The aim of the study was to determine the impact of various methods of oil mixing with wastewater on
properties of synthetic municipal wastewater containing edible oil (SMW+0.02% m/v rapeseed oil). The study was
carried out in 3L glass, cylindrical reactors to which SMW+0.02% were introduced. Various methods of its mixing
with water were applied: mechanical mixing (SMW+0.02%+mixing) and sonication (SMW+0.02%+ultrasounds).
The wastewater was sonicated at 35 kHz for 30 min. The constant temperature conditions were maintained during
the experiment for each mixing method (15°C, 20°C and 30°C). The analysis of parameters (pH, COD, BOD5
and
long chain free fatty acids concentration) of raw wastewater and after 2, 4, 6, 24, 48 and 72 hours of inoculation
was performed to determine the effect of mixing method.
The most signifi cant changes in wastewater chemical parameters after the introduction of the oil were observed
in the case of COD. For SMW+0.02%+ mixing a slow increase in COD within 24 hours of the process was observed.
In the case of SMW+0.02%+ultrasounds the increase and the decrease of COD value were observed in reference
to the initial value. The changes in acids concentrations observed in reactors with SMW+0.02%+ultrasounds were
referred to the ones observed in reactors with SMW+0.02%+mixing but changes were more intense in the fi rst
reactor. The use of ultrasounds in pre-treatment of wastewater resulted in the intense appearance of palmitic acid
for 6 hours. Regardless of the emulsion formation method (mixing or ultrasounds), the concentration of oleic acid
and linoleic acid was reduced. The biggest changes in free fatty acids concentration were observed for palmitic,
oleic and linoleic acids after 24 hours.
The factor which essentially affects sludge biodegradation rate is the degree of fluidization of insoluble organic polymers to the solved form, which is a precondition for availability of nutrients for microorganisms. The phases which substantially limit the rate of anaerobic decomposition include hydrolytic and methanogenic phase.
Subjecting excess sludge to the process of initial disintegration substantially affects the effectiveness of the process of anaerobic stabilization. As a result of intensification of the process of hydrolysis, which manifests itself in the increase in the value and rate of generating volatile fatty acids (VFA), elongation of methanogenic phase of the process and increase in the degree of fermentation of modified sludge can be observed. Use of initial treatment of sewage sludge i.e. thermal disintegration is aimed at breaking microorganisms' cells and release of intracellular organic matter to the liquid phase. As a result of thermal hydrolysis in the sludge, the volatile fatty acids (VFA) are generated as early as at the stage of the process of conditioning. The obtained value of VFA determines the course of biological hydrolysis which is the first phase of anaerobic stabilization.
The aim of the present study was to determine the effect of thermal disintegration of excess sludge on the effectiveness of the process of hydrolysis in anaerobic stabilization i.e. the rate of production of volatile fatty acids, changes in the level of chemical oxygen demand (COD) and increase in the degree of reduction in organic matter. During the first stage of the investigations, the most favourable conditions of thermal disintegration of excess sludge were identified using the temperatures of 50°C, 70°C, 90°C and heating times of 1.5 h - 6 h. The sludge was placed in laboratory flasks secured with a glass plug with liquid-column gauge and subjected to thermal treatment in water bath with shaker option. Another stage involved 8-day process of anaerobic stabilization of raw and thermally disintegrated excess sludge. Stabilization was carried out in mesophilic temperature regime i.e. at 37°C, under periodical conditions. In the case of the process of anaerobic stabilization of thermally disintegrated excess sludge at the temperature of 50°C and heating time of 6 h (mixture B) and 70°C and heating time of 4.5% (mixture C), the degree of fermentation of 30.67% and 33.63%, respectively, was obtained. For the studied sludge, i.e. mixture B and mixture C, maximal level of volatile fatty acids i.e. 874.29 mg CH3COOH/dm3 and 1131.43 mg CH3COOH/dm3 was found on the 2nd day of the process. The maximal obtained value of VFA was correlated on this day with maximal COD level, which was 1344 mg O2/dm3 for mixture B and 1778 mg O2/dm3 for mixture C.