What is wastewater?
Wastewater or sewage is the byproduct of many uses of water. There are the household uses such as showering, dishwashing, laundry and, of course, flushing the toilet. Additionally, industries and commercial enterprises use water for these and many other purposes, including processes, products, and cleaning or rinsing of parts. After the water has been used, it enters the wastewater stream, and it flows to the wastewater treatment plant.
Why treat wastewater?
Pollutants must be removed from the water to protect the environment and public health. When water is used by our society, the water becomes contaminated with pollutants. If left untreated, these pollutants would negatively affect our water and environment. For example, organic matter can cause oxygen depletion in lakes, rivers, and streams. This biological decomposition of organics could result in fish kills and/or foul odors. Nutrients in wastewater, such as phosphorus, can cause premature aging of our lakes, called Eutrophication. Waterborne diseases are also eliminated through proper wastewater treatment. Additionally, there are many pollutants that could exhibit toxic effects on aquatic life and the public.
How do we collect the wastewater?
Sewer collection systems are designed so that the wastewater flows to a centralized treatment location. Durand’s collection system is comprised of sewer pipes ranging in diameter of 8-inches to 24-inches. As the pipes get closer to the treatment facility the pipes become larger in diameter. Where gravity systems are not practical, pumping stations are used to lift the wastewater and pump it to a point where it will flow by gravity. Durand has four of these pumping (lift) stations.
Durand’s collection system consists of approximately 15 miles of sewer mains that collect wastewater from approximately 1,500 homes and businesses. After treatment the water is pumped approximately 3 miles to the Shiawassee River.
What is Inflow & Infiltration (I & I)?
Inflow is rain water that gets into a sewer from surface inlets, holes or leaks in manholes or manhole covers, sump pumps, or roof downspouts. This is relatively clean water that should be discharged to a storm water system. Some communities have “combined sewers” that intentionally receive inflow, instead of having separate sanitary and storm sewers. At one time Durand’s storm and sanitary systems were combined; they are now separate from one another. The last of the combined sewers were separated in the late 1970’s. The City of Durand implemented a sump pump and footing drain separation program in 2000. This program has eliminated approximately 90-95% of the ground water that at one time was pumped into the sanitary sewers.
Infiltration is groundwater that leaks into the sanitary sewer. All sewer pipes have leaking joints or cracks that allow the groundwater to enter the system to some extent. Infiltration is usually most severe in the spring when melting snow and rain saturate the ground.
What happens after collection of the wastewater?
The wastewater continues to flow through the collection system and eventually reaches the wastewater treatment plant. Upon reaching the plant, the flow first encounters preliminary treatment where larger items are removed. Preliminary Treatment is followed by Primary Treatment where smaller items that either float to the surface or settle to the bottom are removed. The wastewater then flows to Secondary Treatment where the water passes over a biological filter and chemicals are added to promote the finer particles to settle out. The solids or “sludge” removed from the wastewater stream also needs to be treated.
What is Preliminary Treatment?
Preliminary treatment processes are the first processes that the wastewater encounters as it enters the treatment process. This involves flow measurement so that the operator can quantify how much wastewater is being treated. Flow monitoring is followed by screenings removal. Screenings are string like materials, rags and large foreign objects like sticks or perhaps an errant golf ball. These materials need to be removed because they can damage machinery or clog processes. Screenings are removed using a bar screen. The next process in preliminary treatment is grit removal. Grit is comprised of inorganic material such as sand, gravel, eggshells, etc. These items are removed to prevent wear and abrasion on pumps and other mechanical equipment. Grit can also plug lines and pipes and does not respond to biological treatment. In this influent area, sampling equipment is used to collect small portions of the wastewater for analysis. Sampling enables the operator to determine the pollutant loadings entering the plant.
What is Primary Treatment?
Primary treatment is a physical settling and floatation process that removes solids. Wastewater that enters the primary settling tank (or clarifier) is slowed down to enable the heavier solids to settle to the bottom. Lighter materials, such as grease, will float to the top of the tank. The settling tanks are designed with mechanisms to remove both the settled solids, as well as the floating solids.
The sludge is removed and pumped to the solids treatment process for ultimate disposal. What’s left after we remove the pollutants that settle and float? The wastewater still has solids remaining after primary treatment. These solids are either dissolved or suspended. Dissolved solids are very small solids (e.g., dissolving sugar in water). You cannot see the solids but they are there. Suspended solids can be likened to the same ends of a magnet. The solids repel each other. These solids are small, but are visible to the human eye. We remove these dissolved and suspended solids through the next phase of treatment: Secondary Treatment.
What is Secondary Treatment?
