Overview Archimedes Screw Pump References


The Headworks at the Arcata Wastewater Treatment Plant is the first step in the treatment of Arcata's wastewater. Wastewater is routed to the Headworks via Arcata's sewer system. The Headworks remove large solids and grit from the wastewater before it reaches the next step of treatment. The next step in treatment is the primary clarifier.The main structure of the Headworks can be seen in Figure 1.

Figure 1: Main structure of the Arcata Wastewater Treatment Plant headworks. (photo courtesy of Dr. Dustin Poppendieck)

Incoming Wastewater

Wastewater entering the treatment plant contains a number of chemicals and suspended solids . The headworks primary purpose is to remove the large and easily trapped solids. Other contaminants are removed at various other steps of the treatment process.

Typical solids collected in the headworks include:

  • Paper products
  • Logs, sticks, and other large pieces of wood
  • Condoms
  • Clothing
  • Sand, silt, glass and other dense solids
  • Moving the Wastewater

    Wastewater routed to the Arcata Wastewater Treatment Plant reaches the Headworks at a sump. The sump is a large concrete tank that maintains a specified wastewater level using a copper float valve. From here two Archimedes screw pumps move the wastewater vertically so that the wastewater may flow through the bar racks by gravity and into the grit chamber (Figure 2).

    Figure 2: Wastewater enters the Headworks at a sump below these two screw pumps.(photo courtesy of Dr. Dustin Poppendieck)

    Removing the Solids

    The main purpose of removing large solids and grit is to protect the pumps and pipes from damage and excessive wear. The Arcata Wastewater Treatment Plant uses two different mechanisms to remove these pump damaging solids.

    Bar Racks

    Bar racks are the first solid removal device that wastewater entering Arcata's Wastewater Treatment Plant encounters. Bar racks are a series closely spaced bars that trap large solids but allow wastewater to flow through. Typically these bars are spaced O.25 to 1.5 inches apart for mechanically cleaned bar racks. To prevent these racks from becoming clogged they must be cleaned periodically. The Arcata Wastewater Treatment Plant uses a mechanical device (Figure 3) that moves up the racks collecting the debris. The debris is passed through a chopping mechanism which breaks the solids into smaller pieces and washes the organic material from the debris. The cleaned and chopped debris are then deposited into a waste dumpster and transported to a landfill (Couch 2008). Some of this collected debris is shown in Figure 4.

    Figure 3: Bar racks and mechanical cleaning mechanism moving up the bar racks. (photo courtesy of Dr. Dustin Poppendieck)

    Figure 4: Mechanical cleaning mechanism and common debris. (photo courtesy of Dr. Dustin Poppendieck)

    Grit Chamber

    After the water has passed through the bar racks the wastewater enters the grit chamber. At this step of treatment the smaller, heavier solids (sand sized) are allowed to settle out of the waste water. The Arcata WWTP uses a horizontal flow grit chamber (Couch 2008). In this type of grit chamber water flows through a horizontal tank at a controlled speed that allows small heavy solids to settle to the bottom of the tank while allowing smaller solids to remain suspended (Davis and Masten 2004). Once the grit has been removed from the wastewater it is washed using a hydrocyclone. This technology spins the grit in a chamber which separates the grit from the organic material. Once the small dense solids are removed the wastewater is then pumped to the primary clarifier.