The Wastewater Treatment Process

Wastewater Flow Diagram
The enclosed flow schematic details both the wastewater process flow and solids flow through the plant. Raw wastewater comes into the plant via 3 different pipes, combining into a single 39 inch diameter pipe that flows into the headworks building where preliminary treatment occurs. From there, water flows to primary settling tanks, trickling filters, aeration tanks, secondary settling tanks, through disc filters and ultraviolet disinfection, and then out a 24 inch pipe to the west end of the Sauk River.

The treatment process produces waste solids at several points in the plant. Grit solids are collected at the headworks building and are disposed of in a landfill. Sludge solids are collected from the bottom of the secondary settling tanks and "co-settled" in the primary settling tanks. The co-settled solids are treated in anaerobic environment in the four digesters and then stored in the million gallon sludge storage tank until they can be land applied as a fertilizer on farm land. The unit processes are further described below.

Preliminary Treatment

  1. Raw wastewater flows through an 18 inches wide Parshall flume for flow measurement, and then into the raw influent wet well at the headworks building. The plant influent flow is displayed locally and on the plant-wide control system monitor in the operations building.
  2. Wastewater in the influent wetwell is lifted and conveyed by a system of two 48-inch diameter screw pumps. The screw pumps provide the lift necessary for wastewater to flow through the primary treatment system
Wastewater Treatment
  1. After the sewage is lifted, it passes through a grinder mechanism. If the grinder needs maintenance, the flow can be by-passed in a separate channel.
  2. The last process in the headworks building is grit removal. The vortex type grit removal system is comprised of a 12 feet diameter grit chamber with a water scour system for conveying grit from the waste stream to a grit classifier located in the headworks building. The grit classifier discharges grit to a dumpster in the lower level.

Primary Treatment

  1. Wastewater flows from the headworks building to five 90 feet long primary settling tanks. The tanks reduce the velocity of the wastewater so that solids can settle out. The waste solids that settle on the bottom of each tank are scraped into sludge hoppers and then pumped to the anaerobic digesters automatically. The process wastewater (primary effluent, or P.E. as shown on the schematic) flows over weirs at the downstream end of each tank and into a wetwell on the north side of the operations building.
  2. The plant has three P.E. pumps that lift the wastewater to the top of the trickling filters, where it flows by gravity through the secondary treatment system. Of the 3 pumps, 2 have variable frequency drives and the whole pumping system is computer controlled through a level measurement system located in the wetwell.
Settling Tanks

Secondary Treatment

  1. The secondary system has 3 major components; trickling filters, aeration tanks, and secondary settling tanks. The formal name of the process is the "Trickling Filter Solids Contract" process and Coldwater was the first plant in Michigan to use it. The process provides both a "fixed film" media in the 3 trickling filters and a "mixed liquor" in the 2 aeration tanks. Both processes require oxygen to complete the biological reactions, which is provided through natural ventilation in the trickling filters and by large blowers of the mixed liquor in the aeration tanks.
  2. The 2 secondary settling tanks located downstream of the aeration tanks provide for settling of the biological solids. Most of these biological solids are recycled to the aeration tanks (RAS) where they provide the micro-organisms that consume the soluble nutrients in the primary effluent. The 2 tanks are "center-feed" and 65 feet in diameter. A polymer may be added in the center-feed well to enhance settling. Settled effluent passes over the perimeter weirs and flows to the filter influent wetwell.
  3. Another step in the secondary treatment process is the addition of a chemical that "binds" with phosphorus, which is a nutrient that the plant is mandated to remove to a very low level. The Coldwater facility currently uses ferric chloride for phosphorus removal. The phosphorus compound that is created settles in the secondary settling tanks along with the biological solids.
  4. Secondary effluent (S.E.) is pumped to the top of the disc filters where it flows by gravity through the filtration process, ultraviolet disinfection, and out to the Sauk River. The pumping system consists of three computer-controlled pumps.
Aeration Tanks
Secondary Settling Tank
Tanks in the Treatment Plant

Advanced Wastewater Treatment & Disinfection

  1. The plant has 3 disc filters, each of which can treat over 2 MGD. The filters have 2 separate automatic cleaning processes to keep the filter cloths clean. It is expected that the filter cloths will need to be replaced once every 5 years.
  1. Filtered wastewater flows into 1 of 2 open channels where ultraviolet (UV) light is used to disinfect the wastewater prior to discharge. Disinfection is achieved by light energy at a wavelength of 254 nano-meters (nm), which interferes with the genetic material of the bacteria, rendering them unable to reproduce.
  2. Prior to discharge, the disinfected final effluent flow is measure at a horizontal weir plate and displayed both locally and on the SCADA system.
Pipes at the Facility

Solids Handling

  1. Waste activated sludge (WAS) from the secondary settling tanks is pumped to the primary settling tanks, where it is co-settled with the primary sludge. Primary sludge, and the co-settled WAS sludge, is pumped on an automatic timed basis to one of the two primary digesters. The reason to control this process automatically is so the digesters are loaded more evenly through-out the day. Also, the operator does not need to be in attendance when this procedure takes place.
  2. The primary and co-settled WAS sludges are stabilized by the anaerobic digestion process. This process takes place at a warm 95 degrees Fahrenheit, and occurs over a period of time (minimum 15 days) so that volatile organic substances are converted to methane (CH4) and Carbon dioxide (CO2). The process takes place in an airtight reactor (digester) and the resulting stabilized solids have a greatly reduced pathogen content.
  3. Stabilized solids are pumped to the two secondary digesters and to the 1 MG sludge storage tank where additional low-rate stabilization occurs. The stabilized solids are land applied on a seasonal basis by an outside contractor.
Digester at the Plant
Land Applied Stabilized Solids


  1. The operation of the plant has been automated by the installation of locally mounted PLCs connected to a central computer with SCADA software. This software allows the operator to control plant operations from a computer in the laboratory or the Superintendent's office. The system also monitors alarms and is programmed to notify plant personnel in the event of an alarm condition.
Instrumentation at the Plant

Sampling & Laboratory Analysis

  1. The plant samples at four locations on a continuous basis. The sample points include: raw wastewater, trickling filter effluent, secondary effluent, and final effluent. Each sample point pumps a continuous sample to an automatic composite sampler for testing purposes. The Coldwater Board of Public Utilities maintains a state-certified water quality testing lab on the premises. The lab is used by various municipalities in the area, and maintains a high level of quality assurance and quality control as required by the State of Michigan.