Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Immersed membrane systems hold many operational and environmental advantages in biological treatment of wastewater. However, immersed membrane filtration have only found application in niche markets to date because of higher capital and operating costs associated with membrane fouling. But with capital costs on the decline as membranes become less expensive, immersed membrane systems are increasingly considered as an attractive alternative to conventional treatment processes. Operating costs remain high however, since energy intensive techniques such as air-sparging are required to limit membrane fouling. Improving the air-scouring efficiency of air-sparged immersed membranes can significantly reduce operating costs and unlock the immersed membrane system technology to wider application. The aim of this study was to identify factors that will improve air-scouring efficiency in order to produce guidelines that will help in the development of an immersed microfiltration membrane system with a resulting lower operating cost. Although, the research was done on a flat-sheet microfiltration membrane, the guidelines obtained can be used for the development of any immersed microfiltration membrane arrangement. An airlift reactor set-up was chosen for this study. Six system hydrodynamic factors were evaluated in a factorial design to determine their effects on the cross-flow velocity profile. They were the downcomer area to riser area ratio, top clearance distance, bottom clearance distance, aeration intensity, water depth and air sparger location. It was found that the air-scouring efficiency was increased by generating a cross-flow velocity profile with increased magnitude and uniformity, but absolute uniformity of the cross-flow velocity profile was found to be a prerequisite for optimisation of air-scouring efficiency. Downcomer area to riser area ratio was found to be 99.9% significant in determining the magnitude of the cross-flow velocity profile. Two models were developed to respectively predict the relative magnitude and uniformity of the cross-flow velocity profile. By using these two models, a methodology was developed to design an airlift reactor set-up that would produce system hydrodynamics with an improved air-scouring efficiency. / AFRIKAANSE OPSOMMING: Gesonke membraanstelsels beskik oor talle bedryfs- en omgewingsvoordele in biologiese behandeling van afvalwater. Maar weens die hoër kapitaal- en bedryfskostes wat gepaardgaan met membraanbevuiling, kon gesonke membraanstelsels tot op hede nog net toepassing in nismarkte vind. Maar soos kapitaalkoste daal met al hoe goedkoper membrane beskikbaar, word gesonke membraanstelsels al hoe aanlokliker as ‘n alternatief vir konvensionele behandelingsprosesse. Bedryfskostes bly egter hoog aangesien energie-intensiewe tegnieke soos lugborreling benodig word om membraanbevuiling te vertraag. Deur die effektiwiteit van die skropaksie wat lugborreling aan gesonke membrane bied te verbeter, kan ‘n beduidende besparing in bedryfskostes teweeggebring word om sodoende die uitgebreide toepassing van gesonke membraanstelsel tegnologie moontlik te maak. Hierdie studie het ten doel gehad die identifisering van faktore wat lugskropaksie effektiwiteit kan verbeter en om riglyne op te stel vir die ontwikkeling van ‘n gesonke mikrofiltrasie membraanstelsel met gevolglik laer bedryfskostes. Alhoewel hierdie navorsing ‘n plat-blad mikrofiltrasie membraan gebruik het, kan die riglyne steeds vir enige gesonke mikrofiltrasie membraanuitleg gebruik word. Daar is besluit op ‘n lugligter-reaktor opstelling vir hierdie studie. Ses stelselhidrodinamika faktore is geëvalueer in ‘n faktoriale ontwerp om hul effekte op die kruisvloei snelheidsprofiel te bepaal. Hulle was die afvloei-area tot opvloei-area verhouding, topruimte-afstand, bodemruimte-afstand, belugtingsintensiteit, waterdiepte en belugterligging. Daar is bevind dat die lugskropaksie effektiwiteit verhoog word wanneer ‘n kruisvloei snelheidsprofiel geskep word met ‘n verhoogde grootte en gelykvormigheid, maar die absolute gelykvormigheid van die kruisvloei snelheidsprofiel is gevind om ‘n voorvereiste te wees vir optimale effektiwiteit. Afvloei-area tot opvloei-area verhouding is gevind om 99.9% beduidend te wees in die bepaling van die snelheidsprofiel se grootte. Twee modelle is ontwikkel om afsonderlik die relatiewe grootte en gelykvormigheid van die kruisvloei snelheidsprofiel te voorspel. Die modelle is in ‘n metodologie vervat vir die ontwerp van ‘n lugligter opstelling met stelselhidrodinamika wat verbeterde lugskropaksie effektiwiteit sal skep.

