Civil Engineering Reference
In-Depth Information
ganese is dependent on the predominant bacteria species and the environmental con-
ditions in the filter. In all cases, the oxidized manganese is deposited as MnO
2
as
described in Table 14-4.
Typical operating conditions for biological manganese removal include the follow-
ing:
Filtration rates of 5 to 15 gpm / ft
2
(12 to 37 m / h), depending on the manganese
•
concentration
Effective size of supporting media of 0.8 to 1.0 mm
•
Media depth of 36 to 60 in. (0.9 to 1.5 m)
•
Saturated dissolved oxygen conditions
•
A start-up period generally of 6 to 12 weeks to establish the biomass
•
A relatively constant flow rate and stable water quality. The process does not react
quickly to rapid changes in the hydraulic loading on the filter or to changes in
water quality.
•
Filter run times of 24 to 72 hours
•
Backwash conditions:
•
•
Unchlorinated backwash supply water
Air scour (usually implemented)
•
Low-rate backwash at 4-6 gpm / ft
2
(10-15 m / h)
•
High-rate backwash at 10-12 gpm / ft
2
(24-29 m / h)
•
Total backwash duration of 5 to 10 minutes
•
Biological manganese removal is more difficult to establish than biological iron
removal. It generally takes 6 weeks to 2 months for the bacteria to seed the reactor,
but once established the biomass is reasonably stable and will recover from back-
washing and short shutdown periods of 1 to 12 hours within 10 minutes at typical
filtration loading rates. As with biological iron removal, it is recommended that the
treatment facility be able to filter to waste.
If the biomass in the manganese removal reactor is kept wet during process stop-
pages, it will recover readily from shutdowns of a few days to weeks or even months.
This recovery time is significantly shorter than the initial seeding time for the reactor,
generally taking 60 minutes to a few days depending on the flow rate and the length
of system shutdown. If the biological filters are allowed to drain and dry out during
process shutdowns, the recovery time will be slower but will still not be as long as
the original seeding and media-ripening period. The operator should plan for this
recovery time and adjust plant operations accordingly to protect consumers when the
plant is restarted following maintenance shutdowns.
In pilot tests of a biological manganese reactor, researchers noted a significant spike
in the manganese levels of the filter effluent following extended shutdowns of several
days to several weeks.
15
On start-up, these levels were two or three times higher than
those of the raw water. The researchers speculated that this was the result of bacteria
sloughing off the filter into the finished water on filter restart. The spike was short,
and the finished water manganese levels dropped rapidly after two or three filter vol-
umes of water had passed through the bed. Utility personnel need to be careful when
restarting the filters after extended shutdowns to prevent this spike from entering the
