Anaerobic mesophilic digestion of sludge with extra-thermophilic and high ph pre-treatment (original) (raw)
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Anaerobic digestion is a technique for activated sludge stabilisation which is applied worldwide. The anaerobic digestion process follows four major steps: hydrolysis, acidogenesis, acetogenesis and methanogenesis with hydrolysis as the rate-limiting step of the overall process during sludge digestion. In the present work, chemical pre-treatment using NaOH has been applied to primary and secondary sludge from a municipal waste water treatment plant in order to improve the anaerobic digestion process. Thermal pretreatment has also been studied applied to secondary sludge. Contact time (30-480 minutes) and NaOH concentration (0.05-0.3 mol/l) were the factors considered during the study of chemical treatment. Thermal treatment was applied to secondary sludge at different temperatures (110-134ºC) and times (20-90 min) in an autoclave. The effectiveness of the two processes was evaluated by means of three parameters: the degree of disintegration using the chemical oxygen demand (% DD COD ), the increment in soluble protein concentration (SPC, in percentage) and the increment of the relationship between filterable volatile solids and the total volatile solids (FVS/TVS, in percentage). Highest NaOH concentration (0.3 mol/l) and temperature (134ºC) tested were found as the best conditions respectively for chemical and thermal treatment in the range of values studied. The time during which the treatment is applied also influences the process but in a less extend.
INFLUENCE OF THERMAL PRE-TREATMENT ON MESOPHILIC ANAEROBIC DIGESTION OF SLUDGES
2008
Abstract. The anaerobic degradation of sludge is usually limited by the rate of hydrolysis of suspended matter and organic solids. Low temperature pre-treatment (below 80 C) results in incomplete destruction of micro-flora but many enzymes maintain their biocatalytic activity. Increasing hydrolysis through thermal and alkali pre-treatment was studied in an anaerobic thermophilic reactor operating between 70 C and 100 C with hydraulic retention times (HRT) between 7 to 30 min followed by the main treatment in an anaerobic mesophilic reactor ...
Thermal pre-treatment of primary and secondary sludge at 70 degrees C prior to anaerobic digestion
Water science and technology : a journal of the International Association on Water Pollution Research, 2005
In general, mesophilic anaerobic digestion of sewage sludge is more widely used compared to thermophilic digestion, mainly because of the lower energy requirements and higher stability of the process. However, the thermophilic anaerobic digestion process is usually characterised by accelerated biochemical reactions and higher growth rate of microorganisms resulting in an increased methanogenic potential at lower hydraulic retention times. Furthermore, thermal pre-treatment is suitable for the improvement of stabilization and could be realized at relatively low cost especially at low temperatures. The present study investigates the effect of the pre-treatment at 70 degrees C on thermophilic (55 degrees C) anaerobic digestion of primary and secondary sludge in continuously operated digesters. Thermal pre-treatment of primary and secondary sludge at 70 degrees C enhanced the removal of organic matter and the methane production during the subsequent anaerobic digestion step at 55 degree...
2011
This study is focused on the effect of thermo alkaline pretreatment on velocity of anaerobic hydrolysis mesophilic and thermophilic of waste activated sludge (WAS), separating the hydrolysis of suspended solids (SS) and dissolved solids (SD). In order to research the hydrolysis of SS, the Sander’s surface model was used, and for the hydrolysis of SD, the Goel’s model of saturation was employed. The pretre tment increased the uperficial rea available for the enzymatic action, due to the reduction of the solids size. The effect of the pretreatment in the anaerobic digestion of the SD in thermophilic conditions, resulted in competitive inhibition over the anaerobic hydrolysis rate, whereas that in mesophilic conditions, resulted in non competitive inhibition. Mathematical simulation of SS to SD and methane consecutive reactions, showed that a 4 % increase in initial thermal-alkaline tr ated sludge di inished the inhibition of SD hydrolysis in the thermophilic anaerobic digestion.
