Regulating Compost Moisture during Hot Composting

Hot composting is most effective at decomposing organic waste when moisture levels are approximately 50%.  A compost with excessive moisture will cause

  • Low temperatures that are below 40°C (104°F);
  • smells foul; and/or
  • brown liquid to be visible or easily freed when squeezing compost.

Excessive moisture while composting does not provide a viable environment for aerobic decomposers to thrive. Too much moisture reduces the oxygen supply to aerobic decomposers; as a result, these decomposers become less active, and compost temperatures remain low and stagnant.  Alternatively, excess moisture creates an inviting environment for the undesired anaerobic decomposers. Anaerobic decomposers are very slow at breaking down organic materials, and they create volatile acids that have a putrid smell. 

This article provides some tips on how to avoid excess moisture while using an Actium Batch Compost Drum for hot composting.

Prepping the composter

When starting a new compost batch in the Actium Compost Drum, first fill the vessel 1/4 full with dry shredded paper or dry sawdust/wood shavings.  Adding these dry and high carbon materials before other organic wastes provide a good start-up environment for aerobic decomposers, and helps the temperature to rise quickly for effective hot composting.

Consider the moisture content of organic inputs

Organic materials such as garden waste, food waste, and deadstock can all be composted; however each of these materials contains water.  For instance, vegetable peelings contain 70% of water, chicken deadstock contains approximated 67% water, and salad or juice pulp contain 90% water.  During the decomposition process, the cells of the “wet” organic materials break and release water into the greater compost pool.  With time and warm temperatures, much of the water will leave the compost through evaporation, and the wet organic materials will continue to decompose – reducing in weight and volume.  These reductions allow us to add organic waste continually over two to four months. The recommended loading rate for the Compost Drum is 50 lb (23 kg) per day.  The Compost Drum has a holding weight capacity of 2000 lb / 907 kg.  The weight capacity of the drum is not the same as the total weight of waste that can be added to the Drum.  The overall weight of waste added during a batch is much more than 2000 lb (close to double the capacity) due to the decomposing process. 

Have dry carbon and bulking agents available

It is a good habit to add dry carbon and/or bulking agents when adding “wet” organic materials to the Compost Drum. The dry carbon absorbs excess moisture, and the bulking agents improve oxygen supply. Dry carbon materials include shredded paper, shredded corrugated cardboard, wood shavings, and coconut husks.  When mixing a bulking agent with the compost, it alleviates compost compaction and allows for aeration. Examples of bulking agents include wood chips, grass clippings, autumn leaves, and chopped corn stalks.  Some bulking agents that contain little moisture can be used to aerate and absorb excess moisture, such as, peanut shells, sawdust, and straw.  As an example, if 10 lbs of food waste were added to the Compost Drum, it would be favourable to include 5 lbs of shredded paper and 2 lbs of wood chips. 

Remove excess  moisture from the composter

The Actium Batch Composter Drum is constructed to maintain high temperatures for hot composting.  The vessel of the Composter Drum has three vents (two along the sides and one on the input door) to allow for gases and water vapour to be released.  If the moisture content is too high due to insufficient additions of dry carbon materials, there will not be enough energy/heat for water to be evaporated and released through the vents as water vapour.   If the compost in the vessel becomes saturated with moisture (such as visible liquid in the vessel or liquid is easily freed when squeezing compost), then there are a couple operations that can be conduction to remove excess liquid from the vessel.

Operation 1

On a warm, dry, and sunny day,  open the input door of the drum. Turn the drum counter clock-wise until the input door is facing upwards or towards the sky.  This action allows for excess moisture to easily evaporate and escape from the vessel. Near the end of daylight hours, return to the composter and turn the vessel clock-wise to its standard position and close the input door.  Continue this operation until the moisture levels are at a point where adding dry carbon materials can correct the moisture content. 

Operation 2

Slide the input door very slightly (i.e. 1-2”) to make the door ajar.  Turn the vessel slowly clockwise. Stop turning the vessel when liquid is exiting from the input door.  Be cautious in preventing compost from pushing the ajar input door to the point that it will open.  Leave the vessel to drain for 6 to 48 hours – depending on the amount of excess moisture.  If interested in saving the liquid (a.k.a. compost tea or liquid fertilizer), collect the liquid using a large container that fits under the drum. When drainage is complete, turn the Composting Drum vessel counter clock-wise to the standard position, add dry carbon and bulking agent materials, then fully close the compost drum and turn the compost. 

The take-home message on compost moisture

Excess moisture hinders decomposition and can reduce the quality of the finished compost. Additionally, excess moisture causes unnecessary weight to the Compost Drum.  In extreme cases, when there is too much liquid in the Compost Drum, the weight can surpass capacity; this can lead to problems with the crank and chain.  Maintaining ideal moisture in the compost (approximately 50%) dedicates the volume and weight capacity of the Compost Drum to transforming up to 4500 lb of organic materials into nutrient-rich compost.

