Sustainable Treatment Techniques for Emerging Pollutants—The Case of Personal Hygiene Products

Abstract

Personal care products (PCPs) enter wastewater primarily through greywater. Treatment plants have not been able to remove this type of contaminant, although PCP abatement techniques have been developed in recent years. The objective of the current study has been to encounter the sustainable technique that keeps the optimal balance between the criteria considered in the comparison. Therefore, a bibliographic review was conducted in scientific databases of the last eight years, demonstrating that co-composting, anaerobic–aerobic sequencing bioreactors and contaminant absorption through the use of carbon nanotubes are the ones with the least environmental impact. Subsequently, the Saaty and Modified Saaty methods were applied, with a comparative criteria of construction costs, maintenance costs, efficiency and the stage of development. The results indicated that the co-composting technique is the best sustainable technique of those studied, with a score of 0.86/1, which means that the criteria analyzed maintain very close values between them. The co-composting technique yields a low environmental impact in eliminating personal care products. This research work constitutes a practical and easy-to-use tool for decision makers, since it allows finding an optimal elimination treatment for PCPs.

1. Introduction

Emerging pollutants (ECs) are pollutants that arise mainly from pharmaceuticals and personal care products (PPCP), which in addition to maintaining and improving the quality of daily life of people are used for the treatment or disease prevention in humans and animals which in addition to maintaining and improving the quality of daily life of people [1,2,3,4,5,6,7,8,9]. ECs are present in parts per billion or less in aquatic matrices, forming than a thousand toxic compounds to date [10,11,12,13,14]. PPCPs have recently been identified as a group of ECs with persistent and bioaccumulative characteristics [15,16,17] that are used with increasing frequency, making their presence in the environment ubiquitous [9,18,19,20,21,22,23,24].

There is a wide variety of chemical components used in the manufacture of sunscreens, lipsticks, shampoos, hair dyes, cosmetic products and more personal care products (PCPs) [25,26]. Due to the greater propensity of consumers to make purchases, the world market for PCPs has grown in the last decade, increasing by 15% in recent years [27]. The compounds discarded by the use of PCPs are not legislated or regulated since very little is known about their effects on the environment [28,29,30]. However, several studies have demonstrated that the main source through which PPCP contaminants are transported in the environment are wastewater treatment plants (WWTPs), since they are not designed to completely remove these compounds during treatment in plants themselves [12,31,32,33,34,35]. Thus, in the investigations conducted by Ahammad et al., Thomas et al., and López et al. [27,36,37], methodologies were applied for the remediation of PCPs in water and soil matrices where the removal of these contaminants was demonstrated in the leachates produced in the soil, in the residual waters with shampoo content and in the removal of heavy metals in aquatic matrices.

In order to determine the impact of these PCPs despite the fact that they are biodegradable substances, some parameters proposed by León et al., Ilyas et al., Zhao et al. and Ahmadkhaniha et al. are considered [38,39,40,41,42]. Therein, they appear as persistence due to their high resistance to photochemical, biological and chemical degradation, bioconcentration, as PCPs have a greater affinity to adipose tissues than to water and reach higher concentrations in them. On the other hand, with bioaccumulation, biomagnification toxicology, and mobility, they generate hazardous substances that potentially transform into more toxic substances [43,44,45,46,47].

Several studies have been conducted in the UK, US, Australia and Japan to better understand the occurrence, behavior and fate of PCPs in the environment [48]. PCPs are not completely and systematically removed during conventional wastewater treatment processes [49] and can often be encountered in surface water or groundwater through soil leaching, posing a potential risk to aquatic organisms and human health [50,51,52,53,54,55]. Cosmetic industries, as well as the manufacturers of PCPs, have used biological techniques for the treatment of wastewater for years, with the most frequently used technique being aerobic activated sludge (AS) [27]. This is followed by physicochemical techniques such as coagulation, flotation and electrocoagulation, the latter being capable of being combined with other biological techniques [56,57,58,59,60,61].

