Backwashing should occur when the head loss reaches about 69 kpa (10 psi.) and the duration of the backwash should be around 10 to 15 minutes allowing the system to unclog the settled insoluble iron and manganese oxides trapped in the filter. Filter cracking can occur which will affect apparent head loss. Filters should be backwashed everyday, but no less than every 2 days to prevent cracking. It is very important not to underfeed the amount of permanganate added to the pretreatment process or else the greensand filter will lose its oxidative properties. However, if the potassium permanganate charge is somehow lost in the filter, the operator can regenerate the greensand manually. The filter must be first shut down. Then, a saturated solution of potassium permanganate (around 5%) is poured into the filters and left to sit for 24 hours.
After
24 hours, the system is backwashed and restarted. Another way the system can be
recharged without shutting down is by increasing the potassium permanganate
dosage until pink water flows out of the bottom of the greensand filter. When
the pink water flows out of the filter, the filter is recharged and regular
doses of potassium permanganate can continue.
The
operator should perform iron, manganese, pH and chlorine residual tests on a
daily basis in order to determine if there are any problems in the system. Remember,
the above is only meant as a guide. Specific backwash requirement are site and
equipment specific. Refer to manufacturer specification and procedures as they relate
to your plant.
For your safety
When mixing, always add chemicals to water. Never add
water to chemicals
|
The
recommend backwash rate for manganese greensand is 12 gpm/sq. ft. of filter
area at 60 degrees Farenheit. This rate is sufficient to expand the bed 35–40
percent. Please note that backwash rates versus filter loading rates can cause
serious problems in smaller treatment units. For example, a small installation
with one 12-inch inside diameter filter, will require the well pump to deliver
12 gpm to properly backwash. However, if high levels of iron are present, that
same unit may only be capable of filtering 2 gpm.
The backwash
cycle is used to remove impurities that have collected in the media bed. When
the backwash cycle is initiated, the backwash inlet valve must not open
instantaneously. With the high flow rates used in the backwash cycle,
"water hammer" will occur if the valve is opened quickly. "Water
hammer" can disrupt the support layer of the greensand filter. The control
system must be able to control the opening speed of this valve to eliminate
"water hammer."
During the
backwash cycle, the valves are oriented to reverse the flow of water from
normal operation. With sufficient flow, impurities are loosened from the media
bed and carried out of the bessed through the inlet distributor and service
inlet. The media bed must be expanded by 30% for the backwash to be effective.
To prevent filter media particles escaping from the vessel, the inlet
distributor must be sufficiently higher than the top of the expanded bed. The
valve configuration used during backwash cycle of the greensand filter system
is shown in below Figure.
- Service Inlet valve closed (to prevent incoming water from flowing against the backwash flow)
- Service Outlet valve closed (to prevent dirty washback water from contaminating downstream equipment)
- Backwash Inlet valve open (to provide a supply of water from the hub/lateral underdrain to backwash the media bed)
- Backwash Outlet valveopen (to set the flow rate and carry away the dirty backwash water from the inlet distributor to drain)
- Rinse Outlet valve closed (to prevent water from the wrong part of the vessel going to drain)
Backwash
continues for a specified time (usually 15 minutes). After the backwash cycle
is complete, the vessel is rinsed and can then return to normal service.
Backwash Flow
The backwash flow rate is equal to the flow rate required to increase the
bed depth by 30%. The flow rate depends on temperature, since the force pushing
the particles up is function of the viscosity of the water, which decreases
with increasing temperature. The sub-surface wash uses the same flow rate as
the backwash. The table below shows the flow rate based on temperature.
Backwash Flow = Backwash Flow Rate x Diameter2 x π/4
Greensand
Filter Backwash Rates Table
Temperature
(°F)
|
Flow
(gpm/ft2)
|
32 to <
40
|
7.2
|
40 to <
50
|
8.4
|
50 to <
60
|
9.7
|
60 to <
70
|
11.0
|
70 to <
80
|
12.8
|
80 to <
90
|
14.3
|
90 to <
100
|
16.0
|
100 to
< 110
|
17.8
|
110 to 120
|
19.6
|
The greensand media has a fixed amount of iron and manganese it can
remove before it gets exhausted and needs to be regenerated. It depends on the
amount of iron and manganese you have and if there is hydrogen sulfide
(“rotten-egg odor”) present.
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