Composting

Composting process monitoring and compost maturity determination

Compost and composting

Composting is a process of biological aerobic decomposition of organic waste by microorganisms. This transformation of organic matter (wastewater treatment sludge, brushwood, household waste) produces a soils enrichment product rich in humic acids called compost.
If composting is a natural process, spontaneously taking place in environment, it must be optimised in order to industrially produce compost.

Composting: a complex natural process

In order to produce compost from organic matter, oxygen, humidity, and a nitrogen/carbon ratio adapted to microbial growth are needed. When all these parameters are properly settled, the aerated and moist organic matter (adapted porosity, wastes reversal, aeration systems) is decomposed by the aerobic microorganisms until compost has been produced. Two consecutive steps take place during this process:

1. A fermentation phase during which, organic matter easily degradable by microorganisms is decomposed. During this phase, mesophile microorganisms respiration increases the temperature up to 40 – 45°C. Mesophile microorganisms are progressively replaced by thermopile microorganisms better adapted to high temperatures. Their respiration increases the compost temperature up to 60 – 70°C or even more. Reaching this high temperature enables to kill pathogenic microorganisms in the compost.
2. After to this fermentation phase, most of the easily degradable organic matter has already been decomposed and so the production of humic acids is now taking place. During this longer phase, the compost temperature slowly decreases down to room temperature. When compost is finally mature it can be spread on cultivable soils.

As long as compost is not mature, it is toxic for plants and can not be used as soils enrichment. It is, then, of first necessity, to know about the compost maturity. In order to determine whether compost is ready or not, several techniques, more or less time consuming and complex, exist:

1. Empiric methods consisting in sensorial observations of the compost’s characteristics (smell, colour, consistence, fauna). Even if this method seems easy to apply, it needs experience.
2. Monitoring physic parameters such as temperature, spectrometric characteristics or oxygen consumption. These methods are not always reliable and are complex as well as expensive.
3. Chemical methods such as pH-metry, sulphides titration, chrome titration, carbon/nitrogen ratio measurement, COD measurement, etc. However, these methods are not perfect and are difficult to apply.
4. Tests on plants to check the compost compatibility with plants such as the seeding test. Even if these tests are representative of the compost toxicity, they are vague and very time consuming.

Most of all, none of these tests consider the biology of compost as composting is a biological process using microorganisms. Quantitative ATP-metry QG21W™ kit is the only test enabling easy and fast (5 minutes) measurement of all the living microorganisms and of their stress (mortality) during the composting process.

The QG21W kit

Adenosine Triphosphate (ATP) is the energy carrier in any biological cell. In particular, ATP is present in all living microorganisms. The QG21W kit, based on quantitative ATP-metry, uses a reaction of bioluminescence to quantify ATP.

This kit enables to measure intra-cellular ATP (cATP™) corresponding to the ATP within living microorganisms and which is a direct indication of active biomass concentration in compost. Intra-cellular ATP is calculated as the difference of total ATP (tATP™) and dissolved ATP (dATP™): cATP = tATP - dATP.

Moreover, QG21W also enables to calculate the Biomass Stress Index – BSI™ – representing biomass health and mortality. The BSI is defined as the ratio of dATP to tATP: BSI (%) = dATP/tATP.

Use of QG21W kit for compost monitoring

Using QG21W kit, monitoring and controlling directly biological activity of microorganisms responsible of compost production is now possible. Studies show that active biomass monitoring via ATP-metry enables to measure evolution of microorganisms concentration within the compost in order to optimize composting process. Using QG21W kit, quantitative measure of ATP gives a direct indication of microorganisms’ health and activity in the composting process. ATP measure is indicative of composting process and can be used for compost monitoring.
Measures carried out by Aqua-tools clearly show a cATP decrease progressively as the compost matures. A mature compost is characterized by a higher BSI. These results are similar to others studies on this subject.
Consequently, using QG21W kit, follow-up of intra-cellular ATP and BSI is a simple, quick and direct way to control composting process and to evaluate compost maturity.

So, measures carried out with QG21W kit enable to follow active biomass activity into compost in order to make sure:

1. the degradation process goes smoothly via cATP parameter,
2. the aeration is homogeneous by checking whether microorganisms, and so cATP, are uniformly distributed within compost mass,
3. no toxic is present which can lead to decreasing microorganisms activity via BSI parameter,
4. the composting process is optimized.

QG21W kit also enables to control compost maturity by observing:

1. the decrease of active biomass concentration during maturation,
2. and in parallel, a cellular death increase.