1. Introduction
Research related to the management of fecal sludge (FS) is of great importance for urban sanitation, as well as for environmental ecology in general [
1,
2]. Various measures are taken to limit the indiscriminate discharge of FS in an urban environment or in transport (trains, planes, buses, etc.) [
3,
4,
5]. The most common way to manage FS is to use chemical toilets, where FS is collected in containers and then disposed of centrally [
6]. The problem with these toilets is microbial activity in FS, accompanied by foul-smelling gas, especially in warm weather [
7,
8].
Various physical and chemical methods are used to control the microbial activity, such as UV irradiation [
9,
10], ozonation [
11], chlorination [
10,
12], use of peracetic acid [
13], performic acid [
14,
15], wood ash [
16], hydrogen peroxide [
17] and various biocides. The use of biocides is most effective in chemical toilets, as it allows not only to control the activity of microorganisms and the release of unpleasant odors but also to avoid the spread of infections [
18,
19,
20].
Quaternary ammonium compounds (QAC) and polyhexamethylene guanidine (PHMG) are widely used for the management of FS in train lavatories [
21]. These substances are biocidal against a number of microorganisms, including fungi, Gram-negative and Gram-positive bacteria, and lipophilic viruses [
22]. However, the disposal of FS containing these biocides through discharge to wastewater treatment plants (WWTP) becomes a problem [
23]. Many studies have shown that discharge of recalcitrant biocides, such as QAC and PHMG, results in the inhibition of nitrification, denitrification and biochemical oxygen demand (BOD) processes in WWTP [
24,
25]. One of the ways to reduce the environmental damage to the activated sludge process is the use of more easily degradable biocides, which can degrade after direct use.
Once in toilets, urea slowly decomposes with the release of ammonia, which alkalizes the FS. Due to the high content of urea in FS, the pH can increase from the initial value of 7.5 to 9.0–10.0 [
26]. This feature can be beneficial for developing a new way of FS management through the use of biocides with controlled decomposition. It is known that some chemical compounds have a high biocidal efficiency, but at the same time they decompose when the medium is alkalized. We selected six such biocidal substances that are odorless, non-toxic in the applicable concentration ranges, and decompose when the pH rises above 7.0. Their structural formulas are shown in
Figure 1.
(1)
Dehydroacetic acid (DA) and its sodium salt (DAN). DA, or 3-acetyl-2-hydroxy-6-methyl-4H-pyran-4-one, is a cyclic ketone that has found wide application in food preservation, pharmaceuticals and cosmetics [
27,
28]. DA is tasteless and odorless and effective under acidic conditions (pH 5.0–7.0 with an optimum of 6.0–6.5) because only the protonated form, which exists at acidic pH, can pass through the cell membrane. Once inside the cytoplasm, DA dissociates, lowers intracellular pH, and disrupts the electrical potential of the microbial cell membranes. Because of its low solubility in water (less than 0.1%), DA is mainly used in the form of a water-alcohol solution, for example, in the preservative «Kem DHA» (Akema Fine Chemicals, Coriano, Italy). [
29]. The optimal dosage of the preservative is 0.2–0.8%, in terms of active ingredient DA, 0.016–0.064%. More often, DA is used in the form of sodium salt (DAN), which has higher solubility in water [
30,
31].
In alkaline conditions, the biocidal activity of DA and DAN decreases, up to complete disappearance, so the use of DA or DAN for the management of FS may be promising.
(2)
Bronopol (B). Bronopol, 2-bromo-2-nitropropane-1,3-diol, is a white or almost white powder, odorless or with a slight characteristic odor, readily soluble in water [
32]. Its aqueous solutions are stable between pH 4.0 and 8.0.
Bronopol has a powerful bactericidal effect [
33]. It is effective against Gram-positive and Gram-negative bacteria, some species of algae as well as yeasts and molds [
34,
35]. Therefore, bronopol is used as a preservative in pharmaceuticals, cosmetics and personal care products individually or in combination with other preservatives at concentrations of 0.01–0.1% [
36].
Aqueous preparations are not stable at alkaline pH and are also capable of hydrolysis and photolysis. This feature makes it promising for use in FS management. Natural alkalinization of FS during urea decomposition will eliminate biocide residues and make the waste safe for the activated sludge of wastewater treatment plants.
