Efficient Phosphorus Cycling in Food Production: Predicting the

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Efficient phosphorus cycling in food production – predicting the phosphorus fertilization effect of sludge from chemical wastewater treatment Anne Falk Øgaard, and Eva Brod J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b05974 • Publication Date (Web): 31 May 2016 Downloaded from http://pubs.acs.org on June 7, 2016

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Journal of Agricultural and Food Chemistry

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Efficient phosphorus cycling in food production – predicting the phosphorus

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fertilization effect of sludge from chemical wastewater treatment

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Anne Falk Øgaard* and Eva Brod

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NIBIO, Norwegian Institute of Bioeconomy Research, P.O. Box 115, N-1431 Ås, Norway

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* [email protected], (+47) 922 18 433

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1

ACS Paragon Plus Environment


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Abstract

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This study examined the P fertilization effects of 11 sewage sludges obtained from sewage

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treated with Al and/or Fe salts to remove P by a pot experiment with ryegrass (Lolium

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multiflorum) and a nutrient-deficient sand-peat mixture. Also it investigated whether

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fertilization effects could be predicted by chemical sludge characteristics and/or by P

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extraction. The mineral fertilizer equivalent (MFE) value varied significantly, but was low for

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all sludges. MFE was best predicted by a negative correlation with ox-Al and ox-Fe in sludge,

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or by a positive correlation with P extracted with 2% citric acid. Ox-Al had a greater negative

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impact on MFE than ox-Fe, indicating that Fe-salts are preferable as a coagulant when aiming

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to increase the plant availability of P in sludge. The results also indicate that sludge liming

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after chemical wastewater treatment with Al and/or Fe salts increases the P fertilization effect.

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Keywords: Sewage sludge, Aluminium, Iron, Mineral fertilizer equivalent, Sequential

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fractionation, P extraction

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INTRODUCTION

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Although rock phosphate is a limited resource, the current utilization of phosphorus (P) is

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non-circular and inefficient. On average, 98% of P in the human diet is discharged to sewage

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as urine and feces.1 In addition, P in industrial and household detergents ends up in sewage. In

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Norway, an estimated 3100 Mg P yr-1 accumulates in sewage, while 8400 Mg P yr-1 are

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applied as mineral P fertilizer to soil.2 This demonstrates that substituting mineral P fertilizer

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by P in sewage can potentially contribute significantly to closing P cycles.

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Sewage treatment systems vary widely between countries. Phosphorus is regarded as the main

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nutrient causing eutrophication in lakes and therefore wastewater treatment plants (WWTP) in

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Norway discharging to sensitive lakes and coastal zones are obliged to reduce P in sewage by

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at least 90%.3 Precipitation of P in sewage with Al and/or Fe salts is effective for P removal,

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and therefore chemical and combined biological-chemical treatment of sewage is well-

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established as the most dominant treatment method in Norway. Some European countries (e.g.

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Switzerland) have prohibited the direct use of sewage sludge on agricultural soil4, but in

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Norway 60% of sewage sludge is today returned to agricultural soil.2 One of the main reasons

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for the interest among Norwegian farmers in applying sludge to their land is the associated

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increase in soil organic matter, which has positive effects on soil structure.5 Sewage treatment

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processes that preserve the organic material in sludge are therefore desired and preferred over

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incineration of sewage sludge. Regulations require sanitisation of the sludge,6 and the known

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risk of soil contamination by organic pollutants, pathogens and heavy metals associated with

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the use of Norwegian sewage sludge has been determined to be low.7

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The P fertilization effect of sewage sludge depends strongly on the sewage treatment process

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and hence varies widely between different sludges. The P fertilization effect of sludge with

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biological P removal can generally be compared with that of mineral P fertilizer, whereas 3



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chemical precipitation of P in sewage with Al and/or Fe salts results in sludge with a reduced

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P fertilization effect.8-13 O’Connor et al.11 suggest that sludges containing >50 g Al and Fe kg-

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1

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speciation techniques such as solid-state

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XANES (X-ray Absorption Near Edge Structure) spectroscopy, chemical P removal with Al

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and/or Fe salts results in sewage sludge containing Al/Fe-phosphates and P adsorbed to Al/Fe-

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(hydr)oxides.14-17 Furthermore, studies using sequential fractionation of chemically

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precipitated P in sewage sludge have found that NaOH-extractable phosphate, which is

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defined to be Al-/Fe-bound P, is often the largest P fraction.11,15,18 Surplus Al and Fe

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precipitation agents applied with sludge can even result in precipitation of soil P.12,19

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Comparing the effects of Al and Fe salts, Huang and Shenker16 found that, when applied to

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sludge, Fe-sulphate is more effective than Al-sulphate in reducing the fraction of water-

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soluble P in sludge, but it is unknown whether Al and Fe salts have different effects on the

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plant availability of P in sludge. There are indications that sanitization and conditioning of

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sewage sludge by liming increase the availability of P in sludge produced by chemical

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precipitation of P in wastewater with Al and/or Fe salts.12,20,21 However, Kahiluoto et al.22

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found reduced plant P uptake after liming of sludge produced by Fe precipitation. Moreover,

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according to Shober et al.17, liming of Al-/Fe-precipitated sludge causes an increase in Ca-P,

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mainly hydroxyapatite, which is characterized by low solubility in soils. On the other hand,

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Huang and Shenker16, report that no crystallized Ca-P minerals were detected in lime-

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stabilized sludge, while Huang et al.23 observed an increase in both labile P and stable Ca-P

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by chemical sequential fractionation of sludge, which was post-treated with CaO. These

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differing results from different studies show that the effect of the chemical wastewater

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treatment process and post-processing on the plant availability of P is complex. Therefore, a

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better understanding is needed of how different parameters in the chemical wastewater

are characterised by low P availability. According to studies using non-destructive 31

P NMR (Nuclear Magnetic Resonance) and

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treatment process and post-processing can be optimized to improve P availability in sewage

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sludge, and at the same time meet the requirements for P removal from sewage.

