Choosing the right deodorant
With so many odour control technologies available, how do you pick the best one? Joanna Burgess and Richard Stuetz from Cranfield University's school of water services help out.
Odour emissions may be controlled by reducing the rate at which odorous compounds are generated, or by containing and treating odorous air. There are three methods of odour treatment;
- biochemical - biofilters, bioscrubbers, activated sludge,
- chemical - chemical scrubbers, thermal oxidation, catalytic oxidation, ozonation,
- physical - condensation, absorption using activated-carbon or clean-water scrubbers.
The choice of technology or combination of technologies is dependent on factors like the site characteristics, treatment objectives, foul-air flowrates and the characteristics and strength of odorous air. Odours from wastewater consist mainly of sulphurous and nitrogenous compounds like hydrogen sulphide and ammonia. Waste gases from other industries have traditionally been treated using physico-chemical processes such as scrubbing, adsorption, condensation and oxidation (see table below); but biological treatment of waste gases has gained support as an effective and economical option in the past few decades. Biological odour treatment received much attention in Europe in the 1990s owing to its efficiency, cost-effectiveness and environmental acceptability.
In all types of waste gas treatment bioreactors, the pollutants diffuse into the liquid phase where micro-organisms degrade them into products such as CO2, H2O and minerals. The use of biotechnologies for odour treatment has grown dramatically because of their ability to destroy the pollutants rather than simply transfer them from the gas to the liquid phase. Published performance data for odour compounds treated using similar process units vary widely, suggesting that research into the parameters affecting the performance of biological air treatment would improve our ability to use odour and air pollution control biotechnologies to their full potential.
The three major types of bioreactor which currently dominate waste gas biotreatment are biofilters, trickling biofilters and bioscrubbers. Alternatives exist for different applications, such as an external loop airlift bioreactor, a spiral bioreactor, membrane bioreactors and activated sludge diffusion.
Odorous compounds are degraded as the contaminated air passes through a bed of naturally-occurring microbes on a support media. The air-stream and media bed are moistened to keep the bacteria active. Odorous compounds in the air-stream provide a source of carbon for the bacteria, while the water sprayed onto the filter has other nutrients such as phosphorus or potassium for the biofilm. Because of the low density of bacteria, which are present as a mixture of species, the specific performance of a biofilter is relatively low.
Closed biofilters are sealed tanks containing an aeration system. Open-soil and compost biofilters consist of a trench, filled with gravel to evenly distribute the foul air, containing a perforated pipe through which the air is pumped. Both systems are covered to prevent operational problems caused by rain. Media employed in biofilters include peat, heather, soil, compost and sand, with soil and compost the more popular choices. Soil is the most common choice owing to its availability, porosity, particle size distribution and existing bacteria. It also has a greater operational life than peat and compost. Soil flora - bacteria and fungi - metabolise biodegradable odorants. The advantages of biofiltration include simple, flexible design, low capital costs and the ability to treat high volumes of low-concentration odorants. The main disadvantages are necessity to regularly replace the media - which means high operating costs, design criteria are still developing, operational control is limited and a large area of land is required to get bed volumes big enough to obtain a long gas residence-time.
The odorous air passes over a mixture of microbes immobilised on support media with a high surface-area-to-volume ratio. Water is recirculated around the media to maintain humidity in the media-bed and allow nutrients to be added. Odorants dissolve into the water and are degraded by the biofilm. Air can pass through a trickling biofilter either co- or counter-currently to the liquid which provides the biofilm with nutrients. Trickling filter media can be ceramic or plastic structures, activated carbon, celite or mixtures of materials. Optimum performance is obtained by recycling the foul air (to dissolve the odorants into the liquid and biofilm), maintaining moderate dissolved oxygen and making sure the wastewater does not short-circuit through the media.
Trickling filters treat STW odours by using a pumping station to transfer the foul air from the headworks and clarifiers to the bottom of a bed of media. As the air passes through the media it is collected from a dome at the top of the filter, the majority for recycling through the filter and a small portion for final treatment in a mist scrubber. Reaction products are washed out of the media so filter bed acidification can be avoided. The major drawback with this system is the problem of dissolving the pollutants in the water, but trickling biofilters can still be effective. The wet (active) area of a filter is usually less than 50% of the total area. This can be improved by increasing the liquid flow rate, but this increases operational costs and, if a filter is also being used to treat wastewater, the level of wastewater treatment can be compromised. Carbon, hydrogen and oxygen are not usually the limiting nutrients present in wastewater, but supplies of nitrogen, phosphorus or micro-nutrients may require control in order to balance bacterial growth and removal efficiency against media clogging.
Bioscrubbing has several advantages over filtration. The process is more easily controlled because the pH, temperature, nutrient balance and removal of metabolic products can be altered in the water of the reactor. Bioscrubbing relies on good gas dissolution, as it needs the pollutants to be in the aqueous phase in a gas/liquid exchange column, followed by degradation in a liquid-phase bioreactor. The bioreactor effluent is recirculated into the absorption column, providing excellent gas cleaning of highly-soluble pollutants.
