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Automated Odor Control Systems A wastewater treatment facility uses pH, ORR flow, and level instruments along with PLC technology to control hydrogen sulfide odors.

Chris Waarvick Rudy Vigilia Ralph J. Titus

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ost odor-producing substances in wastewater applications re- M sult from anaerobic decomvo-

are drawn through a ventilation system for processing from the following odor sources located throughout the facility: tricking filters pumping station, trickling filters, influent building, screening build- ing, solids handling building, and dis- solved air flotation system. Average daily flow is approximately 21 million gallons per day (mgd) with an estimated peak flow of 36 mgd.

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Different methods of odor control were considered for the Yakima WWTP. Based on design criteria, a packed scrubber vessel arrangement was chosen using chemical absorption techniques. The designed system can operate in a completely automatic mode but also allows for operator in- terface in the field and at a local control panel. Using pH, ORP (oxidation re- duction potential), flow, and level in- struments in conjunction with PLC (programmable logic controller) tech- nology, all information can be moni- tored and controlled. The information can be used for current trend analysis and archived for future use by opera- tors and laboratory personnel. Perfor- mance tests conducted at the facility have indicated that 99.71% hydrogen

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Figure 2 . Process instrumen tation diagram (PZD) for the odor control system. Instrument Society of America (ZSA) notation is used as a guideline.

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IilNTECH Applying Technology

sulfide removal is occurring and the system is functioning as designed.

In 1985 a comprehensive plan pre- pared by HDR Engineering for the Yakima Regional WWTP recommended that covers be provided for existing rock media trickling filters due to increased use in meeting nitrification require- ments. Working in conjunction with the odor control system, these covers would act as containment vessels. All odors throughout the WWTP would be brought to these vessels and drawn out into a packed tower scrubbing system.

Past experience at the Yakima Re- gional WWTP indicated that influent wastewater typically did not become septic; therefore, hydrogen sulfide odors were not noticed at the primary clarifiers. If hydrogen sulfide were pre- sent in the wastewater, it would be stripped out as the wastewater flowed over the weir at the primary clarifier ef- fluent launder. It is estimated that the trickling filter odors are caused primar- ily by compounds such as mercaptans, organic acids, or amines. Hydrogen sul- fides are assumed to be produced in the existing DAF and solids hanging build- ings. Ammonia is not anticipated to be an odor problem because the high wastewater pH required to have am- monia stripped out typically does not occur.

Removing Odorous Compounds Activated carbon is used to remove

odorous compounds by means of ad- sorption. Impregnated activated carbon typically is used where hydrogen sul- fide is the predominant cause of odors, while nonimpregnated activated carbon is used for odorous organic compounds. Activated carbon-type scrubber systems require regeneration and eventual re- placement when the carbon reaches ex- haustion. These types of systems gen- erally are used on installations with small air flow rates.

Absorption via chemical treatment is the most common method of removing odorous compounds in large quantities. This consists of capturing odorous gases in a chemical solution which either oxi- dizes or neutralizes odorous com- pounds. Odorous air is pushed through a scrubbing v e s d from bottom to top, counter to a chemical solution flowing in from top to bottom.

With the exception of existing trick- ling filters, all areas in the Yakima Re- gional WWTP, including a screenings building, existing influent building, new trickling filter pumping station, ex- isting dissolved air flotation building, and existing solids handling building, would have odor control provided based on a ventilation rate of twelve air

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Figure 3. The large white horizontal tank is used for caustic storage. Recirculation of the caustic tank is accomplished by the closest white pump. The scrubber recirculation pumps are painted blue. Toward the rear wall behind the blue pumps, the caustic feed pumps are painted white.

changes per hour. The covered trick- ling filters would be designed to have a forced ventilation achieved by drawing air up from a vented subdrain system. Using an oxygen transfer efficiency of 3%, a ventilation rate of 22,000 cubic feet per minute (cfm) would be re- quired for each trickling filter. The total air flow requirement is approximately 48,000 cfm.

A Chemical Treatment-Based System

Taking into consideration design cri- teria, air flow requirements, and high annual operating costs associated with an activated carbon-type system, ab- sorption via chemical treatment was chosen. The system is based on chemi- cal treatment using chlorine and caustic (NaOH) solutions. Together, they have proven to be an effective method of odor control. Adding caustic makes chlorine remain in solution as hypochlorite ions, which prevents chlo- rine off-gassing and odors in dis- charged air.

To meet system requirements, the de- signed odor control system consisted of two scrubber vessels measuring nine feet in diameter by 22 feet high. A de- sign was established with a minimum removal rate of 99 with an inlet concen- tration of 15 parts per million (ppm) of hydrogen sulfide. Within each scrubber vessel, there will be ten feet of media material. Each scrubber vessel can scrub 24,000 cfm of odorous air (Fig. 1).