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When sewage sludge is returned to agricultural soil, it is utilized as an alternative P fertilizer

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only if the mineral fertilizer dose is reduced accordingly. Knowing the P fertilization effect of

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different sludges relative to mineral fertilizer (mineral fertilizer equivalent, MFE) is therefore

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essential when setting up reliable fertilization plans. Growth experiments are the most reliable

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method to determine MFE, but are too time-consuming and expensive to be used as a standard

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procedure. Since there is great variation in the composition of sewage sludges, both between

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WWTP and within single plants due to variations in the quality of the inlet water, it is

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important to identify simple laboratory methods with a good ability to predict MFE for

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sewage sludges. Parameters from characterisation of the sludge and chemical P extractions are

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two possible options to predict MFE. Sequential P fractionation has been applied to sewage

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sludge in earlier studies.18,15,24 However, no previous study has used sequentially extracted P

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in sewage sludge to explain the associated P fertilization effect. Several studies have

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addressed the use of chemical standard soil or fertilizer extractions as a predictor of the MFE

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of waste products, where P is mainly present as Ca-bound P.25,26 However, it is unknown

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whether the results can be transferred to sewage sludge, where P is often present as Al-/Fe-

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bound P in considerable amounts (see above). Elliot et al.27 tested the ability of neutral

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ammonium citrate extraction of P in sewage sludge to predict MFE, but found insufficient

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relationships. Therefore, there is a need to evaluate laboratory methods in terms of their

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ability to predict the P fertilization effect of sludges.

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The objectives of the present study were thus:

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To examine how the P fertilization effect of sewage sludges is affected by parameters of the chemical wastewater treatment process and post-processing, 5



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To predict the P fertilization effect of sewage sludges from their chemical characteristics and different standard soil and fertilization extraction methods.

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To achieve these objectives, a pot experiment with ryegrass (Lolium multiflorum) was

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conducted over six cuts to compare the P fertilization effect of 11 sewage sludges that varied

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widely in amounts and ratios of Al and/or Fe salts used for chemical P precipitation in

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wastewater. Two sludges were limed during post-treatment. Furthermore, with the aim of

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predicting MFE for sewage sludge, the chemical characteristics and extraction methods

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resulting in the best relationship with MFE were identified by regression.

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MATERIAL AND METHODS

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Sludges

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Treatments for production of the different sludges are described in Table 1 and chemical

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characteristics in Table 2. Dry matter (DM) content was determined after drying at 105°C and

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organic matter (OM) content after incinerating at 550°C. For further analyses, the sludges

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were dried at 40°C and milled. The pH was measured in a sample-water suspension with a

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solid-solution ratio of 1:5 (v/v). Total N concentration was determined by the modified

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Kjeldahl method,28 while NO2- and NO3-N and NH4-N concentrations were determined using

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a Konelab analyser after extraction with 2 M KCl in a solid-solution ratio of 1:5 (w/v). Total

114

P, K, Mg, Ca, Fe, Al and heavy metal concentrations were determined by ICP-OES

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(Inductively Coupled Plasma-Optical Emission Spectrometry) after digestion with 7 M nitric

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acid and using an ultraclave.29 Organic P was determined according to Møberg and Petersen.30

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CaCl2-P was extracted with 0.0025 M CaCl2 in a solid-solution ratio of 1:20 (w/v). Oxalate-

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extractable Al (ox-Al) and Fe (ox-Fe) were determined according to van Reeuwijk.31 Based 6


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on heavy metal concentrations (data not shown) and under Norwegian regulations,6 all

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sludges could be applied to Norwegian agricultural land at a maximum rate of 20 tonnes DM

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ha-1 within a period of 10 years.

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Sequential chemical P extraction

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Fractions of P with different solubility in sludges were analysed using a modification and

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simplification of the sequential fractionation scheme of Hedley et al.32 In brief, 1 g dried,

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milled sample was extracted sequentially in 180 ml 0.5 M NaHCO3 (labile P), 0.1 M NaOH

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(Al/Fe-phosphates or P adsorbed to Al/Fe-(hydr)oxides, hereafter referred to as Al/Fe-bound

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P) and 1 M HCl (stable Ca-P), each for 16 h. After centrifugation at 3000 rpm for 10 min,

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total P concentration in the extracts was determined by ICP-OES. Total P in the residual

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samples (Residual P) was analysed by ICP-OES after digestion of the dried residual sample in

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concentrated HNO3 in an ultraclave. The dried residual samples were weighed and the sample

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loss (1 g minus weight of the residual sample) was equally distributed to the NaHCO3, NaOH

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and HCl fractions when calculating the concentrations of different fractions. Phosphorus

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recovery was calculated as the sum of all fractions.

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Single P extractions

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A total of five chemical extraction methods, including standard soil extractions and standard

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fertilizer extractions, were applied to dried (40°C) and milled samples. See Table 3 for

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method description and abbreviations. All extractions were conducted in duplicate on a

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horizontal shaker except extraction with Fe strips, which was conducted without parallels. All

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extracts except Fe strips were filtered through Whatman blue ribbon filters (589/3), pore size 7



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2 µm. Ortho-P was determined in all extracts by colorimetric analysis using the molybdenum

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blue method according to Murphy and Riley.37 In addition to standard AL extraction (1:20),

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two modifications of AL extraction were applied to study the effect of increasing

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sample:solution ratio (1:50, 1:100) on extractable P. The pH was measured in AL-extracts.

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Pot experiment

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The pot experiment was conducted using 5 L pots filled with 6 kg DM of a soil consisting of a

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limed mixture of nutrient-deficient sand and sphagnum peat (10 vol-%). The soil had been

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used in a previous pot experiment with waste products and was thoroughly mixed before use

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in this study. This model soil was chosen to avoid P fertilization effect of fertilizer treatments

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being masked by soil P. Selected chemical properties of the soil are shown in Table 4. The

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particle size distribution of the soil samples was determined by the pipette method.38 Total C

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was analysed on milled samples using a LECO TruSpec CHN analyser.39 Soil pH, P-AL,

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CaCl2-P, ox-Al and ox-Fe were measured as for the sludges described above and in Table 2.

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Ox-Al and ox-Fe were also analysed, as a measure of amorphous Al/Fe-(hydr)oxides which

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govern the P sorption capacity.

