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What Is a Polishing Pond in a Wastewater Facility?

Wastewater treatment facilities are designed to process industrial and domestic inflows biologically. There are various treatment stages that wastewater undergoes before it is discharged into a river or drinking water system. Polishing ponds are in the final stages of water treatment and typically produce water that is suitable to discharge into natural waters.

  1. Objective

    • Polishing ponds are designed to increase the environmental compatibility and quality of effluents from preceding treatments. They act as the final stage in meeting legal and environmental regulatory standards set forth by local and federal governing bodies. Their primary purpose is to increase hygienic quality of the water before it is recharged into public waters.

    Parameters

    • The "lithmus test" to gauge whether the water is suitable to leave the polishing pond is determined by measuring the concentration of helminth eggs propagated by parasitic worms commonly found in feces. However, more significantly, fecal coliforms (FC) are measured as well. Fecal coliforms come from human and animal defecation and take longer to remove than helminth eggs. By virtue of this trait, FC removal is the measuring stick with which the efficacy of a polishing pond is gauged.

    Detention

    • Polishing ponds typically hold water for much shorter periods of time as compared to stabilization ponds or those ponds designed to remove larger suspended solids and organic material that must settle to the bottom. Detention time is typically one to three days.

    Pond Design

    • Depths are generally greater in polishing ponds at 5 to 10 feet deep, compared to stabilization ponds that normally range from 3 to 5 feet deep. The greater depths decrease the ability for algae to grow and subsequently be released into natural waterways. Algae reproduce at high rates and can degrade water quality by forming large mats and shading out native aquatic plants, preventing photosynthesis.

    Fish

    • Grass carp

      Polishing ponds are sometimes stocked with fish such as grass carp to help reduce the amount of organic material being discharged. Grass carp eat submersed weeds and, if no other food options exist, will eat algae.

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