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drop of pH-value. At lower pH values, much more of the VFAs exists in the undissociated form,
which is much more toxic than ion form since its greater membrane permeability. A study from
Amani et al. (2011) showed that under high mixing ratio of methanogens to acetogens (M/A =
3.1) the removal efficiency of propionic (HPr), butyric (HBu), and acetic acids (HAc) decreased
because of the acetogens (propionate- and butyrate-oxidizing bacteria) were not sufficient to
degrade high concentration of propionic (1543.5 mg/L) and butyric acids (2000.8 mg/L), they
inhibited the acetogenic reactions and suppressed the growth of acetogens. At the same time,
increasing M/A from 1.1 to 2.1, the removal efficiency of HPr, HBu, and HAc increased from
10, 18, and 20 to 41, 59, and 46%, respectively.

+
Ammonium ion (NH 4 ) and free ammonia (NH 3) are the two principal forms of inorganic
ammonia nitrogen in aqueous solution come mainly from biodegradation of the nitrogenous
compounds, mostly from proteins and urea. Free ammonia has been suggested to be the main
cause of inhibition since it is freely membrane-permeable. The hydrophobic ammonia molecule
may diffuse passively into the cell, causing proton imbalance, and/or potassium deficiency (Chen
et al., 2008). The study from Cavinato et al. (2012) showed that the methane production rate
3
3
3
3
decreased from 2.0 m /m r d to 1.6 m /m r d since the concentration of ammonia in the second
stage increased to 2 g/L with free ammonia concentration of 916 mg/L. Under thermophilic
conditions, 700 mg/L of free ammonia could be already toxic for methanogenic archae.
Moreover, they also reported that 2 g-N/L of ammonia concentration was responsible of
inhibition for biohydrogen production. Moreover, increasing of ammonia concentration from 960
mg/L to 1976 mg/L, the acetic and butyric acids decreased, while propionic acid increased from
696 mg-COD/L to 1904 mg-COD/L, which was the presumably reason for the observed decrease
in hydrogen yield.

Proteins also had a constituent of both sulfate and sulfur compounds. In anaerobic
2-
2-
condition, sulfate (SO 4 ) is reduced to sulfide (S ) by responsible of two major groups of sulfate
reducing bacteria are comprehensive complete and incomplete oxidizers. The reduce compounds
such as lactate was converted to acetate and carbon dioxide by incomplete oxidizers and
-
complete oxidizers, acetate was completely converted to carbon dioxide and bicarbonate (HCO 3 )
with reducing sulfate as electron acceptor. The competition of sulfate-reducing bacteria does not
occur in the hydrolysis stage since it does not degrade natural biopolymers such as starch,
proteins, and lipids. However, the competition of sulfate-reducing bacteria with methanogenic
archaea for hydrogen and acetate is occurred at low concentrations of sulfate, while the
competition of sulfate-reducing bacteria with acetogenic bacteria for propionate and butyrate is
occurred under high concentration of sulfate.

Light metals ions including sodium (Na), potassium (K), calcium (Ca), magnesium (Mg),
and aluminum required at low concentration as stimulate nutrients for microbial growing.
Cabirol et al. (2003) reported the mechanism of aluminum inhibition due to its adhesion to the
microbial cell membrane and microbial cell wall, which affect microbial growth. Both
acetogenic and methanogenic microorganisms decreased by 50% and 72%, respectively after