Yield response of crops amended with sewage sludge in the field is more affected by sludge properties than by final soil metal concentration

Adverse effects on crop yield or quality have been reported in sewage‐sludge treated soils at soil total metal concentrations below those of the current EU directives. A field trial was set up in Belgium (2002–2004) to assess crop response to the application of sewage sludge below these soil thresholds but with sludge metal concentrations either above (high‐metal) or below (low‐metal) sludge metal limits. Two lime‐stabilized and two raw, dewatered sludges were applied annually at rates of 10, 25 and 50 t dry matter (dm) ha^?1 for 3 years with four rates of N‐fertilizer as a reference. Final soil metal concentrations increased to maximums of 1.6 mg Cd kg^?1 and 225 mg Zn kg^?1 through sludge applications. Maize yield was marginally affected by treatments in year 1, whereas wheat and barley grain yields in subsequent years increased up to threefold with increasing sludge or fertilizer rates and were mainly explained by grain‐N. However, the grain yield of winter wheat in year 2 was reduced by about 14% in lime‐stabilized high‐metal sludge treatments compared with wheat receiving N‐fertilizer at equivalent grain‐N. Wheat grain and straw analysis showed no nutrient deficiencies but Zn concentrations in grain and straw were greater than in N‐fertilizer and lime‐stabilized, low‐metal sludge treatments, suggesting Zn toxicity. Sludge properties other than Cd concentration (e.g. electrical conductivity) affected crop Cd in the first year (maize), whereas significant correlations between Cd application and wheat grain Cd were found in the second year. Wheat grain Cd concentrations reached the international trade guideline of 0.1 mg Cd kg^?1 fresh weight in the plots amended with lime‐treated, high‐metal sludge even though soil Cd remained below EU limits. In the third year, barley grain Cd remained largely below EU limits. We discuss the possibility that sludge properties rather than soil total metal concentrations are related to effects on crops in the initial years after sludge applications. In none of the 3 years were any adverse effects on crops found for sludge meeting current EU regulations.     

Comparative availability to wheat of metals from sewage sludge and inorganic salts

A greenhouse experiment was conducted to evaluate the availability of metals from sewage sludge and inorganic salts, and the effect of pH and soil type on yield and metal (Zn, Cu, Cd and Ni) uptake by wheat (Triticum aestivum L. var. ‘holly’). Soils used in this study were Hartsells sandy loam (fine-loamy, siliceous Thermic Typic Hapludult) and Decatur silty clay loam (Clayey, kaolinitic, Thermic Rhodic Paleudult). Two treatments of sewage sludge containing metals were applied at the rate of 20 and 100 mt ha^?1. Inorganic Salts of Zn, Cu, Cd, and Ni were applied (as sulfate salts) at concentrations equivalent to those found in the 20 and 100 mt ha^?1 sludge. One treatment consisted of inorganic metals plus sewage at the 20 Mg ha^?1 rate. Two soil pH levels, one at field pH (below 6.0) and another pH adjusted between 6.5 and 7.0 were used. Wheat plants were harvested four weeks after germination. Two more subsequent harvests were made at four week intervals. For each harvest, dry matter yield increased as the rate of sludge application increased for both soil types. The soil pH also influenced the dry matter yield. High yield was observed when the pH was adjusted between 6.5 to 7.0 for both soils. An increase in yield was also observed at each subsequent harvest for most of the treatments. Inorganic salt treatments produced lower dry matter yields when compared with the sludge. Both sludge application and metal salts increased plant tissue concentration of Zn, Cu, Cd, and Ni at field pH for both soils. However, increasing the pH of the soil for both sludge and inorganic salt treatments generally decreased the tissue concentration of the above metals.     

Cadmium and zinc additions to wisconsin soils by commercial fertilizers and wastewater sludge application

Cadmium and Zn concentrations were determined on 21 commercial fertilizer samples by atomic absorption. The Cd concentration ranged from 1.5 to 9.7 mg kg^?1, the median being 4.3 mg kg^?1. Zinc showed a much wider range. The Cd added to soil by commercial fertilizers may be as much as 2150 kg annually in Wisconsin, compared to a potential of 1700 kg if wastewater sludges from all municipal sewage treatment plants in the state were disposed of on land. However, because of the higher application rates, sludge Cd on a soil concentration basis is a much more concentrated source of Cd than that from phosphate fertilizers.     