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Sludge doses were calculated based on total P content and 225 mg P pot-1, equivalent to 90 kg

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P ha-1 (assuming 20 cm topsoil depth), were applied. This high rate of P application was used

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to reflect practical application rates in Norwegian agriculture. The fertilization effect of the

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sludges was compared with that of a treatment providing no P fertilization (NoP) and mineral

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control treatments (MinP) that received Ca(H2PO4)2 at a rate of 75, 150 and 225 mg P pot-1,

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equivalent to 30, 60 and 90 kg P ha-1. All other nutrients were applied in amounts regarded as

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sufficient: 500 mg N pot-1 was applied as Ca(NO3)2, 500 mg K pot-1 was applied as K2SO4 8


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and 62.5 mg Mg pot-1 was applied as MgSO4 in addition to Fe, Mn, Cu, Mo, B and Zn. After

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each cut, 250 mg N and K pot-1 were applied as NH4NO3 and K2SO4. All mineral nutrients

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were added in solution. There were three replicates per treatment.

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Italian ryegrass (Lolium multiflorum Lam., cv. Macho) was sown at 0.65 g pot-1 and the pots

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were randomly placed on tables in a greenhouse. Heating was provided if the temperature

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dropped below 18°C during the day (16 h) and 12°C at night (8 h). Artificial light (white light

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from halogen lamps) was used to provide a photoperiod of 16 h day-1. Tap water was applied

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three times a week to about 70% of the water-holding capacity of the soil. The ryegrass was

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cut six times, leaving about 4 cm stubble on each occasion. The first cut was 6 weeks after

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sowing and the next five cuts were at intervals of 4 weeks. Harvested biomass was dried at

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60°C before weighing and milling. Total concentration of P, K, Ca and Mg was determined in

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two of the replicates by ICP-OES after digestion with 7 M ultrapure nitric acid and hydrogen

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peroxide in closed Teflon vials using a microwave oven,40 whereas total N concentration in

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the samples was determined by the Dumas method.41 Uptake of P (mg pot-1) was computed by

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multiplying concentration by aboveground yield, giving duplicate values for P uptake.

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After the second and sixth cuts, five soil cores per pot were extracted from all pots using an

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auger with 9 mm diameter. Cores from the three replicates were pooled into one bulk sample

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per treatment, dried at 40°C, passed through a 2 mm sieve and analysed for pH.

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184

Mineral fertilizer equivalent (MFE)

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To compare the fertilization effect of sludges with that of MinP, MFE was calculated. MFE is

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defined as the relative P fertilization effect of sludges measured as P uptake in aboveground

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biomass compared with the fertilization effect of increasing rates of MinP (Figure 1). MFE 9



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was calculated separately for cut 1, cut 2 and the sum of cuts 3, 4, 5 and 6 (hereafter Σ cuts 3-

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6) to distinguish between readily available P for ryegrass in the establishment phase and P

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available in the longer term, according to:

191 X1

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= 100× P applied

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1 =

(1)

1

(2)

194 195

Where for cut 1:

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P applied = amount of P applied with sludges (225 mg P pot-1);

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X1 = amount of MinP (mg P pot-1) to which P uptake in aboveground biomass after sludge

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application at cut 1 is equivalent;

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Y1 = P uptake (mg pot-1) at cut 1 obtained after application of sludge;

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and a and b = slope and intercept obtained from linear regression with Y = P uptake (mg pot-1)

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at cut 1 obtained after application of increasing rates of MinP and X = P application rate of

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MinP (0, 75, 150 and 225 mg P pot–1).

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For the parameters a and b of the linear response of ryegrass to MinP application in the soil

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used here and an example of the calculation of MFE, see Figure 1a.

205 206

When calculating MFE for cut 2 [and for Σ cuts 3-6 in brackets], P taken up in previous cuts

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was adjusted for by subtracting this amount from the amount of P applied according to:

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P applied = amount of P applied with sludge (225 mg P pot-1) minus P uptake at cut 1 [or

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minus P uptake at Σ cuts 1 and 2];

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Y1 = P uptake (mg pot-1) at cut 2 [or Σ cuts 3-6] obtained after application of sludge;

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and a and b are slope and intercept obtained from linear regression with Y = P uptake (mg pot-

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1

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application rate of MinP (0, 75, 150 and 225 mg P pot–1) minus P uptake at cut 1 [or minus P

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uptake at Σ cuts 1 and 2] (see Figure 1b and 1c).

) at cut 2 [or Σ cuts 3-6] obtained after application of increasing rates of MinP and X = P

215 216

217

Statistical analyses

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Analysis of variance (ANOVA) was performed to study the effect of sludges on MFE (n =

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22). To perform multiple comparisons, the Tukey’s honestly significant difference (HSD)

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multiple comparison test was used (α = 0.05). To study the ability of sludge characteristics,

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sequential extraction and other extraction methods to predict availability of P in sludges,

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simple or multiple linear regressions were applied with Y = MFE after cut 1, cut 2 or Σ cuts 3-

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6 and X = extractable P as % of total P (mean of duplicates) or selected sludge characteristics.

224

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RESULTS

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Characterisation of sludges

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All the WWTP that delivered sludges to this study use chemical treatment for precipitation of

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P and organic matter as the main removal method. Poly-Al-chloride, Al-sulphate, Fe-chloride

229

or Fe-sulphate are used as coagulants either alone or in combination (Table 1). All WWTP

230

except NRA use anaerobic digestion of the sludge for sanitisation, stabilisation and reduction

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of the sludge volume. NRA adds quicklime to the sludge for sanitisation and stabilisation and

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VEAS adds slaked lime for conditioning. There were large differences in P concentration

233

between the sludges (7-37 g P kg-1 DM) (Table 2), because of differing composition of inlet 11



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water, removal methods and sludge treatments. The majority of the P in the sludges was

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inorganic P (only 0-27% organic P). The concentration of ‘water-soluble P’ (CaCl2-P) was

236

low, ranging between 0 and 21.2 mg P kg-1 DM. Different concentrations of P and organic

237

matter in the inlet water resulted in different amounts of coagulant addition to meet the

238

removal requirements. Therefore, there were also large differences in the Al-, Fe- and Ca

239

concentrations (10-110 g Al kg-1 DM, 5-220 g Fe kg-1 DM and 10-250 g Ca kg-1 DM). On

240

average, about 40% of total Al or Fe was oxalate-extractable. According to the sequential

241

chemical P extraction, the largest fraction of labile P was present in the limed sludges (NRA

242

and VEAS), which had 38 and 17 % of total P, respectively, in the labile fraction (Figure 2)

243

(for absolute values, see Supplementary material, Table S1). All other sludges had a low

244

proportion of labile P (≤5%). The limed sludges also differed from the other sludges in terms

245

of other fractions, e.g. they contained a small fraction of Al/Fe-bound P and a large fraction of

246

Ca-P, whereas the other sludges mainly contained Al/Fe-bound P. Residual P accounted for 6-

247

14% of recovered P, and P recovery varied between 87 and 142% of total P in the sludges.

