Anaerobic- EGSB (Expanded Granular Sludge Bed)

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.

Introduction

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 (Expanded Granular Sludge Blanket), 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 bacterium under 20 °C.

The advantages and disadvantages of these anaerobic systems can be seen in the following table:

AdvantageDisadvantages
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 systemPutting the reactor into operation can be slow if you do not have the help of an acclimated inoculum
Under space requirementRequires 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 feedingLow rate of removal of nutrients and pathogens
Tolerance to high organic loadsKinetics 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:

anaerobico-egsb-01

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. 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.

Depending on their load, anaerobic treatment systems can be classified as follows:

Low applied loadSeptic tanks
 Anaerobic lagoons
 Conventional digesters
 
high applied loadUASB
 EGSB
AnMBR

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. 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.

EGSB reactors derive from UASB, with the exception of the type of sludge and the blanket expansion inside of it. EGSB reactors use granulated sludge and the bed is expanded due to the high hydraulic loads applied. This condition intensifies the hydraulic mixture inside the reactor increasing contact between biomass and substrate. The high flow rates before mentioned (5-10 m/h) are reached by high recirculation of the effluent, combined with the use of reactors in a relation height / diameter >20. These reactors are usually used when space is limited.

In this type of reactor, the excessive presence of suspended solids in the incoming effluent may cause maintenance inconveniences in the good characteristics of granulated sludge.

Scheme of EGSB Reactor

anaerobico-egsb-02

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