Membrane Bioreactor (mbr) as an Advanced Wastewater Treatment Technology
Advantages and Drawbacks of MBR Technology
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Advantages and Drawbacks of MBR Technology A need for the development of MBR technology arose mainly from the limita- tions of the CAS process. It is of interest here to describe CAS in more detail, since it has been used successfully for almost a century in wastewater treat- ment. CAS in its most simple manner consists of a primary physical treatment that includes screening of gross solids and sedimentation of settleable solids followed by biological treatment with activated sludge, and subsequent sec- ondary sedimentation where activated sludge in the form of flocs is separated from treated water by gravitational force. The biological step is carried out in an aerated bioreactor in the presence of mixed microbial culture, where pollu- tants from water are degraded by microorganisms and turned into microbial biomass and gases such as carbon dioxide, water, and inorganic nitrogen products. This stage may include an anoxic zone preceding the aerobic zone within the single reactor or a separate post-denitrification reactor to achieve a complete nitrogen removal. Also, preliminary anaerobic zone for biologi- cal removal of phosphorus is available. The settled sludge is returned to the bioreactor while excess sludge additionally grown during the process is being constantly removed. Essentially, the process uses aerobic microbial biodegra- dation of organic matrix presented in wastewater in the same manner as the natural microbial community in the water bodies would, if given enough time 86 J. Radjenovi´c et al. and oxygen. The final products of the process are treated water and excess sludge. Treated water is usually discharged into water bodies such as lakes and rivers, while excess sludge ends up mostly as a fertilizer in agriculture or it is disposed of on land. Some countries like Germany and Switzerland forbade the use of secondary sludge in agriculture and excess sludge is incinerated together with hazardous wastes. In any case, the processing of this sludge, which includes operations like thickening, anaerobic stabilization, chemical conditioning, dewatering and thermal reduction [260], represents a cost and a problem that has to be dealt with. Land application of sewage sludge in agri- culture is very restricted owing to the presence of potentially toxic substances, i.e., heavy metals, pathogens, persistent organic pollutants, etc. Critical short- age of available land coupled with new, more-stringent regulations for design and operation of landfills have caused prices of their sighting, building, and operating to rise sharply. Incineration is usually the final option for sewage sludge treatment due to an abundant ash generation, which has a high content of heavy metals and is generally toxic. Therefore, high sludge production is one of the main drawbacks of CAS. Currently, reduction of sludge wasting is a major challenge of biological wastewater treatment. Excess sludge processing and disposal could account for about 50–60% of the total cost of wastewater treatment [261, 262]. The ideal way to solve the problem of sludge post-treatment and disposal is to reduce its production. To reduce the production of biomass, the wastewater process must be engineered in such a way that substrate utilization is diverted from assimilation of carbon for biosynthesis to non-growth activities of a mi- crobial community. In activated sludge plants, the sludge-yield coefficient (Y) is typically 0.5. [263]. According to Urbain et al. [264], the yield coefficient for an aerobic membrane separation process treating municipal wastewa- ter (488 ± 143 mg COD/L) was 0.23 kgSS kgCOD –1 removed . Pollice et al. [99] reported a production of sludge in an MBR of 0.12 gVSS gCOD –1 removed , which was in accordance with previously reported yields for MBRs [34, 42, 65, 263]. This advantage of MBR, together with the abandonment of energy- demanding sludge recirculation loop in CAS, contribute to better competi- tiveness of membrane technology compared to the conventional one. The limiting step in the conventional treatment is the separation of sludge from the treated water. Without a good sedimentation in secondary settler, tải về 0.95 Mb. Chia sẻ với bạn bè của bạn:
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