BIOTECHNOLOGY - V. H. Gerasymenko - 2006

Part II. Special Biotechnologies

Chapter 22. BIOTECHNOLOGIES FOR WASTE UTILIZATION AND BIORECYCLING IN THE AGRO-INDUSTRIAL COMPLEX

22.3. UNCONVENTIONAL METHODS. BIOTECHNOLOGY OF BIOGAS PRODUCTION VIA ANAEROBIC WASTE DIGESTION

22.3.3. Technical and technological aspects of biogas production

22.3.3.2. Design features of the biogas plant (BGP) reactor

Reactors or Fermentation chambers are core components of a biogas plant. The economic viability of biogas production largely depends on the design Features of the fermentation chamber. When designing it, one must account for both the volume of biomass to be processed and The amount of biogas required to meet the energy needs of the facility.

Currently operating biogas plants utilize oval, cylindrical, cylindrical-conical, cubic, and rectangular parallelepiped reactors. Oval and cylindrical-conical reactors offer the greatest advantages. They facilitate better substrate mixing, sludge discharge, and biogas extraction. Reactors of other shapes present poorer conditions for biomass mixing, breaking up floating crusts, and removing biogas and sludge.

Reactors are constructed from brick, concrete, reinforced concrete, sheet steel, fiberglass, and polymer film. Flexible reactors are built with rubberized Materials or plastic shells and given an oval shape. During operation, to prevent damage, they are either buried in the ground or enclosed within rigid structures.

To enhance biogas production efficiency and reduce costs, industries in the USA and Germany have initiated the manufacture of low-cost, prefabricated, stamped plastic reactors featuring a synthetic thermal insulation layer. These reactors are quick and easy to install on a farm, easy to clean from manure and residual sludge, and possess a low Heat transfer coefficient.

Thus, the fermentation chamber must meet several key requirements: absolute airtightness, excellent thermal insulation, and high corrosion resistance. A constant Temperature must be maintained inside the chamber (depending on the operating regime), as methanogenic microorganisms are highly sensitive to sudden temperature fluctuations. Therefore, the incoming biomass is preheated to the required temperature using specialized devices. This consumes approximately up to 30 % of the produced biogas energy in summer and up to 60 % in winter.

Agricultural waste contains various solid particles. Some of these, having a density higher than that of liquids (such as sand, cement, and clay), tend to settle and form sludge, while others form a surface crust.

During fermentation, the biomass inside the Reactor tends to separate into three fractions. The top layer—the crust—is formed by larger particles lifted by rising bubbles of generated biogas. Over time, this crust can become quite dense and hinder biogas release. The liquid phase accumulates in the middle of the reactor, while the bottom sludge-like fraction settles out. Methanogenic Bacteria are most active in the middle zone. Therefore, to optimize methanogenesis within the fermentation chamber and maximize biogas yield, the biomass must be mixed periodically (at least once a day, and preferably up to six times).

Various mechanical and hydraulic mixing devices are used for agitation, as well as biogas injected into the reactor via a compressor. The movement speed of the biomass during mixing should not exceed 0,5 m/s. Higher speeds can rupture the walls of microbial Cells.

The mixing process allows to:

- maintain a uniform distribution of the fed raw materials and their continuous contact with microorganisms, enabling maximum utilization of fresh nutrients;

- keep the concentration of degradation products low through their uniform distribution throughout the reactor volume; ensure medium uniformity in both temperature and nutrient concentration, which creates favorable conditions for microflora activity;

- eliminate localized concentrations of inhibiting substances in specific Zones of the reactor, thereby limiting their adverse effects on methane fermentation;

- prevent surface crust formation and the buildup of a dense, stagnant sludge layer at the bottom;

- reduce the number of dead zones within the bioreactor where undecomposed solids and spent liquid might otherwise accumulate.