 |
Patented Hall BioreactorSource: Hall, P.J., 2000. "Perchlorate Remediation at a DoD Facility", in Perchlorate Treatment Technology Workshop, 5th Annual Joint Services Pollution Prevention & Hazardous Waste Management Conference & Exhibition, August 21-24, 2000, Henry B. Gonzalez Convention Center, San Antonio, Texas. Project Summary: The following text was excerpted from Hall, P.J., 2000. "Perchlorate Remediation at a DoD Facility", in Perchlorate Treatment Technology Workshop, 5th Annual Joint Services Pollution Prevention & Hazardous Waste Management Conference & Exhibition, August 21-24, 2000, Henry B. Gonzalez Convention Center, San Antonio, Texas: EcoMat, Inc. is a California-based firm that makes and sells advanced biological remediation systems, including systems for denitrification. The bacteria that provide denitrification are readily found in nature. They are facultative anaerobes that can utilize oxygen for metabolic energy from sources other than dissolved oxygen. These bacteria will take oxygen from the easiest supply and then look for more. In water treatment they remove oxygen based upon the following preferred sequence:
EcoMat designed a system to achieve perchlorate removal from the Baker tanks within a period of several months. Initially, without sufficient information to determine the hydraulic residence time to remove perchlorate to non-detectable levels, the system was designed for a residence time of approximately one-half hour with an active volume of 200 liters. Given average tank volumes of 20,000 gallons, this would enable complete reduction in a period of seven days after the bacteria are firmly established. EcoMat had designed and built an identical system and installed it in the John G. Shedd Aquarium in Chicago. Denitrification bacteria that were exposed to perchlorate were placed in the reactors and the skid was transported to southern California. At the site, the system was functioning and reducing perchlorate within a few days, and systems operation was transferred to Earth Tech, with continued telephone consult from EcoMat. After several months during which operating problems were overcome, the tanks were completely clean of perchlorate, below the detectable concentration. The system was then moved to a similar site on the base, where it remains in operation. The system design begins with water from the Baker tank being drawn into the top of the deaeration reactor, reflecting EcoMat's understanding that a two-stage process works best for biological oxygen removal. In the deaeration tank is a large number of ordinary bio-balls that provide surface for bacterial growth. The reactor is designed to reduce the dissolved oxygen concentration from saturation down to a concentration of 0.5--1.0 ppm. This is the optimum concentration for either denitrification or perchlorate remediation. If the dissolved oxygen concentration rises above one ppm, the remediation is ineffective, and if it drops to near-anaerobic concentrations, the threat of sulfate attack arises. Hydrogen sulfide can be injurious to the bacteria, stopping the remediation activity. Although the bacteria can be revived very easily by restarting the process, time is wasted if oxygen levels are not monitored. From the bottom of the deaeration reactor, water is then drawn into the bottom of the patented Hall reactor, the key element of EcoMat's process. The reactor is designed to hold a mass of floating media and maintain continuous circulation of the media along with the water in the reactor. This mixing is attained without any internal moving parts, but rather, by external pump re-circulation. Continuous circulation is extremely important as it provides for uniform, low concentrations of the contaminant under all influent contaminant concentrations. In this manner, no upper limit on the allowable inlet concentrations is needed. The Eco-Link media that fills the Hall reactor is a polyurethane-based sponge that is cut into one-centimeter cubes. The media last for up to several years, and are kept reasonably clean and capable of supporting bacteria colonies by virtue of their gentle collisions with each other and with the walls of the reactor. When functioning to produce a gas, as in denitrification, the size of the interstitial spaces within the sponge is designed to permit passage of gas out, as well as passage of water into, these spaces. At the same time, the surface area involved is sufficiently great to provide for large bacteria concentrations and high interaction efficiency. The overflow from the Hall reactor is recycled back into the deaeration reactor during the startup period to form colonies of bacteria. In normal operation the effluent is discharged from the system. In cases where drinking water purity is desired, a post-treatment system can be added to the process to control the small amount of biosolids that leaves the system. This is the only residual stream that results from the process. In case of upset conditions, water can be returned tot he Baker tanks. Both reactors require feed of a carbon source (electron donor) to feed the bacteria. EcoMat