Estimation of bioaerosol risk of infection to residents adjacent to a land applied biosolids site using an empirically derived transport model

AIM: The purpose of this study was to develop an empirically derived transport model, which could be used to predict downwind concentrations of viruses and bacteria during land application of liquid biosolids and subsequently assess microbial risk associated with this practice. METHODS AND RESULTS: To develop the model, coliphage MS-2 and Escherichia coli were aerosolized after addition to water within a biosolids spray application truck, and bioaerosols were collected at discrete downwind distances ranging from 2 to 70 m. Although coliphage were routinely detected, E. coli did not frequently survive aerosolization. Data on aerosolized coliphage was then used to generate a virus transport model. Risks of infection were calculated for various ranges of human virus concentrations that could be found in biosolids. CONCLUSIONS: A conservative estimate at 30.5 m (assumed to be nearest adjacent residences) downwind, resulted in risks of infection of 1 : 100,000, to the more realistic 1 : 10,000,000 per exposure. Conservative annual risks were calculated to be no more than 7 : 100,000 where as a more realistic risk was no greater than 7 : 10,000,000. Overall, the viral risk to residences adjacent to land application sites appears to be low, both for one time and annual probabilities of infection. SIGNIFICANCE AND IMPACT OF THE STUDY: This study demonstrated a simple approach towards modelling viral pathogens aerosolized from land applied liquid biosolids, and offers insight into the associated viral risk.     

Occurrence of antibiotic-resistant bacteria and endotoxin associated with the land application of biosolids

The purpose of this study was to determine the prevalence of antibiotic-resistant bacteria and endotoxin in soil after land application of biosolids. Soil was collected over a 15 month period following land application of biosolids, and antibiotic resistance was ascertained using clinically relevant antibiotic concentrations. Ampicillin, cephalothin, ciprofloxacin, and tetracycline resistance were all monitored separately for any changes throughout the 15 month period. Endotoxin soil concentrations were monitored using commercially available endotoxin analysis reagents. Overall, land application of biosolids did not increase the percentage of antibiotic-resistant culturable bacteria above background soil levels. Likewise, land application of biosolids did not significantly increase the concentration of endotoxin in soil. This study determined and established a baseline understanding of the overall effect that land application of biosolids had on the land-applied field with respect to antibiotic-resistant bacterial and endotoxin soil densities.     

Field studies measuring the aerosolization of endotoxin during the land application of Class B biosolids

Endotoxins are a component of Gram-negative bacteria cell walls and are known to be present in biosolids. Endotoxins have been shown to be potent stimulators of the innate immune response causing airway irritation and shortness of breath. Class B biosolids are routinely applied to agricultural lands to enhance soil properties and can be used as an alternative to chemical fertilizers. This study investigated the aerosolized endotoxin dispersed during the land application of Class B biosolids on agricultural land and a concrete surface at two sites in Colorado, USA. Aerosolized endotoxin was captured using HiVol samplers fitted with glass fiber filters, polycarbonate filter cassettes (both open and closed) and BioSampler impinger air samplers. Endotoxins were also measured in the biosolids to allow for correlating bulk biosolids concentrations with aerosol emission rates. Endotoxin concentrations in biosolids, impinger solutions and filter extracts were determined using the kinetic Limulus amebocyte lysate assay. Aerosolized endotoxin concentration was detected from all sites with levels ranging from 0.5 to 642 EU/m³. The four types of sampling apparatus were compared, and the HiVol and open-faced cassette samplers produced higher time-weighted average (TWA) measurements (EU/m³) than the impinger and closed cassette samplers. Ambient wind speed was found to be the variable best describing the observed results with optimal wind speed for highest deposition estimated at 5 m s^?1. It is argued that HiVol air samplers are a particularly reliable approach and subsequent analyses relating TWA measurements to wind speed and biosolids characteristics were based on the measurements collected with those samplers.     

