Listeria monocytogenes contamination of crops grown on soil treated with sewage sludge cake

A l -G hazali , M.R. & A l -A zawi , S.K. 1990. Listeria monocytogenes contamination of crops grown on soil treated with sewage sludge cake. Journal of Applied Bacteriology 69 , 642–647.
Listeria monocytogenes was found in the sewage sludge cake which is commonly used as an agricultural fertilizer in Iraq. Soils treated with this material were contaminated with the organism. Pot and field experiment showed that crops grown on treated soil became contaminated with L. monocytogenes and when alfalfa plant was grown on farmland soil treated with sewage sludge cake, listerias were found on 10% of 50 plants sampled at harvest, but the organism was detected only in low numbers on these crops (≤ 5 cells/g). This could add to the risk to animals and man.     

Detection and enumeration of Listeria monocytogenes in a sewage treatment plant in Iraq

Listeria monocytogenes was isolated from a sewage treatment plant in Baghdad, Iraq, at all stages of treatment. The treatment processes did not yield a sewage sludge cake or a final discharge free of listerias. The agricultural practice of using such sewage products as fertilizers could become a route of spreading the organism in Iraq, particularly by infecting animals that consume vegetation in fields spread with such sewage. Dewatering of sewage reduced the number of L. monocytogenes but long periods of exposure to sun would be needed to obtain a 'safe' sewage sludge cake.     

Detection and enumeration of Listeria monocytogenes in a sewage treatment plant in Iraq

Listeria monocytogenes was isolated from a sewage treatment plant in Baghdad, Iraq, at all stages of treatment. The treatment processes did not yield a sewage sludge cake or a final discharge free of listerias. The agricultural practice of using such sewage products as fertilizers could become a route of spreading the organism in Iraq, particularly by infecting animals that consume vegetation in fields spread with such sewage. Dewatering of sewage reduced the number of L. monocytogenes but long periods of exposure to sun would be needed to obtain a 'safe' sewage sludge cake.     

Effects of sewage treatment on the removal of Listeria monocytogenes

Two sewage treatment plants in Baghdad, Iraq, were investigated to assess the effects of the different treatment stages on the removal of Listeria monocytogenes . The bacteria were severely affected after the activation and digestion stages at both plants. A dramatic decrease in numbers of listerias after each of these two stages was noticed during the cold months (September-January). The organisms were able to survive these treatments and were present in the final effluent and even in low numbers in the sewage sludge cake. Sufficient dewatering of sewage sludge is recommended to obtain sewage free of listerias. Improvements in the isolation procedure of L. monocytogenes from such heavily contaminated material is also discussed.     

Effects of sewage treatment on the removal of Listeria monocytogenes

Two sewage treatment plants in Baghdad, Iraq, were investigated to assess the effects of the different treatment stages on the removal of Listeria monocytogenes. The bacteria were severely affected after the activation and digestion stages at both plants. A dramatic decrease in numbers of listerias after each of these two stages was noticed during the cold months (September-January). The organisms were able to survive these treatments and were present in the final effluent and even in low numbers in the sewage sludge cake. Sufficient dewatering of sewage sludge is recommended to obtain sewage free of listerias. Improvements in the isolation procedure of L. monocytogenes from such heavily contaminated material is also discussed.     

Storage effects of sewage sludge cake on the survival of Listeria monocytogenes

Sewage sludge cake is widely used as an agricultural fertilizer in Iraq. Listeria monocytogenes was shown to be present in small numbers in this material despite sewage treatments. In an attempt to reduce the numbers of this pathogen in this sewage end product, the survival of L. monocytogenes was monitored in a heap of sewage sludge cake stored for over 23 weeks on farm land. The organisms were reduced in numbers and eliminated to undetectable limits during 8 weeks of storage under subtropical weathering and did not recover even 2 months after disappearance. Dewatering processes seem to have some affect on the survival of the bacteria. Therefore, solar dewatering by heaping the sewage sludge cake and exposing it to sun for no less than 8 weeks is recommended to obtain a listeria-free product.     