In 1980, Thomas L. Satty designed an Analysis Hierarchy Process (AHP) that is capable of solving complex multi-criteria problems. This quantitative method generates priority scales based on a preference scale, which allows the incorporation of judgments about intangibles into a decision model, representing the dominance or preference of one alternative over another [62,63]. In addition, it is designed with the purpose of supporting decision making and providing an understandable and rational framework to structure a problem in order to relate it to objectives and to evaluate alternative solutions. However, despite the fact that the AHP method helps in decision making, it can be inefficient when multiple qualitative criteria are presented. Therefore, to solve these limitations, using hybrid methods is proposed, such as the Fuzzy Hierarchical Analytical Process, which combines the benefits of AHP with fuzzy logic [64,65,66,67,68,69]. Fuzzy logic is a form of multivalued logic and is able to allow approximate conclusions. For this reason, linguistic variables are used in the definition of the sets [70,71,72,73,74,75,76]. Thus, for example, a linguistic variable such as cost could have values that are low, medium or high. In this manner, low values allow cost elements to be categorized in greater detail by assigning values that vary in membership within the set.

The objective of the present study is to provide the scientific and technical community with the best treatment to abate PCPs in wastewater that represents the least environmental impact. To this end, fuzzy logic methodologies were used that would also constitute tools for daily use, which are easy and fast, for the community, authorities and any other social actor that needs to make decisions.

2. Materials and Methods

Within the framework of this research, various bibliographical compilations were realized in different high-impact scientific databases such as Scopus, SpringerOpen, SciELO and Science Direct, with a publication date of no less than the last ten years, in order to obtain truthful, meaningful and up-to-date information regarding the sustainable removal treatments of PCPs from wastewater. Subsequently, through the modified Saaty methodology, a comparison of the most relevant parameters of each treatment was generated, such as installation costs, efficiency, development stage and maintenance costs, in order to yield the most appropriate, profitable and reliable technique viable in the abatement of PCPs.

Although PCPs are safe for human use, the scarcity of environmental evaluations and tests before they are placed on sale means that there is a lack of regulations that indicate their impact on the environment [77,78,79].

Table 1. List of most common PCPs.

PCPs	Toxic Components	Environmental Impact
Feminine Sanitary Napkins	Aldehydes, Alkanes, Halohydrocarbons, Terpenes, Ketones, PAHs	They generate toxic and mutagenic waste, produce cancer at the time of postuse exposure [80].
Deodorants	Heavy metals	They are bioaccumulative, cause several diseases [81].
Disposable Masks	Microplastics	When receiving UV rays, they release chemical substances that cause adverse effects on the marine ecosystem [82].
Condoms	Parabens (Methylparaben, Propylparaben and Octylparaben)	Endocrine disturbances. Both estrogenic and antiandrogenic effects [83].
Fragrances	Musk Xylene, Musk Ketone, Galaxolide	Musk causes endocrine disruption in aquatic biota and galaxolide is bioaccumulative [84,85].
Soap, shampoo, detergent, toothpaste, sunscreen and deodorant	Triclosan (TCS, 5-chloro-2-(2,4-dichlorophenoxy)phenol)	Alters the composition of benthic bacteria. Toxic for algae species, it presents endocrine disruption in fish [86].
Sunscreen	Octocrylene Oxybenzone	Benzophenones cause DNA alteration, bleaching and death of corals and reefs [86].
Hair dyes	Triclosan (TCS, 5-chloro-2-(2,4-dichlorophenoxy)phenol)	Dye residues accumulate on the seafloor, entering the food chain [86].

displays a list of some personal care products together with their toxic components and the impact that these causes on the environment.

Tests performed with laboratory animals have yielded that exposure to these and other contaminants from PCPs can cause endocrine disruptors, abnormalities in babies and damage to the development of the brain, reproductive systems and thyroid, among others [87].

