Applying reverse stable isotope labeling analysis by mid-infrared laser spectroscopy to monitor BDOC in recycled wastewater

Abstract

Biological stability of treated wastewater is currently determined by methods such as biological oxygen demand, ATP-quantification, or flow-cytometric cell counting. However, the continuous increase in water reclamation for wastewater reuse requires new methods for quantifying degradation of biodegradable dissolved organic carbon (BDOC) ranging from very small to high concentrations of dissolved organic carbon (DOC). Furthermore, direct activity measures or absolute concentrations of BDOC are needed that produce comparable and reproducible results in all laboratories. Measuring carbon mineralization by CO₂ evolution presents a suitable approach for directly measuring the microbial degradation activity. In this work, we investigated the extent of BDOC in water samples from effluent of a wastewater treatment plant and after purification by ultrafiltration over 204 days. BDOC monitoring was performed with the recently introduced reverse stable isotope labeling (RIL) analysis using mid-infrared spectroscopy for the monitoring of microbial CO₂ production. Average BDOC degradation rates ranged from 0.11 to 0.32 mg L^-1 d^-1 for wastewater treatment plant effluent and from 0.03 to 0.22 mg L^-1 d^-1 after ultrafiltration. BDOC was degraded over >90 days indicating the long-term instability of the DOC. Degradation experiments over 88 days revealed first order kinetic rate constants for BDOC which corresponded to 12.7 · 10^-3 d^-1 for wastewater treatment plant effluent and 2.7 · 10^-3 d^-1 after ultrafiltration, respectively. A thorough sensitivity analysis of the RIL showed that the method is very accurate and sensitive with method detection limits down to 10 μg· L^-1 of measured CO₂.

Keywords: Biological stability; Microbial mineralization; Reclaimed water; Stable isotopes; Ultrafiltration; Waste water.