Initial results from long-term field studies at three sites on the effects of heavy metal-amended liquid sludges on soil microbial activity

In a long‐term study of the effects on soil fertility and microbial activity of heavy metals contained in sewage sludges, metal‐amended liquid sludges each with elevated Zn, Cu or Cd concentrations were applied over a 3‐year period (1995–1997) to three sites in England. The experiments were sited adjacent to experimental plots receiving metal‐rich sludge cakes enabling comparisons to be made between the effects of heavy metal additions in metal‐amended liquid sludges and sludge cakes. The liquid sludge additions were regarded as ‘worst case’ treatments in terms of likely metal availability, akin to a long‐term situation following sewage sludge additions where organic matter levels had declined and stabilised. The aim was to establish individual Zn (50–425 mg kg^?1), Cu (15–195 mg kg^?1) and Cd (0.3–4.0 mg kg^?1) metal dose–response treatments at each site, but with significantly smaller levels of organic matter addition than the corresponding sludge cake experiments. There were no differences (P > 0.05) in soil respiration rates, biomass carbon concentrations or most probable numbers of clover Rhizobium between the treatments at any of the sites at the end of the liquid sludge application programme. Soil heavy metal extractability differed between the metal‐amended liquid sludge and metal‐rich sludge cake treatments; Zn and Cd extractabilities were higher from the liquid sludge additions, whereas Cu extractability was higher from the sludge cake application. These differences in metal extractability in the treated soil samples reflected the contrasting NH₄NO₃ extractable metal contents of the metal‐amended liquid sludges and sludge cakes that were originally applied.     

Possibilities of using sewage sludge as nitrogen fertilizer for maize

Abstract

The objective of this study was to determine the effects of nitrogen fertilizer sources of ammonium sulphate and municipal sewage sludge on yield, N content and uptake of the maize (Zea mays L.). Nutrient and heavy metals were determined in soil and plant. The experiment with three sludge rates (256, 513 and 1026 kg total N ha^?1 or 9.5, 18.0 and 38.1 t ha^?1 sludge), two nitrogen rates (80 and 160 kg N ha^?1) and zero-N control were conducted on a clay loam soils under irrigated conditions in Eastern Anatolia region in Turkey. Treatments were arranged in a randomized complete block design with four replications. Yield, N content and total N uptake of maize increased significantly with sludge application. 9.5 t and 19.0 t ha^?1 sewage sludge applications did not significantly affect heavy metal content of leaf and grain. However, 38.1 t ha^?1 sludge applications increased leaf Pb and Zn. DTPA-extractable Cd, Cu, Fe, Pb and Zn concentrations of the soil increased at applications of 38.1 t ha^?1 sewage sludge, whereas applications of 9.5 t and 19.0 t ha^?1 sludge only resulted in elevated levels of Cu and Zn, We conclude that if sewage sludge is to be used in production of maize, applications rate up to 19 t ha^?1 could be accepted. However, this means also that the N requirement of maize crop is not covered by the sludge; therefore, the rest of nitrogen could be supplied as inorganic N.     

Heavy Metals Leachability as Affected by pH of Compost-Amended Growth Medium Used in Container-Grown Rhododendrons

The leaching of heavy metals from plant growth medium, admixed with different amounts of compost (prepared from sewage sludge and yard waste) at pH 5, 6 and 7 was determined over a six-month period. Twelve-week old rhododendron cuttings were planted in 2-L containers and rainfall was supplemented with irrigation to supply two centimeters of water per day. Leachates collected over each two to four week period were analyzed for Cd, Cr, Cu, Ni, Pb and Zn using atomic spectrometry. The concentrations of Cd, Cu, Ni and Zn in the leachates increased with increasing proportions of compost in the medium and decreased with increasing time of leaching. Decreasing media pH dramatically increased the concentrations of Cd, Ni and Zn in the leachates, but had no effect on the Cu concentrations. For example, as the proportion of the compost in the pH 5 medium increased from 0 to 100 percent, the concentrations of metals (μg L^?1) in the leachates collected during the first two weeks increased from 1 to 33 (Cd), 10 to 123 (Cu), 8 to 113 (Ni) and 300 to 24,000 (Zn). Corresponding increases at pH 7 were 0.4 to 0.8 (Cd), 14 to 141 (Cu), 8 to 28 (Ni) and 100 to 400 (Zn) μg L^?1. The concentrations of Cr and Pb in the leachates remained below the detection limits regardless of media pH and amounts of compost.     

