SCCMUA PLANT PROCESS

In this section the following topics will be covered:

  • Definition of Wastewater
  • What is Wastewater Treatment
  • Where does wastewater come from
  • How do treatment plants protect our water
  • How does our treatment plant work   
  • Influent wastewater pumping
  • Primary treatment
  • Secondary treatment
  • Tertiary treatment
  • Sludge disposal

WASTEWATER

Many of us don't give much thought to the water that flows down the drain; out of sight, out of mind.  But what happens to that water is vital to our health and to the environment.

Wastewater treatment purifies our water, removing harmful bacteria, and returning clean water to our lakes, rivers and oceans.  SCCMUA plays an important role in wastewater treatment in the Southern Clinton County area.  Spend some time learning about how wastewater treatment works and what SCCMUA is doing to protect our communities and preserve the environment.

What is Wastewater Treatment?

Wastewater treatment is the process of cleaning used water and sewage so it can be returned safely to our environment.  Wastewater treatment is the last line of defense against water pollution.  If you envision the water cycle as a whole, you can see that the clean water produced by wastewater treatment is returned to our lakes and rivers, from which we get our drinking water.

Where does wastewater come from?

Wastewater is generated by homes, industry, schools, and businesses.  On average, each person in the U.S. contributes 50 to 100 gallons of wastewater everyday.   Wastewater can also be made up of water that comes from storms, through homes with sump systems incorrectly hooked up to sanitary lines, as well as groundwater that enters through cracks in the sewers.

How do treatment plants protect our water?

A wastewater treatment plant uses physical, biological, and chemical means to remove pollutants from water.  It settles solids in large tanks, reduces organic matter and pollutants using helpful bacteria and other microorganisms which consume pollutants in wastewater and are then separated from the water.   Chemicals are used to settle other pollutants.  Treatment plants then return oxygen to the water to ensure that it can support life in the oceans, lakes, and rivers it is returned to.  The settled solids are treated and recycled as nutrients on farmland.

How does our wastewater treatment plant work?

Wastewater treatment is accomplished in two main phases, primary and secondary.  Our plant is designed to treat an average flow of 5.0 million gallons per day, with the capability to treat as much as 10.0 million gallons for a short period of time, (24 hours).  The plant was expected to serve the growing needs of Bath, DeWitt, & Watertown Twp as well as the City of DeWitt for an estimated 20 years.  Slower than expected growth in the 1980's has extended that time frame past the year 2000.

Influent Wastewater Pumping: Wastewater enters the plant at the north end (Herbison Rd side) of the site.  Wastewater from the City of DeWitt and Watertown Twp enters the wet well of the Sandhill pumping station from two separate gravity sewer lines.  Flow meters on each line in the metering chamber record the wastewater discharged from each community.  The wastewater from Bath & DeWitt Twp enters the wet well in a common gravity sewer line.  This combined flow is metered in the metering chamber, while the Bath Township flow is metered at a remote pump station site so that the wastewater flow from these two communities can be calculated.  Prior to entering the lift station wet well, the wastewater from Bath and DeWitt Twp can be diverted into the equalization basins.  Control valves located in the metering chamber are operated remotely from the computer control system located in the main building.  The equalization basins are tanks provided to store wastewater during high flow periods and increase flows through the plant, during low flow periods.  Moderating peak flows is important in an advanced waste treatment plant, it prevents disruption of the biological secondary process, and overloading of the tertiary filters.  The Sandhill pump station is provided with two (2) 6000 gallon per minute pumps operated by a variable speed drive controlled remotely by computer.  There is also a third two speed pump as an emergency standby.  The pumps transport the raw wastewater to the primary treatment portion of the plant in the southwest corner of the main building.

