The effect of ultrasound for fouling mitigation in the microfiltration of an activated sludge-lagoon effluent

Low pressure membrane filtration is playing an increasingly important role in the reclamation of municipal wastewater. However, membrane fouling remains a critical factor affecting the efficiency of the membranes in the filtration of the biologically treated secondary effluent. The potential of ultrasound (US) was investigated for membrane cleaning and feed pre-treatment in the microfiltration of municipal activated sludge-lagoon effluent. Application of US (45 kHz, 107 W) to the fouled membranes for greater than 5 min gave greater cleaning efficiency with flux recovery of over 80% compared with backwashing (74%). The better performance of US was attributed to the cavitational shear forces induced by US which dislodged the cake layer and loosened the materials clogging the membrane pores. When US cleaning was followed by backwashing, flux recovery was enhanced further, this was due to the dislodged foulants being effectively flushed away from the membrane surface and pores by backwashing. However, flux recoveries decreased with successive fouling and cleaning cycles, the decrease being greater for backwashing (64% for US after 5 cycles, three times that for backwashing). These results were consistent with membrane analysis by attenuated total reflectance - Fourier transform infrared (ATR-FTIR) and scanning electron microscopy (SEM). The increased irreversible fouling was due to compaction of the foulants and their increased affinity to the membrane surface and pores during the operation under pressure. When used as feed pre-treatment, sonication led to some dissolution and fragmentation of the particulate matter in the effluent, these smaller particles had a negative impact on permeate flux by causing rapid pore blockage. However, sonication led to decreased irreversible fouling. ATR-FTIR spectra of membranes fouled with sonicated effluent showed that US modified the characteristics of the effluent organic matter (EfOM) by altering the structure/conformation of proteins, which apparently reduced their potential to adsorb to the membranes. US treatment followed by Al3+-based coagulation using aluminium sulphate or polyaluminium chlorohydrate (ACH) achieved a greater increase in permeate flux compared with coagulation alone. This effect was greater at relatively higher turbidity (8.0 NTU) compared with lower turbidity (i.e., 1.0 and 3.6 NTU). This increase was due, at least in part, to the breakdown of the particles to form more nuclei for coagulation. Furthermore, the physico-chemically modified EfOM may have enhanced interaction with the hydrolysed Al species, which led to the formation of a cake layer with lower filtration resistance. These changes influenced the size of the coagulation flocs, increasing the diameter by 20–30%. Flux recovery was greater when US was used before ACH, and decreased only slightly with successive fouling and cleaning cycles. The enhanced cleaning performance may be due to the lower affinity of ACH flocs for the membrane materials. As alum treatment alone achieved good flux recovery, there was not a significant benefit from employing US for mitigating irreversible fouling. This work demonstrated the potential of US for mitigating membrane fouling. Further work on the determination of the detailed mechanisms associated with enhancing coagulation would enable optimisation of the system with regard to process, cost and energy efficiency

Numerical evolution of plane gravitational waves

Unlike electromagnetic waves, gravitational waves self interact. This interaction is non-linear and can have very interesting properties which effect the curvature of space-time. A gravitational plane wave collider, implemented in the Python package COFFEE [20] that been developed in recent years by the Otago relativity group and implements the method of lines, can be reliably used to study this self-interaction. This was shown to work well numerically as profounded by Frauendiener, Stevens and Whale in 2014 [24]. For this reason, COFFEE will be used to study these gravitational wave propagations and subsequently collisions. The Einstein field equations are formulated as a well-posed initial boundary value problem (IBVP) in the Friedrich-Nagy gauge [26] and due to the large class of boundary conditions admitted by this framework, a variety of investigations into the propagation of plane gravitational waves could be carried out. This study focuses on the propagation of plane gravitational waves in the de Sitter (dS) space-time, which is the maximally symmetric solution of the Einstein’s vacuum field equations with a positive cosmological constant λ. There is substantial cosmological evidence that our universe is asymptotically de Sitter, yet no work, analytical nor numerical, has been done on gravitational plane waves propagating on such a space-time, mainly due to the increased complexity from the non-vanishing λ. Firstly, it is found analytically that with an arbitrary cosmological constant λ and a non-vanishing energy momentum tensor, the constraints will propagate. This means that we still have a wellposed IBVP, which is nontrivial since the Friedrich-Nagy gauge has only been shown to lead to a wellposed IBVP without matter [26]. Using this system, we consider one ingoing wave propagating on said space-time in vacuum. The area of the ingoing wave profile is varied and inferences are made about the different phenomena that arise in the curvature of space-time during the evolution. It is found that there exists a critical value of the wave’s area, ac, whereby taking the area below this value the system asymptotes to its initial state, and above the system diverges, indicating the presence of a singularity. Furthermore, we define an expansion parameter H to measure how the gravitational waves influence the accelerated expansion, generalising (numerically) results of Tsamis and Woodard.Thesis (MSc) -- Faculty of Science, Mathematics, 202