248

Deviations from 100% are due to uncertainty arising from the many steps in the sequential

249

extractions.

250

251

Extractable P in sludges

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There was great variation in the amount of P extracted by the seven extraction methods tested

253

(Table 5). Only 0.3-5.2% of total P was extracted by the Olsen P method, whereas 57-100%

254

of total P was extracted by AC. Furthermore, the ranking of the different sludges varied for

255

the different extraction methods, e.g. Fe strips extracted the smallest fraction of total P from

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NRA sludge, whereas use of Olsen P or AL (1:20) extracted the largest fraction from this

257

sludge. The limed sludges (NRA and VEAS) differed clearly from the other sludges in terms 12


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of per cent of total P extracted by Olsen P, with 8-10 times more P extracted from the limed

259

sludges than on average for the non-limed sludges. Moreover, AL (1:50), AL (1:100) and CA

260

extracted significantly more P from the limed sludges than the non-limed sludges in all cases

261

except that CA extracted similar proportions of P from IVAR and from NRA.

262

Increasing the sample:solution ratio from 1:20 to 1:100 for AL extractions considerably

263

increased the amount of extracted P (Table 5). This effect was most pronounced for VEAS

264

sludge, where 3.2% of total P was extracted at the ratio 1:20 and 64.3% at the ratio 1:100. The

265

AL solution was buffered to pH 3.75, but at the sample:solution ratio of 1:20 the buffer

266

capacity was exceeded and the pH in the extracts increased to 4.0-5.8 depending on sludge

267

type (see Supplementary material, Table S2). The highest pH increase was observed in the

268

extracts of the limed sludges. Increasing the sample:solution ratio decreased the pH in the AL

269

extract, and thereby increased the extraction capacity. With a sample:solution ratio of 1:50 the

270

pH varied from 4.0 to 4.5, while with a sample:solution ratio of 1:100 it varied from 3.9 to

271

4.4.

272

273

Effect of sludge on soil pH

274

At cut 2, limed sludges had increased soil pH by approximately one unit, to pH 8.2-8.3,

275

whereas in the other treatments the pH ranged from 6.8 to 7.5 (Table 6). By cut 6, soil pH had

276

decreased to pH 6.6-7.5 for the limed sludges and to pH 5.6-6.1 for the non-limed sludges.

277

This can be attributed to fertilization with NH4NO3 and associated H+ release through plant

278

uptake and nitrification of NH4.

279

280

P fertilization effect of sludges 13



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There was a clear response of ryegrass to P application on the soil used in the pot experiment,

282

giving a linear increase in P uptake in aboveground biomass as a function of increasing

283

mineral P fertilizer application rate (Figure 1).

284

At cut 1, all sludge treatments showed significantly lower DM yields than the MinP treatment

285

(for DM production, see Supplementary material, Table S3). MFE varied significantly

286

between sludges at all cuts, but was low for all sludges (Table 7). It was lowest at cut 1, where

287

it ranged from 2 to 24 %. NRA tended to result in the highest MFE, but the level was not

288

significantly different from that calculated for VEAS and IVAR. At cut 2, MFE increased

289

(range 9-35%) and VEAS tended to result in the highest MFE, even though it was only

290

significantly higher than for Gjøvik and Sandefjord. For Σ cuts 3-6, MFE increased further, to

291

19-52%, again with VEAS tending to result in the highest MFE.

292

The observed fertilization effect of sludges was ascribed to P fertilization effect of treatments

293

rather than N fertilization effects, because in all sludge treatments the N:P ratio in plant

294

biomass (g kg-1) was ≥9 (results not shown).42 Growth limitation by other nutrients was also

295

excluded based on concentrations in aboveground biomass (results not shown).43

296

297

Prediction of P fertilizer value (MFE) by sludge characteristics

298

Sludge characteristics significantly influenced MFE, with ox-Al and ox-Fe in sludge being the

299

best predictors of P fertilization effect (Table 8). Multiple regression analysis revealed a

300

significant negative effect of both ox-Al and ox-Fe (mol kg-1) on MFE at cut 1, with the

301

following regression equation:

302

MFE cut 1 = 24.2 – 10.4 ox-Al – 6.7 ox-Fe (R2 = 0.79***)

(3)

14


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Thus as the coefficient of regression shows, 79% of the variation in MFE for cut 1 could be

304

explained by the concentrations of ox-Al and ox-Fe. The slope coefficient indicates a greater

305

negative impact on MFE by ox-Al concentration than by ox-Fe concentration. A similar

306

regression analysis with total Al and Fe concentrations instead of ox-Al and ox-Fe resulted in

307

a slightly lower coefficient of determination:

308

MFE cut 1 = 25.7 – 4.6 tot Al – 3.0 tot Fe (R2 = 0.73***)

(4)

309

For cut 2, only 38% of the variation in MFE could be explained by ox-Al and ox-Fe, and only

310

the effect of ox-Al was significant:

311

MFE cut 2 = 31.6 – 8.6 ox-Al – 4.0 ox-Fe (R2 = 0.38*)

(5)

312

For MFE for Σ cuts 3-6, the prediction ability of ox-Al and ox-Fe was further decreased (R2 =

313

0.19, p=0.13).

314

The ratio P/(Ox-Al + Ox-Fe) also had a significant positive influence on MFE at cut 1 and Σ

315

cuts 3-6 (Table 8). At cut 2, this relationship was not significant.

316

When predicting MFE by the P fractions from the sequential P fractionation, the limed

317

sludges (NRA and VEAS) were excluded from the regression analyses because they would

318

have had a great influence on the regression lines, coefficients of determination and p-values.

319

They behaved as outliers in the regression models, since their P fractionation varied strongly

320

from that of the other nine sludges, as described above. There was no relationship between

321

any of the P fractions and MFE for cut 1, whereas for cut 2 and Σ cuts 3-6, MFE was

322

significantly negatively related to NaOH-P and significantly positively related to HCl-P

323

(Table 8). In other words, MFE decreased with increasing fraction of Fe-/Al-bound P and

324

increased with increasing fraction of Ca-P when only non-limed sludges were considered.

