Denzil Bowman Wastewater Treatment Plant - Executive Summary

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                                                           EXECUTIVE SUMMARY 
                                          Denzil Bowman Wastewater Treatment Plant 
 
    The Denzil Bowman Wastewater Treatment Plant is located approximately 4 miles outside the corporate limits of the Town of Greeneville.  There are presently eleven employees working three shifts at the Denzil Bowman Wastewater Treatment Plant.  The Greeneville Water Commission has taken an aggressive position relative to chemical safety.  We must file an Emergency Response Plan because we are required to have a Process Safety Program by TOSHA and we have chlorine on site in excess of the threshold limits as defined by the Environmental Protection Agency. 
    Wastewater is transported to the treatment plant by a collection system which consists of 21 pumping stations and several hundred miles of sewer pipes.  When the wastewater arrives at the plant it is raised or lifted up and dumped into a channel so that it will trave 
l through the treatment plant by gravity.  This is done in order that the wastewater does not have to be mechanically moved from one treatment to another. 
    After being placed/dumped into the influent channel the wastewater goes through a mechanical bar screen.  This step is to remove any large objects that might cause damage to the equipment or piping in the plant.  From here the wastewater goes through a Parshall Flume.  A Parshall Flume is a smooth sided device that will not impede the wastewater flow.  The Parshall Flume has an ultra sonic meter that sends a signal down to the surface of the wastewater stream, where it is bounced back to the meter so the flow can be calculated and converted into million gallons per day.  These calculations are based upon the depth of the waste stream going through the Flume at that time.  Once converted the results are electronically relayed to a flow recorder and chart inside the laboratory building.  Next the wastewater flows into the grit cha 
mber.  In the grit chamber air is introduced into the wastewater in such a way as to cause grit, such as sand and gravel, to settle out of the wastewater where it can be mechanically removed so it will not accumulate in the plant.  The wastewater, with most of the large objects and grit removed, now travels to the oxidation ditches. 
    There are two oxidation ditches at the Wastewater Treatment Plant.  Each ditch has a capacity of 2.4 million gallons.  The oxidation ditch type of treatment is an aerobic process.  Aerobic means that air is mechanically placed into the wastewater to enable the aerobic bacteria to break down the solids into a non-harmful, useful substance.  Each ditch has three channels.  The wastewater enters the ditch in the outside channel.  The wastewater is mixed with the water that is already in the ditch and is aerated by the use of several round rollers that have flat paddles on them.  These paddels "slap" the water which causes air to be injected, and keeps the 
water circulating so the solids will not settle out in the ditch.  From the time the wastewater enters the outside channel it takes approximately 8 hours for it to make its way to the inside channel where it leaves the ditch by way of weirs. 
    After flowing over the oxidation ditch weirs, the water flows by underground pipe to the final clarifiers.  The wastewater enters the clarifiers in the center and as it flows toward the outside "V" notch weirs the solids are allowed to settle out of the water.  The clear, solids-free water then flows to the effluent Parshall Flume.  The solids that settle out of the wastewater are moved to the inside of the clarifiers by means of mechanical scrapers that are continually moving the solids back toward the center of the clarifier where it is removed to the sludge pumping station by way of telescopic valves. 
    Once at the return sludge pump station, the sloids are either pumped back to the oxidation ditches or to the digester.  The solids in t 
he ditches are called "mixed liquor suspended solids".  These solids contain the microorganisms that breakdown or "eat" the solids in the raw wastewater.  They are returned to the ditches in order to maintain an acceptable mixed liquor suspended solids level and to breakdown the solids in the raw wastewater as it enters the ditch.  These microorganisms are continually going through a cycle of breaking down solids in the wastewater, leaving the ditches and going to the clarifiers where they are settled out of the water, going to the return sludge pumping station, and being returned to the ditches where they can begin the process again. 
