Responses of indicator bacteria to forest soil amended with municipal sewage sludge from aerated and non-aerated ponds

Biowaste Mixtures Affecting the Growth and Elemental Composition of Italian Ryegrass ()

Biosolids (sewage sludge) can be beneficially applied to degraded lands to improve soil quality. Plants grown on biosolids-amended soils have distinct concentrations of macronutrients and trace elements, which can be beneficial or present a risk to humans and ecosystems. Potentially, biosolids could be blended with other biowastes, such as sawdust, to reduce the risks posed by rebuilding soils using biosolids alone. We sought to determine the effect of mixing biosolids and sawdust on the macronutrient and trace element concentration of ryegrass over a 5-mo period. was grown in a low fertility soil, typical for marginal farm areas, that was amended with biosolids (1250 kg N ha), biosolids + sawdust (0.5:1) and urea (200 kg N ha), as well as a control. Biosolids increased the growth of from 2.93 to 4.14 t ha. This increase was offset by blending the biosolids with sawdust (3.00 t ha). Urea application increased growth to 4.93 t ha. The biowaste treatments increased N, P, Cu, Mn, and Zn relative to the control, which may be beneficial for grazing animals. Although biowaste application caused elevated Cd concentrations (0.15-0.24 mg kg) five- to eightfold higher than control and urea treatments, these were below levels that are likely to result in unacceptable concentrations in animal tissues. Mixing biosolids with sawdust reduced Cd uptake while still resulting in increased micronutrient concentrations (P, S, Mn, Zn, Cu) in plants. There were significant changes in the elemental uptake during the experiment, which was attributed to the decomposition of the sawdust.

Decay of enteric microorganisms in biosolids-amended soil under wheat (Triticum aestivum) cultivation

There is a growing need for better assessment of health risks associated with land-applied biosolids. This study investigated in-situ decay of seeded human adenovirus (HAdV), Salmonella enterica, Escherichia coli, and bacteriophage (MS2) in biosolids-amended soil under wheat cultivation. The biosolids seeded with microorganisms were placed in decay chambers which were then placed in the topsoil (10 cm depth) at three different sites. Sites were selected in arid wheat-growing regions of Australia with loamy-sand soil type (Western Australia) and sandy soil (South Australia). Seeded E. coli and S. enterica had a relatively short decay time (T90 = 4-56 days) in biosolids-amended soil compared to un-amended soil (T90 = 8-83 days). The decreasing soil moisture over the wheat-growing season significantly (P < 0.05) influenced survival time of both bacteria and MS2 at Western Australia (Moora) and South Australia (Mt Compass) sites, particularly in the un-amended soils. Increasing soil temperature also significantly (P < 0.05) influenced the decay of MS2 at these sites. In this study, no notable decline in HAdV numbers (PCR detectable units) was observed in both biosolids-amended and the un-amended soils at all three sites. The HAdV decay time (T90 ≥ 180 days) in biosolids-amended and un-amended soils was significantly higher than MS2 (T90 = 22-108 days). The results of this study suggest that adenovirus could survive for a longer period of time (>180 days) during the winter in biosolids-amended soil. The stability of adenovirus suggests that consideration towards biosolids amendment frequency, time, rates and appropriate withholding periods are necessary for risk mitigation.

Mobility and survival of Salmonella Typhimurium and human adenovirus from spiked sewage sludge applied to soil columns

AIMS

This study investigated the survival and transport of sewage sludge-borne pathogenic organisms in soils.

METHODS AND RESULTS

Undisturbed soil cores were treated with Salmonella enterica ssp. enterica serovar Typhimurium-lux (STM-lux) and human adenovirus (HAdV)-spiked sewage sludge. Following an artificial rainfall event, these pathogens were analysed in the leachate and soil sampled from different depths (0-5 cm, 5-10 cm and 10-20 cm) after 24 h, 1 and 2 months. Significantly more STM-lux and HAdV leached through the soil cores when sewage sludge was present. Significantly more STM-lux were found at all soil depths, at all time periods in the sewage sludge treatments, compared to the controls. The rate of decline of STM-lux in the controls was more rapid than in the sewage sludge treatments. Survival and transport of HAdV were minimal.

CONCLUSIONS

The presence of sewage sludge can significantly influence the transport and survival of bacterial pathogens in soils, probably because of the presence of organic matter. Environmental contamination by virus is unlikely because of strong soil adsorption.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study suggests that groundwater contamination from vertical movement of pathogens is a potential risk and that it highlights the importance of the treatment requirements for biosolids prior to their application to land.

Survival of Escherichia coli and Salmonella spp. after application of sewage sludge to a Pinus radiata forest

AIMS

This study investigated the survival of Escherichia coli and Salmonella spp. in sewage sludge applied to young and old Pinus radiata forest in Spring and Autumn/Winter.