Storage effects of sewage sludge cake on the survival of Listeria monocytogenes

Sewage sludge cake is widely used as an agricultural fertilizer in Iraq. Listeria monocytogenes was shown to be present in small numbers in this material despite sewage treatments. In an attempt to reduce the numbers of this pathogen in this sewage end product, the survival of L. monocytogenes was monitored in a heap of sewage sludge cake stored for over 23 weeks on farm land. The organisms were reduced in numbers and eliminated to undetectable limits during 8 weeks of storage under subtropical weathering and did not recover even 2 months after disappearence. Dewatering processes seem to have some affect on the survival of the bacteria. Therefore, solar dewatering by heaping the sewage sludge cake and exposing it to sun for no less than 8 weeks is recommended to obtain a listeria-free product.     

Persistence of poliovirus 1 in soil and on vegetables grown in soil previously flooded with inoculated sewage sludge or effluent.

Land disposal of sewage sludge and effluent is becoming a common practice in the United States. The fertilizer content and humus value of such wastes are useful for agricultural purposes, and the recycling of sewage onto the land eliminates many of our stream pollution problems. The potential exists for crops grown in such irrigated soil to be contaminated by viruses that may be present in the sewage. Studies were initiated to determine viral persistence in soil and on crops grown under natural conditions in field plots that had been flooded to a depth of 1 inch (2.54 cm) with poliovirus 1-inoculated sewage wastes. Lettuce and radishes were planted in sludge- or effluent-flooded soil. In one study, the vegetables were planted 1 day before flooding, and in another they were planted 3 days after the plots were flooded. Survival of poliovirus 1 in soil irrigated with inoculated sewage sludge and effluent was determined during two summer growing seasons and one winter period. The longest period of survival was during the winter, when virus was detected after 96 days. During the summer, the longest survival period was 11 days. Poliovirus 1 was recovered from the mature vegetables 23 days after flooding of the plots had ceased. Lettuce and radishes are usually harvested 3 to 4 weeks after planting.     

Persistence of poliovirus 1 in soil and on vegetables grown in soil previously flooded with inoculated sewage sludge or effluent.

Land disposal of sewage sludge and effluent is becoming a common practice in the United States. The fertilizer content and humus value of such wastes are useful for agricultural purposes, and the recycling of sewage onto the land eliminates many of our stream pollution problems. The potential exists for crops grown in such irrigated soil to be contaminated by viruses that may be present in the sewage. Studies were initiated to determine viral persistence in soil and on crops grown under natural conditions in field plots that had been flooded to a depth of 1 inch (2.54 cm) with poliovirus 1-inoculated sewage wastes. Lettuce and radishes were planted in sludge- or effluent-flooded soil. In one study, the vegetables were planted 1 day before flooding, and in another they were planted 3 days after the plots were flooded. Survival of poliovirus 1 in soil irrigated with inoculated sewage sludge and effluent was determined during two summer growing seasons and one winter period. The longest period of survival was during the winter, when virus was detected after 96 days. During the summer, the longest survival period was 11 days. Poliovirus 1 was recovered from the mature vegetables 23 days after flooding of the plots had ceased. Lettuce and radishes are usually harvested 3 to 4 weeks after planting.     

Isolation and Enumeration of Listeria monocytogenes from Sewage, Sewage Sludge and River Water

Listeria monocytogenes in sewage, sewage sludge and river water was isolated by enrichment at 4°C with subculture and enrichment in thiocyanate, naladixic acid broth and plating on to Tryptose Agar. The results indicated that L. monocytogenes is present in sewage and sewage sludge in considerable numbers and that this organism survives longer than Salmonella spp. on land sprayed with sewage sludge.     

污泥土地利用对农作物及土壤的影响研究

以沈阳北部污水处理厂污泥为研究对象 ,开展了污泥土地利用对农作物及土壤环境影响的研究 .结果表明 ,污泥土地利用可提高土壤中N、P及有机质含量 .污泥用量为 2 2 .5t·hm-2 和 45t·hm-2 时有利于作物生长发育 ,使水稻生物量与对照相比分别增加了 11.48%、11.83%.污泥施用量只要控制在 45t·hm-2以内 ,污泥中的重金属不会对土壤、农产品质量及地下水产生不良的影响 .     

