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exposed to 100 mg/L Al(OH) 3 for 59 days. The excessive amounts of calcium lead to
precipitation of carbonate and phosphate, which affect biomass activity because of mass transfer
2+
limitations. A study from Yu et al. (2001) showed that the low Ca concentration from 100-300
mg/L to be beneficial for sludge granulation in an UASB reactor. The optimal sodium
concentration for mesophilic aceticlastic and hydrogenotrophic methanogens growth was 230
+
mg/L and 350 mg N /L, respectively (Kugelman and Chin, 1971; Patel and Roth, 1977).
Moreover, a study the effect of potassium and magnesium on microorganism’s growth shows
+
that at high concentration of potassium (0.15 M K ) caused 50% inhibition of acetate-utilizing
methanogens, while under high magnesium ions concentration, stimulate the production of single
cell, which causes loss aceticlastic activity in the digesters. Consequently, under high salt levels
could cause bacterial cells to dehydrate due to osmotic pressure, and therefore suppress microbial
growth, and consequently affect specific growth rate.

Several industrials and domestic wastewater was found a significant concentration of
heavy metals such as zinc (Zn), copper (Cu), cadmium (Cd), nickel (Ni), iron (Fe), chromium
(Cr), and etc. Heavy metals are bio-available and it’s a necessary as nutrients at low
concentration to microbial growth. Heavy metals are toxic when present in ionic form since it’s
attributed to disruption of enzyme function and structure by binding to ion-exchange site on the
cell membrane and form a matrix with extracellular enzymes. Takashima and Speece (1989)
reported the following order of heavy metal composition in the cell of ten methanogenic strains:
Fe >> Zn > Ni > Co = Mo > Cu, and also acidogens are more resistant to heavy metal toxicity
than methanogens. There are several forms of heavy metal such as precipitation as sulfide,
carbonate, and hydroxides, and sorption to the solid fraction. There are, however, only metals in
soluble, free form are toxic to the microorganisms (Oleszkiewicz and Sharma, 1990). The study
from Lin (1993) showed that the relative toxicity of heavy metals in inhibition test on sensitivity
of methanogens as Pb > Ni > Cd > Cu > Cr > Zn, while the relative toxicity of heavy metals on
sensitivity of acidogens is Cu > Zn > Cr > Cd > Ni > Pb (Lin, 1992).