Effect of carbon to nitrogen ratio of feed wastewater and sludge retention time on activated sludge in a submerged membrane bioreactor

Evaluation of membrane fouling in a microalgal-bacterial membrane photobioreactor: Effects of SRT

As a novel system, the microalgal-bacterial membrane photobioreactor (MPBR) has better performance than the conventional MBRs in membrane fouling control. Nevertheless, how the operating conditions affect its fouling performance is rarely reported. In this study, a microalgal-bacterial MPBR was set and continuously operated to treat synthetic wastewater. Effects of solids retention time (SRT, 10, 20, and 30 d) on the membrane fouling were investigated. The results showed that the relationship between membrane fouling and SRT was nonlinear and the fastest membrane fouling was observed at SRT 20 d. The predominant fouling mechanism was gel layer formation. X-ray photoelectron spectroscopy results showed a significant difference in the surface composition of the microalgal-bacterial consortia at different SRTs. The biological flocs at SRT of 20 d had the largest floc size, moderate filament abundance, and the highest content of bound EPS and SMP. The highest membrane fouling at SRT 20 d was mainly attributed to the highest concentration of EPS and SMP. Environmental stresses and fierce competition between microalgae and bacteria are considered to be the underlying reasons for the elevated production of EPS and SMP. In brief, optimizing the SRT value to control the balanced growth of microalgae and bacteria and keep them at an appropriate ratio is critical for delaying membrane fouling in microalgal-bacterial MPBR.

Recovery Techniques Enabling Circular Chemistry from Wastewater

In an era where it becomes less and less accepted to just send waste to landfills and release wastewater into the environment without treatment, numerous initiatives are pursued to facilitate chemical production from waste. This includes microbial conversions of waste in digesters, and with this type of approach, a variety of chemicals can be produced. Typical for digestion systems is that the products are present only in (very) dilute amounts. For such productions to be technically and economically interesting to pursue, it is of key importance that effective product recovery strategies are being developed. In this review, we focus on the recovery of biologically produced carboxylic acids, including volatile fatty acids (VFAs), medium-chain carboxylic acids (MCCAs), long-chain dicarboxylic acids (LCDAs) being directly produced by microorganisms, and indirectly produced unsaturated short-chain acids (USCA), as well as polymers. Key recovery techniques for carboxylic acids in solution include liquid-liquid extraction, adsorption, and membrane separations. The route toward USCA is discussed, including their production by thermal treatment of intracellular polyhydroxyalkanoates (PHA) polymers and the downstream separations. Polymers included in this review are extracellular polymeric substances (EPS). Strategies for fractionation of the different fractions of EPS are discussed, aiming at the valorization of both polysaccharides and proteins. It is concluded that several separation strategies have the potential to further develop the wastewater valorization chains.

Performance and activated sludge characteristics at short solid retention time in a submerged MBR: effects of C/N ratio of wastewater

This study investigated the effect of carbon to nitrogen (C/N) ratio of influent wastewater on performance and activated sludge properties at a sludge retention time of 5 d in a submerged membrane bioreactor. The chemical oxygen demand and ammonia-Nitrogen (NH₃-N) removal efficiencies were found to be over 96.3% and 86.9% in the sMBRs having different C/N ratios. It was found that total extracellular polymeric substances and total soluble microbial products increased with an increase in C/N ratio. It was also observed that critical flux, relative hydrophobicity and zeta potential values decreased, and capillary suction time, particle size and viscosity of sludge increased as the C/N ratio increased.

