Partial replacement of rock phosphate by sewage sludge ash for the production of superphosphate fertilizers

Thermal utilization of sewage sludge through mono‐incineration or gasification results in phosphorus (P) rich sewage sludge ash (SSA) that must be returned to agricultural production systems to fulfill the need for recycling of P resources contained in wastewater streams. As the plant‐availability of P contained in SSA is low, we propose feeding SSA directly into the production of superphosphate fertilizers, thereby opening a further pathway for the recycling of phosphorus (P) from wastewater streams to agricultural production systems by using available technologies. We carried out laboratory‐scale production of superphosphate test‐products, in which rock phosphate (RP) was partially replaced with SSA (gasification) before digestion with concentrated sulfuric acid, and evaluated these products with regard to the solubility of P in H₂O and neutral ammoniumcitrate solution. We further carried out a growth‐chamber experiment (28 d) using maize (Zea mays L. cv. Sulano) as a model plant on a low P (0.4 mg P_CAL 100g^?1), high pH (7.6) substrate to evaluate plant P availability of the test products. Our laboratory‐scale results show that at least 8% of P from RP can be replaced by P from SSA while maintaining both the high solubility of P in the fertilizer product and the growth of maize compared to pure RP digested with concentrated sulfuric acid. Further substitution of RP through SSA decreased the total P concentration of the test products, as well as the relative amounts of P soluble in H₂O and neutral ammoniumcitrate solution, which affected early plant development of maize.     

Increased phosphorus availability from sewage sludge ashes to maize in a crop rotation with clover

A recycling of Phosphorus (P) from the human food chain is mandatory to secure the future P supply for food production. However, many available recycled P fertilizers from sewage sludge do not have an adequate P bioavailability and, thus, are not suitable for their application in soils with pH >5.5–6.0, unless being combined with efficient mobilization measures. The aim of the study was to test the P mobilization ability of red clover (Trifolium pratense L.) from two thermally recycled P fertilizers for a subsequently grown maize. Two sewage sludge ashes (SSA) were investigated in a pot experiment at soil pH 7.5 with red clover differing in its nitrogen (N) supply (added N fertilizer or biological N₂ fixation (BNF)), followed by maize (Zea maize L.). Shoot dry matter of maize was almost doubled when N supply of previous grown clover was covered by BNF, instead of receiving added N fertilizer. Similarly, shoot P removal of maize following clover with BNF was significantly increased. It is suggested that the P mobilization is related to the BNF, and a proton release of N₂ fixing clover roots led to the measured decrease in soil pH and thereby increased P availability of the tested fertilizers.     

Plant availability of phosphorus from dewatered sewage sludge,untreated incineration ashes,and other products recovered from a wastewater treatment system

The objective of the present study was to evaluate the plant phosphorus (P) availability of products derived from new P‐recovery technologies deployed in wastewater treatment systems against sewage sludge and untreated sewage sludge ashes. Eight P sources were evaluated in a six‐week pot experiment with spring barley (Hordeum vulgare L.) and a soil incubation experiment with anaerobically digested and dewatered sewage sludge (Sludge), sewage sludge ash (Ash), thermochemically treated sewage sludge ash (TrAsh), struvite (Struv), concentrate (Conc), and centrifuged concentrate (Prec) from evaporated reject water, with triple super phosphate (TSP), and composted organic household waste (Comp) as references. All sludge‐related materials except struvite came from the same wastewater treatment plant in Denmark. The apparent plant P use efficiency (PUE) of the treatments in the pot experiments was in the following order: Prec (17.0%) > TSP (12.8%) ≥ Conc (12.7%) > Sludge (8.8%) ≥ TrAsh (6.9%) ≥ Struv (6.0%) ≥ Comp (5.8%) > Ash (3.4%). The water‐extractable P (WEP) in the incubation experiment largely supported this order and there was a strong correlation between WEP in the incubation experiment and plant P uptake in the pot experiment. Overall, the results of this study demonstrate that it should not automatically be assumed that products recovered with new treatment technologies for P recovery are more effective P fertilizers than the sewage sludge from which they originate. Furthermore, these results indicate that the measurement of water‐extractable P after soil incubation could be a suitable proxy for plant P availability.     