    When the mixed liquor suspended solids in the ditches goes above what is acceptable, the solids are removed from the "cycle" by being pumped from the return sludge pumping ststion to the digester.  In the digester the solids or sludge is aeriated and then allowed to settle.  When the sloids have settled out, the water on top is removed by way of valv 
es at different levels on the digester and returned to the grit chamber where it flows back into the plant.  The air is then turned back on in the digester and as more solids need to be removed from the "cycle" they are pumped into the digester, the air is turned off, the solids are allowed to settle out, the water is removed and returned to the plant, and the air is turned back on.  This process is repeated until the solids content of the digester sludge gets to the point that it needs to be removed.  The sludge is then fed into a tank called the "thickener" where it settles even more, with the water from the top being returned to the plant. 
    Sludge is removed from the thickener into the dewatering building where it is pumped into the centrifuge.  As the sludge enters the centrifuge, it is mixed with a polymer that acts as a solids magnet and helps separate the solids from the water at an accelerated rate.  The solids are then slung to the outside wall of the centrifuge where it i 
s removed by scrapers and emptied onto a conveyor belt that is located below the centrifuge.  After having the solids removed, the water from the centrifuge process is returned to the plant.  The sludge is carried by the conveyor belt and discharged into a sludge truck were it is taken to state approved disposal sites and land applicated. 
    The amount of water that exits the plant is converted to million gallons by the ultra sonic meter on the effluent Parshall Flume and relayed to a flow recorder and chart located inside the laboratory building.  When the water leaves the Parshall Flume it enters the outfall line that carries it to the receiving stream, in our case the Nolichuckey River at mile 47.5.  Immediately prior to entering the outfall line, chlorine gas is injected into the water to destroy pathogenic organisms, which are disease producing organisms in warm blooded animals. 
    The Wastewater Treatment Plant is currently using an average of 40 pounds of chlorine per day to 
treat a flow of approximately 3.5 MGD (3,500,000 gallons per day).  The chlorine is contained in one ton cylinders which are stored in a building located at the lowest point at the Wastewater Treatment Plant.  There are usually three full one ton cylinders on site at any time.  Of these three cylinders, two are on the scales and one is in reserve.  Of the two on the scales, only one is open at any one time.  The tank that is open is the one that we feed from.  The chlorine tanks are pressurized and the chlorine is used in its gaseous state.  The chlorine gas leaves the chlorine tank and travels through the chlorine feeders and into the injectors.  The chlorine gas then enters the water stream. 
    Chlorine must have an adequate contact time to insure that the harmful pathogens are destroyed.  The outfall line from the Wastewater Treatment Plant to the receiving streat is long enough, approximately 3 miles, to allow for the necessary contact time.  For this reason a chlorine contact c 
hamber was not needed at the Wastewater Treatment Plant. 
    The maximum amount of chlorine residual in milligrams per liter (mg/l) that is allowable in the water that is discharged into our receiving stream is 0.4 mg/l.  This is the maximum limit that is allowed by our current National Pollutant Discharge Elimination System, "NPDES" Permit.  The average amount of chlorine residual that is currently found in the plant effluent is o.4 mg/l.  When converted into pounds this amount translates into approximately 40 pounds of chlorine being used daily to treat a daily average flow of approximately 3.5 million gallons of wastewater. 
    Because we store the chlorine cylinders inside a building, we are considered to have passive mitigation.  This means that in the event of a chlorine leak, approximately 55 percent of the chlorine leaked will be able to reach the outside atmosphere. 
    The worst-case release scenario for the Wastewater Treatment Plant would be to have a tank rupture and re 
lease 2,000 pounds of chlorine.  Because of the mitigation, being inside of an enclosure, only 55 percent or 1,100 pounds of chlorine would be released to the atmosphere.  If this kind of situation were to happen, the chlorine alarm at the chlorine building would sound and alert the operator on duty.  Once alerted the operator would immediately go to the chlorine building to visually confirm a leak.  If the operator determines that there is a chlorine leak he/she would follow the Wastewater Treatment Plant's Emergency Response Plan for the correct action(s) to take. 
    In the event of a worst-case release scenario, everyone within a radius of 0.9 miles of the Wastewater Treatment Plant would need to be evacuated.  This action would be handled by the Emergency Management Agency for the Town of Greeneville and would mean that approximately 150 people would need to be evacuated.  However, given the lay of the land and the location of the chlorine building at the Wastewater Treatment Pla 
nt, the chlorine gas, which is more dense and therefore heavier than air, would more than likely follow the creek bed until it dissipated to the point of being non-harmful to humans or the environment. 
