SUPEROXYGENATION OF FORCE MAINS AND GRAVITY INTERCEPTORS FOR ODOR/CORROSION PREVENTION
Abstract:Many of the problems associated with wastewater collection and treatment are related to the absence of Dissolved Oxygen (D.O.) in the wastewater. The considerable imbalance of BOD and solubility of oxygen from air into water is the underlying cause of this problem. The long residence time of wastewater in force mains in the absence of D.O. results in significant H2S generation which is odorous and corrosive to ductile iron pipe. The subsequent transition of a force main discharge into a gravity interceptor then results in stripping of H2S into the headspace, where the oxic conditions support the microbial conversion of H2S to H2SO4, which corrodes the concrete pipe section above the water line. The limited reaeration in gravity sewers also sets up the conditions for H2S generation. Gaseous oxygen costs about 1/10 as much – on an oxygen equivalency basis - as nitrate or peroxide but the lack of availability of technology for efficiently and effectively dissolving high purity oxygen into raw sewage has prevented exploitation of the considerable economy associated therewith. This paper will describe case studies where gaseous high purity oxygen was completely dissolved into a sidestream of raw sewage at concentrations of 50 to 100 mg/L D.O. and then blended back into the main sewer flow for prevention of odor and corrosion. There is no effervescent loss of D.O. in the process. In force mains with reverse grades that are several miles long there is no gas phase to collect at the high points so that pump head through the force main is not increased and no potentially explosive gas phase can develop in these gas pockets. The economics of the design will be discussed. The short pay back periods of 2 to 3 years allow considerable savings in chemicals for odor control. Additionally for every pound of D.O. added in the collection system for odor/corrosion prevention, there is a pound of D.O. saved in the subsequent aeration requirements for secondary treatment. Not only is H2S generation prevented under oxic conditions, but also any H2S already in the wastewater is microbially metabolized to sulfate within 15 to 30 minutes after oxic conditions are established. The low reaeration rates in gravity interceptors of 3 ft diameter and greater are used to considerable advantage in retaining superoxygenated D.O. concentrations of 20 to 40 mg/L for long distances. For instance, in a 3 ft diameter gravity interceptor flowing half full which has its D.O. raised to 20 mg/L using this technology, only 4 mg/L of D.O. will be lost to the head space above the flowing sewage after flowing over 2 miles or approximately 2 hours. Thus it is practical to apply superoxygenation technology to even larger gravity interceptors for odor/corrosion prevention as well as eliminating serious odors at the head works. The implications of this for extending the useful life of the sewer system are exceptional since the operating costs for superoxygenation are far less than the cost of sewer replacement and the use of nitrate or peroxide. Results from superoxygenation to 40 mg/L D.O. of a force main that is 3 miles long will be discussed. Another case study in which the influent to a 5-mile long force was superoxygenated to over 50 mg/L D.O. for H2S prevention will also be covered. A final case study showing complete elimination of reduced sulfur compounds after superoxygenation of a 25 MGD force main 2 miles long will be considered.
Document Type: Research Article
Publication date: January 1, 2005
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