1 Introduction

Recently, there are growing concerns about potential adverse human and ecological health effects resulting from the production, use, and disposal of many chemicals necessary for agriculture, industry, household, and medical treatment [2, 23]. A major environmental problems today is the hydrocarbon contamination resulting from different industrial activities [4]. Progress in science and technology should not ignore its likely impact on the environment. Phenol is a key precursor in some industrial manufactures and it has been especially used in the synthesis of some plastics and resins such as phenol–formaldehyde and phenol-urea–formaldehyde [12, 16].

During the production of these adhesive and during their applications, phenol finds its way into the wastewater stream with concentrations ranging from 100 to 2300 mg L−1 phenol [5]. Phenol is reported hazardous substance and is toxic to humans and the environment [12]. Its impact implies the inhibition of biological activity of microorganisms that are responsible for the stabilization of the organic load in industrial and domestic effluents. It reacts with chlorine and produce mono-, di-, or trichlorophenols, which impart tastes and odors of waters [17]. Chlorinated phenols have often been detected in treated wastewaters [18] and have been identified in fish [9] and are readily bio-accumulative [17]. Therefore, this wastewater must be efficiently treated before discharging into the environment. Among the physical, chemical, and biological methods available for the removal of toxic contaminants from wastewater, biological processes are preferred due to their flexibility, industrial applicability, reliability, removal of a wide range of contaminants, simplicity of operation and maintenance, environmentally friendly, degradation of contaminants to less toxic or harmful products rather than transferring them into another phase, and cost-effectiveness [3, 11, 16]. Biodegradation is the use of microorganisms to breakdown or degrade or transform environmental pollutants. Biodegradability is an important ecological indicator of a substance since the value of biodegradability gives possibility of predicting and describing the fate of the substance as a pollutant of an ecologic system (air, water, or soil) [26]. The environmental fate of organic pollutants has become an important research area of increasing concern [25].

Biological decomposition of various substrates is performed by a large number of microorganisms [1, 6, 7, 13, 19, 22, 27]. Microbial degradation of phenolic wastewater has been reported by some authors [3, 20, 21]. Biodegradation studies of organic compounds are tedious, costing, and time-consuming. In this study, the correlations between the initial phenol concentration and different biological parameters are provided, which enables estimating the biodegradation parameters without running the costing and time-consuming BOD test. Results of this study are anticipated to guide subsequent biodegradation experiments of other biodegradable organic compounds.

2 Materials and methods

2.1 Reagents and chemicals

Phenol from Sigma-Aldrich, purity 99%, and a stock solution of 1000 mg L−1 were prepared. Solvents and other chemicals used were all reagent-grade.

2.2 Analytical determinations

2.2.1 Determination of phenol

The brominating mixture method [28] for phenol determination was modified by authors [10] in order to make it capable of detecting lower phenol concentrations. The modified method is as follows:

In a 250-ml conical flask, pipette out 25 ml brominating mixture and add 25 ml water, 5 ml conc. hydrochloric acid and 5 ml potassium iodide solution. The solution will become dark brown due to liberation of iodine. Titrate this with sodium thiosulphate solution (0.1 N) until the solution is light yellow in color and then add few drops of starch solution, the solution becomes blue. Add sodium thiosulphate very carefully and note the volume of sodium thiosulphate at the end point marked by the disappearance of the blue color, this blank titration is used to determine the volume of brominating mixture equivalent to 1 ml of sodium thiosulphate solution. Then take 25 ml of standard phenol for titration, add 50 ml water and 5 ml conc. hydrochloric acid. To this, add 0.2 N brominating mixture (prepared by dissolving 5.567 g of potassium bromate and 75 g of potassium bromide in 1 L distilled water) from a burette until the solution is light yellow and no more precipitate of tribromophenol separates out; the brominating mixture reacts with existing phenol to form a precipitate of tribromophenol. Add 2 ml more of the brominating mixture, the excess brominating mixture remains in solution, and note the volume added, and then add 2 ml potassium iodide and titrate to the liberated iodide against sodium thiosulphate using starch indicator as described under blank experiment above. Since for lower phenol concentrations, the resulting tribromophenol will be a small amount and the residual bromine would be too high to be consumed by 2 ml KI. It has been found that it is more convenient to add 3 ml of KI instead of 2 ml; reproducible results have been obtained. Repeat the titration with 25 ml of the unknown phenol solution in the same manner as with the standard phenol solution described above. Use the same volume of brominating mixture for the unknown phenol solution.

Calculations:

  • (i) weight of phenol in the standard solution (W g).

  • (ii) volume of sodium thiosulphate used in blank experiment against 25 ml brominating mixture (V ml).

