**3.2 Anaerobic digestion**

For anaerobic digestion several different types of anaerobic processes and several different types of digesters are applicable. It is hard to say in advance, which digester type is most appropriate for treating the selected organic waste. Digestion of farm waste, for example, should be carried out in decentralized plants to serve each farm separately, to make it an economic and technological unit combined with the farm. In the same sense a town may be a unit in treatment of organic municipal waste. It is important to study the waste of each such unit carefully to be able to determine optimal conditions for substrate digestion. Organic waste can differ very much even in same geographical areas, therefore it is strongly recommended to conduct laboratory and pilot scale experiments before design of the full scale digester is made. Considering the costs of the full scale digester, conducting pilot scale experiments is a minor item, especially if you have no preceding results or experience. The

Anaerobic Treatment and Biogas Production from Organic Waste 15

Alternative processes that treat wet organic waste in solid state is reported in literature as SEBAR - Sequential Batch Anaerobic Digester System (Tubtong et al., 2010). In this case the cycle is also divided into four phases, however somehow different than in an ASBR process. This process requires digesters always to be in pairs. The reactor is almost completely emptied between cycles therefore it requires inoculation through leachate exchange between the two digesters (from the one in the peak biogas production to the one at the start of the process). In the other phases leachate is self-circulated (Fig. 6). Typical cycle time is between 30 and 60 days. Although solid substrate reduces the reactor volume, the volume is still rather large due to long cycle times compared to conventional digesters that process liquid substrates. The advantage of this type of digesters is less complicated monitoring equipment

Fig. 5. Schematic picture of the batch ASBR process

Fig. 6. Batch solid anaerobic digestion

so they are applicable in smaller scale.

biggest economic setback is when a digester is constructed and it does not perform as expected and consequently requires reconstruction.

There are several processes available to conduct anaerobic digestion. Roughly, the digestion process can be divided into solid digestion and wet digestion processes. Solid digestion processes are in fact anaerobic composters. In this process substrate and biomass are in presoaked solid form, containing. 20 % of dry matter and 80 % water. Such processes have several advantages. The main advantage is reducing the reactor volume due to much less water in the system. Four times more concentrated substrate equals approximately four times less reactor volume. It is also possible that some inhibitors (such as ammonium) can have less inhibitory effects in solid digestion process. The biggest disadvantage of solid digestion process is the substrate transport. Substrate in solid form requires more energy for transport in and out of the digesters. It is also a stronger possibility of air intrusion into the digesters, which poses a great risk to process stability and safety. It has been only recently that such processes have gained ground for a wider use. A fine example is the Kompogas® process (Kompogas 2011).

A much larger variety represents wet digestion processes. They operate at conventional concentration up to 5 % of dry solids by mass of the digester suspension. There are several reactor technologies available to successfully conduct anaerobic digestion. Roughly, they can be divided into batch wise (Fig. 5 and Fig. 6) and continuous processes. Furthermore continuous processes can be divided into single stage (Fig. 7) or two-stage processes (Fig. 8). In most of the wet digestion processes microorganisms are completely mixed and suspended with substrate in the digester. The suspended solids of substrate and microorganisms are impossible to separate after the process. If the substrate contains little solids and is mostly dissolved organics liquid, we can apply flow-through processes. In these processes microorganisms are in granules and granules are suspended in liquid which contains dissolved organic material. In such anaerobic processes microorganisms granules are easily separated from the exhausted substrate. Typical representative of such process is the UASB (Upflow Anaerobic Sludge Blanket) process (Fig. 9).

