Enhanced Absorption and Biodegradation
PRD Tech, Inc. has developed an Enhanced Absorber, Bioreactor with associated aeration and submersible pump, liquid recycle pumps and activated adsorbent media for polishing the gas exiting the absorber section. The Enhanced Absorber uses a proprietary formulation, consisting of surfactant/co-solvent and other additives, which increases the solubility of the odorous compounds in the water mixture. The water with the formulation exits the absorber and is treated biologically in an aerated bioreactor, which has high surface area biomedia to increase the biomass concentration. Figure 4, not drawn to scale, shows a schematic of the proposed treatment process.
Figure 4. Enhanced Absorption of VOCs/HAPs with Biodegradation.
The inlet gas is pumped by the inlet gas blower, BL-1, through the absorber (AB-1) and activated media (AM-1) beds. The absorber bed consists of high surface area media, MB-1, which is kept wet by the water spray. The gas exiting the absorber passes through activated adsorbent media, AM-1, that is used to polish the treated gas to remove traces of the odorous compounds. This activated media is designed to last at least 2-3 years.
Water from the sump of the absorber, AB-1, is pumped by pump, P-1, through an automatic backwash filter, BF-1, which periodically uses a backflush to remove accumulated solids on the screen and flushes them back (stream 4) into the absorber vessel sump. Filtered water, passing through the automatic backflush screen is recycled back to the spray heads in the absorber vessel. The automatic backflush filter protects the spray heads from clogging.
Some of the unfiltered liquid from the absorber vessel sump is biologically treated in the bioreactor vessel, BR-1. The bioreactor vessel contains high surface area biomedia, not shown in the figure, which allows active microorganisms to grow on the biomedia as biofilms, that biologically treat the water, and converts the organic contaminants into carbon dioxide and water.
Water from the bioreactor is pumped by the submersible pump, P-2, through an aerator, AE-1, which draws in ambient air and mixes it with the liquid flow. The air-water mixture is then sparged into the bioreactor liquid through eductors that also actively mixes the water within the bioreactor. Treated water from the bioreactor is recycled back to the absorber spray heads in the absorber vessel. The submersible pump is installed inside a self-cleaning screen, not shown in the figure, which filters the water before it gets into the suction of the submersible pump.
Since the air leaving the absorber is completely humidified, water make-up is required to balance the water loss. The make-up water flows through an Additive Addition system, SBA-1, which uses a concentrate of the formulation to add to the make-up water before it gets into the water spray in the absorber vessel.
The absorber is described in this proposal as an “Enhanced Absorber” since it uses a proprietary formulation to enhance the solubilization of poorly-soluble odorous compounds. This proprietary formulation consists of a mixture of surfactants, co-solvents, odor absorbing chemistries, enzymes and bacteria concentrate, that is supplied as a concentrate. This concentrate is used in the Additive Addition system (SBA-1) to mix with the make-up water flow, before the water is added to the flow flowing into the absorber spray heads.
The surfactants and co-solvents used in the proprietary formulation biodegrade very slowly in the bioreactor, while the odorous compounds are biodegraded at a significantly higher rate, thereby regenerating the surfactants and co-solvent, before they are recycled back to the spray heads in the absorber.
There is a periodic blowdown flow, not shown in the figure, which is discharged into the drain line, from the flow exiting pump, P-1, before the automatic backwash filter, BF-1. The purpose of this periodic blowdown is to maintain the total suspended solids within the sump of the absorber below 2,000 mg/L. Some of the surfactants and co-solvent are lost through this blowdown stream.
The main advantages of the Enhanced Absorption and Biodegradation Process are as follows:
- Ability to handle water soluble and water insoluble compounds;
- Gas absorbers are designed with superficial gas velocities in the range of 300-500 ft/minute, which are 3-5 times the superficial gas velocity that can be used in a biofilter, which significantly reduces the diameter of the vessel compared to a biofilter;
- The selection of the absorption liquid and co-solvent can be tailored to the types of odors and VOCs that need to be removed, providing a huge amount of flexibility in the design of the system;
- The water soluble and water-insoluble compounds are simultanoeusly biodegraded in the bioreactor system, wherein advanced oxidation, such as ozone treatment, can be used in conjunstion with biodegradation, to enhance biodegradability of recalcitrant VOCs, such as chlorides, etc.;
- Use of co-solvents enables simultaneous absorption of water soluble and water insoluble compounds, rather than typical absorbers that use water and hence can only remove water soluble compounds;
- The absorption liquid is regenerated in the bioreactor system, thereby minimizing loss of liquid and blowdown; and
- Investment and operating costs are significantly lower than other competing technologies, sucha as biofiltration, activated carbon or water absorption.