Chemical Treatment Of Wastewaters "Conventional Treatment"

INTRODUCTION
The term conventional treatment today is usually interpreted as an old and cost prohibitive technology. In fact, there are many new ways and techniques of achieving better results using these "old" chemical theories. With the use of computer programming and state of the art PLC logic controls a once complex and operator attentive system has now become a smooth and worry free system. More and more established facilities are updating their waste treatments systems by only adding a new control panel and a few automated devices. There is no need to "gut" the system and start from scratch. However, there are new companies and new facilities that require a new treatment system and the best available technology is conventional treatment. These facilities usually have multiple waste steams that deem that the best technology to use is the "old" technology of chemical treatment because it is reliable, consistent, and it works.

THE DESIGN
The Electropolish Finishing industry produces a wide array of wastewaters. The wastewater treatment at this electropolishing facility treats dilute acid rinsewaters, fume scrubber bleed and concentrated acid dumps continuously. The design of this waste treatment process was to economically reduce disposal costs associated with the facility. This facility produces hexavalent chrome, acidic rinsewaters, and metals (such as nickel and copper). The most economic method(s) used to reduce the wastes produced was to chemically treat the waste streams. There are four types of chemical treatment processes implemented at this facility. The first chemical method used is the neutralization of the waste acids. This is preceded by a reduction reaction of the hexavalent chrome to trivalent chrome. Next, is precipitation of the metals, which is then followed by flocculation. These chemically processes used (neutralization, reduction, precipitation, and flocculation) is done "automatically" through modern equipment, including PLC logic, which reduces operator attention. The use of state of the art equipment and technologies insures that the effluent discharge continually meets compliance with permit limitations.

The wastewater treatment system treats flows rates up to 20 GPM. The continuous flow through treatment process encompasses the following technologies:

• Equalization
• Hexavalent Chrome Reduction
• pH Neutralization
• Metal Hydroxide Precipitation
• Flocculation
• Clarification
• Metal Hydroxide Sludge Dewatering
• Final pH Neutralization
• Final cartridge filtration
• Flow Measurement and Automatic Sampling.

All collection and transfer vessels are equipped with appropriate level controls for automatic operation. All centrifugal pump systems are designed in pairs, with one pump as the lead and the other as lag (operator selectable) so that the system will operate regularly with one pump. When the need arises, the lag pump will operate due to a lead pump failure (or a change in process conditions) that would require two pumps operating. High level indication lights and alarms alert the operator of these conditions at all collection tanks. Process reaction vessels requiring pH and ORP controls are automated monitoring devices. Recording monitoring is provided for final pH and final flow before discharging to the sewer.

The control system in the main control panel is a state of the art Allen Bradley PLC system with a Panelview 550 Operator Interface Terminal (OIT) screen. The Operator Interface Terminal (OIT) is used to process the logic required to operate the system. The OIT screen allows the operator to operate this system with greater flexibility using touch screen technology. There are pilot lights on the panel for four transfer pumps and five mixers in the system so that the operator is aware of what equipment is operating throughout the system at all times. Alarm conditions that signal during preset conditions include; high and low levels for transfer tanks, chemical day tanks, and holding tanks, low and high pH, and low and high ORP alarms for all units, as well as motor overload trip alarms.

METALS TREATMENT PROCESS OPERATIONS

EQUALIZATION
The Equalization tank (EQ) collects all dilute acid rinsewaters, fume scrubber bleeds, and other rinses that are pumped from the production process areas. This equalization tank is equipped with two ¾ horsepower centrifugal pumps (CP-1 & CP-2) and an ultrasonic level transducer to transmit (control) the tank level. A visual flow meter is provided on the discharge pipeline of the pumps for the purpose of adjusting the proper flow through the Chrome Reduction Reactor vessel to allow for designed reaction residence times. This will be set at approximately 10 GPM (gallons per minute) or to whatever the process conditions dictate. A strong acid dump tank is used to collect strong acids and then transfer them with a metering pump to the equalization tank. This metering pump is interlocked with CP-1 and CP-2 so that the acid concentrates are only added to the equalization tank when the CP-1 and CP-2 pumps are running. This commingling of waste streams dilutes the acid concentrate dump so that the stream entering the first stage chrome reaction tank are of a consistent chemistry.