(3) 2,2-dibromo-3-nitrilopropionamide (DBNPA). DBNPA is a white crystalline powder, well soluble in water. Aqueous solutions of DBNPA are stable at pH 3.0 to 7.0.
DBNPA is a broad-spectrum biocide [
37]. It is used in the pulp and paper industry (for the treatment of paper and cellulose sludge); the oil production industry (biocide for cooling water and water pumped into wells and hydroplastics), and in membrane technologies [
38,
39,
40,
41].
DBNPA is readily hydrolyzed under both acidic and alkaline conditions, making it promising for FS management. Above pH 7.5, DBNPA starts to decompose to form less or non-toxic compounds: carbon dioxide, ammonia, bromide ions, dibromacetonitrile and dibromoacetic acid. The approximate half-life of DBNPA is 24 h at pH 7.5, 2 h at pH 8.0, 15 min at pH 9.0. The degradation coefficient depends on pH and temperature [
42].
(4)
The preparation Sharomix (SH) is a mixture of isothiazalons. Isothiazalon (IT) and its derivatives (methylisothiazolinone (MI), methylchloroisothiazolinone (MCI), benzisothiazolinone (BIT), octylisothiazolinone (OIT) and dichloroctylisothiazolinone (DCOIT))—powerful biocides that are used for the preservation of detergents, paints and cosmetics, as well as other everyday products [
43]. These compounds are able to diffuse through the bacterial cell membrane and the cell wall of fungi. In the intracellular environment, the electron-deficient sulfur N—S bond of these compounds can react with nucleophilic groups of cellular components, such as cysteine thiols in the active centers of proteins, blocking their enzymatic activity and ultimately causing the death of microbial cells [
44,
45,
46].
Biocidal agents contain both individual isothiazolones and their mixtures. In practice, the biocidal agent SHAROMIX MCI with 1.5% isothiazolones (Methylchloroisothiazolinone 1.1% and Methylisothiazolinone 0.4%) in water is widely used. It is effective in small doses: the minimum inhibitory concentration (MIC) of SHAROMIX MCI against bacteria is 1–3 ppm. The product is stable at pH 4.0 to 8.5; temperature 5 to 50 °C. The biocide decomposition rate increases approximately 10–20 times at 40 °C (compared to 7 °C) and 2000 times when the medium is alkalized to pH 11.0 (compared to 4.5) [
43]. This property can be used in FS management.
(5)
Sodium percarbonate (P). Sodium percarbonate is a crystallosolvate of sodium carbonate and hydrogen peroxide Na
2CO
3-1.5H
2O
2. It is a white granular substance, easily decomposed by heating, and dissolves quickly in hot water [
47]. In cold water, it decomposes very slowly, forming hydrogen peroxide. The end products of sodium percarbonate decomposition in water are water, oxygen and sodium bicarbonate. With the decomposition of sodium percarbonate, the pH of the medium slightly increases [
48].
The main use of sodium percarbonate is in the domestic sphere as an oxygenated bleaching component in synthetic detergents and stain removers [
49]. Sodium percarbonate is also used in: (1) the chemical industry, as an oxidizing agent [
50]; (2) in the textile industry in technological processes of dyeing fabrics and rinsing [
51]; and (3) in medicine and other fields, as a means for removing all types of contaminants and disinfecting surfaces and materials [
52].
Sodium percarbonate can be used as an environmentally friendly biocidal agent for bio-toilets, as, on the one hand, it has antimicrobial properties and, on the other hand, it decomposes quickly in the aquatic environment.
(6)
Silver citrate (SC). Silver citrate is a water-soluble salt of silver and citric acid (chemical formula Ag
3C
6H
5O
7), produced by an electrochemical process and containing 2400 ppm of silver ions [
53]. Silver citrate is an effective antibacterial compound widely used in medicine [
53,
54,
55] and veterinary medicine [
56]. The recommended concentration of SC is 0.5% and higher [
57]. If necessary, SC can be used in combination with other biocides [
55].
Silver citrate is sensitive to light and is also unstable at a pH above 7.0 and temperatures above 50 °C. Accordingly, it is expected to decompose when added to FS, the pH of which rises to 9.0–9.7 over time due to the decomposition of urea.
