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City of Newton - Utilities (Water Pollution Control & WaterWorks)

Posted on: June 20, 2023

What You Should Know About Water Pollution Control Preliminary Treatment


In our wastewater treatment plant, a critical step called preliminary treatment plays a vital role. It is located at the beginning of the treatment process and aims to protect the plant's overall operations by removing large materials from the wastewater. These materials, such as cans, bottles, scrap metal, sticks, rocks, bricks, plastic toys, lids, toothpaste tube caps, razor blades, needles, towels, rags, and sand, are commonly found in the initial wastewater that enters the plant.

These materials can cause various issues. Pieces of metal, rocks, and similar items have the potential to block pipes, damage or clog pumps, disrupt the mechanisms that collect sludge in settling tanks called clarifiers or interfere with other processes in wastewater treatment. Sand, eggshells, and similar materials, called grit, can clog pipes, lead to excessive wear in pumps, and occupy valuable space in sludge digesters and aeration tanks.

The preliminary treatment facilities are crucial for the effective and efficient performance of subsequent treatment processes and ultimately determine the quality of the treated wastewater. Some typical processes involved in preliminary treatment include flow measurement, screening, grit removal, pre-aeration, and emergency storage.

  • Flow measurement is essential for evaluating the plant's efficiency, monitoring the loadings, and maintaining records as required by the Iowa Department of Natural Resources.
  • Screening is a part of the preliminary treatment that focuses on removing larger debris like rocks, cans, bottles, and rags. This process involves passing the wastewater through parallel bars or slotted drums.
  • Grit removal eliminates heavy inorganic wastes such as sand, gravel, coffee grounds, and rice. Grit is separated by reducing the velocity of the wastewater, causing the heavier particles to settle at the bottom of specialized channels or hoppers.
  • Pre-aeration involves adding air to the wastewater, which helps separate oils, fats, and greases.
  • Emergency storage is necessary in two situations. If the incoming flow of wastewater becomes too excessive for the plant's capacity, or if the influent contains toxic waste that cannot be treated, the excess flow or toxic waste must be redirected to emergency storage.

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