Effect of Sodium Chloride-induced Salinity on Phyto-availability and Speciation of Cd in Soil Solution

A greenhouse experiment with two levels of Cd (0.5 and 10 mg Cd kg^?1, in the form of CdCl₂), and five salinity levels of irrigation water (0, 8.6, 17.1, 34.2 and 68.4 mM NaCl) in triplicate was conducted to determine the effect of NaCl-induced salinity on the solubility and availability of Cd in clay loam and sandy calcareous soils. Corn seeds (Zea mays L.) were sown in pots. Forty-five days after planting, the shoots were harvested, and their Cd concentration was determined. The post-harvest electrical conductivity (EC_e), pH, and concentrations of cations and anions were determined in soil saturation paste extracts. Increasing irrigation water salinity resulted in significant increases in the total soluble Cd concentration in both studied soils. A positive correlation was found between the total soluble Cd and the chloride concentration in the soil solution.Solution speciation, calculated with MINEQL+ (a chemical equilibrium modeling system), predicted that Cd was present mainly as free Cd²⁺ ions followed by CdCl⁺ and $ {\text{CdSO}}^{0}_{4} $ in the soils irrigated with deionized water. However, Cd species in the soil solution were significantly altered by increasing chloride concentration, with Cd–chloro complexes becoming the dominant Cd species in the soil solution. Increasing the salinity level resulted in significant decreases in the shoot dry matter and increases in the shoot Cd concentration. Shoot Cd concentration was positively correlated with both the total Cd and Cd–chloro complexes in the soil solution.     

Initial results from a long-term, multi-site field study of the effects on soil fertility and microbial activity of sludge cakes containing heavy metals

In a long‐term study of the effects on soil fertility and microbial activity of heavy metals contained in sewage sludges, metal‐rich sludge cakes each with high Zn, Cu or Cd concentrations were applied annually for 4 years (1994–1997) to nine sites throughout Britain. These sites were selected to represent agricultural soils with a range of physical and chemical properties, typical of those likely to be amended with sewage sludge. The aim was to establish individual total Zn (approx. 60–450 mg kg^?1), total Cu (approx. 15–200 mg kg^?1) and total Cd (approx. 0.2–4 mg kg^?1) metal dose–response treatments at each site. Sludges with low metal concentrations were added to all treatments to achieve as constant an addition of organic matter as possible. Across the nine sites, soil pH was the single most important factor controlling Zn (P < 0.001; r² = 92%) and Cd extracted with 1 m NH₄NO₃ (P < 0.001; r² = 72%), and total iron content the most important factor controlling Cu extracted with 1 m NH₄NO₃ (P < 0.001; r² = 64%). There were also positive relationships (P < 0.001) between soil organic carbon (C) concentrations and soil biomass C and respiration rates across the nine sites. Oxidation of sludge C following land application resulted in approximately 45% of the digested sludge cake C and approximately 64% of the ‘raw’ sludge cake C being lost by the end of the 4‐year application period. The sludge cake applications generally increased soil microbial biomass C and soil respiration rates, whilst most probable numbers of clover Rhizobium were generally unchanged. Overall, there was no evidence that the metal applications were damaging soil microbial activity in the short term after the cessation of sludge cake addition.     

Cadmium and zinc availability in soil irrigated with sludge containing a cationic conditioner

