# New Approach in COD Fractionation Methods

^{*}

## Abstract

**:**

_{S}was calculated on the basis of the biochemical degradation rate determined in studies (k) for raw wastewater, whereas the S

_{I}fraction was calculated from the difference between SCOD and BOD

_{Tot}of filtered treated wastewater. BOD

_{Tot}of the treated wastewater was calculated taking into account the rate of biochemical degradation determined in the studies (k) for treated wastewater. The shares of individual COD fractions in raw wastewater calculated on the basis of the standard and modified procedure differed by approx. 10% in the case of suspension fractions. Modification of the methodology to determine the COD of the treated wastewater S

_{S}fraction significantly influenced the contents of all fractions in treated wastewater.

## 1. Introduction

_{250}nm) is a measure of the content only of organic compounds that absorb UV radiation, e.g., phenol, surface-active substances. A dependence was determined between COD and UV

_{250}represented by the formula Equation (2). [3]:

_{250}+ 13.6

_{t}= BOD

_{Tot}· (1 − e

^{−k · t})

- BOD
_{t}—biochemical oxygen demand after time t, - k—coefficient of the rate of kinetic reaction,
- BOD
_{Tot}—total oxygen demand for the 1st phase of decomposition.

^{−1}[7].

_{2}dm

^{−3}) needed to oxidize organic compounds and certain inorganic compounds to simple mineral forms [6,8]. The analytical method for determining COD can be schematically represented as:

_{4}. The COD index gives a good representation of the total organic content in wastewater. However, it should be remembered that numerous organic compounds are not determined under the conditions of this test, e.g., some aromatic (benzene, pyridine) and aliphatic (n-hexane, n-heptane) hydrocarbons. During the determination, it is possible to lose volatile organic compounds before they are oxidized. COD is a summary measure of the content of organic compounds in wastewater both subject to and not subject to biochemical degradation, but does not allow separate determination [9,10,11].

_{5}, which allows to initially estimate the biodegradability of pollutants. Wastewater can be considered susceptible to biodegradation if 1.5 < COD/BOD

_{5}< 2.5. A high value of the COD/BOD

_{5}ratio (>2.5) indicates a slow decomposition and a high content of organic substances that are hardly decomposable or biologically indecomposable, which may be caused by a large proportion of industrial wastewater in municipal wastewater. In turn, the value of COD/BOD

_{5}< 2.0 indicates a significant content of biologically degradable contaminants [5,12,13] (Table 1).

_{S}+ S

_{I}+ X

_{S}+ X

_{I}, g O

_{2}m

^{−3}

- S
_{S}—COD of soluble readily biodegradable substrates, - S
_{I}—COD of inert soluble organic substrates, - X
_{S}—COD of particulate slowly biodegradable substrates, - X
_{I}—COD of inert particulate organic substrates,

_{LKT}S

_{F}+ S

_{I}+ X

_{S}+C

_{X}+ X

_{H}+ X

_{I}, g O

_{2}m

^{−3}

- S
_{LKT}—COD of volatile fatty acids, - S
_{F}—COD of fermentable organic substrates, - C
_{X}—COD of slowly biodegradable colloidal substrates, - X
_{H}—COD of heterotrophic biomass fraction.

_{S}fraction can be determined by filtration through a 0.1 or 0.45 μm filter, combining coagulation with Zn(OH)

_{2}and filtration through a 0.45 μm filter, based on tests of the oxygen uptake rate of the activated sludge, or most often with the use of BOD

_{5}measurements [1,17,18]. In the case of fraction S

_{I}, only physico-chemical methods are known [16]. The fraction of inert particulate organic substrates (X

_{I}) can be determined on the basis of the mass balance of the sludge in the system, taking into account active sludge, degradation products, and accumulation of fraction X

_{I}in the sludge accumulated in the secondary wastewater settling tank and lost with effluent [17] or by the direct weight method [19]. Concentration of fraction X

_{S}can be determined on the basis of the amount of oxygen used for the distribution of activated sludge, determined on the basis of batch or continuous tests.

