Ultrasonic disintegration, as a method of sludge pre-treatment (before the stabilization process), causes changes in their physicochemical characteristics. The aim of this study was to determine the influence of ultrasonic disintegration conditions (sonication) on the changes in the physicochemical characteristics of sonicated sludge, i.e. an increase in the content of organic substances in the supernatant, sludge dewaterability and flocs structure. Thickened and non-thickened excess sludge from the municipal wastewater treatment plant in Gliwice was disintegrated. The process was conducted, using a high-power disintegrator equipped with a lenticular horn. In order to determine the most favorable conditions, the sewage sludge was sonicated at a wave frequency of f=25 kHz (as a function of time), with a different samples volume (V1=0.5 and V2=1 L) and emitter position of 1 and the 2.5 cm from the bottom of the chamber in which the process was conducted. The disintegration of sewage sludge was carried out with a specific energy density (EV) in the range from 10 to 30 kWh/m3. The evaluation of the disintegration effects was based on changes in the physicochemical characteristics of the sludge and/or supernatant at the end of the process, expressed by commonly used and author’s disintegration indicators. The best results were obtained for the sludge disintegrated with a volume of V2=1 L and the emitter position of 2.5 cm from the bottom of the chamber. The study confirms that in various operating conditions of ultrasonic disintegration, there is a possibility for obtaining different effects which may influence the course of anaerobic stabilization and quality of the final products of the process.
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.
Sewage sludge (municipal, or industrial) treatment is still a problem in so far that it is not satisfactorily resolved in terms of cost and final disposal. Two common forms of sludge disposal are possible; the first being direct disposal on land (including agriculture) and the second being incineration (ash production), although neither of these methods are universally applied. Simplifying the issue, direct sludge disposal on land is seldom applied for sanitary and environmental reasons, while incineration is not popular for financial (high costs) reasons. Very often medium and large wastewater treatment plants apply anaerobic digestion for sludge hygiene principles, reducing the amount to be disposed and for biogas (energy) production. With the progress in sewage biological treatment aiming at nutrient removal, primary sludge has been omitted in the working processes and only surplus activated sludge requires handling. Anaerobic digestion of waste activated sludge (WAS) is more difficult due to the presence of microorganisms, the decomposition of which requires a relatively long time for hydrolysis. In order to upgrade the hydrolysis effects, several different pre-treatment processes have already been developed and introduced. The additional pre-treatment processes applied are aimed at residual sludge bulk mass minimization, shortening of the anaerobic digestion process or higher biogas production, and therefore require additional energy. The water-energy-waste Nexus (treads of) of the benefits and operational difficulties, including energy costs are discussed in this paper. The intensity of pre-treatment processes to upgrade the microorganism’s hydrolysis has crucial implications. Here a low intensity pre-treatment process, alkalisation and hydrodynamic disintegration - hybrid process - were presented in order to achieve sufficient effects of WAS anaerobic digestion. A sludge digestion efficiency increase expressed as 45% biogas additional production and 52% of the total or volatile solids reduction has been confirmed.
The article describes problems related to intensiﬁcation of energy production at a sewage treatment plant. The authors analyze anaerobic co-digestion of sludge from a water treatment plant and sewage treatment plant. The authors proposed a methodology of the research and analyzed the preliminary results, which showed that co-digestion of sewage and water sludge enhanced biogas production. The authors hope that the results of the study will provide a basis for development of methodology for sludge control and disposal.
This paper presents the content changes in the Chemical Oxygen Demand (COD) solubilised in hydrolisates obtained from thermally disintegrated municipal waste biofractions. A series of tests related to biowaste undergoing thermal treatment at the following temperatures: 55, 75, 95, 115, 135, 155 and 175°C were conducted for 0.5, 1 and 2 hours. The highest increase in COD solid fraction solubilisation (238%) was observed for the samples disintegrated at 175°C for 2 hours. The values of the reaction rate coefficient k20 = 0.6 d-1 and temperature coefficient θ = 1.023 were determined. Statistical analysis of the multiple regression (correlation coefficient R = 0.89) showed that the temperature has a greater impact on COD solid fraction solubilisation - determined β = 0.66. The multiple correlation coefficient for the treatment time was β = 0.61.
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.