Modified Saaty Method

The modified Saaty methodology is a decision theory method that combines quantitative and qualitative criteria. It compares the different choices made by policy makers for each criterion, comparing the criterion with each choice in a similar way. To accomplish this, it uses a preference table with values ranging from 1 (nothing preferred) to 9 (very preferred) [30]. The use of the sine and cosine functions are usually used in the modified Saaty method in order to observed the behavior of the variable, as is demonstrated in Equations (1) and (2):

$${\mathsf{\mu }}_{\mathrm{A}}\left({\mathrm{V}}_{\mathrm{o}}\right)=\cos \left(\frac{\mathsf{\pi }}{2}\times \frac{{\mathrm{V}}_{\mathrm{o}}-{\mathrm{V}}_{\min }}{{\mathrm{V}}_{\mathrm{m}á\mathrm{x}}-{\mathrm{V}}_{\min }}\right)0\le {\mathsf{\mu }}_{\mathrm{A}}\left({\mathrm{V}}_{\mathrm{o}}\right)\le 1$$

(1)

$${\mathsf{\mu }}_{\mathrm{A}}\left({\mathrm{V}}_{\mathrm{o}}\right)=\sin \left(\frac{\mathsf{\pi }}{2}\times \frac{{\mathrm{V}}_{\mathrm{o}}-{\mathrm{V}}_{\min }}{{\mathrm{V}}_{\mathrm{m}á\mathrm{x}}-{\mathrm{V}}_{\min }}\right)0\le {\mathsf{\mu }}_{\mathrm{A}}\left({\mathrm{V}}_{\mathrm{o}}\right)\le 1$$

(2)

where μ_A (V_o) is the behavior of the variable, A is the criterion or variable that is represented, (V_o) is the original value, (V_min) is the minimum value and (V_máx) is the maximum value.

Equation (1) is used when the value (V) of the variable is inversely proportional to the evaluation variable, while Equation (2) is used when there is a directly proportional relationship between (V) and the evaluation variable. For the values obtained by applying Equation (1), one must be subtracted from the calculated value so that the results obtained in both equations are normalized. The rating range of the criteria or variables ranges from 0 to 1, with one being the highest rating [88]. When the criterion is closer to 1, this means that it better approaches the stated objective of sustainability.

In the present investigation, the diffuse Saaty method was applied individually in each treatment technique. By evaluating the criteria (A) of maintenance cost, construction cost, efficiency and development stage, qualitative variables were assigned a quantitative value (weighting) on a scale of 1 to 9 to calculate the initial value (V_o) of the weighting assigned to each variable, and Equation (1) or Equation (2) is applied depending on the proportionality of each variable. AHP was used to provide a weight to each variable, first providing them with values between 1 and 9 according to the degree of importance, to finally multiply the weights obtained in the AHP with the value of μ_A (V_o) of their respective criteria, and the calculated values of the four variables are then added [88].

3. Results

Three studies are presented in which different representative sustainable treatment techniques were applied for the removal and/or elimination of contaminants from PCPs.

3.1. Technique 1: Co-Composting

Co-composting in several investigations demonstrates the efficiency of PCP elimination. Taking as reference the investigation performed by Hiba et al. [87] in an introductory manner, it is defined that PCPs in different wastewater treatments cannot be eliminated completely. They determined that, by applying co-composting methodology in a wastewater treatment plant, high rates of triclosan (TCS) degradation were achieved, and these were mixed with sawdust in two different proportions 1:2 and 1:3, as well as collected concentrations of TCS of 2 mg/kg in relation to the dry weight of the residues. The results of the current study indicate how contaminant reduction was more complete in the compost samples where a mixture ratio of 1:3 was used and an 81% reduction was reduction, while in the 1:2 mixture, they obtained 55% [89].

Once the potentiality of the method was defined, it was taken as a reference, the research realized by Thomas et al. [36] involved a methodology used for the removal of triclosan and carbamazepine, which consisted of the application of co-composting in a container. The pollutants used were obtained from the wastewater discharge point of the WWTP in Nesapakkam, Chennai, and Tamil Nadu. Mixed organic waste was collected from the canteen of the IIT Madras hostel, Chennai. Cow manure and coconut fiber marrow were used for composting, and they were obtained from a town near the study region. The experiment was performed using systems in vertical vessels with a specific diameter of 320 mm, a height of 450 mm and an effective volume capacity of 30 L, as demonstrated in Figure 1.

