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

3.4. Temperature management and control 
Temperature is a critical factor that affects the reaction rate, process 
stability and microbial activity during AD. The optimal temperature 
ranges for the growth of various anaerobic microorganisms can differ 
significantly 
[78,79]
. For example, psychrophilic AD without active 
D. Wu et al.

---

Fuel 303 (2021) 121282
9
heating may lack financial feasibility due to slow microorganism activity 
and low biogas yield. Heating and insulation are straightforward 
methods of maintaining an optimal mesophilic (30–37 
◦
C) or thermo-
philic (50–55 
◦
C) temperature at the industrial level 
[61]
, and these two 
common ranges are more economically viable than intermediate ranges. 
According to a financial analysis of a commercial medium-sized biogas 
plant, changing the operating temperature from a mild temperature 
(47 
◦
C) to a temperature favoring thermophilic operation (55 
◦
C) 
improved the biogas production from 0.45 to 0.62 m
3
/kg VS. Although 
the total cost of digester operation was increased to 719,885 
€
/yr, the 
energy capacity reached 8,789 MWh/yr, while benefits reached 
1,933,473
€
/yr 
[80]
. 
An operating temperature outside the preferred range of the micro-
bial community can disturb their metabolic activity and affect the 
thermodynamics and kinetics of their biological processes. When the 
rate of temperature change exceeds 1 
◦
C per day, process instability may 
be encountered, especially in plants operating at thermophilic temper-
atures. Multiple studies have demonstrated reduced microbial diversity 
during operation at thermophilic temperatures, and the optimal tem-
perature range for thermophilic operation is relatively narrow 
[81–83]
. 
Therefore, thermophilic anaerobic reactors are relatively sensitive to 
changes in temperature, and even small change of a few degrees can 
have a strong detrimental effect on overall performance. However, due 
to the advantages of reduced HRT and a higher rate of methane gener-
ation, thermophilic operation is still popular in full-scale commercial 
applications 
[78,84]
. The thermophilic temperature range is reported to 
be preferred in most full-scale and farm-scale Danish biogas plants 
performing co-digestion of manure and organic waste 
[80]
. Generally 
speaking, the selection of an operating temperature depends on multiple 
factors, including the characteristics of feedstock, energy demand, 
financial support and sanitization requirements. However, the primary 
concern during the construction and operation of any AD system should 
be temperature management and control. 
With recent improvements in system design and construction prac-
tices, as well as an increasing number of qualified companies developing 
biogas projects, techniques focusing on temperature management and 
control have become relatively mature and well-developed 
[34,45]
. 
However, unexpected temperature changes caused by mechanical mal-
functions may be responsible for subsequent process instability. Moeller 
et al. 
[39] 
reported that a sudden rise in operating temperature from 
35 
◦
C to 38 
◦
C led to increased microorganism death and the release of 
mucilage and storage substances such as extracellular polymer sub-
stances (EPS), which increased the viscosity of the reactor contents and 
contributed to the formation of foam, consequently decreasing biogas 
production. In other cases, the activity of methanogens and VFA con-
version rate were significantly slowed as the temperature decreased due 
to a power outage. In turn, the metabolic activity and growth of 
methanogens were inhibited by accumulated VFAs, and the substrate 
was not digested. Once the temperature recovered, rapid hydrolysis 
resulted in the accumulation of a high concentration of VFAs, increasing 
the potential risk of process instability 
[13,85,86]
. Note that with the 
application of a remote alarm system, the probability of long-term 
process failure caused by temperature fluctuation may be reduced. 
In addition to unexpected heating and accidental power failure 
caused by mechanical malfunction, the application of pre-treatment 
methods along with seasonal variations in ambient temperature can 
also contribute to changes in the operational temperature of anaerobic 
digesters. As mentioned in 
Section 3.1.2
, removal of a portion of the 
lipids contained in FW prior to AD operation is widely accepted as an 
effective method of minimizing the negative effects of LCFAs and 
enhancing the operational stability of anaerobic digesters. However, if 
the process is performed on summer days with a relatively high ambient 
temperature, the substrate stored in the adiabatic container or equal-
ization basin usually has a high temperature after the application of the 
high-temperature extraction pretreatment. The effects of the factors 
described above slow the process of substrate cooling, and consequently, 
the substrate may not be able to match the temperature required by 
biogas plants operating under mesophilic conditions. If the feeding 
procedure continues without countermeasures such as heating exchange 
[87]
, a significant fluctuation in operating temperature can occur. Thus, 
more attention should be paid to achieving proper utilization of neces-
sary pretreatment methods while considering the temperature re-
quirements of the mainstream AD process. 
In conclusion, as one of the most important operational parameters 
of AD, maintaining a constant temperature within the optimal range and 
ensuring that no large variations occur are major requirements for long- 
term stable and effective biogas production. In theory, with the wide 
application of process monitoring and a remote alarm system, deviation 
from optimal operating temperatures caused by mechanical malfunction 
and the application of pretreatment procedures can be completely 
avoided.