Civil Engineering Reference
In-Depth Information
the biomass is not always stable and the predominant species can change over a period
of time. The operator has little control over the species that populates the filter. Slight
changes in either pH, dissolved oxygen levels, or redox potential in the feedwater may
result in one species becoming predominate over another. The common iron-oxidizing
bacteria species all remove iron with similar efficiency, and the operator is usually not
concerned with the specific species in the filter. However, some species may require
slight adjustments in the operation of the plant, especially backwashing, and the op-
erator needs to be on the lookout for these changes.
The biological iron removal process is dependent on the pH and the redox potential
of the source water. If the redox potential is allowed to go too high, the iron will be
chemically oxidized and the process efficiency will drop. Because the filters are not
designed to handle a chemical precipitate, this condition can lead to filter breakthrough
of nonsoluble iron. The pH and redox potential conditions conducive to biological
oxidation were charted by Mouchet.
5
The conditions required for biological iron re-
moval are described in Table 14-4. Among the optimum conditions are a pH of 6 to
7.6 and a redox potential of 0 to 400 mV (NHE). A schematic for a typical two-step
high-rate biological process is shown in Figure 14-4.
Typical operating conditions for biological iron removal include:
Filtration rates of 5 to 15 gpm / ft
2
(12 to 37 m / h), depending on the iron con-
•
centration
Supporting media with an effective size of 1.0 to 1.5 mm
•
Media depth of 36 to 60 in. (0.9 to 1.5 m)
•
A start-up or seeding period generally of 2 to 7 days to establish the biomass in
the reactor
•
A relatively constant flow rate or stable water quality. The process does not react
quickly to rapid changes in the hydraulic loading on the filter or changes in source
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
•
Recovery of the biomass after normal operational shutdowns of 1 to 12 hr or after
backwashing is rapid, generally requiring less than 10 minutes. It is recommended that
a filter-to-waste cycle be employed on each start-up of the filter system following
short-term shutdowns and backwashing. For process shutdowns of several days, weeks,
or even months, the restart time is significantly longer; however, it is usually faster
than the initial reactor seeding. To ensure a high-quality finished water following a
system shutdown of more than 48 hr, it is necessary to provide for a start-up cycle
that could last from 10 minutes to several hours.
If the medium in a biological iron filter is kept wet during a filter shutdown, it will
recover rapidly from shutdowns of a few days to weeks or even months. If the filters
are drained and dry out during process shutdowns, the biomass will have to reestablish
