Biogas Safety first!

General requirements
5.1. Hazard assessment for specific
operating states
Normal operation for a plant is the state in which the
plant is operated within its design parameters. It is
not appropriate to consider start-up/shut-down or
maintenance work on a continuously running biogas
plant as normal operation. Statistical evaluations of
personal injuries at biogas plants reveal a significantly
high accident rate in connection with maintenance
work and start-up or shut-down procedures. These
operating states should therefore be considered separately,
with their own specific operating instructions.
5.2. Start-up / commissioning
It is always advisable to draw up a start-up plan before
starting commissioning, and to follow it closely
during the start-up phase.
Before the digester is filled for the first time, all work
on it including the associated pipework must be
completed in order to prevent potential damage or
injury. Precise knowledge of the plant load requires
the feedstock materials to be properly weighed. This
is highly important both for start-up operation and
for ongoing process control. In addition, regular
chemical analysis of the feedstock materials and
in particular of the digestion mixture is a sensible
control measure for speedy start-up. However, if the
organic loading rate rises too fast during the phase
of establishing the digestion biology, the process can
quickly become overloaded; ultimately this can extend
the duration of start-up operation. Incompletely
filled tanks can lead to an uncontrolled escape of
biogas. During filling it is therefore important to take
care that the filling level is sufficient to ensure that
the substrate feeding equipment is fully immersed
in the liquid phase.
It should also be noted that during start-up of the
plant an explosive gas mixture is temporarily present
because of the increase in the proportion of methane
in the biogas (with a volume fraction of 6–22 %
biogas in air).
5.3. Maintenance and repair work
Only persons who have the necessary specialist
knowledge and experience for the relevant work and
of whom it can be expected that they will perform
their task reliably should be allowed to maintain
and repair biogas plants. The protective measures
required for the maintenance and repair of biogas
plants must be defined by the client on the basis of
the hazard assessment (see section headed Hazard
assessment and see Figure 6) and must be applied
when the work is carried out (TRGS 529, 2016).
The necessary protective measures must be
determined and documented as part of the
hazard assessment before the work is carried out.
The documentation must also include written work
instructions, and in addition, in the case of work
with ignition hazards, must contain a permit-towork
system (see Annex 2 ‘Instruction record for
subcontractors and employees for maintenance,
installation and repair work’).
During maintenance work on biogas plants, in addition
to the areas subject to explosion hazard designated
in the explosion protection document under
normal operation (see section headed Explosion protection
document) there may be further areas with a
hazardous explosive atmosphere (and depending on
the H 2
S content of the biogas, areas posing a health
hazard) for the duration of the work (for example
when membrane roofs are opened in order to replace
the agitator).
Appropriate protective measures include:
Establishing and marking or cordoning off areas
where a risk of fire/explosion or a health hazard
is to be expected.
Shutting down electrical and other
non-explosion protected systems.
Removal of biogas from parts of the plant.
Selection of appropriate explosion-proof
equipment and tools.
Clearance measurement, i.e. the determination
of the concentration of hazardous materials or
oxygen content using appropriate measuring
techniques in a certain area. The purpose of this
is to classify the surrounding atmosphere as safe
for employees or to initiate further protective
measures.
Use of appropriate personal protective
equipment.
Ensuring adequate ventilation.
Appointment of a supervisor.
Employees engaged in activities in the vicinity of
maintenance and repair work must be notified of
the timing, location and nature of the planned work
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