The effect of composting process on changes in quantity of yeast and mould fungi population

Osady ściekowe są nieodłącznym produktem ubocznym procesów zachodzących w trakcie mechanicznego, chemicznego i biologicznego oczyszczania ścieków. Zgodnie z obowiązującym prawem zaliczane są do jednej z kategorii odpadów [1, 3]. Skład osadów ściekowych zależy od rodzaju oczyszczanych ścieków oraz stosowanych procesów ich oczyszczania [3]. Osady ściekowe charakteryzują się korzystnym składem chemicznym, stąd mogą odgrywać znaczną rolę w procesach glebotwórczych. W związku z powyższym celowe jest ich wykorzystanie w rolnictwie jako nawozu organicznego dostarczającego, podobnie jak pomiot kurzy, obornik, czy gnojowica, cennych i niezbędnych związków zarówno dla roślin, jak i drobnoustrojów zymogennych [14]. Czynnikami limitującymi rolnicze wykorzystanie osadów ściekowych jest zawartość metali ciężkich, szkodliwych związków organicznych, chorobotwórczych drobnoustrojów oraz konsystencja [21]. Osady zawierające dopuszczalne ilości niepożądanych związków chemicznych mogą zostać poddane obróbce biologicznej, w wyniku której zostaną przekształcone w nawóz spełniający wymagania agrotechniczne [21]. Jednym z najstarszych, a zarazem najbardziej powszechnych, sposobów zagospodarowania osadów ściekowych jest ich kompostowanie z dodatkiem materiału roślinnego, np. słomy.Sewage sludge is a valuable fertilizer, but its agricultural usage may be difficult because of the presence of pathogenic bacteria, viruses and fungi. Fungi present in the sewage sludge may cause fungal infections, allergic reactions and mycotoxin poisoning. Accordingly, the hygienisation of sewage sludge is necessary prior to their application on fields. One of the oldest and most widely used methods is composting of sludge with the addition of the plant material. The aim of the study was to investigate quantitative changes in populations of yeast and mould fungi in composted sewage sludge from municipal sewage treatment plants. The research material was digested sewage sludge from municipal sew-age treatment plant. The experience was carried out in a technical scale. Before starting the composting process, the sewage sludge was mixed with straw and sawdust in proportions of 1.0: 0.7: 0.3. The material prepared this way, was piled in a hall and periodically mechanically mixed for aeration. The experiment was conducted in three research cycles - cycle I (autumn-winter season), cycle II (winter season) and cycle III (spring season). Samples for testing were taken at each cycle from bottom, middle and top part of pile after 7, 16, 20 and 45 days counting from the date of its construction. Three replications were made for each location and each sampling term. Each time the measurement of temperature, humidity and pH in particular parts of the pile was made. Fungi were counted with the plate method using Sabouraud agar with 4% dextrose and Sabouraud agar with chloramphenicol. Yeast were counted after 3 days of incu-bation and mould fungi were counted after 7 days of incubation at 25°C. Obtained results were statistically analyzed with SAS 9.2 PL. Measured temperatures were different depending on the research cycle and part of pile. In autumn-winter season (cycle I) temperature was 22.5-49.4°C, in winter season (cycle II) it ranged from 24.5 to 47.7°C and in spring season (cycle III) it was on the level of 35.3-55.4°C. In cycle I and II the highest temperatures in all sampling terms were noted in top part of pile and in cycle III in the middle part. The pH of the tested material was slightly alkaline, and its value ranged from 7.7 to 8.9. In all cycles and parts of pile pH tended to in-crease, which last up to 20th day of composting. The humidity of pile ranged from 63.9 to 78.0% and was slightly higher than the optimum provided for the composting process. The number of yeast at 7thday of composting ranged from106 to 107 cfu/g and at 45th day was lower and shaped on the level of 102-106 cfu/g. The lowest decrease of these fungi was noted in cycle I, and the highest one in cycle III. In all cycles and sampling terms the most of yeast occurred in the bottom part of pile. The greatest number of mould fungi was also noted in the bottom part of pile. The number of these fungi was on the level of 105-107 cfu/g at 7th day of composting, and it decreased to 102-104 cfu/g at 45th. In cycle I and II, mold fungi were not isolated in the middle part of the pile and in the top part starting from 16th day. In cycle III the decline in the number of these fungi was at the level of 2-4 log. Positive correlation was found between the number of yest and humidi-ty of prism and a negative correlation between the number of mould fungi and the pH of the tested material. Concluding it can be stated that during the research, in most cases, there was no full hygienization effect in composted sewage sludge