ODOR CONTROL IN WASTE WATER TREATMENT PLANTS
The collection and processing of sewage and industrial wastewater is often a source of unpleasant odors.
This water contains dissolved organic material and particles, nitrogenous compounds (including ammonia)
and phosphorous, whose purification by-products (sludge and grease) may directly or indirectly cause
unpleasant odors due to the biological process of fermentation which occurs in oxygen-deprived media.
Similar odor emission can be found in waste management facilities and composting areas.
THE THREE MAIN ODOR CAUSING MOLECULES ARE:
- Sulfurated compounds (H2S, R-SH,…)
- Nitrogenous compounds (NH3, R-NH2,…)
- VOC (Aldehydes, ketones, organic acids,…)
Detection levels, fluctuating concentrations, various process steps, personal perception, as well as type
of odor are all considered in order to determining the best equipment selection to control your odor emissions.
TWO AREAS ARE THE MAIN ODOR SOURCES IN A WWTP
Covered pretreatment systems require only small air flows to control the low concentration which are released from lifting stations, screening areas, fat removal and fat digesters, filtration units, and buffer tanks.
The last steps in the wastewater treatment process is sludge treatment and storage, which generally includes, filter press, draining tables, centrifuges, liming, sludge tanks, containers, and the general sludge area.
Typical pollutants levels are:
|H2S||=||2||to||3 ppm||NH3||=||1||to||2 ppm||R-SH||=||0.5||to||1 ppm|
TCA's (Activated Carbon Tower) are used on small airflows between 150 and 6,000 cfm, mainly on pre-treatment processes and stand alone storage tanks. They are ideal solutions for:
- Easy installation (it's delivered as a "plug and play" skid)
- No need for water, chemicals, low power consumption
- Compact installation
- No maintenance (only the exchange of the spent carbon after
1 or 2 years of operation)
- Offers a good efficiency on the odorous compounds
TCA's can also be used as the main control device in a small WWTP. If there is a sludge area, treatment by wet scrubbers or biofiltration is necessary because of the larger flow rate.
Typical pollutants levels are:
|H2S||=||5 ppm||NH3||=||5 ppm||*||R-SH||=||1 ppm|
*(NH3 can obtain 20 ppm if there is liming)
Wet scrubbers are most often used as a global treatment for all the WWT-processes, because they can handle higher flow rates and pollutant concentrations. Wet scrubbers should be used if higher removal efficiency is required. The wet scrubbing process, is a specific treatment that needs to be installed in a multiple stage arrangement to treat the various compound families in the air. (Nitrogenous, sulfurated, ...)
The different types of compounds are treated using separate packed towers or horizontal cross flow scrubbers.
- Treatment with Sulphuric Acid for nitrogenous compounds (NH3, R-NH2
- Treatment with caustic soda + bleach (usually 2 scrubbers in a row) for sulphurated compounds (H2S, R-SH), organic acids (R-COOH) and some aldehydes and ketones
- Treatment with bisulphite or thiosulfate for chlorate residues (Cl2) + aldehydes and ketones
- A final polishing stage using activated carbon may be used
Biofiltration is based on the degradation of the pollutants by micro-organisms (bacteria,…).
The principal advantage resides in the simplicity of the process, however, following the parameters should be met.
- Temperature between 40 to 100 °F
- Moisture > 70%
- Stability of the pollutant concentration
- Biodegradable compound
- Absence of dust
Additonal points to concider:
- No dust accepted
- Low NH3 concentrations
- Stability of the concentrations
In this case, a pre-treatment with a scrubber has to be placed before the biofilter (an acid scrubber is also needed if it is the deodorization of a limed sludge storage for example).
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