Spectrometric characterization of effluent organic matter of a sequencing batch reactor operated at three sludge retention times

Effluent organic matter (EfOM) from activated sludge systems is composed primarily of influent refractory compounds, residual degradable substrate, intermediate products and soluble microbial products (SMPs). Depending on operational conditions (hydraulic and sludge retention time (SRT)), the quantity and quality of EfOM significantly changes. The main objective of this research was to quantify and characterize the EfOM of a lab-scale activated sludge sequencing batch reactor (SBR), which was operated at three SRTs and fed glucose, an easily biodegradable substrate. EfOM was followed with two direct-quantification methods (chemical oxygen demand (COD) and dissolved organic carbon (DOC)), three spectrometric methods (ultraviolet absorbance at 254 nm (UVA254), excitation-emission matrix (EEM) fluorescence and parallel factor analysis (PARAFAC)) and three organic matter (OM) indices (specific UVA254 (SUVA), SUVA–COD, COD/DOC ratio). The significant increment of UVA254 and OM indices after treatment indicated an accumulation of refractory high-molecular-weight humic-like compounds in the EfOM, which demonstrated that EfOM was composed mainly by SMPs and not glucose. On the other hand, as the SRT increased, the amount of EfOM decreased, but SUVA, SUVA–COD and fluorescence intensity increased; these trends indicated the accumulation of SMPs of increased molecular weight and aromaticity. Increasing SRT in the SBRs reduced the amount of EfOM, but increased its aromaticity and reactivity. Visual analysis of EfOM EEMs showed two protein- and one humic-like peak, which were attributed to SMPs generated within the SBRs. PARAFAC determined that a two-component model best represented EfOM EEMs. The two-components from PARAFAC were mathematically correlated to the visually identified protein- and humic-like SMPs peaks.

► Effluent organic matter (EfOM) of a lab-scale activated sludge SBR was characterized.
► The increment of UVA254 after treatment showed an accumulation of humic-like EfOM.
► The EfOM was composed mainly by soluble microbial products (SMPs).
► Increasing the SRT in the SBR reduced the EfOM, but increased its aromaticity and reactivity.
► Visual and PARAFAC analysis of EEMs showed protein- and humic-like SMPs peaks in EfOM.