Pretreatments for enhancing sewage sludge reduction and reuse in lipid production

The impact of radicals on physicochemical properties of waste activated sludge during hydrodynamic cavitation treatment

In this study, laboratory-scale Pinned Disc Rotary Generator of Hydrodynamic Cavitation was used to treat waste-activated sludge with a Total Solids concentration of 0.7 %. Five different rotor-stator arrangements were tested, focusing on waste-activated sludge physicochemical and rheological parameters of industrial relevance: general chemical analysis, rheometry, dewaterability, interfacial tension, UV-Vis and FTIR spectroscopy. Radical formation in all five arrangements was confirmed using salicylic acid dosimetry before sample testing. Three of the arrangements generated twice the radical concentration of the other two and achieved a disintegration degree three times higher (17 % compared to 5 %). Capillary Suction Time tests demonstrated a 14-fold reduction in filterability across all arrangements, accompanied by an increase in interfacial tension exceeding 10 %. Statistically significant changes in the UV-Vis spectra indicated alterations in dissolved organic matter humification, aromaticity, and molecular size of colorimetric dissolved organic matter, DNA, and RNA. FTIR analysis revealed characteristic peaks at 1537 cm-1 and 1648 cm-1, signifying microbial cell wall damage. Rheological analysis showed a reduction in apparent viscosity within the low shear stress zone (τ < 5 Pa) and a shift in the yield stress point to lower shear stresses (τ < 0.14 Pa compared to τ = 0.17 Pa for the untreated samples). Pearson's correlation test revealed strong, statistically significant correlations between cell wall damage (as identified by FTIR) and hydrodynamic conditions in the reactor, while the correlation with radical formation was not statistically significant. This suggests that hydrodynamic forces were the primary drivers of cell wall damage, with potential synergetic effects from radicals.

Predicting membrane fouling in membrane bioreactor systems using viscosity: Impacts of environmental conditions and antifouling agents

This study attempted to establish a viscosity-based prediction of membrane fouling. Various factors, including pH, temperature, MLSS concentration, and the addition of NaOCl and citric acid were identified, and their effect on sludge properties such as EPS concentration and wastewater viscosity were estimated. There was a very good correlation between these parameters with EPS concentration and viscosity. The increase in EPS concentration and viscosity significantly affected the membrane flux and filtration time for all the different experimental conditions. However, there were fluctuations in results obtained from experiments related to change in pH, including the addition of antifouling agents NaOCl and citric acid. Such variations accompanied by low correlation in these experiments indicated the influence of pH that may pose difficulty in a viscosity-based estimation of membrane fouling. However, if such large variations in operating conditions could be avoided and the reactor could be operated under optimal conditions, a much better correlation could be obtained between viscosity and membrane fouling. Data from continuously operated MBR systems support this observation, where even a linear equation defining relation between viscosity and transmembrane pressure (TMP) could be obtained. Overall, findings from this study provide a great insight into membrane fouling prediction using viscosity-based methods.

Methane yield response to pretreatment is dependent on substrate chemical composition: a meta-analysis on anaerobic digestion systems

Proper pretreatment of organic residues prior to anaerobic digestion (AD) can maximize global biogas production from varying sources without increasing the amount of digestate, contributing to global decarbonization goals. However, the efficiency of pretreatments applied on varying organic streams is poorly assessed. Thus, we performed a meta-analysis on AD studies to evaluate the efficiencies of pretreatments with respect to biogas production measured as methane yield. Based on 1374 observations our analysis shows that pretreatment efficiency is dependent on substrate chemical dominance. Grouping substrates by chemical composition e.g., lignocellulosic-, protein- and lipid-rich dominance helps to highlight the appropriate choice of pretreatment that supports maximum substrate degradation and more efficient conversion to biogas. Methane yield can undergo an impactful increase compared to untreated controls if proper pretreatment of substrates of a given chemical dominance is applied. Non-significant or even adverse effects on AD are, however, observed when the substrate chemical dominance is disregarded.

Newly isolated lysozyme-producing strain Proteus mirabilis sp. SJ25 reduced the waste activated sludge: Performance and mechanism

To promote aerobic digestion of sludge, a lysozyme-producing strain was screened and identified as Proteus mirabilis sp. SJ25. The results of response surface methodology (RSM) showed that at the temperature of 30.8 °C, pH of 6.69, and the inoculum amount of 2.81%, the sludge reduced by 26.58%. Compared with the control group, the removal efficiency of suspended solids (SS) from sludge in the experimental group increased by 14.60%, the release of soluble chemical oxygen demand (SCOD) increased by 2.21 times, and the release of intracellular substances increased significantly. Actinobacteriota, Chloroflexi, Proteobacteria, Bacteroidota, and Firmicutes were the main phyla involved in the sludge reduction process. Strain SJ25 enhanced the degradation rate of sludge by releasing lysozyme lysis to lyse bacteria, enhancing the metabolism and membrane transport of carbohydrates and amino acids. This study provides a new perspective in the field of efficient degradation of waste sludge.

Novel strategies towards efficient molecular biohydrogen production by dark fermentative mechanism: present progress and future perspective

In the scenario of alarming increase in greenhouse and toxic gas emissions from the burning of conventional fuels, it is high time that the population drifts towards alternative fuel usage to obviate pollution. Hydrogen is an environment-friendly biofuel with high energy content. Several production methods exist to produce hydrogen, but the least energy intensive processes are the fermentative biohydrogen techniques. Dark fermentative biohydrogen production (DFBHP) is a value-added, less energy-consuming process to generate biohydrogen. In this process, biohydrogen can be produced from sugars as well as complex substrates that are generally considered as organic waste. Yet, the process is constrained by many factors such as low hydrogen yield, incomplete conversion of substrates, accumulation of volatile fatty acids which lead to the drop of the system pH resulting in hindered growth and hydrogen production by the bacteria. To circumvent these drawbacks, researchers have come up with several strategies that improve the yield of DFBHP process. These strategies can be classified as preliminary methodologies concerned with the process optimization and the latter that deals with pretreatment of substrate and seed sludge, bioaugmentation, co-culture of bacteria, supplementation of additives, bioreactor design considerations, metabolic engineering, nanotechnology, immobilization of bacteria, etc. This review sums up some of the improvement techniques that profoundly enhance the biohydrogen productivity in a DFBHP process.

Bioprocesses for the recovery of bioenergy and value-added products from wastewater: A review

Wastewater and activated sludge present a major challenge worldwide. Wastewater generated from large and small-scale industries, laundries, human residential areas and other sources is emerging as a main problem in sanitation and maintenance of smart/green cities. During the last decade, different technologies and processes have been developed to recycle and purify the wastewater. Currently, identification and fundamental consideration of development of more advanced microbial-based technologies that enable wastewater treatment and simultaneous resource recovery to produce bioenergy, biofuels and other value-added compounds (organic acids, fatty acids, bioplastics, bio-pesticides, bio-surfactants and bio-flocculants etc.) became an emerging topic. In the last several decades, significant development of bioprocesses and techniques for the extraction and recovery of mentioned valuable molecules and compounds from wastewater, waste biomass or sludge has been made. This review presents different microbial-based process routes related to resource recovery and wastewater application for the production of value-added products and bioenergy. Current process limitations and insights for future research to promote more efficient and sustainable routes for this under-utilized and continually growing waste stream are also discussed.