_{S}is defined as the difference of total BOD, calculated on the basis of BOD

_{5}raw unfiltered wastewater and the rate of biochemical degradation and the immediately decomposed dissolved fraction on the basis of the assumed kinetic value of the decomposition rate k. The share of industrial or transported wastewater in a municipal wastewater system affects the rate of biochemical degradation of organic pollutants [21]. Therefore, the calculation of the COD fraction based on the assumed, and not determined for a given wastewater value of the coefficient of biochemical degradation rate k = 0.1 d

^{−1}may be miscalculated.

## 2. Aims and Methodology of the Study

_{S}, S

_{I}, X

_{S}and X

_{I}fraction is based on the determination of COD and BOD

_{5}in samples of filtered (0.45 µm) and unfiltered raw and treated wastewater, where:

- The COD of inert soluble organic substrates S
_{I}is defined as COD in treated filtered wastewater. - The COD of soluble readily biodegradable substrates S
_{S}is calculated from the difference in the concentration of dissolved organic pollutants S_{COD}determined in raw filtered wastewater and fraction S_{I}: S_{S}= S_{COD}− S_{I}. - The COD of particulate slowly biodegradable substrates X
_{S}is defined as the difference of total BOD, calculated on the basis of BOD_{5}raw unfiltered wastewater and the rate of biochemical degradation and the easily decomposed dissolved fraction: X_{S}= (BOD_{5}/ k1) − S_{S}. - The fraction of inert particulate organic substrates XI is determined from the dependence: X
_{I}= X_{COD}− X_{S}where X_{COD}is the total COD of organic suspensions. - The total COD of wastewater is the sum of all fractions: TCOD= S
_{I}+ S_{S}+ X_{S}+ X_{I}.

_{S}is calculated on the basis of the constant rate of biochemical degradation k = 0.1 d

^{−1}, for which BOD

_{Tot}=BOD

_{5}/ 0.6. The calculation also assumes that the fraction S

_{I}corresponds to the COD value in filtered treated wastewater. This assumption is reflected in the calculation of the S

_{S}fraction because it is calculated based on the difference S

_{COD}− S

_{I}, which in treated wastewater is equal to zero in each case.

_{S}in the modified method was calculated on the basis of the biochemical degradation rate determined in studies (k) for raw wastewater, whereas the S

_{I}fraction was calculated from the difference between S

_{COD}and BOD

_{Tot}of filtered treated wastewater. BOD

_{Tot}of the treated wastewater was calculated taking into account the rate of biochemical degradation determined in the studies (k) for treated wastewater.

^{3}d

^{−1}(approx. 27,000 P.E.), working in the technology of low-load activated sludge. Wastewater is supplied to the wastewater treatment plant by sanitary sewerage and transported by septic tankers.

## 3. Results

_{2}dm

^{−3}, BOD

_{5}= 641 ± 84 mg O

_{2}dm

^{−3}, and TOC = 267 ± 93 mg O

_{2}dm

^{−3}. The average efficiency of removal of organic substances was 94.6 ± 1.5% for COD, 97.8 ± 0.7% for BOD

_{5}, and 94.5 ± 1.8% for TOC.

_{5}quotient for raw wastewater were from 1.8 to 2.0, which at the efficiency of COD removal above 90% classifies organic contaminants contained in wastewater entering the treatment plant as easily biodegradable (Table 1).

_{5}= 3.6–6.2) calculated for treated wastewater indicates that the compounds were slowly biodegradable (COD/BOD

_{5}= 3.6–5) or non-biodegradable (COD/BOD

_{5}> 5–6.2). Estimated on the basis of changes in the COD/BOD

_{5}value, susceptibility to biodegradability of wastewater has been extended by the COD fraction values expressing susceptible and biodegradable fractions, taking into account the presence of contaminants (dissolved, undissolved).

