**3. Anaerobic digestion technologies**

Block scheme of anaerobic digestion (Fig. 3) shows that technological process of typical anaerobic digestion. It consists of three basic phases: i) substrate preparation and pretreatment, ii) anaerobic digestion and iii) post treatment of digested material, including biogas use. In this section all of the processes will be elaborated in detail.

Fig. 3. Block scheme of anaerobic digestion and biogas/digestate utilisation

#### **3.1 Pretreatment**

In general, all types of biomass can be used as substrates as long as they contain carbohydrates, proteins, fats, cellulose and hemicellulose as main components. It is however

Anaerobic Treatment and Biogas Production from Organic Waste 13

digestion. The research revealed that higher temperatures of pretreatment (120-160°C) enabled finishing of the pretreatment process in 1-2 hours; however the need for a

Thermal pretreatment rewards with up to 30 % more biogas production if properly applied. This process occurs at temperature range of 135-220°C and pressures above 10 bar. Retention times are short (up to several hours) and hygienisation is automatically included. Pathogenic microorganisms are completely destroyed. The process runs economically only with heat regeneration. When heat is regenerated from outflow to inflow of the pretreatment process, it takes only slightly more heat than conventional anaerobic digestion.

It is also possible to use biological processes as pretreatment. They are emerging in the world. Disintegration takes place by means of lactic acid which decomposes complex components of certain substrates. Recently also disintegration with enzymes has been quite successful, especially using cellulose, protease or carbohydrases at a pH of 4.5 to 6.5 and a

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

Such process is very appropriate for cellular material such as raw sewage sludge.

retention time of at least 12 days, preferably more (Hendriks and Zeeman 2009).

pressurised vessel in such case did not outweigh the time saving.

Fig. 4. Effectiveness of thermo-chemical pretreatment

**3.2 Anaerobic digestion** 

important to consider several points prior to considering the process and biomass pretreatment. The contents and concentration of substrate should match the selected digestion process. For anaerobic treatment of liquid organic waste the most appropriate concentration is between 2 - 8 % of dry solids by mass. In such case conventional single stage digestion or two stage digestion is used. If considering the treatment of solid waste using solid digestion process, the concentration substrate is between 10 and 20 % by mass. Organic wastes can also contain impurities which usually impairs the process of digestion. Such materials are:


The presence of impurities in the substrate can lead to increased complexity in the operating expenditure of the process. During the process of digestion of liquid manure from cattle the formation of scum layer on the top of the digester liquid can be formed, caused by straw and muck. The addition of rumen content and cut grass (larger particles than silage) can contribute to its formation. If the substrate consists of undigested parts of corn and grain combined with sand and lime the solid aggregates can be formed at the bottom of the digester and can cause severe clogging problems.

In all such cases the most likely solution is pre-treatment to reduce solids size. Naturally, that all the non-digestible solids (soil, stones, plastics, metals...) should be separated from the substrate flow in the first step. On the other hand grass, straw and fodder residue can contribute to the biogas yield, when properly pretreated, so they are accessible to the digestion microorganisms. Pretreatment can be made by physical, chemical or combined means.

Physical pretreatment is the most common. The best known disintegration methods are grinding and mincing. In grinding and mincing the energy required for operation is inversely proportional to the particle size. Since such energy contributes to the parasitic energy, it should be kept in the limits of positive margin (the biogas yield increased by pretreatment is more than energy required for it). In the case of organic waste the empirical value for such particle size is between 1 and 4 mm.

Chemical pre-treatment can be used when treating ligno-cellulosic material, such as spent grains or even silage. Very often chemical treatment is used combined with heat, pressure or both. It is common to use acid (hydrochloric, sulphuric or others) or an alkaline solution of sodium hydroxide (in some cases soda or potassium hydroxide). Such solution is added to the substrate in quantities that surpass the titration equilibrium point and then it is heated to the desired temperature and possibly pressurized. Retention times are generally short (up to several hours) compared to retention times of the anaerobic digesters. The pretreated substrate is then much more degradable. The shortage of this pretreatment is low energy efficiency and the cost of chemicals required. It rarely outweighs the costs of building a bigger digester. Therefore it is used mostly in treating industrial waste (such as brewery) where there is plenty of waste lye or acid present and waste heat can be regenerated from the industrial processes as well. Fig. 4 presents the results of our research done on spent brewery grain, where up to 70% of organic matter could be, by means of proper pretreatment, extracted from solid to liquid form, ready for flow-through anaerobic

