Everything you should know in the composting and waste treatment business.

Respiration activity after 4 days (AT4)


Respiration activity is a parameter for MBT plants used to determine the biological stability/activity of waste. The limit values must be met in order for the waste to be disposed of in a landfill after the stabilization process. It is measured over a period of 4 or 28 days.



Pre-treatment is the treatment of the input material before the actual composting process in order to obtain optimum decomposition. Examples of this include the separation of impurities, homogenization with or without crushing of the materials, and the addition of nutrients or aggregate material.



A Bioaerosol is an air mixture of solid and liquid particles that contain living organisms. The living organisms are tiny microorganisms, such as fungal spores, bacteria, and viruses that live on dust or in water vapor, for example. They are distributed in the environment through the air.

Biochemical oxygen demand (BOD5)


Biochemical oxygen demand (BOD5, also referred to as biological oxygen demand) is a sum parameter for the contamination of sewage with organic substances which can be degraded biologically within 5 days. The BOD shows the amount of oxygen [mg/l] necessary for the biological degradation of carbon and nitrogen compounds under certain conditions within 5 days.



A biofilter is a relatively simple and inexpensive technology for cleaning exhaust air that contains odorous substances. In the process, microorganisms break down the pollutants and odorous substances contained in the exhaust air into harmless products, such as CO2 and water. The requirement for a good cleaning performance is to maintain optimum living conditions for the microorganisms.

C/N ratio


The C/N ratio is defined as the ratio of carbon and nitrogen. Substrates are mixed together until the optimum ratio of 25:1 and 35:1 is achieved. If it is outside the optimal conditions, increased emissions, a slower start of the process, loss of nutrients, etc. will result.

Chemical Oxygen Demand (COD)


Chemical oxygen demand (COD) is defined as a sum parameter for the contamination of sewage with organic substances. COD measures the amount of oxygen [mg/l] required for chemical oxidation of organic substances in the sewage.



In some countries, it is allowed to use the fine fraction (< 15 mm) of the stabilized waste as landfill greening as long as the foreign particles do not exceed a maximum limit value. This fraction is referred to as CLO ("Compost Like Output").

Landfill criteria


The output material of an MBT plant must fulfill the landfill criteria of the European Landfill Directive 1999/31/EC to be allowed to dispose the treated waste in a landfill. These parameters are specific for each country and may include:

  • Respiration activity of the material (AT4 and/or GB21)
  • Maximum calorific value
  • TOC (Total Organic Carbon)
  • DOC (Diluted Organic Carbon)



In waste management, pollutants may be released from the waste by water. These may take the form of a liquid, paste or slurry and are referred to as eluate.



Emission is the release of disturbing factors into the environment. These can be of natural or anthropological (man-made) origin. There are different types of emissions:

  • Gaseous emissions (air pollutants, bioaerosols)
  • Liquid emissions (waste water, leachate)
  • Dust emissions
  • Noise emissions

Fresh mass (FM)


Fresh mass is the weighed mass of a material. It is composed of water mass and dry matter (DM).

FM = DM + water mass

Gas formation rate in 21 days (GB21)


The gas formation rate (GB21) refers to the rate of formation of biogas under anaerobic conditions within a minimum of 21 days. Besides the determination of respiration activity (AT4), it is a parameter for the stability of waste. However, its measurement requires an extended period of time.

Loss on ignition


Loss on ignition (also referred to as organic dry matter) indicates the mass loss that results from incineration at 550°C. It contains the organic compounds that can be biologically degraded.



The term "sanitization" refers to the killing of pathogenic bacteria and the inactivation of plant seeds. Sufficient process heat is released during the composting process, allowing sanitization to take place with an adequate temperature-time regime.



Composting (also referred to as rotting) refers to the biological process of nutrient cycling in which organic waste is degraded by bacteria and fungi (microorganisms) under the influence of atmospheric oxygen (aerobic). In addition to carbon dioxide, this process also releases water-soluble minerals, e.g. nitrates, ammonium salts, phosphates, and potassium and magnesium compounds, which act as fertilizers. Some of the intermediate products produced during this decomposition process are transformed into humus.

The following conditions are important for the composting process:

  • Water content 40-60%
  • Material density max. 650 kg/m³
  • C/N ratio 30:1

Compost windrow


A compost windrow (or windrow in short) is an elongated heap of compost in which biodegradable (organic) materials in the correct composition undergo an aerobic degradation process. There are different types of compost windrows:

  • Triangle-shaped windrows (most common)
  • Trapezoid windrows
  • Table windrows

MBT plant


An MBT (mechanical-biological waste treatment) plant is a plant consisting of a mechanical and a biological treatment process.

The mechanical treatment takes place before or after the biological treatment process and may include the following steps:

  • Conditioning of the waste for biological treatment
  • Sorting/screening of the waste into different fractions
  • Separation of valuable materials from the waste (recycling)

The purpose of an MBT plant is to

  • Stabilization of the waste to produce a landfillable material according to the European Landfill Directive
  • Reduction of climate relevant emissions (greenhouse gases, leachate, ...)
  • Better utilization of the landfill volume

Organic dry matter (oDM)


Organic dry matter is the proportion of the dry matter that contains organic compounds and can therefore be biologically degraded by microorganisms. It is determined by incinerating the sample at 550°C (the mass loss (loss on ignition) corresponds to the oDM mass).

oDM = mass loss on ignition / mass DM

Pore volume


The pore volume is the ratio of the available cavity to the total volume of a fill/windrow. It can be filled with either air or water, though water ALWAYS prevails. For this reason, care must be taken to maintain an appropriate water content.

Pore volume = cavity ratio / total volume

Quality assurance


Quality assurance refers to all measures taken to ensure consistent product quality. In the process, the characteristics of a product / process are checked. In case of deviations, corrective measures can be implemented. Quality assurance can either be carried out by the company itself or by an external body.



Recultivation is the production of a soil layer on which plants can grow. A rootable soil layer that is beneficial for plant growth is placed on top of the subsoil. Areas of application for recultivation include landfill revegetation of filled landfills or lawns along roads.

Residual waste


Household waste and household-type industrial waste collected via garbage collection.



Leachate is the excess water that is not absorbed by the compost windrow and either seeps through or runs off along the slope of the material. Depending on the type of plant, leachate can have various origins:

  • Precipitation (with open plants)
  • Irrigation water
  • Condensate with negatively aerated windrows

Feeding the leachate back into the composting process can significantly reduce the leachate volumes.

Dry matter


The dry matter is the proportion of fresh mass that remains after the drying process. It is measured at 105°C until the mass no longer changes (constant mass).

Dry matter = wet mass - water mass [m]

Dry matter = dry mass / wet mass [%]



Turning is the intensive mixing of the compost material with the help of a turning machine. The purpose of turning is to allow

  • material from the peripheral zones to enter the hot windrow core
  • the material to loosen
  • air to enter the material
  • gases stored in the windrow to escape



Scrubbers are used to separate solid and/or gaseous substances from the exhaust air. Scrubbing water is distributed in the scrubbing tower via nozzles, whereby the substance to be removed is transferred from the exhaust air to the liquid medium.

There are different types of scrubbers:

  • Acid scrubbers: pH of the scrubbing water < 7
  • Neutral scrubbers: pH value of the scrubbing water is equal to 7
  • Alkaline scrubbers: pH value of the scrubbing water > 7

Water content (WC)


The water content is the amount of water contained in a mass. For composting, the water content should lie between 40% and 60% [m/m].

DM + WC = 100%

WC = water mass / fresh mass * 100 [%]