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Project Name: Denim WWTP pH Control System Design
Country: USA
Description: This leading manufacturer of denim operated a textile dyeing and finishing wastewater treatment facility at it's North Carolina plant. Dyeing processes used at the manufacturing facility required the use of sulfur dyes and high concentrations of caustic. Oxidation of the dyes and discharge of the caustic results in high alkalinity and high dissolve solids concentrations in the wastewater. These conditions have been determined to contribute to aquatic toxicity. A toxicity reduction investigation was performed during which laboratory and full scale trials demonstrated that the addition of a strong acid such as sulfuric acid to the wastewater would provide several benefits including: reduction in alkalinity, reduction in aquatic toxicity, improved pH control; and reduction in operating costs. A pH control system described below was designed. This system provided more precise control of pH resulting in optimal performance with respect to the benefits stated above. The major design features include: sulfuric acid for influent pH neutralization by means of a chemical feed system in conjunction with an automatic closed-loop pH sensing and control system and an influent neutralization reactor consisting of a concrete basin to provide time for chemical reaction and control as well as a degree of chemical equalization. A detailed listing of design features follows: 1. Acid Pump Building; a concrete block building, to contain the Acid Feed System comprised of two pairs of different capacity diaphragm metering pumps. The system was designed such that one split-range pair (one small and one large) is to provide adequate acid delivery for normal operation. The other pair is for redundancy and/or future peak design conditions. Coordinated control of the split-range pumps was accomplished by a PLC, which receives a proportional control signal from a pH controller and sends appropriate control signals to the metering pumps. The building also contains an emergency eye wash/shower, ventilation fan, electric heater, and lighting. 2. Acid Storage Tank Containment Structure; a concrete containment structure surrounding a 10,000 gallon Acid Storage Tank. The structure provided sufficient volume for the entire tank contents, an eight inch rainfall event, and leave three ft freeboard. 3. React Basin; a concrete structure with a volume of 36,000 gallons which provides 6 minutes of detention at an ultimate flowrate of 6,000 gpm. At the WWTP design flowrate of 2.5 mgd (1735 gpm) the React Basin provided 20 minutes detention. There is a flow splitter structure included in the side of the React Basin with two equal elevation weir plate of equal length to split flow equally to the two Aeration Basins. Slide gates are provided to allow blocking flow to either basin. There is a floating mixer centrally located in the React Basin which pumps liquid from the surface downward toward the bottom in a toroidal motion. New piping intercepts the existing force main and redirect flow to the React Basin. Valving provides to by-pass the React Basin through the existing force main piping and basin inlets. 4. Control System; an Integrated Monitoring and Control System, based on a programmable logic controller (PLC) and two dedicated loop controllers provided automatic control and overall system monitoring of the pH adjustment process and its support sub-systems. Each of the large pumps has its own electronic variable speed drive unit which allows either local manual control or remote automatic control. In manual control mode, pump operation is controlled via start/stop pushbuttons with output flowrate determined by a speed selector pot with pump speed indicated on the controller's display panel. In the auto mode, speed is determined from a remote proportional signal provided by the PLC via a remote I/O (input/output) unit located in the Acid Pump Building. Each of the small pumps has a three position toggle selector switch which allows either local manual control or remote automatic control or neither (off). In manual control mode, pump output flowrate determined by a speed selector knob and by a stroke length selector knob. In the auto mode, speed is determined from a remote discrete signal provided by the PLC via a remote I/O (input/output) unit located in the Acid Pump Building. There are alarms provided by the system for all major process conditions, each annunciated by the Operator Panel on the main control panel accompanied by an audio-visual horn/light system.
 

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