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MBRs are generally viewed as providing a higher cost solution than conventional treatment, while also achieving a higher treated water quality and incurring a lower process footprint. A full needs analysis is required to assess whether an MBR offers the most cost-effective solution for a given application, taking everything into account.
Cost factors to take into account when considering MBR technology include: capital cost (CAPEX), operating cost (OPEX), overall cost (TOTEX).
The key components of OPEX are the energy consumption, staffing requirements, membrane replacement, chemicals consumption, and waste management and disposal. The most significant of these is the energy consumption, primarily from aeration.
Aeration often represents the most significant contributor to the energy consumption. Aeration is used for both the membrane and process tanks in an immersed MBR. For 'pumped' sidestream MBRs aeration is normally used only for the process tanks.
Sludge (or mixed liquor) pumping from the process tank to the membrane modules can be the most significant contributor to energy consumption in a sidestream MBR, the pressure applied and crossflow generated both promoting the permeate flux. In an immersed MBR sludge pumping is often the second most significant contributor to MBR energy demand after aeration.
Costs associated with the pumping of water through the membrane relate to the transmembrane pressure (TMP), the flux and the operating protocol. The energy demand for membrane permeation tends to be relatively low for an immersed MBR. For a sidestream MBR the flux is promoted by the mixed liquor pumping.
Total costs of an installation comprise the capital (or investment) expenditure (CAPEX) and the operating expenditure (OPEX). The OPEX in $ per m3 permeate is given by the combined cost of energy demand, critical component replacement, chemicals consumption, waste management/disposal and labour.
Capital (or investment) costs are strongly location dependent, but are normally slightly lower for an MBR due to the reduced footprint of the tanks compared with a conventional process.
The primary contributors to the energy consumed by an MBR during its operation comprise the aeration of the process biology, and either the membrane air scour (for an immersed MBR) or the pumping of mixed liquor through the membrane modules (for a sidestream MBR).
Apart from the principal MBR OPEX components of energy expenditure and labour effort, the other key contributors are membrane replacement, chemicals consumption and waste management.