A national study on the residential impact of biological aerosols from the land application of biosolids

AIMS: The purpose of this study was to evaluate the community risk of infection from bioaerosols to residents living near biosolids land application sites. METHODS AND RESULTS: Approximately 350 aerosol samples from 10 sites located throughout the USA were collected via the use of six SKC Biosamplers. Downwind aerosol samples from biosolids loading, unloading, land application and background operations were collected from all sites. All samples were analysed for the presence of HPC bacteria, total coliform bacteria, Escherichia coli, Clostridium perfringens, coliphage, enteroviruses, hepatitis A virus and norovirus. Total coliforms, E. coli, C. perfringens and coliphage were not detected with great frequency from any sites, however, biosolids loading operations resulted in the largest concentrations of these aerosolized microbial indicators. Microbial risk analyses were conducted on loading and land application operations and their subsequent residential exposures determined. CONCLUSIONS: The greatest annual risks of infection occurred during loading operations, and resulted in a 4 x 10(-4) chance of infection from inhalation of coxsackievirus A21. Land application of biosolids resulted in risks that were <2 x 10(-4) from inhalation of coxsackievirus A21. Overall bioaerosol exposure from biosolids operations poses little community risk based on this study. SIGNIFICANCE AND IMPACT OF THE STUDY: This study evaluated the overall incidence of aerosolized micro-organisms from the land application of biosolids and subsequently determined that microbial risks of infection were low for residents close to biosolids application sites.     

Diversity of aerosolized bacteria during land application of biosolids

AIMS: The purpose of this study was to determine the diversity of bacterial communities associated with bioaerosols generated during land application of biosolids using 16S ribosomal RNA (16S rRNA) PCR. METHODS AND RESULTS: Anaerobically digested Class B biosolids were land applied to an agricultural site located in South Central Arizona. Aerosol samples were collected downwind of the biosolids operations and were collected via the use of SKC Biosamplers and subsequently extracted for the presence of bacterial community DNA. All DNA was amplified using 16S rRNA primers, cloned and sequenced. All sequences were aligned and phylogenetic trees were developed to generate community profiles. The majority of aerosolized bacterial clone sequences belonged to the Actinobacteria and alpha- and beta-proteobacterial taxa. Aerosol samples collected downwind of soil aerosolization produced similar profiles. These profiles differed from upwind and background samples. CONCLUSIONS: No one clone sequence isolated from the aerosol samples could be solely attributed to biosolids; on the contrary, the majority appeared to have arisen from soil. SIGNIFICANCE AND IMPACT OF THE STUDY: This study demonstrates that in dry, arid climates the majority of aerosols associated with biosolids land application appear to be associated with the onsite soil.     

Year-long assessment of airborne endotoxin at a concentrated dairy operation

In this study, we monitored total airborne endotoxins at upwind and downwind sites at a large open-lot dairy each month for a year. At the upwind site, the average airborne concentration was 28.5 endotoxin units (EU) m⁻³, while at the downwind edge of the lot and 200 m from the lot edge, the average concentrations were 169 and 72 EU m⁻³, respectively. At the downwind edge of the lot, there was a significant correlation between the airborne endotoxin concentration and wind speed or air temperature. A comparison between total and inhalable airborne endotoxin concentrations, near the end of the study, revealed no significant differences between the two endotoxin collection methods. Our data suggest that endotoxin exposure can be reduced as one increases their distance from the open-lot dairy.     

Dynamics of carbon,nitrogen and phosphorus in soil amended with irradiated,pasteurized and limed biosolids

Sewage biosolids contain high concentrations of pathogens, which limits their use as soil amendment. This study investigated how application of lime (Ca(OH)2), irradiation, or pasteurization reduced pathogens in biosolids and how its application affected soil characteristics. A soil sampled outside the canopy of Mesquite trees (Prosopis laevigata) and from a pasture at Lerma (Mexico) was amended with treated or untreated biosolids, characterized and incubated aerobically while dynamics of carbon (C), nitrogen (N) and phosphorus (P) were monitored. Heavy metals concentrations in the biosolids were low, so it was of excellent quality (USEPA). The amount of pathogens in the biosolids made it a class "B" (USEPA) which can be used in forests. Only irradiation sufficiently reduced faecal coliforms to make it a class "A" biosolids without restrictions in application. C mineralization increased significantly when biosolids were added, but not concentrations of available P (P < 0.05). Ammonium (NH4+) concentrations in soil amended with biosolids were higher compared to unamended soil, but not the concentrations of nitrate (NO3-) except when biosolids treated with Ca(OH)2 was added to the Lerma soil.     