The influence of solid retention time on IFAS-MBR systems: analysis of system behavior

A University of Cape Town Integrated Fixed-Film Activated Sludge Membrane Bioreactor (UCT-IFAS-MBR) pilot plant was operated at different values of the sludge retention time (SRT). Three SRTs were investigated at different durations: indefinitely, 30 and 15 days. The organic carbon, nitrogen and phosphorus removal, kinetic/stoichiometric parameters, membrane fouling tendency and sludge filtration properties were assessed. The findings showed that by decreasing the SRT, the pilot plant could maintain excellent carbon removal efficiencies throughout the experiments. In contrast, the biological carbon removal showed a slight nitrification and was slightly affected by the decrease of the SRT, showing high performance (approximately 91%, on average). Thus, the biofilm might have helped sustain the nitrification throughout the experiments. The average phosphorus removal performance increased slightly with a decrease in SRT, achieving the maximum efficiency (61.5%) at a SRT of 15 days. After a 30-day SRT, an increase in resistance due to pore blocking and a general worsening of the membrane filtration properties occurred.

Understanding and optimization of the flocculation process in biological wastewater treatment processes: A review

In the operation of biological wastewater treatment processes, fast sludge settling during liquid-solids disengagement is preferred as it affects effluent quality, treatment efficiency and plant operation economy. An important property of fast settling biological sludge is the ability to spontaneously form big and dense flocs (flocculation) that readily separates from water. Therefore, there had been much research to study the conditions that promote biological sludge flocculation. However, reported findings have often been inconsistent and this has possibly been due to the complex nature of the biological flocculation process. Thus, it has been challenging for wastewater treatment plant operators to extract practical information from the literature. The aim of this review is to summarize the current state of understanding of the factors that affect sludge flocculation so that evaluation of such information can be facilitated and strategize for intervention in the sludge flocculation and deflocculation process.

Biological performance and sludge filterability of anaerobic membrane bioreactors under nitrogen limited and supplied conditions

The impact of nitrogen on biological performance and sludge filterability of anaerobic membrane bioreactors was investigated in two lab-scale cross-flow anaerobic membrane bioreactors that were fed with cheese whey at two different COD:TKN ratios (50 and 190). Nitrogen deprivation adversely affected the biological treatment performance and reactor stability, as indicated by volatile fatty acids accumulation. On the other hand, nitrogen (urea) supplementation resulted in a reduced sludge median particle size and decreased sludge filterability. Standard filterability parameters such as capillary suction time and specific resistance to filtration tended to rapidly increase in the nitrogen supplemented reactor. The critical fluxes in the nitrogen limited and supplemented reactors were 20 and 9 L m^-2 h^-1, respectively. The rapid deterioration of sludge filterability under nitrogen supplemented conditions was attributed to abundant growth of dispersed biomass. Thus, the COD:TKN ratio of wastewater affected both bioconversion and filterability performance in the anaerobic membrane bioreactors.

The investigation of paper mill industry wastewater treatment and activated sludge properties in a submerged membrane bioreactor

The paper mill industry produces high amounts of wastewater and, for this reason, stringent discharge limits are applied for sustainable reclamation and reuse of paper mill industry wastewater in many countries. Submerged membrane bioreactor (sMBR) systems can create new opportunities to eliminate dissolved substances present in paper mill wastewater including. In this study, a sMBR was operated for the treatment of paper mill industry wastewater at 35 h of hydraulic retention time (HRT) and 40 d of sludge retention time (SRT). The chemical oxygen demand (COD), NH₃-N and total phosphorus (TP) removal efficiencies were found to be 98%, 92.99% and 96.36%. The results demonstrated that sMBR was a suitable treatment for the removal of organic matter and nutrients for treating paper mill wastewater except for the problem of calcium accumulation. During the experimental studies, it was noted that the inorganic fraction of the sludge increased as a result of calcium accumulation in the reactor and increased membrane fouling was observed on the membrane surface due to the calcification problem encountered. The properties of the sludge, such as extracellular polymeric substances (EPS) and soluble microbial products (SMP), relative hydrophobicity, zeta potential and floc size distribution were also monitored. According to the obtained results, the total EPS was found to be 43.93 mg/gMLSS and the average total SMP rejection by the membrane was determined as 66.2%.