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Potable Water Biotechnology, Membrane Filtration and Biofiltration 501
the data showed microbial activity to be a significant contributor to the removal process for
TOC and THMFP over the long term. THMFP was defined as THM formation after chlorination
at 20 mg/L, at a pH of 6.5, a temperature of 26 2 C, and a 3-day incubation time.
The study was run for 83 weeks. At the beginning of the study (weeks 0–21), both sets of
columns achieved approximately 80% TOC and THMFP removal. These removals were
primarily due to physical adsorption of the PM. In the final phase of the project (weeks
65–83), TOC removals averaged 24 and 27% in the GAC and O3-GAC columns. During
the same time period, THMFP removals averaged 24 and 30%, respectively, in the GAC and
O3-GAC columns. By this time, the GAC columns had each passed more than 50,000 bed
volumes, and their adsorptive capacity was exhausted. In the absence of adsorption, TOC and
THMFP removals during weeks 65–83 were deemed to be due to biological activity.
5.2. Bench-Scale Studies
A series of three bench-scale studies was performed to examine the impacts of ozone dose,
water type, and attached versus suspended bacteria on the biological removal of DOC,
THMFP, and HAAFP. The first study investigated the effect of ozone dose and biological
treatment on PM control (122). THMFP and HAAFP were measured by chlorinating at
12 mg/L, at a pH of 7.5–8.0, and holding for 7 day at 20 C. Bench-scale biological treatment
was performed in batch recycle tests (see Fig. 11.2). The water sample was circulated
continuously through a bed of bioacclimated sand for 5 day. Oxygen was provided by
applying a vacuum to the head space of the sample chamber and drawing in air through a
water trap, which removed foreign bacteria. The 5-day contact time was considerably longer
than the 5–10 min typical of pilot- or full-scale biological filters. As a result, estimates of
biological removal in the batch test should be viewed as “ultimate” or “potential” removals,
which will be higher than flow-through removals at equivalent organic matter concentrations,
compositions, and ozone pretreatment.
Fig. 11.2. Bench-scale
biofiltration apparatus
(120).
vacuum
sample
sand
air