Source tracking aerosols released from land-applied class B biosolids during high-wind events

DNA-based microbial source tracking (MST) methods were developed and used to specifically and sensitively track the unintended aerosolization of land-applied, anaerobically digested sewage sludge (biosolids) during high-wind events. Culture and phylogenetic analyses of bulk biosolids provided a basis for the development of three different MST methods. They included (i) culture- and 16S rRNA gene-based identification of Clostridium bifermentans, (ii) direct PCR amplification and sequencing of the 16S rRNA gene for an uncultured bacterium of the class Chloroflexi that is commonly present in anaerobically digested biosolids, and (iii) direct PCR amplification of a 16S rRNA gene of the phylum Euryarchaeota coupled with terminal restriction fragment length polymorphism to distinguish terminal fragments that are unique to biosolid-specific microorganisms. Each method was first validated with a broad group of bulk biosolids and soil samples to confirm the target's exclusive presence in biosolids and absence in soils. Positive responses were observed in 100% of bulk biosolid samples and in less than 11% of the bulk soils tested. Next, a sampling campaign was conducted in which all three methods were applied to aerosol samples taken upwind and downwind of fields that had recently been land applied with biosolids. When average wind speeds were greater than 5 m/s, source tracking results confirmed the presence of biosolids in 56% of the downwind samples versus 3% of the upwind samples. During these high-wind events, the biosolid concentration in downwind aerosols was between 0.1 and 2 microg/m3. The application of DNA-based source tracking to aerosol samples has confirmed that wind is a possible mechanism for the aerosolization and off-site transport of land-applied biosolids.     

The impact of sludge amendment on gas dynamics in an upland soil: monitored by membrane inlet mass spectrometry

Studies of the land disposal of biosolids and municipal sewage have focused largely on the potential pollution of the soil with pathogens, toxic compounds or heavy metals. Little is known about the impact of sludge amendment on carbon source and sink concentrations in soils. In this study gas concentrations in Scottish soil cores (from limed and unlimed plots) were monitored continuously at 3 cm depth before, during and after sludge application using membrane inlet mass spectrometry (MIMS). Following sludge application to soil cores, significant and sustained increases in CH4 (for 8 days) and CO2 (for between 16 and 120 days) concentration were observed. This suggested short-term stimulation of indigenous methanogens, provision of a new methanogenic inoculum, or inhibition of methane oxidizers (for example by heavy metals or NH4 in sludge). Soil microbial fermentative activity was enhanced over periods of a few months as shown by elevated CO2 concentrations.     

Movement of Endotoxin Through Soil Columns

Land treatment of wastewater is an attractive alternative to conventional sewage treatment systems and is gaining widespread acceptance. Although land application systems prevent surface water pollution and augment the available water supplies, the potential dangers to human health should be evaluated. Since sewage may contain high amounts of bacterial endotoxin, the removal of endotoxin from sewage by percolation through soil was investigated. It was found that 90 to 99% of the endotoxin was removed after travel of sewage through 100 to 250 cm of loamy sand soil. When distilled water was allowed to infiltrate into the soil to simulate rainfall, the endotoxin was mobilized and moved in a concentrated band through the soil column. On testing samples from actual land treatment sites, as much as 480 ng of endotoxin per milliliter was found in some groundwater samples. The presence of endotoxin in potable water is known to be a potential problem under some circumstances, but the importance of endotoxin in water supplies has not been fully assessed. Therefore, the design, operation, and management of land application systems should take into account the fate of endotoxin in groundwater beneath the sites.     

