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News: Plant Spotlight

Plant Spotlight: Mebane Bridge WWTP

Tuesday, June 13, 2017  
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Plant staff

 

 

Aeration basin

 

 

Fine screen

 

 

Headworks

 

 

Plan effluent

 

 

Solids handling facility

 

Originally published in the Spring 2017 issue of NC Currents magazine.

General

Located in the central part of the state, just south of the NC/VA state line, the City of Eden is the largest city in Rockingham County. The City of Eden was incorporated in 1967 when three other towns merged. It has the nickname “Land of Two Rivers” because of the Smith and Dan Rivers flowing together on the south side of Eden. In 2011 the city received one of the All-America City Awards.

The City of Eden currently operates one WWTP that services a population of 15,488. It was put into operation in 1967 and is publicly owned. The WWTP currently employs a total of nine staff, consisting of 13 operations and maintenance staff and one laboratory staff, and has an annual operating cost of approximately $1.5 million.

The plant’s effluent discharges into the Dan River, part of the Roanoke River Basin. After leaving the WWTP, the Dan River flows back into Virginia and then eventually to the Kerr Reservoir on the Roanoke River. The WWTP has a design flow of 13.5 mgd and operates with an average daily flow of 4.5 mgd. Peak flow is 14 mgd.

 

Basic Treatment Processes

The city has 20 pump stations that pump all wastewater to the plant. Currently, the Mebane Bridge plant operates at one-third its capacity and is using half of the plant for treatment, with the other half ready for backup or peak flows. The plant has mechanical bar screens to remove larger inert material and a grit removal system following the bar screens. A fine screen has been added after the grit removal system to further remove any material that is missed by prior treatment systems. Extended aeration using activated sludge is the next process to reduce and remove biochemical oxygen demand. It consists of 12 brush aerators per basin and three SolarBees® for mixing, allowing the operators to turn off part of the aerators during peak power demand. The sludge is separated from treated water by circular clarifiers. Collected sludge or biosolids are wasted to the aerobic digester or returned to the aeration system. The biosolids from the digester are then dewatered and land applied on permitted sites. Treated water to the effluent leaves the clarifiers and is disinfected with chlorine and then dechlorinated. The treated effluent is then returned to the Dan River, meeting all state permit discharge requirements.

The last plant expansion was in 1992. There are no plans for expansion, but the staff is planning to redo the solids handling section of the plant and do away with the digester.

The plant has limited automation through SCADA. Alarms for high water near the bar screens are monitored along with low DO in the basins. Aerators can be controlled on and off in the aeration basins. Influent and effluent flow can be monitored along with the equipment run time.

The primary sources of influent are domestic, due to the decline of textile manufacturing in the area. There is one textile plant left, along with a chemical recycling plant, a plastic recycling plant, and a small metal finishing boiler plant.

 

Solids Treatment

Currently, the plant has one digester that handles all of the wasted sludge. It is original to the plant and was not part of the 1992 upgrade. The plan is to eliminate this basin and run all of the wasted sludge through a Clean B system before being dewatered.

The biosolids management program includes a 2-meter belt filter press (BFP) that dewaters all of the sludge. It is then stored onsite on a covered storage pad until it can be land applied on permitted fields in the area. All dewatering and land application is handled by Synagro. The entire system was built through a design-build-operate contract.

 

Disinfection

The plant’s disinfection system utilizes chlorine gas for disinfection. The system includes two 500 pounds per day (PPD) flow proportional gas feeders, one 500-PPD manual gas feeder and three baffled chlorine contact basins. Before being released to the river, the effluent is dechlorinated with sodium bisulfite. A flash mixing chamber with a turbine-type mixer provides the required mixing.

The operations personnel have a very active risk management and process safety plan to monitor the safety issues associated with having gas chlorine onsite.

 

Challenges and Unique Features

The plant operations team faced a difficult problem when it lost its largest industry, one that contributed 3.6 mgd, which was half of the plant’s flow. Operations had to quickly figure out how to adjust treatment in a plant with very little flexibility. They were able to use this adjustment period to get the basins cleaned out and the clarifiers rehabbed while there were still funds available. They have still had to make adjustments during low flows, but they are able to handle high peak flows from rain events with no issues.

 One of the most unique or interesting things about the facility is the SolarBee® mixers. They are the only ones in the state that are used for mixing assistance in an aeration basin. A number of trials were performed to address this issue, and it has proven to be a huge savings to the electrical budget by providing flexibility to turn aerators off for several hours every day.

 

Personnel

The WWTP offers personnel development programs that encourage cross training for all employees in any area of interest.

The plant personnel includes five Grade IV, one Grade III, two Grade II, and one Grade I operators. In addition to this, three have a pretreatment license and three have certifications in lab analysis, one in maintenance tech, and one in land application. As well, the superintendent has B Surface, B Distribution, and Collections II certifications. One member of the staff is cross-trained at the water plant and one is working on his backflow/cross connection and collections certifications.

The plant’s safety and health program personnel have been Safety and Health Achievement Recognition Program (SHARP) certified through the NC Occupational Safety and Health Administration (OSHA) for the past five years. Operators rotate terms on the City’s Safety Committee and assist with updating the Safety Manual. They are also active in keeping process safety manuals up to date.

 

Awards

George W. Burke Safety Award

Melinda Ward, William D. Hatfield Award

 

Contact Information

Melinda Ward, Wastewater Plant Superintendent Phone: 336-627-1009 E-mail: mward@edennc.us



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