A Second Life for Sewage Sludge

When asked to think about sludge, lush farmland might be the farthest thing from your mind. But cities in Montana and across the country are working to make that connection by using sewage sludge–a byproduct of wastewater treatment plants–to generate compost that can then be sold or given back to communities for use in farming. This process started to make the news in the early 2000s, when state courts and the EPA realized that sewage sludge generation was only increasing with population growth, and turning it into a compost product (also called biosolids) could prevent thousands of tons of waste from being tossed into landfills. Since then, the composting process has only grown, with cities like Milwaukee, Seattle, Austin, Washington D.C., Houston, Boston, and Baltimore all building their own facilities. By all accounts, biosolids are booming.

One of the reasons cities like biosolids is that they’re fairly easy to produce. There’s no need for specialized equipment or chemicals. Generally, composting of sludge requires only a common byproduct of logging operations: sawdust. Sawdust and bark are mixed with arriving sewage sludge and stored in warehouses where air currents are modified as necessary to heat the mixture until it reaches a temperature high enough to kill any dangerous microbes. The sludge can also be mixed with other types of organic waste, like yard clippings, to create a mixed-waste compost that recycles even more material that would otherwise end up in a landfill.

Here in the Flathead Valley, a company called Glacier Gold has long been committed to recycling human waste products by transforming them into nitrogen-rich compost. The compost is generated under indoor, controlled conditions and thoroughly tested to ensure it meets state and national standards. The City of Kalispell ships roughly 6,000 cubic yards of sludge to Glacier Gold’s facilities in Olney every year, with only one problem: according to this Daily Interlake article, Glacier Gold hit its sludge acceptance limit from the city back in 2007. This means that the leftover sludge, an amount that increases every year as Kalispell grows, must instead be sent to landfills or other facilities. As of 2018, Kalispell’s Public Works department is looking into starting an in-house composting operation to process the overflow. A city-run operation of this type could be a boon for local farmers, especially small-scale operations trying to cut costs. As this NPR article states, about 50% of biosolids end up being returned to farmland, likely due to the U.S. Composting Council’s program intended to provide biosolid compost to community gardens for free. And any solution to ever-growing waste generation besides putting it in landfills provides a welcome alternative. Flathead County’s Solid Waste Board reported in August that the district landfilled almost 26% more waste in July 2018 than it did in July 2017, with no sign of slowing.

However, sewage sludge composting is not without its critics. Some environmentalists believe that the relatively high concentrations of heavy metals, like lead, in sewage sludge result in a more dangerous compost product than those generated from agricultural waste. Others point to studies showing increased numbers of antibiotic-resistant bacteria in fields fed with sewage sludge-based compost as an indicator of dangerous contamination by pharmaceuticals and human pathogens. To date, using biosolid compost makes farmers ineligible for the USDA’s National Organic Program, barring them from marking their produce as certified organic. And some argue that having biosolids meet EPA regulations means little, given that the amount of contaminants allowed by the EPA is far greater than that allowed in European countries. For example, the EPA lists the maximum allowable concentrations of cadmium and mercury in sludge applied to farmland to be 85 mg/kg and 57 mg/kg respectively. The European Union, by contrast, allows a maximum of 20-40 mg/kg cadmium and 16-25 mg/kg mercury in sewage sludge used in agriculture. Therefore, though studies of sewage sludge usage at European farms have shown no negative effects, this could be attributed to Europe’s more stringent standards.

According to most scientists, the data is inconclusive. So far, there’s been no evidence to suggest that using sewage sludge to grow food results in threats to human health. Back in 2002, the National Academy of Sciences published a report stating that despite anecdotal evidence of people getting sick after exposure to biosolids, there was no concrete evidence that the biosolids were the cause. However, there’s one big reason that biosolids will continue to play a part in US waste management–we’re generating too much waste and we’re running out of places to put it. For smaller cities like Whitefish, the problem of sewage sludge can be avoided by simply spreading the sludge across a fallow field and letting it dry and turn to dust in the sun. For larger cities, the sheer amount of sludge produced prevents the use of such methods and requires more creative solutions. What’s certain is that sludge, in one way or another, will be a part of the Flathead’s future.