Monitoring of the Respiration Activity of Sewage Sludge and Biofractions of Municipal Waste in the Composting Process

Introduction: Rational waste sludge disposal means utilizing its fertilizing and calorific properties to their maximum. The ability to convert sewage sludge depends on many factors. Fermented or aerobically stabilized sludge contains components easily assimilable by plants, but its utility in agriculture or gardening is limited. However, stabilization of sewage sludge in the composting process ensures a beneficial modification of its sanitary and epidemiological condition, as well as significantly improving the organoleptic properties, such as odor, color, and granulation; this stabilization also influences the fertilizing value. Before utilizing the compost created from sewage sludge or other bio-waste, it is necessary to check the degree of its stabilization by evaluating the biological activity. One of the methods of evaluating the biological activity of compost is the respiration test, e.g. using the AT4 test (Static Respiration Index). Literature data of respirometric tests show the availability of substrates for microorganisms; in other words, the susceptibility of a substrate to further biodegradation.

Aims: This article analyzes the results of biological activity tests (AT4) of compost from installations for the aerobic stabilization of sewage sludge and biofractions of municipal waste. The aim of the study was the observation of changes to the degree of stabilization of substrates, during a period of 10 weeks of aerobic processing.

Methodology: The studies on a technical scale were conducted at the mechanical and biological processing of municipal waste (MBP) and selectively gathered green waste and bio-waste processing (composting) installation. The fermented sewage sludge was composted together with the organic substrate acquired from municipal waste. The process was conducted during a period of 10 weeks. Every 7 days samples were taken from the aerated heap in order to determine the moisture, ignition loss, and AT4 parameter. The measurements of the compost’s biological activity were conducted using an OxiTop® Control measurement system.

Results: The mixture of municipal waste biofractions and fermented sewage sludge that made up the heaps turned for the composting process was characterized by the following parameters: moisture – 61%, organic substance content – 58% of dry matter, AT4 – 64.5 gO2/kg of dry matter. Continued monitoring of temperature changes (average temperature 39°C/61-67°C/55-35°C respectively) in the heaps confirmed the correct course of the composting process in three phases. After 10 weeks of the process a reduction of organic substances by 54% was achieved, while the value of the AT4 parameter in the stabilized compost was 4.9 gO2/kg of dry matter.

Conclusion: The study results show that changes to the content of organic substances in waste subjected to the composting process are directly proportional to changes of the AT4 parameter, which characterizes the respiration activity of substrates. This correlation can be fairly accurately described by a linear equation (R2=0.93).