A scrubber system removes harmful materials from exhaust gases before they are released into the environment. There are two primary methods of scrubbing exhaust: wet and dry scrubbing. Both systems have the same basic methods of operation and the same final goal; they simply reach them through different materials. Scrubber systems are one of the primary ways of removing acid gasses from exhaust before they are released into the sky where they cause acid rain.
Wet scrubbing was the original type of scrubbing system. In a common wet scrubber system, the gas is funneled through an area where a wet substance is actively sprayed. Water is used when the gas needs dust and particulate matter removed. Sometimes, other chemicals are added that specifically react with certain airborne contaminants. Since this process adds so much vapor to the exhaust, if the gas is vented, it typically looks like billowing white smoke.
The sprayed liquid collects in the bottom of the area. This liquid is funneled away from the spraying chamber and collected for disposal. Since the liquid contains a wide range of potentially harmful materials, it cannot be reused or simply poured down the drain. The volume and weight of the liquid are very great and are some of the main reasons that dry scrubbing was developed.
Dry scrubber systems spray a collection of dry reagents into the exhaust. These reagents can have a number of different effects depending on the material they target. Some will simply neutralize a harmful material through a chemical reaction. Others cause a material to react and accrete into a different substance that is large enough to fall out of the gas stream or be caught in a particle screen. Since the vapor in these gasses is very low, they are typically dark or invisible when vented.
The system produces very little waste material, at least when compared to a wet scrubber. Most of the material sprayed into the exhaust stream is allowed to leave, burn off in the heat of the stream or be caught in a filter. As a result, the collection requirements for a dry scrubber system are much less than for a wet one. This reduces the cost of the system as well as removes the cost of transporting and storing the waste water.
In modern plants, dry scrubbing is more common by far, but wet scrubbing still has its uses. Some contaminants, such as mercury, are extremely harmful and can only be removed by a wet compound. The other common use for a wet system is heat dissipation and recycling. A wet system will collect, and potentially repurpose, heat before gas is vented. This is both better for the environment and allows a company to recoup some of the expense of the wet scrubber system.