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9.18 CHAPTER NINE

30

BFO BF3.4 BF 3F27, BF20
25
,., ,, ......-......... -----

o,
20
I / / oo
I
. , ,** ,,,,,," A....~25 psi Available Head
' ~ ,,'*o °* .
15
• o o og
"~ Fixed He ad Loss
~
10
0 10 15 ' 20 25 30 35 40
Time, h
FIGURE 9.8 Effect of varying body feed addition.

are a mixture of relatively inert matter and solids that are predominantly organic. If source
water is filtered through the precoat alone, the buildup of solids and compression of the
accumulated cake quickly reduce filter cake porosity, and head loss increases at an ex-
ponential rate. This may be avoided by adding body feed to the source water in sufficient
amounts to produce a constant flow versus head loss relationship (Figure 9.8).
Although the rate of flow does not affect effluent quality or turbidity breakthrough,
the flow rate for precoat filters should generally be limited to about 2 gpm/ft 2 (7.8 m/h).
The shape of the precoat filter head loss curve that reflects both feed and flow conditions
is, therefore, an important feature to control effective filter run performance.

Supplementary Treatment. Supplementary measures may be added to the basic DE fil-
ter process to enhance the filtration process and to expand the process to remove some
nonparticulate constituents.
Natural color in source water supplies can be caused by either organic or mineral mat-
ter. Color can result from the decay of plant matter or from the solubilization of iron in
the soil, and in many instances the mineral and organic matter may be bound together.
Therefore color can be present either in particulate form or in solution. Particulate color
consists mostly of negatively charged colloids, and even though the precoat medium has
low pore size, charged colloids pass through unless the charge is neutralized. The use of
a strong oxidant such as ozone has been demonstrated to be effective in conditioning color
for removal.
When color is particulate rather than dissolved, DE filters reduce source water color
of about 25 color units (CU) and less to below 5.0 CU. With source color between 25
and 60 CU, filter effluent is generally no higher than 10 CU. Supplemental treatment such
as preozonation or alum-coated media may be required to improve removal of particulate
color and to reduce dissolved color.
Dissolved iron may be precipitated by aeration or by adding a strong oxidant so that
the iron may be removed as a particulate in DE filtration. The use of magnesite (magne-
sium oxide) has been found to facilitate removal of some forms of iron. Magnesite mixed
along with body feed is held for about 10 rain to form a negatively charged suspension
of magnesium oxide (MgO) that gradually undergoes hydration and solution.