The use of staged scouring aeration to reduce energy for membrane fouling was studied using one pilot-scale submerged membrane bioreactor to treat municipal wastewater. The experiments were conducted by varying each of permeate fluxes, scouring air scouring intensities and sequence during both permeation and relaxation periods while keeping other factors same. The critical and recoverable fluxes were measured by the stepwise flux method. Mixed liquor, permeate and filtrate was characterized by analysing COD, cTOC, SMP contents, etc. The recorded transmembrane pressure data were used to calculate the fouling resistance after relaxation and fouling rate of each cycle. The results showed that when operated at relatively high permeate flux rate, membrane fouling could be effectively controlled by using relatively lower air scouring intensity and/or less infrequent aeration sequence during the permeation combined subsequently with more vigorous and frequent air scouring during the relaxation. At lower permeate flux rate with good permeability sludge, membrane fouling was effectively controlled by relatively low air scouring intensity and/or relatively infrequent aeration sequence during both permeation and relaxation periods. For each sludge condition, an optimal combination of cyclic air scouring intensity and sequence existed which could minimize the aeration energy consumption while maintaining effective fouling control. The frequency of aeration sequence plays a more dominant role than the air scouring intensity during the permeation in aeration optimization. / GE Water & Process Technologies Natural Sciences and Engineering Research Council of Canada

Membrane BioReactor (MBR) is a wastewater treatment method which combine both biological and physical cleaning. In the membrane tank there is membrane fibres which physically separate wastewater from containing organic material. This causes the membrane fibres to easily build fouling. A measure that can describe membrane fouling, is the pressure across the fiber membranes (transmembrane pressure). The higher the pressure, the dirtier the fiber membranes. Therefore the fiber membranes need to be cleaned frequently. A current applying cleaning method, membrane air scouring, works by releasing air bubbles from the bottom of the tank to remove organic fouling from the membrane fibres. Membrane air scouring accounts for a big part of the total energy consumption. Therefore research is needed to find development opportunities to counteract membrane fouling and to make membrane air scouring more energy efficient. In this master thesis, two different air scouring strategies are used in the membrane tank to reduce the development of the membrane fouling. The first aeration strategy, with lower air supply, was operated at a constant air supply of 5.04 m3air/m3permeate. The second aeration strategy, with higher air supply, varied instead the air supply linearly with increased incoming flow. The different aeration strategies were then compared with each other, and it was looked at which aeration strategy is the most energy efficient regarding energy consumption per permeate volume produced, to reduce membrane fouling.For three weeks, each aeration strategy was applied in the pilot plant at Himmerfjärdsverket. During the strategies, maintenance cleaning was done with oxalic acid, as the commonly used chemical sodium hypochlorite was not available. The transmembrane pressure in the membrane tank was analysed during the aeration strategies to investigate how the membrane fouling developed. The sludge properties of the water in the pilot plant were also analysed, to see if these had any influence on the membrane fouling. Energy calculations were also done to for quantify the energy consumption from the membrane air scouring.The thing that had the biggest impact on energy consumption was the size of the incoming flow. Then irreversible fouling of inorganic substances that clog the membrane