Land application of treated sewage sludge: quantifying pathogen risks from consumption of crops

AIMS: To predict the number of humans in the UK infected through consumption of root crops grown on agricultural land to which treated sewage sludge has been applied in accordance with the current regulations and guidance (Safe Sludge Matrix). METHODS AND RESULTS: Quantitative risk assessments based on the source, pathway, receptor approach are developed for seven pathogens, namely salmonellas, Listeria monocytogenes, campylobacters, Escherichia coli O157, Cryptosporidium parvum, Giardia, and enteroviruses. Using laboratory data for pathogen destruction by mesophilic anaerobic digestion, and not extrapolating experimental data for pathogen decay in soil to the full 30-month harvest interval specified by the Matrix, predicts 50 Giardia infections per year, but less than one infection per year for the other six pathogens. Assuming linear decay in the soil, a 12-month harvest interval eliminates the risks from all seven pathogens; the highest predicted being one infection of C. parvum in the UK every 45 years. Computer simulations show that a protective effect from binding of pathogens to particulate matter could potentially exaggerate the observed rate of decay in experimental systems. CONCLUSIONS: The results confirm, assuming pathogens behave according to our current understanding, that the risks to humans from consumption of vegetable crops are remote. Furthermore the harvest intervals stipulated by the Safe Sludge Matrix compensate for potential lapses in the operational efficiency of sludge treatment. SIGNIFICANCE AND IMPACT OF THE STUDY: The models demonstrate the huge potential impact of decay in the soil over the 12/30-month intervals specified by the Matrix, although lack of knowledge on the exact nature of soil decay processes is a source of uncertainty. The models enable the sensitivity of the predicted risks to changes in the operational efficiency of sewage sludge treatment to be assessed.     

Comparison of a cultural method with ListerScreen plus Rapid'L.mono or PCR-ELISA methods for the enumeration of L. monocytogenes in naturally contaminated sewage sludge

Cultural methods used to count Listeria monocytogenes in sewage sludge are laborious and time consuming, and alternative methods are needed to reduce analysis time and improve detection limits. In this study, a survey of L. monocytogenes in sewage sludge is presented with a comparative study between a cultural method and immunomagnetic separation using a ListerScreen test followed by identification of L. monocytogenes with Rapid'L.mono agar or PCR-ELISA. These two alternative methods improved the detection of L. monocytogenes in different types of sludge, irrespective of their physical and chemical characteristics. The ListerScreen method coupled with detection of L. monocytogenes on Rapid'L.mono offers the advantage of being less sophisticated than the molecular method and allows isolation of the organism, which may be useful in epidemiological studies. However, ListerScreen coupled with PCR-ELISA proved best for high-sensitivity detection of L. monocytogenes in sewage samples.     

Effect of soil pH and sewage sludge on VA mycorrhizal infection of soybeans

Summary Small plots were amended in 1976 or 1978 with four kinds of sewage sludge. The sludges represented samples considered to be relatively free of heavy metals as well as sludges highly contaminated with heavy metals. Sludges were added to a silt loam soil at rates of 224 or 448 Mgha⁻¹. The soils were maintained at a high or low pH regime. In 1984, soybeans (Glycine max L. Merril. var. ‘Clark’) were planted and grown to the R4 stage. After harvest, roots were removed from the soil, washed, and examined for VA mycorrhizal infection. It was found that the heavy metal content of the sludge alone was generally not related to determining the extent of mycorrhizal infection. A heat treated sludge, high in heavy metals, exhibited the highest degree of mycorrhizal infection when the soil was maintained at a pH of 6.2. With this treatment, 52% of the root segments examined were infected by mycorrhiza. When the same sludge was added to a soil with a slightly lower pH (5.7) none of the roots examined were infected by mycorrhiza. When soybean roots were examined from soils that received no sludge and were maintained at either a low (5.6) or high (6.2) pH, there was no significant difference in mycorrhizal infection between the pH regimes. These results therefore indicate that sewage sludge may inhibit mycorrhizal infection if the sludge contains a high concentration of heavy metals and the sludge is applied to the soil with a low pH. Scientific Article No. A-4093 and Contribution No 7078 of the Maryland Agric. Exp. Stn., Dept. of Agronomy, University of Maryland, College Park, MD 20742.     

