Composting: Process Inhibition and Key Performance Indicators

Blog ECS Staff 01/04/2019

ECS focuses on controlling the process conditions to facilitate efficient composting. This article outlines some of the key performance indicators (KPI’s). When operators manage these KPI’s well, systems tend to operate well. In contrast, poorly managed KPI’s often result in inefficient and malordorous systems.

ECS and other industry experts have identified the following KPI’s for maintaining efficient aerobic composting.


Increasing decomposition rate from 0C up to 60C with reduction in decomposition rate beyond 65-70C (due to reduced oxygen solubility in water and other factors like ammonia inhibition etc). Severe rate limitation beyond 80C.

A proper aeration design is needed to ensure >90% of the compost on a pad is uniformly temperature controlled, and ideally controlled dynamically with temperature feedback, preventing overheating and over cooling.


Oxygen and temperature are in a tight relationship. A good reference table is linked below. Aerobic processes move much faster than anaerobic processes (which smell bad). Anaerobic decomposition prevails below 2-3 ppm oxygen at the biofilm of decomposing waste.

At 60C and 5% oxygen measured in the pile with a probe, the process is largely anaerobic and well inhibited (~1ppm O2 – see link below). This is a very common place for windows or generic ASPs to operate at. This is also a fully inhibited, slow moving and smelly place to operate (fairy dust or not)

ECS designs all systems to operate with 3-6ppm O2 for >90% of the primary and secondary processing time. This requires a skookum (Canadian slang for excellent) aeration delivery and aeration floor system). This is the #1 way to ensure the fastest decomposition (whilst paying attention to a BMP mix and all the other potential rate inhibitor possibilities)


Dramatic inhibition of decomposition rate when pH is <6 (acidic feedstocks) and temperatures are >45C. Mesophiles work well on acid feedstocks, but thermophiles do not. Major effort needed here in facility design and operation to avoid this rate limitation zone (which is also a very smelly place to be).

No inhibition on decomposition rate when pH is >6, good zone for thermophiles (and thermophilic temperatures)

This effect is published by Sundberg, a Swedish researcher who did her PhD on this topic. It is easy to web search her publications.

To get pH out of acidic zone, must aerate hard at the beginning, or add a basic conditioner (like ash, digestate, or otherwise).


Excess ammonia can inhibit decomposition in the range of published values from 2000-4000 mg/kg.

It is difficult to measure free ammonia (as opposed to ionized NH4+), so if you can smell ammonia when shoveling / driving loader, know that you’ve got to much ammonia, and its impacting your decomposition rate. The hotter your process is (and higher in pH like 9) the more impact NH3 has on inhibiting your compost.


A majority of windrow composting facilities seem to have been water deficient resulting in excessively long retention times to reach stability. Its hard to add enough water to the surface of a steep sided windrow and have it penetrate. Its hard to carry enough water on the back of a windrow turner to make more than a squirrel’s flatulence of a difference. The only really good way to get enough water into a windrow in a hot and dry climate is to hook up a long water reel hose to the turner, have a huge pond of pathogen free water, drive slowly, and figure out how to not get the hose tangled. This was figured out at Kelowna’s Glenmore Landfill.

<40% moisture and process pretty much grinds to a halt (not going to smell, but not decomposing).

>60% and its likely to be anaerobic (and very slow and stinky)

Heavy Metals

Heavy metals and other chemical inhibitors are rarely high enough in concentration to impact rate.

In summary, ECS advises clients to focus on managing potential inhibiting variables. Measuring and managing these key performance indicators can help sites maintain efficient compost operations.

Reference Table Link:

Oxygen solubility in compost