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 (Absorption_{550 nm}), measured by transmitted light, was investigated in relation to the AC residue. Linear correlations for turbidity (R² = 0.95) and Absorption_{550 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.