1 Introduction

Wastewater treatment requires continuous monitoring of pollutants, therefore, analyses criteria for this type of water have been established, i.e., temperature, fats and oils, sedimentable solids, pH, conductivity, inorganic ions concentration, etc. Organic matter determination represents one of the most significant challenges since it includes all carbonaceous compounds and some other elements, such as hydrogen, oxygen, and nitrogen. Thus, determining their presence and concentration requires a different kind of analysis, traditionally measured by two tests, Chemical Oxygen Demand (COD) (Federation W. E., & APH Association [26] – 5220) and Biochemical Oxygen Demand (BOD5) (Federation W. E., & APH Association [26] – 5210). Nevertheless, currently the Total Organic Carbon (TOC) (Federation W. E., & APH Association [26] – 5310) analysis is being extensively more used than the other two [93].

COD is the standard analysis for measuring oxygen consumption during the decomposition of organic and oxidizing of inorganic matter, through oxidizing agents such as potassium dichromate in a short time (equivalent to hours). On the other hand, BOD5 is the standard method for determining the concentration of oxygen demand required by microorganisms to decompose organic matter, which occurs in a minimum period of 5 days [71]. TOC is considered a potential alternative for COD and BOD, as it is faster, more precise, sensitive, and environmentally friendly [21].

Several authors have extensively studied the COD, BOD5, and TOC analyses in an attempt to improve standard protocols (Visco et al. [27, 39, 78] since they present some disadvantages such as the use of toxic reagents, waiting time, or the sensitivity of the response; hence, these analyses have been modified over time with the objective of optimizing them, and although those changes have not managed to replace the standardized methods, they may be considered as a possibility for ascertaining these parameters, so it is convenient to analyze the way they have evolved through time.

2 Relationship between the COD, BOD5, and TOC Parameters to Determine Organic Matter In Water

COD and BOD5 are routine analyses for wastewater. Respectively, each of them is the most accurate quantification of the amount of organic matter that can be oxidized utilizing chemical or biological processes. These techniques are closely related, and it is highly recommendable to use them together. The values obtained by COD are generally higher than those obtained by BOD5 and depend on the type of water analyzed. The BOD5/ COD ratio must be smaller or equal to 1.0. However, this proportion is only an indicator of the fraction of biodegradable organic matter present in wastewater [50, 71, 93].

Similarly, it is also possible to calculate BOD5/TOC and TOC/COD. For example, Dubber and Gray [21] show that the BOD5/TOC relationship for the influent is linear, this proportion was proposed initially by Eckenfelder [23], and it shows a decrease during water treatment that is explained by the generation of recalcitrant residues. Therefore, it is recommended to have at least the value of BOD5 before performing a TOC analysis [5, 17], Lee et al. [49].

3 Chemical oxygen demand (COD)

Water pollution is a problem that affects society. Water treatment processes require covering some analytical parameters that allow finding out the initial and final degree of contamination. Of all the possible pollutants, organic matter is considered necessary, and this is why there are several techniques to find out the amount of organic matter present in water. Chemical Oxygen Demand (COD) is one of the fastest tests available to monitor water quality during treatment [9, 20].

COD represents the degree of contamination in water bodies due to organic matter. It is defined as the oxygen measure equivalent to the content of organic matter present in a sample that strong chemical oxidants, potassium permanganate or potassium dichromate, can oxidize. Each of these oxidants is used to calculate COD in different types of water: CODMn (COD Manganese) is utilized in determinations for slightly contaminated bodies of water. In contrast, CODCr (COD Chrome) is used in determinations of heavily contaminated water or wastewater due to the high oxidizing power of the reagent [52, 62].

3.1 Standardized method

In the standardized method described in International Standards, ISO 6060-[34] and ISO 15705–2002, organic matter (i.e., potassium biphthalate) is oxidized in the presence of potassium dichromate and sulfuric acid, in 2-h reflux at a temperature of 150 °C (open or closed). HgSO4 are added to eliminate the presence of chloride ions and silver salts to catalyze the reaction.