Environmental and nutritional responses of a Pinus radiata plantation to biosolids application

Since the mid-1990s, a Pinus radiata (D. Don) plantation growing on a sandy, low fertility soil at Rabbit Island near Nelson, New Zealand received aerobically digested liquid biosolids. An experimental research trial was established on the site to investigate the effects of biosolids applications on tree growth, nutrition, soil and ground water quality. Biosolids were applied to the trial site in 1997 and 2000, at three application rates: 0 (control), 300 (standard) and 600 kg N ha⁻¹ (high). Biosolids application significantly increased tree growth. This was mainly attributed to improved N supply, demonstrated by the enhanced N concentration in the tree foliage. Soil analysis indicated that biosolids application have not caused significant changes in concentrations of most nutrients. However, biosolids treatments significantly increased the available P (Olsen P). Of the heavy metals only total Cu concentrations in the soil increased after biosolids application. Groundwater quality, which was monitored quarterly, has not been affected by biosolids application. The concentrations of nitrate and heavy metals in groundwater were well below the maximum acceptable values in drinking water standards. Biological treatment of sewage and digestion of sewage sludge resulted in the enrichment of ¹⁵N in the biosolids (δ¹⁵N values between 5.0 and 8.7‰). Such enrichment was used as a tracer to study the fate of biosolids derived N. The elevated δ¹⁵N in biosolids treated pine foliage indicated that a considerable amount N was sourced from biosolids. Analysis of δ¹⁵N in understorey plants showed that both non-legume and legume understorey plants took up N from the biosolids, and acted as a N sink, reducing N availability for leaching. Our study showed that application of biosolids to a plantation forest can significantly improve tree nutrition and site productivity without resulting in any measurable adverse effect on the receiving environment.     

Field investigations on the survival of Escherichia coli and presence of other enteric micro-organisms in biosolids-amended agricultural soil

AIMS: To measure the survival of enteric micro-organisms in agricultural soil amended with conventional and enhanced treated biosolids in relation to environmental and edaphic conditions. METHODS AND RESULTS: Escherichia coli, Salmonella and F-specific RNA bacteriophage were enumerated in sludge and amended soil. Salmonella was not detected and only small numbers of bacteriophages were found in conventional, dewatered mesophilic anaerobically digested biosolids (DMAD). Neither organism was detected in soil. Escherichia coli numbers in soil increased with DMAD application compared with the unamended control, or soil receiving enhanced treated, thermally dried digested (TDD) and composted (CPT) biosolids. Empirical statistical models were developed summarizing the relationship between soil temperature, moisture content and time and E. coli populations. Background numbers of E. coli declined with increasing soil temperature and decreasing soil moisture responding to seasonal patterns in environmental conditions. Time following application was the only significant explanatory variable of E. coli numbers and decay in DMAD-amended soil. CONCLUSIONS: E. coli are an indigenous component of the microbial community in field soil and populations increased in cool, moist soil during autumn-winter and declined in warm, dryer soil during spring-summer. Enhanced treated biosolids were not a source of E. coli, but reduced the size of the indigenous population possibly by stimulating the activity of predatory and competing soil flora because of the organic substrate input from sludge. Conventionally treated biosolids increased E. coli numbers in soil. However, introduced bacteria declined rapidly and survival was limited to 3 months, irrespective of the timing of sludge application or environment. SIGNIFICANCE AND IMPACT OF THE STUDY: The results provide assurance that residual numbers of pathogens applied to soil in treated biosolids decay to background values well within cropping and harvesting restrictions imposed when sewage sludge is spread on farmland.     

Comparison of coliphage and bacterial aerosols at a wastewater spray irrigation site.