Composting Temperatures

Compost Thermometer

Hot composting:

In-vessel composters, such as the Compost Drum that is manufactured by Actium Composting Equipment, creates compost quickly by using the hot-composting (also known as active-composting) technique. The key to successful hot-composting is to create a composting environment that has ample oxygen, and achieve temperatures that reach 40°C – 65°C (104°F – 149°F). This environment promotes the activity of aerobic (oxygen-loving) microorganism that rapidly break-down organic matter with temperatures that most pathogens cannot survive. Alternatively, cold-composting (also known as passive-composting) technique utilizes different microorganisms that decompose organic materials at a significantly slower rate than hot composting. Cold-composting occurs when organic materials are added to a pile and left without frequent turning – mature compost using this technique can be ready between one to two years. Cold composting typically follows the day to day air temperatures – potentially reaching higher temperatures in the center of the pile. Both aerobic and anaerobic microorganisms exist within a cold compost pile. Anaerobic microorganisms are not as efficient at breaking down organic materials compared to aerobic microorganisms. Anaerobic microorganisms flourish where there is limited to no oxygen available. Low oxygen supply occurs when compost is not turned regularly and/or there is a high supply of moisture. This article details the benefits of using the hot-composting technique, along with how temperature influences decomposition.

Benefits and drawbacks of hot-composting:

The main benefits of hot-composting include:

  • producing mature compost quickly;
  • generating a greater amount of compost in less time than a cold-compost system;
  • reaching high temperatures (> 55°C, > 131°F) within the compost that can effectively kill most weed seeds and larvae;
  • reaching high temperatures (> 55°C, > 131°F) that kill most pathogens when composting; and
  • decomposing organic materials that are difficult to decay (i.e. bones and corn cobs) at a quicker rate than when cold-composted.

The main drawback of hot-composting compared to cold-composting is that monitoring and some maintenance is needed to warrant a favourable environment for rapid aerobic decomposition throughout the composting process; which includes:

  • diversifying and mixing different organic materials to ensure a carbon to nitrogen (C:N) ratio remains within the range of 20:1 to 30:1;
  • maintaining moisture levels in the range of 40-70%;
  • ensuring adequate oxygen levels (for example, turning the organic materials regularly (after materials are added or at least once a week)
  • monitoring compost temperature to make sure it stays within the range of 40°C – 65°C (104°F – 149°F), and to observe stages of decomposer activity; and
  • monitoring compost temperature to make sure it reaches > 55°C (131°F) for at least three days for the sterilization of weed seeds, larvae, and human pathogens.

Temperature ranges for composting:

Different types of microorganisms thrive at different compost temperature ranges. The three types discussed in this article are the psychrophiles, mesophiles, and thermophiles.

Psychrophiles:

Psychrophilic microorganisms live within the cool temperature range of -10°C to 21°C (14°F to 70°F), and they actively decompose organic materials at temperatures ranging from 13°C to 21°C (55°F to 70°F). Psychrophiles are utilized when cold composting, as they decompose material at ambient temperatures. In a hot-composting environment, psychrophiles will exist only briefly – during the beginning of Spring when air temperatures become warmer, or when the very first materials are added to start composting. When the psychrophiles decompose organic materials, the
microorganisms will release heat, raising the temperature of the compost. When the compost temperature surpasses 21°C (70°F), it is no longer a viable environment for the psychrophiles, and the mesophiles begin to populate the compost. When temperatures get too warm for the psychrophiles, they either become dormant, die-off, or move to a section of the pile that is cooler.

Mesophiles:

Mesophiles will inhabit both cold-compost and hot-compost types. They live within environments that have moderately warm temperatures, ranging from 20°C to 45°C (68°F to 113°F). They are most active at decomposing materials when temperatures are between 21°C to 32°C (70°F to 90°F). Though mesophilic decomposers thrive in this temperature range, so do human pathogens (such as Escherichia coli, Staphylococcus aureus, Listeria monocytogenes, and Salmonella sp). For these pathogens to die-off, temperatures need to increase to a point where they cannot survive.

Thermophiles:

The decomposing activity of mesophiles heat-up the compost to the point that surpasses their habitable range, and then thermophiles take over. Thermophiles can live in temperatures that destroy most pathogens. The temperature range for thermophiles is 40°C to 71°C (104°F to 160°F), and the temperature for pathogen sterilization is > 55°C (> 131°F). At high temperatures, both sterilization and decomposition are occurring. Thermophiles are very efficient decomposers − they accelerate the breakdown of complex carbohydrates, fats and proteins, and convert organics into humus – a complex material that is nutrient-rich. In a cold-compost, thermophiles are less likely to be present, but if they are, it will be most likely at the center of the pile. For hot-composting, fostering thermophiles is key to killing pathogens and decomposing organic matter rapidly. Having the ability to turn a batch or in-vessel composter is beneficial for hot composting because:

  • turning aerates the compost, providing oxygen supply to the aerobic thermophiles;
  • mixing organic matter allows thermophiles to have greater access to materials in need of sterilization or further decomposition, and;
  • turning permits equal temperature distribution, ensuring all compost materials have been sterilized and underwent thermophilic decomposition.