In the "open reflux" procedure, the excess of dichromate can be measured by titrating the \({K}_{2}{Cr}_{2}{O}_{7}\) that was not reduced, using a solution of ferrous ammonium sulfate (Mohr's salt), with ferroin as an indicator [27, 52].

According to Li et al. [52], the initial reaction involves biphthalate, which reacts with dichromate:

$${2KC}_{8}{H}_{5}{O}_{4} + {10K}_{2}{Cr}_{2}{O}_{7} + {41H}_{2}{SO}_{4} \to {16CO}_{2} + {46H}_{2}O + {10Cr}_{2}({SO}_{4}{)}_{3} + {11K}_{2}{SO}_{4}$$
(1)

After this reaction, dichromate (\({K}_{2}{Cr}_{2}{O}_{7}\)) is replaced by oxygen (\({O}_{2}\)). After digestion (reflux), the excess of non-reduced dichromate is titrated with an iron solution (\({Fe}^{2+}\)), according to the following reaction:

$${Cr}_{2}{O}_{7}^{2-} + {6Fe}^{2+} + {14H}^{+} \to {2Cr}^{3+} + {6Fe}^{3+} + {7H}_{2}O$$
(2)

In the "closed reflux" procedure, the excess of dichromate is detected via spectrophotometric methods at a wavelength of 420 nm or the increase of trivalent chromium ions (Cr(III)) at a wavelength of 600 nm [59].

The methods above are reproducible and reliable, though they use expensive, highly corrosive, highly toxic reagents. In addition, using them involves other problems such as the interference and incomplete oxidation of volatile compounds. Therefore, it has been necessary to consider other alternatives for the modification of this technique. These new approaches are more environmentally friendly, and in some cases, they have reduced digestion time.

3.2 Modifications to the standardized method

Initially, the COD technique was improved in processes such as scaling [11]. Besides, the cost could be lowered, and the importance of adding silver salts (or not) was discussed [11, 19]. Moreover, the characteristics of COD allow us to divide it into different stages, all of which can be modified. That is the case of the elimination of interferences, other oxidation or digestion methods, the elimination of mercury, or the measurement of the response. Adding, new novel alternatives have emerged, e.g., the use of thermal biosensors [27, 52, 59].

Table 1 summarizes some papers related to the modifications to the standardized method reported since 2010. The modification kind was divided into a) standardized method, b) standardized method: oxidation processes, c) standardized method: detection methods, and d) new techniques.

Table 1 Techniques used as alternatives for COD
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The standardized method (5220D) uses the dichromate (Federation W. E., & APH Association [26] as oxidizing agent, which decomposes the greatest amount of organic matter, so the reaction is accelerated and interferences are avoided. A catalyst (Ag +) and HgSO4 should be used together with the dichromate. Digestion is carried out by refluxing (150 °C). The excess of dichromate can be determined by titration or spectrophotometry, and using mercury salts to eliminate the interference caused by chlorine is needed. Dichromate is considered as the best oxidizing agent and the prepared solution is stable for a long time, the major problem that shows is the generation of highly toxic waste [12, 59].

Standard method scaling utilizes the same highly toxic reagents; nevertheless, a diminishing of volumes of reagent needed and the processing time of standardized technique allow minimizing the impact of waste generation. Also, costs are reduced while obtaining statistically comparable results.

However, the use of the same highly toxic reagents is a disadvantage for this technique, so some researchers have preferred to modify the standardized method using less toxic oxidizing reagents, such as the permanganate, which is used to obtain a rapid and generalized approximation of the contamination level of water discharge [33, 86].

The amount of organic matter present in the sample is calculated by the time required for the permanganate ion to reduce, generating manganese dioxide. This compound catalyzes the decomposition of permanganate, thus increasing its consumption of organic matter. The process is carried out in hot acidic solutions but may not oxidize all organic matter, even requiring large volumes of sulfuric acid (E et al. [22]; Kolb et al. [45]).