Tomato plants (Lycopersicum esculentum Mill, ‘Red Cherry Small’), grown in 15 cm diameter plastic pots with a standard greenhouse medium (1:1:1, by volume, soil:peat:sand) were irrigated for 15 weeks with liquid sewage sludge containing a liquid cationic conditioner (Petroset SB, Phillips Petroleum Company, Bartlesville, Oklahoma) to determine the effect of the conditioner on Cd and Zn availability. Half of the plants received 50 ml week^?1 liquid digested sludge with no conditioner and half of the plants received 50 ml week^?1 sludge containing 0.25 ml conditioner (200:1, by volume, sludge:conditioner). Plant height was measured weekly. Plants were harvested 3, 6, 9, 12, and 15 weeks after sludge treatments began and separated into roots, shoots, and fruits for dry weight determination and Cd and Zn analyses. Soil and sludge crusts were sampled at the same times and analyzed for extractable concentrations of Cd and Zn. Dry weights of plants grown with conditioned sludge were similar to those grown with nonconditioned sludge. Plants with conditioner flowered and fruited one and two weeks earlier, respectively, than plants without conditioned. Six weeks after treatments began, when the plants had grown to their greatest height, Cd concentrations in sludge crusts, soil, and roots receiving conditioner were 2.0, 1.5, and 2.1 times greater, respectively, than crusts, soil and roots not receiving conditioner. After the six weeks sampling time, Cd concentrations in crusts, soil, and roots receiving conditioned sludge were similar to those in crusts, soil, and roots receiving nonconditioned sludge. At the third-week sampling time, shoots of plants grown with conditioner had 2.6 times more Cd than shoots of plants grown without conditioner. Cadmium concentrations in shoots from both treatments were similar at later sampling dates. Cadmium content of fruits was the same for both treatments all sampling times. Zinc content of roots, shoots, fruits soil, and sludge crusts was no affected by the conditioner. Results showed that a cationic conditioner, added to sludge, increased the availability of Cd, but not of zn, for tomato plants until maximum height was reached.     

Long-term changes in soil and plant metal concentrations in an acidic dredge disposal site receiving sewage sludge

A long-term experiment was conducted to determine the distribution of sludge-borne metals applied to a revegetated acidic dredge spoil disposal site. The initial soil was infertile and highly acidic (pH 2.4). Sewage sludge and lime were applied in 1974 at the rates of 100 and 23 mt ha^?1, respectively, and tilled into the soil to a depth of 20 cm. In 1974 an adjacent site was also revegetated with topsoil and lime but without sludge. Soil and plants were sampled 2, 4 and 16 yr following seeding. After 16 yr the total and DTPA-extractable Cu, Zn, Cr, Pb, Ni and Cd decreased in soils to nearly the levels of the control soils. Concentrations of metals in plants also decreased. Decreases in tissue concentrations ranged from 40 to 70% for Cu, Cr, Pb, Ni and Cd and up to 90% for Zn. The results showed that a single 100 mt ha^?1 application of sewage sludge containing high concentrations of metals was a cost-effective method for improving plant growing conditions on highly acidic soils.

     

After effects of metals derived from a highly metal-polluted sludge on maize (Zea mays L.)

High Cd and Ni concentrations in sandy soils were built up in a field experiment, receiving an unusually metal-polluted sewage sludge between 1976 and 1980, at Bordeaux, France. The study evaluates the availability of metals and their after effects on maize at one point in time, the 8th year following termination of sludge application (1988). Plant parts (leaves, stalks, roots, grains) and soil samples were collected from plots which received 0 (Control), 50 (S1) and 300 Mg sludge DW ha^?1 (S2) as cumulative inputs. Dry-matter yield, plant metal concentrations, total, and extractable metals in soils were determined. Metal inputs resulted in a marked increase in total and extractable metals in soils, except for extractable Mn and Cu with either 0.1 N Ca(NO₃)₂ or 0.1 N CaCl₂. Total metal contents in the metal-loaded topsoils (0–20 cm depth) were very often lower, especially for Cd, Zn, and Ni, than the expected values. Explanation was partly given by the increases of metal contents below the plow layer, particularly for Cd at the low metal loading rate, and for Cd, Ni, and Cu at the high one (Gomez et al., 1992). In a control plot beside a highly metal- polluted plot, Cd, Zn, and Ni concentration in soil increased whereas the concentration of other metals was unchanged; lateral movement, especially with soil water, is plausible. Yield of leaves for plants from the S2 plot was reduced by 27%, but no toxicity symptoms developed on shoots. Yields of stalks for plants in both sludge-treated plots numerically were less than the controls but the decrease was not statistically significant. Cd and Ni concentrations increased in all plant parts with metal loading rate while Mn concentrations decreased. Leaf Cd concentration in plants from sludge-treated plots (i.e. 44 and 69 mg Cd kg^?1 DM for S1 and S2) was above its upper critical level (i.e. dry matter yield reduced by 10%: 25μg Cd g^?1 DM in corn leaves, Macnicol and Beckett, 1985). Yield reduction at the high metal-loading rate was probably due to 3 main factors: Mn deficiency in leaves, the accumulation of Ni especially in roots, and the increase of Cd in leaves. The amount of metal taken up by plants from the control plot ranked in the following order (mole ha^?1): Fe(22)? Mn(7)>Zn (5.6)?Cu (0.7), Ni (0.6), Cd (0.4). For sludge-treated plots, the order was (values for S1 and S2 in mole ha ^?1): Fe (16, 15)>Zn (7.9, 7.7)>Ni (4.3, 4.7)>Cd (1.9, 2.1)>Cu (1.0,1.2), Mn (1.5, 1.1). Zn and Cd had the greatest offtake percent from the soil to the above ground plant parts. Cd or Ni uptake by maize were correlated with extractable metals by unbuffered salts (i.e. 0.1 N Ca(NO₃)₂ and 0.1 N CaCl₂). It is concluded that part of the sludge-borne Cd and Ni can remain bioavailable in this sandy soil for a long period of time (e.g. 8 yr) after the termination of metal-polluted sludge application.     

