international: Sweden Creates Sludge Products

Stina Cassidy

What began as a pilot project in Sweden for recycling municipal sludge in 1995 now is creating products such as biofuel, fertilizer and recycled coagulants. In addition to eliminating the need for sludge disposal, the process also generates energy and economic surpluses.

Known as the Kemwater Recycling Process (KREPRO) project, it is a cooperative effort of the Kemwater division of Kemira Kemi AB, Alfa Laval Separation and the city of Helsingborg, site of the pilot plant. The project also is financed partially by the Swedish Environmental Protection Agency.

KREPRO transforms municipal sludge into usable products. The goal "is to end up with zero sludge for waste disposal," says Rene Bouwman of Kemwater, KREPRO's marketing manager.

After pilot and full-scale trials were completed in 1997, Helsingborg decided to install the process permanently.

In its simplest form, KREPRO creates three products:

* organic sludge with a high dry solids (DS) content, which can be used as a biofuel;

* inorganic sludge with a high phosphorus content, which can be used for agricultural fertilizer; and

* liquid-containing soluble organic matter and the precipitation chemical used in phosphorus removal.

The resulting liquid is returned to the treatment plant where its organic content provides a carbon denitrification source, and its precipitation chemicals are reused during the phosphorous removal process.

The project more than pays for itself by eliminating the costs of sludge disposal. It also provides an energy surplus, because the biofuel's energy value exceeds the energy consumed during the process. If the products' values are considered, then a monetary surplus also is attainable.

During processing, the sludge initially is acidified to a pH of about 1.5, then boiled at 140 degrees Celsius. Next, the insoluble organic matter is separated in a decanter centrifuge.

Reject water from the centrifuge is pH-adjusted, which causes the inorganic fraction to precipitate. The organic matter then is separated in another centrifuge. The reject water from this second centrifuge constitutes the liquid product, which is returned to the treatment plant.

The DS content of the organic sludge can reach 45 percent to 50 percent, making it an excellent biofuel. The energy content and market value is nearly equal to that of wood chips. In its basic variant KREPRO concentrates the heavy metal content of the original sludge into this organic sludge.

The organic sludge's heavy metal content does not pose a problem for biofuel use because it is designed for systems that can eliminate heavy metals their emissions, such as district heating plants, Bouwman says.

The inorganic sludge, in this basic KREPRO version, consists mainly of ferric phosphate, with a DS content of approximately 35 percent. The heavy metal content is extremely low, and the phosphorous content can reach as high as 15 percent of DS. The high phosphorus level is ideal for agriculture use, either directly or after further treatment.

Trials have shown that the phosphorus in the ferric phosphate is accessible for a variety of crops. In terms of heavy metal content, the inorganic sludge meets national legal requirements.

In fact, according to Bouwman, "it's actually cleaner than the domestic sludge currently being used in agriculture and is comparable with commercial fertilizers."

As for cost, "the cheapest phosphorus source for use in fertilizers costs about 10,000 Swedish kroners (SEK) per ton/pure phosphorus (P)," he says. The market value for the ferric phosphate is approximately 400 SEK per ton.

The liquid product can be reintroduced to a treatment plant's particular configuration and process flow. In the best case scenario, KREPRO's liquid product is used as a carbon source, and the wastewater's nitrogen content is reduced further by 5 milligrams (mg) to 6 mg Ntot/l (total nitrogen content per liter).

The product also can recycle 90 percent of the precipitation chemical used in phosphorus removal.

Currently, the cheapest precipitation chemical is iron sulfate, which sells for between SEK 1,500 and SEK 1,800 per ton of iron, according to Bouwman. The market value for the carbon source/coagulant is approximately 30 SEK per ton.

In addition to producing commercially viable products, this process also can replace anaerobic digestion as the final standard treatment for municipal sludge.

"In our Helsingborg pilot plant, the KREPRO process starts with digested sludge, but in principle, we could use raw sludge," Bouwman says. "KREPRO both reduces the volume and pasteurizes the sludge, which eliminates harmful bacteria and viruses - so in effect, it's doing the same job as anaerobic digestion." The process also can serve as an alternative to drying and/or incineration: It doesn't involve evaporation, a big energy consumer in other sludge handling methods.

Currently, the process has been fully automated for operation 24 hours per day. KREPRO is beneficial to Helsingborg's sewage treatment plant because of stricter demands on sludge disposal, especially heavy metals.

Each year, the plant also produces approximately 3,500 tons of organic sludge, 1,700 tons of ferric phosphate and 70,000 tons of carbon source/ coagulant, for a value of approximately 3.5 million SEK. Total operational costs are 3.6 million SEK per year.

Landfill Gas Facts and Figures

* Each person in the United States throws away about 4.5 pounds of trash per day, most of which goes to solid waste landfills.

* Nearly a ton of waste materials per person is landfilled each year.

* There are approximately 160 landfill gas (LFG) energy recovery facilities in the United States.

* The U.S. Environmental Protection Agency estimates that about 700 U.S. landfills could install economically viable LFG energy recovery systems.

* As of October 1997, Landfill Methane Outreach Program (LMOP) participants have credited the LMOP with assisting more than 30 LFG energy recovery projects.

* These projects have helped improve air quality, removing the equivalent of more than 47,000 cars from the road, which is equal to planting more than 100,000 acres of trees.

Acquisition Med/Waste Inc., Miami Lakes, Fla., has signed an agreement to acquire Health Care Waste Services Inc., New York.

Awards Natural Sciences Inc., Augusta-Richmond County, Ga., has been awarded Southeastern Technology Center's (Augusta) Phytoremediation project.

Sen. John H. Chafee, R-R.I., has been chosen as the first recipient of the National Recycling Coalition's (Alexandria, Va.) Recycling Leadership Award for his commitment to natural resources conservation and pollution reduction.

The Delaware Solid Waste Authority, Dover, has received an "A" rating on its municipal bonds.

Call for Papers Papers are invited for SWANA's 10th Annual Waste Reduction, Recycling and Composting Symposium and 3rd Annual Collection Symposium, January 11-15, 1999, in Seattle. Abstracts of 500 words or less and biographical sketches of 100 words or less, and questions should be sent to: Chris Voell, P.O. Box 7219, Silver Spring, Md. 20907-7219. Phone: (301) 585-2898 ext. 255. Fax: (310) 589-7078. E-mail: [email protected]