The essence of the methane fermentation course is the phase nature of changes taking place during the process. The biodegradation degree of sewage sludge is determined by the effectiveness of the hydrolysis phase. Excess sludge, in the form of a flocculent suspension of microorganisms, subjected to the methane fermentation process show limited susceptibility to the biodegradation. Excess sludge is characterized by a significant content of volatile suspended solids equal about 65 ÷ 75%. Promising technological solution in terms of increasing the efficiency of fermentation process is the application of thermal modification of sludge with the use of dry ice. As a result of excess sludge disintegration by dry ice, denaturation of microbial cells with a mechanical support occurs. The crystallization process takes place and microorganisms of excess sludge undergo the so-called “thermal shock”. The aim of the study was to determine the effect of dry ice disintegration on the course of the methane fermentation process of the modified excess sludge. In the case of dry ice modification reagent in a granular form with a grain diameter of 0.6 mm was used. Dry ice was mixed with excess sludge in a volume ratio of 0.15/1, 0.25/1, 0.35/1, 0.45/1, 0.55/1, 0.65/1, 0.75/1, respectively. The methane fermentation process lasting for 8 and 28 days, respectively, was carried out in mesophilic conditions at 37°C. In the first series untreated sludge was used, and for the second and third series the following treatment parameters were applied: the dose of dry ice in a volume ratio to excess sludge equal 0.55/1, pretreatment time 12 hours. The increase of the excess sludge disintegration degree, as well as the increase of the digestion degree and biogas yield, was a confirmation of the supporting operation of the applied modification. The mixture of reactant and excess sludge in a volume ratio of 0.55/1 was considered the most favorable combination. In relation to not prepared sludge for the selected most favorable conditions of excess sludge modification, about 2.7 and 3-fold increase of TOC and SCOD values and a 2.8-fold increase in VFAs concentration were obtained respectively. In relation to the effects of the methane fermentation of non-prepared sludge, for modified sludge, about 33 percentage increase of the sludge digestion degree and about 31 percentage increase of the biogas yield was noticed.

Here One important aspect of the process of anaerobic stabilisation of sewage sludge in medium and large sewage treatment plants, in addition to sludge mineralisation, is the acquisition of a valuable source of energy, which is biogas. There are well-known methods of intensifying the process of methane fermentation by subjecting sludge to disintegration using physical factors, i.e. ultrasonic field. Acetate production is the ratelimiting step in the acetate consumption pathway and affects the efficiency of the anaerobic stabilisation process. The product of the first stage of the process is also the substrate for the next stage. Therefore, it is advisable to subject sewage sludge to disintegration, which increases its susceptibility to biodegradation. Sludge modification with the above-mentioned method causes a significant increase in the concentration of organic substances in the supernatant liquid. The reflection of the physical and chemical transformations of sludge in the disintegration processes is the change in their structure expressed by the increase in the degree of particle dispersion. The disintegration of sludge using sonolysis is an effective process solution, both in terms of technology and energy, in terms of obtaining biogas, which is a valuable source of energy.