Heavy metal contents of vegetable crops treated with refuse compost and sewage sludge

Refuse compost and sewage sludge were mixed with a loamy sand at various rates in pots and sown withBrassica chinensis, Daucus carota andLycopersicon esculentum in a glasshouse. A commercial fertilizer was also applied to the same soil for comparison. Dry matter production of the three crops and contents of Cd, Cu, Mn, Pb and Zn in the harvested tissues were determined at the end of the experiment. In general, crop yield in refuse compost treatment was improved over that in sandy soil alone, but was less than that in the sludge and fertilizer treatments. Despite the relatively high heavy metal contents of refuse compost, crops grown on compost-treated soils accumulated lower levels of metal than those grown on sludge-treated soils. This is probably due to the high pH and organic matter content of the composted refuse. Higher levels of heavy metals were found in the roots than in the aerial parts ofB. chinensis andL. esculentum, but the reverse was found inD. carota. In the edible tissue of the three crops,L. esculentum accumulated metals to a lesser extent than the other two.     

Using Maize (Zea mays L.) and Sewage Sludge to Remediate a Petroleum-Contaminated Calcareous Soil

Phyto-stimulation, the use of plants to stimulate activity of microorganisms in a root zone, has been proposed as an approach to promote the degradation of petroleum hydrocarbons and thus the remediation of petroleum-polluted soils. In this study, we investigated the potential use of sewage sludge to enhance phyto-stimulating effects of maize (Zea mays L.) on the elimination of an aged petroleum contamination in a calcareous soil. In a pot experiment, maize was grown on the experimental soil for two months at three levels of sewage sludge application (0, 20, and 50 g dry matter of sludge per kg soil). The amendments increased root and shoot growth of the experimental plants approximately by a factor of two at the lower sludge treatment level and by a factor of five at the higher sludge treatment level. In a separate incubation experiment, sludge application also led to an immediate stimulation of soil respiration, which then further increased over time. The initial stimulation was three times larger at the higher than at the lower treatment level, but the rate of subsequent increase was similar in both treatments. The two sludge treatments also accelerated TPH elimination in the contaminated soil, and again the effect was approximately three times stronger at the higher than at the lower treatment level. The sludge effect on TPH elimination was much stronger than the effect of the plants. More than half of the initial contamination was reduced in combined treatment with maize and sludge application at the highest rate. The results show that sewage sludge can substantially enhance the remediation of petroleum-contaminated soil, especially when applied in conjunction with a suitable plant such as maize.     

Using event trees to quantify pathogen levels on root crops from land application of treated sewage sludge

AIMS: To quantify the incremental exposure of root crops, at point of harvest, to enteric pathogens from sewage sludge applied to agricultural land according to current regulations and guidance (Safe Sludge Matrix). METHODS AND RESULTS: A quantitative risk assessment based on the Source-Pathway-Receptor approach is developed for Cryptosporidium and salmonellas. Event trees are constructed to model the partitioning of pathogens present in raw sewage into sludge at the sewage treatment works and to model to the pathways by which root crops may be exposed to those pathogens after treatment and land application of the sludge. The main barriers are sewage sludge treatment, and decay and dilution of the pathogens in the soil. The exposures are expressed in terms of the arithmetic mean. This represents the total loading and accommodates fluctuations not only in the levels of pathogens present in sewage but also in the removal efficiencies by the various barriers. One source of uncertainty is the degree of by-pass of sludge treatment at operational scale. CONCLUSIONS: The models predict that land application of sewage sludge treated by conventional processes (achieving 2-log removal) increases the exposures of root crops to salmonellas and Cryptosporidium oocysts by counts of 0.070 and 0.033 kg(-1), respectively. These predictions are based on decay periods in the soil of 5 and 12 weeks, respectively, and are therefore worst case in not allowing for the full extent of no harvesting periods. A Monte Carlo simulation predicts that 0.01% of 1-kg batches contained > 50 salmonellas and demonstrates that, for risk assessment, it is acceptable to use the arithmetic mean exposure directly in the dose-response curve. SIGNIFICANCE AND IMPACT OF THE STUDY: The predicted numbers of pathogens on root crops at point of harvest provide a basis for modelling the excess risks to humans consuming such crops. The approach underpins scientifically the Safe Sludge Matrix.     

