So what is a scrubber?
A Biological Scrubber is a type of wet odor control scrubber that treats and removes contaminants from an air stream that only utilizes caustic typically to control the pH of the re-circulation solution. There are several types of odor control and chemical fume scrubbers on the market today and each of them play a role in the treatment of noxious or corrosive gases in the industry. Biological scrubbers are commonly utilized in municipal applications for the treatment of hydrogen sulfide (H2S) gases that are removed from a water or waste water treatment process. In water treatment equipment such as a “degasification” or “decarbonation” tower strips the hydrogen sulfide gas from the water and exhaust the gas from an exhaust port. These gases are captured and sent via an air duct system to the biological scrubber. Hydrogen gases captured at a wastewater treatment process which may include the treatment plant, lift station or head-works facility are also sent using a PVC or FRP duct system to the biological scrubber.
So how does a Biological Scrubber work?
Interesting enough a biological scrubber utilizes tiny microorganisms (bacteria) to breakdown and digest contaminants and the bacteria feed on the contaminants and utilize this as a feed source to live and grow. When utilizing a biological scrubber for hydrogen sulfide (H2S) treatment the by-product waste is acid from the digested H2S. This lowers the pH and requires the use of caustic to buffer the water and nutrient solution that is
recirculated within the scrubber to maintain a neutral pH. The captured gas containing contaminants enters into the bottom of a vertical biological scrubber much like the gas enters any other type of chemical scrubber or other type of single or dual pass odor control scrubber. The gas stream then travels upward and passes over a media bed that has been cultured to grow live microorganism on its surface. In a way you could say that a biological odor control scrubber already has “artificial intelligence” because of the millions of microbes colonies it supports. Unlike a traditional chemical scrubber or an ammonia scrubber the biological media bed is specially designed to support the bacteria colony growth and allow air movement with the least amount of back pressure. In addition, the biological scrubber has far less media packing required to treat the contaminate in the air stream because the contaminant is being consumed and digested as opposed to oxidized.
The best application for a Biological Scrubber is when
The concentration of the gas for a biological scrubber is relatively low < 10 PPM and the concentration should not fluctuate or vary rapidly. Chemical scrubbers can adjust quickly to changes in concentrations within the air flow contaminant because they rely upon the immediate increase in chemical reagents like caustic or acid, but a biological scrubber needs to adjust more slowly over time and acclimate due to the size of the bacteria colony or risk unbalancing the scrubber. When levels remain constant the bacteria colony acclimates to the amount of contaminant that it can consume and treat and the caustic feed rate is adjusted accordingly but when there is a sudden or rapid increase then there may be insufficient bacteria to consume the increase contaminant load allowing a breakthrough (untreated contaminants leave the scrubber) to occur. Therefore you will normally see and apply biological scrubber technology with the treatment of off gases where the concentrations are known and not expected to change and are typically under 10 ppm. When fluctuations are expected additional nutrients can be added to support a larger bacteria colony.
Quite often the actual ability to remove the odor
is not the most important factor behind the technology selection. There are several different methods of treating odors in waste water applications, but this short overview focuses primarily on biological odor control scrubbers. The most prevalent types on the market today to treat hydrogen sulfide (H2S) are either “chemical” or “biological” and both types have been widely used over the past decade in municipal, industrial, and petroleum markets. DeLoach Industries a leader in odor control and off gas treatment designs and manufactures a variety of systems and is now incorporating artificial intelligence (AI) into their systems to further improve component to component communications and reporting on all critical functions. Imagine an operator being alerted if there is too much acid or not enough caustic in a supply tank in advance or having the scrubber contact the local supplier or support facility to respond before a problem becomes critical.
When choosing what type of scrubber, it is best to first understand the parameters of your air waste stream you need to treat. It is always recommended to have the odor gases analyzed to understand the total loading rate and what type of chemicals are creating the odorous gases and to work with manufactures with design experience. Many times, you will find methane and hydrogen sulfide combined with other organics so understanding the chemical matrix of the gas is the first step in the selection process.
Make sure you to obtain more than just one air sample and over several time periods
Before making the selection of not only what technology to use for odor control but also the correct design and size of the odor control scrubber or air emission scrubber if treating another form of volatile organic chemical (VOC) gas stream. If after you collect your data, you see that you have concentrations that have wide variations over a day or even an hour with more than a 5% change than a chemical scrubber may be your best choice. Biological scrubbers perform best when the concentrations remain steady and do not fluctuate. When concentrations change rapidly the biological growth cannot adjust fast enough so the scrubber will experience “break-through” of untreated gases unless it is a two-stage scrubber with a carbon polishing unit.
If the concentration levels fluctuate, are extremely high, or have a wide variety of compounds or contain any type of contaminant that may be harmful to the bacteria colony in the gas mixture than a chemical scrubber (single or dual pass) utilizing caustic or chlorine (for hydrogen sulfide) is often the best selection. The other two key components to the selection process is the anticipated annual operating cost of the scrubber and what type of service will the scrubber receive. If the scrubber is located where there is not an active operator and it may be left at times unattended than this can drive the decision back towards selecting a biological scrubber that has fewer components that need maintaining and typically only requires caustic for pH control and occasionally a nutrient feed supply. It is always best to gather as much information on the specific location and obtain good analytical data before making the final section process.