pores. Finally, reversible fouling of organic material that forms cakelayers on the fibres. The investigation showed that the aeration strategy with the lowest air supply was enough to successfully suppress the membrane fouling and maintain a high permeability. In pilot-scale, consumed the air strategy with lower air supply 74 Wh/m3, while the higher air supply aeration strategy consumed 92 Wh/m3for the pilot-scale membrane air cleaning. At full scale, the aeration strategy with lower air supply is expected to consume 46 Wh/m3. In full scale, more frequent maintenance cleaning may need to be done, which means poorer filtration performance during washing periods. The recommendation is to test the aeration strategy with lower air supply and at the same time have more frequent maintenance cleaning, to investigate whether it is the most energy efficient option in the long term. / Membrane BioReactor (MBR) är en vattenreningsteknik som tillämpar både biologisk och fysisk vattenrening. I membrantanken finns det membranfibrer som fysiskt separerar avloppsvattnet från det innehållande organiska materialet. Detta får membranfibrerna att lätt bli nedsmutsade. Ett mått som kan beskriva membrannedsmutsningen är trycket över membranfibrerna (transmembrantrycket). Ju smutsigare membranfibrerna är, desto sämre genomsläpplighet får de och trycket som krävs att suga vattnet igenom dem ökar. Membranfibrerna behöver därmed rengöras ofta. I dagsläget görs det med hjälp av luftbubblor som skaver bort smutsen som finns på fibrerna. Att lufta membrantanken kallas membranluftrengöring och står för en stor del av reningsteknikens totala energiförbrukning. För att göra MBR-processen till en mer energieffektiv vattenreningsteknik, behöver kunskapen öka om hur nedsmutsningen av membranen kan motverkas och membranluftrengöringen bli mer effektiv.I detta examensarbete tillämpas två olika strategier för att lufta membrantanken. Den första luftningsstrategin, med en lägre lufttillförsel, drevs med en konstant lufttillförsel på 5,04 m3luft/m3permeat. Den andra luftningsstrategi, med en högre lufttillförsel, varierade istället lufttillförseln linjärt vid ökat inkommande flöde av avloppsvatten. De olika luftningsstrategierna jämfördes sedan med varandra med avseende på vilken luftningsstrategi som är mest energieffektiv med avseende på energiförbrukning per producerad permeatvolym.I tre veckor tillämpades varsin luftningsstrategi i pilotanläggningen på Himmerfjärdsverket. Under tiden utfördes det underhållsrengöring, dock med oxalsyra, eftersom den vanligtvis använda kemikalien natriumhypoklorit inte var tillgänglig. Transmembrantrycket i membrantanken analyserades under luftningsstrategierna för att utreda hur membrannedsmutsningen utvecklat sig. Slamegenskaperna för vattnet i pilotanläggningen analyserades också, för att se om dessa hade någon påverkan på membrannedsmutsningen. Det gjordes därefter energiberäkningar för att ta reda på membranluftrengöringens energiförbrukning.Det som hade störst påverkan på energiförbrukningen visade sig vara storleken pådet inkommande flödet. Därefter irreversibel nedsmutsning som igensätter membranporerna. Sist kom reversibel nedsmutsning av organiskt material som bildar beläggningar på fibrerna. Utredningen visade att luftningsstrategin med lägst lufttillförsel var tillräcklig för att lyckas dämpa membrannedsmutsningen och bibehålla en hög genomsläpplighet. I pilotanläggningen förbrukade luftningsstrategin med lägre lufttillförsel 74 Wh/m3, medan luftningsstrategin med högre lufttillförsel förbrukade 92 Wh/m3. I fullskala förväntas luftningsstrategin med lägre lufttillförsel förbruka 46 Wh/m3. I fullskala kan mer frekvent underhållsrengöring behöva göras, vilket innebär sämre filtreringsprestanda under tvättperioderna. Rekommendationen är att testa luftningsstrategin med lägre lufttillförsel och samtidigt ha mer frekvent underhållsrengöring, för att utreda om det är det mest energieffektiva alternativet på lång sikt.