The first figure represents the different experiments that were conducted in this co-composting process, where TCS represents the contaminants of the personal care products that were used and CBZ represents the contaminants of the pharmaceutical products. In the investigation, a mixture of a single TCS and CBZ and of multiple TCS and CBZ was used to determine the aforementioned contaminant load. The average levels of leachate found in the dump were observed on day 0 and day 20, with their respective average concentrations of 5, 50 and 100 ppm. In this study, tests were conducted in order to be able to evaluate and identify the degradation processes of one and multiple contaminants. They used an average of contaminant concentrations in ppm (mg/kg) in the order of 5, 50 and 100 ppm of contaminant in dry weight so that the impact of the load of these contaminants in the container where they are stored can be observed. They also used control reactors (without contaminants) as a reference.

The leachates generated on the floor of the container were passed through a cartridge, with a speed of 1 mL/min. The withdrawn extracts were reconstituted with 1 mL of methanol after drying under a stream of nitrogen. In order to determine the concentrations of PCPs in the leachates, Milli Q water and acetonitrile were used in a ratio of 20:80 in order to later pass them through the spectrophotometer at a wavelength of 280 nm. All leachate samples of the three concentrations presented in the study were centrifuged and filtered, and the supernatants were subjected to solid phase extraction.

This methodology was able to demonstrate how the reduction in one and multiple TCS and CBZ contaminants occurred. Taking as a reference the data obtained from Table 1 by Thomas et al. [36], the results of the experiment in three types of cases were indicated, including the Average Concentration of Micropollutants, Load of Micropollutants and Reduction in Micropollutants by Weight. Although the study realized by Thomas et al. was conducted in a laboratory, this technique has been commercialized for several years due to the ease of its implementation and its low cost of construction and maintenance [90]. The results demonstrated that the co-composting method had an efficiency of 86.4% in the removal of a single contaminant and 83.7% in the removal of multiple contaminants at a concentration of 5 ppm of contaminant per dry weight. For the case of 50 ppm, the removal efficiency of a single contaminant was 68.2% and 22.6% for multiple contaminants. Finally, for 100 ppm, the lowest removal efficiency was determined for one and multiple contaminants, with results of 5% and 12.8%, respectively.

3.2. Technique 2: Combined Anaerobic–Aerobic Method

The main objective of implementing a combined system of anaerobic–aerobic reactors was to reduce energy consumption and reduce the cost of maintenance and biomass production in addition to achieving high efficiency for the elimination of organic matter and lowering the specific production of excess of sludge. Hereby, the studies conducted by Ahammad et al. [25] identified that the remediation method used in most PCPs industries was the aerobic method treated with activated sludge. However, the application of this technique presents a high energy demand and a high cost of construction and maintenance. Therefore, the application of anaerobic–aerobic sequencing bioreactors was presented as a solution, which allow the improvement in energy efficiency in the treatment of waste in the PCP industry and, therefore, contribute to a reduction in maintenance costs.

