Scattered and transmitted light as surrogates for activated carbon residual in advanced wastewater treatment processes: Investigating the influence of particle size.

The use of powdered activated carbon (PAC) is a common process in advanced wastewater treatment to remove micropollutants. Retention and separation of PAC is essential as PAC loaded with micropollutants should not be released into the environment. Determining the activated carbon (AC) residual in the effluent poses a challenge, as there is currently no on-line measurement method. In this study, the correlation between turbidity, measured by scattered light, and absorption at wavelength of 550 nm (Absorption550 nm), measured by transmitted light, was investigated in relation to the AC residue. Linear correlations for turbidity (R2 = 0.95) and Absorption550 nm (R2 = 1.00) to AC concentrations were observed in both laboratory and full-scale experiments in a pilot plant where superfine PAC was added prior to Pile Cloth Media Filtration (PCMF). Decreasing the particle size (d50) while maintaining the same AC concentration leads to increased turbidity: Therefore, a fourfold reduction in d50 results in a 2- to 3-fold increase in turbidity, whereas a 30-fold reduction in d50 leads to a 6-to 8-fold increase. Furthermore, the original wastewater turbidity led to a parallel shift in the linear correlation between turbidity and AC. Coagulant doses of up to 400 mg Me3+/g AC resulted in a 50% reduction in turbidity. However, higher concentrations from 400 to 1,000 mg Me3+/g AC resulted in increased turbidity with only a 30% reduction compared to the initial turbidity. The study also highlights the significance of AC particle size in optical measurements, impacting result accuracy.

Semi-crystalline microplastics in wastewater plant effluents and removal efficiencies of post-treatment filtration systems.

Microplastics (MPs) are ubiquitous in the environment and have been found in every environmental compartment. Wastewater and wastewater treatment plants (WWTPs) have been identified as possible point sources contributing to the emission of microplastic particles (MPP) into the aquatic environment. So far, MPP in wastewater effluents have mainly been analyzed by spectroscopic methods resulting in concentrations as number per volume. In this study, we present mass concentrations in the secondary effluents of four German municipal WWTPs, removal efficiencies of seven post-treatment systems and the resulting load emissions. Differential Scanning Calorimetry (DSC) was used for the analysis of semi-crystalline MPs. The concentrations of secondary effluents ranged from 0.1 to 19.6 µg L-1. Removal efficiencies > 94% were found for a microfiltration membrane (MF), two cloth types of a pile cloth media filter (PCMF), a micro strainer, a discontinuous downflow granulated activated carbon filter (GAC) and a powdered activated carbon (PAC) stage with clarifier and rapid sand filtration. A rapid sand filter (RSF) at WWTP B showed a removal efficiency of 82.38%. Only a continuous upflow GAC filter at WWTP C proved to be unsuitable for MP removal with an average removal efficiency of 1.9%.