What is the best process for hydrogen sulfide treatment? Biological odor control Scrubber or a Chemical Scrubber?
Many industrial water treatment, municipalities and private customers whoare responsible for water and wastewater treatment often generate “hydrogen sulfide” off gas odors. Professionals who either provide the design engineering or maintain the water or waste water collection systems are often faced with the need to address and control “hydrogen sulfide” odors. Many times, they find themselves in a situation where they must select what type of odor control system will perform for their customer or at their location. A chemical odor control scrubber may utilize “chlorine” or “caustic” as a scrubbing agent or just “caustic” alone. Typically, “acids” are not utilized when treating “hydrogen sulfide” gas but “acid “may be utilized when treating other types of off gases such as ammonia. There are many variables that should be considered when choosing what type of odor control technology for any given application or process including the use of incorporating “artificial intelligence” into the machinery to provide more rapid responses to operators for the prediction of maintenance and other operational functions. If the process air to be treated is being generated from either a “decarbonation” tower or a “degasification” tower for the purpose of removing either “carbon dioxide (CO2) or “hydrogen sulfide (H2S) it is important to understand the basics of either of the processes in order to allow the design professional to properly select the best type of “odor control” scrubber to utilize and make the decision to either select a chemical scrubber that uses “caustic” and “chlorine” or a biological scrubber for the treatment of the hydrogen sulfide that only requires the use of “caustic” to buffer the re-circulation water. In other types of processes involving treatment such as the use of ion exchange a design professional or owner must understand that the process may also produce an off gas that requires treatment. “Ion exchange” can be used as a standalone treatment process to treat hard water utilized as a post treatment process after reverse osmosis. Regardless of when “ion exchange” is utilized it is always recommended to try and remove the carbon dioxide (CO2) prior to the process to prevent the formation of carbonic “acid” and to extend and save the life of the of the Ion Exchange resin. It may also be necessary to adjust the pH of the water either pre or post treatment by injecting either a “acid” solution when lowering the pH or injecting a “caustic” solution when raising the ph. It is important to remember that the efficiency of the process depends on proper pH control.
High pH and hydrogen sulfide will not convert or be removed by “degasification” and too low and the “carbon dioxide” cannot be removed.
The process of decarbonation and “degasification” will also remove other types of contaminates from the water depending on if the water being treated is for the upstream process or if the water and any contaminants like “acids” that may be in the water that was generated as the result of a downstream industrial wastewater process. The injecting of “caustic” to raise the pH in an attempt to make the water more “neutral” the design professional or owner should be aware that other contaminates that were previously in a dissolved state in suspension may begin to drop out as a solid during the raising of the ph. If the water is considered a wastewater, then it may also contain. Other types of contaminants including VOC's (Volatile Organic Chemicals) that will be removed and stripped as a gas along with the other “degasification taking place to remove the hydrogen sulfide and or carbon dioxide. These off gases often require treatment, and a design professional must decide to use either a chemical or biological process to neutralize the harmful and odorous gases.
The selection of what type to of scrubber to use and what type is the most economical and operationally cost effective is the challenge a professional or owner faces in addition to meeting the “air emissions” needs.
There are many variables that should be considered when choosing what type of odor control technology for any given application. Quite often the actual ability to remove the odor and neutralize the gas is not the most important factor behind the technology selection. There are several different methods of treating odors in water and wastewater applications, but this short overview focuses primarily on odor control gas scrubbers. The most prevalent types on the market today are either “chemical” or “biological” and both types have been widely used over the past decade and they utilize “caustic, chlorine, and in some cases acid as a chemical reagent or for pH control. 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 off gas containing the “odor” analyzed by a laboratory to understand the total loading rate, the various contaminants, and what type of chemicals or reactions are creating the odorous or corrosive gases that need to be treated. Many times, you will find methane and hydrogen sulfide (H2S) combined with other organics so understanding the chemical matrix of the gas is the first step in the selection process.
The next step is to make sure you gather more than just one sample and over a larger period.
This is critical in the selection of technology for odor control. 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 utilizing either “chlorine” or “caustic” may be your best choice when treating hydrogen sulfide. Biological scrubbers perform best for the treatment of “hydrogen sulfide” when the concentrations remain steady and do not fluctuate and they only require “caustic” to buffer the recirculation water to maintain a pH close to neutral for the biological colony to thrive. When concentrations change rapidly the biological growth cannot adjust fast enough so the scrubber and it will experience “break-through” of untreated gases unless it is a two-stage scrubber with a carbon polishing unit. Remember to monitor the pH of the inlet feed water because pH adjustment of industrial, municipal, or water requiring “reverse osmosis” can fluctuate creating different removal efficiencies and load rates. If the gas stream concentration levels fluctuate, are extremely high, or have a wide variety of compounds in the gas mixture than a chemical scrubber (single or dual pass) is often the best selection.
The other two key components to the selection process are the anticipated annual operating cost of the scrubber and what type of service will the scrubber receive.
In the past odor control scrubbers are often met with challenges because they require constant attention and maintenance for vital instrumentation that must function properly. DeLoach Industries is now incorporating “Artificial Intelligence” into their line of odor control scrubbers and“degasification/decarbonation” systems the equipment is becoming more independent and will communicate directly to operators and owners alerting them on normal maintenance to prevent critical problems or to alert operators about consumption of “caustic”, chlorine, or “acid” chemicals. 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. It is always best to gather as much information on the specific location and obtain good analytical data before making the final section process.
If the odor stream is going to be pulled into the odor control scrubber, you typically will have a fresh air vent allowing make up air to enter the tank. The proper balancing of the fresh air, make up air to extracted air is critical as it may impact the concentration levels of the gases you intend to treat in the scrubber, and this may influence the type of odor control scrubber you select. For more information or to learn more contact the professionals at DeLoach Industries Inc. at (941) 371-4995.