuation coefficient was only 0.0046 (Figure 5). This shows that the UF–tea polyphenol disinfection process is also the most advantageous in providing lasting antibacterial ability.
Cost analysis
The all-inclusive disinfection costs for different disinfection methods (including the costs of pharmaceuticals, equipment, water consumption, electricity and labor) are different. Since the main cost of the tea polyphenol disinfection process is the reagent cost, the total cost for the tea polyphenol disinfection process was calculated and is shown in Table 3. The tea polyphenols dose as a disinfectant for UF effluent is 5 mg/L, and the all-inclusive disinfection cost is only 0.9 CNY/ton. As a new type of disinfectant, tea polyphenols have not been widely promoted and applied. As such, when compared to more traditional disinfection methods, the all-inclusive disinfection cost of tea polyphenol disinfection methods is still relatively high. However, with promising developments in the tea polyphenol extraction process, it is anticipated that total costs can be further reduced.
All-inclusive disinfection costs for different disinfection methods
Disinfection methods . | All-inclusive disinfection costs (CNY/ton) . | References . | |
---|---|---|---|
Traditional disinfection | Chlorine | 0.021 | Shi et al. (2020) |
Sodium hypochlorite | 0.0414 | Carnimeo et al. (1994) | |
Ozone–chlorine | 0.072 | Liu et al. (2019b); Shi et al. (2020) | |
Ozone–UV–chlorine | 0.065 | Shi et al. (2020) | |
UV–chlorine | 0.021 | Shi et al. (2020) | |
UV–sodium hypochlorite | 0.054 | Huang et al. (2022); Carnimeo et al. (1994) | |
UV–chloramine | 0.161 | Cao et al. (2022) | |
Tea polyphenol disinfection | Tea polyphenols (100 mg/L) | 18 | Feng et al. (2016a) |
ozone–tea polyphenols (20 mg/L) | 3.64 | Feng et al. (2020) | |
UV–tea polyphenols (75 mg/L) | 13.51 | Liu et al. (2020) | |
UF–tea polyphenols (5 mg/L) | 0.90 | Wei et al. (2021) |
Disinfection methods . | All-inclusive disinfection costs (CNY/ton) . | References . | |
---|---|---|---|
Traditional disinfection | Chlorine | 0.021 | Shi et al. (2020) |
Sodium hypochlorite | 0.0414 | Carnimeo et al. (1994) | |
Ozone–chlorine | 0.072 | Liu et al. (2019b); Shi et al. (2020) | |
Ozone–UV–chlorine | 0.065 | Shi et al. (2020) | |
UV–chlorine | 0.021 | Shi et al. (2020) | |
UV–sodium hypochlorite | 0.054 | Huang et al. (2022); Carnimeo et al. (1994) | |
UV–chloramine | 0.161 | Cao et al. (2022) | |
Tea polyphenol disinfection | Tea polyphenols (100 mg/L) | 18 | Feng et al. (2016a) |
ozone–tea polyphenols (20 mg/L) | 3.64 | Feng et al. (2020) | |
UV–tea polyphenols (75 mg/L) | 13.51 | Liu et al. (2020) | |
UF–tea polyphenols (5 mg/L) | 0.90 | Wei et al. (2021) |
Xu et al. (2022) found that the integration of tea markets in various countries reduces the fluctuation of tea prices caused by climate or pests in a certain country or region. Stable tea prices favor the promotion and application of tea polyphenols. Expanding production may lower tea prices locally or globally. In fact, with economic development, the global tea production and planting areas are growing steadily. In the past 30 years, tea consumption in China has increased by nearly 13 times, and global tea production has increased by about 2.5 times (Xu et al. 2021). From a market perspective, the ever-increasing tea production in the world will inevitably lead to a continuous reduction in tea prices. Concomitantly, to add value and gain a further advantage in the fierce international market, enterprises will shift their focus on tea as a primary raw material to investing more resources on tea extracts. Taking the Chinese market as an example, the price of tea polyphenols (80%) has dropped from 400 CNY/kg in 2005 to 180 CNY/kg today. In the future, with the continuous demand for better water quality, continuous expansion of the tea market and progress in the extraction process will further drive the price of tea polyphenols to lower figures. More importantly, tea polyphenols, as a potential disinfectant with beneficial effects to the consumer and limited or no DBP production, may have a large market in the secondary water supply disinfection of upscale hotels and residences even if the current price for the service is high.
CONCLUSIONS AND OUTLOOK
The ability of tea polyphenols to inhibit bacteria involves different mechanisms. However, most studies in the past have only focused on specific mechanisms of action. Further investigations are necessary to understand if these mechanisms occur simultaneously and how much they contribute to bacterial inhibition or killing still require further study. Since it is difficult to extract and characterize the tea polyphenols–bacterial structures complexes, the binding mode of tea polyphenols needs to be further verified by more sophisticated methods. Existing studies have shown that the complex-formation and destruction of various bacterial structures by tea polyphenols mainly involve hydrogen bonding and hydrophobic interaction rather than the strong oxidation of traditional disinfectants, and this approach also limits the production of DBPs normally detectable with the traditional methods. This lays a theoretical foundation for the use of tea polyphenols as a new type of green drinking water disinfectant. Although there is no clear safe intake dose for tea polyphenols, many reports have attested to the possible safe dosage for tea polyphenols to be utilized in the disinfection process, which would not be harmful to the human body. Therefore, further research on tea polyphenols as a drinking water disinfectant is of high value and practical significance for safe and high-quality drinking water suitable for humans to consume.
It is difficult to use tea polyphenols as the only disinfectant due to the influence of dosage and chromaticity. When tea polyphenols are used as a supplementary disinfectant together with ozone and UV, it can be administered at a lower dosage with reduced chromaticity and also effectively inhibit the formation of biofilm in the pipe network while killing a variety of pathogenic bacteria and chlorine-resistant bacteria. As the main disinfectant for UF effluent, tea polyphenols can achieve a lower dosage while ensuring the microbiological safety of drinking water. This process is currently the most likely to be put into practical application. Although the comprehensive disinfection cost of the tea polyphenol disinfection process is relatively high, there is still a lot of room for a drop in pricing in the future. This technology may also have a big market in the secondary water supply disinfection of upscale hotels and residences. The disinfection effect of tea polyphenols in the pipe network is greatly affected by the pipe materials. In the future, further research needs to be carried out from the perspective of chemical mechanisms and reaction kinetics to expand the application scope of the tea polyphenol disinfection process. The potential large-scale application of tea polyphenols still needs to solve a series of problems, including low-cost mass production of tea polyphenols, the availability of suitable disinfection equipment and the standardization of the production and application of tea polyphenols disinfectants. However, with the development of technology and the demand for safe drinking water, tea polyphenols have broad development prospects.
ACKNOWLEDGEMENTS
This work was supported by the National Natural Science Foundation of China under Grant number 51678026, Beijing University of Civil Engineering and Architecture Postgraduate Innovation Project under Grant number PG2022061 and Open Project of Key Laboratory of Urban Stormwater System and Water Environment, Ministry of Education, Beijing University of Civil Engineering and Architecture under Grant number 2020. We would like to express our gratitude to EditSprings (https://www.editsprings.com/) for the expert linguistic services provided.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.