has studied a variety of available sources as find the best one is methanol. Methanol residual of less than 2 ppm is considered non-hazardous and EcoMat's systems normally run at undetectable concentrations (below 0.5 ppm). Methanol is not only the lowest cost commercially available carbon source but is also maintains the lowest level of biosolids. Alternative carbon sources, such as ethanol, tend to "gum up" the works. The major requirement for methanol is for removal of dissolved oxygen in the deaeration reactor, as oxygen levels are so much greater than perchlorate levels in the first stage of the process. For fire safety reasons, the methanol is dissolved in water (generally 50%). The rate of feed of methanol is so small that even if it were to exit unused, the concentration would not reach hazardous levels. While the bacteria involved in denitrification are hardy, best operations are realized when temperatures are controlled between limits of 8 deg. C. and 35 deg. C. During normal flow, the influent water maintains adequate temperature control. During startup, when recirculation is 100%, care should be taken to turn on the circulation pump in the Hall reactor for a relatively small time period each day. The way the system works is that the bacteria can "eat" a constant rate of contaminant. Thus the flow rate of water through the system isn't a significant parameter in the design. The most significant system size factor, which determines the basic system size, is the total amount of material that is to be removed per day. This number is the product of the flow times the concentration. For example, for a system that will remediate 1,000 gpm of water having a concentration of 10 ppm, the amount of contaminant to be removed is 120 pounds per day. For this example, EcoMat estimates that it can build, own, and operate this system, at the currently demonstrated sizing criteria, at total cost to the customer of $.50 per thousand gallons. The system is built on a four by four foot skid. Startup operations involve continuously recycling the water through the reactors while feeding methanol and assuring that there is adequate perchlorate in the water. This recirculation need not be constant, and in warm weather, when the bacteria may overheat, it is best to circulate for no more than a few hours per day. Periodic measurements are made of the dissolved oxygen levels leaving the deaeration reactor. When the dissolved oxygen level is below 1.0 ppm, the system can be opened in stages, until it is wide open. After startup, operations remain continuous, and it is only necessary to check the system once daily to be sure that no spurious upset has taken place. The methanol source only needs to be replenished every few weeks. At this DoD site there were a number of upsets, particularly during the early operating days. First, someone driving by pulled the main power plug. A few days passed before the operators realized that there was something wrong. During that time, the bacteria used up all of the oxygen and perchlorate and started producing hydrogen sulfide. The system turned black and smelled characteristically of that material. The system was re-started and within a few days it returned to normal operation. Earth Tech was not concerned with optimizing the time for performing the remediation of the water from the Baker tanks. With a retention time of one half-hour, the remediation proceeded sufficiently rapidly. However, based upon EcoMat's denitrification experience, much shorter retention times may be feasible for perchlorate remediation, further reducing the cost of new systems. EcoMat is pursuing this possibility. Measurements were made by Earth Tech on a regular basis. As a result of the "closed loop" feature, it was possible to control the outlet so that only when the effluent perchlorate concentrations were below the allowable level (ND) would water be discharged to a cleaned water baker tank. Initial results during the startup period were as follows (in ug/L):
** New tank When the Baker tanks were emptied, the system was moved to another location at the DoD site, where it is presently in operation. Reactors 15 times the size of the subject reactor are currently in operation, and EcoMat has designed reactors as large as 100 cubic meters. The reactors may be ganged together to provide adequate volume for any flow rate. EcoMat plans to offer its perchlorate remediation process to customers as a build-own-operate package, with pricing in the range of $.50/1,000 gallons. For very large systems it would be cost effective to implement on-line measurement capabilities with SCADA systems to transmit data to a remote operations center, facilitating satisfactory operations. Additional Info Source: Hall, P.J., 2000. "Perchlorate Remediation at a DoD Facility", in Perchlorate Treatment Technology Workshop, 5th Annual Joint Services Pollution Prevention & Hazardous Waste Management Conference & Exhibition, August 21-24, 2000, Henry B. Gonzalez Convention Center, San Antonio, Texas. |
|
||||||||||||||||||||||||||||||||||
      
|
||||||||||||||||||||||||||||||||||||