As efforts to reduce non-sustainable waste disposal methods increase, the agricultural use of stabilized sewage sludge seems to be a logical alternative. It is known to be a beneficial source of organic matter and plant nutrients. However, land application of biosolids often faces public opposition due to the resulting emission of malodors.
To better understand the source of these odors, a research group from the Agricultural Research Organization of Israel, led by Yael Laor, has investigated emissions associated with field application of sewage sludge before and after alkaline treatment.
Researchers found that dimethyl disulfide and dimethyl trisulfide were most responsible for the odor concentrations of soils treated with biosolids. The complete results from this study were published in the September-October 2011 issue of the Journal of Environmental Quality.
Stabilized municipal biosolids are defined as Class A or Class B, depending on their level of pathogen reduction. Class B biosolids have pathogen levels high enough to face some application restrictions. These biosolids are stabilized through the process of anaerobic digestion, usually in waste treatment facilities. Class A biosolids are alkaline-stabilized, and contain minute pathogen levels. They have hardly any usage restrictions.
For this experiment, samples of anaerobically digested Class B sewage sludge were taken from wastewater treatment plants of three municipalities in Israel along the Mediterranean coastline in late 2007. A portion of the collected sewage sludge was converted to a Class A biosolid by adding fly ash, a fine powder recovered from gases created by burning coal, and quicklime, also known as calcium oxide.
Several small field plots were established in Jezreel Valley, a fertile area in northern Israel that is thought to have once been a channel connecting the Dead Sea and Mediterranean Sea. Class A and Class B biosolids were manually spread across the valley’s fine, clayey soil, and sampled at pre-determined intervals.
Odor concentrations at the plots were then evaluated on site with a Nasal Ranger field olfactometer, a portable nasal instrument used to detect and measure odors. The Nasal Ranger was mounted atop a surface chamber constructed with a metal frame and nylon bag, which was sealed with soil at the bottom. After the chamber was set up for 15 minutes, panelists used the Nasal Ranger to evaluate odor emissions from that specific plot.
Alkaline stabilization of the anaerobically digested biosolids using lime and coal fly ash resulted in substantial odor emissions associated with high concentrations of ammonia and nitrogen-rich compounds. Laor’s team notes that odorous compounds resulting from the application of biosolids can be generated under anaerobic conditions beneath the soil’s surface, and additional odor emission events may be recorded weeks after incorporation.
Laor’s research team also narrowed down the specific compounds responsible for odor emissions, and suggests that understanding the concentration of these compounds is important to future biosolids-related studies.
“We propose that dimethyl disulfide and dimethyl trisulfide, which seem to be most related to the odor concentrations of biosolids-treated soil, be used as potential chemical markers for the odor annoyance associated with incorporation of anaerobically digested sewage sludge,” says Laor.
Doing so would allow scientists to quantify odor emissions resulting from the application of biosolids, and ultimately help to reduce the odor annoyance responsible for public opposition of sewage sludge.
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