The MBR Book: Principles and Applications of Membrane

In short, the design calculation demonstrates the principle of MBR process design
and can provide trends in operating costs without necessarily providing absolutely
correct values.
3.3.4 Design and O&M facets
Design 159
As well as those facets already discussed, a number of practical aspects of MBR plant
design and operation impact upon performance. Some of these are listed below, and
further details can be found in WEF (2006).
3.3.4.1 Process control and software
● As with other process plant for water and wastewater treatment, feedback
control and alarm triggering relies on monitoring of key parameters such as
TMP (for indicating membrane fouling condition and triggering a cleaning
cycle), DO (for biological process control) and turbidity (for membrane
integrity).
● Unless they are specifically designed to be free from clogging, it is essential that
the maintenance schedule includes cleaning of the aerators. This can be
achieved by flushing with a dilute hypochlorite solution. For some technologies
the aerator flush is scheduled at the same time as the maintenance clean.
● The principle impact of added process complexity is on the software and programmable
logic controllers (PLCs), and on ancillary hardware such as pumps,
valves and actuators. For example, intermittent aeration at 10 s means that
each valve is actuated pneumatically over 3 million times per year.
● Foaming control and abatement procedures are particularly important in an
MBR since aeration is more intense than for an ASP. Designs should include
surface wasting or spraying.
● Sludge wasting for SRT control can be based on on-line MLSS measurement,
although instruments have only recently developed for this.
3.3.4.2 Pre-treatment and residuals management
● The basic process can be modified in the same way as a conventional ASP to
achieve denitrification, chemical phosphorous removal (CPR) and biological
nutrient removal (BNR), as indicated in Section 2.2.6. Additional tanks (or
tank volume) and sludge transfer pumps must then be sized accordingly,
based on similar biokinetic principles as those used for the design for the core
aerobic process.
● Pre-clarification can be used to reduce aeration demand (for agitation of biomass)
by reducing solids (“trash”) loading, but this significantly adds to footprint.
● Upgrading screens is essential, especially for HF iMBRs (Section 2.3.9.1). Grit
removal is desirable for smaller plants if no capacity is available to allow the
grit to settle out before the membrane tank.
● MBR sludge is generally less settleable than ASP sludge, with floc sizes generally
being smaller (Section 2.2.6) and sludge volume index (SVI) values
higher. Conventional gravity thickening is therefore less effective for MBR
sludges. Membrane thickening can and has been used for this duty, albeit
operating at necessarily low fluxes.