Abstract
Determination of the important wastewater parameter chemical oxygen demand (COD) in international standard procedures is still based on the use of mercury sulfate and potassium dichromate, which are hazardous to health and the environment (REACH). In order to overcome this environmental paradox, alternative oxidizing agents were compared, and a new wet-chemical method using Mn(III) as an oxidant was developed that works without Cr(VI) and Hg(II). Oxidation of potassium hydrogen phthalate as a reference substance was reproducible in a COD range of 20–500 mg/L with a mean recovery of 88.7% compared with the standard Cr(VI)-method. A high correlation (R2 = 0.9935) to the standard Cr(VI)-method with a mean recovery of 78.1% (± 5.2%) was found for a series of industrial and municipal wastewater samples. Although the results of the new method were not 100% of the standard method, its high correlation with the latter and its reproducibility offer an environmentally benign alternative wet chemical method.
TopIntroduction
Pollutants in wastewater (WW) are various and of inorganic nature like heavy metals, toxic ions, and fertilizing compounds (nitrate and phosphate), and organic ones like persistent organic compounds, pesticides, detergents, personal care products, etc. Whilst some of these pollutants are regulated as individual chemicals, many others are difficult to analyze one by one due to practical reasons. Therefore, some group parameters are defined in water legislations and regulations, among which the Chemical Oxygen Demand (COD) is the most important one (Duca et al, 2021). COD is used worldwide to determine the organic load in wastewater, especially for the characterization of the WW and the operation of wastewater treatment plants (WWTP). Furthermore, in the case of mainly industrial WW producers, the taxation of the discharged pollution into the public sewerage is based often on COD.
The determination of the COD as a group parameter is based worldwide since the 1970s on the total oxidation of organic compounds to carbon dioxide and water with the help of strong oxidants after the following redox reactions, e.g., for oxalic acid and for ethanol:
Oxalic acid: C
2H
2O
4 + 0.5 O
2 → 2 CO
2 + H
2O
(1) Ethanol: C
2H
6O + 3 O
2 → 2 CO
2 + 3 H
2O
(2)If one wants to quantify the amounts of these two organic compounds in WW, there are two opportunities to measure: (i) the amount of oxygen needed to completely oxidize the compounds (COD), and (ii) the total organic carbon that is oxidized to and collected as the amount of CO2 (TOC). But as can be seen in Eq. 1 and Eq. 2, the two different chemical compounds produce the same amount of CO2 due to their different oxidation states. Contrary to TOC, the amount of oxygen needed is different: (i) 0.5 mol O2 for oxalic acid (Eq. 1) and (ii) 3 mol O2 for ethanol (Eq. 2). This is the reason that TOC is unsuitable and, therefore, usually not used to measure the organic load in WW.
The ratio of COD/TOC depends on the average oxidation state of the organic compounds in WW. Additionally, in the case of COD, phosphorus and sulfur in the molecules are oxidized and contribute to the amount of oxygen needed for total oxidation. In fact, the theoretical oxygen demand for the total oxidation of an organic molecule is based on Eq. 3:
C
nH
mO
eX
kN
jS
iP
h +
bO
2 →
nCO
2 + [(
m – k – 3
j – 3
h)/2]H
2O +
kHX +
jNH
3 +
iH
2SO
4 +
hH
3PO
4(3) with X = halogen atoms; b = n + (
m – k – 3
j – 2
i -3
h)/2 –
e/2 + 2
i + 2
hIn daily practice, the ratio COD/TOC can vary in a range of 2 to 8, why TOC is not suitable to replace the COD, although it was investigated often to do so due to toxic chemicals needed for the determination of COD after international standards (DIN 38409-41 (1980), ISO 6060 (1986)). Further alternatives to the established wet chemical COD method could be electrochemical, thermal, and photocatalytic oxidation. Here also some devices are already on the market (e.g., from LAR, Mantech, UVT). These devices are offered for online monitoring but have not yet found their way into monitoring laboratories. An important reason for this is the high acquisition costs, especially for smaller laboratories. Another disadvantage is that suspended solids (SS), which are an important component of the total COD, cannot be detected as they can clog tubes and other equipment parts.
Key Terms in this Chapter
Phasing out: Strategy of the EU to ban chemicals that are harmful for human beings and the environment step by step, unless the use is essential.
Standard method: Analytical methods developed by consensus from results of science and technology, formulated and issued by recognized organizations and their committees, and made available to the public. The aim is a national or international standardization of the applied methods and thus a better comparability of analytical results.
Total organic carbon (TOC): A sum parameter for the total amount of organic carbon in a water sample. The determination is carried out via combustion or total chemical oxidation of the carbon content. The amount of carbon dioxide produced is measured, from which the carbon content in mg/L is calculated.
Chemical Oxygen Demand (COD): A sum parameter for the sum of all substances present in water that can be oxidized under certain conditions usually with potassium dichromate as the oxidant. It indicates the amount of oxygen (in mg/L) that would be required for their oxidation if oxygen were the oxidizing agent.
Chloride: interference: The interference of the COD value by the co-oxidation of chloride ions in water samples by the oxidant.
Cuvette test: Analytical method in which chemical parameters of solutions are determined in photometer cells (cuvettes). The analysis solution is reacted with reagents specific to the parameter of interest. This reaction causes a change in the color or other optical properties of the solution. These changes can then be measured by means of photometry and used for quantification.
Digestion: Chemical decomposition of organic ingredients under harsh reaction conditions (strong oxidant, high temperature, low pH).
Theoretical COD: Amount of oxygen in mg/L required for the complete oxidation of an organic compound to carbon dioxide and water, which can be calculated stoichiometrically.