Environmental Engineering Reference
In-Depth Information
economically or energetically sustainable to transport feedstock such as
animal slurries further than 5 km and energy crops further than 15 km (Epp
et al., 2008). This means that a biogas plant processing these types of
feedstocks should be located inside an average radius of 15 km from the
available feedstock. In many cases, agricultural biogas plants are built in the
proximity of animal farms so that liquid animal slurries can be piped
directly from the livestock sheds to the plant. For other feedstock types such
as the organic fraction of MSW or catering waste, the economically
sustainable distances of biomass transport are often determined by the waste
collection systems and the gate fees for the waste treatment. When large
amounts of agro-industrial wastes are available at one place, AD facilities
could be set up at the agro-industrial site, with the advantage of very short
transport distances for which even piping systems could be used.
Apart from the distance between the feedstock source and the biogas
plant, the transport of digestate has to be considered. For the most common
use of digestate as agricultural fertilizer, it is also recommended to only
transport digestate within a radius of about 15 km around the biogas plant
(Epp et al., 2008). However, in areas with a high concentration of
agriculture and problems of over-fertilization, digestate may be transported
further. In order to reduce transport costs, additional treatment of digestate,
such as liquid-solid separation or drying may be suitable alternatives. If
liquid-solid separation is used, the solid fraction may be further composted
and sold as compost or used on agricultural or landscape areas. The liquid
fraction can be further processed and used as nitrogen-rich liquid fertilizer.
A good overview on different options for digestate treatment is given by
Fuchs and Drosg (2010) and more details are available in Chapter 12 of this
topic.
Feedstock storage serves primarily to compensate for the seasonal
fluctuations of feedstock supply. It also facilitates mixing different co-
substrates for continuous feeding of the digester (Al Seadi et al., 2008). The
type of storage facilities depends on the feedstock used. Storage facilities can
be bunker silos for solid feedstock like maize silage (Fig. 2.4), often covered
with plastic sheets to minimize the exposed face, and storage tanks for liquid
feedstock, frequently used for liquid manure and slurries. Usually, bunker
silos have a storage capacity of more than one year while for storage tanks it
is usually several days. Details on ensiling of substrates can also be found in
Chapter 4 of this topic.
The dimensioning of storage facilities is determined by the quantities to be
stored, delivery intervals and the daily amounts fed into the digester.
Depending on the logistical concept, feedstock storage facilities as well as
digestate storage facilities may be located at the biogas plant or
decentralized in the agricultural surroundings of the biogas plant, which is
more often the case for digestate storage. Decentralized storage of feedstock
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