As the drawback is using a strong oxidant coupled with toxic reagents, modifications to the standard technique begin to use advanced oxidation processes like photocatalysis and Fenton reaction that promote hydroxyl radicals (●OH) are generated, able to accelerate the decomposition of persistent (Li et al., [51]. Similar strategies can be used (UV/H2O2/Fe2+), UV/H2O2/ultrasonic energy, UV/TiO2, ZnO, ZnS, CdS WO3, CeO2, ZrO2, O3/UV, and, O3/H2O2/UV [2, 7, 12, 24, 52]. These techniques are more environmentally friendly, that avoids the use of toxic reagents. Nevertheless, the disadvantage of this kind of modification is that an approximation of the pollutant’s concentration is required to identify the appropriate parameters, to name a few: exposure time to light and peroxide concentration [48, 61].

In addition to modifying the reagents of the original technique, other authors have preferred to work the detection method directly or indirectly by measuring the portion of organic matter in water with specific sensors, approaching the COD. Some of these detection methods include chemiluminescence (CL) and fluorescence; the first one is an environmentally friendly technique than when using it, Cr3+ is produced during COD reagents reaction, which is measured in the presence of a luminol-H2O2 mixture. The luminol-H2O2-Cr3+ reaction is measured with a photodiode detector developed to allow its instrumentation to be simple and monitoring can be online [30, 57, 75, 90].

On the other hand, fluorescence uses fluorescent chemical elements, such as Cerium (III) (Ce3+). Its fluorescence intensity changes in response to the different concentrations of organic matter in the solution. As a result, the concentration of Ce3+ can indirectly determine COD. Nevertheless, it requires expensive equipment and reagents and is suitable only for slightly polluted waters (Li et al. [10, 51].

Other options for determinate COD are the electrochemical sensors, photocatalytic sensors, and the photo chemiresistor sensor. Wang et al. [80] proposed a highly sensitive electro photocatalytic detection method to determine COD in surface and lightly polluted water. The photogenerated electrons react with organic matter in a redox-type reaction. Also, an electrode is irradiated with UV light to generate electrons that will be transferred to a working electrode that produces an analytical signal, which may quantify COD values. For some authors, the disadvantages of this detection are the small specific surface areas and limited use of UV light to generate wide bands [63, 72, 82, 83].

Other techniques like thermal biosensors use sensors to monitor COD online at long-term in water samples. The biosensor measures the amount of heat generated during the oxidation of organic matter. The samples cross through a column with H3IO6 and generate a thermal signal to determine COD. These robust systems have a higher tolerance to interference, but oxidation may be incomplete, limiting its potential application [91, 92].

None of the modified techniques presented in Table 1 has been able to displace the standardized procedure; despite being cheaper or more sensitive, they do not offer the robustness of the standardized technique, which hinders its wide use in different types of wastewaters. These disadvantages are mainly the low capacity to achieve all the oxidation of organic matter as dichromate does. As a result, the elimination of chlorine interference in the samples is only accomplished using dichromate, mercury, and silver reagents [27, 52]. In the case of advanced oxidation processes, the diminishing or substitution of toxic compounds has been attained,however, larger amounts of reagents and prior knowledge of the possible contaminants present in the sample are required (Li et al. [7, 12, 51],Bri et al. [10, 48, 53, 61].

Methods focusing on the detection system have not yet been fully developed, as their only function is to make the final response more sensitive. Therefore, most do not directly modify the standard method; their applications aim to obtain the answer so that it can be more quickly detected and established, in some cases on online systems.

4 Biochemical Oxygen Demand (BOD)

Biochemical Oxygen Demand (BOD5) measures the amount of organic matter present in water that has the potential to be decomposed by a microbial consortium in 5 days. Nagel et al. [62] define BOD5 as a measure of the organic contamination in the water that can be biologically degraded. It corresponds to the amount of oxygen present divided between the system's volume; it is usually expressed in milligrams of O2 per liter.

The oxidation of organic matter is a biological process that requires the presence of microorganisms. Kono [46] expresses this process (4) as follows:

$${X}_{I}+{O}_{2}+ C \to {X}_{f}+ {CO}_{2}+ {H}_{2}O$$
(4)

where: Xi and Xf are the initial and final microbial populations; C is the organic matter present in the sample, which in contact with water and dissolved oxygen, is transformed into biomass, CO2, and water.