According to Gašpariková et al. [91] this methodology was used because anaerobic systems represent a sustainable technology with low operation and maintenance costs for industrial wastewater treatment. Despite this, Ahammad et al. [25] conducted an evaluation of seven treatment systems where energy and performance terms were implemented for the treatments caused by the residues of the shampoo. Aerobic methodologies were included, where anaerobic and anaerobic–aerobic reactor designs obtained decisive results, which demonstrated a fairly high removal of chemical oxygen demand (COD) in anaerobic–aerobic systems. The results of these combined reactors presented a removal of the contaminant of 87.9 ± 0.4% and 86.8 ± 0.5% and, in turn, used 69.2% and 62.5% less energy than the use of a single process, regardless of whether it was anaerobic or aerobic. The results obtained in the research for the treatment of shampoo waste are promising within the combination of these anaerobic–aerobic sequence reactors [27]. Therefore, the use of an anaerobic pretreatment system and aerobic systems for post-treatment was proposed. In China, according to Zhang et al. [90], this methodology has allowed the development of the use of a combined system of anaerobic baffle reactor (ABR) and upflow aerated biological filter (UBAF) for the treatment of wastewater from cosmetic industries. The investigation highlights the efficiency of COD removal, mainly in the front compartments of the ABR reactor, where the effluent COD load was set at 1.5 g COD/L.d and reached a COD removal efficiency that reached a maximum in forward compartment number 1. The highest COD removal efficiency was reached with 2.0 g COD/L.d for the complete ABR reactor. Under the optimal experimental conditions mentioned, the COD removal efficiency of UBAF was 69.5–82.6% and after the combined treatment of ABR and UBAF, the cosmetic wastewater effluent effectively complied with the discharge standard in China [92].

Figure 2 illustrates the process of aerobic and anaerobic reactors used during the investigation. The AnCSTR reactor had a capacity of 5 L, while HUASB and AHR units had 1.5 L. In addition, six suspended cell aerobic reactors were used, where the AR1, AR2 and AR3 units received effluents from the AHR reactors, HUASB and AnCSTR, as indicated in sections (a)–(c), respectively. In section (c) of Figure 2, the remaining units AR4, AR5 and AR6 that were used in the aerobic system with a volume capacity of 1.2 L, 1.5 L and 3 L were observed. Parameters such as Dissolved Oxygen (DO), Chemical Oxygen Demand (COD), Total Suspended Solids (TSS), Volatile Suspended Solids (VSS) and Total Carbon (TC) were used to verify the performance of each reactor. The microbial conditions of the anaerobic units were also characterized, collecting replicate samples at the beginning and end of the reactor operations.

In order to determine energy consumption, the six operating combinations were evaluated in relation to the use of energy in the aeration processes based on the height of the liquid and air flow. On the contrary, mixing and pumping were realized based on of the power ratings of the centrifugal pumps. The results of the comparison of the removal rates of COD and energy that were used in the bioreactor systems are presented in Figure 2, which corresponds to the performance of each reactor in the study performed [25].

Based on the laboratory study performed by Ahammad et al. [27], it was indicated that aerobic reactors needed eight times more energy per COD removed than the combined reactors, coinciding with what was indicated in the study conducted by Popli and Upendra [93] in 2015. There, it was mentioned that the use of combined systems is a profitable process and is ecological for the degradation of hair dyes compared to traditional methods, suggesting that the use of combined anaerobic–aerobic treatment systems is an optimal option for the treatment of wastewater with PCPs that contain shampoo residues [94].

The results of the combined anaerobic–aerobic reactor method in the investigation showed that the overall COD removal rates were higher for the HUASB-aerobic and AHR-aerobic treatment combinations with a COD removal efficiency of 88% and 87%, respectively. Regarding energy savings, the results indicated that the treatment design with the highest energy efficiency and for COD eliminated was the combined HUASB-aerobic system, since it used almost 70% less energy than the aerobic treatment systems that are normally used in the PCP industry. The AHR-aerobic combination also presented good results with an energy reduction of 62.5%.

3.3. Technique 3: Adsorption Method

The adsorption method is the most frequently used physical process to remove trace amounts of EC from PCPs. There are different adsorption methods such as adsorbents based on activated carbon, graphene and nanotubes [85]. In the recent advances in nanotechnological sciences, Yit Thai Ong et al. [92], in his review article demonstrated the contribution of carbon nanotubes (CNT) from the perspective of sustainable environment and green technologies. It is implemented in wastewater treatment, air pollution control, biotechnologies, renewable energy technologies, supercapacitors and green nanocomposites. Carbon nanotubes for large-scale application were also discussed from the point of view of cost and potential hazards [94]. Although the installation costs of adsorption methods are relatively low and have highly effective results, the cost of manufacturing carbon nanotubes is high [87]. CNTs have a structure similar to a sheet of graphene rolled up on itself. Depending on the number of layers, they are divided into two types of CNTs: those with single walls (SWCNTs) and those with multiple walls (MWCNTs), as illustrated in Figure 3 [37].