15



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325

326

Prediction of the P fertilizer value (MFE) of sludges by single P extractions

327

In prediction of MFE by single P extractions, the limed sludges (NRA and VEAS) were again

328

excluded from the regressions if they behaved as outliers in the regression models (Table 8: n

329

= 9 when NRA and VEAS are excluded; n = 11 when all sludges are included). At cut 1, CA

330

resulted in the best fit and explained 69% of the variation in MFE (n =11; Table 8). With

331

NRA and VEAS excluded, there were also significant positive relationships between MFE

332

and Fe strips, AL (1:20) and AL (1:50) at cut 1, explaining 26-53% of the variation in MFE.

333

For cut 2 (n=11), there were significant positive relationships between MFE and CA, Fe strips

334

and NAC, explaining 19-42% of the variation in MFE. For Σ cuts 3-6, n=11, there were

335

significant positive relationships between MFE and CA and Fe strips, where Fe strips resulted

336

in the best fit and explained 41% of the variation in MFE. Increasing the sample:solution ratio

337

of AL from the standard 1:20 to 1:50 improved the prediction ability of AL for cut 1 (n=9),

338

whereas further increasing the sample:solution ratio decreased the prediction ability.

339

340

DISCUSSION

341

P fertilization effect of sludges

342

Our study confirmed that the plant availability of P in sludges produced from wastewater

343

using Al- and/or Fe-salts as coagulants is often low compared with that in mineral fertilizer.

344

Low plant availability of P in sewage sludges has also been found in other studies.8,10-13

345

Delin44 found some higher MFE for both Fe- and Al-precipitated sludge compared with the

346

present study; 37 and 33% for first cut of ryegrass, respectively. However, these sludges had

347

lower Fe- and Al-concentrations than the sludges in our study. In contrast, Kahiluoto et al.22 16


Page 17 of 43


348

claimed that P in sewage sludge is more recyclable than mineral P fertilizer, but this claim

349

was valid only for sludges with moderate use of Fe-salts as coagulant and low molar Fe/P

350

ratio (1.6). The sludges in the present study had considerably higher (Fe + Al)/P ratio, ranging

351

from 2.8 to 13.0. In the present study, MFE was lowest in the first growth period, which

352

included establishment of the plants (Table 7). Low plant availability of P in sludges was

353

reflected by very low concentrations of easily releasable P, measured as P extractable in

354

0.0025 M CaCl2. Lowest MFE in the first growth period was also found by Brod et al.26 for

355

waste materials with low concentration of water extractable P. When roots are still small,

356

plants are dependent on a high P concentration in the soil solution, giving higher P transport

357

to the root surface by diffusion and mass flow. In later growth periods, the whole soil volume

358

is penetrated by roots and a larger part of sludge P is in the vicinity of the roots, where

359

rhizosphere processes help to acquire P. Therefore MFE increased for later cuts in this study.

360

361

Effects of Al and Fe precipitation salts on MFE

362

Our results indicate that P in sludge is more plant available when produced by precipitation

363

mainly with Fe-salts as coagulants than mainly with Al-salts as coagulants. This can be seen

364

from the multiple regression with ox-Al and ox-Fe as predictors, where the slope coefficients

365

showed a greater negative impact on MFE by ox-Al than by ox-Fe. Furthermore, FREVAR

366

sludge, which was produced by precipitation of P with Fe-salts only, did not result in lower

367

MFE than the other sludges, even though it had by far the highest (Fe+Al)/P ratio of all

368

sludges (Table 2). There were also indications that P in IVAR sludge (precipitation of P with

369

Fe-salts only) had higher plant availability than P in Bekkelaget sludge (precipitation of P

370

with both Al- and Fe-salts), even though both had approximately the same (Fe+Al)/P ratio.

371

More P was extracted from IVAR by the extractions, with a significant relationship to MFE 17



Page 18 of 43

372

for cut 1 (CA, AL (1:20 and 1:50), Fe strips), than from Bekkelaget sludge. Higher MFE for

373

Fe-precipitated sludge compared to Al-precipitated sludge was also found by Delin44. A

374

greater negative impact on MFE by Al than by Fe can be explained by a lower solubility of

375

Al-phosphate compared to Fe-phosphate.45 The solublity of these salts is also reflected in the

376

stability of phosphate adsorbed to the respective metals.46 Recently, Achat et al.47 showed,

377

based on a global compilation of the literature, that posphate concentration in soil solution

378

(Cp) was generally stronger negatively related to ox-Al than to ox-Fe.

379

380

Effect of sludge liming on sludge P characteristics and MFE

381

Previous studies have indicated that liming can increase the P fertilization effect of

382

sludges.12,20,21 Our results partly confirm this. The chemical P fractionation showed that the

383

majority of P in the non-limed sludges was Al-/Fe-bound P (Figure 2). Similarly, Shober et

384

al.17 found by XANES spectroscopy that P in sludge precipitated with Al- or FeCl3 was

385

mainly bound to Al/Fe-hydroxides and Frossard et al.48 found that 60-67% of P was bound in

386

vivianite (Fe-phosphate) in two FeSO4-flocculated and anaerobically digested sewage

387

sludges. Liming of sludges in the present study led to a shift from Al-/Fe-bound P (NaOH-

388

extractable P) to the labile P fraction (NaHCO3-extractable P) and Ca-P (HCl-extractable P)

389

(Figure 2). The solubility of Al/Fe-bound P increases with increasing soil pH.45 Therefore, a

390

pH increase by liming promotes P release from Al/Fe-bound P in sludge, and the increased Ca

391

concentration together with increased pH results in precipitation of Ca-phosphates.45 Shober

392

et al.17 found that lime stabilisation of sludge precipitated with Fe-salts increased the content

393

of P in hydroxyapatite and reduced the amount of P bound to Fe-hydroxides. In contrast to

394

Shober et al.,17 Huang and Shenker16 did not find crystalline Ca-P minerals after CaO

395

treatment and attributed this to dissolved organic carbon coating the Ca-P compounds and 18


Page 19 of 43


396

thereby inhibiting crystal growth. As in our study, Huang et al.23 found an increase in both

397

labile P (NaHCO3 extractable P) and stable Ca-P (HCl extractable P) at the expense of Al/Fe-

398

bound P (NaOH extractable P) after CaO application to chemically precipitated sludge. The

399

extraction solution for labile P (0.5 M NaHCO3) dissolves non-crystalline Ca-P,33 and thereby

400

explains the higher portion of labile P in the limed sludges.