Primary Treatment: The total raw wastewater flow is metered and sampled for analysis prior to entering the primary treatment processes.  This provides the operator with information on the characteristics of the influent wastewater and the metering data is used to determine the pollutant loading and the amount of chemical additives required in later processes.  The wastewater first passes through coarse and fine bar screens which remove the rags and debris from the wastewater.   Next the wastewater enters an aerated grit chamber where the heavy non-organic substances such as sand, stones, etc., and heavy organic solids (such as garbage disposal waste) are settled out of the wastewater.  This material is pumped out of the tank by an air lift pump, cleaned in a grit washer, and disposed of with the rags and debris in a landfill. The wastewater flows out of the grit tank into a channel which carries the wastewater to the primary clarifier tanks.  Here the flow is split evenly between the two tanks if necessary, (current flow volume requires one clarifier for good treatment).  The primary clarifiers are settling tanks and the suspended organic solid material in the waste settles to the bottom of the tank.  These solids are referred to as sludge.  A mechanical collection system on the bottom of the tanks moves the sludge to the collection hopper in the center of the tank.  An upper sweep arm skims grease from the surface and deposits it in a scum hopper.  The sludge and grease is pumped into the anaerobic digesters.  The treated wastewater is sampled and analyzed again after the primary stage to calculate treatment efficiency and improve operational control.

Secondary treatment: Large amounts of polluting substances still remain in the wastewater after treatment in the primary clarifier's.  This light suspended and dissolved material is removed by biological treatment of the wastewater.   Forty-two rotating biological contactors (RBC's) - six trains of seven RBC's - are utilized in the biological process, (at current flow levels only four tanks are necessary).  A population of micro-organisms is grown on the surface of the RBC's   These micro-organisms utilize the dissolved and suspended organic pollutants in the wastewater as a food source.  The first section of RBC's in each train removes Biochemical Oxygen Demand (BOD), substances which require oxygen to become stabilized.  The second section removes ammonia nitrogen, a plant nutrient that is toxic to fish.  The RBC's are rotated in the wastewater by air released under each RBC.  The air accumulates in air cups mounted on the surface of each RBC and the buoyancy of the air rotates the biological contactor.  The micro-organisms receive oxygen as they pass through the air and absorb the pollutants while passing through the wastewater in the tank.  The air driving the RBC's is provided by three 125 horsepower blowers.  In the course of their natural life cycle the micro-organisms slough off the RBC's and form clumps of organic matter called floc.  The floc is carried in suspension through channels and pipes to the next stage of the secondary system, the secondary clarifier's.  While flowing through the channels and pipes to the secondary clarifier's a chemical, ferric chloride is added to assist in settling and removing phosphorous from the wastewater.   The wastewater flow is split between the final clarifier's, (only one is needed at present).  Organic and phosphorus solids are settled out.  The solids are mostly the micro-organisms washed out of the RBC tanks.  The sludge from the final clarifier's is pumped to the primary clarifier's for co-settling and treatment with the primary sludge.  The secondary clarifier effluent is sampled again for analysis as it flows to the inlet pipes of the tertiary filters.

Tertiary (Advanced) Treatment:  In the tertiary filters the flow cascades out of the inlet pipes and is distributed over the surface of the filter beds.   The wastewater is filtered through a mixed bed of anthracite coal, sand, and gravel to remove the remaining pollutants.  When the filter media becomes dirty or plugged with pollutant material, the filter is backwashed, (cleaned by reversing the flow through the filter with 5,000 gallon a minute backwash pumps using chlorinated effluent water).   The backwash water along with all building drains flows by gravity to the mud well tanks.  The mud well tanks are pumped to the equalization system for treatment.   After being sampled and analyzed again the clean filtered water flows to the chlorine contact basins where it is disinfected utilizing chlorine gas.  The treated water enters the reaeration basin where if necessary a mechanical aerator is used to raise the dissolved oxygen concentration in the water to meet effluent water quality requirements.  After disinfection sulfur dioxide is added to dechlorinate (chlorine is toxic to fish).  The effluent is sampled and analyzed one last time at this point for several parameters to ensure that the river is properly protected.  The treatment of the liquid portion of the wastewater is then complete and it enters the Looking Glass River through a 24 inch outfall pipe.

Sludge Disposal:  The sludge from both the primary and final clarifier's is anaerobically digested.  The anaerobic digesters reduce organic solids using anaerobic bacteria, producing primarily, methane, carbon dioxide, and water.   The digested sludge is pumped to the sludge/biosolids storage tank.   Periodically the biosolids are hauled and injected by specially designed implements into local cropland for recycling as fertilizer.  The land application of biosolids is regulated by federal and state law, which requires rigorous testing of many parameters to insure the quality of the biosolids.  As an alternative, sludge is pumped to the main building for mechanical dewatering (drying).  Two belt filter presses literally squeeze the excess water out of the sludge.  The "dried" sludge is then hauled to a landfill for final disposal.

Flow Schematic

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