Published: Mar 16, 2011 02:00 AM
Modified: Mar 15, 2011 05:43 PM
Is sewage sludge safe on crops?
by Betty Cross
A recent article in The Cary News highlighted a new dewatering device at the Cary wastewater treatment plant. The new equipment will separate liquid from solids resulting in dewatered sewage sludge, which can then be labeled as "fertilizer." There is much more to know about the composition of sewage sludge than is revealed in the story. Wastewater treatment plants were never designed to produce fertilizer. Wastewater treatment plants were designed to clean the water discharged from the plant.
Though sewage sludge contains nitrogen and phosphorous beneficial to crops, it also contains myriad other chemicals, heavy metals and pathogens harmful to the environment and human health. The Environmental Protection Agency sets limits for only nine heavy metals (out of 28) and certain pathogenic bacteria. Only a few organic chemicals are measured out of thousands in use. Because our waste streams have been channeled into centralized wastewater treatment plants, everything sent down the drain from homes, hospitals and businesses, along with stormwater runoff and landfill leachate, winds up concentrated in sewage sludge.
Two examples of chemicals harmful to humans, and for which limits in sludge have not been set, are the antibacterials triclocarban and triclosan. These endocrine disruptors are found in many consumer products and concentrate in sewage sludge. A study at Johns Hopkins' Bloomberg School of Public Health determined that approximately 75 percent of the ingredients washed down the drain by consumers persist during wastewater treatment and accumulate in sewage sludge. In the EPA's Targeted National Sewage Sludge Survey, triclocarbon was found in 100 percent of the samples taken and triclosan was found in 94 percent. These two chemicals interfere with hormones needed for proper brain and reproductive system development in children. A recent study at Ohio State University shows that both chemicals can enter the food chain through sewage sludge used as fertilizer on agricultural fields. There is nothing in the dewatering process that removes triclocarban or triclosan from sewage sludge.
Another disturbing finding from the Water and Environment Research Foundation (WERF) is that dormant bacteria in dewatered sewage sludge can reactivate when exposed to air and water. The researchers suspect that anaerobic digestion and high centrifuge separation renders some bacteria viable but nonculturable, therefore non-measurable. Once the bacteria leave the treatment process, they can multiply exponentially. It seems there is no way to assure consumers that dewatered sewage sludge is free of pathogenic bacteria.
If shopping for compost or fertilizer this spring, whether in bags or in bulk, ask what's in it. If the answer is sewage sludge, please be aware, no one really knows what it contains or if it is safe to use. It hasn't been tested.
Cross is the co-chairwoman of the Sewage Sludge Action Network.