_{S}) was on average 787.6 ± 131.8 mg O

_{2}dm

^{−3}, and COD of the dissolved fraction S

_{S}280.4 ± 56.3 mg O

_{2}dm

^{−3}. The average COD values of non-biodegradable, suspended and dissolved fractions were X

_{I}= 61.2 ± 19.2 mg O

_{2}dm

^{−3}, and S

_{I}= 34.4 ± 10.4 mg O

_{2}dm

^{−3}, respectively. The share of individual fractions in the total COD was as follows: X

_{S}= 67.6 ± 5.3%, S

_{S}= 24.2 ± 4.3%, X

_{I}= 5.2 ± 1.1% and S

_{I}= 3.0 ± 0.9%.

_{I}= 34.4 ± 10.4 mg O

_{2}dm

^{−3}, X

_{S}= 23.4 ± 10.2 mg O

_{2}dm

^{−3}and X

_{I}= 4.6 ± 3.2 mg O

_{2}dm

^{−3}, and the corresponding share in the total COD was as follows: S

_{I}= 55.9 ± 7.2%, X

_{S}= 37.3 ± 7.0% and X

_{I}= 6.8 ± 2.1%. According to the standard methodology for determining the COD fraction in treated wastewater, no SS fraction was recorded.

^{−1}, taken as a standard in calculations in accordance with ATV-A131. The mean values of the biochemical degradation rate constants were, respectively, for raw wastewater k = 0.225 d

^{−1}and for treated wastewater k = 0.174 d

^{−1}, which corresponds to raw wastewater BOD

_{Tot}= BOD

_{5}/ 0.68, and in treated BOD

_{Tot}= BOD

_{5}/ 0.58.

_{S}= 656.3 ± 121.5 mg O

_{2}dm

^{−3}and S

_{S}= 292 ± 55.4 mg O

_{2}dm

^{−3}, while the average COD of non-biodegradable, suspended and dissolved fractions were at the level of: X

_{I}= 192.5 ± 27.8 mgO

_{2}/dm

^{3}and S

_{I}= 22.8 ± 12.5 mg O

_{2}dm

^{−3}. The share of individual fractions in the total COD was as follows: X

_{S}= 67.6 ± 5.3%, S

_{S}= 24.2 ± 4.3%, X

_{I}= 5.2 ± 1.1% and S

_{I}= 3.0 ± 0.9%. In treated wastewater, the average COD values for the analyzed fractions were: S

_{I}= 22.8 ± 12.5 mg O

_{2}dm

^{−3}and X

_{S}= 9.3 ± 10.9 mg O

_{2}dm

^{−3}, S

_{S}= 11.7 ± 4.4 mg O

_{2}dm

^{−3}, X

_{I}= 18.7 ± 4.6 mg O

_{2}dm

^{−3}, and the corresponding share in the total COD was as follows: S

_{I}=35 ± 13.2%, S

_{S}= 20.9 ± 10.2%, X

_{S}= 12.5 ± 9.5% and X

_{I}= 31.6 ± 8.5%.

## 4. Discussion of Results

_{S}and S

_{S}(biodegradable fractions), and X

_{I}and S

_{I}(non-decomposed fractions) (Figure 5). The shares of the dissolved fractions were slightly different, and equalled respectively S

_{S}and S

_{I}: 24.2 and 3%, and 25.3 and 1.9%. However, the difference for the COD value of the suspension fraction was over 10%. The shares of this fraction are respectively X

_{S}= 67.6% and X

_{I}= 5.2%, and X

_{S}= 56.3% and X

_{I}= 16.5%.

_{I}= 55.9%, X

_{S}= 37.3% and X

_{I}= 6.8%. Modification of the methodology made it possible to determine the share in COD of wastewater of the treated S

_{S}fraction, which significantly affected the share of all factions that were at the level of: S

_{I}= 35%, S

_{S}= 20.9%, X

_{S}= 12.5% and X

_{I}= 31.6%.

