New insights on destruction mechanisms of waste activated sludge during simultaneous thickening and digestion process via forward osmosis membrane

This study delved into the efficacy of sludge digestion and the mechanisms involved in sludge destruction during the implementation of forward osmosis process for sludge thickening and digestion (FO-MSTD). Utilizing a lab-scale FO membrane reactor for the thickening and digestion of waste activated sludge (WAS), the investigation explored the effects of sludge thickening and digestion in FO-MSTD processes using draw solutions of varying concentrations. The findings underscored the significance of hydraulic retention time (HRT) as a pivotal parameter influencing the swift thickening or profound digestion of sludge. Consequently, tailoring the HRT to specific processing objectives emerged as a key strategy for achieving desired treatment outcomes. In the investigation, the use of a 1 M NaCl draw solution in the FO-MSTD process showcased enhanced thickening and digestion capabilities. This specific setup raised the concentration of mixed liquor suspended solids (MLSS) to over 30 g/L and achieved a 42.7% digestion efficiency of mixed liquor volatile suspended solids (MLVSS) within an operational timeframe of 18 days. Furthermore, the research unveiled distinct stages in the sludge digestion process of the FO-MSTD system, characterized by fully aerobic digestion and aerobic-local anaerobic co-existing digestion. In the fully aerobic digestion stage, the sludge digestion rate exhibited a steady increase, leading to the breakdown of sludge floc structures and the release of a substantial amount of nutrients into the sludge supernatant. The predominant microorganisms during this stage were typical functional microorganisms found in wastewater treatment systems. Transitioning into the aerobic-local anaerobic co-existing digestion stage, both MLSS concentration and MLVSS digestion efficiency continued to rise, accompanied by a decreasing dissolved oxygen (DO) concentration. More organic matter was released into the supernatant, and sludge microbial flocs tended to reaggregate. The localized anaerobic environment within the FO-MSTD reactor fostered an increase in the relative abundance of bacteria with nitrogen and phosphorus removal functions, thereby positively impacting the mitigation of total nitrogen (TN) and total phosphorus (TP) concentrations in the sludge supernatant. The results of this research enhance comprehension of the advanced FO-MSTD technology in the treatment of WAS.

A Review on the Fate of Legacy and Alternative Antimicrobials and Their Metabolites during Wastewater and Sludge Treatment

Antimicrobial compounds are used in a broad range of personal care, consumer and healthcare products and are frequently encountered in modern life. The use of these compounds is being reexamined as their safety, effectiveness and necessity are increasingly being questioned by regulators and consumers alike. Wastewater often contains significant amounts of these chemicals, much of which ends up being released into the environment as existing wastewater and sludge treatment processes are simply not designed to treat many of these contaminants. Furthermore, many biotic and abiotic processes during wastewater treatment can generate significant quantities of potentially toxic and persistent antimicrobial metabolites and byproducts, many of which may be even more concerning than their parent antimicrobials. This review article explores the occurrence and fate of two of the most common legacy antimicrobials, triclosan and triclocarban, their metabolites/byproducts during wastewater and sludge treatment and their potential impacts on the environment. This article also explores the fate and transformation of emerging alternative antimicrobials and addresses some of the growing concerns regarding these compounds. This is becoming increasingly important as consumers and regulators alike shift away from legacy antimicrobials to alternative chemicals which may have similar environmental and human health concerns.

Organic overloading affects the microbial interactions during anaerobic digestion in sewage sludge reactors

There is still a lack of information about microbial interactions of anaerobic digestion microbiome during process disturbance which limits our ability to predict the mechanisms that drive community dynamics on these events. This paper aims to determine how an organic overloading affects these interactions and to characterize in detail the microbiome structure and diversity in sewage sludge anaerobic reactors during an acidosis event. Two identical sewage sludge anaerobic reactors were subjected to an organic loading shock by adding glycerol waste. As consequence, volatile fatty acids accumulated after only 24 h (up to 2.5 g/L) while Bacteroidales and Methanomicrobiales became displaced by Firmicutes and Methanosaeta sp, showing that reactor acidosis can occur without an immediate decline of this methanogen. Network analysis revealed 9 clusters of co-occurring microorganisms with different behaviors during overloading. At first, Veillonellaceae family, the main glycerol degrading, associated with Candidatus Cloacimonetes, volatile fatty acids fermenters, increased their relative abundance in detriment of the syntrophic bacteria; although as conditions become more acidic, these groups were displaced by other fermenters like Porphyromonadaceae and Chitinophagaceae. Eventually, the methanogenesis failed 72 h after organic overloading, when pH reached values lower than 6. Overall, our results showed a succession of functionally redundant microorganisms, most likely because of niche specialization during organic overloading. The detailed temporal analysis elucidated the processes governing the dynamics anaerobic digestion microbiome, a knowledge required to develop anaerobic digestion management strategies based on its microbiome during process disturbances.

Operational experience with a full-scale anaerobic baffled reactor treating municipal wastewater

The aim of the study was to investigate the influence of moderate temperatures on the efficiency of a full-scale anaerobic treatment system consisting of a primary sedimentation unit (PST) and an anaerobic baffled reactor (ABR, approx. 10 m³ ). Therefore, two trials with different test setups in series were carried out (1st trial: continuous load; 2nd trial: diurnal variation load). The plant was fed with municipal wastewater and operated at temperatures between 8 and 24°C. The mean efficiency of the ABR was low, compared to the one of the PST. At 10°C, only 10% of the COD was removed. The COD (chemical oxygen demand) removal efficiency of the plant (PST + ABR) averaged 50%. At low temperatures, volatile fatty acids began to accumulate. In both trials, the contents of total suspended solids in the sludge bed differed distinctly and influenced the effort for desludging. PRACTITIONER POINTS: The operation of ABR in combined collection systems depends on the efficiency of the pretreatment unit. At cold temperatures, an ABR has no advantages compared to conventional pretreatment processes. For use under moderate conditions, the design must be adjusted.