Thus, the above biocidal compounds (
Figure 1) are promising for the treatment of FS. Five of them:sodium dehydroacetate (DAN), bronopol (B), 2,2-dibromo-3-nitrilopropionamide (DBNPA), sharomix (Sh), and silver citrate (SC), significantly reduce their activity when the pH of the medium is increased to alkaline values. In the case of sodium percarbonate, the decrease in the biocidal effect is not so much dependent on the pH of the medium as it occurred during incubation due to decomposition with the formation of a mixture of hydrogen peroxide (which eventually decomposes into water and oxygen), Na
+, and CO
32−.
The purpose of this work was to assess the feasibility of using these biocides to temporarily inhibit microbial activity in fecal sludge for its subsequent more environmentally friendly disposal. In accordance with the purpose of the work, (1) the biocidal effect of selected biocides were tested against test-microorganisms; (2) the ability to self-degrade after alkali addition to the medium or as a result of FS incubation was evaluated; and (3) the biocidal activity of selected biocides in relation to the FS microbiota was analyzed and the degradation of these biocides after a long incubation (10 days) using the BOD5 test was assessed.
2. Materials and Methods
2.1. Fecal Sludge, Biocides, Microorganisms
Fecal sludge (FS) was sampled at the beginning of June 2019 from the environmentally safe toilet complexes (ESTC) of railway trains of the North-Western branch of the Joint-stock company «Federal Passenger Company» (JSC «FPK», Russia). For this study, FS was sampled from ESTC, that was not supplemented with any biocides.
The following chemicals with antimicrobial properties were used as biocides:
Sodium dehydroacetate (DAN, Sigma-Aldrich, Buchs, Switzerland);
Bronopol (B, St. Petersburg, Russia);
2,2-dibromo-3-nitrilopropionamide (DBNPA, Sigma-Aldrich, Buchs, Switzerland),
Sharomix (SH, Ashdod, Israel);
Sodium percarbonate (P, Krasnodar, Russia);
Silver citrate (SC, Darmstadt, Germany);
Biocide “Latrina” produced by Limited Liability Company «Rail Chemical» (Russia). “Latrina” is widely used in the ESTC of JSC «FPK» and by Russian Railways as a toilet chemical additive. The composition of “Latrina” included: didecyldimethylammonium chloride (0.24%) and PHMG (6.5%) (total 6.74%), surfactants (surfactants), perfume and water.
The following test-microorganisms from the collection of Research Center of Biotechnology RAS were used for the determination of minimum inhibitory (MIC) and bactericidal (MBC) concentrations:
Gram-negative bacteria
Pseudomonas aeruginosa 4.8.1 and
Alcaligenes faecalis DOS7 [
58];
Gram-positive non-sporulating bacteria Staphylococcus aureus 209P and Micrococcus luteus NCIMB 13267;
Gram-positive spore-forming bacteria Bacillus subtilis 534, yeast (eukaryotes) Yarrowia lipolytica 367-2.
2.2. Cultivation of Test-Microorganisms
The test-microorganisms were grown in lysogeny-broth medium (LB) (Broth, Miller, VWR Life Science, Radnor, PA, United States). Cultivation was carried out in 250-mL flasks with 50 mL of nutrient medium with mixing on an orbital shaker (120 rpm) for 24 h (to the stationary phase) at a temperature of 28 °C. Inoculum (culture at the beginning of the stationary growth phase) was introduced in an amount of 0.25 mL per 50 mL of medium (0.5% vol.).
2.3. Determination of MIC and MBC
Biocide aliquots at various final concentrations were added to 25 mL glass test tubes with cotton stoppers containing 5 mL of LB medium. The tubes were then inoculated with test-microorganisms at stationary growth phase and incubated in a thermostatically controlled shaker (28 °C, 120 rpm). After 2 days of incubation, the growth of microorganisms was assessed visually by the appearance of turbidity. The lowest concentration of the biocide, at which no growth of the test-microorganisms was observed, was taken as the MIC. Additionally, after 2 days of incubation, aliquots were plated on an agar LB nutrient medium from test tubes in which no visual microbial growth was observed. The MBC was taken to be the lowest concentration of the biocide in the test tube, in which no growth of microorganisms was observed in the agar medium.