HEXAVALENT CHROME REDUCTION
The first stage Chrome Reduction Reactor Vessel (Cr I) is equipped with a mixer, pH and ORP controllers and sensors. The influent pH range from the equalization vessel is approximately 1.5 to 2.5 units. There is a sulfuric acid pump controlled automatically through the pH control unit and is turned on if the pH is above the 2.5 pH units. The reduction of hexavalent chrome to trivalent chrome (Cr+6 ==> Cr+3 ) is accomplished by the use of sodium metabisulfite (a reducing chemical) which creates a reduction reaction in the controlled pH environment. The ORP is controlled at a range of 250 to 300 mV. The flow from this reactor then flows to the next Chrome Reduction Reactor (CR II) for a continued reduction reaction. This second stage chrome reduction vessel is to ensure that any hexavalent chrome that escaped treatment in CR I is reduced to trivalent chrome. The CR II reactor is equipped with a mixer, pH and ORP controllers and sensors. The pH is maintained at 2.2 to 3.0 units via metering pumps which add sodium hydroxide or sulfuric acid as controlled by the low and high set points of the pH controller. The ORP is maintained at a range of 250 mV to 300 mV as in CR I. At this point, the hexavalent chrome reduction reactions are complete and the wastewater containing trivalent chrome and other metals [such as nickel (Ni+2) and copper (Cu+2)] flow by gravity to the pH I Reactor.

pH NEUTRALIZATION
The pH I Reactor is equipped with a mixer, pH control and sensor unit. The pH is maintained at 7. 5 to 9.0 units via metering pumps, which add sodium hydroxide or dilute sulfuric acid, controlled by the low and high set points of the pH controller. The flow continues on to the pH II Reactor that has the same equipment as the first stage pH reactor. The pH is maintained between 8.5 and 9.5 units.

METAL HYDROXIDE PRECIPITATION
A coagulant chemical, containing calcium chloride and other proprietary ingredients, is fed into the inlet pipe of the pH II Reactor. This chemical helps the precipitating metal hydroxide flocculate particles created by pH neutralization. These particles become larger by coagulating and more amenable to building a larger flocculation particle with the addition of an anionic polymer. The anionic polymer solution (polyelectrolytes) is fed automatically to this tank by the polymer feed pump that is interlocked with the equalization vessel feed pumps, CP-1 and CP-2. Therefore, the polymer is only added when there is flow from the equalization pumps. The polymer is added after the coagulant addition to enhance metal hydroxide flocculent particle size. The larger flocculent will speed the settling time in the clarifier. The flocculated wastewater is then gravity fed to the Lanco Gravity Inclined Plate Settler unit.

CLARIFICATION
The Lanco Gravity Plate Settler unit has 100 ft2 of settling plate area. This will allow the settler to operate at 0.20 gpm / ft2, at the system flow rate of 20 gpm. The metal hydroxide flocculated wastewater flow enters the Clarifier by gravity at the top. The flow then travels to the first baffled section that contains a high-speed mixer for a flash mixing to build a better floc, if needed. The flow continues to the flocculation compartment where there is an air mixer that will be operated at a very low speed so that the floc particles do not break up. This slow mixing action further enhances the flocculent particles to collide and become larger, thus heavier so they settle faster. The flow is then forced, or directed by design, down through the center of the Clarifier and up through a slant plate section. This design allows the metal hydroxide flocculent particles to settle to the cone bottom hopper of the plate settler, creating a sludge bed. The liquid flow is directed upwards through the slant plates settling section and out over a weir section at the top of the Clarifier that is separated from the inlet of the unit. The flow then cascades over the end of the weir to the discharge section. The clarified effluent, with very low suspended solids, flows by gravity out the discharge port of the Plate Settler to the Final pH Adjust Tank.