Phosphorus bioavailability in ash from straw and sewage sludge processed by low‐temperature biomass gasification

Reuse of phosphorus (P) from waste streams used for bioenergy conversion is desirable to reduce dependence on nonrenewable P resources. Two different ash materials from low‐temperature biomass gasification of wheat straw and sewage sludge, respectively, were investigated with regard to their P bioavailability. A set of pot experiments with spring barley was carried out to compare the ash P fertiliser value with mineral P fertiliser and the sewage sludge feedstock. An indirect radioactive labelling approach with ³³P was used to determine the amount of P taken up from the fertiliser materials. Depending on the application rate, straw gasification ash produced a fertiliser response comparable to mineral P. However, P uptake from the ash was generally less than uptake from equivalent amounts of mineral P, and the calculated relative effectiveness was 44% after 6 weeks of plant growth. In contrast, the P fertiliser value of Fe‐rich sewage sludge after low‐temperature gasification was practically zero. These results suggest that ash from low‐temperature gasification could be developed into alternative P fertilisers; however, as the P bioavailability depends greatly on the feedstock used, a greater emphasis on feedstock composition is required.     

Barley growth in response to potassium fertilization of soil with long term application of sewage sludge

Application of sludges to agricultural land depressed plant growth despite its beneficial effects on crop production. Hence it remained to be determined whether sludge application induced any adverse effect on crop nutrition or growth. Thus the growth response of barley on an Andosol to which two kinds of sewage sludge compost had been applied for 17 y was tested in pot experiments with and without application of chemical K fertilizer. The addition of K in the sewage sludge composts resulted in the improvement of plant growth, high dry matter production and increase of total K content both in plant and soil compared to the materials without K application. It was assumed that long term application of sewage sludge requires chemical fertilizer amendment for crop production.     

Effects of the application of heated sewage sludge on soil nutrient supply to plants

Abstract

We previously reported that heating sewage sludge significantly changes the rate of N mineralization. The present study was undertaken to examine the extent to which these changes affect plant growth and nutrient supply after application to different soils. A pot experiment in which komatsuna plants (Brassica campestris L. var. rapa) were grown in a Fluvisol or an Arenosol indicated that the amount of N taken up by the plants increased significantly by heating air-dried sludge at 120°C and decreased significantly by heating at 180°C. Heat-drying of the sludge at 120°C or 180°C also increased the N uptake significantly. These plant responses could be explained by the heat-induced changes in the release of inorganic N from the sludge. In contrast to N, the sludge materials containing Fe as a coagulant immobilized soil soluble P. When the sludge was applied to an Andosol, the N-supplying effects on plant growth were offset by P adsorption onto the sludge and soil. In a successive pot experiment conducted without additional sludge application, it was further suggested that frequent sludge application is required to maintain plant growth. However, it will also lead to the accumulation of sludge components in the soil because less than 40% and 15% of the sludge N and P, respectively, were apparently recovered in two harvests of the plants. In conclusion, heated sludge materials can act as an effective organic N fertilizer provided that they are applied to a suitable soil and that the short-term effects on soil productivity are balanced with the long-term effects on environmental quality.     

Nitrogen transformation and plant growth in response to different urea‐application methods and the addition of DMPP

The differences in soil inorganic‐nitrogen (N) concentration and distribution, plant biomass, and root growth in the presence or absence of the nitrification inhibitor 3,4‐dimethylpyrazole phosphate (DMPP) under different urea‐application methods (placement versus homogeneously applied) were explored in a short‐term microcosm experiment. Spring wheat (Triticum aestivum L.) was grown in a microcosm with six different treatments: no amendment (CK), DMPP homogeneously applied (DMPP‐hom), urea homogeneously applied (Urea‐hom), urea with DMPP homogeneously applied (Urea + DMPP‐hom), urea placement (Urea‐place), and urea with DMPP placement (Urea + DMPP‐place). After 28 d, plant biomass, soil inorganic nitrogen content, distribution of soil inorganic nitrogen and plant roots in the soil were analyzed. The soil inorganic N and plant roots tended to be distributed asymmetrically in the placement treatment but were distributed symmetrically in the homogeneous treatment. DMPP addition significantly increased the soil NH$ _4^+ $ ‐N content and decreased the NO$ _3^- $ ‐N content, especially near the fertilized zones in the placement treatment. Compared to the urea‐only treatments, DMPP application significantly increased the shoot biomass and root lengths of the wheat in the homogeneous treatment but decreased them in the placement treatment. Therefore, homogeneously applied urea and DMPP may produce a more uniform nutrient distribution, leading to greater nitrogen retention in the soil and thus accelerating wheat growth.     

