Application: used to reduce emissions from buildings
- Very effective on multiple gases, odors and particulate matter.
- Stages can be designed to address specific pollutants.
- Expensive to implement and maintain.
- Creates another effluent stream requiring disposal.
- Requires a higher level of water usage that may be a challenge in some locales.
- May require special consideration to not negatively impact ventilation rates.
|Figure 1. Nozzles spraying water on to a filter bank.
(Photo courtesy of Big Dutchman)
Wet scrubbers are used on ventilation air as it exits livestock housing. They use liquid to collect dust or particulate matter (PM) and absorb gases from air by using a wet surface, spray system, or wet material bed. Some scrubbers use water which is partially recirculated, while others use some type of acid or base to shift the pH in order to absorb specific gases in the air. The liquid effluent then has to be treated as a waste product.
Scrubbers generally are either acid scrubbers, bioscrubbers, or water-only scrubbers. Multiple stages which combine these different scrubbers to treat multiple pollutants are typical. Acid scrubbers: These scrubbers trap alkaline material, such as ammonia, in a sulfuric acid solution that is circulated over a packed bed at a pH of 2 to 4. The ammonia removal efficiency tends to be over 90%, while the odor removal rate is around 30%. This same technology can be used with a base solution if hydrogen sulfide was the targeted chemical for removal.This would require separate scrubber stages, one with an acid targeting ammonia and one with a base targeting hydrogen sulfide.
Bioscrubbers: Bioscrubbers work much like a biofilter in that bacteria growing on biomass within the scrubber converts ammonia into nitrate and nitrite. Nitrogen in the water has to be kept below levels that will inhibit bacteria. They tend to use 8 to10 times more water than acid scrubbers. The ammonia removal efficiency averages approximately 70%, and the odor removal efficiency averages 50%.
Water-only scrubbers: These scrubbers are configured similar to the acid scrubbers or bioscrubbers except they solely use water and not biomass or acid. This is less effective in ammonia scrubbing but still a viable choice.
|Figure 2. Schematic of a multi-stage wet scrubber.
(Melse et al 2012)
The actual scrubber mechanics can use various configurations. In spray scrubbers droplets of water are sprayed into the air stream and the dust is captured by the liquid, which then settles out. Packed-bed scrubbers move air through a moist packed material which is normally porous and made of inert plastic which traps particulate matter. Water is recirculated through the bed and the material is occasionally flushed to remove trapped dust. Other units use hanging fiber cloths to pads that are sprayed with water or acid solutions. Dust adheres to the surface and is cleaned by the surface spray. Multi-stage scrubbers are useful when multiple pollutants are a concern and may have one stage to remove dust, one for ammonia and one for odors. Retention time of air within the systems is an important consideration with efficiency increasing with retention time.
Several challenges exist with the usage of scrubbers. Much like a biofilter, scrubbers are connected to the ventilation system and measures need to be taken to not stifle the ventilation rate due to high pressure losses. This could result in heat stress or air quality problems within the building. Handling effluent, some of which could be an acid (or base), can also be a challenge. Acids could cause corrosion of metal parts and also represents an “unusual” disposal protocol. Water usage can be high in some units which could be a challenge in some regions where water supply is limited. Using scrubbers in a cold winter climate may also represent some challenges.
|NH3||70 to 90%||Depends on configuration, retention time and number of stages|
|Odor||30 to 60%|
60 to 90%
|Volatile Organic Compounds (VOC)||50 to 90%|
There are a wide variety of scrubber configurations so initial cost and maintenance/operating costs vary greatly.Scrubber systems are generally not inexpensive. One source (Melse et al. 2008) estimates that to provide scrubbing for up to 35 cfm/pig in a building the initial investment would be $45 to $70 per head with yearly expenses of $15 to $20 per pig space. Some options, such as spray options in chimneys, may be less expensive.
Aarnink, A.J.A., W.J.M. Landman, R.W. Melse, Y. Zhao, J.P.Ploegaert and T.T.T. Huyhn. 2011. Scrubber capabilities to remove airborne microorganisms and other aerial pollutants from exhaust air of animal houses. Trans of the ASABE 54(5):1921-1930.
Melse, R.W., N. Ogink and B. Bosma. 2008. Multi-pollutant scrubbers for removal of ammonia, odor and particulate matter from animal house exhaust air. In: Mitigating Air Emissions from Animal Feeding Operations Conference Proceedings. Iowa State University. p. 162-168.
Melse, R.W., P. Hofschreuder, and N.W.M. Ogink. 2012. Removal of particulate matter (PM10) by air scrubbers at livestock facilities: results of an on-farm monitoring program. Trans of the ASABE 55(2): 689-698.
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