The elimination of microplastic particles (MPP) and microplastic fibers (MPF) was investigated in the final treatment stage (sand filtration) in two municipal wastewater treatment plants (WWTP) and the final treatment stage (sand filtration) at a process WWTP of a manufacturer of polyvinyl chloride (PVC). At each sampling site, three samples on three different days were taken (before/after sand filtration). The samples were filtered through a 10 μm stainless steel cartridge filter utilizing a stainless steel centrifugal pump. Microplastics (MP) were separated from the wastewater matrix by oxidative treatment and density separation and analyzed by Raman microspectroscopy. Due to precautionary measures, procedural blanks were very low with a mean number of 4.3 ± 2.7 MPP and 0.88 ± 0.56 MPF within eight blank samples. The municipal WWTPs were able to eliminate 99.2% ± 0.29% and 99.4% ± 0.15% of MP in the sand filtration stage. The sand filtration of a PVC manufacturer eliminated 99.2%–99.9%.

The effects of mechanical disintegration on anaerobic digestibility of sewage excess sludge in downflow stationary fixed film (DSFF) digesters were investigated on laboratory scale. Mechanical pretreatment using a high pressure homogenizer led to significantly enhanced concentrations of soluble proteins and carbohydrates in the feed sludge. Using DSFF digesters with two different tubular plastic media as support material it was shown that a stable digestion process could be achieved at hydraulic retention times (HRT) down to 5 days. Compared to conventional digesters at 10 d and 15 d HRT respectively, the degradation of volatile solids was enhanced up to 25%, also resulting in a higher specific biogas production. Further investigations on degradation of soluble proteins and carbohydrates showed that a slowly degradable fraction of carbohydrates was released via disintegration. Using the distribution of chain length and the concentrations of volatile fatty acids as process parameters, the dependability on the HRT and the degree of disintegration (the release of soluble COD) predominated the effects of specific surface area of the support media.

This study focuses on identifying the optimum conditions of sewage sludge hydrothermal carbonization by Box–Behnken Design and on the effects of volatile solids on heating value and on process water load. To get insight into the solid and process water characteristics, we applied the Box–Behnken Design on the hydrothermal reaction temperature (190, 220, 250 °C), reaction time (0.5, 2.25, 4 h) and pH (3.9, 5, 6.1). The response surface of the liquid phase revealed decreasing dissolved organic carbon (DOC) concentrations with increasing temperature from 9446 mg/L (190 °C) to 7402 mg/L (250 °C) at 4 h reaction time. For the same hydrothermal conditions, NH₄-N concentration increased from 754 to 1230 mg/L. Reaction temperature was identified as the most important process parameter, whereas reaction time and pH had only minor effects. Moreover, linear coefficients of the models were more decisive than the interrelation and quadratic coefficients. Volatile solids (VS) of the feedstock were found to significantly influence both the load of the process water and the change in heating value of the hydrochars. Process water load increased steadily with higher VS. The heating value only increased with more than around 65–80% VS in feedstock.

Aeration is an energy-intensive process of aerobic biological treatment in wastewater treatment plants (WWTP). Two-stage processes enable energy-efficient operation, but oxygen transfer has not been studied in depth before. In this study, α-factors were determined with long-term ex situ steady-state off-gas measurements in pilot-scale test reactors (5.8 m height, 8.3 m³) coupled to full-scale activated sludge basins. A two-stage WWTP with more than 1 Mio population equivalent was studied over 13 months including rain and dry weather conditions. Operating data, surfactant concentrations throughout the two-stage process, and the effect of reverse flexing on pressure loss of diffusers were examined. The values of α_mean, α_min, and α_max for design load cases of aeration systems were determined as 0.45, 0.33, and 0.54 in the first high-rate carbon removal stage and as 0.80, 0.69, and 0.91 in the second nitrification stage, respectively. The first stage is characterized by a distinct diurnal variation and decrease in α-factor during stormwater treatment. Surfactants and the majority of the total organic carbon (TOC) load are effectively removed in the first stage; hence, α-factors in the second stage are higher and have a more consistent diurnal pattern. Proposed α-factors enable more accurate aeration system design of two-stage WWTPs. Fouling-induced diffuser pressure loss can be restored effectively with reverse flexing in both treatment stages.