2.4. Evaluation of Changes in the Antimicrobial Properties of Sharomix, Bronopol and DBNPA
Aliquots of biocidal agents were added to 25 mL glass tubes with 5 mL of LB nutrient medium and cotton stoppers to final concentrations in the medium corresponding to MIC or higher. Then, 40 μL of 1 N NaOH was added to one part of the tubes, increasing the pH of the medium to 9.0. In another part of the tubes, the pH was first raised to 9.0 by adding 1 N NaOH, then after 10 min incubation, 40 μL of 7% hydrochloric acid was added to restore the pH to 7.0.
The tubes were then inoculated with test-microorganisms as described above. The tubes were placed on a shaker at 28 °C. After 4 days, the growth of microorganisms was assessed visually by the appearance of turbidity. The lowest concentration of the biocidal agents at which no growth of test cultures was observed was taken as the MIC.
2.5. Evaluation of Changes in the Antimicrobial Properties of Sodium Percarbonate during Long-Term Storage
Three batches of 25 mL glass tubes with 5 mL of LB nutrient medium and cotton stoppers were prepared in which sodium percarbonate (in the form of powder) was added to the final concentrations in the medium, corresponding to MIC or higher. The tubes were thoroughly mixed and stored for 10 days at 28 °C. After 5, 8, and 10 days, inoculation with test-microorganisms of one of the batches was performed as described above. After inoculation, the tubes were placed on a shaker at 28 °C, and microbial growth was evaluated after 4 days. The lowest concentration of the sodium percarbonate at which no growth of test cultures was observed was taken as the MIC.
2.6. Evaluation of the Effect of Biocides on the Microbial Activity of FS under Aerobic and Anaerobic Conditions
A total of 10 mL of FS and a certain amount of biocide were added to 120 mL glass vials. In the control vials, no biocide was added to the FS. For anaerobic experiments, the vials were purged with argon, then sealed with butyl rubber and aluminum caps. For aerobic experiments, the vials were first sealed with butyl rubber and aluminum caps and then 50 mL of extra air was added to the vials using a syringe to avoid a lack of oxygen for aerobic microorganisms in the FS. Incubation was performed for 10 days (the time during which the biocide must perform its biocidal function in the ESTC, according to the Rail Chemical terms) on a thermostatically controlled shaker at 110 rpm and 28 °C. Under anaerobic conditions, the rate of oxygen consumption was taken as the criterion for the rate of FS biodegradation, and under anaerobic conditions, the rate of carbon dioxide accumulation. The experiments were performed in triplicate.
The specific rate of oxygen consumption by the microbial community of FS (in mM O
2/(mL FS ∗ day)) was calculated based on the concentration of oxygen in the headspace of the vials at the beginning and after 10 days of incubation, according to Equation (1):
where
cO2 init—initial concentration of oxygen in the headspace, %vol.;
cO2 fin—final concentration of oxygen in the headspace, %vol.;
VG—volume of the headspace, mL;
50—volume of the extra air added to headspace, mL;
100—per cent, %;
22.4—molar volume of a gas at standard temperature and pressure, L/mol;
VL—volume of the liquid phase, mL;
10—incubation time, day.
The specific rate of carbon dioxide production by the microbial community of FS (in mM CO
2/ (ml FS ∗ day)) was calculated according to Equation (2):
where
cCO2 fin—final concentration of carbon dioxide in the headspace, %vol.;
VG—volume of the headspace, mL;
100—per cent, %;
22.4—molar volume of a gas at standard temperature and pressure, L/mol;
VL—volume of the liquid phase, mL;
10—incubation time, day.
2.7. Determination of the Total Number of Colony Forming Units (CFU)
Aliquots of FS pretreated with a biocide after assessing microbial activity under aerobic and anaerobic conditions were plated after appropriate dilution on agar LB nutrient medium. The cultures were incubated at 28 °C for 3 days. The number of CFU in the corresponding dilutions was counted and the cell number (CFU/mL FS) was determined.
2.8. Evaluation of the Decrease in Biocidal Activity in FS
A total of 80 mL of the native (non-pretreated) FS and a certain amount of biocide were added to 120 mL glass vials. Control vials did not contain biocides. In anaerobic experiments, the headspace was purged with argon before incubation. In aerobic experiments, vials were closed with cellulose plugs to avoid a lack of oxygen for aerobic microorganisms. Incubation was performed for 10 days on a thermostatically controlled shaker (110 rpm and 30 °C). The degree of degradation of the biocide was then assessed in a five-day biochemical oxygen demand (BOD5) test.
