USAB is short form of Up-flow Anaerobic Sludge Blanket. The advent of UASB process enabled the design engineer to evolve an anaerobic treatment system for medium strength wastewaters like those from the dairy industry. In the UASB concept, treatment is carried out in an upflow reactor. The wastewater is taken either to a distributing inlet chamber of 4.5-5 m depth and released it uniformly in the lower part of the reactor allowing it to rise at a desired velocity up to the outlet which is at the upper periphery of the unit or alternatively,the feed could also be pumped up from the bottom. A schematic diagram of a UASB reactor is shown in figure. In the lower part of the tank, a blanket of active anaerobic sludge is maintained. The wastewater is evenly distributed over the reactor bottom and is forced to percolate upwards through this bed of anaerobic sludge. During passage through the sludge bed suspended solids are entrapped(sedimentation and adsorption of organic waste matter in the sludge bed) and biodegradable material is consequently digested.
Key feature of the UASB is the formation of a dense sludge bed in the bottom of the reactor in which all biological processes take place. This sludge blanket is initiated by accumulation of incoming suspended solids and bacterial growth. If proper conditions are maintained during start up, different species of bacteria are observed to form flocs or granules in upflow anaerobic systems. These dense aggregates have good settling properties and are not susceptible to washout from the system under practical reactor conditions. Retention of active sludge, either granular or flocculent, within the UASB reactor enables good treatment performance at high organic loading rates. Natural turbulence caused by the influent flow and biogas production provides good wastewater-biomass contact in UASB systems.
Dissolved organics are removed from the solution by the anaerobic bacteria and converted into biogas and a small fraction in new bacterial biomass. The biogas provides a gentle mixing in the sludge bed, therefore no mechanical mixing is required. In the upper part of the reactor, A-shaped structures are placed that collect the produced biogas, from where it is withdrawn. Consequently, betweentwo adjacent ‘A’s a quiescent zone is provided, that serves as an internal settler,where the treated water is freed of sludge particles.
The water-sludge mixture enters the settling compartment where the sludge can settle and flow back into the digestion compartment. After settling the treated water is collected in gutters and discharged. Since the ‘A’ structures separate the treated wastewater from gas and sludge, they are commonly referred to as ‘GLS(gas, liquid, solids) separators’. A salient feature of the UASB concept is that anaerobic flocculent or granular type of sludge inherently has or will attain good settling properties provided the process is operated in the proper way during the reactor start-up.
Higher organic loading can be applied in UASB systems than in aerobic processes.Therefore, less reactor volume and space is required while, at the same time, high-grade energy is produced as biogas. The UASB reactor may replace the primary settler, the anaerobic digester and the secondary settler of a conventional aerobic treatment plant.However, the effluent from UASB reactor usually needs further treatment, in order to remove remaining organic matter, nutrients and pathogens. This post treatment can be accomplished in conventional aerobic systems.
The advantages of UASB is that you do not need to fill with any stone or other media in reactor and no mixers or aerators are required, thus conserving energy and giving very low operating costs.The gas produced can be collected and used if desired. Anaerobic systems function satisfactorily when temperature inside the reactor/ digester is above 18-20 o C.However, optimum temperature is 38-40 o C under mesophilic and 50-60 o C under thermophilic conditions. Thus, in most parts of India, temperature is no problem. In colder countries, the reactor needs to be heated and hence the use of UASB is generally limited to high BOD industrial wastes from which gas recovery is more than adequate for heating.
Excess sludge is removed from time to time through a separate pipe and sent to a simple sand bed for drying. The nutrients, nitrogen and phosphorus are not removed but are in fact conserved in the process and, to that extent, make irrigation use of the effluent more valuable.
Key feature of the UASB is the formation of a dense sludge bed in the bottom of the reactor in which all biological processes take place. This sludge blanket is initiated by accumulation of incoming suspended solids and bacterial growth. If proper conditions are maintained during start up, different species of bacteria are observed to form flocs or granules in upflow anaerobic systems. These dense aggregates have good settling properties and are not susceptible to washout from the system under practical reactor conditions. Retention of active sludge, either granular or flocculent, within the UASB reactor enables good treatment performance at high organic loading rates. Natural turbulence caused by the influent flow and biogas production provides good wastewater-biomass contact in UASB systems.
Dissolved organics are removed from the solution by the anaerobic bacteria and converted into biogas and a small fraction in new bacterial biomass. The biogas provides a gentle mixing in the sludge bed, therefore no mechanical mixing is required. In the upper part of the reactor, A-shaped structures are placed that collect the produced biogas, from where it is withdrawn. Consequently, betweentwo adjacent ‘A’s a quiescent zone is provided, that serves as an internal settler,where the treated water is freed of sludge particles.
The water-sludge mixture enters the settling compartment where the sludge can settle and flow back into the digestion compartment. After settling the treated water is collected in gutters and discharged. Since the ‘A’ structures separate the treated wastewater from gas and sludge, they are commonly referred to as ‘GLS(gas, liquid, solids) separators’. A salient feature of the UASB concept is that anaerobic flocculent or granular type of sludge inherently has or will attain good settling properties provided the process is operated in the proper way during the reactor start-up.
Higher organic loading can be applied in UASB systems than in aerobic processes.Therefore, less reactor volume and space is required while, at the same time, high-grade energy is produced as biogas. The UASB reactor may replace the primary settler, the anaerobic digester and the secondary settler of a conventional aerobic treatment plant.However, the effluent from UASB reactor usually needs further treatment, in order to remove remaining organic matter, nutrients and pathogens. This post treatment can be accomplished in conventional aerobic systems.
The advantages of UASB is that you do not need to fill with any stone or other media in reactor and no mixers or aerators are required, thus conserving energy and giving very low operating costs.The gas produced can be collected and used if desired. Anaerobic systems function satisfactorily when temperature inside the reactor/ digester is above 18-20 o C.However, optimum temperature is 38-40 o C under mesophilic and 50-60 o C under thermophilic conditions. Thus, in most parts of India, temperature is no problem. In colder countries, the reactor needs to be heated and hence the use of UASB is generally limited to high BOD industrial wastes from which gas recovery is more than adequate for heating.
Excess sludge is removed from time to time through a separate pipe and sent to a simple sand bed for drying. The nutrients, nitrogen and phosphorus are not removed but are in fact conserved in the process and, to that extent, make irrigation use of the effluent more valuable.
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| Schematic diagram of UASB Reactor |
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