Treatment Plant / Headworks
 
Raw wastewater delivered from the collection system to the treatment plant is first received at the headworks facility. It is called this because it is at the head
  
 
              Headworks Building
of the plant. At the headworks five  primary functions are performed: sampling, flow measurement, chemical addition, screening, and grit removal. Some refer to the headworks as a pretreatment facility because no biological treatment occurs. Its purpose is to condition the wastewater for treatment through the processes of screening and grit removal. 
Large objects such as cans and pieces of lumber are taken out of the flow stream by automatic bar screens. Such objects would cause problems if allowed to continue into pumps and piping.



        Headworks Step Screens
Grit is defined as heavy inorganic matter such as sand and gravel. Because it is inorganic, it doesn’t require biological treatment. It is also potentially damaging to pumps and piping because of its abrasiveness. If grit is not removed it begins to accumulate in the digester tanks, thereby reducing their effective treatment and storage capacity for organic waste.

The District’s headworks has two grit removal basins. The liquid depth of these basins is approximately 13 feet. They are referred to as aerated grit basins because air is used in the grit removal process. Air is introduced near the bottom of the liquid depth through diffusers located to one side of the basin. As the air bubbles rise on the one side of the basin they induce a horizontal rolling motion to the liquid contained in the basin. The liquid travels upward near one wall of the basin, across the top towards the other side, and downward near the opposing wall. The volume of air used is regulated to achieve just the right amount of agitation and rolling motion. The goal is to keep the finer and lighter organic solids in suspension so that they pass through to further treatment and to allow the coarser and heavier inorganic solids (grit) to settle. As grit settles and accumulates, it is removed by pumping, dewatered in a screw type conveyor called a grit classifier, and transported by conveyor to a dumpster. The grit is taken to a landfill for final disposal.


© North Davis Sewer District 2006