Leaching of heavy metals and nutrients from calcareous sandy‐loam soil receiving municipal solid sewage sludge

Leaching column experiments were conducted to determine the degree of mobility of heavy metals (HMs) and nutrients after the addition of municipal solid sewage sludge (MSS) in a sandy‐loam soil. Treatments were (1) soil application of low metal content MSS, (2) soil application of metal‐enriched municipal solid sewage sludge (EMSS), and (3) control. The MSS application represented a dose of 200 Mg dry weight (dw) ha^–1. Soil columns were incubated at room temperature for 15 d and were irrigated daily with distilled water to make a total of 557 mm. Leachates were collected and analyzed for HMs and nutrients. The Ni and Pb added to soil via MSS and EMSS were found to be leached through the 20 cm columns of calcareous sandy soil although Ni and Pb concentrations in the percolate were small relative to the total amounts of metals applied. Losses of K⁺ from the EMSS, MSS, and control were 92.5, 82.0, and 52.5 kg ha^–1, respectively. Losses of Mg²⁺ were in the range from 104.4 (control treatment) to 295.2 kg ha^–1 (EMSS), while the loss of Ca²⁺ was in the range from 265.0 (control treatment) to 568.2 kg ha^–1 (EMSS). The results showed that the amounts of P leached from EMSS (3.02 kg ha^–1) and MSS (2.97 kg^–1 ha^–1) were significantly larger than those from the control treatment (1.54 kg ha^–1). The geochemical code Visual MINTEQ was used to calculate saturation indices. Leaching of P in different treatments was controlled by rate‐limited dissolution of hydroxyapatite, β‐tri‐Ca phosphate, and octa‐Ca phosphate. The results indicate that application of MSS to a sandy soil, at the loading rate used in this study, may pose a risk in terms of groundwater contamination with Ni, Pb, and the studied nutrients.     

Effect of an artificial soil mix from coal fly ash and sewage sludge on soil microbial activity

An artificial soil mix was prepared from coal fly ash and sewage sludge and an experiment was performed to evaluate their effects on soil microbial respiration. Coal fly ash at 0%, 5%, 10%, 35% and 50% w/w was mixed with dewatered sewage sludge and then each ash-sludge mixture was incubated with a sandy soil at 1:1 v/v at 28°C for 42 days. All treatments showed the same carbon dioxide production pattern with a peak production at day 7 to day 14. Addition of ash-sludge mixtures to soil resulted in an increase in carbon dioxide production but the production rate decreased according to the ash amendment rate. The high pH of coal fly ash and the dilution effect of the sludge were the major reasons for the decrease. However, the ecological dose 50% values sharply increased from 26% at day 3 to 39% ash at day 14. This indicates the rapid acclimatization of microorganisms to the fly ash-sludge mixtures. Therefore, a brief stabilization period may be required for the establishment of soil microbial populations in soil amended with ash-sludge mixtures.     

Effects of thermal drying on phosphorus availability from iron‐precipitated sewage sludge

Thermal drying of sewage sludge implies sanitation and improves practical handling options of the sludge prior to land application. However, it may also affect its value as a fertilizer. The objective of this study was to assess whether thermal drying of sewage sludge, as well as drying temperature, affects plant P availability after application to soil. The experiment included dewatered sewage sludge (20% DM) and thermally dried sewage sludge (95% DM) collected at a Danish wastewater treatment plant, as well as laboratory oven‐dried (70, 130, 190, and 250°C; DM > 95%) subsamples of the dewatered sludge, and a triple superphosphate as a reference. Plant P availability was studied in a 197 d soil incubation experiment, with sampling for Diffusive Gradients in Thin films (DGT) and water extractable P (WEP) analyses over time, and in a pot experiment with spring barley (Hordeum vulgare L.). In both experiments, thermal drying reduced P availability, as shown by 37 and 23% lower DGT and WEP values, respectively, and a 16% lower P uptake by barley in the pot experiment. The specific drying temperature did not appear to have much effect. Overall, our results suggest that thermal drying of iron‐precipitated sewage sludge is not an optimal treatment option if the aim is to optimize plant P availability.     