  • (iii) volume of brominating mixture added to 25 ml standard phenol solution (V\ml).

  • (iv) volume of sodium thiosulphate used for known phenol solution (V1ml).

  • (v) volume of sodium thiosulphate used for unknown phenol solution (V2ml).

V ml sodium thiosulphate solution ≡ 25 ml brominating mixture, hence:

1 ml of sodium thiosulphate solution ≡ 25/V ml brominating mixture.

∴ V1ml of sodium thiosulphate solution ≡ (25/V) × V1ml of brominating mixture.

Hence, the volume of brominating mixture used for 25 ml of standard phenol solution = V\—[25 V1/ V] ml. Similarly, the volume of brominating mixture used for 25 ml of unknown phenol solution = V\—[25 V2/V].

Hence, the weight of phenol in the unknown solution = 4 × W × {V\—[25 V2/ V]} / {V\—[ 25 V1/V]} g/L.

2.3 Biodegradation studies

The biodegradation studies were based on using synthetic wastewater (which contains Bacto-peptone which is a common component in microbiological media as an organic nitrogen source for bacteria; in addition to essential growth elements other than carbon which will be driven from phenol, the synthetic wastewater was clear and pale yellow in color) seeded with 10% municipal wastewater (collected from the municipality of Mansoura city, to the north of Egypt, latitude and longitude coordinates are 31.037933, 31.381523) and dosed with phenol. Controls were prepared in the same way but without the addition of phenol. The biodegradability at 25 °C was followed using a respirometric BOD controller. Experiments were done in dark (required for the optimal bacterial growth and to avoid decomposition by direct light). All bottles were dosed with 10% seed (municipal wastewater) to inoculate them with microorganisms; experiments were done in duplicates. Microbial growth medium (Table 1) was adjusted to pH 6.8 (pH 6.8 is suitable for bacterial cells and is the mean pH for natural water streams in the region). In all experiments, treatments and controls were done under the same experimental conditions. Seed-corrected data were obtained by subtracting blank values from corresponding values in other experiments, which gave BOD values corresponding to the phenol added as “pure substrate” only.

Table 1 Composition of the synthetic wastewater used in the experiments [24]
Full size table

After the appropriate time of incubation, the supernatant was separated to determine the final phenol concentration.

3 Results and discussion

In this study, the biodegradability data of wastewaters containing phenol, monitored on long period BOD exertions at 25 °C, is studied.

Different doses of phenol ranging from 11.8 to 146.7 mg/L were added to synthetic wastewater samples; their long period BOD exertion curves are shown in Figs. 1 and 2. It was found that phenol is completely utilized in all bottles, whereas partial utilization of phenol was found at concentrations above the reported values.

Fig. 1
figure 1

BOD exertion curves of synthetic wastewater containing different phenol concentrations

Full size image
Fig. 2
figure 2

BOD exertion curves of synthetic wastewater with different phenol concentrations

Full size image

In this article, we are concerned only with experiments where a complete removal of phenol has been achieved. The plateau BOD values were calculated according to the procedure outlined by Parisod and Schroeder [15], as the oxygen uptake at the plateau minus the oxygen uptake of a seeded blank flask at the same point in time. The purpose of the seed correction in this method is to account for oxygen uptake resulting from extraneous organics present in the used seed solution. The form of the seed-corrected BOD progression curves as found by the previous method where phenol is used as pure substrate is given in Fig. 3.

Fig. 3
figure 3

Typical BOD curves for different concentrations of phenol as pure substrate

Full size image

In this figure, the line extension of the pre- and post- plateau curves is shown to demonstrate the method of determining the plateau value. From this figure, the data in Table 2 are extracted.

Table 2 The relation between initial phenol concentration ratios and plateau BOD ratios
Full size table

The data given above shows that there is a correspondence between substrate ratios and plateau ratios. This correspondence provides excellent evidence of the stoichiometric validity of the plateau concept for phenol. When calculating the correlation coefficient, r, between the initial phenol concentration and the seed-corrected plateau BOD values, using Eq. (1) [14], it was found that the correlation coefficient r = 0.9999.

$$r=\frac{n{\Sigma }_{\mathrm{i}=1}^{n}{X}_{i}{Y}_{i}-\left({\Sigma }_{\mathrm{i}=1}^{n}{X}_{i}\right)\left({\Sigma }_{\mathrm{i}=1}^{n}{Y}_{i}\right)}{\sqrt{\left(n {\Sigma }_{\mathrm{i}=1}^{n}{X}_{1}^{2}-{\left({\Sigma }_{\mathrm{i}=1}^{n}{X}_{i}\right)}^{2}{\left(n {\Sigma }_{\mathrm{i}=1}^{n}{Y}_{1}^{2}-{\Sigma }_{\mathrm{i}=1}^{n}{Y}_{i}\right)}^{2}\right)}}$$
(1)

where:

n:

number of pairs of values

X:

plateau BOD value

Y:

phenol concentration

This value of the correlation coefficient, r, is indicating a complete correlation between plateau BOD values and the initial phenol concentrations, Fig. 4.

