Agriculture Reference
In-Depth Information
ability of different species of earthworms to grow and reproduce in sludge, (3) the need for methods
of preprocessing sludge to make it acceptable to earthworms, and (4) the effects of mixed earthworm
species on rates of sewage sludge breakdown in a vermistabilization process or system. For
earthworms to be commercially useful in stabilizing sludges, they must increase the stabilization
rate, which can be demonstrated best if the presence of earthworms in sludge causes an increase
in the rates of volatile solids reduction. A maximum rate of reduction of volatile solids is a major
goal of any sludge stabilization system.
Research at SUNY followed two complementary lines: (1) basic studies to identify fundamental
factors that affect the performance of the vermistabilization process with sludges and (2) applied
studies to determine design and management relationships for sludge systems (Neuhauser et al.
1988). The earthworm
has been shown to accelerate the rates of destruction of sludge
volatile solids in aerobic sludges greatly. This increase in the rate of destruction of volatile solids
decreases the probability of putrefaction occurring in the sludge because of creation of anaerobic
conditions. The main cause of increased rates of degradation of the sludge is probably the increased
aeration and turnover of the waste by the earthworms. A series of experiments determined the most
desirable moisture contents for optimal procession of sludges used in vermistabilization units
because both excessive and insufficient moisture can have an adverse impact on the growth of
earthworms (Edwards 1988).
A range of sewage sludges with different moisture contents were tested, using
E. fetida
E. fetida,
at a
temperature of 25
C (Neuhauser et al. 1988). Earthworm growth at the lower and higher total solids
levels (6.3 to 7.9% and 17.9 to 25.1% solids) differed statistically at the
A
.01 probability level
from the earthworm growth rates in the middle range of solids (9.3 to 15.9% solids). This indicated
that optimum earthworm growth occurred over a range of total solids in the media on a wet basis
ranging from about 9 to 16% (i.e., 84 to 91% moisture contents). However, more liquid wastes
than these can be used in the vermistabilization process as long as they are added slowly to the
system and the liquid can drain readily from the sludge, the organics are retained, and aerobic
conditions are maintained. The desirability of maintaining the total solids content of the media in
the range quoted above does not mean that wetter sludge slurries cannot be processed with
earthworms. In fact, in practice it might be necessary to add moisture to wastes in a vermistabili-
zation process if the media should become too dry. It is also important to correlate rates of earthworm
growth with the age of the sludge, that is, the time after the sludge was removed from the aerobic
reactor and dewatered (Neuhauser et al. 1988). As sludge ages, its nutritive value to earthworms
decreases rapidly, whereas the ash content of the sludge increases with time, which is a further
indication of sludge stabilization.
The practical feasibility of using earthworms to stabilize wastewater treatment sludges was
investigated (Loehr et al. 1984). These workers concluded that a liquid sludge vermistabilization
process was feasible and provided data on the rates of stabilization and the physical and chemical
characteristics of the residual stabilized solids resulting from vermistabilization, a conclusion that
was supported by Pincince et al. (1980).
There have been various practical attempts to utilize earthworms as a main component of
sewage breakdown in the United States (Pincince et al. 1980; Green and Penton 1981; Loehr
et al. 1984), in the U.K. (Edwards and Neuhauser 1988), and in Italy (Tomati and Grappelli
1984). However, full-scale, long-term successful vermiprocessing of sewage sludge has not yet
been achieved. The potential of commercial operations was reviewed by Hartenstein (1978),
Appelhof (1981), and Edwards and Neuhauser (1988), who all concluded that a major obstacle
in the systems to be overcome is the possibility of exposure of the earthworms used in
vermistabilization to toxic chemicals that might enter the wastewater stream accidentally.
However, the use of biosolids as a substrate for vermicomposting is well established and holds
considerable potential now that it is known that human pathogens are eliminated during ver-
micomposting (Eastman et al. 2001).
p
≤
