The use of respiration indices in the composting process: a review

Respiration is directly related to the metabolic activity of a microbial population. Micro-organisms respire at higher rates in the presence of large amounts of bioavailable organic matter while respiration rate is slower if this type of material is scarce. In the composting process respiration activity has become an important parameter for the determination of the stability of compost. It is also used for the monitoring of the composting process and it is considered an important factor for the estimation of the maturity of the material. A wide range of respirometric protocols has been reported based either on CO2 production, O2 uptake or release of heat. The most common methods are those based on O2 uptake. Respirometric assays are affected by a number of parameters including temperature, humidity, and both incubation and pre-incubation conditions. Results from respirometries are generally expressed as 'respiration indices', most of them with their own units and basis. In consequence, some confusion exists when referring and comparing respiration indices. This is particularly important because current and future legislations define and measure the biological stability of waste on the basis of respiration activity of the material. This paper discusses and compares most common respiration indices currently used.