401

In our study, the higher labile P concentrations in limed sludges were only partly confirmed as

402

an increased P fertilization effect in the pot experiment. The solubility of Ca-P minerals

403

decreases with increasing soil pH.45 In the present study, soil pH was around 7 for non-limed

404

sludges and 8.2-8.3 for limed sludges in the first part of the pot experiment (Table 6), and

405

thereby at a pH level that is more favourable for availability of Al/Fe-bound P than for Ca-P.

406

The limed sludges were still among the sludges with the highest MFE values in the beginning

407

of the pot experiment. This may be attributable to acidifying processes in the rhizosphere,

408

which can result in release of Ca-P.49 The difference in MFE between limed and non-limed

409

sludges can thus be expected to be even larger when soil pH is lower than in the present pot

410

experiment.

411

412

Prediction of P fertilizer value of sludges

413

Knowing the P fertilization effect of different sludges is essential when setting up reliable

414

fertilization plans. Therefore, it is important to identify simple parameters with close

415

relationships to the P fertilizer value of sludges. Here, we discuss the ability of chemical

416

characteristics and different standard soil and fertilization extraction methods to predict

417

fertilizer values of sludges.

418 19



Page 20 of 43

419

Prediction of P fertilizer value by sludge characteristics

420

Reflecting the importance of Fe and Al concentrations in sludges for P availability as

421

explained above, MFE at cut 1 was best predicted by multiple regression with ox-Al and ox-

422

Fe as predictors. Ox-Al and ox-Fe are ascribed to amorphous Fe- and Al-(hydr)oxides, which

423

are known to be the part of total Fe and Al most responsible for P sorption. However, there

424

were close relationships between ox-Fe and total Fe (R2 = 0.99) and between ox-Al and total

425

Al (R2 = 0.98) in these sludges. Therefore, prediction from regression analysis using total

426

concentrations of Al and Fe did not differ much from the prediction when using ox-Al and ox-

427

Fe. A quite close negative relationship between plant availability of P in sewage sludges and

428

total concentrations of Fe and Al was also reported by Elliot et al.27 García-Albacete et al.50

429

found that the short-term solubility of P in a wide range of biowastes of different origins and

430

treatments, including animal manure, sewage sludge and municipal solid waste, was better

431

predicted from the negative relationship with total molar concentration of [Ca + Fe + Al] than

432

from total P. Miller and O’Connor51 reported that the P saturation index (ox-P/(ox-Al+ox-Fe)

433

in sewage sludges was well correlated with the plant availability of P. Even though different

434

parameters were used in the present study and in the studies cited, all the results indicate that

435

the concentration of precipitation salts in sludge can be used for predicting the P fertilization

436

effect of sludges. In Norway, the current regulations require sludge to be analysed for total P

437

and heavy metal concentrations, amongst other parameters. Additional analysis of total Fe and

438

Al concentrations should therefore be less costly than oxalate extraction of Al and Fe and we

439

suggest using total Al and Fe concentrations for predicting MFE with equation (4) presented

440

above.

441

442

Prediction of P fertilizer value by extractable P 20


Page 21 of 43


443

MFE could also be predicted by chemical P extraction and CA was the extraction method that

444

showed the closest positive relationship with MFE at cut 1, when all sludges were included.

445

This can be explained by CA-extractable P showing a close negative relationship with ox-Al

446

(R2 =0.69***), which again had a negative relationship with MFE, as explained above. CA-

447

extractable P showed a diminishing relationship with MFE after cut 1 (Table 8). This

448

coincided with a decreased influence of ox-Al on MFE after cut 1 and might be a

449

consequence of decreasing soil pH during the pot experiment, which in turn resulted in higher

450

availability of Ca-P. In contrast, Brod et al.26 found a strong relationship with CA extraction

451

only for late-season fertilization effects in a study with nine different P-rich waste products,

452

but in these waste products P was mainly present as Ca-P. Kratz et al.25 found strong

453

correlations for both short- and long-term P uptake and CA extraction of P in different waste

454

products such as meat and bone meal, sewage sludge and sewage sludge ash. However, in

455

both these investigations the correlations were better for NAC-extractable P. In the present

456

experiment, NAC-extractable P showed a poor relationship with plant P uptake from sewage

457

sludges. This is in line with Elliot et al.,27 who found that P extracted with NAC was not

458

correlated with plant availability of P in sewage sludge. They attributed this to NAC

459

dissolving Al/Fe (hydr)oxides, which are the components that adsorb P and reduce plant

460

availability of P in Al/Fe-precipitated sewage sludges. Extraction with NAC seems therefore

461

to be most suitable method for characterising the P solubility in acid soils of fertilizer

462

products containing high amounts of stable Ca-P.26,52 Consequently, the plant availability of P

463

in fertilizers with high amount of Al/Fe bound P must be evaluated by other extraction

464

methods than for fertilizers with dominating Ca-bound P. Furthermore, the optimum P

465

extraction method for MFE prediction will probably depend on the pH of the target soil

466

(calcareous or acid soil).26

21



Page 22 of 43

467

Phosphorus extraction with Fe strips has been mentioned as a method that works across a

468

broad range of soil types.53 The concept for this method is that Fe strips mimic the effect of

469

plant P uptake on P release from soil and fertilizers by acting as a sink for P released from the

470

substrate. The P concentration in the extraction solution is thereby kept low and P is released

471

by equilibrium reactions. However, for the sludges in the present study there was a poor

472

relationship between P extracted with this method and MFE for cut 1 when the limed sludges

473

(NRA and VEAS) were included. NRA sludge, which tended to have the highest MFE for cut

474

1, released the smallest amount of P by extraction with Fe strips. This was probably caused by

475

very high pH (>11) in NRA sludge, which in turn could be expected to increase pH in the

476

non-buffered extraction solution and thereby result in low solubility of Ca-P. This is in

477

accordance with Brod et al.,26 who found nearly no P in wood ash (pH 13) extracted with Fe

478

strips. VEAS sludge had a lower pH (8.2) and relatively more P was therefore released by Fe

479

strips. When limed sludges were excluded, extraction with Fe strips was one of the extraction

480

methods with the best model fit for cut 1 and cut 2. Similarly, Brod et al.26 found that Fe strips

481

were the best extraction method to predict early-season P fertilization effect.

