Connections: Back To Composting School

Sally Brown

Sally Brown
BioCycle September 2018, Vol. 59, No. 8, p. 63

Kids are going back to school — little kids with brand new lunchboxes and pencil sets, ready to learn the three “Rs”. Here I am referring to reading, writing and arithmetic, although Reduce, Reuse and Recycle are also critical to learn. A solid foundation in the basics means they are ready to progress to such advanced topics as trigonometry, linear algebra, calculus and Shakespeare.

For those of us who work with organics, we have three “Ts” instead of the three “Rs” — temperature, turning and time. Those three ‘“Ts” are the keys to making high quality composts. If you understand the basis for those letters, you will understand the critical processes required to make a quality product. That doesn’t mean that certain processes that modify or skip one of those letters won’t work for you. However, to know if it will or if it won’t, it is required to understand the reasons behind those letters. Just like those kids ready to tackle the three “Rs”, it is time for us composters, old timers as well as newbies, to review the basics.

The Three “Ts”

The first is temperature. Organics will break down over time at ambient temperatures if you let them. But the regulations governing municipal biosolids and food scraps (in many cases), and the National Organics Standards, require that your compost reach 55°C for a specified period of time. The primary reason that the standards require this is to kill any pathogens in the material. Pathogens come in all shapes and sizes and varying degrees of resistance to die off based on temperature. Most pathogens that we are concerned about are enteric, or used to living in a nice warm intestinal tract where the temperature is about 37°C. The regulatory value of 55°C is given as the number that relatively consistently kills them all when exposed to it for a sufficient amount of time.

Much of the testing to determine what temperature kills all the E. coli, salmonella, viruses, protozoa and helminths, has been done under controlled conditions. That is where the second “T”, turning, comes in. In a lab environment it is relatively easy to maintain constant temperature for a fixed time for a finite sample. Try that in a windrow in the winter and you will see it is not so simple. The turning requirements for windrow composting systems were designed to insure that all particles in the pile will be subject to the 55°C during the composting process. If you have a bunch of salmonella hanging out on the top of the pile where the temperature is a cozy and comfortable 39°C, they will not only live, but they may even prosper. By requiring multiple turns where the inside of the pile reaches and maintains 55°C, you are doing your best to make sure that every pathogen in that pile has had the opportunity to experience the conditions that killed its brethren under controlled conditions.

The final part of the “Ts”, time, comes into play here. You put any pathogen in extreme enough conditions and they will die in seconds. If the conditions are pretty damn hot but not boiling over, it can take several hours and even up to days for all of the critters to expire. The time portion of the three “Ts” is to make sure that the time required to kill the pathogens is well exceeded. A fairly recent review (Wichuk and McCartney, 2007) goes into more detail on the different pathogens, what the mechanisms are that render them harmless, and the different times and temperatures required to do so.

So, there you have the three “Ts”. If you are composting in a system other than a windrow, you can potentially reach the temperature and time required for pathogen kill without the turns. If you are not reaching the temperature you have to really extend your time to kill the pathogens. And even this is not guaranteed — certain high pathogen feedstocks are required to go through the time, turns and temperature for this exact reason.

Basics With Benefits

Now you know the basics of the three “Ts”. Destroying pathogens are the reason that these are required. It turns out that you get a bunch of additional benefits if you follow these basics. While pathogens are what can make people sick from improperly treated residuals, they are typically not what raises the general public’s concern — particularly if you are dealing with biosolids compost. These days, people who voice concerns about biosolids tend to be worried about things like pharmaceuticals and personal care products in the compost more than the pathogens.

It turns out that the three “Ts” help to degrade these as well. Here, it is not the temperature that degrades the compounds but the accelerated rate of microbial activity. The compounds are eaten, not burnt. Studies have found near complete mineralization of a range of antibiotics, phthalates and estrogen disrupting compounds in the compost pile. As with the pathogens, different compounds are more resistant to degradation than others and can require different amounts of time to decompose. However, for a substantial number of the compounds in this category, composting is a great way to reduce concentrations and concerns.

Finally, adhering to the three “Ts” gives you a better product. No one wants to grow a crop of noxious weeds with their lettuce. If you operate your compost pile to achieve full pathogen reduction you will also kill all weed seeds. Like pathogens, different weeds are more or less resistant to dying. There are almost as many types of weeds as there are pathogens and they all have different sensitivities. For example, a study tested the viability of 18 weed species in a cattle manure compost that was maintained with a core temperature of 55°C in Canada (Larney and Blackshaw., 2003). Some of the weeds tested were wimps, dead after 7 days at 39°C. Two others were tough guys — only dying after the piles reached 60°C. Another study showed that the temperature was also able to kill soil borne blight (Neher et al., 2016).

So not only does adhering to the three “Ts” keep the people who use the compost healthy, it keeps the plants growing in the compost healthy. This September, you may not need to get new notebooks and pencils, but it doesn’t hurt to make sure you have a solid foundation in composting basics. It may not get you a star from the teacher, but it will get you regulatory approval and happy customers.

Sally Brown is a Research Associate Professor at the University of Washington in Seattle ( and a member of BioCycle’s Editorial Board.

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