The course of the methane fermentation process of dry ice modified excess sludge

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 fl occulent suspension of microorganisms, subjected to the methane fermentation process show limited susceptibility to the biodegradation. Excess sludge is characterized by a signifi cant content of volatile suspended solids equal about 65 ÷ 75%. Promising technological solution in terms of increasing the effi ciency of fermentation process is the application of thermal modifi cation 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 modifi ed excess sludge. In the case of dry ice modifi cation 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 fi rst 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 confi rmation of the supporting operation of the applied modifi cation. 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 modifi cation, 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 modifi ed sludge, about 33 percentage increase of the sludge digestion degree and about 31percentage increase of the biogas yield was noticed. The course of the methane fermentation process of dry ice modifi ed excess sludge 51 It should be emphasized that the pre-treatment increases the amount of biogas produced, which can be used in the cogeneration process for the production of heat and power generation, which amortizes part of the costs incurred to carry out the process of disintegration of sludge (Vinay Kumar Tyagi, Shang-Lien Lo 2011). Commonly used parameter to evaluate the effectiveness of disintegration is soluble chemical oxygen demand (SCOD) (Neumann et al. 2016). As reported by Bougrier et al. (Bougrier et al. 2006) there is a relationship between COD solubilization and biodegradation. In addition, there is a relationship between an increase in the SCOD value and the VFAs concentration. However, as reported by Nazari et al. and Sapkaite et al. (Nazari et al. 2016, Sapkaite et al. 2017) higher SCOD value, due to the excessive increase in the charge of dissolved organics, did not result in higher biogas production. The freezing/thawing technique is considered a method contributing to the release of extracellular polymers and the disruption of sludge cells (Gao 2011). The freeze/thaw (F/T) treatment could facilitate mass transfer from the solid phase into the aqueous phase (Kai Hu et al. 2011). As reported by Kai Hu et al. (Kai Hu et al. 2011) freeze/thaw (F/T) treatment is an effi cient pretreatment method of excess sludge, increasing their biodegradability. According to Parker and Collins (Parker and Collins 1997) curing is a storage process of frozen sludge under subfreezing temperature. During the freezing process, the term “curing stage” is called the process during which bulk sludge is frozen and tiny unfrozen regimes in the ice matrix are continuously dehydrated by surrounding ice fronts. Therefore, the F/T treatment could enhance sludge organic matter solubilization. The curing process contributes to a limited growth of SCOD and ammonium nitrogen release. The crystallization of intraaggregate is responsible for the destruction of cell membranes and release of intracellular substances to the sludge liquid (Parker and Collins 1997). In the case of the freeze/thaw method, the formation of intracellular and extracellular ice crystals is responsible for the lysis process. The ongoing process of destroying cell walls of microorganisms and the release of intracellular substances has a mechanical basis and occurs as a result of the so-called thermal shock (Gao 2011, El-Kest et al. 1992). Figure 1 shows the process of freezing/thawing of sludge (Parker and Collins 1997). Thermal shock of bacteria occurs usually under specifi c conditions such as rapid freezing and in the case of the log phase of cell growth. The occurrence of thermal shock is conditioned by the cells concentration and the nature of the medium in which they are undergoing freezing. Changes are usually not evident immediately after chilling but appear to an increasing degree with extended storage near 0°C. Microbes undergo the so-called thermal shock and are injured when the ambient temperature drops sharply. During this process, frozen cells can be injured mechanically by intraand extracellular ice crystals. During the thawing the growth of ice crystals which occurs can physically affect cells (El-Kest and Marth 1992). Freeze-thaw pretreatment increases signifi cantly the concentrations of proteins, carbohydrates, and cations in the sludge liquid, what is the consequence of cell disruption and releases intracellular substances (Örmeni and Vesilind 2001). The aim of the study was to determine the effect of dry ice disintegration on the course of the methane fermentation process of modifi ed excess sludge.