EL PASO — Breathe easy. New York City sludge spread on a ranch in windy West Texas isn’t stirring pathogens into the air, a new study has found.
“The most important finding is that the population in Sierra Blanca is not being impacted by the sludge application,” said Dr. Suresh Pillai, environmental microbiologist at Texas A&M University’s Agricultural Research Center at El Paso. “This sludge application poses little risk under the conditions.”
Sierra Blanca, with a population of about 700 people, is only about 4 miles from the ranch site which has been a repository for New York City sludge since 1993. Pillai studied the location, taking air samples in the spring and fall of 1995 over an eight-month period, to see if people were breathing air contaminated by disease-causing organisms.
His lab analyzed the samples for salmonella, fecal coliforms, viruses, and sludge indicators called hydrogen sulfide producers and clostridia, but no significant amounts of any of those pathogens were found.
“Air quality is the last issue that we didn’t have data on,” said Bob Carlile of College Station, technical director for MERCO Joint Venture, the company which is applying the sludge. “Previous studies have shown that we are improving the soil, the quality and quantity of vegetation on the project and that we are not impacting the water. We didn’t have scientific data on the air until now.”
Sludge is being applied at a rate of 3 tons per acre per year to about 18,000 acres of a 120,000-acre ranch near Sierra Blanca, about 75 miles east of El Paso in the Chihuahuan desert. The Chihuahuan Desert spreads over some 175,000 square miles in southern New Mexico, the Trans-Pecos Region of Texas and much of northern Mexico. Some residents near the site were concerned about the possibility of pathogens becoming airborne because it is an arid, windy region.
Carlile said that the company will continue to conduct research on the location because it is the largest biosolids recycling effort — in terms of land area being used — in the nation.
“It is a controversial project being looked at by a lot of different people. That’s why we’re the standard bearer. We are spending tremendous amounts of dollars to get the answers,” he said.
The company also has sought research on the extent to which the applied sludge is moved offsite by wind erosion, according to Dr. B.L. Harris, Texas Agricultural Extension Service soil specialist.
“The overall amounts of both mineral and organic materials being moved onto and off the application areas is almost too small to measure,” Harris reported. “Applying biosolids reduces wind erosion by roughening the surface, thereby increasing the boundary layer of air just above the ground surface.”
Harris also said the application of biosolids reduces wind erosion because the material contains nutrients which promoted the growth of plant species which serve as windbreaks that also filter particulate matter out of the air as it crosses the site.
“Based on these studies to date, there should be little concern about potentials for the applied biosolids to be moved offsite by wind,” Harris said.
Yet because of its location in an arid, windswept region, company officials wanted to make sure that no pathogens were being stirred up in the air. At Sierra Blanca, wind speeds of up to 40 miles per hour are common in the spring.
Municipal sewage sludges are routinely used on agricultural lands in various parts of the world, Pillai said. In the United States, as much as 33 percent of the municipal sludge produced is applied onto agricultural lands. Pillai said there has been “only limited information on the occurrence of airborne microbial pathogens during sludge application.
“With the ban on ocean sludge dumping and the increasing restrictions on landfills, disposal onto land surfaces becomes almost the only alternative and is expected to increase in the future,” he noted.
Pillai collected air from five locations — upwind, at the rangeland-population interface, at the old application site, at the current application site and at the hopper loading site — throughout the two study periods. The collection devices, called impingers, where placed on poles and set to collect at a rate similar to the breathing patterns of humans — about 5 feet in height and three gallons of air in 20 minutes. Microbiological analyses then were performed on the concentrated air samples at Pillai’s El Paso lab.
“We looked for the total numbers of bacteria in the samples, looked for specific pathogens such as salmonella, then we did some genetic fingerprinting of airborne clostridia to help us determine their sources,” Pillai said.
He explained that clostridia are bacteria that forms spores. Since sludges have gone through a heat digestion process, the clostridia originating from sludges are heat tolerant. So, by comparing the genetic fingerprint of airborne heat tolerant clostridia with that of the clostridia isolated directly from the sludges and surrounding soils, it is possible to identify the sources of airborne clostridia, according to Pillai.
“This new molecular fingerprinting has tremendous application in doing exposure studies to trace the origins of airborne bacteria,” Pillai said. “Future studies of this type will need to include the analysis of clostridia in addition to the other pathogens so that sources could be identified.