Estimation of biological oxygen demand and chemical oxygen demand for combined sewer systems using synchronous fluorescence spectra

Abstract

Real-time monitoring of water quality for sewer system is required for efficient sewer network design because it provides information on the precise loading of pollutant to wastewater treatment facilities and the impact of loading on receiving water. In this study, synchronous fluorescence spectra and its first derivatives were investigated using a number of wastewater samples collected in sewer systems in urban and non-urban areas, and the optimum fluorescence feature was explored for the estimation of biochemical oxygen demand (BOD) and chemical oxygen demand (COD) concentrations of sewer samples. The temporal variations in BOD and COD showed a regular pattern for urban areas whereas they were relatively irregular for non-urban areas. Irrespective of the sewer pipes and the types of the areas, two distinct peaks were identified from the synchronous fluorescence spectra, which correspond to protein-like fluorescence (PLF) and humic-like fluorescence (HLF), respectively. HLF in sewer samples appears to be associated with fluorescent whitening agents. Five fluorescence characteristics were selected from the synchronous spectra and the first-derivatives. Among the selected fluorescence indices, a peak in the PLF region (i.e., Index I) showed the highest correlation coefficient with both BOD and COD. A multiple regression approach based on suspended solid (SS) and Index I used to compensate for the contribution of SS to BOD and COD revealed an improvement in the estimation capability, showing good correlation coefficients of 0.92 and 0.94 for BOD and COD, respectively.

Keywords: BOD; COD; fluorescence; prediction; sewer system.

Figure 1.

Temporal variations in BOD and COD concentrations of the sewer samples from sewer pipes for urban and non-urban areas.

Figure 2.

Typical synchronous fluorescence spectrum of the sewer sample from the sewer systems (Δλ = 30 nm).

Figure 3.

Temporal variations in protein-like fluorescence (PLF) and humic-like fluorescence (HLF) intensities (QSE unit) of the sewer samples from sewer pipes for urban and non-urban areas.

Figure 4.

Typical first-derivative of synchronous fluorescence spectrum of the sewer sample from the sewer systems.

Figure 5.

Correlations between a selected fluorescence index (Index I) and BOD or COD data.

Figure 6.

Correlations between Index I and suspended solid concentrations (SS) (a) and between Index I and the fluorescence intensity at excitation/emission wavelengths of 633/633nm (b).

Figure 7.

Correlation between measured BOD (left) and COD (right) and the predicted values by a multiple regression method using Index I and SS concentrations (n = 101, SS measurements were not taken for some samples).