2.9. Analytical Methods
The pH was determined using an FE20 pH meter equipped with an InLab
® microelectrode (both Mettler Toledo, Switzerland). The oxygen and carbon dioxide concentrations in the headspace of the vials were determined by gas chromatograph Crystal 5000.2 (Chromatec, Yoshkar-Ola, Russia) as described earlier [
59]. BOD
5 was determined by the OxiTop respirometric BOD measuring system (WTW, Weilheim in Oberbayern, Germany), according to manufacturer’s recommendations.
2.10. Statistical Methods
All experiments were performed in triplicate. Statistical analysis was carried out using standard mathematical methods (Student’s t-test and calculation of the standard deviation) using the Microsoft Excel program. The data group was considered homogeneous if the mean square deviation σ did not exceed 10 percent. The differences between the data groups were considered valid under the probability criterion p < 0.05.
4. Discussion
Thus, the six biocides studied in this work had a strong antimicrobial effect on test microorganisms. Four substances selected for further studies: DBNPA, bronopol, Sharomix and sodium percarbonate were able to suppress the activity of aerobic and anaerobic microflora of the FS for 10 days, which is required for environmentally safe toilet complexes (ESTC) of Russian railway long-distance trains. The antimicrobial activity of biocides depended on the dosage. The use of the highest concentrations resulted in the death of part of the FS microbiota, as indicated by the absence of viable cells in the pretreated FS. However, lower concentrations were also feasible in suppressing the activity of the FS microbiota. Optimal concentrations of biocides were determined by comparing (1) the rate of decrease in O
2 consumption under aerobic conditions, (2) the rate of decrease in CO
2 production under anaerobic conditions, and (3) the number of viable cells under both aerobic and anaerobic conditions, with the “Latrina” biocide, which is currently used in ESTC (
Table 7,
Figure 7).
Application of “Latrina” at a working concentration of 700 mg/L reduced specific oxygen consumption rate by 27.7%, and at a concentration of 1400 mg/L—by 38.6%. The same or greater decrease in the specific rate of oxygen consumption was observed for new biocides at the following concentrations: DBNPA—200 mg/L; bronopol—30–60 mg/L; Sharomix—500 mg/L and sodium percarbonate—6000 mg/L (
Figure 7).
Under anaerobic conditions, to obtain results similar to the action of “Latrina” in reducing the specific rate of carbon dioxide production, DBNPA can be used at a concentration of 500 mg/l; bronopol 30 mg/L; Sharomix 500 mg/L and sodium percarbonate 1000 mg/L (
Figure 7).
The antimicrobial activity of the new biocides under aerobic conditions in all tested concentrations was stronger than that of “Latrina”: the cell number in the FS after 10 days of incubation was lower than 3.3 × 10
5 CFU/mL (
Figure 7). Viable microorganisms were not found at all in the presence of the highest concentrations of the new biocides. Under anaerobic conditions, the effect of new biocides on the reduction of cell numbers was also significant, in most cases surpassing the effect of “Latrina” (
Figure 7).
Despite their biocidal effectiveness, the active ingredients of “Latrina”, DDAC and PHMG, are very stubborn compounds and slowly decompose in the environment. Once discharged to wastewater treatment plants, they can become a major problem in activated sludge operation. Moreover, undegraded biocides discharged with treated water from WWTP can persist in water bodies for a long time and cause the emergence of biocide-resistant microorganisms, which can induce resistance to many different antimicrobial agents. The environmentally friendly biocides used in this study have unique properties. DBNPA, bronopol and Sharomix begin to decompose when the pH rises above 8.0. Decomposition of sodium percarbonate occurs at any pH over time. The pH of FS after 10 days of incubation has been shown to increase to 8.78–8.90 in control, and to 8.64–9.50 in biocide-treated FS (
Table 8). This enhances the decomposition of biocides. Using the BOD
5 test, it was shown that after 10 days of incubation, the biocides lose their antimicrobial properties against the FS microbiota. The values of BOD
5, indirectly reflecting the health of the microflora, for the biocide-pretreated FS samples (bronopol 30 mg/L and Sharomix 600 mg/L) were close to control (without the use of biocides). On the contrary, “Latrina” continued to exhibit antimicrobial activity. A decrease in the toxic effect of the studied biocides could also be judged from the change in the slope of the curves reflecting the dynamics of O
2 consumption in aerobic experiments and CO
2 production in anaerobic experiments. It was noted that after 7–9 days, the slope of these curves (mainly corresponding to low concentrations of biocides) had an upward trend, reflecting an increase in the rate of O
2 consumption or CO
2 release.
