Status and Trends

In 1970, the Detroit Wastewater Treatment Plant began removing phosphorus from its effluent using pickle liquor (ferrous chloride) and polymer to meet the 1 mg/L phosphorus effluent standard for all major plants (3.8 million liters; one million gallons per day or greater). Pickle liquor was obtained from local steel mills and pumped or fed by gravity into plant headworks, while polymer was injected into channels leading to the primary clarifiers. Aeration facilities for secondary biological treatment were constructed during 1973-1976. Through this process, ferrous chloride is converted to ferric chloride, which has been found to more effectively precipitate phosphorus. During 1979-1980, staff at the plant implemented an alternative sludge removal process which increased sludge handling capability and indirectly increased the plant’s ability to remove phosphorus.

Two statewide phosphorus control initiatives were also implemented. In 1971, Michigan enacted a phosphorus limitation of 8.7 percent by weight on all cleaning agents. Michigan’s phosphorus detergent ban was implemented in 1977, restricting the phosphorus content of household laundry detergents to no greater than 0.5 percent by weight.

The combined influence of these phosphorus control efforts can be seen in Figure 2a and 2b below. The result was a greater than 90 percent reduction in phosphorus concentration and loading from the Detroit Wastewater Treatment Plant. Similar reductions occurred in other wastewater treatment plants; however, because of the Detroit plant’s 2,461 million liters (650 million gallons) per day flow, the impact on Lake Erie was substantial. Phosphorous reductions at the Detroit Wastewater Treatment Plant would become the single largest cause of the reversal of cultural eutrophication of Lake Erie during the 1970s and 1980s. Lake Erie responded with dramatic improvements in water quality.

With the passing of the Clean Water Act in 1972, and the issuance of the first National Pollutant Discharge Elimination System (NPDES) permit, there were a number of problems with consistent and adequate operation of the plant. These problems climaxed in a 1977 Federal Consent Judgment, which outlined the specific deficiencies, areas requiring improvement, and target dates for achieving compliance. A full-scale evaluation of the plant was performed in 1979 to identify the treatment capacity and capability, additional capital improvements, and to help ensure adequate operation of the plant. It was not until 1981, when the construction and modification of secondary settling tanks were completed, that the plant would achieve consistent operation sufficient for secondary treatment standards.

 

The story of reductions in phosphorus loadings and the subsequent reversal of cultural eutrophication of Lake Erie is one of the greatest success stories of water resource management. Indeed, the US-Canada phosphorus control program is heralded as an international model.

By the early 1990s, zebra and quagga mussels invaded Lake Erie causing major changes in the food web and lake ecosystem. Additionally, farming practices changed over a similar time frame, with the advent of no-till farming and increased use of drain tiles. In recent years, blooms of blue-green algae have occurred in the Western Lake Erie Basin, particularly in the mouth of the Maumee River, raising new concerns. The exact reasons for these blooms are uncertain. Did increased discharge of nutrients occur? Did zebra mussels change the water quality and favor productivity of blue-green algae? Will this lead to more taste and odor problems in drinking water supplies? Research needs to be focused on answering such questions.

In the meantime, Michigan implemented a statewide residential fertilizer phosphorous ban in 2012. Concurrently, GLWA has moved forward with a set of treatment process measures to further mitigate phosphorus impact on the Detroit River. These measures include refinements in the Ferric Chloride dosing and secondary treatment processes including:

1. Maintaining a minimum of 0.5 mg/l phosphorus in the influent to secondary treatment to maintain a healthy level of nutrients for the secondary biological process that results in a better overall phosphorus removal;
2. Maintaining solids inventory below 500 dry tons;
3. Maintaining solids retention time close to 24 hours; and,
4. Keeping dissolved oxygen levels between 2.0 and 4.0 ppm in the last-bay of aeration basins.

Since 2006, phosphorus discharge concentrations have been reduced by an additional 40 to 50 percent (on average from approximately 0.8 to 0.4 mg/l). This average concentration approaches the lower limit of phosphorus reduction that can be achieved through technology-based treatment. Figure 3a and 3b better illustrate the effectiveness of these measures over the last 10 years.

Reflecting the success of the WRRF in reducing phosphorus discharge limits, the NPDES permit issued March 2013 was modified to reduce the phosphorus discharge limits from 1.0 mg/l to 0.7 mg/l effective in 2015. In 2016, the permit further reduced the phosphorus discharge limits to 0.6 mg/l during the Growing Season.