3.3. Solids content
An optimized SS-AD process is able to treat more waste in terms
of dry mass than a liquid AD plant of the same size. The initial
substrate concentration influences the mesophilic anaerobic
digestion of OFMSW at batch conditions. A 2008 study found that
when the total solid concentration increased from 20% to 30%, the
COD removal of the SS-AD decreased from 80.69% to 69.05%. The
methane yield at 30% solids content was 17% less than that at 20%
solids content [47].
3.4. Temperature
As SS-AD under mesophilic conditions exhibited a poor start-up
performance, thermophilic operation of AD was developed later
and it has been established as a reliable and accepted mode of SSAD.
Operating SS-AD systems at thermophilic conditions (55 8C)
can accelerate the AD process [48]. It also provides the added
benefit of increased pathogen reduction during the anaerobic
phase. The added amount of heat required for thermophilic
operation can be offset by the higher gas production yields and
rates [2].
Thermophilic operations have been proved to be a reliable and
acceptable option for digestion of organic urban wastes [49,50].
There is also considerable interest in applying SS-AD at thermophilic
conditions (55 8C) to treat the organic fraction of municipal
solids waste (OFMSW) [51]. The biogas yield of anaerobic digestion
of OFMSW at thermophilic conditions is much higher than that in
mesophilic conditions [52].
3.5. C/N ratio
Most of the literature recommends an operating C/N ratio
range of 20/1 to 30/1 with an optimal ratio of 25/1 for anaerobic
bacterial growth in an AD system [36,53]. Improper C/N ratios
could result in high total ammonia nitrogen (TAN) released and/or
high VFA accumulation in the digester. Both TAN and VFAs are
important intermediates and potential inhibitors in the AD
process [53]. High concentrations of TAN and VFAs in the digester
would decrease the methanogen activity and cause possible
failure of the AD process [54].
The optimal C/N ratio varies with the type of feedstock to be
digested. Yen and Brune [54] used waste paper to balance the high
nitrogen concentration of algal sludge for methane production and
results showed that the optimized C/N ratio for the co-digestion
was 20/1 to 25/1. Romano and Zhang [55] recommended the C/N
ratio be maintained at 15 for the co-digestion of onion juice and
digested sludge. When corn stover was inoculated with digested
sewage sludge, the digestion process worked well with a C/N of 15
to 18 but failed with a C/N of 21 or higher because the pH
dramatically decreased in the first 7 days at 37 8C [56].