The Bloomington Sludge Ponds
Following World War II, demand for housing in the Twin Cities area led to a building boom in Bloomington. Because there was no municipal water or sewer service at the time, homes were built with their own water wells and septic systems. The water wells usually consisted of a length of pipe driven into a shallow aquifer about 12 or 15 feet below ground. Unfortunately, the septic systems were built just above this and contamination soon began leaching into the residential drinking water.
To address the problem, the City of Bloomington developed a municipal sanitary sewer and water supply system. Water mains were laid in 1960 and the City began purchasing lime-softened water from the City of Minneapolis. Later, reservoirs and water towers were constructed to better accommodate the peak demands of the growing City.
By the early 1970s, Minneapolis could no longer supply all of the City of Bloomington’s water needs. So in 1973, the City of Bloomington constructed the Sam H. Hobbs Water Treatment Plant. This lime-softening plant operates around the clock, 365-days a year. Improvements made in 2002 allow the plant to produce up to 14,000,000 gallons of treated, drinkable water each day.
Since Bloomington’s water comes both from groundwater and the Mississippi River, it must be processed in order to convert this "raw" product into a safe, drinkable form. One step in the process is lime softening. Lime softening involves a series of chemical reactions. The goal of these reactions is to change the naturally occurring calcium and magnesium compounds in the water into calcium carbonate (basically limestone) and magnesium hydroxide which are easily settled out.
Calcium bicarbonate is the most common calcium compound in water. At the treatment plant, when slackened quicklime is added to the water, the calcium compounds react with the lime and carbonate hardness is removed from the water. This is represented in the following chemical equation: