Civil Engineering Reference
In-Depth Information
pH
Adjustment
Backwash
Waste
Backwash
Waste
Air
Biological
Iron Filter
Biological
Manganese
Filter
Primary
Disinfection
Cl
2
Cl
2
Well or
Water Source
Unchlorinated
Backwash
Supply
Blower
Unchlorinated
Backwash
Supply
Finished Water
to Distribution System
Filter Loading = 10-15 gpm/ft
2
Filter Run Lengths = 48 hr
Filter Loading = 5-15 gpm/ft
2
Filter Run Lengths = 48 hr
Filter Efficiency = 99%+
Area Required = 47 ft
2
/MG
Filter Efficiency = 99%+
Area Required = 47 ft
2
/MG
Fig. 14-4.
Biological filtration for iron and manganese removal
itself for the process to regain its efficiency. The actual length of time required for
this reseeding will depend on the conditions at the facility.
Mouchet reported solids retention on biological iron filters to be 3 to 5 times greater
than for conventional oxidation filtration processes.
5
This additional loading capacity
results in longer filter runs and produces a denser backwash floc with superior settling
and handling characteristics. The backwash water supply must be unchlorinated to
protect the biomass on the filter. The backwashing process could be described as more
of a rinsing or flushing that removes the old and dead bacteria and allows new bacteria
to take over. The low backwash rates used for biological iron filters reduce the required
size of the backwash supply system and backwash storage basin, and they increase
the water production efficiency of the treatment plant. The backwash solids can readily
be handled in the sanitary sewer system or can be dried and disposed of in a landfill.
Biological Manganese Removal
Biological manganese removal requires signifi-
cantly different conditions than biological iron removal. Manganese bacteria are strictly
aerobic organisms and prefer a higher pH than iron bacteria. Saturated dissolved ox-
ygen conditions are required for manganese bacteria to grow and thrive. As mentioned
earlier, to allow the high filter loading rates normally used with biological filtration,
manganese and iron are not generally removed in the same biological reactor. Figure
14-4 illustrates a two-stage high-rate biological process for removing iron and man-
ganese.
Manganese oxidation by bacteria takes place by any one of three processes: (1)
primary intracellular oxidation by enzymatic action; (2) adsorption of the dissolved
manganese at the surface of the cell membrane, followed by oxidation by enzymatic
action; or (3) simple catalysis in the vicinity of the cell, under the influence of the
biopolymers secreted by the bacteria.
5
The actual process used to oxidize the man-