482

483

Concluding remarks

484

The larger negative impact observed on mineral fertilizer equivalents by ox-Al concentration

485

than by ox-Fe concentration in sludge indicates that when aiming at increasing plant

486

availability of P in sludge, Fe-salts should be used as the coagulant instead of Al-salts. It also

487

appears to be advantageous to lime this type of sludge, to increase the P fertilization effect.

488

The P fertilization effect of sludge was found to be best predicted by the concentration of ox-

489

Al and ox-Fe in sludge or by P extracted by 2% citric acid (CA). Neutral ammonium citrate

490

(NAC), which has been recommended for predicting P fertilization effect in waste products 22


Page 23 of 43


491

with mainly Ca-bound P, failed to predict the P fertilization effect of sludges produced with

492

Al- and/or Fe salts as coagulants.

493

494

Acknowledgements

495

This work was part of the research projects ‘Biosolids in Food Production – Phosphorus

496

Recycling and Food Safety’, funded by the Research Council of Norway (RCN) (Grant No.

497

207811) and industry partners and ‘CenBio’ (Bioenergy Innovation Centre) (RCN, Grant No.

498

193817).

499 500

The authors declare that they have no conflict of interest.

501 502

23



Page 24 of 43

503

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504 505

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506 507 508 509

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8. Frossard. E.; Sinaj, S.; Zhang, L.-M.; Morel, J.L. The fate of sludge phosphorus in soilplant systems. Soil Sci. Soc. Am. J. 1996, 60, 1248–1253.

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14. Hinedi, Z.R.; Chang, A.C.; Yewinowski, J.P. Phosphorus-31 Magic Angle Spinning Nuclear Magnetic Resonance of wastewater sludges and sludge-amended soil. Soil Sci. Soc. Am. J. 1989, 53, 1053–1056.

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15. Frossard, E.; Tekeley, P.; Grima, J.Y. Characterization of phosphate species in urban sewage sludges by high-resolution solid-state 31P NMR. Eur. J. Soil Sci. 1994, 45, 403– 408.

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16. Huang, X.-L.; Shenker, M. Water-soluble and solid-state speciation of phosphorus in stabilized sewage sludge. J. Environ. Qual. 2004, 33, 1895–1903.

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17. Shober, A.L.; Hesterberg, D.L.; Sims, J.T.; Gardner, S. Characterization of phosphorus species in biosolids and manures using XANES spectroscopy. J. Environ.Qual. 2006, 35, 1983–1993.

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19. Soon, Y.K.; Bates, T.E. Extractability and solubility of phosphate in soils amended with chemically treated sewage sludges. Soil Sci. 1982, 134, 89–96.

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20. Montgomery, M.B.; Ohno, T.; Griffin, T.S.; Honeycutt, C.W.; Fernanadez, I.J. Phosphorus mineralization and availability in soil amended with biosolids and animal manures. Biol. Agric. Hort. 2005, 22, 321-334.

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21. Bøen, A.; Haraldsen, T.K. Meat and bone meal and biosolids as slow-release phosphorus fertilizers. Agr. Food Sci.2013, 22, 235-246.

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25. Kratz, S.; Haneklaus, S.; Schnug, E. Chemical solubility and agricultural performance of P containing recycling fertilizers. Landbauforsch. vTI Agric. For. Res. 2010, 60, 227–240. 25



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26. Brod, E.; Øgaard, A.F.; Haraldsen, T.K.; Krogstad, T. Norwegian waste products as alternative phosphorus fertilizer: Part II: Predicting P fertilization effects by chemical extraction. Nutr. Cycl. Agroecosyst. 2015, 103,187–199.

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27. Elliot, H.A.; Potter, J.M.; Kang, J.H.; Brandt, R.C. Neutral ammonium citrate extraction of biosolids phosphorus. Commun.Soil Sci. Plant Anal. 2005, 36, 2447–2459.

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28. NS-EN 13654-1. Soil improvers and growing media - Determination of nitrogen - Part 1: Modified Kjeldahl method. 2001. Standards Norway.

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30. Møberg, J.P.; Petersen, L. Instructions for exercises in geology and soil science II. Den Kongelige Veterinær- og Landbohøyskole, Copenhagen, Denmark, 1982. (In Danish).

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31. Van Reeuwijk, L.P. Procedures for soil analysis. 12-2. Acid oxalate extractable Fe, Al, Si. International Soil Reference and Information Centre, Wageningen, The Netherlands. 1995. ISBN 90-6672-052-2.

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32. Hedley, M.J.; Stewart, J.W.B.; Chauhan, B.S. Changes in inorganic and organic soil phosphorus fractions induced by cultivation practices and by laboratory incubations. Soil Sci. Soc. Am. J. 1982, 46, 970–976.

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38. Elonen, P. Particle-size analysis of soil. Acta Agral. Fenn. 1971,122, 1-122. 26


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42. Liebisch, F.; Bünemann, E.K.; Huguenin-Elie, O.; Jeangros, B.; Frossard, E.; Oberson, A. Plant phosphorus nutrition indicators evaluated in agricultural grasslands managed at different intensities. Eur. J. Agr. 2013, 44, 67–77.

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43. Bergmann, W. Ernährungsstörungen bei Kulturpflanzen: Entstehung, visuelle und analytische Diagnose. Gustav Fischer: Jena, 1993.

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44. Delin, S. Fertilizer value of phosphorus in different residues. Soil Use Manage. 2015, 32, 17-26.

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45. Lindsay, W.L. Chemical equilibria in soils. New York: John Wiley & Sons. 1979.

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46. McBride, M. Environmental chemistry of soils. New York: Oxford University press. 1994.

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47. Achat, D.L.; Pousse, N.; Nicolas, M.; Brédoire, F.; Augusto, L. Soil properties controlling inorganic phosphorus availability: general results from a national forest network and a global compilation of the literature. Biogeochemistry 2016, 127, 255-272.

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48. Frossard, E.; Bauer, J.P.; Lothe, F. Evidence of vivianite in FeSO4-flocculated sludges. Wat. Res. 1997, 31, 2449–2454.

