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Membrane Systems Planning and Design 381
inlet structure until the system is fully commissioned. Any state requirements governing the
disposal and potential recycle of the water produced during the start-up phase should be
incorporated into the planning process.
7.2. Flushing and Testing Without Membranes
Prior to installing the membrane modules, any debris introduced during construction
should be flushed from the system. This flushing is usually conducted by running the
appropriate pump(s) at high velocity and low pressure through the piping and discharging
the water to a suitable drain. Typically, the flush water can be discharged to the sanitary
sewer, although the utility should comply with any state requirements for disposal. Because
the intent of the flushing process is to remove debris from the system, this water should not be
recycled unless pretreatment processes would be expected to remove contaminants flushed
from the system.
After the piping system has been flushed, the operational sequences and chemical addition
systems should be tested to ensure that they are operating properly before installing the
membrane modules. Because of the complexity of the equipment involved, it is suggested
that a plan for conducting these diagnostic checks be developed in advance. In general, the
recommended testing can be divided into the following categories (1):
1. Mechanical equipment – Inspect automated mechanical equipment to ensure that it is properly
installed and that there are no leaks in the piping system. Operate mechanical equipment in
manual mode and then in automatic mode to verify that it is working as designed.
2. Instrumentation – Verify that the instrumentation is properly installed and calibrated. Confirm that
the instruments are operating correctly and are responsive to the control system.
3. Control system – Verify control inputs and outputs (both digital and analog), instrumentation
alarm limits, programing logic, instrumentation loops, and operational sequences.
4. Membrane units and related systems – Verify that the membrane filtration system and each
respective membrane unit can be both started and shutdown smoothly. Operate the chemical
feed systems (if any) to verify that they are each in proper working condition and that all
chemicals are delivered at the proper dosages.
For NF/RO systems, the procedure for testing the operating sequences is commonly known
as the “48-h test,” as it may take two or more days to complete. Unlike the initial flushing, the
48-h test should be conducted at actual operating parameters, including both typical pressures
and flows. Therefore, it is common to insert flow restriction devices in the system in place of
the membrane modules during this operation to simulate the anticipated system backpressure.
These devices typically consist of orifice plugs placed in the permeate (i.e., filtrate) port
inside the pressure vessel. The orifice plugs are sized to simulate the flow and pressure
parameters of the operating membrane filtration system. Alternatively, an isolation valve on
the permeate piping could be throttled to the appropriate backpressure. Although the term
“48-h test” is less commonly used in association with MF and UF, these systems are also
generally tested either with “dummy” modules that have similarly designed orifices or by
throttling filtrate effluent valves to create sufficient backpressure during the testing
sequences.