Civil Engineering Reference
In-Depth Information
distribution system. It is recommended that several filter volumes be wasted following
extended filter shutdowns.
Typical filter loading rates for biological manganese filters are dependent on the
manganese concentration and generally range from 4 gpm / ft
2
(10 m / h) for 2.0 mg / L
Mn to 16 gpm / ft
2
(40 m / h) for 0.5 mg / L or less Mn.
4
Filter runs are typically 24 to
72 hr, depending on the raw water quality.
As with biological iron removal, solids retention on a biological manganese filter
is 3 to 5 times higher than with conventional filtration. With these filters, the objective
of backwashing is not to clean the filters but to provide additional space for bacteria
to grow. The backwash water is unchlorinated to protect the biomass and is applied
at a lower rate than for conventional filter backwashing. The dead or old bacteria are
rinsed or flushed from the filter during the backwashing cycle, which is of short du-
ration. This reduced backwash requirement results in smaller facilities in terms of both
backwash supply and backwash storage and handling. The backwash residuals can
either be wasted to the sanitary sewer system or dried and disposed of in a landfill.
Biological Ammonium Nitrification
If ammonium is present in the water and bi-
ological manganese treatment is going to be used, the ammonium will be nitrified first
before the manganese can be oxidized. The designer must be aware of these conditions
and construct the system to handle the ammonium present. The biological conversion
of ammonium to nitrates is a common wastewater process that is well documented in
textbooks and the literature. The removal or nitrification of ammonium in drinking
water is not a common process. This is due to the fact that ammonium usually occurs
in very low concentrations in a water source and these low concentrations are oxidized
during chlorine addition. Because of its interference with biological manganese re-
moval, the process is described here. Nitrification of the ammonium is accomplished
by obligate autotrophic aerobic bacteria that oxidize the ammonium in the presence
of oxygen.
14
The conversion of ammonium to nitrates is described by the reaction
given in Table 14-4.
Typical operating conditions for ammonium nitrification include the following:
Filtration rates of 2 to 5 gpm / ft
2
(5 to 12 m / h), depending on the
NH
4
concen-
•
tration. Higher filtration rates may be possible for low
NH
4
concentrations.
For
NH
4
effective size, with a media depth of 36 to 60 in. (0.9 to 1.5 m)
concentrations less than 2.0 mg / L, supporting media of 1.0 to 1.5 mm
•
For
NH
4
provide sufficient dissolved oxygen for nitrification
concentrations greater than 1.0 mg / L, a filter bed that is aerated to
•
For
NH
4
monia removal process to reduce ammonia concentration below 2 mg / L
concentrations greater than 2.0 mg / L, provide special nitrification / am-
•
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 lengths of 48 to 96 hours
•
Backwash conditions:
•
•
Unchlorinated backwash supply water
