Abstract:
In recent years, it has been reported that numbers of Escherichia coli increase
significantly following centrifugation of sludge during the treatment process. E.
coli is used as an indicator of the microbiological quality of sludge-derived
products destined for agricultural recycling and of the efficacy of the sludge
treatment processes. The re-growth phenomenon is of concern because of the
potential for additional treatment requirement / higher disposal costs and loss of
consumer confidence associated with a compliance failure. It is hypothesised
that a competitive exclusion treatment could be the solution wherein the
digestate be exposed to a ‘probiotic’ or defined mixture of micro-organisms, to
effectively out compete or eliminate any resident E. coli remaining following
treatment. The competitive exclusion principle as a treatment method has
already seen application in various industrial sectors, the most well-known
being the poultry industry. In experiments it was determined that an
antimicrobial producing organism would be most likely to succeed. From the
candidates screened, Lactobacillus reuteri proved the most promising. L. reuteri
is a known producer of reuterin in the presence of glycerol and organic acids as
a part of its normal metabolic activity. In sludge derived nutrient broth in the
presence of glycerol and low pH, L. reuteri addition resulted in a reduction of E.
coli to undetectable levels. In sludge cake under the same conditions, L. reuteri
was less successful. However the addition of glycerol and L. reuteri to sludge
cake restricted E. coli growth to a 2 log increase from the initial concentration of
E. coli recorded following pasteurisation (an average of around 1x102 cfu/gDs),
in comparison in the positive control a 4 log increase was recorded. From this
result the sludge cake could be defined as conventionally treated. It can be
concluded that competitive exclusion and L. reuteri show promise as a
treatment for reducing E. coli re-growth in sludge cake