Achieving high temperatures is crucial for some compost operations for them to meet specific quality standards. For instance, to ensure pathogen removal, the Ontario Compost Quality Standards require temperatures to be monitored daily until > 55°C (131°F) occur for at least three consecutive days when composting most organic materials. In-vessel composters by Actium Composting Equipment are designed to help sustain temperatures where thermophiles can
thrive, and sterilization can occur; they are made with an insulated outer-layer to minimize heat escape and an optional electrical heater for maintaining a warm compost environment when operated in colder climates. As well, long-stem thermometers (Reotemp Instruments, USA) come with Actium Composters to measure compost temperatures, and to monitor when sterilizing has occurred.

Compost curing:

After the thermophiles have completed their cycle, the compost gradually cools down, and the mesophilic and psychrophilic microorganisms return. The decomposing process slows as less material is available for microorganisms to
consume. The compost is not yet complete or ready to be added to the garden. At this later stage, the compost is stabilizing as it cures. Temperatures will continue to decrease until it is near ambient temperatures, and decomposing microbial activity will eventually stop. The curing process can last a minimum of 21 days. When composting using a drum or vessel, the compost can be removed during the curing stage and placed in a secondary compost area or bin, protected from the rain.

Essentials for a Healthy Compost

Compost Food Scraps

Macro- and micro-organisms are the workers that alter and decompose those rotting vegetables and raked leaves, into the earth smelling, dark brown crumbly substance that we call compost.  The quality of the finished compost depends on the environment we create for the organisms that do the decomposing business (a.k.a decomposers).  Just like ideal environments for many living species,  compost should provide an environment with sufficient food, temperature, water, and air.  This article briefly introduces each of these environment essentials needed for a healthy compost. 

Food:

The organic materials we add to compost can have a wide range of nutrients and minerals.  These nutritious components are why compost is considered a natural fertilizer for gardens and fields.  A healthy diet for decomposers consists of a mix of materials that contain carbon and nitrogen.  Materials such as dry fall leaves, straw, wood shavings, pine needles, twigs, newsprint, and shredded paper are high carbon sources.  Materials that have a lot of carbon are also considered “browns” in composting terminology.  These high carbon or “browns” are carbohydrate-rich and provide the energy for the decomposers.  Food scraps, livestock mortalities ( in the case of farm animal operations), grass clippings, egg shells, and coffee grounds are materials that are considered “greens” in composting terminology. These green materials contain high amounts of nitrogen, which provides protein for decomposers and helps these organisms multiply.  Both types of materials – greens and browns – should be layered and mixed to build the decomposer community and allow them to continually turn your organic waste into compost.  For instance, 1 lb of vegetable waste should be combined with at least 1 lb of newsprint.

Temperature:

With a good mix of materials and sufficient water and air, compost temperature should increase quickly.  The process of organisms decomposing materials causes the temperature of compost to rise quickly (within a few days).  Compost temperature while decomposing can range from 13°C to 71°C.  Throughout the composting process, different microbial communities take over decomposing materials, and each community prefers a different temperature range.  The ideal temperature range for efficient composting is between 32-55°C.  Below 32°C, the process of decomposition occurs but is slow.  Having compost reaches 55°C or above, the majority of microorganisms cannot survive. However, compost reaching 55°C or above for at least three days aids in sterilizing weed seeds and destroy pathogens.  

Water:

Decomposers prefer compost to have between 40% – 60% moisture.  If the compost is too dry, decomposition will slow down.  If the compost is too dry, water may need to be added or materials with a lot of moisture. More often, composts can end up being too wet. Composts with greater than 60% moisture create an anaerobic environment (low oxygen) that is inviting for undesirable micro-organisms that release bad smells in the compost.  Adding very dry “browns” to the compost mixture helps prevent the compost from becoming too wet.

Air

Aerobic decomposers (oxygen-loving organisms) are preferred for composting.  Keeping within the ideal moisture range, and turning the compost will ensure there is enough oxygen for the decomposers. Turning a batch composter every time materials are added, or once a week will help keep the compost aerated.  Furthermore, turning helps break up materials, and improves decomposers access to recently added materials. 

Managing these four factors – food, temperature, moisture, and air – are key to creating quality compost in a timely manner. Composting is not a complicated process, but these four factors should be considered if your compost starts to smell bad or if the decomposition process is slow or stagnant.