Winona’s Vital Wastewater Infrastructure
The World Health Organization says that every year more than 3.4 million people die as a result of water related diseases, making it the leading cause of disease and death around the world. Most of the victims are young children, the vast majority of whom die of illnesses caused by organisms that thrive in water sources contaminated by raw sewage.
By contrast water borne diseases caused 17 deaths in the US in 2013–2014.
The US population is about 327 million and the world population is about 7.53 billion. How is it that a country that has over 4% of the world population has only about 0.0005% of the world’s deaths from waterborne diseases?
One main reason for this disparity is that a very high percentage of the wastewater which contains waterborne pathogens, generated in this country, is fully treated and disinfected prior to reintroduction into the environment. This is the case with the wastewater generated in Winona.
European settlement began in Winona in the 1850s. As streets were laid out storm sewers were put in place to carry runoff from the city into the Mississippi River. In 1892–93 the first sanitary sewers were put in place to collect and convey wastewater from inside homes and businesses. These sanitary sewers connected to and combined with the storm sewers to carry unwanted water away from the city via the river. The result of the use of these combined sewers was the contamination of the river.
In the 1930s the City uncombined the storm and sanitary sewers and built a primary wastewater treatment plant to treat the sanitary sewage prior to discharging it to the river. This reduced pollution entering the river from Winona in sanitary sewage by about half.
In 1972 the Congress passed the Clean Water Act which, among other things, required that all cities provide full treatment for municipal wastewater. In 1972 a new primary and secondary treatment plant went on line in Winona which is consistently removing over 90% (up to 99%) of the contaminants in the wastewater prior to discharging to the river.
This infrastructure, because most of it is out of sight, is invisible to the average Winona resident and it is my intention in writing this article to create a greater awareness of this infrastructure, so it is not underappreciated or taken for granted.
The two major components of wastewater infrastructure are collection and treatment.
Wastewater Collection
In order to fully treat the wastewater generated in Winona it must first be safely and efficiently collected and conveyed to the Wastewater Treatment Plant (WWTP). If you look in the basement of your home, you will probably be able to see components of your house’s sewer collection system. These cast iron or PVC pipes are connected to a house main that usually passes through a basement wall facing the street. This sewer line, owned and maintained by the property owner, conveys your wastewater to the City sewer which is usually buried under the middle of the street.
The City maintains about 130 miles of sanitary sewer lines that graduate from a minimum of 8 inches to a maximum of 42 inches. This system is mostly a gravity system, so the pipes are laid at a slope of about 0.5%. The minimum depth of the lines is about 6’ to insure it stays below the frost line and the maximum depth is about 20’, below which sewer maintenance is not practical. In order to be able to keep the collection system operating workers must have access to it. This access is provided by the installation of manholes every 100 yards or less. The word “manhole” is misleading because workers rarely enter these structures in Winona. But these structures allow access to the sewer lines for the rodding, jetting, and flushing done to keep the lines open and flowing freely. The City maintains about 2500 sanitary sewer manholes.
Because the gravity sewer lines in Winona operate within a 14’ cross section of the soil (between 6’and 20’ depth), the wastewater collected in various locations around the City must be lifted (pumped back up to 6’ depth) at key locations in order to get it to the WWTP. The City operates 14 of these lift stations as part of the collection system. A lift station consists of a wet well (large tank) that collects the wastewater, sensors that sense the wastewater depth in the wet well, pumps that lift the wastewater, and controls that operate the pumps.
The collection system then consists of 130 miles of sewer lines, 2500 manholes, and 14 lift stations.
Wastewater Treatment
The four main components of municipal wastewater treatment are primary treatment, secondary treatment, disinfection, and biosolids recycling. The first three processes occur in the Winona WWTP located at 1400 Shives Road on the eastern edge of the City. The fourth process: treatment, storage, and land application of biosolids, starts at the WWTP and extends to fields in Winona Country within a 25-mile radius of Winona.
Primary treatment is a series of physical processes designed to remove solid materials from the wastewater so that solid materials do not accumulate downstream in the WWTP interfering with plant efficiency. By the end of the primary treatment processes roughly 50% of the contaminating pollutants have been removed from the wastewater.
Secondary treatment is biological treatment designed to remove the organic materials left dissolved in the wastewater after primary treatment. In the secondary treatment units, the organisms growing in the wastewater consume dissolved organic materials like carbohydrates, fats, and proteins making them easy to remove in settling tanks at the end of the process. By the end of the secondary treatment processes an average of 95% of the contaminating pollutants have been removed from the wastewater.
Disinfection is the process whereby any pathogens (infectious agents) remaining in the wastewater are killed prior to discharge into the river. In Winona we use chlorine to disinfect our wastewater. Any residual chlorine left in the wastewater after disinfection is neutralized prior to discharge so that no disinfecting agents that could harm the organisms in the river are discharged to the river.
All the organic materials removed during the treatment processes are pumped to digesters that digest these materials for 30–35 days. This extensive digestion process converts this material to a valuable and safe biosolids product that is used as a fertilizer on farms in Winona County. The City spreads the biosolids on permitted farms and the nutrients in the biosolids are taken up in crops like corn and soybeans. The City can meet only a fraction of the demand for this material which replaces the use of inorganic fertilizers on the fields where it is applied.
Summary
The City of Winona collects and treats 3–4 million gallons per day of wastewater. This produces about 3500 tons of biosolids that are recycled each year in crops grown in Winona County. Operating these systems protects the citizens of Winona and the people downstream. It also protects the water quality of the Mississippi River and provides for the beneficial reuse of the nutrients removed in the treatment processes.
In 2018 the City spent about 6.3 million dollars providing these services. All the money required to construct, operate, and maintain these systems is collected directly from the people who use them. The current charge for wastewater collection and treatment in Winona is about $20 per 7500 gallons or about one cent per 3.75 gallons. This is one of the lowest rates in the nation for cities of comparable size. The average shower in the US uses about 17.2 gallons of water. The sewer use customer in Winona pays 4 or 5 cents to have the wastewater from that average shower collected and treated. This helps put the cost of collection and treatment of wastewater in Winona in perspective.
Conclusion
The residents of Winona receive great value for the money they spend on wastewater collection and treatment. Although these systems are well designed and maintained they deteriorate over time and, like all infrastructure, must be renewed in a timely fashion. Like all cities, Winona must reinvest in this infrastructure if it is going to continue to serve and protect its residents. In order to accomplish this, sewer use rates will have to be significantly increased (beyond the rate of inflation) in the coming years. The people who pay for these services could easily become alarmed and resistant to these increases. But there is no free lunch. If these valuable services are going to be provided, then they must be payed for. It is my hope that, by better understanding the services provided, the people who pay for these services will conclude that they are getting good value for their investment as they help prevent the incidence of waterborne diseases in Winona (and downstream) and protect the integrity of the ecosystems in the Mississippi River valley that serve us all.
Francis Bacon famously said that knowledge is power. If that is true, then ignorance introduces weakness. Knowledge of the systems that we build and operate can help us set priorities and make wise decisions about the use of limited resources. Ignorance of these systems can make us vulnerable to poor planning and decision making which could have catastrophic consequences in some cases.
