Preliminary Treatment

Often the first step in wastewater treatment is coarse or fine screening to remove large materials (rags, logs and other large objects) or small material (food scraps) from the wastewater that could interfere with the treatment process. Racine has 4 automatic bar screens with one-half inch openings to perform the initial screening. Each bar screen has a washing press to reduce organic content, moisture content and volume.

The next step is to remove smaller objects (such as sand, broken glass, silt and pebbles). If these objects are not removed, they can damage pumps and other mechanical devices. These objects also have a tendency to settle in corners and bends, thus reducing flow capacity and eventually clogging pipes and channels. Racine employs the use of 2 vortex grit removal units each rated at 70 MGD to accomplish this task.

Grit Concentrators

Racine uses two grit concentrators to remove water and organics from the material coming from the vortex grit removal system.

Primary Clarifier

After screening the solids and removing the grit, the wastewater still contains light organic suspended solids. Some of these can be removed by gravity in a sedimentation tank. These tanks are either 8 ft or 10.5 feet deep and hold the wastewater for 2-5 hours, depending on the flow. What settles out is called sludge. The sludge is removed from the primary treatment tank with mechanical scrapers and pumps. Grease, oil, and other floating substances rise to the top, where they are removed by surface skimming equipment. Racine has 12 primary clarifiers.

Secondary Treatment

Secondary treatment removes soluble materials that require oxygen for decay, as well as further removal of suspended solids.

Many of the pollutants in wastewater, if not treated and dumped directly into streams, kill fish and other wildlife by depriving them of oxygen. These pollutants naturally rot, or decompose, on their own; but require oxygen to do so. These pollutants steal dissolved oxygen from the water as they decompose, and fish and other aquatic wildlife die from oxygen starvation.

The amount of these oxygen-stealing pollutants present in wastewater is measured in terms of biochemical oxygen demand (BOD, pronounced “bee-oh-dee”). BOD is the amount of oxygen, usually measured in milligrams of oxygen per liter of wastewater, required to decompose the organic pollutants found in the wastewater.

Because these organic pollutants decompose on their own in the presence of oxygen and common microorganisms, these pollutants can be decomposed by merely forcing oxygen into the wastewater and giving these pollutants time to decompose by natural processes. This is the procedure used in secondary treatment.

Aeration / Activated Sludge

Activated sludge is another method of providing secondary treatment to wastewater, whereby a mixture of wastewater and biological sludge (microorganisms) is agitated and aerated. The biological solids are then allowed to settle out.

The name “activated sludge” comes from the biological mass formed when oxygen (in the form of air) is continuously injected into the wastewater. In this process, microorganisms are thoroughly mixed with organics under conditions that stimulate their growth. As the microorganisms grow and are mixed by the agitation of the air, the individual microorganisms clump (or flocculate) together to form a mass of microbes called activated sludge. About eight cubic feet of air are required for every cubic foot of wastewater.

In the activated sludge process, wastewater flows continuously into an aeration tank where air is injected into the wastewater to mix the wastewater with the activated sludge, and also to provide the oxygen needed for the microorganisms to break down the organic pollutants. The mixture of wastewater and activated sludge is called mixed liquor. The mixed liquor flows to a secondary clarifier (settling tank) where the activated sludge settles out. Some (usually 40% to 50%) of the settled sludge is returned to the aeration tank (and hence is called return sludge) to maintain a high population of microbes to break down the organics.

Since more activated sludge is produced than is needed for return sludge, the excess sludge is removed and disposed of.

Final Clarifier

Regardless which method of secondary treatment is used, the end result is a mixture of microorganisms and partially treated wastewater that is essentially free of dissolved and suspended organic material. The organic material that was dissolved or suspended in the wastewater has now been broken down and consumed by these microorganisms. These microorganisms, while very small, are still large enough to settle out of the wastewater. Wastewater leaving trickling filters and activated sludge aeration tanks is then sent to secondary clarifiers (settling tanks) where the microorganisms settle out. At this point, the wastewater treatment process is nearly completed.

UV Disinfection

Disinfection is considered to be the primary mechanism for the inactivation/destruction of pathogenic organisms to prevent the spread of waterborne diseases to downstream users and the environment.

It is important that wastewater be adequately treated prior to disinfection in order for any disinfectant to be effective.

An Ultraviolet (UV) disinfection system transfers electromagnetic energy from a mercury arc lamp to an organism’s genetic material (DNA and RNA). When UV radiation penetrates the cell wall of an organism, it destroys the cell’s ability to reproduce. UV radiation, generated by an electrical discharge through mercury vapor, penetrates the genetic material of microorganisms and retards their ability to reproduce.

The effectiveness of a UV disinfection system depends on the characteristics of the wastewater, the intensity of UV radiation, the amount of time the microorganisms are exposed to the radiation, and the reactor configuration.

Wastewater Treatment Process

There are four basic stages to cleaning wastewater at the Racine Wastewater Treatment Plant:

In the first stage, wastewater enters preliminary treatment where screens and grates remove large objects, sand, gravel and other grit. These materials can collect in plant tanks reducing capacity, and are destructive to plant pumps and equipment.

In step two, primary treatment, the wastewater enters large settling tanks where we remove grease and oil after it floats to the top. Heavier material sinks to the bottom of the tank and is removed.

In secondary treatment, microscopic organisms, or “bugs”, breakdown the majority of organic material that remains in the wastewater. Keeping organic material out of rivers and lakes is important because it can consume large amounts of oxygen that fish and plants need to live.

Finally, the water goes through disinfection. It’s here where radiation destroys the cell’s ability to reproduce. The treated wastewater is then discharged to Lake Michigan and measured against permitted limits established by the DNR and EPA.