Bioavailability and crop uptake of trace elements in soil columns amended with sewage sludge products

In order to assess the potential impact of long-term sewage sludge application on soil health, the equivalent of about 25 years of agronomic applications of low-metal (`EQ') sewage sludge products were made to greenhouse soil columns. After a 6-year period of `equilibration', during which time successive crops were grown with irrigation by simulated acid rain, the plant-available quantities of trace elements were estimated in the soils by extraction with 0.01 M CaCl₂ at 90 °C, and measured directly by uptake into a crop of red clover (Trifolium pratense L.). Soil pH had a strong influence on the level of extractable and plant-available metals, and because the tested sludge products affected soil pH differently, pH was directly factored into the comparison of different sludge treatments with controls. CaCl₂-extractable levels of several metals (Cu, Zn, Mo), sulfur and phosphorus were found to be higher in the soils amended with organic-rich sludge products than in the control soils. However, extractable Cd and Ni were not significantly elevated by the sludge amendments, presumably because of the low total loading of these metals. Copper, Zn and Mo applied in the form of sludge ash had low soil extractability, suggesting that these trace metals were trapped in high-temperature mineral phases formed during sludge incineration, and resisted subsequent weathering in the soil environment. Extractable soil metals in the alkaline-stabilized sludge treatment were also generally low. Phytotoxicity from the sludge metal loadings (Zn≤125, Cu≤135 kg/ha), was not clearly indicated as long as soil pH was maintained in the 6–7 range by lime amendment. Nevertheless, unexplained depressions in yield were noted with some of the sludge products applied, particularly the dewatered and composted materials. On limed soil columns, the most consistent effect of sludge product amendment on red clover composition was a marked increase in plant Mo.     

Effects of Municipal Sewage Sludge Doses on the Chlorophyll Contents and Heavy Metal Concentration of Sugar Beet (Beta vulgaris var. saccharifera)

The present study was conducted to assess the suitability of sewage sludge amendment (SSA) in soil for Beta vulgaris var. saccharifera (sugar beet) by evaluating the heavy metal accumulation and physiological responses of plants grown at a 10%, 25%, and 50% sewage sludge amendment rate. The sewage sludge amendment was modified by the physicochemical properties of soil, thus increasing the availability of heavy metals in the soil and consequently increasing accumulation in plant parts. Cd, Pb, Ni, and Cu concentrations in roots were significantly higher in plants grown at 25% as compared to 50% SSA; however, Cr and Zn concentration was higher at 50% than 25% SSA. The concentrations of heavy metal showed a trend of Zn > Ni > Cu > Cr > Pb > Cd in roots and Zn > Cu > Ni > Cr > Pb > Cd in leaves. The only instance in which the chlorophyll content did not increase after the sewage sludge treatments was 50%. There were approximately 1.12-fold differences between the control and 50% sewage sludge application for chlorophyll content. The sewage sludge amendment led to a significant increase in Pb, Cr, Cd, Cu, Zn, and Ni concentrations of the soil. The heavy metal accumulation in the soil after the treatments did not exceed the limits for the land application of sewage sludge recommended by the US Environmental Protection Agency (US EPA). The increased concentration of heavy metals in the soil due to the sewage sludge amendment led to increases in heavy metal uptake and the leaf and root concentrations of Ni, Zn, Cd, Cu, Cr, Pb, and Zn in plants as compared to those grown on unamended soil. More accumulation occurred in roots and leaves than in shoots for most of the heavy metals. The concentrations of Cd, Cr, and Pb were more than the permissible limits of national standards in the edible portion of sugar beet grown on different sewage sludge amendment ratios. The study concludes that the sewage sludge amendment in the soil for growing sugar beet may not be a good option due to risk of contamination of Cr, Pb, and Cd.     

Oxygen diffusion and microbial activity in the composting of dehydrated sewage sludge cakes

The effective diffusivity for oxygen within sewage sludge cakes was measured, and in turn used for the study of microbial activity in the sludge cake composting. The diffusivity was found to be proportional to the one and a half power of the sludge cake porosity. Composting experiments were done using two types of sludge cakes, 5 mesh sieved and pelletized. It was found that the microbial activity in the inner core region of the pelletized sludge cake was significantly less than that in the outer region, even through the rate of oxygen diffusion was fast enough to support the microorganisms' activities. It appears, therefore, that microorganisms acting to degrade organic matter grow primarily on the macroscopic interface between solids and air.