Fig. 4
figure 4

Relation between Plateau BOD values and initial phenol concentrations

Full size image

These results are supporting the validity of the plateau calculation method, and the plateau concept as associated with the termination of the biodegradation of the substrates’ carbon. It was of interest to investigate the relation between the initial phenol concentration and the ultimate oxygen required for complete removal of the phenol. Although our measurements are showing a complete removal of phenol at the plateau BOD, it is concluded by Kim et al. [8] that 64.4% of the phenolic carbon removed is calculated to be used for cell synthesis. Parisod and Schroeder [15] indicated that the ultimate BOD of a wastewater can be determined by adding the plateau BOD and the BOD of the cells produced up to the plateau. A theoretical BOD of 1.42 g O2/g cells was used in the estimation of the BOD of cells produced up to the plateau based on a cell formula C5H7O2N [15]. The complete chemical oxidation demand of phenol (COD) requires 2.383 g of oxygen per gram of phenol according to the following oxidation reaction, Eq. (2):

$${\mathrm C}_6\;{\mathrm H}_{5\;}\mathrm{OH}\;+\;7\;{\mathrm O}_2\rightarrow6\;{\mathrm{CO}}_{2\;}+\;3\;{\mathrm H}_2\mathrm O$$
(2)

Values of the theoretical COD for different phenol concentrations which are calculated based on complete chemical oxidation of phenol are shown in Table 3.

Table 3 The amount of oxygen required for chemical oxidation of different phenol concentrations
Full size table

Values of the ultimate BOD for different phenol concentrations are calculated by adding BODcell produced up to the plateau and the measured plateau BOD as indicated in Table 4.

Table 4 The ultimate BOD values for different phenol concentrations
Full size table

Comparing the results of ultimate BOD mg/L (Table 4) with the values of the oxygen required for chemical oxidation of phenol (Table 3) shows very close values for the same initial phenol concentration. Calculating the linear correlation coefficient, using Eq. (1), between the initial phenol concentration and the ultimate BOD values yields r = 0.9999, Fig. 5.

Fig. 5
figure 5

Relation between ultimate BOD values and initial phenol concentrations

Full size image

Calculating the linear correlation coefficient between the initial phenol concentration and the calculated biomass produced up to the plateau, using Eq. (1), is giving r = 1.0002, Fig. 6.

Fig. 6
figure 6

Relation between biomass produced and initial phenol concentrations

Full size image

Calculating the linear correlation coefficient, r, between BODultimate and CODtheoretical, using Eq. (1), yields r = 0.9999 indicating a complete correlation between BODultimate and CODtheoretical values, Fig. 7.

Fig. 7
figure 7

Relation between ultimate BOD and theoretical oxygen required for chemical oxidation (COD) of phenol

Full size image

4 Conclusion

This study is providing a correlation between the initial phenol concentration and the corresponding plateau BOD, the biomass produced up to the plateau, the ultimate BOD, and the oxygen required for chemical oxidation, which enables a good BOD estimation and in turn a good prediction of the fate of phenol in wastewater, by knowing the initial phenol concentration, without having to run the costing, time-consuming BOD experiments. It paves the way for establishing machine learning programs that provide enough data about the biodegradation and fate of pollutants in the environment from their concentrations. The data obtained in this study shows that 9 days are enough for complete phenol degradation and for phenol concentrations less than 150 mg/L; using the data in Table 4, it can be concluded that:

  • The BOD of the biomass produced up to the plateau is calculated to be 70% of the corresponding initial phenol concentration.

  • The plateau BOD value mg/L is calculated to be 169.47% of the corresponding initial phenol concentration.

  • The ultimate BOD mg/L is calculated to be 239.47% of the corresponding initial phenol concentration.

  • The biomass produced up to the plateau is calculated to be 49.32% of the corresponding initial phenol concentration.

  • The ultimate BOD mg/L is calculated to be 141.3% of the corresponding plateau BOD value.

  • The oxygen required for ultimate biological degradation of phenol was found to be very close to the oxygen required for chemical oxidation; it gives a way for calculating biological treatment parameters from the oxygen required for the chemical oxidation of the substrate even if the initial concentration of the substrate is not known.

  • The modification introduced to the brominating mixture method for phenol determination makes it capable of detecting lower phenol concentrations in wastewater samples.