Environmental Engineering Reference
In-Depth Information
of the buffer varies due to expansion of the membrane itself. The shape is
typically that of a sphere. In the double-membrane version, the inner
membrane is protected from weather influences by an outer membrane. The
space in between the membranes is flexible and is ventilated with ambient
air. Instead of EPDM, polyethylene is used as membrane material in these
buffers. In both cases, any diffusion or leakage through the inner membrane
will lead to emission of methane.
Bu¨ eler investigated the magnitude of the permeation through the
membrane under clearly defined laboratory conditions. The potential
influences of a number of circumstances were determined, as follows.
.
Thickness of the membrane. Applied membranes vary in thickness
between 0.8 and 2mm, generally being around 1.5mm; the thickness
was found to be inversely proportional to the permeation rate.
.
Effect of aging. No clear dependence between age and diffusion rate
could be established.
.
Effect of organic acids in the fermenter. An indication of increasing
permeation with increasing acid concentrations was shown, but this was
not considered statistically significant.
.
Effect of substrates treated in the digester. No influence of variations in
substrates was determined.
.
Influence of expansion of the membrane. Expanding the membrane by
40% of its original dimensions increased the permeation by 23-24%.
This is a result of
the decrease in thickness connected with the
expansion.
The actual emissions for biogas plants were estimated based on the
permeation rate, the degree of expansion of the membrane and the
dimensions and gas production at ten different biogas plants. The
conclusion drawn for single-membrane gas buffers made from EPDM was
that the methane emission by permeation through the membrane was 0.26%
of the average volume present in the biogas buffer. For double-membrane
buffers, where polyethylene was applied instead of EPDM, the permeability
of the polyethylene was stated to be only 10% of the value for EPDM.
It is also important to note that the figure of 0.26% refers to the average
volume in the buffer. Generally, biogas buffers are designed to hold a
capacity equal to a number of hours of biogas production (4-6 hours). This
means that the biogas volume that passes through the buffer is 4-6 times
larger than the volume of the buffer itself. In relation to biogas production,
the methane emission would only be 0.040-0.065% (earlier literature
mentions a value of 0.1% of the methane production).
Thus, the overall conclusion of this particular study was that methane
emission by permeation through the EPDM membrane was between 0.040
and 0.065% of total biogas production. Where polyethylene membranes are
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