Microbiological aerosols were measured on a spray irrigation site at Fort Huachuca, Ariz. Indigenous bacteria and tracer bacteriophage were sampled from sprays of chlorinated and unchlorinated secondary-treatment wastewaters during day and night periods. Aerosol dispersal and downwind migration were determined. Bacterial and coliphage f2 aerosols were sampled by using Andersen viable type stacked-sieve and high-volume electrostatic precipitator samplers. Bacterial standard plate counts averaged 2.4 x 10(5) colony-forming units per ml in unchlorinated effluents. Bacterial aerosols reached 500 bacteria per m3 at 152 m downwind and 10,500 bacteria per m3 at 46m. Seeded coliphage f2 averaged 4.0 x 10(5) plaque-forming units per ml in the effluent and were detected 563 m downwind. Downwind microbial aerosol levels were somewhat enhanced by nighttime conditions. The median aerodynamic particle size of the microbial aerosols was approximately 5.0 micrometer. Chlorination reduced wastewater bacterial levels 99.97% and reduced aerosol concentrations to near background levels; coliphage f2 was reduced only 95.4% in the chlorinated effluent and was readily measured 137 m downwind. Microbiological source strength an meteorological data were used in conjunction with a dispersion model to generate mathematical predictions of aerosol strength at various sampler locations. The mean calculated survival of aerosolized bacteria (standard plate count) in the range 46 to 76 m downwind was 5.2%, and that of coliphage f2 was 4.3 %.     

Application of biosolids in mineral sands mine rehabilitation: use of stockpiled topsoil decreases trace element uptake by plants

Mineral sands mining involves stripping topsoil to access heavy-mineral bearing deposits, which are then rehabilitated to their original state, commonly pasture in south-west Western Australia. Organic amendments such as biosolids (digested sewage sludge) can contribute organic carbon to the rehabilitating system and improve soil chemical fertility and physical conditions. Use of biosolids also introduces the risk of contamination of the soil-plant system with heavy metals, but may be a useful source of trace elements to plants if the concentrations of these elements are low in unamended soil. We expected that biosolids amendment of areas mined for mineral sands would result in increased concentrations of metals in soils and plants, and that metal uptake would be decreased by adding stockpiled topsoil or by liming. A glasshouse experiment growing a mixed annual ryegrass (Lolium rigidum)-subterranean clover (Trifolium subterraneum) sward was conducted using two soil materials (residue sand/clay and conserved topsoil) from a mineral sands mine amended with different rates of biosolids (0, 10, 20, 50 dry t/ha), and including a liming treatment (2 t/ha). Total concentrations of metals (As, Cd, Co, Cr, Cu, Ni, Pb and Zn) in soil increased with increasing rate of biosolids application. Metal uptake was generally lower where topsoil was present and was decreased by liming. With increasing biosolids application, plant metal concentrations increased for Cd, Ni and Zn but decreased or were erratic for other elements. In clover, biosolids application removed the Zn deficiency observed where biosolids were not applied. Plant uptake of all elements increased with increasing biosolids application, suggesting dilution by increased plant biomass was responsible for erratic metal concentration results. Despite the observed increases in uptake of metals by plants, metal concentrations in both species were low and below food standard thresholds. It is unlikely that a single application of biosolids in this system posed a threat from heavy metal contamination of soils or plants, and was beneficial in terms of Zn nutrition of T. subterraneum.     

Dispersal of Erysiphe graminis and Lycopodium clavatum spores near to the source in a barley crop