    The alternative release scenario for a chlorine leak at the Wastewater Treatment plant would mean a release of 200 pounds or less of chlorine gas.  Because of the mitigation, being inside a building, the amount of actual pounds of chlorine leaked would be 110 pounds or less.  If this situation were to occur, the chlorine alarm at the chlorine building would sound and alert the operator on duty.  Once alerted the operator would immediately go to the chlorine building to visually confirm a chlorine leak.  If the operator determines that there is a chlorine leak, he/she would follow the Wastewater Treatment Plant's Emergency Response Plan for the correct action(s) to take. 
    In the event of an alternative release scenario everyone within a radius of 0.2 miles of the Wastewater Treat 
ment Plant would need to be evacuated.  This action would be handled by the Emergency Management Agency for the Town of Greeneville and would mean that only approximately 50 people would need to be evacuated.  The alternative release scenario, as with the worst-case release scenario, given the lay of the land and the location of the chlorine building at the Wastewater Treatment Plant, the chlorine gas which is more dense and therefore heavier than air, would more than likely follow the creek bed until it dissipated to the point of being non-harmful to humans or the environment. 
    The Wastewater Treatment Plant complies with the OSHA Process Safety Management rule which outlines the correct procedures to follow for any circumstance that may occur.  The Wastewater Treatment Plant has a written Emergency Response Plan as required by the Environmental Protection Agency and is included in the written plans of the Emergency Manage,ent Agency for the Town of Greeneville.  The personnel at  
the Wastewater Treatment Plant are trained in the latest and most up to date procedures concerning chlorine safety and handling.  This training is updated annually to guarantee that all employees know the correct actions to take in any situation that might arise. 
    There has not been a major chlorine gas leak at the Wastewater Treatment Plant during the last five years or for that matter during the life time of the Wastewater Treatment Plant.  Occasionally when hooking up a full container of chlorine a lead washer will not seal properly and when the operator turns the valve on to check for leaks a small amount of chlorine gas will escape.  The amount is so minute that it usually will not set off the alarm and will not leave the confines of the chlorine building.  When this happens the operator immediately shuts off the valve and replaces the lead washer and then rechecks for leaks.  When there is no leakage detested the container can be placed into service. 
    The response plan at 
the Wastewater Treatment plant consists of the following components.  The first is the Emergency Response Plan fir the Wastewater Treatment Plant which spells out what actions are to be taken in the event of a chlorine gas leak and who is to take the action.  If any leaks are detected we have a two man Emergency Response Team designated to respond.  The two members of our Emergency Response Team will perform any needed repairs while their backup, which will consist of a two member team from the Greeneville Firew Department are outside on standby.  The Emergency Response Plans are included in the written response plan of the Town of Greeneville's Emergency Management Agency.  Training and drills for all Wastewater Treatment Plant employees are arranged and provided by the Greeneville Water Commission to insure that leaks will not occur, but in the event one should occur, employees would be able to respond to it, both efficiently and safely, to better protect the public.  This training  
consists of the proper fitting, use, care, and maintenance of respirators and protective clothing that might be needed during a chlorine leak.  This training and drills is both classroom and hands-on, and also involves training in the use of any repair tools that might be needed to repair a faulty chlorine tank.  Public notifications and alert systems will be handled by the Emergency Management Agency. 
    There has never been a reportable leak at the Denzil Bowman Wastewater Treatment Plant.  To help insure the public safety, automatic shut-off valves are going to be installed on all one ton chlorine cylinders at the Wastewater Treatment Plant.  These shut-off valves are battery operated and will automatically shut off the flow of chlorine gas in the event of a pipe rupture.  These preventive measures will aid in insuring that chlorine leaks cannot happen.  In adition, more frequent and up-to-date classroom and hands-on training will be scheduled for all employees involved with chlor 
ine handling or the Emergency Response Plan. 
    In closing we want to point out there has never been, either a worst-case release scenario or a alternative release scenario, chlorine gas leak at the Denzil Bowman Wastewater Treatment Plant.  Furthermore, we are confident that, with these additional measures, there should be no reportable chlorine gas leaks in the future.
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