After a brief career in pharmaceuticals, Zimmermann settled in Rothschild, WI, in 1935, and established a productive relationship with the Marathon Paper Corp., now the Weyerhaeuser paper mill.

Zimmermann formed Salvo Chemical Co., taking spent pulping liquor from Marathon Paper to produce artificial vanilla (vanillin) using wet oxidation; by 1940, Salvo was producing 70 percent of the United States supply.
 
 
In 1942, Salvo was purchased by Sterling Drug, Inc., a large international pharmaceutical firm, and with Sterling's support, Zimmermann and his staff developed a completely new method of obtaining vanillin directly from pulping liquor by partial oxidation of the ligno-sulphonic acids. The technology was known as wet air oxidation.

Zimmermann's group perfected wet air oxidation (or the "Zimmermann Process" as it was known) and expanded it to other applications, including pollution control. The technology was introduced to the pulp and paper market in 1955, and to the municipal sewage sludge market in the late '50s and early '60s. Naming the company after the Zimmermann process, Sterling established "Zimpro" at Rothschild in 1961, building the present engineering and research center along the Wisconsin River.

• 1950  Zimmermann applies for wet oxidation patent.
• 1959  The largest wet oxidation plant built to-date begins operation in Norway at the Borregaard Paper Company.
• 1961  Zimpro installs a 3 ton/day (dry solids) wet oxidation plant at the Wausau, WI, Wastewater Treatment Plant to demonstrate the destruction of sludge. A wet oxidation unit, having a capacity of 5.6 tons/day (dry solids) is installed and operated at Wheeling, WV. Zimpro also installs a 200 ton/day (dry solids) wet oxidation system at the West-Southwest Treatment Plant of the Metropolitan Sanitary District of Greater Chicago for sludge destruction.
• 1962 - 1965  Wet oxidation systems are installed at Rye, NY, and South Milwaukee, WI, for sludge treatment.
• 1966  An industrial wet oxidation unit is commissioned at the Associated Pulp and Paper Mill in Burnie, Tasmania, Australia, for the recovery of soda from spent pulping liquor.
• 1967  The first Zimpro Low Pressure Oxidation (LPO or Thermal Sludge Conditioning) system begins operation at Levittown, PA.
• 1968  Zimpro obtains patents on the application of wet oxidation for the regeneration of powdered activated carbon and other adsorbents.

By 1972, the year the U.S. Clean Water Act was signed into law, the new company had designed and installed about 30 wet air oxidation systems for municipalities and industries, with units in the U.S., Europe and Japan. Our LPO systems were also being used extensively for thermal sludge conditioning at municipal wastewater treatment plants.

Growth in the 1970s
After establishing a firm foothold in the sludge processing market with wet air oxidation in the '60s, Zimpro branched out in the '70s, developing and acquiring several new products on its way to becoming a major wastewater treatment systems supplier.

With Sterling Drug supplying support, and the large municipal market, Zimpro perfected wet air regeneration of spent powdered carbon and in the process developed the PACT® (powdered activated carbon treatment) system. In 1974, Zimpro acquired the Adrian Construction Co. of Golden, CO, and began to market Adrian's multiple hearth furnaces for sludge combustion. And in 1975, the Hydro-Clear filter was added to the line when the Hydro-Clear Corp. was acquired.

• 1973  Manufacturing building erected in Rothschild, WI.
• 1974  19,000-square-foot engineering wing added at Rothschild.
• 1975  Largest wet oxidation system reactor set at Ontanagon, MI, for Hoerner-Waldorf Paper Co.
• 1975-1983 Wet oxidation systems, supplied by Niigata Engineering (a licensee of Zimpro) are extensively used in Japan for the treatment of a variety of industrial wastewaters, including wastewater from the production of acrylonitrile, caprolactum, coke oven gasification liquors and ethylene spent caustic.
• 1976  Founder Zimmermann dies in Florida.
• 1977  Papierfabrik in Biberist, Switzerland, starts up a wet oxidation system for  filler clay recovery.
• 1978  Passaic Valley Sewer Commission signs contract for $50 million for twelve wet oxidation systems-the largest ever built.
• 1979  First PACT-Wet Air Regeneration system in U.S. starts up in Vernon, CT.

Changes in the New Decade, and New Owners
Change was the keyword of the '80s for Zimpro. In 1980, we were in our 38th year as part of Sterling Drug, and federal funding was pouring into the municipal wastewater treatment market. However, in January of that year, Congress substantially cut the federal portion of projects.

At Zimpro, marketing efforts refocused on the industrial wastewater market. We acquired all PACT system patents and introduced package and batch-operated units. We revitalized wet air oxidation for industrial waste applications, introducing manufactured modular systems for industrial applications, and we were contracted to own and operate a wet oxidation system for commercial hazardous waste treatment.

