Commercial biogas plants: Review on operational parameters and guide for performance optimization

2. General operational status and performance of commercial 
biogas plants 
From the perspective of commercial biogas production, the ideal 
operational mode of biogas plants requires stable production of consis-
tent quantities of the desired products, as well as the development of a 
process that has low input requirements and reduced internal energy 
consumption 
[27–29]
. Clearly, all of these requirements are closely 
related to the configuration of the operational parameters of AD. For 
example, the configuration of HRT, characteristics of the feeding sub-
strate, and OLR directly influence the initial capital investment and 
subsequent revenue. Moreover, operational and maintenance costs 
largely depend on other operational parameters such as heating to 
maintain an optimal temperature, adjustment to a proper ratio of 
feeding substrate, mixing strategy, pumping rate, and feeding frequency 
[30,31]
. 
Process instability is frequently reported during the operation of 
commercial biogas plants. To obtain a more comprehensive overview of 
process instability at the industrial level, reported medium- and large- 
scale cases are summarized in 
Table 2
. As shown by the case over-
view, with the exception of common equipment malfunctions such as 
those related to pumps and mixing devices, most instability occurs as a 
result of inadequate or faulty operating procedures, including organic 
overload, changes in feeding substrate composition, temperature vari-
ation, and shortened HRT (
Fig. 2
). In theory, process stability of AD 
depends mainly on the delicate balance between the production and 
consumption of intermediate products such as volatile fatty acids (VFAs) 
[32]
. However, the microorganisms involved in each stage are distinct 
and significantly different with respect to growth rate, physiology, 
nutritional needs, and sensitivity to environmental changes 
[33]
. 
Accordingly, the sensitivity of different members of the microbial 
community to changes in operational parameters also differs greatly. In 
most cases, unrestrained operational procedures directly lead to large 
deviations in operational parameters, so that optimal conditions for the 
entire microbial community in AD cannot be maintained. Disruption of 
the microbial balance causes AD to become unstable, followed by pro-
cess instability that can lead to a total crash, resulting in severe financial 
consequences. As reported by Labatut and Gooch 
[34]
, due to poor 
anaerobic digester performance and system failure, some AD systems in 
New York State generate less than 60% of their electrical energy po-
tential. Therefore, to allow further sustainable development of com-
mercial biogas plants, process stability is the first priority. 
AD systems in full-scale biogas plants are often operated under 
suboptimal conditions (such as a relatively lower OLR and longer HRT) 
to reduce the potential for process instability 
[31,35]
. However, such 
safety precautions are usually taken at the cost of economic loss and 
capacity utilization. For example, a financial analysis of a 500-kW 
electricity equivalent biogas plant indicated that operating at subopti-
mal conditions caused a 10% reduction in biogas yield, which resulted in 
an 11% loss of annual revenue from electricity sales 
[36]
. Another study 
focusing on suboptimal operational conditions in biogas plants showed 
that up to 30% of the potential in the residual remained unexploited 
[11]
. Thus, the general prevalence of suboptimal operational conditions 
indicates that there remains significant potential for process optimiza-
tion to improve the overall performance of anaerobic digesters. For this 
reason, the demand for the construction of new biogas plants is currently 
decreasing, as shown in 
Fig. 1
, and the focus of the industry is turning to 
process optimization with the goal of improving the efficiency of exist-
ing biogas plants. 
In summary, both process stability and process optimization are 
closely related to the configuration of operational parameters. There-
fore, focusing on these aspects from the perspective of commercial 
biogas production, a comprehensive discussion and related suggestions