Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE credits
The use of membrane bioreactors in municipal wastewater treatment is of growing importance,since stricter rules and regulations regarding design and effluent limits are trends in theenvironmental protection programs applied throughout the world. The idea of the membranebioreactor is a replacement of the secondary clarifier and additional stages like sand filter anddisinfection present in conventional wastewater treatment plants using the activated sludge process.The MLSS in a MBR is very high, which involves the advantage of a small reactor volume. One ofthe features of the membrane bioreactor is a reduced production of excess sludge, which generallyoccurs if the sludge loading ratio is low. As a consequence, biologically accumulated phosphoruswill stay within the reactor for a long period of time. Since growth of biomass is a phosphorusconsuming process and a reduced production of excess sludge will result in a lower storage ofphosphorus in the sludge, the most probable way to reach low contents of phosphorus in theeffluent will be achieved by means of chemical precipitation, either simultaneous or preprecipitation.
The operation of two MBR pilot plants, situated at the Soers wastewater treatment plant in Aachen,has led to the need of investigating the ability of the plants to keep a limit of 1 mg/l phosphorus inthe effluent. Especially when the sludge load is low, the limit may be exceeded due to biomassstarvation. In this project, the ability of the plants to eliminate phosphorus by means ofsimultaneous and pre-precipitation at normal and half flux is investigated in order to simulatenormal and bad conditions with respect to sludge load. The influence of simultaneous precipitationon the active sludge nitrification, suitable precipitant dosage, critical substrate concentration andluxury uptake ability are studied. The outcome of the project is the development of a concept forthe removal of phosphorus when applying membrane bioreactors in municipal wastewatertreatment.
The gathered results obtained in the investigations indicate that a lack of substrate after the presettlingoperation could amount to starvation of the biomass and an impaired denitrification, whypre-precipitation should be used with great care. The optimum conditions for a successfulelimination of phosphorus would instead be with the use of simultaneous precipitation at normalflux. A sufficient β ratio would be at about 0.6. However, when the flux through the plant isreduced to the half and the sludge load is reduced accordingly, the β ratio used in the simultaneousprecipitation should lie at about 1.5 since the elimination of phosphorus is more difficult the lowerthe flux. With these conditions, the critical limit of 1 mg/l P will be possible to keep without anydamages on the active sludge or the membranes.
At very low flows for a long period of time, simultaneous precipitation would be insufficient due toa release of phosphorus by the biomass. In this extreme situation, additional measures must beapplied.