Harris County Municipal Utility District #170 - Executive Summary |
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The Harris County Municipal Utility District Numbers 168 and 170 share a wastewater treatment plant to biologically treat and disinfect wastewater prior to discharging to White Oak Bayou. Because district number 170 is responsible for the operation, this plant is commonly referred to as the Harris County Municipal Utility District No. 170 (HCMUD #170) Both districts produce domestic wastewater. District number 168 is primarily residential and produces the majority of the flow. District number 170 includes commercial operations and a hospital. The permitted average flow of the treatment plant is 2.5 mgd. Treatment operations include bar screening, activated sludge with extended aeration, clarification, disinfection with chlorine, and de-chlorination using sodium bisulfite. Only one of the 140 chemicals on EPA's regulated substance list is stored at HCMUD #170 above the threshold quantity. This is the compressed liquid chlorine used in the disinfection process. . The maximum am ount of chlorine contained onsite at any given time is 4500 lbs. This is stored in two one-ton tanks and up to three 150 cylinders. The tanks and cylinders are located in the central part of the facility. The threshold quantity for chlorine is 2,500 lbs. HCMUD #170 performed an analysis of the processes and risks and found the worst case release scenario to be a sudden release of the entire contents of a 1 ton cylinder of chlorine ( the largest vessel in the process). The release rate would be at a maximum of 200 lbs/min and would affect all areas surrounding the tank out to a distance of 1.3 miles. The population within this area is approximately 7100. Two alternate release scenarios, a rupture in the flexible hose used to connect the chlorine cylinder to the system, both with and without automatic shutdown controls, were identified as more likely to occur than the worst case scenario. These two scenarios resulted in a radius of influence of 0.2 miles for the flexible hose rup ture and no impact for the flexible hose rupture with immediate shutdown due to the proper functioning of the automatic switch-over valve The HCMUD #170 has not had any incidents in the past five years involving the chlorine storage tanks. |