Effects of a cadmium-contaminated sewage sludge compost on dynamics of organic matter and microbial activity in an arid soil

 An incubation experiment lasting 120 days was carried out to ascertain the effect on the soil microbial activity and organic matter mineralization of adding a sewage sludge compost contaminated with two different levels of Cd to an arid soil. Two composts, with a low (2 mg kg^–1) and high (815 mg kg^–1) Cd content, respectively, were used in this experiment. Both composts increased the total organic C, humic substance and water-soluble C contents, the beneficial effects still being noticeable after 120 days of incubation. The most labile C fraction (water-soluble C) was the most sensitive to the high Cd content. The high Cd concentration decreased soil microbial biomass C and stimulated the metabolic activity of the microbial biomass, the metabolic quotient (qCO₂) revealing itself to be a very sensitive index of the stress that the incorporation of a Cd-contaminated sewage sludge compost causes in a soil. The effect of Cd contamination on enzyme activities (urease, protease that hydrolyse N-α-benzoil-l-arginamide, phosphatase, and β-glucosidase) depended on the enzyme studied. Received: 10 September 1997     

Effect of sewage sludge application on cotton yield and contamination of soils and plant leaves

Abstract

In a two‐year field experiment, dry sewage sludge was applied to fields plots at rates of 0,26,42,58, or 77 tons ha^‐1 year^‐1 on a clay loam soil (calcixerrolic xerochrept), well drained with a pH value of 8.15. Cotton was the cultivated plant (Gossypiumhirsutum variety korina). Sequential extraction was used to separate the different forms of the metals (exchangeable, organically bound, carbonates and residual) in the soil‐sludge mixtures. Cotton yield increased in the second year of experimentation compared with the control treatment (without fertilization and no application of sewage sludge). Most of the metals studied [cadmium (Cd), zinc (Zn), copper (Cu), and nickel (Ni)] were found in the organically bound, carbonate or residual forms. From the elements in the soil fractions, only Zn in the residual form was correlated with the Zn content of cotton leaves. The diethylenetriaminepentaacetic acid (DTPA) extraction of the plant‐available levels of the elements showed only for Cd a simple linear correlation, between concentration in soils and cotton leaves.     

Effects of time and temperature on the bioavailability of Cd and Pb from sludge-amended soils

A pot experiment was conducted to compare the behaviour and bioavailability of Cd and Pb from two soils mixed with sewage sludge at three rates (0, 50 and 150 t ha^?1) and maintained at two contrasting ambient temperatures (15°C and 25°C) over a period of one year following the treatments. Ryegrass (Lolium perenne) accumulated Cd and Pb in the sewage sludge treated soils, although accumulation was significantly lower in the soils treated at the high rate (150 t ha^?1) compared to the low rate (50 t ha^?1). Ryegrass grown in the warm environment (25°C) accumulated significantly higher levels of Cd and Pb than that grown in cooler conditions (15°C). Samples of the soils spiked with nitrate salts of Cd and Pb at equivalent rates of metal loading resulted in the ryegrass accumulating much higher levels of both the metals than on the sludge treated soils. Metal uptake by the ryegrass from the sludge treatments increased over successive harvests while that from metal salt treatments decreased. The observed trend of increasing plant metal uptake over time coincided with a trend of decreasing pH in the sludge treatments. However, the concentrations of Cd and Pb extracted by DTPA failed to predict the changes in plant metal uptake. The importance of sewage sludge as both a source and a sink of pollutant metals and the trend of increasing bioavailability over time shown by this experiment are discussed.     