Principles and potential of the anaerobic digestion of waste-activated sludge
Progress in Energy and Combustion Science, 2008
When treating municipal wastewater, the disposal of sludge is a problem of growing importance, representing up to 50% of the current operating costs of a wastewater treatment plant. Although different disposal routes are possible, anaerobic digestion plays an important role for its abilities to further transform organic matter into biogas (60-70 vol% of methane, CH 4 ), as thereby it also reduces the amount of final sludge solids for disposal whilst destroying most of the pathogens present in the sludge and limiting odour problems associated with residual putrescible matter. Anaerobic digestion thus optimises WWTP costs, its environmental footprint and is considered a major and essential part of a modern WWTP. The potential of using the biogas as energy source has long been widely recognised and current techniques are being developed to upgrade quality and to enhance energy use. The present paper extensively reviews the principles of anaerobic digestion, the process parameters and their interaction, the design methods, the biogas utilisation, the possible problems and potential pro-active cures, and the recent developments to reduce the impact of the problems. After having reviewed the basic principles and techniques of the anaerobic digestion process, modelling concepts will be assessed to delineate the dominant parameters. Hydrolysis is recognised as rate-limiting step in the complex digestion process. The microbiology of anaerobic digestion is complex and delicate, involving several bacterial groups, each of them having their own optimum working conditions. As will be shown, these groups are sensitive to and possibly inhibited by several process parameters such as pH, alkalinity, concentration of free ammonia, hydrogen, sodium, potassium, heavy metals, volatile fatty acids and others. To accelerate the digestion and enhance the production of biogas, various pre-treatments can be used to improve the rate-limiting hydrolysis. These treatments include mechanical, thermal, chemical and biological interventions to the feedstock. All pre-treatments result in a lysis or disintegration of sludge cells, thus releasing and solubilising intracellular material into the water phase and transforming refractory organic material into biodegradable species. Possible techniques to upgrade the biogas formed by removing CO 2 , H 2 S and excess moisture will be summarised. Special attention will be paid to the problems associated with siloxanes (SX) possibly present in the sludge and biogas, together with the techniques to either reduce their concentration in sludge by preventive actions such as peroxidation, or eliminate the SX from the biogas by adsorption or other techniques. The reader will finally be guided to extensive publications concerning the operation, control, maintenance and troubleshooting of anaerobic digestion plants.
Effects of thermochemical pretreatment on the anaerobic digestion of waste activated sludge
1997
Effects of pretreatment on the anaerobic digestion of waste activated sludge (WAS) were investigated in terms of VSS solubilization and methane production by batch experiments. The methods of pretreatment studied are NaOH addition (chemical), heating (thermal) and heating with NaOH addition (thermochemical) to the domestic WAS and to the combined WAS from domestic, conunercial and industrial wastewaters. The thermochenucal pretreatment gave the best result among three methods in the combined WAS, i.e., the VSS was solubilized by 40-50% and the methane production incresed by more than 200% over the control when the WAS was heated at 130°C for S minutes with the dose 0.3 g NaOHlg VSS. In the domestic WAS, the VSS solubilization rate was 70-80% but the increase of the methane production was about 30% after thermochemically pretreated. The domestic WAS consists of 41 % protein, 25% lipid and 14% carbohydrate on COD basis, and the solubilization rate of protein, which is the largest constituent of the WAS, was 63% in the thermochemical pretreatment. Although the effect of the thermochemical pretreatment on the methane production was higher to the combined WAS than to the domestic WAS, the methane production rate was 21.9 ml CHJg VSSw,u•day in the domestic WAS and 12.8 ml CHJg VSSWAS•day in the combined WAS.
Aerobic Thermophilic and Anaerobic Mesophilic Treatment of Sludge
Journal of Environmental Engineering, 2000
ÐLaboratory experiments were conducted to investigate two-stage aerobic thermophilic and anaerobic mesophilic treatment of swine waste. The two-stage system included a 1-day sludge retention time (SRT) aerobic thermophilic reactor operating at 628C and with 1.0 mg dissolved oxygen/l, followed by a 5, 9, and 14-day SRT anaerobic mesophilic digester operating at 378C. A single stage anaerobic mesophilic digester operating at 6, 10, and 15-day SRT and 378C was used as the control. Feed swine waste slurry average composition included total solids (TS)=4.3%; volatile solids (VS)=67% of TS; supernatant chemical oxygen demand (COD)=14,330 mg/l; fecal coliform density=7.2 Â 10 8 MPN/g TS; pH=7.1; and alkalinity=2700 mg CaCO 3 /l. Two-stage system operating at 6, 10, and 15-day system SRT reduced VS by 46, 54, and 61%, respectively, and was signi®cantly better than the control at each SRT. Supernatant COD reduction by the two-stage system (56±67%) was signi®cantly better than that obtained in the control (44±60%). Fecal coliform density was reduced to <10 3 MPN/g TS at all SRT by the two-stage system, whereas, the control did not reduce the fecal coliform density below 10 5 MPN/g TS at all SRT. The two-stage system anaerobic digester produced 0.56±0.64 m 3 CH 4 /kg VS destroyed compared to lower levels of 0.47±0.51 m 3 CH 4 /kg VS destroyed by the control, both operating at 6, 10, and 15-day system SRT. The methane gas production by the twostage system of 0.26, 0.32, and 0.39 m 3 /kg VS fed at 6, 10, and 15-day system SRT, respectively, was signi®cantly higher than that by the control system (0.17, 0.22, and 0.25 m 3 /kg VS fed at 6, 10, and 15day SRT, respectively). The biogas produced by the two-stage system anaerobic digester contained 353±387 ppm (v/v) H 2 S content compared to 569±609 ppm (v/v) H 2 S content in the biogas from the control anaerobic digester. The time-to-®lter values of the product sludge from the two-stage system (245, 197, and 158 s at 6, 10, and 15-day system SRT, respectively) were about 50% lower than those of the product sludge from the control (355, 295, and 250 s at 6, 10, and 15-day system SRT, respectively), indicating better dewaterability of the two-stage system product sludge.