In a study conducted by López et al. [37], the absorption capacity of heavy metals was tested by using different types of CNTs, including commercial, multi-walled with different degrees of purity and others fused with carboxylic groups. For his study, López et al. [37] prepared aqueous solutions of In³, Cu², Co², Ni² and Zn² with a metal concentration of 0.01 g/L. The solution was treated with 0.1 g of CNTs at a stirring speed of 1500 rpm at a constant temperature of 20 °C and with a pH of 4. The solutions were obtained by an aqueous dissolution of different soluble salts. In order to determine the metal content in the solution after the interaction with CNTs, it was initially expected that the contact time would be fulfilled and that the suspensions of CNTs would be filtered. Later, by means of atomic absorption spectrometry, the percentage of remaining metals would be calculated in aqueous solutions. Furthermore, while ascertaining the preference of absorption in metals possessed by CNT, López et al. [37] prepared a multielement aqueous solution of pH = 4 containing 0.01 g/L of elements In³, Cu², Co², Ni² and Zn², treating the solution with 0.05 g of CNTs and stirring at 1500 rpm for 30 and 60 min. After the solution was filtered as with previous solutions, the remaining metal content in the solution was determined.

The results obtained in the multielement sample of 0.05 g and the individual sample of 0.1 g showed that the heavy metal that was mostly absorbed by the CNT was In³ with an absorption percentage greater than 99%, followed by Cu² with 75% and Zn² by 44%. However, for Co² and Ni², the percentage of removal varied, and it is mostly absorbed in the individual sample of 0.1 g in 40% and 34%, respectively, while for the multi-element sample of 0.05 g, the percentages of absorption of Co² were 36 % and 22% Ni².

Factors such as temperature and pH López et al. [37] proved to be of great importance since CNTs had a greater absorption capacity in alkaline media, although they are also capable of absorption in acid media, only with less efficiency. Finally, López et al. [37] acknowledged that more research is still needed regarding the application of these nanotubes, since aspects such as the type of nanotubes, model, dimensions and versions also had a considerable influence on the absorption capacity of heavy elements; thus, this technique is still in the testing phase in the laboratory.

It is also necessary to mention that, according to López et al. [37], the metals absorbed by CNTs can be recovered by elution in an acid medium, thus providing new use to the absorbed metals and allowing the reuse of CNTs. In the study by Gui et al. [95], it was mentioned that magnetic carbon nanotube sponges have 99% volume sorption capacity and are able to maintain their original structure, high capacity and selectivity even after 1000 sorption cycles and recovery.

The results show the CNT adsorption method in the removal of heavy metals contained in PCPs; the results show that the “speed” at which these elements are adsorbed will depend on the metal in question. Metals such as Co (II), Cr (III), Cd (II) and Ni (II) in the multi-element sample, were absorbed by 36%, 52%, 45% and 22%, respectively, which is a lower absorption amount than that obtained in the individual samples of 40%, 75%, 59% and 34%, respectively. They have been exempted from this case by In (III), Cu (III) and Zn (II), in which the percentage of adsorption was 99%, 75% and 44%, respectively, in both samples.

3.4. Modified Saaty Analysis

For the analysis of the evaluation criteria, the AHP hierarchical analysis method developed by Saaty was applied, for which the order of importance was assigned according to a panel of experts with the following values: maintenance cost (MC) 9, construction cost (CC) 7, efficiency (Ef) 5 and stage of development (ED) 3.

Table 2. Results based on the hierarchical analysis method (AHP).