Disadvantages of this system are based on the requirement to dissolve the pollutants during the short residence time in the absorption column, a major consideration as many air pollutants and odorants are volatile and don't dissolve easily in water. In addition, biomass growth has to be controlled to reduce solid waste output and increase gas treatment efficiency. Sewage can be used as the bioreactor liquid, although this reduces the operator's control over the nutrients available. This is probably the reason bioscrubbers are less popular than biofilters. Newer bioscrubber applications have developed because it is possible to biologically de-sulphurise large volumes of gas (up to 2 x 106 m3/h) and bioscrubbers are one of very few means of anaerobic gas treatment which avoiding aeration costs.
In membrane bioreactors the pollutants are transferred from the gas to the liquid phase - where they are degraded - via a membrane. Two membrane material types are available for treating odours; dense - silicone rubber, and hydrophobic microporous - polysulphone. Dense materials are more selective and microporous materials more permeable.
There are also two types of biomass; fixed-film cultures (biofilms), and suspended growth cultures. The membrane forms the gas/liquid interface and therefore its size can be closely controlled. Passage of the contaminated air across a membrane surface allows passive diffusion of contaminants through the membrane into the liquid on the other side, driven by the concentration gradient. The mass-transfer coefficient inside a membrane depends on the solubility and diffusivity of the compound in the material matrix. Solubility and diffusivity vary between each contaminant and mass-transfer resistances of different membrane materials vary for the same compound. This allows waste gas compounds to be selectively extracted from or retained within the gas phase by careful choice of membrane material. This presents an advantage over thin-film and bubble diffusion, in which it is impossible to select removal of certain components of the foul air. The presence of the membrane prevents microorganisms from contaminating the gas phase.
Activated sludge diffusion is also used for treating odours from wastewater treatment processes. Contaminant removal mechanisms in activated sludge diffusion of waste gas include;
- absorption - the solution of gases into the mixed liquor, limited by bubble size and gas residence time,
- adsorption - high molecular mass compounds with low solubility adsorb onto bacteria,
- condensation - volatile compounds in warm air condense on contact with the cooler mixed liquor, followed by biodegradation.
Foul air is collected from source and transferred via blowers through a pipework system to submerged nozzles in the aeration tank. The odorous air bubbles diffuse into the mixed liquor where the contaminants dissolve and are subsequently adsorbed onto the floc or absorbed into bacterial cells and biodegraded.
Problems with activated sludge treatment of waste gases vary. Main concerns are corrosion of pipework and blowers by the moist, acidic air; the transfer of the foul air from the gas to the liquid phase and potential toxic effects of odorants in solution. But PVC, fibreglass and stainless steel are all suitable for the foul air delivery system.
Activated sludge diffusion of odorous air reduces the presence of liquid-phase odorants via biological oxidation, but can produce odours via gas stripping, especially where systems become overloaded. Odours are significantly reduced at all wastewater treatment sites, as the odour monitored at site boundaries is the product of the entire site as opposed to the activated sludge tanks alone.
Biological degradation processes involve biological oxidation of the offending compounds, so its action is decisive - odours are destroyed rather than masked or moved from the air to the water. However, before the compounds can be oxidised, the gas-phase odorant must be dissolved into the liquid phase in order to become bioavailable. This lies behind most advantages and disadvantages of the five biotechnologies and dictates their suitability to different odour compounds. For example, biofilters treat air pollutants effectively with air/water partition coefficients of less than 1.0 because there is no liquid phase and water solubility of the pollutants is of relatively little importance, but they can only cope with relatively low loads of hydrogen sulphide as the media quickly acidifies. In trickling biofilters, reaction products are washed out of the media and acidification can be avoided. The major drawback with this system is the problem of transferring the pollutants from the air to the liquid phase, but they can still be effective in treatment of gaseous compounds with an air/water partition coefficient of less than 0.1.
Both types of filter suffer problems with media clogging and maintenance of humidity, whereas suspended-growth reactors (bioscrubbers, membrane bioreactors and activated sludge) avoid such problems. In addition, the cost of setting up a new treatment unit at many odorous sites cannot be justified, particularly where the odours result from waste treatment processes.
For wastewater treatment sites, activated-sludge diffusion offers a low-cost option. By collecting odorous gas and diverting it into an activated sludge aeration basin, odours can be eliminated with relatively cheap technology. Biological odour treatment has been shown to be reliable and to have lower capital and operating costs than traditional physical or chemical methods.
|Odour treatment - chemical technologies|
|Treatment process||Advantages||Disadvantages||Economical treatment range (H2S)||Level of use|
|Packed tower wet scrubber||Proved effective and reliable, moderate costs||Blowdown of unused chemical, high maintenance||<100ppm||High|
|Fine-mist wet scrubber||Can be designed for VOC removal, lower chemical cosumption and hence chemical costs||Higher capital cost than a packed tower, scrubber water requires softening, larger footprint||<100ppm||Moderate|
|Activated carbon||Consistently high performance, mechanically simple||High cost (carbon replacement/regeneration), only for low contaminant loads (to ensure acceptable carbon life)||<10ppm||High|
|Thermal oxidiser||Effective||Very high capital and energy costs, only economical for high-strength recalcitrant air streams or VOCs||>600ppm||Low|
|Chemical counteractant||Simple||Marginal effectiveness (<40% odour reduction), high ongoing chemical costs||Very low levels||High|