To assess the economic feasibility of using the proposed biocides for the treatment of FS and to select the most cost-effective ones, an assessment of the costs of their use was carried out. For this, the cost of a single treatment of one ESTC with a volume of 1000 L was calculated, taking into account the cost of biocides at the moment and their recommended dosages (
Table S1). The calculation did not take into account the cost of water, containers, dyes, flavors, surfactants, etc., which may be needed for the production of a biocidal preparation for its commercial use. Since Sharomix is a 1.5% aqueous solution of two Isothiazolones (Methylchloroisothiazolinone 1.1% and Methylisothiazolinone 0.4%),
Table S1 shows the calculations for its concentrate.
Bronopol and Sharomix appeared to be the least expensive, followed by DBNPA and sodium percarbonate. Further studies will be aimed at testing the action of biocides, in particular bronopol, Sharomix and DBNPA, under conditions close to real. It would also be interesting to find new means and methods of conservation of FS that bring the least harm to the environment.
The findings of this work could be of great ecological importance. To the best of our knowledge, this is the first study of its kind. Probably, due to commercial interest, such results may not be published. The works related to the development of methods to reduce the anthropogenic load on the ecology are very important. By using more gentle methods and means, we can maintain balance on our planet. If there is no possibility to refuse biocides, we should choose the ones that bring less harm. In this regard, the use of biocides with controlled degradation and low residual biocidal action for the temporary inhibition of microbial activity in fecal sludge can have great practical appeal and be environmentally friendly.
5. Conclusions
Quaternary ammonium compounds and guanidine derivatives are currently used in many chemical toilet additives, such as “Latrina”, to control microbial activity in FS. Despite their biocidal effectiveness, these substances are very persistent and slowly degrade in the environment. Thus, the disposal of such biocide-pretreated FS in WWTPs has the adverse effect of suppressing nitrification, denitrification and BOD processes. In this study, it is proposed to use other biocidal substances that are easily degraded after a certain time. Based on the literature data, five biocidal compounds were chosen as such substances: sodium salt of Dehydroacetic acid (DAN), bronopol, DBNPA, Sharomix, and silver citrate, which can rapidly decompose in an alkaline environment. Another substance, sodium percarbonate, naturally decomposes upon prolonged incubation in a liquid medium. Using MIC and MBC tests, it was shown that these substances have good antimicrobial activity. However, it turned out that it was necessary to use high concentrations of silver citrate and especially DAN, and given the high cost of these substances, they were excluded from further studies.
At pH 9.0, the MIC values of bronopol, DBNPA and Sharomix increased 1.5–4 times. This suggests that the change in pH caused partial inactivation (destruction) of the biocide in the medium, which can be used in practice to reduce the biocidal effect before discharging these substances into WWTPs. This is all the better because as urea contained in FS decomposes, pH rises from the initial 7.5 to 9.0–10.0. Additionally, experiments have shown that incubation of sodium percarbonate for 10 days leads to a decrease in its biocidal effect against test microorganisms by 1.25–1.50 times.
DBNPA, bronopol, Sharomix and sodium percarbonate were able to suppress the activity of the aerobic and anaerobic microflora of the FS for 10 days, which is required for Russian long-distance railway trains. Optimal biocide concentrations were determined based on comparison with “Latrina”: DBNPA—200 mg/L; bronopol—30–60 mg/L; Sharomix—500 mg/L and sodium percarbonate—6000 mg/L. The calculation of the estimate showed that the most cost-effective was the use of bronopol and Sharomix, then DBNPA and sodium percarbonate. Further research will be aimed at a detailed study of the application of the proposed biocides in real conditions. In addition, to reducing harm to nature, it is necessary to continue the search for new methods for controlling microbial activity in FS and similar waste.