Contribution of sewage sludge to erosion control in the rehabilitation of limestone quarries

The aim of this study was to evaluate the effects of high doses of municipal sludge on soil aggregation and to assess its value for soil erosion reduction, both under natural and in simulated rainfall conditions. Doses of 200 and 400 Mg ha⁻¹ sewage sludge (dry weight) were applied to the soil of experimental plots situated on a 28 degree slope. Two sludge application procedures were tested: pre-mixing into the soil before disposal on the slope, and direct application on the soil surface. Sheet erosion was measured by collecting the sediment carried down to a Gerlach trough situated at the base of the plots. Simulated rainfall, with an intensity of 64 mm h⁻¹, was applied to evaluate soil erodibility. When the soil had no vegetation, the erosion measured on plots treated with sludge represented less than 10 per cent of the erosion from the control plot. Even when the vegetation was well developed, the erosion was also lower in the plots where sludge had been applied. In simulated rainfall, the soil loss was inversely proportional to the sludge dose, and when sludge was applied directly on the soil surface the erosion rates and particle mobilization caused by raindrop impact were minimal. Sludge amendments increase infiltration rates and improve soil structure, increasing the mean weight diameter of aggregates and their water resistance.     

Effect of freezing and thawing processes on some physical properties of saline–sodic soils mixed with sewage sludge or fly ash

Dispersion of saline–sodic soils was rather difficult to leach. Therefore, negative effects of freeze–thaw on soil physical properties should be reduced by inexpensive and practical methods. This study investigates the effect of freeze–thaw cycles (3, 6, and 9) on wet aggregate stability, bulk density, and permeability coefficient in three soils with different electrical conductivity and exchangeable sodium percentage levels (soil I: 5.30 dS m⁻¹, 47.51%; soil II: 42.80 dS m⁻¹, 55.45%; soil III: 36.30 dS m⁻¹, 59.34%) which consist of different proportions of sewage sludge and fly ash by volume (10%, 20%, and 30%). The experiment was conducted under laboratory conditions using disturbed and non-cropped soil samples mixed with sewage sludge and fly ash. Soils mixed with sewage sludge produced higher aggregate stability and permeability coefficients and lower bulk density values as compared to the soils mixed with fly ash. Sewage sludge added with a rate of 30% eliminated the negative effects of freeze–thaw processes on wet aggregate stability. Freeze–thaw processes did not affect the bulk density of the soils II and III, which were mixed with sewage sludge. However, fly ash addition decreased the bulk density of these soils very significantly after nine freeze–thaw cycles. Addition of sewage sludge or fly ash with rates of 20% and 30% significantly increased the permeability coefficients in soil I after nine freeze–thaw cycles. Results indicated that addition of sewage sludge and/or fly ash to saline–sodic soils could be alternative way for reducing negative effects of freezing–thawing on soil wet aggregate stability, bulk density, and permeability coefficient.     

Behavior of heavy metals from sewage sludge in a Chernozem of the dry belt in Saxony‐Anhalt/Germany

