Odour minimisation methods
As odour controls are tightened, Peter Fish of Prisma Services discusses some new developments in dry scrubbing media technology
Eliminating odours at WwTWs and pumping stations has always been a major concern within the industry. This concern will continue to grow as control requirements are tightened and the penalties increase for non-conformity to these requirements. There are basically three main methods used to remove odours:
- wet scrubbing,
- biological filtration,
- dry chemical scrubbing.
Wet scrubbers are particularly useful in applications with high airflows and high gas concentrations. Their basic principle of operation is simple (although often operationally complex) because they utilise intimate contact between odorous gases and the liquid solvent via packing material. When a wet scrubber is in operation, gases are drawn from the storage area to a tower that holds packing material and the caustic solution. The gases then flow counter-current to the liquid caustic solution, which is typically potassium hydroxide, hydrogen peroxide or sodium hydroxide. The caustic material neutralises the gases and removes odours from the air stream. The two main biological filtration methods are the bio-filter and the bio-scrubber.
The bio-filter uses a large surface area which supports micro-organisms. Odorous air is passed through this mass and the organisms break down the hydrogen sulphide (H2S). In a bio-scrubber, a microbiological film is immobilised in a gas/liquid transfer media, which also enables the organisms to break down the H2S. A fourth method, the use of perfumed sprays to mask odours, does not strictly eliminate odours because it is intended only to reduce the adverse effects of unwanted odours. In any case, there is no single solution that can satisfy all odour applications.
There are advantages and disadvantages in all three technologies and sometimes the best solution to an odour problem is to consider a solution that combines two of the technologies (see Figure 1).
However, any solution should satisfy a number of criteria:
- system performance must meet specification, for example, it eliminates odours not just H2S,
- capital cost is acceptable,
- lifetime running cost is acceptable,
- reliability/maintainability is acceptable,
- system's environmental impact is acceptable. The purpose of this article is to review some new developments in dry scrubbing media technology. This technology is now widely used with great success in European countries such as the Netherlands, France, Italy, Spain and Denmark as well as in the US.
Two main processes can be used to remove odours:
- adsorption - a physical process which is reversible,
- chemisorption - a chemical process which is irreversible.
Adsorption is a process where the odour is attracted by Van der Waals's forces to the surface of a media. The odour adsorbing capacity of the media is therefore maximised by maximising the surface area. Chemisorption takes the adsorption process one stage further by using a chemical reaction to neutralise the adsorbed odour.Activated carbon
Activated carbon is the most well known dry scrubbing media. It contains a combination of macropores (100,000Å wide) and micropores (20Å wide), which result in a massive surface area of approximately 1,000m2/g.
However, virgin activated carbons have limited performance with low molecular weight gases such as H2S. Odour consists of many different types of compound such as mercaptans, other sulphur compounds and sometimes amines, all of which are of high molecular weight. These heavier compounds easily adsorb to the macropores but block the path of lighter molecules to the micropores, which are responsible for most of the surface area. The disposal of activated carbon also presents major problems since it will desorb and therefore have a negative environmental effect.Impregnated carbon
Impregnated carbon is simply activated carbon impregnated with an agent such as potassium hydroxide (KOH). It is a great improvement on virgin activated carbon and can be very effective in eliminating H2S. But again we have to remember that in many cases the odour we are trying to eliminate is a combination of compounds which cannot all be eliminated by KOH.
There are some practical drawbacks to the use of virgin and impregnated activated carbon. It is very sensitive to humidity (maximum 45-50% RH), can support the growth of fungi and creates a high pressure drop, which results either in the need to install bigger air systems or the cross sectional area of the carbon filter has to be increased to decrease the pressure drop, thus increasing the cost (see Figure 3). There is also evidence the efficiency of activated carbon drops dramatically when the H2S concentration exceeds levels of about 40-45ppm. This has to be taken into account when dealing with levels of 100-200ppm, which prevail in areas handling sludge.Engineered medias
It is clear there is very rarely a single dry scrubbing media solution to an odour problem. Over the last 40 years a range of engineered medias has therefore been developed and are still being developed to enable an effective chemisorption process to be available for the whole range of odour compounds. The design of these medias is based on an activated alumina pellet.
