The effectiveness of disinfection, as well as microbiological composition of water, are assessed by microbiological tests. The analysis usually takes from 24 to 48 hours. During this time the water produced at the water treatment plant has already been fed to the water supply system. In order to reduce the risk of bacteriological contamination, higher concentrations of disinfectants are maintained. Considering the fact that water supply systems use retention tanks with retention times of from 5 to 12 hours, microbiological tests should give the result in shorter time than water retention time in the water tanks. The aim of the study was to determine whether the bacterial community in static liquid medium with the control of absorbance change at 600 nm (OD600) can be applied as a diagnostic method at water treatment plant laboratories. The research on water and wastewater samples has shown that the duration of lag time λ depends on the approximate amount of bacteria and the initial conditions in liquid media at the start of the experiment. It has been shown that the method can be used in the future for rapid monitoring of bacteriological contamination in the water.

It is often only at the operation stage of a wastewater treatment plant that there is a need to adjust the treatment process in terms of variable hydraulic capacity, increased pollutant load, high/low concentration of suspended biomass, or the unfavorable phenomenon of reduced sedimentation capacity of the activated sludge. One of the ways to improve the treatment process efficiency is to increase the biologically active surface by using bio-carriers in the form of fibers, materials, or bio-balls. This paper presents the results of a wastewater treatment plant operation during the period of six months after the implementation of the integrated fixed-film activated sludge (IFAS) technology. The research showed that microorganisms developed both in the activated sludge and on the fibers, positively influencing the activated sludge condition. During the start-up of the IFAS process, ciliates predominated over the other species. However, as oxygen content was high (2 mg/dm³ and more) and textile beds were used, the protozoan population developed intensively, and small metazoans became increasingly common. Throughout the research period, nitrifying and phosphorus-accumulating bacteria were observed both in the activated sludge and on the fibers. Between the 59th and 184th day of operation, numerous microorganisms were detected on the fibers and in the activated sludge, testifying to low biological oxygen demand, good aerobic conditions for nitrification, and long sludge age. However, the process seemed to break down after day 72, when the occurrence of metazoan led to reduced sludge production; after day 88, chemical oxygen demand and total suspended solids in the outflow increased, and oligochaetes and rotifers dominated the suspended sludge and fibers. Results also showed that the textile bed and low ammonia concentration became an excellent substrate for the development of Stentor sp. With regard to chemical and biological oxygen demand, total nitrogen- and total phosphorus-effluent concentrations were mostly within the legally permissible limits throughout the 184 days of operation.

While chromium–nickel steel is known to be extremely resistant to corrosion, the occurrence of certain factors can unfortunately initiate an uncontrolled corrosion process. This paper presents samples made of 304 stainless steel containing delta ferrite that have been exposed to wastewater for 18 months. Samples placed above the surface of the wastewater (A-series) were intensively corroded. Samples half-submerged in the wastewater and periodically fully submerged at higher effluent flows through the screenings and grit separator (B-series) only suffered minor mechanical erosion. No significant changes in the tested surface were observed on samples fully submerged in wastewater (C-series). The results indicated that the observed pitting corrosion of samples placed above the surface of the wastewater was a consequence of the presence of bacteria in a wet hydrogen sulfide environment. The fluorescence in situ hybridization method showed that either the sludge taken from the wastewater, or from the surface of samples submerged in wastewater exhibited increased amounts of bacteria from the δ-proteobacteria class, indicating the presence of microorganisms involved in the reduction of sulfur or sulfate compounds. A new approach to microbiological evaluation by determining classes of bacteria may be a promising tool for evaluating wastewater in terms of aggressiveness and recognizing favorable corrosive conditions.

The increasing volume of sewage sludge generated by wastewater treatment plants poses a major environmental and economic challenge, highlighting the need for sustainable treatment and valorization strategies. Hydrothermal carbonization (HTC) converts wet biomass, such as sewage sludge, into value-added products; however, the aqueous post-reaction phase (HTC-AP) remains largely underutilized. The presented study aimed to evaluate HTC-AP derived from sewage sludge as a medium for Yarrowia lipolytica to produce microbial lipids, thereby addressing the gap in integrated sludge-to-value bioprocesses. HTC was performed at 180-300 °C for 60-120 min to generate liquid fractions, which were subsequently tested as substrates for lipid biosynthesis by different Y. lipolytica strains. The composition of HTC-AP, strains tolerance, volatile fatty acid (VFA) assimilation, lipid accumulation, and fatty acid profiles were assessed. In addition, techno-economic and life cycle assessments were conducted, supported by integrated experimental and process modeling approaches. The HTC liquid fraction contained primarily acetic (4.9 g/L) and propionic acid (6.6 g/L), together with phenolic and heterocyclic compounds, which could potentially have an inhibitory effect. Among the tested strains, Yarrowia lipolytica A101 and the double mutant ΔEYD DGA1 (knock-out of erythritol dehydrogenase and overexpression of diacylglycerol acyltransferase) exhibited the highest tolerance to HTC-AP and effectively assimilated VFAs, particularly under HTC conditions of 180 °C for 60 min. Lipid profiles showed the oleic (C18:1) and linoleic (C18:2) acids being produced more predominantly, regardless of the cultivation scale. Techno-economic analysis demonstrated the feasibility of the integrated process, yielding an estimated net profit of approximately $292.5 per ton of treated sludge (TT180-RT60), while life cycle assessment indicated a relatively low environmental impact of ∼0.38 kg CO₂-eq per kg of produced lipids. These findings demonstrate that coupling hydrothermal processing with metabolically engineered Y. lipolytica fermentation allows for an effective sewage sludge valorization. Furthermore, integrated process modeling facilitates scale-up and industrial implementation, while additional optimizations such as pre-dewatering and heat recovery may further improve process efficiency and environmental performance.