Washing agents from sewage sludge: efficiency of Cd removal from highly contaminated soils and effect on soil organic balance

Purpose

To compare Cd removal from different soils with three washing agents recovered from sewage sludge (dissolved organic matter (DOM), soluble humic-like substances (HLS), soluble humic substances (SHS)). Also, to investigate how washing with these agents changes soil organic-matter composition (OM).

Materials and methods

Sandy clay loam (S1) and clay (S2) highly contaminated with Cd (300 mg kg^?1) were washed with DOM, HLS, or SHS solutions at various pHs, and with various washing times and washing modes (single or double). Cd distribution and OM composition were determined (including content of humic substances (HS), fulvic fraction (FF), labile humic acids (L-HA), and stable humic acids (S-HA)).

Results and discussion

Cd removal proceeded with pseudo-second-order kinetics. Equilibrium was reached in 30 min (S1) and 60 min (S2). DOM, HLS, and SHS removed 75–82% of Cd from S1, and 80–87% from S2. The most mobile fraction of Cd was removed after one wash. S2 retained more OM, including HS, than S1. Although washing did not change the HA/FF ratio in most variants, washing with DOM and HLS increased the percentage of L-HA in both soils. Washing with SHS increased S-HA content in both soils, but the percent content of S-HA was similar to that in the unwashed soil.

Conclusions

DOM, HLS, and SHS derived from sewage sludge can effectively remediate clay and sandy clay soils highly contaminated with Cd. Washing with an SHS solution can increase the content of the most stable carbon forms (HA), which is beneficial for carbon sequestration in remediated soils.

     

Soil amendment with city sewage sludge increases soil extractable cadmium,nickel and zinc more than tannery,pharmaceutical and paper mill wastes

Abstract

The distribution of DTPA-extractable Cd, Ni and Zn in four profile samples collected from areas contaminated with wastes from tannery, city sewage, pharmaceutical and paper mills located at different places in Bangladesh was investigated. Soil samples were analysed for the total and their DTPA-extractable metal contents. The total concentration of metals in the soil horizons ranged from 0.07 to 0.62 mg kg^?1 for Cd, 31 to 54 mg kg^?1 for Ni and 59 to 838 mg kg^?1 for Zn, respectively. These metal concentrations were highest in the surface and lowest in the subsurface horizons. This trend was also observed for the DTPA-extractable amounts of these metals. The relative extractability, expressed as the ratio of DTPA to total contents (aqua regia-extractable) was 33 to 46% for Cd, 2 to 10% for Ni and 3 to 28% for Zn, respectively, in the A1 horizon, while in the B and C horizons the ratios decreased gradually as did total concentration, indicating that metal contamination was primarily limited to the surface horizon. For all three metals, the above mentioned ratio was highest in the city sewage soil and the lowest in the paper mill soil. In general, the extent of contamination among the profiles investigated was in the following order: city sewage>tannery>pharmaceutical>paper mill soil. Higher ratios of DTPA-extractable Cd, Ni and Zn in the city sewage soil than the other soils may create a risk for the contamination of agricultural products and ground water.     

Effect of sewage sludge on some chemical properties of calcareous sandy soils

Abstract

City sewage sludge was applied to the surface layer (0–10 cm) of two sandy soils, slightly calcareous with 8.9% CaCO₃ and moderately calcareous with 26.7% CaCO₃, at the rates of 0, 25, 50, 75, and 100 Mg ha^‐1. The effects of sewage sludge and its rates on total soluble salts, pH of soils and concentration and movement of some heavy metals within soils were investigated. Soil samples were packed at bulk density of 1.5 g cm^‐3 in PVC columns and incubated for 19 weeks. The results indicated that total soluble salts (EC) of the treated layer increased with increasing sewage sludge rates. Soluble salts also increased with an increase in soil depth for both soils. The pH values of treated layers in two soils decreased with increasing sewage sludge rates. With increasing sewage sludge rates, concentrations of heavy metals [cobalt (Co), nickel (Ni), cadmium (Cd), and leaf (Pb)] increased in the treated layers compared to the untreated layers and their mobility was restricted mostly to the upper 30‐cm depth. Movement of Co and Pb in both the soils was predominately limited up to a depth of 40 cm for Co and 5 cm for Pb below the treated soil layer. Nickel and Cd movement was mostly limited to a depth of 10 cm in slightly calcareous soil and 5 cm in moderately calcareous soil. Metal movement in the respective soils is ranked as Co>Ni=Cd>Pb and Co>Ni=Cd>Pb. The low concentrations of heavy metals and the restricted mobility with soil depth, suggest that this material may be used for agricultural crop production without any toxic effect on plants.     