4.1 Standardized method

In 1936, the American Public Health Association (2005) recognized BOD5 as a standard method to estimate the biodegradability of compounds present in wastewater. The technique consists of placing the samples contaminated, possibly by organic matter in special bottles containing a mixture of previously aerated water, nutrients, and an unknown microbial inoculum, with approximately 105 cells/mL Patnik [60, 66]. First, the measurement of initial dissolved oxygen is carried out,then, the bottles are hermetically closed and incubated in darkness at 20 °C for 5 days. The determination of final dissolved oxygen according to ISO 5815: "Water quality determination of biochemical oxygen demand after the nth day (BODn)—Dilution and seeding method" is undertaken (1983).

This method aims to calculate the amount of dissolved oxygen consumed by the microorganisms; this oxygen is associated with the amount of oxidized organic matter. However, this technique presents practical problems related to response time (5 days), temperature or oxygen concentration. Besides, some compounds present in the sample may be toxic, whereas the concentration and conformation of the microbial inoculum could add a substantial variability in the results (> 20%) [15, 39, 42, 65].

Due to these drawbacks, different alternative techniques to carry out BOD analyses have been developed.

4.2 Alternatives for BOD5 analyses

According to Jouanneau et al. [39], alternative methods for the standardized BOD5 technique can be classified in the following areas: (i) biosensors with a redox mediator; (ii) biosensors based on bacterial bioluminescence; (iii) biosensors based on immobilized bacterial cells; (iv) microbial fuel cells; and, (v) bioreactors.

Table 2 shows the comparisons between various alternative techniques to ascertain BOD5. Additionally, two more techniques are considered; measurement by fluorescence (a previously studied method, adding more recent applications), and miniaturized biological systems.

Table 2 Comparison of the different techniques used as alternatives for BOD5
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Modifications to the standard method include a decrease in the response time to achieve a faster test, and an increase in the sensitivity of the response to biodegradation. However, the disadvantages they present are the cost of sophisticated equipment compared with traditional standardized test kits, their maintenance, and the cost of special microbial inoculums [1, 37], Wang et al. [81] Costa et al. [18], which also present various problems such as the variation of the inoculum, the death, and replacement of the microorganisms in the cells, which generates short operational periods and the use of reagents that accelerate the degradation of organic material to allow greater bioavailability [14, 37, 64].

Systems that modify the way to obtain the biodegradation response have opted to use more sensitive techniques such as fluorescence spectroscopy or optical sensors. In the first case, the results allow us to consider fluorescence spectroscopy as an alternative to ascertain BOD5; albeit it is not applicable to optimize systems, since it is also a laborious and slow technique that can compromise the development of microorganisms with the reagents used [10, 13, 39]. On the other hand, optical sensors seem more promising in detecting the consumption of organic matter, but they are still under development, and at present, they are only reported in miniaturized systems [69].

5 Total Organic Carbon (TOC)

Unlike the techniques COD and BOD described above, Total Organic Carbon (TOC) is the most complete analysis if the objective is to detect all kinds of organic matter present in water: it measures the organic carbon present in all the dissolved compounds. Therefore, it approximates the carbon value of which organic pollutants are constituted (Federation, W. E., & APH Association, [26], however, this technique cannot replace COD or BOD, and it is considered as a complimentary analysis with great importance for water treatment.

To calculate how many carbon bonds there are in a water sample, it is necessary to break these bonds to convert the compounds into a more straightforward molecular form, one that could be quantitatively detected and measured as CO2 (Federation, W. E., & APH Association, [26]. Methods capable of achieving a higher degree of conversion of organic matter into CO2 employ high temperatures, catalysis, chemical oxidation, UV radiation, or combining several of these processes. Once CO2 is released, it can be measured, resorting to various methods, e.g., titration, non-dispersive infrared (NDIR), or colorimetry [8], Visco et al., [79].

