Primary clarifiers at the Jones Island plant.
Click here to enlarge imageThe 10-year United Water contract, which began in March 1998, will result in $140 million savings over the term and has already contributed to a 16 percent rate decrease for its facilities, according to Milwaukee Commissioner Antonio Riley, who was at WaterWorld's Energy Efficiency Conference in Philadelphia with United Water Asst. Project Manager John Cheslik to accept the award. In addition, the Jones Island plant has a cogeneration plant which provides 30 MW of power, Riley said.
Milwaukee reports that it has saved over $300 million over the years from revenue generated by the sale of Milorganite.
"With the kind of revenue that Milorganite raises, it is certainly the most effective method of use of the biosolids for Milwaukee," according to Paul Schlecht, contract compliance administrator for MMSD's operations group. Schlecht is responsible for overseeing the production of the biosolids for Milwaukee. "And there are costs or savings that aren't as tangible as dollars and cents just in the sense that we're beneficially reusing the solids instead of landfilling them... you obviously can't put a dollar cost on that, but I think it figures into the decisions that have been made over the decades here in Milwaukee as to how we've dealt with the biosolids."
Although the program does not make a profit, it almost pays for itself and is a real bargain when compared with the cost of landfilling dewatered biosolids, Schlecht said. Overall, the program has saved over 200,000,000 cubic feet of landfill space. And MMSD has not had to landfill biosolids since 1996, Spence said.
"We did an audit of our options in 1996, based on 1995 costs, and clearly landfilling was the most expensive of the options that we had available to us," he added.
Milwaukee's two wastewater treatment plants are connected by a 12-mile underground sludge pipeline. Jones Island WWTP handles both plants' Milorganite production, and South Shore WWTP handles production of the Class B liquid product, AgriLife, Schlecht said.
Milorganite, which gets its name from parts of the words MILwaukee ORGanic NITrogEn, is sold bagged or unbagged to commercial and residential customers and can be used on edible crops.
The 8,000 dry tons of AgriLife that South Shore makes per year come from the plant's anaerobically digested biosolids. It is transported to farm land in Southeast Wisconsin in stainless steel tankers, where it is moved into an applicator and injected into the soil. This process avoids odor and visual signs of the biosolids, which in turn prevents complaints and resident fears.
The district also has an arrangement with a local power plant to produce a lightweight construction aggregate using a combination of biosolids and fly ash.
The basic process to make Milorganite begins with activated sludge from the aeration tanks at Jones Island and South Shore. The sludge moves to the drying and dewatering facility, a $200 million facility built in 1994 to replace the original plant, which has been in use since 1926.
There, it undergoes a series of dewatering processes including gravity thickening (settling), centrifugal concentration (spinning), blending and conditioning.
Belt filter presses squeeze the water out until the biomass is a thick filter cake comparable to wet cardboard. The filter cake is approximately 20 percent solids and 80 percent water.
The filter cake is mixed with already dried product and moved into large rotary kiln dryers. The material tumbles approximately 40 minutes on average at a temperature of 800 to 1,200 degrees F, high enough to kill any pathogens present in the filter cake. When it is complete, what remains are the granular remains of heat-dried microorganisms.
A screening process controls the particle size and removes dust. Particles that do not meet size specifications are recycled through the process. The product is then cooled and sent to storage silos or the packaging facility and prepared for shipment.
The end result has a nutrient ratio of 6-2-0 with 4% iron. Since most of the iron is naturally chelated, it is safe for application frequently and on wet turf, something that cannot be done with products containing inexpensive iron salts.
Milwaukee has gone to a great deal of trouble to keep up Milorganite's good public image. Milwaukee spends $750,000-800,000/year in advertising for Milorganite, according to O&M budget figures.
The district is part of the initial pilot project for the National Biosolids Partnership and imposes strict standards on its process which help ease fears of the public. The district limits metals and industrial pollutants in wastewater with a pretreatment program.
"The point is to guarantee to the public that we are doing everything possible to create a safe and valuable product," Schlecht said.
The product has its own web site which explains in further detail the process behind Milorganite. To see this, visit www.milorganite.org.
