The installation in McMinnville, Oregon perhaps best demonstrates our capabilities as the levels of odorous compounds to be treated were among the highest ever reported from the wastewater treatment environment, and provided a real challenge to any odour control system.
We were called in there to a 6-month old, $25,000,000 U.S.
wastewater treatment plant, designed by CH2M Hill, that had an
extreme odour problem that was threatening the plantís very
existence. They had an ATAD (Autothermal Thermophyllic Aerobic
Digestion) system in place to treat the sludge, and the chemical
scrubber used to control odours was not capable of handling the
levels involved. They were getting 15 to 20 odour complaints per
day, and a local citizens group was about to take the city to
court over the odour problem. The odour was very strong, with a
smell. Some of the odour complaints came from residents several
miles from the plant, and buzzards were a common sight circling
the stack looking for the rotting flesh. The levels of reduced
sulphur compounds in the air were extremely high - up to 140 ppm
methyl mercaptan, 10 to 20 ppm each of dimethyl disulphide and
dimethyl sulphide, and about 1100 ppm ammonia. The odour
thresholds for these reduced sulphur compounds are very low, all
less than 20 ppb (Table 1).
There was no time for a pilot run, so we over-designed the biofilter to be sure to remove the odours. The odour complaints stopped about 3 days after we started up the biofilter.
The biofilter treats 3000 cfm of undiluted ATAD air and was
installed in 1997. The air is first humidified and
to lower the ammonia to manageable levels (250 ppm) and reduce
the temperature to below the thermophylic range, and is then
treated in the biofilter with an empty bed residence time of 3
minutes. On subsequent installations on ATADs, we have
successfully lowered this to around 45 seconds (Table 1). We now
have biofilters on over 7 ATADs all over North America
This biofilter was not tested for odour removal, but there is essentially no odour on top of the bed, so the compounds mentioned above must be at or below their odour threshold after the biofilter, which suggests >99.9% removal, and there have been no odour problems since in almost 8 years of operation. The filter material was changed after four years, only because the plant is very pro-active. It would probably have lasted many more years.
|Compound||Long Sault Ontario†||Cardinal Ontario||McMinnville Oregon†||Franklin Indiana||Princeton Indiana||Odour Threshold|
|Mercaptan (ppm)||~ 10.0||~ 6.0||128||~ 80||~ 10||0.02|
|Dimethyl disulfide (ppm)||~ 1.0||n.a.||14.8||n.a.||n.a.||0.002|
|Dimethyl sulfide (ppm)||~ 1.0||n.a.||9.63||n.a.||n.a.||0.01|
|Residence Time (s)||28||28||180||48||33|
|Air flow (cfm)||1500||1500||3000||600||600|
|% of air flow from ATAD||14||14||100||100||100|