A biological solution to odour control
There are many options available for removing odours from gas streams before discharge and in this article Chris Smith from Bord na Mona discusses the biological techniques available.
A biofilter treating odourous gases from a sewage treatment works.
Odours from wastewater treatment plants are the result of volatile or semi-volatile compounds being released to atmosphere. They are often generated as a result of septicity where anaerobic decomposition of carbohydrates, proteins and fats have occurred. Sulphates are biologically reduced under such conditions leading to sulphide production, released to atmosphere as hydrogen sulphide.
All these odorous compounds are formed in solution. Their release into the atmosphere is dependent on chemical properties, physical state, partition coefficients, biodegradability and sorption tendency. Biological treatment simply reverses this process and treatment involves absorption into liquid and biological oxidation of the odorous chemicals.
There are a number of biological treatment systems available and they are marketed as biofiltration, bioscrubbers and bioreactors. Each of these techniques offer a slightly different approach. Biofiltration is the most established technique. It uses organic media to act as a support for microbial growth and provide a surface for microbial treatment.
Bord na Mona originally used a peat fibre and heather based mix. Other media include bark, compost and wood chips. A biofilter houses this media and supports it on a false floor. This floor creates a plenum chamber beneath the media to allow the gas to pass uniformly across the media. A sprinkler system keeps the media moist to allow adsorption of the odorous compounds.
Bioscrubbers are counter current scrubbing towers filled with plastic rings on which a film of microbial growth occurs. Adsorption of odorous compounds is achieved by creating turbulence between gases flowing upwards through the bioscrubber and water flowing downwards. Nutrients may also be added to maintain microbial growth in the bioscrubber. Bioreactors are hybrids of bioscrubbers and stirred aerated tanks.
Absorption of odorous compounds is achieved in the bioscrubber element of the bioreactor and microbial treatment then occurs in a separate stage similar to an activated sludge plant.
The treatment efficiencies of biological odour control systems continue to be improved through manufacturers' re-search and development programmes. Much of the research has focused around improving the media for microbial growth.
Increasing the surface area, maximising contact between microbes and odorous gases, has been an important objective. In addition the problem of souring has been addressed. Souring in a biological system is often due to significant decreases in pH as oxidation of sulphides leads to the formation of sulphuric acid. Microbial oxidation of the sulphides occurs best at neutral pH and in the past addition of lime to control pH, or a water sprinkling system to wash off excess sulphuric acid have been employed.
While these methods have proved successful, prolonged exposure to high levels of sulphides have still resulted in souring, causing major drops in system performance.
Recent developments resulted in the development of high performance media which increase treatment efficiency. A high performance media must provide an environment that optimises microbial activity in the reactor. This involves ensuring that the microbes readily come into contact with the compounds they are treating and that optimum pH and nutrient levels are provided.
These high performance media increase the loading parameters for biological reactors. MONASHELL is an example of such a media developed to handle high and variable levels of hydrogen sulphide.
Typical design limitations for conventional media would be 30 ppm H2S. To treat higher concentrations dilution of the gas stream or reductions in gas flow rates are required. These result in increases in the size of the reactor with ensuing increases in cost.
MONASHELL is routinely used to treat 150 ppm H2S and one system is effectively removing sustained levels of 1000 ppm H2S. High performance media are typified by their ability to provide self-buffering for the products of microbial metabolism and offer mechanical strength to increase their service life.
In addition such media should have an open structure to assist gas flow through the reactor with minimum pressure drop. With these increases in performance biological reactors are now decreasing in size and small footprint reactors are on the horizon. Typically, use of high performance media has halved reactor sizes. Manufacturers are also investigating methods of treating compounds with low water solubility. Nodular peat systems with low moisture content are performing well in pilot trials with superior VOC removal characteristics.
As the technology for treating odours improves and the consent limits for discharge tighten biological treatment offers an increasingly cost effective abatement technology. Low running costs in terms of staff time, chemical and energy usage make biological treatment attractive. This is particularly true for wastewater treatment plants and low temperature industrial off gases.
Now sporting a successful track record as efficient, robust and low maintenance systems, the market for biological treatment systems is rapidly growing. Where you are searching for a solution to odour control it is now worth thinking biologically.