SILAGES FOR ANAEROBIC REACTORS The microorganisms that

SILAGES FOR ANAEROBIC REACTORS
The microorganisms that grow in silage consist of part of the epiphytic microflora of
the forage plant, to which a more or less significant amount of contaminating
microflora is added.
Epiphytic microflora consists of bacteria, yeasts, moulds, microalgae and related
spores.
In particular, sporogenic bacteria and many species of fungi may be present only in
the form of spores and in that case they are “transitory” organisms: that is, such
bacterial spores (endospores) and fungal spores (ascospores and conidiospores) may
subsequently germinate, in the ensiled product or during desiling.
However the leaf surface is colonised above all by bacteria, which on average form
populations of 106-107 cells/cm2 (Lindow and Brandl, 2003). Although the good
ensiling process is based on lactic fermentation, accomplished by specific lactic acid
bacteria (LAB), bacterial plate counts show that LAB are numerically underrepresented, and present only in small numbers, around 200-300 CFU/cm2 in
Gramineae grasses (Fenton, 1987).
Hence, the practice of inoculating forage with an adequate number of LAB cells
(starters) at the ensiling stage is justified, in order to anticipate and boost lactic
fermentation, avoiding abnormal fermentations (butyric fermentations, and
putrefactions) as well as severe dry matter and energy losses.
In particular, in the case of silages for feeding anaerobic digester systems, acetic acid
production by facultatively heterofermentative LAB starters has to be evaluated as a
key element in the choice of inoculating strains, because acetic acid is a substrate for
the production of methane by some Archea.
In addition, acetic acid has strong anti-yeasts and anti-moulds properties and it helps
to stabilize the silage after the opening, reducing dry matter losses.
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METHANE PRODUCTION INTO ANAEROBIC REACTORS.
Methane production is a complex process, which can be divided up into four phases:
1) hydrolysis;
2) acidogenesis;
3) acetogenesis;
4) methanation.
At the end of the degradation chain, two groups of methanogenic Archea produce
methane: one from acetate, the other from hydrogen and carbon dioxide.
In any case, both malfunctions and poor methanation yields often do not find a clear
understanding because of limited knowledge on microbial communities present in
anaerobic reactors.
On the other hand, these microbial communities have no definite composition, and
the influence of material introduced into the digester (chemical composition, particle
size, pH) and fermentation temperature on type/speed of microbial growth are
enormous. The biogas production can occur in mesophilic (35-42° C) or thermophilic
digesters (45-60° C).
Important parameters for the anaerobic process are:
- Volatile fatty acids (VFA). The production of VFA (acetic acid, propionic acid,
butyric acid, etc.) can be stimulated by a proper dosage of macro-nutrients (nitrogen,
phosphorus, sulphur and metals) provided that they are lacking, and by adjustment of
pH around 6.0. Acetic acid can be added by means of a silage fermented by a suitable
LAB culture. After the digestive process, the VFA concentration should be at least 45 meq/liter. Higher concentrations indicate that the process of digestion had some
problems and so it’s necessary to dose nutrients or alkali in order to keep pH under
control.
- Alkalinity. Alkalinity of bicarbonates is another important control parameter. The
optimal range of alkalinity of the reactor effluent is between 15 and 20 meq/liter. If
lesser, you must correct the pH, for example with lime or soda. Obviously, VFA
concentration, bicarbonates, and pH, are interrelated parameters.
- Temperature. The optimal range of temperature of anaerobic digestion (mesophilic
digesters) is 35-38° C, but the degradation process could be satisfactory even at
slightly lower temperatures. At a temperature of 42° C the activity of mesophilic
Archea decreases significantly, and above 45° C they undergo to an irreversible
cellular damage. It is important to maintain constant thermic conditions to ensure the
best yields.
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CENTRO SPERIMENTALE DEL LATTE S.p.A. con socio unico
Società soggetta a Direzione e coordinamento della Capogruppo Granarolo S.p.A
- Sede Legale: Strada per Merlino, 3 – 26839 Zelo Buon Persico (LO) - ITALIA
CAPITALE SOCIALE € 2.620.000,00 i.v. - R.E.A. LO-372111 - Codice Fiscale e Part. I.V.A. n. 00886520154
Tel. +39 (02) 90696.1 – Fax +39 (02) 90696.99 – E-mail: [email protected] - www.csl.it
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CENTRO SPERIMENTALE DEL LATTE S.p.A. con socio unico
Società soggetta a Direzione e coordinamento della Capogruppo Granarolo S.p.A
- Sede Legale: Strada per Merlino, 3 – 26839 Zelo Buon Persico (LO) - ITALIA
CAPITALE SOCIALE € 2.620.000,00 i.v. - R.E.A. LO-372111 - Codice Fiscale e Part. I.V.A. n. 00886520154
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CENTRO SPERIMENTALE DEL LATTE S.p.A. con socio unico
Società soggetta a Direzione e coordinamento della Capogruppo Granarolo S.p.A
- Sede Legale: Strada per Merlino, 3 – 26839 Zelo Buon Persico (LO) - ITALIA
CAPITALE SOCIALE € 2.620.000,00 i.v. - R.E.A. LO-372111 - Codice Fiscale e Part. I.V.A. n. 00886520154
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CENTRO SPERIMENTALE DEL LATTE S.p.A. con socio unico
Società soggetta a Direzione e coordinamento della Capogruppo Granarolo S.p.A
- Sede Legale: Strada per Merlino, 3 – 26839 Zelo Buon Persico (LO) - ITALIA
CAPITALE SOCIALE € 2.620.000,00 i.v. - R.E.A. LO-372111 - Codice Fiscale e Part. I.V.A. n. 00886520154
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