Civil Engineering Reference
In-Depth Information
have been added at the inlet of the flocculation basins; this has normally been done
with untreated spent-filter backwash water, because solids in the recycle are thought
to improve flocculation and sedimentation of the main process flow.
Because of concerns about potential effects on finished water quality, separate treat-
ment of the liquid waste streams may be required before they are recycled. In Cali-
fornia, the state's Safe Drinking Water Act
77,78
required that new treatment plants have
solids removal for spent-filter backwash water recycle streams, although no specific
treatment methods are specified.
Treatment options for recycled flows include:
Lagooning, with or without chemical addition
•
Batch sedimentation, with or without chemical addition
•
Continuous flow conventional or high-rate sedimentation, with or without chem-
ical addition and flocculation
•
Dissolved air flotation
•
Granular bed filtration
•
Microfiltration
•
The recycled flows may also be disinfected following the above processes and before
introduction into the main treatment process.
Information presented by Cornwell and Lee
79
indicates that very low overflow rates
must be used in settling spent-filter backwash water without chemical addition. For
70-80 percent removal of
Giardia
- and
Cryptosporidium
-size particles, without chem-
ical pretreatment, overflow rates of less than 0.05 gpm / ft
2
(0.1 m / h) were required.
The addition of a nonionic polymer showed improvement to 90 percent removal at
overflow rates of 0.2 to 0.3 gpm / ft
2
(0.5 to 0.8 m / h). At another plant, virtually no
removal of
Giardia-
size particles was detected at 1.0 gpm / ft
2
(2.5 m / h), but the ad-
dition of less than 1 mg / L of nonionic polymer resulted in greater than 90 percent
removal of the same-size particles; the removal of larger particles, such as flow con-
taining microbial contaminants, must also be considered when examining pathogen
removal.
More recent work
80
achieved different results in treating spent-filter backwash water
by sedimentation. No significant improvement in the removal of total suspended solids
(TSS) was shown with polymer addition. Some improvement in 2- to 10-micron (
)
particle removal was achieved by adding polymer. However, the greatest level of par-
ticle removal (about 90 percent) was achieved through a combination of pH adjustment
and the addition of both ferric chloride and polymer, suggesting that particle charge
is an important factor in settling spent-filter backwash water.
In addition to the reduction of particulates, turbidity, and pathogens, waste stream
treatment may also be necessary to prevent the recycle of DBP precursors, TOC, and
AOC. Cornwell and Lee
79
examined the removal and recycle of these contaminants at
full-scale plants. Although waste streams had high levels of TTHMFP and TOC prior
to settling, the settled waste streams had TTHMFP and TOC levels near those found
in the raw water. The TTHM precursors and TOC were apparently associated with
solids in the waste streams and were removed by sedimentation.
At two WTPs, high AOC levels were found in the untreated waste streams. Contrary
to what was seen with TTHMFP and TOC, little AOC removal was achieved with
settling. The end result is that high levels of AOC were recycled to the head of the