#### **3.2.1 Batch processes**

In the batch process all four steps of digestion as well as four stages of treatment process happen in one tank. Typically the reaction cycle of the anaerobic sequencing batch reactor (ASBR) is divided into four phases: load, digestion, settling and unload (Fig. 5). A stirred reactor is filled with fresh substrate at once and left to degrade anaerobically without any interference until the end of the cycle phase. This leads to temporal variation in microbial community and biogas production. Therefore, batch processes require more precise measurement and monitoring equipment to function optimally. Usually these reactors are built at least in pairs, sometimes even in batteries. This achieves more steady flow of biogas for instant use. Between the cycles the tank is usually emptied incompletely (to a certain exchange volume), which is up to 50% of total reactor volume. The residue in the tank serves as microbial inoculum for the next cycle. This makes batch reactors volume larger than of the conventional continuous reactors; however they do not require equalization tanks and the total reactor volume is usually less than in conventional processes. They can be coupled directly to the waste discharge; however this limits the use to more industrial processes (for example food industry) and less to other waste production. Typical cycle time is one day.

biggest economic setback is when a digester is constructed and it does not perform as

There are several processes available to conduct anaerobic digestion. Roughly, the digestion process can be divided into solid digestion and wet digestion processes. Solid digestion processes are in fact anaerobic composters. In this process substrate and biomass are in presoaked solid form, containing. 20 % of dry matter and 80 % water. Such processes have several advantages. The main advantage is reducing the reactor volume due to much less water in the system. Four times more concentrated substrate equals approximately four times less reactor volume. It is also possible that some inhibitors (such as ammonium) can have less inhibitory effects in solid digestion process. The biggest disadvantage of solid digestion process is the substrate transport. Substrate in solid form requires more energy for transport in and out of the digesters. It is also a stronger possibility of air intrusion into the digesters, which poses a great risk to process stability and safety. It has been only recently that such processes have gained ground for a wider use. A fine example is the Kompogas®

A much larger variety represents wet digestion processes. They operate at conventional concentration up to 5 % of dry solids by mass of the digester suspension. There are several reactor technologies available to successfully conduct anaerobic digestion. Roughly, they can be divided into batch wise (Fig. 5 and Fig. 6) and continuous processes. Furthermore continuous processes can be divided into single stage (Fig. 7) or two-stage processes (Fig. 8). In most of the wet digestion processes microorganisms are completely mixed and suspended with substrate in the digester. The suspended solids of substrate and microorganisms are impossible to separate after the process. If the substrate contains little solids and is mostly dissolved organics liquid, we can apply flow-through processes. In these processes microorganisms are in granules and granules are suspended in liquid which contains dissolved organic material. In such anaerobic processes microorganisms granules are easily separated from the exhausted substrate. Typical representative of such process is

In the batch process all four steps of digestion as well as four stages of treatment process happen in one tank. Typically the reaction cycle of the anaerobic sequencing batch reactor (ASBR) is divided into four phases: load, digestion, settling and unload (Fig. 5). A stirred reactor is filled with fresh substrate at once and left to degrade anaerobically without any interference until the end of the cycle phase. This leads to temporal variation in microbial community and biogas production. Therefore, batch processes require more precise measurement and monitoring equipment to function optimally. Usually these reactors are built at least in pairs, sometimes even in batteries. This achieves more steady flow of biogas for instant use. Between the cycles the tank is usually emptied incompletely (to a certain exchange volume), which is up to 50% of total reactor volume. The residue in the tank serves as microbial inoculum for the next cycle. This makes batch reactors volume larger than of the conventional continuous reactors; however they do not require equalization tanks and the total reactor volume is usually less than in conventional processes. They can be coupled directly to the waste discharge; however this limits the use to more industrial processes (for example food industry) and less to other waste production. Typical cycle time

expected and consequently requires reconstruction.

the UASB (Upflow Anaerobic Sludge Blanket) process (Fig. 9).

process (Kompogas 2011).

**3.2.1 Batch processes** 

is one day.

