Questions and Answers from the Webcast
Q: Do you see any future regulation of biological filtration?
A: I am not aware of any planned regulations regarding biological filtration in the U.S. There
were references to biological filtration in the draft Stage 1 DBPR but these references are
absent in the final. Rather than regulations, a more likely scenario will be guidelines and/or
standards of practice for biological filtration.
Q: Is biofiltration performance temperature dependent? Does it vary seasonally? How do the
utilities handle this variation during winter months?
A: I have seen little impact of temperature on biological nitrate/perchlorate removal (down
to 5°C). Aerobic biofiltration, which are lower growth systems, can be greatly impacted by
temperature. Longer contact times can be required during colder months. Fortunately, water
demand tends to go down in colder months as well, which can allow a utility to increase
contact times while still meeting demand. Media selection may help as well (e.g., GAC tends
to hold more biomass, which can help improve degradation kinetics during cold periods).
Q: What is the realistic replacement frequency of GAC media?
A: For a biological filtration use, GAC does not have to be theoretically replaced at all. However,
to accommodate attrition of media, 10 to 15 percent additional media should be added per
year to top off the filter bed. At that rate, the equivalent replacement frequency would be
in the seven to 10 years. Some utilities are of the opinion to replace media in two to five
years to take advantage of slow adsorption of trace contaminants. If GAC is used as a filter
absorber for TOC removal, the replacement frequency will be in months (six to 18 months).
Q: Has biofiltration changed your removal efficiency of turbidity and particle counts?
A: All filtered turbidity results from the demonstration filters are well below 0.1 NTU.
Q: What is the typical dosage for acetic acid for the anaerobic systems discussed? And what is
this dosage based on?
A: The acetic acid dose is determined by the raw water dissolved oxygen and nitrate
concentration. Stoichiometric acetic acid demand is calculated, and then 60 to 70 percent
excess is added to that to account for cell synthesis (determined empirically). As an example,
we’ve been removing 3 mg/L dissolved oxygen and 70 mg/L nitrate (as NO3-) with an acetic
acid dose of 76 mg/L.