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6.6.9 Backwash of UF membrane
This section refers to fig.: 6-8.
A backwash (BW) is performed to remove fouling on the surface
and in the pores of the UF membrane (13). There are several
backwash types:
Included backwash steps:
Backwash type
1
2
X
3
4
X
5
6
7
X
8
X
In general, a backwash consists of the following steps, which are
carried out in listed order:
1. Air scouring
2. Draining
3. Backwash of the bottom of the UF membrane
4. Backwash of the top of the UF membrane
5. Forward flushing of the UF membrane
The backwash types 6, 7 and 8 are also referred to as Chemically
Enhanced Backwash (CEB), because sodium hypochlorite
solution is added to the backwash flow. For these backwash
types a chlorination module is required.
Activation
The task can be activated by different criteria:
•
Activation each time a predefined volume of water has passed
UF membrane (13).
•
Activation after long idle time.
•
Activation at the end of production runtime. See also section:
6.6.3 Procedures for start and end of
•
Manual activation, when the system is in semi-automatic
mode.
For each of these criteria a different backwash type can be
predefined at the system. Observe section:
parameters.
Furthermore, the following criteria must be met:
•
All conflicting tasks must be inactive. If
is stopped.
•
There must be enough water in the internal tank (46).
– If there is not enough water in the internal tank, the system
starts
Production
until enough water is prepared and then
activates the task.
•
No active alarms.
Backwash at the end of production runtime
The backwash at the end of production runtime is performed to
make sure, that fouling is removed from the UF membrane, and
that the UF membrane is filled with clean water while the system
is in a passive state. Typically type 5 backwash is chosen. If a
backwash type including forward flushing is chosen, feed water is
flushed along the outside of the membrane fibres as the last step.
The feed water increases the fouling in the membrane while the
system is passive, which is undesirable. Backwash at the end of
production runtime should never include forward flushing.
22
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
day.
6.6.4 System
Production
is active, it
Adaptive activation volume
Once a BW has been activated, the counter for the water that has
run through the membrane and the counter for the idle time are
reset. The volume of water that must run through the membrane
before a BW is activated is automatically changed by the system
to optimise the performance. The system changes the water
volume, if the Trans-Membrane Pressure (TMP) value for the UF
membrane increases too fast.
Alarm handling
If an alarm is activated while the task is active, the following
sequence is activated:
1. Pumps (GP1) and (GP2) are stopped. If present, the dosing
pump (GP5) is stopped as well.
2. Compressor (GP4) is stopped.
3. All motor valves (MK) are closed.
The system remains stopped, until the problem is remedied and
the [Reset] button is pressed at the HMI. Alternatively, certified
service staff can reset the alarm in the service menu. If the
system is in automatic mode and the alarm is reset, the task is
repeated.
6.6.10 Internal CIP
This section refers to fig.: 6-9.
This task is only relevant, if an internal CIP module and a
chlorination module are part of the system.
The
Internal CIP
task is an enhanced variant of the
UF membrane
task and removes fouling in the UF membrane.
Internal CIP
is less frequently activated and more time-
consuming.
Internal CIP
establishes a closed loop with sodium
hypochlorite solution around the UF membrane. The system
circulates the sodium hypochlorite solution through the UF
membrane to dissolve the fouling. The
of the following steps, which are carried out in listed order:
1. Draining
2. Backwash of the bottom of the UF membrane
3. Backwash of the top of the UF membrane together with
enabled circulation pump (GP3).
4. Sodium hypochlorite solution is added to the water
5. The chlorinated solution is circulated in a closed loop around
the UF membrane. The circulation is interrupted cyclically for
soaking.
6. Dilution where the sodium hypochlorite solution is mixed with
feed water.
7. Backwash type 1.
Activation
The task can be activated by different criteria:
•
Manual activation, when the system is in semi-automatic
mode.
•
Activation by internal CIP flag.
•
Activation by increase in TMP value. If the TMP value has
increased more than a predefined value since the last internal
CIP, a new internal CIP is activated.
•
Periodic activation. The automatic activation must be carried
out with a minimum periodic interval, e.g. once every 14 days.
The time interval in days is a predefined parameter.
Activation by internal CIP flag occurs, when the backwash water
volume has reached its minimum limit and the internal CIP flag is
set.
Internal CIP
must be carried out at the first possible
opportunity, which is always at the end of production runtime.
Internal CIP
is also activated, when the TMP value of the UF
membrane has increased more than a predefined value since the
last internal CIP.
Periodic activation can also occur, if an internal CIP has not been
activated within the last YY days (e.g. 14 days). It is not desired
to have the internal CIP activated too often. The frequency limit
(max one internal CIP every 7 days) is determined by a
predefined parameter. If the frequency limit is reached, the feed
pump flow is reduced to mitigate the fast fouling. If a minimum
flow is reached, the system requires an external CIP and stops.
Backwash of
Internal CIP
task consists
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