METAL HYDROXIDE SLUDGE DEWATERING
The sludge layer that accumulates in the cone bottom of the Plate Settler is drawn off periodically by an air diaphragm pump to the Sludge Thickener Tank. This operation is performed automatically by a duration timer in the Panelview Timer Section that is adjusted accordingly to system loading conditions. The sludge withdrawal from the Clarifier can also be done manually through operation on the Panelview. The dewatering operation commences with the draw off, or decanting, of the clear layer of liquid on top of the settled sludge in the thickener tank using valves located on the sidewall. From this tank, the sludge containing 2-4% solids is pumped to a filter press as needed to remove a large percentage of the water in the sludge, producing a sludge cake. The filtrate and decant water from the press and the sludge thickener collect in the Decant Lift Station. This lift station is equipped with a sump pump and pumped to the equalization tank. The sludge cake from the filter press usually contains 40 - 50% solids and is disposed of at an EPA permitted off site TSD Facility.

FINAL pH NEUTRALIZATION
The effluent from the Clarifier/Plate Settler flows to the Final pH Adjust Feed Tank. The Final pH Tank is equipped with a mixer and pH control unit with a sensor. The pH is maintained between 7.0 and 10.0 units through the addition of sodium hydroxide or dilute sulfuric acid. This is automatically controlled from the pH unit within the control panel. The effluent flows by gravity to the Sewer Lift Station. This tank is equipped with two 1.0 HP centrifugal pumps (CP-3 and CP-4) and a pressure level transducer to control the tank level. A visual flow meter is provided on the discharge pipeline of the pumps for the purpose of adjusting the proper flow to the sewer. The only other flow that enters this transfer tank is the permeate of an Ultrafiltration Treatment System. It will not need pH control.

FINAL CARTRIDGE FILTRATION
There is a cartridge filter unit that holds four 10" poly-wound filters. The micron size used can vary from 10 to 50 microns depending on system effluent conditions. There are two automatic valves located before this filter unit for the purpose of directing the flow of the effluent from the pumps. One automatic valve directs the flow through the filter and then to the sewer. The other automatic valve directs the flow back to the equalization tank. This second valve is necessary to avoid any out of spec pH conditions (high or low) that may occur. The valves are controlled by the high and low alarms set at the Final pH Meter in the control panel. The low alarm set point is set at 6.8 units and the high alarm set point is set at 10.2 units. Any out of spec condition will open the valve to equalization tank and close the valve to the sewer and then sound an alarm. When the condition is satisfied both valves switch automatically to their normal operating positions.

FLOW MEASUREMENT & AUTOMATIC SAMPLING
The flow that is pumped through the filter unit to the sewer encounters a paddle wheel type flow sensor. The paddle wheel, when moved, sends a signal to the final flow computer in the panel. The flow transmitter processes the signal from the paddle wheel and displays a digital readout on the display screen. Through the display screen the instantaneous flow rate or flow total can be viewed. The flow microprocessor also transmits a 4-20 mA output signal. The 4-20 mA output signal is sent to a Honeywell 10" Circular Chart Recorder Unit for the purpose of keeping flow records. In the same outlet pipe, after the flow sensor, is an in-line pH sensor that measures the actual final pH discharge to the sewer. This transmitter is locally mounted on the wall near the sensor. This unit has a 4-20 mA output for the purpose of recording the final pH. This pH signal is sent to another Honeywell 10" Circular Chart Recorder. Records are kept on 7-day chart paper. There is an ISCO Auto Sampler Unit that is used to collect composite samples of the effluent for analysis. This will assure that the effluent complies with permit regulations. This unit operates under a flow composite pulse signal from the flow computer from the ISCO Programmer.

CONCLUSION
Through the use of conventional treatment the wastewater from an electroplating finishing industry was neutralized and disposed of in a viable and economic fashion. The wastewaters that contained hexavalent chrome was reduced to trivalent chrome and then precipitated out of solution. Other metals such as nickel and copper were also precipitated out of the waste stream. By using state of the art PLC logic and control equipment ensures that the effluent will be continually discharged into sewer at a "clean" and consistent chemistry.