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Modernization of the UNOX-U Plant Instrumentation and Controls Water and Sewer Authority of Cabarrus County Rocky River Regional WWTP Concord, North Carolina

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In the mid 1970s a two step oxygen activated sludge (OAS) nitrafication system was put on line at the Rocky River Plant of the Water and Sewer Authority of Cabarrus County in Concord, North Carolina. The system consisted of a 1st step for carbonaceous removal consisting of three trains with three stages each and a 2nd step for nitrogenous removal consisting of six trains with three stages each. The installed nameplate horsepower in the 1st step was 150, 75, and 60 and in the 2nd step was 40, 30, and 30. The oxygen supply was from a cryogenic distillation plant with a distillation column nominal capacity of 80 tons per day and a compressor capacity of 70 tons per day at 70°F and 50% RH. The control system was manually set and implemented by analog pneumatic controllers. The design was based on the following influent conditions:
1st STEP 2nd STEP
Flow, MGD 31.5 33.6
BOD5, mg/l 250 28
COD / BOD5 2.3 4.9
TSS, mg/l 100 85
NH3-N, mg/l 15
TKN-N, mg/l 20
Alkalinity 300
Temperature 10-28°C

The plant operated successfully in this process mode for over twenty five years.

In the late 1990s Black & Veatch studied future options for the facility. A decision was made to retrofit the existing 2 Step process into a 1 Step process. The new system would have 12 similar trains. Designated oxygenators would then be retrofitted to 30 nameplate horsepower with new gearboxes. All analog pneumatic controllers would be converted to electronic controllers. New electronic instrumentation would be provided. The oxygen activated sludge (OAS) system and cryogenic distillation oxygen production plant would be put on integrated direct digital control via PLCs. Control then would be automatic with manual inputs by the operators to adjust for changing influent load conditions. Consideration was given to potential future effluent requirements for total nitrogen control. Thus a criterion was set to maintain dissolved oxygen in the final oxygen activated sludge system stages of 2-4 mg/l. This would allow any future anoxic stages to be designed at a considerably reduced size; since last stage dissolved oxygen levels in OAS systems usually have dissolved oxygen levels upwards of 15-20 mg/l.

The new design was based on the following influent conditions:
Minimum Month Maximum Month Peak Day
Flow, MGD 10 34 78.4
BOD5, mg/l 80 151 32
TSS, mg/l 60 170
TKN-N, mg/l 18 25.5 16
Alkalinity 100-400 100- 400 100-400
Temperature 16-28° 16-28°C 16-28°C

The retrofit of oxygen dissolution equipment and instrumentation/controls was accomplished during the period of 2002-2003. There were some initial problems during the initial test period in matching the required oxygen transfer rates and mixing requirements to maintain KLas necessary to attain a dissolved oxygen level of 2-4 mg/l; however the final performance test indicated good control of dissolved oxygen within the range specified. The automatic control system is running and while operator intervention is required when load changes occur it is functioning well. This is the first instance where both the oxygen dissolution and cryogenic distillation oxygen production subsystems have been integrated and put on automatic PLC control at any of the more than 75 such facilities installed.
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Document Type: Research Article

Publication date: 2004-01-01

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