Boston Recycles Sludge Into Fertilizer

ERIC BUEHRENS strides across the concrete floor of the newly built fertilizer plant and digs his hands into the pile of black, soil-like material that rolls onto the conveyor belt.

"On the scale of things, this is pretty good-looking stuff," he says with a satisfied smile. "That's pretty dry, and that's good."

Mr. Buehrens, residuals manager of the Massachusetts Water Resources Authority (MWRA), is inspecting "sludge cake," the partially finished product churned out by this plant that turns heat-dried municipal sludge into fertilizer pellets.

In a world where almost anything can be made into something, even one of society's least-valued resources - like municipal sewage - can take on a new and useful purpose.

That holds especially true at this $87-million facility south of Boston, which may soon be one of the nation's largest manufacturers of sludge fertilizer.

The plant, which began operating last month, is part of the ambitious $6.1-billion Boston Harbor cleanup program. The plan includes revamping an outdated sewage system that serves 43 cities and towns in the Boston area.

The sludge plant here is also a crucial element in a move last month to shut down a sludge-discharge pipe, which had been dumping 450,000 to 500,000 gallons of the smelly, dirty liquid into Boston Harbor daily for four decades. Sludge, which is made up of more than 90 percent water, is the byproduct of treated human waste.

Environmentalists applaud the move to recycling the sludge rather than dumping it into the harbor. "The worst environmental damage in the harbor was directly connected to that ... pollution stream," says Peter Shelley, senior attorney of the Boston-based Conservation Law Foundation. The sludge pipe shutdown "is a major milestone," he says.

Compared with the alternatives of incineration or landfill dumping, recycling is considered the most environmentally conscious way of handling urban sludge.

According to Mr. Shelley, reusing sludge as fertilizer is a better way to handle it than "simply disposing of it in already-crowded disposal facilities or an incineration facility that would discharge [burned material] broadly throughout the atmosphere."

Inside the plant is a complex conglomeration of storage tanks, drying drums, and different-size, multicolored pipes. But the operation is simple; it is really just a drying-out process, Buehrens says.

"Almost entirely what goes on here is the process of getting rid of the water," he says. "We don't really change the character of the material. All we do is dry it up."

The operation is divided into different, organized stages. Special attention is made to contain the pungent sewage smell to only certain areas of the plant and to keep it from escaping outside before it is treated, says Buehrens.

"Experience in handling this stuff shows that if you don't try to get a grip on odor issues up front ... you're going to create a real problem that people are not going to forgive you for," he says.

The sludge, shipped from two sewage-treatment facilities on Boston's Deer Island and Nut Island, is pumped directly from barges into storage tanks inside the plant.

The material is then transferred to the belt-filter-press area, where water is mechanically squeezed out of the sludge by wide fabric belts.

The filter belts squeeze the incoming sludge - which can contain as much as 97 percent water - down to a ratio of roughly 75 percent water and 25 percent solid.

"This stuff comes in as dirty water and it leaves as stuff that looks like mud," says Buehrens.

After that, the sludge cake is moved by conveyor belt to the drying area.

Sludge is heated and dried through large rotating heat dryers, which convert the sludge into small, hard pellets. The drying process also destroys odor and harmful bacteria.

The pellets will then be sold as a low-grade fertilizer or will be blended with other synthetic nutrients to form a more complete fertilizer, says Buehrens. The MWRA hopes to produce 21,000 dry tons of fertilizer pellets per year. Production could increase to 65,000 dry tons by 1998.

Converting sewage sludge into fertilizer pellets is not a new idea. Milwaukee, for example, has been producing high-quality, nutrient-rich fertilizer for the past 65 years. Cities all over the United States have been operating sludge processing plants similar to the MWRA plant since the late 1970s.

In agricultural areas, sludge is used directly on land as a fertilizer with little processing. But cities in more densely populated regions are more apt to favor pelletizing because the pellets are easy to transport and have a long shelf life.

In Boston, the MWRA has a contract with a Maryland-based company, Enviro-Gro, to run its sludge plant. Karl Sattler, general manager of Enviro-Gro, says there is a growing interest in sludge-processed fertilizer.

"We certainly have had a lot of interest in the process, an interest from communities," Mr. Sattler says. "We are getting more inquiries about [the plants] than say three years ago."

But in Boston, critics wonder whether there is a strong enough demand for the fertilizer pellets. Others are concerned about lax enforcement of regulations to keep companies from dumping toxics and heavy metals into the sewer system.

"The MWRA has to guarantee that their pellets are safe, that they are contamination free. And the only way they can do that is through a vigilant enforcement program," says Paul Keough, deputy regional administrator in the Environmental Protection Administration office in Boston. "The cleaner the waste stream, the cleaner the pellets, and the cleaner the pellets, the more marketable they will be as a soil enhancer."

Buehrens says the new sludge-fertilizer plant is a not money-making operation. While processing the sludge costs about $750 per dry ton of pellets, that amount sells for between $40 to $100.

The pellets will be shipped out of state for use as fertilizer for agricultural or landscaping use. The pellets can't be sold in Massachusetts because of strict regulations on the use of cadmium, a metal found in municipal sludge. But those standards are in the process of being revised, says Buehrens.

"For the moment, that's a problem for us. We're registered for sale now pretty much in every state in the country," he says. "But meeting the Massachusetts cadmium standard has been a problem for us, because it's five times more restrictive than any other state in the United States."