Lebanon Wastewater Treatment Plant Operation

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 Wastewater Treatment Plant Operation Flow Chart

NOTE: Please send an E-mail to taxless3@comcast.net if you wish to have a "hard copy" of the Flow Chart below delivered to you - the scanned Flow Chart below may be a little difficult to read.

Operation During Normal Conditions
(Compiled September 7, 2014)

NOTE: Most of the numbers and letters on the "Wastewater Treatment Plant Operation Flow Chart" at the top of this web page are referenced in this description of how the Plant operates during normal conditions.

The Lebanon Wastewater Treatment Plant is located at 802 Lafayette Avenue. This Class III Plant is a 3.3 million gallons per day (MGD) activated sludge treatment plant with a process peaking factor of 8.6 MGD. The Plantís collection system is 100% sanitary sewer by design, meaning that the sanitary sewer collection system is not designed to accept storm water runoff. Listed next is a description of how the Plant operates during normal conditions (when there is no significant rain event).

(1) Wastewater influent is fed to the Plant through a 48-inch concrete pipe.

(2) The head works provides fine screening immediately upstream of four variable speed raw sewage pumps with a total pumping capacity of 18 MGD. Influent to the Plant begins by entering the influent channel and passing through a gate into a bar screen where most debris and solids are removed.

(3) The wastewater flows into a wet well where the raw sewage pumps lift the wastewater up and direct it into a splitter tank.

(4) The splitter tank directs the wastewater flow to the grit tank via a manually operated weir gate or to the equalization basin.

(5) Grit removal in the grit tank consists of an aerated grit tank.

(6) After grit removal, wastewater is introduced into the oxidation ditch splitter box where it combines with the return activated sludge.

(7) The wastewater stream is then directed to one of the four oxidation ditches for secondary bio treatment. The oxidation ditches provide an extended air secondary treatment for the biosolids. (Note: Only two of the four oxidation ditches are in operation.) Each oxidation ditch has a 700,000 gallon capacity, and it takes 12 to 15 days for the microorganisms to complete the secondary bio treatment.

(8) The bio treated wastewater goes to a collection box.

(9) The wastewater then travels from the collection box to a splitter box.

(10) The wastewater is directed from the splitter box into two secondary clarifiers for settling out the heavier biosolids and separating the clear water. The secondary clarifiers are served by three return activated sludge pumps (B). The heavier biosolids are pumped into a return line (C) and a wasting line (D). The activated sludge in the return line goes to the splitter box in front of the oxidation ditches; this flow helps maintain the bug types and population numbers needed to reduce the solids from the influent stream. The waste line goes to the digesters (E). The digesters are aerobic tanks that further break down the solids. "Soup" is pulled off the digesters, which further increases the solids and separates the clear water. Digesters are filled equally to maintain consistency of air feed to the tanks. From the digesters, excess sludge is moved to sludge storage (F). All sludge is land applied rather than transported to a landfill.

(11) The clear water from the clarifiers goes to a collection box.

(12) A portion of the wastewater from the collection box is directed to rapid sand filtration to supply the non-potable water system that is used for pump seals.

(13) The remaining effluent from the collection box is sent to the chlorine contact tank for disinfection.

(14) From the chlorine contact tank the effluent flows into an old chlorine tank where more chlorine can be added to the effluent. Sulfur dioxide is also added to remove the chlorine so none of the microorganisms in Prairie Creek are killed. Oxygen is then added to the effluent just before release into Prairie Creek to make sure a safe amount of oxygen is sustained for the aquatic life. An influent flow meter is used to measure the raw sewage as well as an effluent meter to measure what is released into Prairie Creek. These meters provide a way to record for the state the pounds of material removed from the wastewater.

(15) The fully treated effluent is discharged into Prairie Creek.

Operation During Significant Rain Events
(Compiled September 7, 2014)

NOTE: Some of the numbers and letters on the "Wastewater Treatment Plant Operation Flow Chart" at the top of this web page are referenced in this description of how the Plant operates during significant rain events.

While the sanitary sewer collection system is not designed to accept storm water runoff, storm water does make its way into the Lebanon Wastewater Treatment Plant during significant rain events through (a) improper storm water connections to the cityís sanitary sewer system by some Lebanon Utilities customers, (b) leaks in the sanitary sewer lateral connections from some Lebanon Utilities customers to the cityís sanitary sewer system, and (c) some leaks through the sanitary sewer pipes and manholes maintained by the city. Listed next is a description of how the Plant operates during a significant rain event).

The wet well in the head works (2) has four variable speed pumps, two of which are all that is needed for normal plant capacity. The other two will act as storm pumps (A) during a significant rain event and are used to pump influent to the equalization basin (G) when flows exceed the 8.6 MGD process peak. The influent is held until it can be reintroduced to the influent sewer for full treatment

In the event flows exceed the 8.6 MGD peak capacity and the equalization basin storage capacity (750,000 gallons), overflow occurs from the equalization basin (G) to the chlorine contact tank (13). Air is added to remove most of the grit and also remove the floatables before the influent overflows into the chlorine contact tank.

In the event the hydraulic capacity of the chlorine contact tank (13) is reached, the equalization basin overflows into the dechlorination tank (14).

The disinfected and dechlorinated overflow effluent is discharged into Prairie Creek (15). Effluent sampling incorporates the combined flow for both the overflow effluent and the normally treated effluent to determine the discharged effluent quality.

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This page was last updated on 09/13/14 .