Excessive nitrogen and insufficient carbon mean an imbalance diet for
microorganisms, and possibly result in ammonia accumulation
and subsequent inhibition to microbial activity (Zhang et al.,
2015). Besides low C/N ratios, low-organic-content sludge is
another negative factor influencing the benefits of anaerobic digestion.
This kind of sludge is commonly derived from the wastewater
treatment plants (WWTPs) using long sludge retention time or collecting
some rainfall, industrial wastewater and wastewater from
construction sites. The typical organic contents (expressed as
volatile solids content in total solids, VS/TS) in sludge range from
60% to 80%, and the corresponding organic degradation rates are
generally from 56% to 65% when sludge retention time (SRT) is
15–30 days (Appels et al., 2008).
In contrast, low-organic-content sludge usually leads to much worse performance. For example,
the organic degradation rate decreased to only 25–35% when
sludge VS/TS was lower than 50% (Liao and Li, 2015).
This eliminates the economic feasibility of bioenergy recovery from sludge anaerobic digestion seriously.