When manufactured correctly these pellets have a large number of macropores, which are able to adsorb a wide range of odour compounds. The pellet also has a high capacity to store a range of chemical impregnates that will eliminate H2S, mercaptans, amines, etc. KOH and sodium bicarbonate (NaHCO3) eliminate H2S and potassium permanganate (KMnO4) eliminates mercaptans and amines (see Figure 4). Since no single engineered media has yet been developed that can neutralise the whole range of odour compounds, the solution is to combine a number of medias in a specific order and in a single filter so the odorous compounds mentioned in the previous paragraph can be neutralised. The design of these filters has been developed over the last ten years and, for instance, takes into account factors such as the difference in ratios by weight of hydrogen sulphide and mercaptans typically found in odorous gases.
Development of engineered medias continues. For instance, a new media with a capacity for H2S of more than 25% by weight is about to be released. This media is a refinement of a media originally introduced 14 years ago and contains a combination of activated alumina impregnated with KOH, NaHCO3, some activated carbon and chemical binders. Engineered media filters also have a further advantage over activated carbon in that the pressure drop across them is much lower and this leads to simpler and cheaper air handling systems.Air Handling System
Within the water industry, dry scrubbing media is used for two main applications: l corrosion control - where the requirement is to pressurise a room or an electri-cal/electronic cabinet with clean air in order to keep acid gases such as hydrogen sulphide, which are in the environment, out of the equipment, l odour control - where the requirement is to stop odour that is inside equipment (this could be a primary settling tank, sludge holding tank, etc) from getting out into the environment.
It can therefore be seen that the requirements for the control of corrosion and odour are almost diametrically opposite one another and it is therefore a mistake to use the same air handling approach for both types of application. Corrosion control will use room air change rates of up to ten times the room volume per hour in order to create a positive pressure in the room which is able to overcome infiltration.
Odour control requires only enough negative pressure in the equipment to prevent odours from escaping and the air flows needed to do this are an order of magnitude lower than for corrosion applications. In fact, in some odour applications where, for instance, a holding tank is well sealed, there is no need to create a negative pressure at all because the positive pressure created in the tank as it fills up can be used to push the odorous air through a 'passive' (for example, without fan) dry scrubber. The low pressure drop across engineered medias is a vital part of this passive filter solution.Distributed Control
There is a simple fact of life - the more contaminated air you pass through a dry scrubber, the faster the media will be used up. Finding a filter that eliminates the odour is therefore only the first step.
The next step is to satisfy the four remaining criteria that were mentioned at the beginning of this article and minimising the air flow is a vital factor in meeting all four criteria - it reduces the capital cost, the running cost, improves reliability of the dry scrubbing system and reduces the environmental impact in a number of ways, such as reducing waste, noise and power usage.
The new approach, which is working in hundreds of applications around Europe and the US, is to abandon the large centralised odour control system wherever possible and to adopt the more versatile, cheaper and smaller distributed system. This is analogous to what has happened in the process control industry where process control used to be handled by a large centralised computer system but is now handled by smaller systems distributed around the manufacturing plant.
Process risk is reduced and solving any individual problem becomes a much simpler task. Where previously a number of tanks were having large quantities of odorous air, with a wide range of concentrations taken from them into a centralised odour filtration system, it is now possible to fit a small, inexpensive passive or draw-through system to each tank and set up the system for its particular application (see Figure 2). Draw-through systems can be provided with small inverter drives so that if personnel access is required the air flow can be increased on a temporary basis to clear the area before access takes place. The use of these small systems also makes it possible to achieve effective long-term odour control in small pumping stations (see Figure 5).
This has been virtually impossible up until now due to the size and cost of the equipment required. In order to meet increasingly strict environmental requirements for odour control we need to take advantage of new developments, which are proven and cost effective. The use of distributed odour control systems using engineered medias is an important, cost-effective step forward in meeting these requirements