Criteria	CM	CC	Ef	ED	Ci	Wi	${\lambda }$
CM	1.00	1.29	1.80	3.00	1.62	0.38	1.00
CC	0.78	1.00	1.40	2.33	1.26	0.29	1.00
Ef	0.56	0.71	1.00	1.67	0.90	0.21	1.00
ED	0.33	0.43	0.60	1.00	0.54	0.13	1.00
Summation	2.67	3.43	4.80	8.00	4.33	1.00	4.00

lists the development of AHP, where Wi is the weight that each criterion will have when formulating the equation and the sum of lambda (λ) is the number of criteria compared in the matrix.

After knowing the weight that each criterion has, we proceeded to formulate Equation (3).

$$\mathrm{Technique}=0.38\times \left(\mathrm{CM}\right)+0.29\times \left(\mathrm{CC}\right)+0.21\times \left(\mathrm{Ef}\right)+0.13\times \left(\mathrm{ED}\right)$$

(3)

In order to evaluate the behavior of each criterion, we proceeded with the application of the modified Saaty method. For example, for the efficiency variable, the values of 1 for bad, 3 for regular, 5 for good, 7 for very good and 9 for excellent were assigned. After providing it with weighting, the weight of these subcriteria was calculated. In the case of the co-composting technique, the efficiency criterion was “very good”, and it obtained a high value, with a V_o of 0.28, V_min of 0.04 and V_max of 0.36. Equation (2) was applied, as it had a direct relationship between the analysis variable and the study method. That is, the greater the efficiency, the greater the benefit for the application of the technique.

$${\mathsf{\mu }}_{\mathrm{Efficiency}}\left({\mathrm{V}}_{\mathrm{o}}\right)=\sin \left(\frac{\mathsf{\pi }}{2}\times \frac{0.28-0.04}{0.36-0.04}\right)=0.92$$

In the case of the maintenance cost variable, the assigned values were 9 for low, 5 for medium and 1 for high. The value obtained from the maintenance cost variable was for the co-composting technique, with “low” possessing quantitative values V_o = 0.60, V_min = 0.07 and V_max = 0.60. In this case, the relationship between the analysis variable and the study method is inversely proportional. The higher the cost to be paid for maintenance, the lower the value of the variable; thus, Equation (1) is applied.

$${\mathsf{\mu }}_{\mathrm{Cost}.\mathrm{Maintenance}}\left({\mathrm{V}}_{\mathrm{o}}\right)=\cos \left(\frac{\mathsf{\pi }}{2}\times \frac{0.60-0.07}{0.60-0.07}\right)=0$$

The calculations detailed above were realized with the two remaining variables (construction costs and development stage) for each treatment technique. Once all the results were obtained, they were normalized so that one minus the calculated value was subtracted from the values that were calculated using Equation (1). In this manner, the four criteria remained in the same proportionality, as shown in

Table 3. Behavior of the criterion using modified Satty.

Technique	Criterion
	Inversely Proportional				Directly Proportional
	Calculated		Normalized		Calculated
	CM	CC	CM	CC	EF	ED
Co-Composting	0	0	1	1	0.924	0
Anaerobic-aerobic	0.707	1	0.293	0	0.924	0
CNT	0	1	1	0	1	0

.

Table 4. Comparison of the viability percentages of the techniques studied.

Technique	=	$0.375\times \mathbf{CM}$	+	$0.292\times \mathbf{CC}$	+	$0.208\times \mathbf{Ef}$	+	$0.125\times \mathbf{ED}$	=	Results
Co-composting	=	0.3750	+	0.2917	+	0.1925	+	0.000	=	0.86
Anaerobic–Aerobic	=	0.0855	+	0.0000	+	0.1155	+	0.000	=	0.20
CNT	=	0.2083	+	0.0000	+	0.0000	+	0.000	=	0.21

lists the values achieved by each technique after multiplying the behavior of the variable or criterion with the weights obtained in the AHP and applying Equation (3).