important to consider several points prior to considering the process and biomass pretreatment. The contents and concentration of substrate should match the selected digestion process. For anaerobic treatment of liquid organic waste the most appropriate concentration is between 2 - 8 % of dry solids by mass. In such case conventional single stage digestion or two stage digestion is used. If considering the treatment of solid waste using solid digestion process, the concentration substrate is between 10 and 20 % by mass. Organic wastes can also contain impurities which usually impairs the process of digestion. Such materials are:

The presence of impurities in the substrate can lead to increased complexity in the operating expenditure of the process. During the process of digestion of liquid manure from cattle the formation of scum layer on the top of the digester liquid can be formed, caused by straw and muck. The addition of rumen content and cut grass (larger particles than silage) can contribute to its formation. If the substrate consists of undigested parts of corn and grain combined with sand and lime the solid aggregates can be formed at the bottom of the

In all such cases the most likely solution is pre-treatment to reduce solids size. Naturally, that all the non-digestible solids (soil, stones, plastics, metals...) should be separated from the substrate flow in the first step. On the other hand grass, straw and fodder residue can contribute to the biogas yield, when properly pretreated, so they are accessible to the digestion microorganisms. Pretreatment can be made by physical, chemical or combined

Physical pretreatment is the most common. The best known disintegration methods are grinding and mincing. In grinding and mincing the energy required for operation is inversely proportional to the particle size. Since such energy contributes to the parasitic energy, it should be kept in the limits of positive margin (the biogas yield increased by pretreatment is more than energy required for it). In the case of organic waste the empirical

Chemical pre-treatment can be used when treating ligno-cellulosic material, such as spent grains or even silage. Very often chemical treatment is used combined with heat, pressure or both. It is common to use acid (hydrochloric, sulphuric or others) or an alkaline solution of sodium hydroxide (in some cases soda or potassium hydroxide). Such solution is added to the substrate in quantities that surpass the titration equilibrium point and then it is heated to the desired temperature and possibly pressurized. Retention times are generally short (up to several hours) compared to retention times of the anaerobic digesters. The pretreated substrate is then much more degradable. The shortage of this pretreatment is low energy efficiency and the cost of chemicals required. It rarely outweighs the costs of building a bigger digester. Therefore it is used mostly in treating industrial waste (such as brewery) where there is plenty of waste lye or acid present and waste heat can be regenerated from the industrial processes as well. Fig. 4 presents the results of our research done on spent brewery grain, where up to 70% of organic matter could be, by means of proper pretreatment, extracted from solid to liquid form, ready for flow-through anaerobic

Soil, sand, stones, glass and other mineral materials

Cords, wires, nuts, nails, batteries, plastics, textiles etc.

digester and can cause severe clogging problems.

value for such particle size is between 1 and 4 mm.

 Wood, bark, card, cork and straw Skin and tail hair, bristles and feathers

means.

digestion. The research revealed that higher temperatures of pretreatment (120-160°C) enabled finishing of the pretreatment process in 1-2 hours; however the need for a pressurised vessel in such case did not outweigh the time saving.

Fig. 4. Effectiveness of thermo-chemical pretreatment

Thermal pretreatment rewards with up to 30 % more biogas production if properly applied. This process occurs at temperature range of 135-220°C and pressures above 10 bar. Retention times are short (up to several hours) and hygienisation is automatically included. Pathogenic microorganisms are completely destroyed. The process runs economically only with heat regeneration. When heat is regenerated from outflow to inflow of the pretreatment process, it takes only slightly more heat than conventional anaerobic digestion. Such process is very appropriate for cellular material such as raw sewage sludge.

It is also possible to use biological processes as pretreatment. They are emerging in the world. Disintegration takes place by means of lactic acid which decomposes complex components of certain substrates. Recently also disintegration with enzymes has been quite successful, especially using cellulose, protease or carbohydrases at a pH of 4.5 to 6.5 and a retention time of at least 12 days, preferably more (Hendriks and Zeeman 2009).