In a small‐plot trial different doses of sewage sludge (equivalent 82‐330 tons of dry matter per hectare) were incorporated in 0—25 cm depth (1982—1985). The aim of the investigations was to study the fate of the heavy metals Zn, Cd, Cu, Ni, Pb, and Cr, to determine their concentration in different soil fractions using a sequential extraction method and to ascertain their uptake by Zea mays L. plants. Eleven years after the last application the metals supplied with the sludge had moved as far as 50 cm in depth. The concentrations of Zn, Cd, Cu, Ni, and Cr in the saturation extract of the sampled soil layers were closely correlated with the concentrations of dissolved organic carbon (DOC). This result suggests that the heavy metal displacement was partly connected with the DOC movement in the soil. Considerable amounts of Zn and Cd coming from sewage sludge were found in the mobile fractions of the soil. Cu, Ni, and Pb were located especially in organic particles, and Cr was obviously bound by Fe‐oxides. Nine years after the last application the binding species of heavy metals were still different compared with those in the untreated soil. The whole withdrawal of heavy metals by plants yielded <1 % of the applied amounts. In the case of Zn the uptake from the sludge amended soil decreased during the experimental period. No similar tendency was observed for the other elements. In any case their annual variations of uptake exceeded the effect of sludge application.     

Localized application of sewage sludge improved plant nitrogen and phosphorus uptake by rhizobox‐grown spring wheat

Sewage sludge (SS) can be used as an alternative fertilizer in agriculture. It is normally broadcasted and plowed into soil, but it is not clear if it has a potential as a placement fertilizer. A rhizobox experiment was conducted to investigate the placement effect of SS and mineral nitrogen (N) fertilizer on shoot and root growth as well as nutrient uptake of spring wheat (Triticum aestivum L.). The treatments included localized SS, mixed SS, localized SS and ammonium, localized ammonium, and a control without addition of SS and ammonium to examine the effect of SS placement and, further, if ammonium co‐localization would enhance the placement effect. The results show that SS fertilization improved soil N and P availability, which significantly increased plant N and P uptake and enhanced shoot growth, while root length was significantly reduced compared to the control. Localized SS increased root proliferation in the placement region, resulting in enhanced uptake of P from the SS patch compared to homogenous application. However, co‐localized application of ammonium with SS significantly depressed plant shoot and root growth. Localized ammonium markedly restricted root proliferation in the placement region and reduced soil pH in both bulk soil and placement region, contributing to decreased nutrient uptake and plant growth.     

Phosphorus availability of sewage sludge‐based fertilizers determined by the diffusive gradients in thin films (DGT) technique

The plant‐availability of phosphorus (P) in fertilizers and soil can strongly influence the yield of agricultural crops. However, there are no methods to efficiently and satisfactorily analyze the plant‐availability of P in sewage sludge‐based P fertilizers except by undertaking time‐consuming and complex pot or field experiments. We employed the diffusive gradients in thin films (DGT) technique to quantify the plant P availability of various types of P fertilizers with a novel focus on sewage sludge‐based P fertilizers. Mixtures of fertilizer and soil were incubated for 3 weeks at 60% water holding capacity. DGT devices were deployed at the beginning of the incubation and again after 1, 2, and 3 weeks. Two weeks of incubation were sufficient for the formation of plant‐available P in the fertilizer/soil mixtures. In a pot experiment, the DGT technique predicted maize (Zea mays L.) biomass yield and P uptake significantly more accurately than standard chemical extraction tests for P fertilizers (e.g ., water, citric acid, and neutral ammonium citrate). Therefore, the DGT technique can be recommended as a reliable and robust method to screen the performance of different types of sewage sludge‐based P fertilizers for maize cultivation minimizing the need for time‐consuming and costly pot or field experiments.     

Qualitative and quantitative analysis of water‐soluble root exudates in relation to plant species and development

The composition of root‐derived substances is of great importance for the understanding of processes in the rhizosphere. Therefore, methods allowing a comprehensive collection and chemical analysis of the organic root exudates are necessary. In this study, we compare different methods with regard to their suitability to collect and characterize root exudates. Because the percolation or water logging method failed to quantitatively extract root exudates, a dipping method was developed which allowed an almost complete sampling of coldwater‐soluble root exudates. By ¹⁴CO₂ labeling of the shoots the composition of root exudates was found to be influenced by plant species and growth stage. In comparison to pea plants maize plants had a higher share of carboxylic acids and a lower share of sugars. Younger maize plants exuded considerably higher amounts of ¹⁴C labeled organic substances per g root dry matter than older ones. During plant development the relative amount of sugars decreased at the expense of carboxylic acids. The described methods are well suited for the elucidation of the influence of growth factors on root exudation.