Fig. 6. Batch solid anaerobic digestion

Alternative processes that treat wet organic waste in solid state is reported in literature as SEBAR - Sequential Batch Anaerobic Digester System (Tubtong et al., 2010). In this case the cycle is also divided into four phases, however somehow different than in an ASBR process. This process requires digesters always to be in pairs. The reactor is almost completely emptied between cycles therefore it requires inoculation through leachate exchange between the two digesters (from the one in the peak biogas production to the one at the start of the process). In the other phases leachate is self-circulated (Fig. 6). Typical cycle time is between 30 and 60 days. Although solid substrate reduces the reactor volume, the volume is still rather large due to long cycle times compared to conventional digesters that process liquid substrates. The advantage of this type of digesters is less complicated monitoring equipment so they are applicable in smaller scale.

Anaerobic Treatment and Biogas Production from Organic Waste 17

is 2.0-3.0 kgm-3d-1. Typical value for thermophilic digesters is 5.0 kgm-3d-1. Maximum OLR depends very much of the substrate biodegradability; mesophilic process can rarely achieve higher loads than 5.0 kgm-3d-1 and thermophilic 8.0 kgm-3d-1. Locally in the digester for a short period of time higher loads can be achieved, however due to inherent instability it is

To achieve better biodegradation efficiency and higher loads stage separated process can be applied (Fig. 8). In this case the whole substrate or just portions of the substrate which are not easily degradable are treated first in hydrolysis-acidogenic stage reactor and after that in the methanogenic reactor. By separating the biological processes in two separate tanks each can be optimised to achieve higher efficiency with respect to one tank, where all stages of the digestion processes occur simultaneously. Many research data have been published giving considerable attention to this kind of processes (Dinsdale et al., 2000; Song et al., 2004; De Gioannis et al., 2008; Ponsá et al., 2008). Both stages can be either mesophilic or thermophilic, however it is preferred that the hydrolysis-acidogenic reactor is thermophilic and methanogenic is mesophilic. Typical HRT for the thermophilic hydrolysis-acidogenic reactor is 1-4 days, depending on the substrate biodegradability. Typical HRT for the methanogenic reactor is 10 - 15 days (mesophilic) and 10 - 12 days (thermophilic). Advantages of this process beside shorter HRTs are higher organic load rate (20 % or more). Many authors also report slightly better biogas yields (Messenger et al., 1993; Han et al., 1997; Roberts et al., 1999; Tapana and Krishna, 2004). The only disadvantage is more

sophisticated equipment and process control, yielding the operation more expensive.

The flow-through processes, such as UASB process (Fig. 9) are used only for substrates where most of the organic material is in dissolved form with solids content at maximum 1-5 gL-1. In this substrate category are highly loaded wastewaters of industrial origin (e.g. from

not advisable to run continuously on such high loads.

Fig. 8. Two stage anaerobic digestion

**3.2.3 Flow-through processes** 

beverage industry).

#### **3.2.2 Continuous processes**

Most of the commercial biogas plants use conventional continuous anaerobic digestion process. By conventional it is meant fully mixed, semi-continuous or continuous load and unload reactor at mesophilic temperature range (35-40°C) - Fig. 7. In majority of the cases, the substrate is loaded to the reactor once to several times a day, rarely is it loaded continuously. Continuous load can lead to short circuit, which means that fresh load can directly flow out of the reactor if mixing is too intense or input and output tubes are located improperly. The digester is usually single stage, although they are built in pairs, they do not function as a stage separated process. Usually digesters are equipped with a preparation tank, where various substrates are mixed and prepared for the loading, which also serves as a buffer tank. In many cases also a post-treatment tank is added (it is also called postfermenter), where treated substrate is completely stabilized and prepared for further treatment. The post-treatment tank can also serve as a buffer towards further treatment steps of the substrate. Generally post-fermenters do not contribute much to overall biogas yield (up to 5 %) if the digester operates optimally. The size of the preparation and posttreatment tanks are determined according to the necessary buffer capacity for continuous operation. The size of the digester is determined with the necessary Hydraulic Retention Time (HRT) and with Organic Loading Rate (OLR) that should be determined in pilot tests. HRT is defined as digester volume divided by substrate flow rate and represents time (in days) in which a certain unit volume of the substrate passes through the reactor. For mesophilic digesters the usual values are between 20 - 40 days, depending on the substrate bio-degradability. In thermophilic digesters to achieve the same treatment efficiency HRT is smaller (between 10 and 20 days).