4. Discussion

Table 2 indicates the weights obtained by the four criteria evaluated, with maintenance costs having the highest weight with 0.38, followed by construction cost with 0.29, efficiency with 0.21 and the development stage with 0.13, with the latter being the lower weight. The weight of each criterion allowed the generation of priority scales based on the authors’ judgments made with comparisons using a preference scale.

When applying modified Satty for each of the four criteria and normalizing the values obtained, from the three techniques individually, the maintenance cost (MC) was 1 for both the co-composting technique and the CNT absorption technique, while that for the anaerobic–aerobic technique was 0.293. In the construction cost criterion (CC), the technique of combined anaerobic–aerobic reactors and CNT absorption obtained a value of 0. On the other hand, the co-composting technique had a value of 1. For the efficiency factor, the co-composting and anaerobic–aerobic techniques obtained a value of 0.924, and the CNT adsorption method obtained a value 1, as evidenced in Table 3.

Regarding the development stage criterion, the three techniques had a zero rating, as the three experiments were conducted in the laboratory; thus, its value is negligible. However, it is fundamental to mention that the co-composting technique in other studies is already in the operation phase and has been marketed for several years [90].

In Table 4, the results determined that the most profitable technique was co-composting with a score of 0.86/1; therefore, it can be interpreted that it is the most sustainable technique that maintains an optimal balance between the criteria considered in the comparison of techniques for the reduction in PCPs. It is a technique with great potential to reduce the concentration of multiple contaminants such as triclosan, a compound commonly used in PCPs (soaps, creams, toothpastes and detergents) [96]. However, in several case studies, it has been shown that the co-composting technique does not help in completely degrading the concentration of TCS, since it occurs only partially during the composting period of one month, indicating that longer periods are needed for the most complete removal of these compounds [96]. In the same manner, there are cases in the co-composting process where the transformation of triclosan into methyltriclosan is promoted, a compound that is extremely persistent to greater biodegradation and is strongly absorbed in solids that have a potential for accumulation in the soil [97].

Most of the PCPs residues are in a liquid state, forming what is known as leachate; thus, the co-composting method is not very suitable for the elimination of these contaminants, since it has a greater efficiency of treatment of solid waste. Despite this, the co-composting method has been well received by the reuse that can be given in the elimination of contaminants due to its sustainable viability and its easy accessibility. On the other hand, the anaerobic–aerobic method is an excellent option for the elimination of contaminants from PCPs present in aquatic biota, since it converts matter into simpler products through biological processes when working with activated sludge. On the other hand, the technique that least meets the balance between the four criteria analyzed is the combined anaerobic–aerobic method, with a score of 0.20/1. In this case, two of the four criteria evaluated had a zero value. Therefore, its only weight was the maintenance cost and efficiency criteria, while the carbon nanotube absorption method had a weight of 0.21/1, which is the construction cost value that can influence a reduction in its weighting due to the high value required for the manufacture of the nanotubes and the weight that this variable had in the Saaty calculation, which was 0.

5. Conclusions

The current study encountered, in a bibliographical manner, three sustainable techniques for the abatement of personal care products, which were anaerobic–aerobic reactors, adsorption treatments and co-composting. It was determined that the co-composting technique has a lower environmental impact. This is verified, by applying the modified AHP and Saaty methodology, when the treatment reached a total of 0.86/1. That is, the analyzed variables had a good relationship with each other. For example, efficiency is 86.4%, the maintenance cost is low, the construction cost is low and it is implemented on a large scale, which allows it to be defined as a sustainable abatement technique. It is fundamental to note that this method has the ability to reduce the concentrations of multiple contaminants such as triclosan, a compound commonly used in PCPs. In the same manner, due to the ease of its implementation and low cost, this technique has been marketed for several years.

The application of the AHP and modified Saaty techniques allow readers to clearly identify co-composting as a sustainable technique and that should be disclosed to the community about its existence, suitability and easy application.

The authorities and other actors in society must be aware that techniques for the abatement of contaminants that are sustainable must be researched. This research work provides a practical, easy-to-use and technical tool to choose a PCP abatement treatment.