In experiments to study dispersal of spores in a crop of barley, a 4-m wide strip of the cultivar Zephyr (a mildew-susceptible variety) was a source of mildew (Eyrsiphe graminis) conidia. Small suction traps, previously calibrated in a wind tunnel, were used to measure spore concentration within and above the crop. Large concentrations of conidia were measured in the crop at the downwind edge of the Zephyr strip but these decreased rapidly with distance downwind. At 1 m concentrations were halved and by 4 m they were no greater than the background concentration measured in the crop upwind of the source. Next to the source, concentrations were much greater within than above the crop and net spore movement (flux) was upwards out of the crop; by 4 m downwind concentrations were greater above the crop and spore flux was reversed. Lycopodium clavatum spores were released in the same crop from a line of point sources. Concentrations also decreased rapidly downwind but, with no background of spores, numbers remained greater within than above the crop further from the source than for E. graminis. Even so by 7 m downwind concentrations in the crop had declined to less than those above. Deposition of L. clavatum spores onto horizontal glass slides in the crop agreed with that expected by settling. However, impaction onto vertical cylinders among plants was much greater than predicted. The reason is not known although turbulent air-flow around plants may, in some way, enhance impaction. Many E. graminis conidia near the source were deposited in clumps. This prevented any accurate prediction of deposition rates as fall speeds of clumps (necessary for prediction) were not known. Not surprisingly, deposition on horizontal slides often exceeded that expected from settling of single spores although it was not always greatest where clumps predominated. The proportion of spores deposited on vertical cylinders and horizontal slides located among plants ranged from 0–02 to 0–27 and from 0–019 to 0–127 of the area dose, respectively. Although these may seem to be small trapping efficiencies, the same deposition rates in a crop with many leaves and stems would rapidly filter most spores from air in the crop and can explain why concentrations were observed to decline so rapidly.     

Polycyclic Aromatic Hydrocarbons (PAHs) Pollution in Agricultural Soil in Tianjin,China

Representative soil samples (n = 60) collected from suburban agricultural land in Tianjin were analyzed to determine 16 PAHs in this study. Accelerated solvent extraction, GPC (Gel Permeation Chromatography), and SPE (Solid Phase Extraction) clean-up procedures were employed for PAH preparation prior to analysis with gas chromatography–mass spectrometry. The concentrations of the total PAHs (T-PAHs) ranged from 228.6 ng/g to 14722.1 ng/g with the mean value of 613.1 ng/g. Bap concentrations in many sites exceeded the suggested standards. Spatial variation of PAHs in soil was illustrated; the pollution status and comparison to other cities were also investigated. Severe PAH soil pollution was observed in some sites near urban areas. Higher PAH concentrations were detected at the downwind side of the urban areas, indicating the influence of human activities. Two indicative ratios (Fl/(Fl+Pyr, Baa/(Baa+Chr)) and principal component analysis were used to identify the possible sources of PAHs. These suggested that coal combustion was still the most important source of PAHs in Tianjin, which coincided well with the previous studies. These data can be further used to assess the health risk associated with soils polluted by PAHs and can help local government find proper ways to reduce PAHs’ pollution in soils.     

Evaluation of Heavy Metals and Associated Health Risks in a Metropolitan Wastewater Treatment Plant's Sludge for Its Land Application

The objectives of this study are to monitor the heavy metal concentrations in sludge samples collected from the Ankara Central Wastewater Treatment Plant (ACWWTP) in Turkey, check if these concentrations comply with the Turkish Regulation (Regulation Regarding the Use of Domestic and Urban Sludges on Soil), and evaluate possible health risks of heavy metals in sludge due to ingestion of sludge by a child. Monthly sludge samples were collected from the ACWWTP during 2012 and analyzed for seven heavy metals (Cd, Cr, Cu, Hg, Ni, Pb, and Zn). According to the results from the study, heavy metal concentrations showed no common seasonal trend. All heavy metal concentrations, except for one sample in which Zn was found to be at the limit value, are below the Turkish Regulation limits. In addition, health risks calculations for the “child ingesting biosolids” pathway, which is one of the most critical pathways identified by the U.S. Environmental Protection Agency (USEPA) for land application, were carried out. Among the seven heavy metals, Pb contributed the most to the cumulative non-cancer health risks throughout 2012. Nevertheless, the results showed that cumulative non-cancer health risks associated with this pathway are within the acceptable non-cancer health risk level suggested by USEPA.     