The odor control system actually con- sists of two separate systems that can be operated simultaneously, but inde- pendently of each other. All equipment and instruments are identified in Fig. 2.

A backup scrubber recirculation pump and a chemical feed pump are common to both control systems. In ad- dition, a caustic storage system sup- ports both systems (Figs. 2 and 3). Each odor control system includes an auto- matic scrubber level control. Depend- ing on the scrubber tank level, as mea- sured by each admittance probe, makeup water is automatically injected into the scrubber system. Flow through each scrubber and caustic recirculation system is monitored. System alarms monitor general performance criteria plus equipment failures. All analog and digital points in both systems are mon- itored through an Allen-Bradley pro- grammable controller located at a local control panel.

Each scrubber system can operate in an automatic or manual mode. In auto- matic, the PLC system takes all infor- mation, processes it, and makes control decisions based on current system pa- rameters. Manual operation provides the operator with the choice of control- ling individual system components from a local control panel or from the field. Individual pumps can be turned on or off, and valves opened or closed, independent of system requirements. System alarms still notify the operator of abnormalities. The PLC system al-

lows information management of all system parameters for current or future analysis. It is tied into an existing su- pervisory control and data acquisition (SCADA) system which allows plant personnel to monitor system perfor- mance from remote locations.

System Operation System operation dictates that odors

are brought back to each packed tower assembly from various sitewide sources. In this type of packed tower scrubbing system, the objective of the odor con- trol system is to transfer pollutants, in the form of a saturated gas, to a scrub- bing liquid. This is accomplished by as- suring a successful mix of pollutants and cleansing liquid in the vessel scrub- ber. Contact is achieved by the gas and cleansing liquid flowing in opposite di- rections through a symmetrically arranged packing medium.

Sodium hypochlorite (NaOClj’ solu- tion commonly is used in removal of

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the odorous hydrogen sulfide (H2S) compound in these scrubber vessels. To increase the solubility of sulfide, a caus- tic solution, usually sodium hydroxide (NaOH), is used to maintain typical pH ranges of 10.5 to 12. In this pH range, mass transfer of sulfide occurs from the gas phase to the liquid phase. Ushg pH and OW sensors to monitor NaOCl and NaOH compounds makes system odor control possible.

The pH and ORP sensors are the cen- tral features of the control system. The pH sensor monitors the amount of NaOH in the scrubber, while the ORP sensor monitors the amount of NaOC1. The sensors are encapsulated, with in- tegral two-wire transmitters.

These two sensors use a differential electrode technique for measurement, in which voltages are measured and transmitted by electronic amplifiers (Fig. 4). A voltage potential is formed by the process electrode (E,) and the ground electrode (E3). This voltage is a

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function of the solution, in this case NaOCl or NaOH. The other voltage is formed by a standard electrode (E2) and the ground electrode (E3). The standard electrode consists of a pH electrode in a chemical with a fixed pH of 7. The stan- dard electrode uses a double junction salt bridge to isolate the standard solu- tion from the process. The voltage of the process electrode potential differ- ence is subtracted from the standard electrode potential difference, and is ex- pressed as:

[El - E31 - [E2 - E31 = E, - E, The result is a differential measure-

ment between an electrode in the process compared with a standard elec- trode in a solution of pH 7. Tempera- ture compensation is achieved auto- matically, using a temperature-sensitive resistor. Moisture and high humidity problems are eliminated by completely encapsulating the high-impedance elec- tronics. The 4-20 mA differential mea- surement is output by an integral two- wire transmitter.

Each odor control system pH and OW sensor is connected to an integral transmitter which sends 4-20 mA sig- nals to individual controllers. Each con- troller, located at the local control panel, has built-in recording and digital indi- cation capability for operator conve- nience.

With the pH and ORP values known at each system scrubber, the controllers compare field signals with user ad- justable set points for each process vari- able. Each controller then sends a signal to increase or decrease the amount of NaOH or NaOCl chemical solution. For instance, if the pH is low, the respective system chemical feed pump will start, injecting NaOH into the system. ORP below its set point will result in addi- tional chlorine injected into the odor control system. Conversely, if pH is too high, the respective system chemical feed pump will stop, while a high ORP will result in the odor control system chlorine valve closing. IT

BEHIND THE BYLINE Chris Waarvick is the division manager for the City of Yakima Wastewater Facili- ties in Yakima, WA.

Rudy Vigilia is a project engineer with HDR Engineering, Inc., Bellevue, WA.

Ralph J. Titus is an electrical/controls project engineer with HDR Engineering.

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