5.1 Standardized method: TOC in water

Federation, W. E., & APH Association [26] considers three methods to ascertain TOC, namely: (i 5310B, combustion at high temperatures; (ii 5310 C, oxidation with persulfate (Persulfate / UV and Heated-Persulfate Oxidation Method; and (iii 5310 D, wet oxidation. Table 3 shows the differences and applications of each one.

Table 3 Summary of the methods proposed by Federation, W. E., & APH Association [26] to ascertain TOC
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5.2 Alternatives for the analysis of TOC in water

The alternatives proposed by Federation, W. E., & APH Association [26] for the determination of TOC present different detection limits, which is a problem when other sample parameters are unknown, such as COD or BOD, because a possible detection limit for the unknown sample. In addition, special equipment and materials are required to perform these tests. Other analytical techniques have been developed to measure TOC, leading to more sensitive methods. Table 4 summarizes the investigations focused on the modifications applied to the standardized method to ascertain TOC in water. Most of the research focuses on the detection of CO2, not on the oxidation process. The kind of samples that can be determined by each method are identified, and the detection limits for each one to make them comparable with the standardized methods shown in Table 3.

Table 4 Alternative detection systems for the detection of Total Organic Carbon in water
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Being TOC a sensitive technique, it is challenging to accomplish a significant change in the methodology; this way, the applied modifications focus on different possibilities to detect the carbon present in the sample. However, prior treatments are required to accomplish the oxidation of all the organic matter present, and the levels of carbon detection are above those obtained through the standard technique, so they could hardly displace it; though, the use of various detection techniques allows their implementation in experimental laboratories as an alternative to traditional ones.

6 General discussion

The traditional techniques to determine COD and BOD are still widely used because they allow reliable results to be obtained. Despite the use of highly toxic reagents, COD is used because the reactions ensure the total oxidation of the raw material, unlike proposals where the change in oxidizing agents does not allow this type of reaction. The most current modifications suggest the use of more sophisticated equipment that may well be unfeasible if one considers the ease of obtaining and using a spectrophotometer necessary to carry out the technique.

On the other hand, the BOD, despite the very long response time, has not been able to be replaced by any other technique either because some require materials (including biological materials) that are difficult to maintain in the laboratories that could occupy them, so maintaining microbial cultures to BOD biosensors is not an easy task that can be compared with the ease that the traditional technique allows in this sense.

For its part, COT has been the only technique that has been able to be perfected over time, however, there was no change in the initial technique, only changes emerged in the detection method. The possibility of using more and more sophisticated equipment for this technique and not for the others can be explained in the acquisition of the equipment by the laboratories. When purchasing equipment with the modifications already preloaded, its use and maintenance is easier.

Due to the above, it is necessary to test the various alternative techniques, at least for COD and BOD, in different areas such as cost–benefit, validation, efficacy and sensitivity with the aim of proposing one as a substitute for the reference techniques.

7 Conclusions

Modifying the various analytical techniques reviewed in this summary focuses on reducing times, reagents, and waste generation and also searching for greater sensitivity.

COD presents a larger number of modifications than the other cases in all the different steps that it is necessary to carry out. The central point of most of these modifications is to diminish the use of highly toxic reagents or to change to others that may be more environmentally friendly. The biggest problem with several of the alternatives is their high cost and the time they require compared with the standardized method.

The analysis of BOD5 methods shows there is unification in the systems, which allows them to be performed in situ or, in the best of cases, with an online identification of the results. Most of the studies reviewed focus on the use of microorganisms or biological systems, which can ascertain BOD in less than 1 day. They also focus on using equipment that could increase the sensitivity of the measurement of biodegradable organic matter.

On the other hand, only modifications in the detection systems are discussed in the analysis of TOC. These systems are expensive, but they have high sensitivity, so it was unnecessary to carry out further modifications.

In conclusion, it is preferable to highlight the importance of TOC and the possibility of using this technique as a complementary tool to verify the results obtained by other techniques.

It is essential to recognize the advance and evolution in the modifications of the standardized techniques; nevertheless, none of them has been able to displace traditional methodologies entirely.