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 of the plant. The headworks performs five primary functions:
- flow measurement
- chemical addition
- grit removal
Some refer to the headworks as a pretreatment facility because no biological treatment occurs.
Large objects such as cans and pieces of lumber are taken out of the flow stream by automatic bar screens, to avoid the problems caused if allowed to continue into pumps and piping.
Grit is defined as heavy inorganic matter such as sand and gravel that doesn’t require biological treatment. With the potential to damage pumps and piping grit needs to be removed as early into the process as possible because of its abrasiveness. Also if grit is not removed it accumulates in the digester tanks, reducing the storage capacity and effective treatment of the organic waste.
The District’s headworks has two aerated grit removal basins approximately 13 feet deep filled with liquid. Air is introduced near the bottom of the basin 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 the coarser and heavier inorganic solids (grit) to settle. As grit settles, it is removed by pumping, dewatered in a screw type conveyor called a grit classifier, and transported by conveyor to a dumpster which is taken to a landfill for final disposal.
Once larger materials and grit are removed from wastewater, industrial pumps force the water on to the next step.