Fig. 7. Conventional single stage anaerobic digestion process

Organic load rate OLR (sometimes also called volume load) is defined as mass of organic material fed to the digester per unit volume per day. Typical value for mesophilic digesters

Most of the commercial biogas plants use conventional continuous anaerobic digestion process. By conventional it is meant fully mixed, semi-continuous or continuous load and unload reactor at mesophilic temperature range (35-40°C) - Fig. 7. In majority of the cases, the substrate is loaded to the reactor once to several times a day, rarely is it loaded continuously. Continuous load can lead to short circuit, which means that fresh load can directly flow out of the reactor if mixing is too intense or input and output tubes are located improperly. The digester is usually single stage, although they are built in pairs, they do not function as a stage separated process. Usually digesters are equipped with a preparation tank, where various substrates are mixed and prepared for the loading, which also serves as a buffer tank. In many cases also a post-treatment tank is added (it is also called postfermenter), where treated substrate is completely stabilized and prepared for further treatment. The post-treatment tank can also serve as a buffer towards further treatment steps of the substrate. Generally post-fermenters do not contribute much to overall biogas yield (up to 5 %) if the digester operates optimally. The size of the preparation and posttreatment tanks are determined according to the necessary buffer capacity for continuous operation. The size of the digester is determined with the necessary Hydraulic Retention Time (HRT) and with Organic Loading Rate (OLR) that should be determined in pilot tests. HRT is defined as digester volume divided by substrate flow rate and represents time (in days) in which a certain unit volume of the substrate passes through the reactor. For mesophilic digesters the usual values are between 20 - 40 days, depending on the substrate bio-degradability. In thermophilic digesters to achieve the same treatment efficiency HRT is

**3.2.2 Continuous processes** 

smaller (between 10 and 20 days).

Fig. 7. Conventional single stage anaerobic digestion process

Organic load rate OLR (sometimes also called volume load) is defined as mass of organic material fed to the digester per unit volume per day. Typical value for mesophilic digesters is 2.0-3.0 kgm-3d-1. Typical value for thermophilic digesters is 5.0 kgm-3d-1. Maximum OLR depends very much of the substrate biodegradability; mesophilic process can rarely achieve higher loads than 5.0 kgm-3d-1 and thermophilic 8.0 kgm-3d-1. Locally in the digester for a short period of time higher loads can be achieved, however due to inherent instability it is not advisable to run continuously on such high loads.

To achieve better biodegradation efficiency and higher loads stage separated process can be applied (Fig. 8). In this case the whole substrate or just portions of the substrate which are not easily degradable are treated first in hydrolysis-acidogenic stage reactor and after that in the methanogenic reactor. By separating the biological processes in two separate tanks each can be optimised to achieve higher efficiency with respect to one tank, where all stages of the digestion processes occur simultaneously. Many research data have been published giving considerable attention to this kind of processes (Dinsdale et al., 2000; Song et al., 2004; De Gioannis et al., 2008; Ponsá et al., 2008). Both stages can be either mesophilic or thermophilic, however it is preferred that the hydrolysis-acidogenic reactor is thermophilic and methanogenic is mesophilic. Typical HRT for the thermophilic hydrolysis-acidogenic reactor is 1-4 days, depending on the substrate biodegradability. Typical HRT for the methanogenic reactor is 10 - 15 days (mesophilic) and 10 - 12 days (thermophilic). Advantages of this process beside shorter HRTs are higher organic load rate (20 % or more). Many authors also report slightly better biogas yields (Messenger et al., 1993; Han et al., 1997; Roberts et al., 1999; Tapana and Krishna, 2004). The only disadvantage is more sophisticated equipment and process control, yielding the operation more expensive.
