Determination of the relative and absolute humidity in Compost piles by differential pectroscopy using Tuneable diode lasers

The composting of biological waste is an important step towards the establishment of closed cycles of matter. A constantly high quality of the compost is, however, of utmost importance for the deliberate re-introduction of compost into the environment. The quality of the compost produced is determined by a very complex biological process during which the organic wastes are converted with the aid of bacteria, actinomycetes and fungi. The microbes require optimum conditions for the conversion of the organic mass, the temperature regime and oxygen supply to the microbes are playing an important role. The microbes are, however, unable to draw the oxygen directly from the air. Only oxygen dissolved in water can be taken up and assimilated by them. A sufficient humidity content of the biomass therefore is vital for the microbes` subsistence.

Any variation of a parameter from optimum conditions results in the microbial activity being limited accordingly. The conversion then is not optimal and the compost produced is not of the desired steady quality. This limits the utilisation, for example as peat substitute material in horticulture. Accordingly, optimum moistening of the biomass is a very important parameter of composting. However, the determination of this quantity still poses a serious problem today. All conventional methods of humidity determination are considerably limited in this field of application. Tests with known conventional humidity sensors have proved that there methods are too inaccurate, too slow or not practicable.

The present contribution describes a measuring instrument which allows the humidity of the organic material to be determined without influencing the composting process. The method for the determination of the water concentration is based on the absorption of electromagnetic radiation by the water molecules.