Part 2 in a series on sewer systems in the Huron River watershed

In our Fall 2024 issue, we introduced the topic of wastewater treatment and discussed the use of private septic systems along with some of the problems they can cause. Here we discuss the development of municipal-scale wastewater treatment plants (WWTP), how they work and their impact on the Huron River watershed.

History of treatment plants

As people settled into agricultural based communities that grew more dense and connected over time, they quickly recognized the need to transport waste away from settlements to prevent the contamination of drinking water sources and the spread of disease. As these settlements grew dense, there was not enough land available for septic systems. The recognized need for a community-wide solution coincided with the period of rapid industrial and medical advancement in the 1800s. The first documented, community-scale wastewater treatment plant was developed in Scotland in the early 18th century.

However, large-scale adoption of centralized sewage treatment did not start until the late 19th and early 20th centuries. Most conveyance systems combined sewage flows with stormwater flows. Many combined systems still exist in older cities, like Detroit, and they frequently overflow, releasing untreated sewage into natural waterways. Luckily, in the Huron River watershed, all municipal systems were built as separate systems, keeping stormwater from overwhelming sewage treatment plants.

How do wastewater treatment plants work?

Modern treatment plants employ 4-5 steps of treatment targeting different components of waste. First, sewage is transported (usually by gravity but also using pumps in lower elevations) from homes, businesses and other facilities within the city to the “headworks” of the treatment plant. There the liquid is filtered through different sized screens in “pretreatment” stations to remove large objects like plastics and other trash, including so-called “flushable” wipes (which can lead to clogs through the conveyance system).

The sewer water then travels to primary treatment, which takes it through a series of settling chambers that remove large to increasingly smaller solid particles. The last step in primary treatment is clarification, which removes fats, oils and greases that float to the surface. Then it is on to secondary treatment. In this step, the partially treated water is aerated in a series of chambers and the heavier sludge is activated with bacteria that consume organic matter and then settle to the bottom for removal. A final, tertiary treatment step can be added that may include physical filtration, membrane filtration, or advanced chemical treatment to remove a wide range of chemical pollutants. Before discharge to waterways, chlorine or ultraviolet disinfection is used to kill bacteria.

Where are municipal treatment plants in the watershed?

Wastewater treatment plants exist in a variety of sizes and treat a range of populations. Michigan’s “MiEnviro Portal” shows over 60 WWTPs in the Huron River watershed area. Most of the bigger ones are run directly by municipalities, like in Chelsea, Brighton, South Lyon, Wixom, Dexter, and Ann Arbor, or by municipal authorities like the Ypsilanti Community Utility Authority and the South Huron Valley Utility Authority. The rest are smaller plants (usually privately operated) that serve smaller communities, industrial facilities, or even a single housing development. Many public facilities offer tours or have annual open houses allowing the public to see how they work.

Do these plants solve the wastewater problem?

The short answer is yes, most of the time. Under dry conditions, operating within capacity and when properly maintained and operated, treatment plants can accept and treat much larger volumes of sewage than single-user septic systems. Under the Clean Water Act, these plants must remove or digest organic pollutants in our waste stream down to levels that protect receiving waters for all uses by humans and aquatic wildlife. If the plant incorporates tertiary treatment, it can even remove or break down a range of inorganic pollutants—such as PFAS—from Industrial processes.

Problems can arise, however, when treatment plants receive excess volume during large rain storms (in the case of combined systems or older, leaky systems), when they push their capacity limits (due to population growth), when they take on wastewater with pollutants they are not equipped to treat, or when they are not properly operated or maintained.

All wastewater treatment plants must routinely apply to Michigan’s Department of Environment, Great Lakes, and Energy (EGLE) for discharge permits which establish limits on a range of pollutants in the release waters or effluent. For example, since the 1990s EGLE has required all plants to meet a phosphorus limit of 1 mg/L to prevent eutrophication. Plants must regularly monitor their effluent and report to EGLE. In contrast, septic systems are mostly unregulated. Not all plants are able to meet their limits under all conditions. However, EGLE can require cities to invest resources to upgrade their plants to meet these limits.

Beyond the plant itself, there can be problems within the collection system. Older sewer lines can get blocked and backup or break, causing spills. Stormwater can infiltrate into older sewer lines and cause backups and spills as well. All spills are required to be reported, but many are not. Because spills can vary in volume, it is difficult for the media and public to determine which spills are consequential.

It is clear that centralized wastewater treatment is better for the Huron River and other water resources.

WWTPs are designed to clean and filter water in multiple ways and are strictly regulated by state and federal law, as opposed to septic systems. WWTPs are also the only current solution for treating waste from concentrated development zones, plus they can filter toxic pollutants (e.g. Wixom’s WWTP capture of hexchrome). In fact, WWTPs provide a double benefit: by allowing for concentrated development, more land can be protected. Pollution prevention combined with pollution treatment is the best long-term solution for the Huron River.

—Ric Lawson

This blog post is also published in the Huron River Report, Spring 2025.