Anaerobic – Upflow Anaerobic Sludge Blanket (UASB)
A great amount of industries such as food, beverage, agribusiness, meat processing, pulp and paper, are characterised for generating effluents with high biodegradable organic loads (high BOD and flow). Aerobic treatments such as activated sludge, MBR o MBBR for these uses are defined by the high energy consumption due to the air required for degrading the organic load present in the BOD. Anaerobic treatments are a very interesting alternative to apply in these cases, not only for their low power consumption but also for the biogas (methane) produced by the reaction. If treated properly, biogas can be used in industrial process in boilers, or turned into electrical power for industrial use.
As the knowledge in the anaerobic treatment field has progressed, some settings such as UASB reactors (Upflow Anaerobic Sludge Blanket) have been wining acceptance in the last few decades in tropical and subtropical countries, unlike the 1980s where these technologies were scarcely used. Even in recent times maximisations of the concept of UASB have been found, developing into high rate technologies such as fluidised bed reactors like EGSB, and the latest developments in anaerobic high rate membrane reactors (AnMBR) or anaerobic MBBR.
In theory, all organic compounds are feasible to be treated in anaerobic ways, that when compared to aerobic processes they consume less energy, and that in both cases, as more biodegradable are these compounds, the bigger will be their efficiency. An important consideration is the temperature of the effluent, the reason why these reactors are more efficient in warm regions where they can forego the expensive insulation and heating systems, due to the low activity of the anaerobic bacteria under 20 °C.
The advantages and disadvantages of these anaerobic systems can be seen in the following table:
|Low production of sludge (3 to 5 times less than an aerobic system)||Anaerobic biomass susceptible to inhibition by numerous compounds|
|Low energy required compared to an aerobic system||Putting the reactor into operation can be slow if you do not have the help of an acclimated inoculum|
|Under space requirement||Requires a post treatment to meet current rollover limits|
|Methane generation (high calorific value fuel)||Possibility of generation of bad odors|
|Possibility of preservation of biomass without long-term feeding||Low rate of removal of nutrients and pathogens|
|Tolerance to high organic loads||Kinetics highly dependent on the temperature of the effluent (more marked than in an anaerobic system)|
|Low nutrient consumption|
The conversion of aerobic vs. anaerobic systems can be seen in the following scheme:
As can be seen, in the anaerobic systems most of the biodegradable organic load present in the effluent is turned into biogas (70-90%), which is removed from the liquid phase and leaves the reactor as gas. Only a little amount of the organic charge is turned into new biomass (5-15%), which represents the sludge excess of the system. In spite of the little amount of generated sludge, it is concentrated and has a higher dewatering degree when compared to the aerobic sludge. The load not turned into biogas or new cell material leaves the reactor without being treated (10-30%).
The kinetic of organic charge removal is given by a complex chain of reactions with the involvement of diverse bacteria species. These reactions are classified by phases and are known as: hydrolysis phase, acidogenic phase,acetogenic phase and methanogenic phase; being the latter the only and ultimately responsible of the conversion of the previous acidified organic load into methane and carbon dioxide.
Anaerobic treatment systems:
With the scientific advances, pilot tests at universities and experimental small scale reactors, in recent times the so called “high rate” reactors were developed. In essence, its main characteristic is the ability of holding solids (biomass) in the system, in contrast to the older systems without these mechanisms, such as anaerobic lagoons or conventional reactors. This way, as it is able to hold solids even with high hydraulic load, reactors of lower volume can be run keeping its efficiency.
According to their load, anaerobic treatment systems can be classified as:
|Low applied load||Septic tanks|
| ||Anaerobic lagoons|
| ||Conventional digesters|
|high applied load||UASB|
In this classification, the most used at industrial level in the region are the three mentioned in the high load applied. Anaerobic membrane reactors (AnMBR) are still fewer worldwide and they are cause of study and development in some industrialised countries such as Japan and The Netherlands.
In the sanitary sector, the UASB take the lead by many installed units at tropical countries.
In the UASB reactors, the process consists mainly of an upward flow going through a thick layer (blanket) of suspended sludge with high biological activity. Solids profile inside the reactor varies from dense and granulated particles with great settling characteristics at the bottom of the reactor to more flocculent or lighter sludge in the upper region (sludge blanket). Conversion of organic load takes place throughout the reactor and the system mixture (effluent + sludge) is favoured by the upward flow and biogas generation. The effluent to be treated goes inside by the bottom of the reactor and goes out of it passing by a group of settling plates in the upper part (phase separator).
This L-S-G phase separator holds the solids or biomass inside the reactor, channels biogas generated in the process and lets the treated liquid phase pass. One of the main bases of this process is the capacity to develop high activity biomass. This biomass can be found in granular or flocculent form (1-5 mm). This type of reactor can reach solids concentration at the bottom of 40-100 gST/L. The surface velocities in these reactors are of 0.5-1.5 m/h.
UASB Reactor Scheme