## 5. Conclusions

_{5}ratio in raw and treated wastewater, combined with a COD removal, allows to control the correctness of the biochemical process of oxidation of organic pollutants contained in wastewater. However, it does not provide information on the share of susceptible and biodegradable compounds in wastewater. The division of COD into fractions is a significant extension of wastewater characteristic in terms of its degree of biodegradation. The methods of determination of biodegradable fractions described in the literature require a long time to obtain a result and additional analytical procedures deviating from standard determinations carried out in raw wastewater.

_{S}).

_{S}and S

_{I}while taking into account the kinetic coefficients determined for raw and treated wastewater makes it possible to determine the shares of individual fractions with greater accuracy. Calculation of the S

_{I}fraction concentration from the difference between S

_{COD}and BOD

_{Tot}of the treated filtered wastewater on the basis of a constant decomposition rate allows to determine the concentration of the S

_{S}fraction in treated wastewater, which in turn affects the value of share of other fractions.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**Schematic diagram of the chemical oxygen demand (COD) fractions and their fates in a biological wastewater treatment plant [14].

**Figure 2.**Particle size distribution in raw wastewater samples (

**a**) and after filtration through 0.45 μm filter (

**b**).

**Figure 3.**The course of BOD curves for raw and treated wastewater. (

**a**) Raw wastewater; (

**b**) treated wastewater.

**Figure 4.**Total share of COD biodegradable and non-biodegradable fraction in raw and treated wastewater. (

**a**) Methodology in accordance with ATV-A131; (

**b**) modified calculation methodology.

**Figure 5.**Average fraction shares in total COD of raw and treated wastewater. (

**a**) Methodology in accordance with ATV-A131; (

**b**) modified calculation methodology.

COD/ BOD_{5} | Value decrease COD, [%] | Assessment of Susceptibility to Biochemical Biodegradation |
---|---|---|

<2.0 | >90 | easily biodegradable |

2.0–2.5 | 50–90 | Biodegradable |

2.5–5.0 | 10–50 | slowly biodegradable |

>5.0 | <10 | resistant to biodegradation |

**Table 2.**Share of COD fraction in municipal raw wastewater and municipal wastewater with a significant share of industrial wastewater.

COD fractions, % | ||||

S_{S} | S_{I} | X_{S} | X_{I} | |

Municipal wastewater | ||||

10–20 | 7–11 | 53–60 | 7–15 | Kappeler, Gujer, 1992 [19] |

9.0 | 4.0 | 77.0 | 10.0 | Sozen, 1998 [22] |

50.0–61.7 | 2.2–6.0 | 22.0–34.4 | 8.0–16.2 | Płuciennik-Koropczuk, Myszograj, 2017 [23] |

20–25 | 8–10 | 60–65 | 5–7 | Ekama, 1986 [15] |

24–32 | 8–11 | 43–49 | 11–20 | Henze, 2002 [17] |

Municipal wastewater with a significant share of industrial wastewater | ||||

Textile industry wastewater | ||||

25.0 | 14.0 | 59.0 | 2.0 | Baban et al. 2004 [24] |

Dairy industry wastewater (10%) | ||||

38.8 | 2.3 | 45.5 | 14.8 | Struk-Sokołowska, 2015 [25] |

Baking industry wastewater (10%) | ||||

38.8 | 1.0 | 44.2 | 15.2 | Struk-Sokołowska, 2017 [26] |

Oil processing wastewater | ||||

29.2 | 9.9 | 37.4 | 23.5 | Chiavola 2014 [27] |

Paper industry wastewater (25%) | ||||

4.2 | 39.5 | 43.1 | 13.2 | Choi 2017 [14] |

Wastewater Sample | Range | COD mg O_{2} dm^{−}^{3} | BOD_{5} mg O_{2} dm^{−}^{3} | TOC mg C dm^{−}^{3} |
---|---|---|---|---|