Sorption of Cd and Pb by selected temperate and semi-arid soils: effects of sludge application and ageing of sludged soils

Five soils from semi-arid regions of India and 5 soils from England were compared with regard to their ability to sorb Cd and Pb when mixed with sewage sludge and as unsludged controls. The application of sewage sludge at 150t ha^?1 significantly increased the amount of Cd retained by the soils. The sorption data were statistically best-fit to the linearized Freundlich equation and the slopes of the isotherms were steeper with sludge application, indicating an increased affinity for Cd in the soil-sludge mixtures. A similar trend in Pb sorption was also observed for the English soils following the sludge treatment. In contrast, most of the Indian soils showed a decline in Pb sorption following the sludge application. Liming an acidic English soil to pH 7.0 was shown to increase its metal sorption capacity. The longer-term persistence of these observed effects of sewage sludge addition on metal sorption by soils was investigated in two sludge-soil mixtures maintained under experimental conditions for up to 450 days. Samples of these soils taken 1,60 and 450 days after the mixing with the sludge were batch equilibrated with Cd and Pb and it was found that the metal retention in both soils decreased significantly over this time period.     

Medically used radionuclides in sewage sludge

The concentration of medically used radionuclides has been studied in sludge from the sewage treatment plant serving the borough of Malmo. In this area all nuclear medicine procedures are carried out in one hospital and almost all patients live in the borough. Therefore, the input of medically used radionuclides into the sewage system can be estimated with good accuracy. Samples of digested sludge have been taken once or twice a week during half a year. Iodine-131 (physical half life (T) = R.05 d) was detected in all samples. The ¹³¹I-activity concentration due to medical use varied between (0.03±0.01) and (0.12±0.02) nCi kg^?1. The ratio between the total output of ¹³¹I via the sludge and an adherent input of the radionuclide into the sewage system was determined to (2.6 ± 0.6) × 10^?3, which is equivalent to a ratio of (2 ± 1) × 10^?2 for stable I. Occasionally measurable activities of ¹⁹⁸Au (T=2.7 d) and ²⁰¹T1(T=3.l d) have been found. The radioactivity concentration of medically used radionuclides in the sludge is low and constitutes no health problems for the persons involved. The sludge however has proved to be a very sensitive and suitable integrator of radioactive material released from a large urban area.     

Magnesium deficiency in Douglas-fir and Grand fir growing on a sandy outwash soil amended with sewage sludge

Soil and plant samples were collected from chlorotic plantations of Grand fir (Abies grandis) and Douglas-fir (Pseudotsuga menziesii) in Winter, 1989. The soils had been amended in 1981 with an average of 300 dry Mg ha^?1 of municipal sewage sludge. The sludge amendment resulted in an N application rate of approximately 8000 kg ha^?1. Foliage analysis indicated a severe Mg deficiency (0.25 g kg^?1 in sludge-treated vs. 0.93 g kg^?1 in untreated area) might be the cause of chlorosis. No other nutrient showed concentrations in the deficient or toxic ranges. Trace metal levels in foliage were elevated significantly for Ni, Cd and Cr in sludge-treated sites, but not toxic levels. Soil samples taken to a depth of 1.4 m indicated the potential for soil acidification (up to 0.9 pH unit) in soil surface horizons. In addition, exchangeable Ca, Mg and K may have been depleted in surface horizons. Exchangeable Al and Fe were greater in the surface of sludge-treated sites. These observations and the loss of much of the nitrogen added during the sludge amendment indicated that nitrification and cation leaching was likely the mechanism for acidification and depletion of exchangeable cations. Fertilization of the plantation with MgSO₄ or dolomitic limestone was carried out in Spring 1990. New foliage collected in June, 1990 was non-chlorotic and significantly higher in Mg concentration than unfertilized foliage (1.1. vs. 0.7 g kg^?1, respectively). Results of this study indicate that it is important to assess the potential for initiating a nutrient deficiency due to secondary effects of sludge application in forest systems.