MONITORING OF FOOD SAFETY IMPORTANT MICROBES AND THE HEAVY
METAL STATUS IN THE SEWAGE SLUDGE TREATED AGRICULTURAL SOILS
Borbala BIRÓ1, Judit BECZNER2
Research Institutes for 1Soil Science and Agricultural Chemistry of H.A.S., 2Central Food
Science, Herman O. ut 15, H-1022 Budapest, Hungary; E-mail: biro@rissac.hu
Some of the human pathogens, which are important from food safety reasons, are
naturally present in the soils (i.e. Bacillus cereus, Clostridium botulinum, Cl. perfringens,
Listeria monocytogenes). After the sewage-sludge treatment, microbes with initiative ability
of several human-diseases may occur and the risk of heavy metal pollution is increasing.
Sewage sludge can be utilised in the agriculture as nitrogen-, phosphor-, microelement- and
organic matter supplements, which are normally removed from the soil by the common
agricultural practises. Biological treatments of the sludge generally reduce the abundance of
human pathogens, nevertheless absolutely pathogen-free status rarely reached. These
microbes may survive in the sludge and the soils for long periods.
Four representative Hungarian soil-types (calcareous chernozem-, acidic forest-,
calcareous sandy- and acidic sandy soils) were treated with two different sewage-sludge
forms (municipal sludge with high zinc content and composted sludge from a leather factory
with high chromium content) in a pot-experiment by using four replicates. Annual doses of 0,
2.5, 5.0, 10 and 20 g sludge d.m. per kg dried soil were used for three consecutive years. Soilsamples were investigated for the abundance of several microbes (at the 3rd year), such as the
aerobe total heterotrophs, micromycetes (moulds, yeasts), coliforms, E. coli, sulphitereducing clostridia, Salmonella, Listeria, Bacillus cereus by using a selective plate-dilution
assay, which are generally accepted in the food microbiological-practice. No
essential
difference was found between the effect of two sludge-types on the microbiological status of
the soils in general. Quantified microbes, however, were found to be more abundant as a
function of increasing sludge doses. Salmonellae was assumed to be eliminated during the
sewage sludge treatment. Microbes survived, however indicated a faecal contamination, i. e
the coliforms (without the proved presence of E. coli) and microbes representing the
Enterobacteriaceae. Micromycetes preferred the more acidic soil-circumstances. The
abundance of sulphite-reducing clostridia increased as a function of sludge doses, more in the
sandy than in the loamy soils. Spores of Bacillus cereus occurred in all type of soils.
Salmonellae, however were un-detectable from the pots (except one sample). Listeria spp.
occurred in each soils and treatments, neither the presence nor the absence could be
unanimously proved, however.
The sludge-doses used have increased the microbial abundance of the soils and concomitantly
enhanced the potentiality of pathogens on the edible plants. At the 3rd year of experiment,
however the increase of microbial abundance was not significant, which suggest a fast
elimination of the introduced microbes after the sludge-deposition. This phenomenon might
be a clear consequence of the activity of indigenous competitive microflora, or some other
affecting environmental factors. The heavy metal accumulation and the plant uptake might be
monitored regularly for reducing the environmental risk on a long-term level.
The support of the Hungarian Research Fund (OTKA T0 46610) is highly acknowledged.