Biogas production facilities use a variety of reactor configurations. In this blog piece, Markus Voelklein, ATBEST researcher at University College Cork, discusses and compares single stage and two stage reactors.
The anaerobic microbial conversion of organic carbon takes place in a four phase degradation process. It consists of hydrolysis, acidogenesis, acetogenesis and methanogenesis, which theoretically can be identified via the presence of characteristic bacteria in each phase.
One-stage reactor system
In waste water treatment and agricultural plants the one‑stage digestion is dominating due to its decreased capital costs. This is probably the most important reason why most facilities have previously been designed for one-stage operation. Nevertheless, non optimum growth conditions prevail in this reactor especially for the bacteria involved in the first two phases (hydrolysis, acidogenesis) of the degradation process. Furthermore, since all four steps of fermentation take place simultaneously in one reactor the generated biogas during fermentation results in a mixture of all gas compounds of each phase. The digester of a one‑stage biogas plant is typically implemented as a cylindrical vessel, whose principle is shown in figure 1.
Two-stage reactor system
The two-stage biogas production has not commonly found its way into the biogas technology. The main reason lies in considerably higher capital costs regarding the construction of reactors and stirring technology. The requirements caused by decreased pH values for vessels, fittings and equipment in terms of corrosion have to be considered during the construction. Nevertheless, this technology is sometimes used in large scale industrial applications for example in waste water treatment or in the processing of easy-acidifying waste products in the food industry.
A two-stage biogas plant excels itself trough the spatial separation of process phases, which arise as a consequence of the different microbiological requirements of the four phase anaerobic process. In an upstream reactor acidification takes place and in a second reactor methanogenic bacteria produce methane. The two-stage system possesses the advantage of adjusting the environmental settings to the optimum requirements of the microorganisms. First of all, the pH value and loading rate can be adjusted individually. The spatial separation allows a directed acidification of the input substrate in a first vessel. The following Figure 2 illustrates a schematic layout of a two-stage biogas plant.
The substrate is fed into the first reactor where acidification takes place leading to hydrogen production and accumulation of acids and alcohol, which are precisely the precursor for the acetogenic microorganisms in the second stage. The hydrolysis phase is either carried out batch wise or continuously and connected to the second methanogenic reactor. The ratio of both reactor volumes is defined by the different retention time and generation time of the bacteria. This requires smaller volumes in the hydrolysis and bigger in the methane reactor, because the retention time in hydrolysis is significantly lower. At fibrous and high solid content enriched input substrate, mixing problems could occur in the hydrolysis phase and form a swimming layer. This can result in incomplete hydrolysis of the substrate. In order to prevent this condition, proper stirring technology has to be chosen in the design phase. Due to conversion of solid carbon into the liquid phase a liquefaction of the substrate occurs.
After a certain retention time of 1 to 5 days the acidified substrate is pumped into the methane reactor. Because of the already pre-liquefied substrate and the existing digested substrate in the methane reactor, the solid content is relatively low and requirements concerning the stirring technology decline. In contrast to the one‑stage process and as a result of the already largely liquefied carbon, a rapid degradation to biogas takes place, which leads to shorter overall retention times. An increase of methane yield or an extension of the input substrate range at higher loading rates is consequently feasible. Also poorly degradable lignocellulosic materials can be partly broken down trough the prevailing conditions in hydrolysis phase. Thus the bioavailability of these materials increases significantly and can be digested to biogas.
Biogas composition of two-stage digestion
The most fundamental difference and substantial advantage compared to the one‑stage system is represented by the separate gas collection of each vessel. This allows an active influence on the concentration of individual gas compounds or even a separated utilisation of the produced biogas. Figure 3 shows a comparison of a one and two-stage biogas plant system.
The gas compounds of an acidification reactor consists mainly of carbon dioxide, hydrogen sulphate and hydrogen. The composition can be influenced by parameters like retention time, loading rate, pH value and temperature in order to gain high carbon dioxide stripping. Due to this removal during acidification, a biogas with enhanced methane content is received in the methane reactor. For substrates like food waste or renewable raw materials a methane content of 60 to 70 % can be achieved. Therefore, the previous carbon dioxide stripping in a two-stage digestion system, enhances the efficiency and reduces the costs of a following biogas purification facility.
However, the loss of hydrogen and carbon dioxide in the first phase can contribute to reduced overall energy yields. On the other hand, the degradation rate rises due to the partial break down and a more complete utilisation of the substrate.
Comparison of both technologies
The one‑stage digestion constrains the biochemical settings of a digester. Inevitably non optimum growth conditions for hydrolysis bacteria prevail in this reactor. As a consequence an optimization should take this limiting step into consideration. The two-stage operation allows an individual optimization due to a separation into two-phases. Two-stages anaerobic digestion has been developed to minimize the inhibition of the hydrolysis and acidogenic microorganism by creating acidic conditions at a pH of 5.5. Slow growing methanogenic bacteria, which require a more neutral pH, are cultured in the second phase at longer retention times of 15 to 20 days. The investment costs for the two-stage fermentation are higher than for a one‑stage fermentation. The exceeding costs have to be refunded by advantages regarding the process stability and higher rate of substrate degradation, leading to higher biogas yields from the same amount of substrate.