Evaluation of the environmental impact of microbial aerosols generated by wastewater treatment plants utilizing different aeration systems

Using three sampler devices (SAS, Andersen Six-Stages and All Glass Impinger), the environmental impact of bacterial and fungal aerosols generated by municipal wastewater treatment plants operating with different methods of sludge oxygenation were evaluated. The highest microbial concentrations were recovered above the tanks (2247 cfu m-3) and in downwind positions (1425 cfu m-3), where a linear correlation (P < 0.05) was found between the quantity of sewage treated and the entities of microbial aerosol dispersion. Moreover, an exponential increase (P < 0.05) in the bacteria recovered from the air occurred at increasing times of treatment. However, after long-term plant operation, high bacterial and fungal concentrations were found in almost all of the sites around the plant. Coliforms, enterococci, Escherichia coli and staphylococci were almost always recovered in downwind positions. Considerable fractions (20-40%) of sampled bacteria were able to penetrate the final stages of the Andersen apparatus and thus, are likely to be able to penetrate the lungs. The plant operating with a fine bubble diffused air system instead was found to generate rather low concentrations of bacteria and fungi; moreover, staphylococci and indicator micro-organisms were almost absent. Finally, salmonellae, Shigella, Pseudomonas aeruginosa and Aeromonas spp. were not detected in either of the plants. The results indicate a remarkable dispersion of airborne bacteria and fungi from tanks in which oxygen is supplied via a mechanical agitation of sludge, and suggest the need to convert them to diffused aeration systems which pose a lesser hazard for human health.     

Heavy Metal Phytotoxicity to Radish (Raphanus sativus L.) in a Digested Sludge-amended Gangetic Alluvium

A limiting factor in land application of sewage sludge is the resultant heavy metal accumulation in soils followed by biomagnification in the food chain, posing a potential hazard to animal and human health. In view of this fact, pot experiments were conducted to evaluate the effect of digested sludge application to soil on phytotoxicity of heavy metals such as Cd, Cr, Ni, and Pb to radish (Raphanus sativus L.) plants. Increasing sludge levels resulted in increased levels of DTPA-extractable heavy metals in the soil. Cadmium was the dominant metal extracted by DTPA followed by Ni, Pb, and Cr. The extractability of metals by DTPA tended to decrease from the first to the second crop. Dry matter yield of radish increased significantly as a function of increasing sludge treatments. Soil application of sludge raised the concentration of one or more heavy metals in plants. Shoots contained higher concentrations of Cd, Cr, and Ni than the roots of radish plants. Shoot concentrations of Cd, Cr, Ni, and Pb were within the tolerance levels of this crop at all rates of sludge application. Shoot as well as root concentration of Cd was above 0.5 mg kg^?1, considered toxic for human and animal consumption. The levels of DTPA-extractable Cd and Ni were less correlated while those of Cr and Pb were more correlated with their respective shoot and root contents. The results emphasize that accumulation of potentially toxic heavy metals in soil and their build-up in vegetable crops should not be ignored when sludge is applied as an amendment to land.     

Wetland-based phytoremediation of biosolids from an end-of-life municipal lagoon: A microcosm study

This study examined the effectiveness of a wetland system for phytoremediation of biosolids from an end-of-life municipal lagoon. The microcosm experiment tested the effects of one vs. two harvests of cattail per growth cycle in biosolids without (PB) or with (PBS) the addition of soil on phytoremediation. Cattail (Typha latifolia) seedlings were transplanted into pots containing 4.5 kg (dry wt.) of biosolids, above which a 10-cm deep water column was maintained. Results showed that two harvests per growth cycle significantly increased N and P phytoextraction relative to a single harvest. Overall, the three cycles of cattail removed ～3.7% of N which was originally present in the biosolids and ～2% of the total P content. Phytoextraction rates are expected to be higher under field conditions where biomass yields are much higher than those obtained under growth room conditions in this study. These results indicate that wetland-based phytoremediation can effectively clean up nutrients from biosolids, and therefore presents a potential alternative to the spreading of biosolids on agricultural land, which may not be readily available in some communities. Phytoextraction rates of trace elements, however, were much lower (0.02–0.17%). Nonetheless, trace element concentrations were not high enough to be of significant concern.