Top Photo: Courtesy of Adobe Express

Sally Brown

Yes! Compost makes a wonderful soil conditioner. It can bring dead soils back to life, increase yields and conserve water. The composting process itself is also pretty amazing, turning slime into sweet smelling soil. If it is a commercial process, it can also melt bones, making that final product rich in calcium and phosphate. 

You wouldn’t stick your hand into smelly food waste without elbow length gloves or a strong financial incentive, but you would be happy to go up to your elbows in finished compost. It is no wonder that people are happy to add compost to their soil and that many municipal and community composters rely on this process as a way to convert wastes into resources.

Public health officials also love compost. While it may not seem that way when you are trying to get a permit to open a site, these officials rely on the composting process to protect public health. In addition to the yuck factor associated with food scraps and the exponentially greater yuck factor associated with human waste, both materials can be a home for pathogens. The composting process, through a combination of heat and time reliably destroys those pathogens and takes away any public health concerns related to untreated wastes. No industrial process is required, just time, air and bugs.

Pathogen Destruction Refresher

Most of the pathogens that we worry about are very comfortable in our guts (see Box). Low oxygen, high humidity and nice and warm (kind of like Baltimore in the summer) is what makes them happy. Put them in a compost pile where there are millions of other bugs – including those that like oxygen and real heat – and those pathogenic microbes are literally and figuratively cooked.  Thermophilic conditions are required to reach pathogen destruction in commercial composting facilities. That means 15 days at or above 55°C/131°F with a minimum of 5 turns for windrow systems to make sure that even any E. coli on the edge of the pile are cooked. 

This is a U.S. EPA regulated process and is referred to as the Process to Further Reduce Pathogens (PFRP). These processes were initially accepted for treatment of biosolids but have been adopted for compost that contains food scraps. EPA recognizes aerated static piles and turned windrows as acceptable composting methods to reach pathogen reduction standards.

Vermicomposting

Then there is vermicompost. Within the composting world, using worms to do the work has almost a cult status. Expressed in terms of chocolate, vermicompost is the finest Belgian whereas your standard static pile comes in well under Nestle’s. Worm castings or worm poop, which is really what vermicompost is, is an excellent source of both nutrients and structure for soils. In one study I did, plants grown in vermicompost outyielded every other compost including my beloved Tagro biosolids product (Una et al., 2022). Worms are magical creatures but they don’t reach temperatures to assure PFRP. At the recent US Composting Council conference, one talk expressed concern that small-scale composting, here referring to vermicomposting, did not provide adequate pathogen destruction. 

Worms destroy pathogens but they do it without heat. When a worm eats, the first step is grinding of the food. This increases the surface area and makes it easier to digest. Who knew that worms have teeth (or their equivalent)? I for one have never been bitten by a worm. The worm then excretes enzymes that aid in decomposition. Worms are very generous hosts and their guts are also home to a range of microbes. These microbes also exude enzymes that further decompose the food. A portion of the carbon in the food is mineralized and/or assimilated at this point in the process. 

The next stage is the actual poop or release of the casts. The casts themselves are rich in microbes that continue the degradation process. In a certain sense, the same way that composting has two phases (thermophilic and then curing or mesophilic), vermicomposting also has two phases. The first occurrs within the worm and the second within the earthworm casts.  This all makes for a great soil amendment but what does it do for the pathogens? 

How Worms Meet PFRP

The pathogens that can make us sick are just another food source to the worms and their microbial house guests.  Multiple studies have shown complete (or nearly so) pathogen kill with vermicomposting.  A review paper published in 2018 summarized the data on pathogen kill across different types of wastes (Swati and Hait, 2018).  Here is a summary of the summary:

• Total coliforms: 85-100% destruction
• Fecal coliforms: 93-100% destruction
• E Coli: 97.6-100% destruction
• Salmonella: 85.7-100%
• Helminth: 75-100%
• Fecal enterococci: 99.7%

 The precise way that earthworms kill these pathogens apart from “they eat them” is not fully understood. Enzymatic activity, preferential treatment for invited guests (endosymbiotic microbes), assimilation and competition against/destruction of by the endosymbiotic microbes are all seen as the ways that pathogens meet their end during vermicomposting. Worms also produce antibiotics that kill a range of bacteria. Those are the mechanisms inside the gut itself.  Secondary treatment occurs in the casts and in the wastes. The casts are coated in mucus – the same mucus that the worms produce to coat themselves. This is food for microbes that provide a second line of attack.  Finally, the burrowing action of the earthworms in the waste provides increased aeration which also helps kill pathogens. 

EPA recognizes vermicomposting for biosolids as a finishing step after pathogens are killed using time and temperature standards. They don’t have the faith in the worms to trust them with the whole process. Understand here that fecal material has been a public health threat for over a century. In areas where centralized treatment is a thing of dreams rather than an operating reality, pathogen loads in fecal material can be very high. In these places, vermicomposting can offer a potential low cost and relatively low infrastructure treatment option that will kill most if not all pathogens.

What About Food Scraps?

Is vermicomposting safe for food scraps?  Food scraps can contain pathogens and can also be of concern as an attractive nuisance. You may not think that they smell good but you also probably don’t find rats and cockroaches appealing. The quantity of pathogens in food scraps is almost certainly lower than that found in partially treated sewage. Remember that the vast majority of pathogens originate in fecal material and that fecal contamination of food scraps is not (ideally) a common occurrence. 

Here are some ranges for comparison. Populations are typically expressed as Most Probable Number or MPN per gram. 

• In mixed piles of food scraps, manure and bulking agents, initial concentrations of fecal coliform were 400-450  MPN per dry g of feedstock (Cekmecelioglu et al., 2005).
• In human feces this number is 13 million MPN per g.  For pig poop that number is 3.3 million (Metcalf & Eddy, 1995).  Understand that the material from water reclamation facilities in the U.S. that could be finished with vermicomposting likely will have been through several treatment stages with much lower fecal coliform concentrations.

Commercial-scale vermicomposting facilities have regulatory oversight. They have to test for pathogens and meet regulatory limits. If they don’t meet PFRP they can’t sell their castings. Community scale (or backyard scale) is a different story. At this smaller scale composters are less likely to test the final product, may not have room to store the compost for additional curing, and may not operate as carefully as a commercial facility that has regulatory oversight. Despite those factors, the likely low pathogen populations going in and the power of the worms are strong indications that the finished product will likely be as safe as it is beautiful. If you have any doubt, just let them sit in a pile or in the soil for a while.  No reason to go back to Nestles if Belgian is an option. 

 Sally Brown, BioCycle Senior Advisor, is a Research Professor at the University of Washington in the College of the Environment.