• 1982  Zimpro installs the first wet oxidation unit in the U.S. to treat ethylene spent caustic.
• 1983  Zimpro installs and operates a wet oxidation system at Casmalia Resources in Santa Maria, CA, for the treatment and detoxification of hazardous wastes.
  Our efforts were boosted by the 1984 amendments to the Resource Conservation and Recovery Act (RCRA). Many hazardous wastewater streams, almost exclusively deep-welled, landfilled or disposed of in surface ponds, would now have to be treated and destroyed.

Then in the mid '80s, a change in ownership pulled us out from under Sterling's wing and led to our most significant product line expansion. We were acquired by the Ventures Group of Houghton, MI, and in 1987, we merged with the U.S. Passavant operation which Ventures had also acquired. We became the Passavant licensee for North America, offering a broad line of wastewater treatment equipment. We also made a strong move in the service business, by launching our in-house, commercial analytical lab, Enviroscan Services. In addition to meeting the needs of our in-house research and pilot testing facilities, Enviroscan Services now served outside customers to grow the analytical services business.

Approaching the Millennium
By the end of the 1980s, Zimpro had become a leader in treating spent caustics, and had started up units at Fina Neste in Belgium and Quantum Chemical in the United States.  In 1989-90, Zimpro was contracted by engineers regarding growth in the ethylene spent caustic market. As a result, we supplied a newly-designed wet oxidation system specifically for the treatment of ethylene spent caustic to the Chinese Petroluem Corporation, Kaohsiung, Taiwan. This project became the starting point for the construction of new systems worldwide for ethylene spent caustic treatment, and was the beginning of the full commercialization of modular wet oxidation units specifically designed for ethylene spent caustic treatment.

• 1990  Zimpro becomes affiliated with the Black Clawson Co., world leader in wastewater recycling systems and pulp and paper machinery. 
• 1992  Zimpro installs a wet oxidation system at Sterling Organics in Dudley, Northumberland, UK, for pharmaceutical wastewater treatment.
• 1995  Zimpro ships the smallest commercial wet air oxidation unit built to Refinaria de Petroleos de Manguinhos, a Rio de Janiero producer of liquid petroleum gas, gasoline, fuel oil and solvents.  The project was also our first wet oxidation unit in South America.
• 1996  Along with a number of other major acquisitions, USFilter announced that it would purchase Zimpro and Enviroscan. Also in this year, the wet oxidation pilot plant in Rothschild, WI, is re-configured to a permanent position, becoming the largest wet oxidation pilot plant facility in the world.
• 1996-1999  Zimpro continues to build and install ethylene spent caustic treatment units.

A Truly Global Company
In April, 1999, Paris-based Vivendi announced the acquisition of USFilter, later changing the name of the parent company to Veolia Environnement. And then in 2003, German-based industrial giant Siemens purchased USFilter's Water & Wastewater Systems and Water Services & Products businesses from Veolia.

• 1999  Zimpro supplies a wet oxidation system for the treatment of crystallizer blowdown from an ammonium sulfate crystallizer used in production of methylmethacrylate.
• 2000  Zimpro completes start-up of the first spent caustic system in Saudi Arabia. Zimpro is also awarded an order to supply a wet air oxidation system for spent caustic treatment in Qatar, and we are selected to supply inclined plate separators to clarify surface water at one of the largest municipal water plants in the U.S., in Detroit, Michigan.
• 2001  Zimpro is awarded an order to supply a wet air oxidation system to Fujian Petrochemical Co., Ltd., our first project in the People's Republic of China.
• 2002  Zimpro begins selling systems and services to the oil and gas industry.
• 2003  Zimpro supplies a water injection system, with sulphate removal, to Total E&P for the Dalia field, offshore Angola.
• 2003-2004  Zimpro supplies DeKalb County, Georgia, and the Gwinnett County F. Wayne Hill WRC with inclined plate separators, as part of multi-million dollar orders. 
• 2005  Zimpro supplies fifth wet air oxidation for ethylene spent caustic treatment to Formosa Petrochemical Company in Taiwan. Also, Shaw Environmental and the U.S. Army choose a wet air oxidation system to treat a waste stream generated by the destruction of chemical warfare material, as part of the U.S. Army Non-Stockpile Chemical Materiel Project (NSCMP). In 2005, the company also acquires Monosep Corporation, strengthening our presence in the oil and gas industry.

A New Era, A New Name
Siemens has positioned USFilter as the cornerstone for the Siemens Water Technologies global business. Throughout 2006, the USFilter name changed to Siemens Water Technologies. But while our name has changed, our technologies, services and people remain the same.