Environmental Engineering Reference
In-Depth Information
treatment (soaking or spraying with 1.5%-2% NaOH solution) and nitrogen
enrichment with ammonia or urea (Schiere and de Wit 1995). The combination of
treated straw and protein-rich food processing wastes (oil cakes) can replace hay or
silage and make it possible to feed beef or dairy cattle without claiming farmland
for concentrate and roughage crops. This option has the highest appeal in land-
scarce Asian countries.
The excellent water absorption capacity of cereal straw (equal to two to three
times their mass for uncut residues, at least three times for chopped material) makes
it also the preferred material for animal bedding. Besides keeping the coni ned
animals clean and comfortable, bedding residues make manures easier to handle
and limit the leaching loss of absorbed nutrients; where straw is plentiful, about
250 kg are used for each 1 t of excrement. Recycling of this nitrogen-rich material,
often after composting, remains a key component of organic agriculture (Kristiansen,
Taji, and Reganold 2006). An even more important use of crop residues is achieved
through direct recycling, in which cereal stubble, chopped corn stover, and legume
and tuber vines are left on the ground to protect against wind and water erosion
and are incorporated into soil, maintaining its organic content and increasing its
water retention capacity.
Even in some modern settings straw continues to be used as fuel and construction
material. Denmark has been a leader in straw burning: about 1.4 Mt of wheat straw
(of the total annual output of about 6 Mt) are burned annually in small household
ovens and for centralized district heating and electricity generation (Stenkjaer 2009).
Clean shredded straw (most often wheat or rice) is used to make boards by com-
pressing the material (it can also be fused with internal resins) and bonding it to
external paper or sandwiching it between laminated strand lumber. Stramit Inter-
national and Agriboard Industries are the leading makers. There is also a fringe
interest in straw-bale buildings as a form of frugal architecture (Steen et al. 1994;
Strawbale 2010).
Straw pulps are similar to short hardwood i bers, but their low density, the high
collection and transportation costs of crop residues, and their relatively high silica
content make more tree-free paper unlikely. Another marginal use of crop residues
has been as a feedstock in biogas generation (Smil 1993; Demirba¸ 2009), and
residual phytomass can be also tapped as feedstocks for extracting organic com-
pounds such as furfural (used as a selective solvent in crude oil rei ning and in
bonded phenolic products) from pentosan-rich corn cobs, rice hulls, or sugarcane
bagasse, or directly from corn stover and cereal straws (Staniforth 1979; Di
Blasi, Branca, and Galgano 2010). Yet another common use of cereal straws is in