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49. Hinsinger, P. Bioavailability of soil inorganic P in the rhizosphere as affected by rootinduced chemical changes: a review. Plant and Soil 2001, 237, 173–195.

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50. García-Albacete, M.; Martín, A.; Cartagena, C. Fractionation of phosphorus biowastes: Characterisation and environmental risk. Waste Management 2012, 32, 1061–1068.

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51. Miller, M.; O´Connor, G.A. The longer-term phytoavailability of biosolids-phosphorus. Agronomy Journal 2009, 101, 889–896.

630 631 632

52. Schick, J. Untersuchungen zu P-Düngewirkung und Schwermetallgeghalten thermochemisch behandelter Klärschlammaschen. Dissertation Technischen Universität Carolo-Wilhelmina, Braunschweig, Germany. 2009.

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53. Beegle, D. Assessing soil phosphorus for crop production by soil testing. In: Phosphorus: Agriculture and the Environment; T.J. Sims, Sharpley A.N., Eds.; ASA-CSSA-SSSA Publishers, USA, 2005; pp. 123-143. ISBN 9780891181576.

28


Page 29 of 43


636

Figure captions

637

Figure 1. P uptake in aboveground biomass as a function of increasing MinP fertilization rate

638

(0, 75, 150 and 225 mg P pot-1) at: a) Cut 1, b) cut 2 and c) Σ cuts 3-6. X1 and Y1 are examples

639

showing how the values in equation 2 (see Materials and methods) are obtained. Error bars

640

represent standard deviation of the means.

641 642

Figure 2. Distribution of total P in the 11 sludges tested into different P fractions.

643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658

29



Page 30 of 43

Table 1. Description of sludges included in the study. WWTP = wastewater treatment plant. WWTP

Coagulant/s

Sludge treatments

HIAS

Aluminium sulphate

Thermal hydrolysis (approximately 160°C for

20

min),

anaerobic

mesophilic

digestion, dewatering MOVAR

Polyaluminium chloride + Anaerobic iron chloride

Sandefjord

Ullensaker

digestion,

dewatering

Polyaluminium chloride + Pasteurisation, iron sulphate

Larvik

mesophilic

anaerobic

mesophilic

digestion, dewatering

Polyaluminium chloride + Anaerobic

mesophilic

digestion,

iron chloride/sulphate

pasteurisation, dewatering

Polyaluminium chloride

Thermophilic and mesophilic anaerobic digestion, dewatering

NRA


VEAS

Polyaluminium chloride + Anaerobic iron chloride

Bekkelaget Iron

FREVAR

sulphate

Quicklime (Orsa method) mesophilic

digestion,

conditioning with slaked lime, drying +

some Thermophilic

polyaluminium chloride

dewatering

Iron chloride + sea water

Pasteurisation,

anaerobic

digestion,

thermophilic

anaerobic

digestion, dewatering Gjøvik


Anaerobic mesophilic digestion, drying, pelleting

IVAR

Iron chloride

Anaerobic mesophilic digestion, drying, pelleting

30


Page 31 of 43


Table 2. Selected chemical properties of sludges. DM = dry matter, OM = organic matter, Po = organic P, Nmin = mineral N (NO3- and NH4+), P/(Fe+Al) = molar ratio with 1 = HIAS, 2 = MOVAR, 3 = Sandefjord, 4 = Larvik, 5 = Ullensaker, 6 = NRA, 7 = VEAS, 8 = Bekkelaget, 9 = FREVAR, 10 = Gjøvik, 11 = IVAR.

pH -1

DM

g 100g

OM

g 100g-1 DM -1

1

2

3

4

5

6

7

8

9

10

11

8.3

7.7

7.5

7.3

8

>11

8.2

8

7.6

8.1

7.5

21

40

26

27

22

28

52

30

35

91

89

58

57

54

52

57

43

32

50

41

54

50

P

g kg DM

31

27

30

18

37

7

16

31

11

22

24

Po

% of total P

8

13

11

9

6

27

9

9

0

12

0

-1

CaCl2-P

mg kg DM

21.2 6.7

0.9

1.0

2.1

0.0

10.5 9.8

1.6

0.5

2.9

N

g kg-1 DM

52

35

35

26

38

25

19

31

24

33

34

-1

16

8

8

6

16

1

6

9

5

3

3

-1

3

2

2

1

3

2

5

4

6

3

3

Ca

-1

g kg DM

29

22

14

16

19

250

170

24

10

17

21

Al

-1

g kg DM

84

99

91

100

110

33

25

59

18

99

10

Fe

-1

g kg DM

16

34

58

80

8

5

28

78

220

17

140

Ox-Al

-1

g kg DM

33

42

37

40

46

6

10

19

5

43

4

Ox-Fe

-1

8

14

25

34

3

1

11

32

91

6

59

3.4

4.9

4.6

8.8

3.5

5.7

2.8

3.6

13.0 5.6

Nmin K

g kg DM g kg DM

g kg DM

(Fe+Al)/P molar ratio

3.7

31



Page 32 of 43

Table 3. Description of chemical extractions applied to sludges. Code

Fe strips

Method

Extraction with 0.005 M CaCl2 by

Time

Ratio

Reference

h

g:ml

24

1:80

33

0.5

1.5:30

34

1.5

1:20

35

simultaneous adsorption to five 2 cm by 10 cm iron-oxide impregnated filter papers (0.6 M FeCl3). Extraction of P adsorbed to filter papers in 40 ml 0.2 M H2SO4 for 4h and 2x washing of filter papers with 20 ml. Olsen P

Extraction with 0.5 M NaHCO3 adjusted to pH 8.5.

AL

Extraction with 0.1 M ammonium lactate and 0.4 M acetic acid adjusted to pH 3.75.

CA

Extraction with 2% citric acid.

0.5

2:200

36

NAC

Extraction at 65°C with ammonium citrate

1

1:100

36

adjusted to pH 7.

32


Page 33 of 43


Table 4. Texture and chemical characteristics of the soil used in the pot experiment. Sand Silt

Clay

Tot C

%

%

%

%

97

2

1

0.26

pH

Ox-Fe -1

7.2

Ox-Al -1

Total P -1

CaCl2-P

P-AL -1

g kg

g kg

mg kg

mg kg

mg kg-1

0.27

0.11

219

30

0.47

Sand = 0.06-2 mm, silt = 0.002-0.06 mm, clay =

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