Optimierung der Energiebilanz von Membranbioreaktoren

• Optimizing the energy balance of membrane bioreactors

Krebber, Katrin Nikola; Pinnekamp, Johannes (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2013, 2014)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2013

Abstract

When used to treat municipal wastewater, membrane bioreactors (MBR) attain excellent water quality and an extensive retention of bacteria and viruses through filtration processes. In comparison to conventional wastewater treatment, MBR have other advantages, such as a smaller footprint. Disadvantageous is the increased electricity consumption. During filtration, a cake layer occurs on the membranes, which has to be constantly removed through energy-intensive aeration underneath the membrane module. The energy balance of MBR is also worsened since they are dimensioned and operated with a high sludge age as simultaneous aerobic stabilizing plants. This increases electricity consumption due to the microorganisms’ increased oxygen demand; furthermore, no electricity is generated by anaerobic sludge digestion. Within the scope of this work, measures to optimize the energy balance of MBR were developed and substantiated with examples from the field. To accomplish this, data from the multi-year operation of seven large-scale MBR plants were used. Moreover, a model plant was developed to test the approaches for optimization of MBR regarding their effects upon energy efficiency and to investigate new concepts theoretically. The mean specific electricity consumption of the MBR investigated lies, depending upon the limiting conditions and aggregates recorded, between 0.73 and 1.83 kWh/m3, or approx. 49 and 208 kWh/(PE•a)). The membrane blowers consume the most electricity, on average 53% of the overall electricity consumption. There are various approaches to optimize them. The intensity of the membrane aeration is given by the manufacturer and independent of the flux. Therefore, the operation with as high and constant flux as possible is advantageous. This is, however, dependent upon several limiting conditions specific to the plant itself and constantly a compromise between electricity consumption and formation of the cake layer. Furthermore, the intensity of the membrane aeration needs to be checked in order to avoid over-aeration and to adapt it, if necessary, to the requirements, e.g. with frequency conversion. As aeration with coarse bubbles, this conversion should never be activated for oxygenation. After a long downtime of a membrane chamber, the membrane aeration is used to prevent anaerobic conditions and the activated sludge from settling. To limit this forced aeration, the active membrane chambers should be changed, if possible, after every filtration interval. Filtration occurs in a pulse-pause interval in order to remove cake layers effectively. Thus, the filtration is interrupted periodically, but aeration continues. For this reason, filtration cycles should not be broken off ahead of schedule so as to counteract an unfavorable ratio between aerated filtration pauses and permeate gained. Before filtration begins, sufficient wastewater must be stored for a complete filtration cycle. With other aggregates, such as permeate pumps, recirculation pumps or stirrers, potential for optimization has also been identified. Compared to the integrated arrangement in the aeration tank, the arrangement of the membrane modules in separate membrane chambers has disadvantages regarding energy consumption, but it also offers operational advantages. The specific electricity consumption falls as the hydraulic load increases. This is, however, low - on average 14% to 45% in the MBR under investigation. Thus, operation management that takes the amount of plant influent into account makes sense regarding energy consumption. Initially, the number of filtrating modules has to be adapted to the influent. Furthermore, the operation of energy-intensive aggregates should be adapted to the influent. If the running time of the aggregates correlates with the hydraulic load of the plant, this signals that the mode of operation is oriented towards energy consumption. Previously, MBR have been built as plants with simultaneous aerobic sludge digestion with a design sludge age of 25 d without primary sedimentation. The oxygen demand increases as the sludge age increases since organic components continue to be mineralized. Partially, the sludge age is very high in the MBR under investigation and there is potential to optimize the process by reducing the sludge age. Operating a primary sedimentation has a positive effect for power consumption as well as for the stable operation of an MBR, as the experience with two of the MBR investigated and the calculations using the model plant suggest. The investigations on anaerobic sludge digestion using the model plant resulted in a total possible savings of between 0.25 kWh/m³ (16 kWh/(PE•a)) and 0.32 kWh/m³ (20 kWh/(PE•a)), which corresponds to 39.5% to 53.6%, respectively, when compared to the conventional configuration. It is recommended, based on energy consumption, to build every larger MBR with primary sedimentation and anaerobic sludge digestion and to utilize the digester gas.