Secondary treatment is a biological treatment process used to stabilize the dissolved solids.
Microorganisms (i.e., bacteria and protozoa) feed on the organic solids (food) in the wastewater and convert the organics into a cellular or biological mass that can later be removed. These biological processes are aerobic (need oxygen) processes. Durand uses two trickling filters or bio-towers to accomplish this phase of treatment. The wastewater is sprayed (or trickled) over a plastic media where microorganisms feed on the waste in the water.
Oxygen must be provided for these aerobic organisms to work properly and efficiently. A large amount of fresh air is blown through the media to keep the “bugs” happy and doing their job.
An integral part of secondary treatment processes is another set of settling tanks or clarifiers. These secondary clarifiers (final clarifiers) remove the biological mass that has grown during biological treatment. The resulting sludge is combined with the sludge collected in the primary treatment process.
The sludge is pumped to the solids treatment system for further processing.
All treatment plants in Michigan are required to remove phosphorus and many are required to reduce ammonia due to the possible negative impacts on the receiving stream. Durand is required to monitor and remove both. This advanced process is accomplished concurrent with secondary treatment.
Where do all the solids go?
Solids that settle out in the primary and secondary clarifiers are referred to as sludge. Sludge that has been processed to reduce disease-causing organisms is referred to as biosolids. Sludge is the byproduct of treating the liquid wastewater. Proper solids handling is of paramount importance. At the Durand wastewater treatment plant sludge is pumped to a gravity thickener, treated to reduce odors, and dewatered. The dewatered solids are treated using liquid lime for stabilization and stored in a 225,000 gallon tank. The stabilized sludge, now called biosolids, is injected into the ground on non-food crop fields as a fertilizer in the spring and fall of each year.
Where does the water go after treatment?
The treated wastewater is referred to as effluent. Durand’s effluent is discharged to the Shiawassee River. Before the effluent is discharged to the river, it undergoes a process to reduce disease-causing organisms called disinfection.
Chlorination is used to disinfect the water prior to discharge. Chlorination involves injecting chlorine into the effluent then holding it in a contact tank for 20 to 40 minutes. Chlorine can harmful to fish and other aquatic life. After the water has been held in contact with the chlorine, it is then dechlorinated before testing and discharge. Testing the effluent before discharge provides assurance that the wastewater has been properly treated and meets all the requirements to be released to the river.
A state-issued permit establishes the level of pollutants allowed that will be protective of the public health and the environment. The ability of the receiving water to remain unharmed is a major factor in setting these limits. The limitations are established in a public permit issuance process that allows anyone to comment on or object to the issuance of the permit.
What are common wastewater terms?
In wastewater vernacular, there are acronyms for many processes. Some of the most common terms are listed below with a brief description.
Aerobic: A process that requires dissolved oxygen to operate properly. The microorganisms need the oxygen to “eat” the food properly.
Anaerobic: A process that can operate or needs to operate without oxygen being present. A good example is an anaerobic digester used for solids handling.
Biochemical Oxygen Demand Test (BOD5): A test that measures the oxygen-consuming substances or the organic strength of a sample of wastewater. It provides information on the organic load or how much “food” there will be for organisms. The load can be either to a treatment plant unit or to a receiving water body.
Clarifier or settling tank: Tanks designed for the physical separation of wastewater floatable solids and settleable solids. These two terms are widely used interchangeably.
Disinfection: Killing disease-causing organisms, differing from sterilization, which kills all organisms.
Dissolved Oxygen Test (DO): A test usually performed by an electronic meter that measures the amount of oxygen dissolved in a sample of water. The sample may have been from natural waters or from a part of the sewage treatment process. It is important because many of the treatment processes require oxygen (aerobic) to operate properly. Too much oxygen can mean that money is wasted through excess energy consumption to provide the oxygen, which is relatively insoluble in water. It is important in natural waters because fish and other desirable aquatic life are just as dependent on oxygen as people.
Effluent: Wastewater or other liquid, partially or completely treated, flowing from a reservoir, basin, treatment process, or treatment plant.
Influent: Wastewater or other liquid flowing into a reservoir, basin, or treatment plant.
Parts per million (ppm) or milligrams per liter (mg/L): A measurement of how much of a substance is contained in water. These terms refer to the results of analyses such as TSS, BOD5 or DO. These terms are used interchangeably and mean exactly the same thing.
Total Suspended Solids (TSS): Particles that are suspended in water (not dissolved) This test measures by weight how much particulate material is contained in water samples by filtering the sample through a special filter.
Some of this information was developed by the New York State Department of Environmental Conservation, the US EPA Region 2, Environmental Finance Center at Syracuse University, And the New York Water Environment Association February 2007