Odor Treatment Methods
Odor Treatments are used either to treat the odor producing compounds in the waste water or to treat the foul air. Some principle methods are:-
- Chemical scrubbers
- Activated carbon absorbers
- Vapour phase biological treatment
- Thermal process
Chemical scrubbers
Design objective is to provide enough contact between air, water and chemicals to enable oxidation and entrainment of odorous compounds. Most commonly used scrubbing liquids are sodium hypochlorite , potassium permanganate and hydrogen peroxide solutions. Sodium hydroxide is also used in scrubbers where H2S concentration is high in gas phase.
Simplified scrubbing reactions:
With sodium hypochlorite
- H2S + 4NaOCl + 2NaOH —- Na2SO4 + 2H2O + 4NaCl
- H2S + NaOCl —- S + NaCl + H2O
With potassium permanganate
- 3H2S + 2KMnO4 —- 3S + 2KOH + 2MnO2 + 2H2O
- 3H2S + 8KMnO4 —- 3K2SO4 + 2KOH+ 8MnO2 + 2H2O
With hydrogen peroxide
- H2S + H2O2 ——- S + 2H2O pH<8.5
Reaction products that can occur, depending on the local waste water chemistry , include elemental sulfur, sulfate thionates, dithionates, and manganese sulfide. Potassium permanganate is generally used in smaller units as it is expensive. Hypochlorite scrubbers are expected to remove oxidizable odorous gases when other gases concentration is minimal. In case where the concentration of odorous components in the exhaust gas from the scrubber are above desired levels, multistage scrubbers are often used.
Activated Carbon Adsorbers
Rate of adsorption for different constituents or compounds will depend on the nature of the constituents (polar Vs. Non polar). Removal of odors also depends on the concentration of the hydrocarbons in the odorous gas, as hydrocarbons are removed preferentially before polar compounds. As carbon beds are limited and needs to be regenerated thereby for prolonged life it is used in two stages followed by wet scrubber. For effective use of activated carbon the composition of odorous gases to be removed must be known.
Vapor phase biological treatment
Two principal biological processes used for treatment of odorous gases present in the vapor phase are
- Bio filters
- Conventional biological processes
As the odorous gases move through the packing in the biofilter, two processes occur simultaneously
a) sorption (absorption/ adsorption)
b) bio-conversion
Odorous gases are absorbed into the moist surface of biofilm layer and the surface of bio filter packing material. Than microorganisms, principally bacteria, actinomycetes and fungi oxidize the absorbed gases and renew the treatment capacity of the packing material. Moisture content and temperature are important environmental conditions and must be maintained to optimize micro-organism activity
The type and composition of the packing material
- sufficient porosity and near uniform particle size
- particle with large surface areas
- ability to support large micro-flora population
Facilities for gas distribution
- perforated pipes
- prefabricated under drain system
Maintenance of moisture within the bio-filter. Temperature control
Conventional biological treatment processes
The ability of micro-organism to oxidize hydrogen sulfide and other similar odorous compounds dissolved in the liquid under aerobic conditions is the basic concept. In the activated sludge process, the odorous compounds are introduced into the aeration basin. But a major concern with this method is high rate of corrosion in air piping and blowers that occurs due to presence of moist air containing hydrogen sulfide
Thermal processing
Three thermal processing techniques are in use:
a) Thermal oxidation
b) Catalytic oxidation
c) Recuperative and Regenerative thermal oxidation
If the gas to be combusted does not liberate enough heat to sustain the combustion process, it is usually necessary to use an external fuel source. As because most of the waste streams have low concentration of odorous combustible gases hence sustainable thermal oxidation is seldom possible. To maintain the combustion temperatures needed to eliminate odors, large amounts of fuel is required
Thermal oxidation involves preheating the odorous gases before passing them into the combustion chamber so that complete oxidation can be achieved
Catalytic oxidation is a flameless oxidation process that occurs in presence of catalyst (palladium, platinum, rubidium etc.)
- Decrease in temperature reduces the energy requirements significantly.
- As the catalyst can become fouled, the gases to be oxidized must not contain particulate matter or constituents that will result in residue
Recuperative and Regenerative thermal oxidation processes are used to reduce fuel consumption by preheating the incoming air. In recuperative oxidizers, thin wall tubes are used to transfer heat recovered from exhaust air to incoming air. In regenerative oxidizers, ceramic packing material is used to capture the heat from the hot exhaust gases and release it to the incoming air.
Selection and design of odor-control facilities
- Determine the characteristic and volumes of the gas to be treated
- Define the exhaust requirements for the treated gas
- Evaluate climatic and atmospheric conditions
- Select one or more odor control and treatment technology
- Economic analysis