To Automate Where no Plant has Automated Before: Acid Washing of Packed-Tower Chemical Scrubbers

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ABSTRACT
The Orange County Sanitation District (Sanitation District), under a capital improvement project, developed an automated procedure to perform acid washing on packed tower chemical scrubbers. Packed-tower chemical scrubbers operate with an alkaline solution, composed of sodium hydroxide, an oxidant, and make-up water. The make-up water may be chlorinated secondary treated wastewater or city water, which are both hard water sources and contribute to scale build up on the random pack media in the chemical scrubbers. In addition, the use of sodium hydroxide contributes to scaling on the packed media. Acid washing is necessary to maintain optimum performance within the chemical scrubber.

INTRODUCTION
The Orange County Sanitation District (Sanitation District) operates two wastewater treatment plants with a total flow of approximately 250 million gallons per day (MGD). Between both plants, there are currently a total of 29 operating scrubbers capable of treating over 850,000 cubic feet per minute (cfm) of foul air from plant treatment processes and the sewer trunk lines. Packed-tower chemical scrubbers operate with an alkaline solution, composed of sodium hydroxide, an oxidant, and make-up water. The make-up water may be chlorinated secondary treated wastewater or city water, which are both hard water sources and contribute to scale build up on the random pack media in the chemical scrubbers.

Due to aging systems and the need to capture and treat more foul air, the Sanitation District initiated several capital improvement projects to demolish existing scrubbers and construct new odor treatment facilities. For new facilities, it is the Sanitation District’s goal to incorporate automation when possible.

During the design of the new chemical scrubber systems, the Sanitation District decided to automate scrubber acid washing into future projects. This was in line with the agency’s longterm vision, and would make it easier for Operators to perform cleaning. The automated acid washing strategy has been designed into capital projects, which are currently in the design and construction phases. Future capital improvement projects for chemical scrubbers will also use this automated procedure.

Regular acid washing of chemical scrubbers is necessary to maintain clean media in the chemical scrubbers. The media is generally composed of small pieces of plastic that allows airflow through it and has a large surface area to maximize the liquid-air interface where the chemical reaction takes place. The reaction may produce precipitated solids. After operating at elevated pH levels, solids in the liquid will deposit on the plastic media. Over time this causes higher pressure drops, reduced airflow, and poor process control due to an additional source of reactants. Acid washing is necessary to remove the build-up on the media. Excessive build-up that can occur on scrubber media is shown in Figure 1. Excessive media scaling has occurred in the chemical scrubbers at the Sanitation District and more frequent acid washing likely would have prevented this.

If the solids become excessive and acid washing is no longer effective, then the media may need to be replaced to return the scrubber to normal operating conditions. Figure 2 shows media replacement at a Sanitation District packed-tower chemical scrubber where the media had become solidified due to scaling. Replacing the media requires taking the scrubber out of service and a confined-space entry. Excessive scaling on the media may necessitate power tools to separate pieces of the media and remove it from the tower. A media change out can be time consuming and expensive, so it is preferable to avoid this through more frequent acid cleanings.

In ideal situations, acid washing is performed regularly to dissolve and remove the build up. There are several factors that contribute to infrequent acid washing:

  • No back-up system to capture and treat foul air
  • Insufficient staff time to perform acid washing; and,
  • Other physical constraints that make it inconvenient to perform acid washing

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