raw | minimum | 1000 | 548 | 182.7 |

maximum | 1340 | 738 | 451.3 | |

average | 1164 ± 157 | 641 ± 84 | 267 ± 93 | |

treated | minimum | 40 | 7.7 | 5.7 |

maximum | 98 | 24 | 22.1. | |

average | 62 ± 22 | 14 ± 6 | 14 ± 5 |

Wastewater Sample | Value | S_{S} | S_{I} | X_{S} | X_{I} | Total COD |
---|---|---|---|---|---|---|

mg O_{2} dm^{−3} | ||||||

Raw | Minimum | 230 | 20 | 663 | 47 | 1000 |

Maximum | 352 | 48 | 990 | 95 | 1340 | |

Average | 280.4 ± 56.3 | 34.4 ± 10.4 | 787.6 ± 131.8 | 61.2 ± 19.2 | 1164 ± 157 | |

Treated | Minimum | 0 | 20 | 13 | 2 | 40 |

Maximum | 0 | 48 | 40 | 10 | 98 | |

Average | 0 ± 0.0 | 34.4 ± 10.4 | 23.4 ± 10.2 | 4.6 ± 3.2 | 62 ± 22 | |

% | ||||||

Raw | Minimum | 18.2 | 1.9 | 62.8 | 4.2 | (-) |

Maximum | 29.5 | 4.0 | 75.1 | 7.1 | ||

Average | 24.2 ± 4.3 | 3.0 ± 0.9 | 67.6 ± 5.3 | 5.2 ± 1.1 | ||

Treated | Minimum | 0 | 49.0 | 27.1 | 5.0 | |

Maximum | 0 | 66.7 | 45.0 | 10.2 | ||

Average | 0 ± 0 | 55.9 ± 7.2 | 37.3 ± 7.0 | 6.8 ± 2.1 |

**Table 5.**COD fractions in raw wastewater and treated wastewater according to the modified ATV-A131 method.

Wastewater sample | Value | S_{S} | S_{I} | X_{S} | X_{I} | Total COD |
---|---|---|---|---|---|---|

mg O_{2} dm^{−3} | ||||||

Raw | Minimum | 243 | 7 | 545 | 165 | 1000 |

Maximum | 359 | 41 | 845 | 236 | 1340 | |

Average | 292 ± 55.4 | 22.8 ± 12.5 | 656.3 ± 121.5 | 192.5 ± 27.8 | 1164 ± 157 | |

Treated | Minimum | 7 | 7 | 3 | 12 | 40 |

Maximum | 16 | 41 | 29 | 24 | 98 | |

Average | 11.7 ± 4.4 | 22.8 ± 12.5 | 10.9 ± 9.3 | 18.7 ± 4.6 | 62 ± 22 | |

% | ||||||

Raw | Minimum | 18.7 | 0.7 | 51.2 | 15.2 | (-) |

Maximum | 30.8 | 3.1 | 64.1 | 17.6 | ||

Average | 25.3 ± 4.5 | 1.9 ± 0.9 | 56.3 ± 5.3 | 16.5 ± 0.9 | ||

Treated | Minimum | 16.7 | 6.9 | 6.5 | 21.7 | |

Maximum | 51.2 | 33.3 | 29.3 | 42.6 | ||

Average | 23.4 ± 10.2 | 35.0 ± 13.2 | 12.5 ± 9.5 | 31.6 ± 8.5 |

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**MDPI and ACS Style**

Płuciennik-Koropczuk, E.; Myszograj, S.
New Approach in COD Fractionation Methods. *Water* **2019**, *11*, 1484.
https://doi.org/10.3390/w11071484

**AMA Style**

Płuciennik-Koropczuk E, Myszograj S.
New Approach in COD Fractionation Methods. *Water*. 2019; 11(7):1484.
https://doi.org/10.3390/w11071484

**Chicago/Turabian Style**

Płuciennik-Koropczuk, Ewelina, and Sylwia Myszograj.
2019. "New Approach in COD Fractionation Methods" *Water* 11, no. 7: 1484.
https://doi.org/10.3390/w11071484