METHODS AND RESULTS

Large numbers of E. coli were present in sludge applied to the forest blocks but Salmonella spp. numbers were low or nondetectable. In the mature stand in Spring, numbers of E. coli returned to back-ground after 3 weeks and die-off was significantly correlated with per cent solids of sludge. E. coli survived longer in mature and young stands in Autumn/Winter where numbers did not significantly decrease until weeks 5 and 13, respectively. Salmonella spp. was detectable in the mature stand until week 4 and in the young stand until week 11 in Autumn/Winter.

CONCLUSIONS

Microbial die-off was related to desiccation of the sewage sludge, and was faster in warmer, drier conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

In many countries, environmental and health risks associated with the application of sewage sludge to land are minimized by 'best management practice' guidelines, where risks are managed by restriction of public access to these sites. This study provides supporting evidence that withholding periods of greater than 6 months are sufficient to reduce microbial contaminants to background levels.

Effect of irrigation on the survival of total coliforms in three semiarid soils after amendment with sewage sludge

Sewage sludges are increasingly used in soil amendment programmes, although not without risk since they contain, among other potential hazards, high concentrations of total coliform bacteria. In this paper we have studied the effect of irrigation on the survival of total coliforms in three semiarid degraded soils with different agricultural practices. Fresh sewage sludge was added at 50 g kg(-1) soil, and incubated in both the presence and absence of irrigation. The absence of irrigation led to a sharp decrease in the number of total coliforms in all soils, with the bacteria disappearing in 40 days. Irrigation produced a substantial initial increase in the number of coliforms in the three soils, although after 80 days there was none growing in any of the soils. The results showed that there were significant differences in the survival of coliform bacteria due to the presence or absence of irrigation.

Fate of plasmid-bearing, luciferase marker gene tagged bacteria after feeding to the soil microarthropod Onychiurus fimatus (Collembola)

In order to study the potential impact of the soil microarthropod Onychiurus fimatus (Collembola) on the microbial community, we analysed the fate of luciferase marker gene tagged bacterial strains fed to young adult specimens in petri dish microcosm experiments. In faeces collected from O. fimatus, Escherichia coli S17-1/pRP4luc and Sinorhizobium meliloti L33 were only detectable for 2 days after feeding whereas strain HR2/pRP4luc, a close relative of Stenotrophomonas maltophilia, isolated from another collembolan species, could be detected for 16 days. The amount of shed cells of strain HR2 increased during the frequent releases of the cast-off skins (exuvia). In order to analyse whether gut associated bacteria could serve as recipients for mobile genetic elements, plasmid-bearing E. coli donor strains were incubated with faeces in filter mating-like experiments and, in other experiments, directly fed to O. fimatus specimens. Transconjugants were obtained with both the conjugative self-transferable broad host range plasmid pRP4luc and the mobilisable (Mob(+)) broad host range plasmid pSUP104luc, the latter, however, only with a mobilising donor strain. No transfer was detected with the narrow host range plasmids pSUP202luc (Mob(+)), pUC18luc (Mob(-)), or with the broad host range transposon delivery plasmid pUTluxCDABE (Mob(+)). Transconjugants of pRP4luc were detected within one day of the beginning of a feeding experiment and then throughout the incubation period of two weeks, with gaps of no detection after 5, 12 and 14 days, probably caused by moulting. The results of this study indicate that feeding activities of collembola can modify the structure of soil-inhabiting microbial communities and enhance the spread of plasmids from non-indigenous to indigenous soil bacteria.

Clover and cress as indicator species of impacts from limed sewage sludge and landfill wastewater land application

This study evaluated the performance of cress and clover as bioindicators to evaluate the impact of limed sewage sludge and landfill wastewater application on soil. The use of these two species, and the subsequent parameters measured, were also compared for their effectiveness in evaluating potential effects. Sewage sludge was added to the soil at a rate of 23 mg ha-1 and also at twice this dose. Wastewater was added at a level of 2 and 10% of the soil volume. Germination rate, height increase per week and final dry biomass were measured for both species. Heavy metal (Cd, Pb, Fe, Cu, Ni, and Zn) concentration was determined only for the shoot and root of clover. In general, the results showed that, compared to the control, the overall performance improved for plants exposed to both treatments. However, plants appeared to respond better to the wastewater addition than the sewage sludge amendment. The germination rate as well as other traits indicate that clover may be more sensitive to these fertilizers than cress. The lime addition did not appear to influence growth, nor heavy metal accumulation in clover. There were major differences in heavy metal accumulation in the root vs. shoot of clover. Hence, the concentrations of Cd, Cu, Fe, Zn, and Pb did not vary in the shoots of the plants under either treatments. The concentrations of the heavy metals in the roots however, greatly varied according to the type of metals. The concentrations of these metals appeared to increase with the increase of the addition treatments. The results suggest that for both species, the levels of fertilizers used in the present study were below the toxicity levels. Since the types of soils and the doses of fertilizers influence the rate of heavy metal absorption and their effects on plants, methods for rapid evaluation/monitoring as well as the types of indicators used should be explored as in the present study.