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 DOI: 10.1177/0734242X13492005

2013 31: 792 originally published online 17 June 2013Waste Manag ResY Dilsad Yilmazel and Goksel N Demirer

silage via struvite precipitationNitrogen and phosphorus recovery from anaerobic co-digestion residues of poultry manure and maize

  

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Introduction

Anaerobic digestion of organic wastes, including animal manure, has been applied successfully to convert them into profitable by-products, as well as to reduce the pollution of air, water and soil caused by these wastes (Gungor-Demirci and Demirer, 2004; Huang and Shih, 1981). Owing to intrinsic process characteristics, the effluent stream of anaerobic digesters is rich in nutrients and therefore can be used as a liquid fertilizer. However, if applied at a higher rate than plant uptake it may cause several environmental problems, such as eutrophication in nearby water bodies, saliniza-tion in semi-arid areas and build-up of toxic concentrations of heavy metals (Burkholder et al., 2007). Therefore, to avoid adverse environmental impacts due to nutrient leaching and runoff, the nutrients must be removed from anaerobic digester effluents.

Moreover, there is an increasing awareness of limited natural resources and importance is given to sustainable treatment tech-niques, and, for this reason, control over the sources of nitrogen (N) and phosphorus (P) shifted from removal to recovery. This shift can be explained by the dependency of modern agriculture on P derived from phosphate rock. There is no substitute of P in agriculture (U.S. Geological Survey , 2009) and rock phosphate is a non-renewable resource. A recent study estimated peak P

production by 2035 (Cordell, 2009). With the present P utilization rates of 40 million tons of P as phosphorus pentoxide each year, the available resources of P are expected to be exhausted in 100–250 years (U.S. Geological Survey, 2009). However, as a basic building block of plant protein, N is an essential element for agri-culture, and there is a growing demand for the nitrogenous ferti-lizers in the world (Mulder, 2003). Worldwide nitrogenous fertilizer demand is expected to increase from a total of 105 mil-lion tonnes in 2011 to 113 million tonnes in 2015, with an annual

Nitrogen and phosphorus recovery from anaerobic co-digestion residues of poultry manure and maize silage via struvite precipitation

Y Dilsad Yilmazel1* and Goksel N Demirer2

AbstractAnaerobic digestion is commonly used for the stabilization of agricultural and animal wastes. However, owing to the stringent environmental criteria, anaerobic digester effluents need to be further treated to reduce nutrient loads to the receiving water bodies. Struvite precipitation is one of the promising techniques applied for this purpose. Yet, in the majority of cases, struvite precipitation is only applied to the liquid phase of anaerobic digester effluents. This study investigated the recovery of nutrients from both the liquid and the solid phases of the phase-separated effluent of a full-scale biogas plant co-digesting poultry manure and maize silage. Struvite precipitation in the liquid phase led to 72.1% and 95.1% average removal efficiencies of ammonium-nitrogen (NH4-N) and orthophosphate respectively. Changing the external phosphorus source did not make any statistically significant difference in nutrient removal. An acidic phosphorus-dissolution process was applied to the solid phase sample to obtain a phosphorus-enriched solution. More than 90.0% of both NH4-N and PO4-P were recovered from the phosphorus-enriched solution with the amendments of magnesium and phosphorus. In the experiments performed without any addition of external magnesium- and phosphorus-containing chemicals, almost complete (99.6%) PO4-P recovery and partial (14.6%) NH4-N recovery were obtained. The results of this study could contribute to the understanding of nutrient recovery from anaerobic digestion residues of manure and agricultural wastes by struvite precipitation.

KeywordsAnaerobic co-digestion, poultry manure, maize silage, acidic dissolution, nutrient recovery, struvite

1 Civil and Environmental Engineering Department, Villanova University, Villanova, PA, USA

2 Department of Environmental Engineering, Middle East Technical University, Ankara, Turkey

* Y D Yilmazel was a visiting scientist at the Institute for Sanitary Engineering, Water Quality and Solid Waste Management (ISWA), University of Stuttgart, Germany during the time of the study.

Corresponding author:Goksel Demirer, Department of Environmental Engineering, Middle East Technical University, Universiteler Mah, Dumlupinar Bulv., Ankara, 06800, Turkey. Email: goksel@metu.edu.tr

492005WMR31810.1177/0734242X13492005Waste Management & ResearchYilmazel and Demirer2013

Original Article

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growth rate of 1.7% (Food and Agriculture Organization of the United Nations, 2011).

Forced precipitation of magnesium ammonium phosphate (MgNH4PO4.6H2O, struvite or MAP) is one of the possible tech-niques used for simultaneous removal and recovery of N and P from wastewaters (Doyle and Parsons, 2002). This approach is based on resource recovery because struvite is a slow releasing fertilizer, and when it is used as a fertilizer P rock mining can be reduced. Several studies have reported that the use of experimen-tally-derived struvite precipitant is possible, as it meets the regu-latory limits for fertilizer use (Munch and Barr, 2001; Uysal et al., 2010 ). To assess the possible use of obtained struvite as a fertilizer, an evaluation of its heavy metal content is required because the presence of heavy metals in fertilizers is strictly reg-ulated. For example, German Fertilizer Ordinance sets limits for heavy metals such as arsenic, lead (Pb), cadmium (Cd), chro-mium (Cr), nickel (Ni), mercury (Hg), thallium (Tl) and zinc (Zn) and Turkish Regulations set limits for Ni, Zn, Cr and Hg (DüMV, 2008; MoARA, 2004). Owing to its resource-saving nature, struvite precipitation is preferred over conventional nutri-ent removal methods. The reaction of struvite formation is as follows:

Mg2+ + NH4+ + PO4

3– + 6H2O → MgNH4PO4.6H2O

Struvite can form and precipitate when ammonium (NH4+), phos-

phate (PO43-) and magnesium(Mg2+) ion activities exceed the ion

activity product at equilibrium or the thermodynamics solubility product (Doyle and Parsons, 2002). The success of struvite pre-cipitation depends on two major factors: the Mg:N:P molar ratio and the pH of the solution (Munch and Barr, 2011). In the case of digested poultry manure, as shown in a recent work of the authors (Yilmazel and Demirer, 2011), and also later in this study, the molar concentration of NH4-N was found to be more than 50 times higher than dissolved PO4-P concentration in the liquid phase. Whereas the molar Mg concentration is the lowest and therefore Mg is the limiting constituent for struvite formation. Hence, addi-tion of the external sources of both Mg and PO4-P is necessary for increasing struvite precipitation and NH4-N recovery from anaero-bic digestion residues of poultry manure. However, struvite pre-cipitation is also influenced by other factors related to the chemical composition of the wastewater, such as ionic strength, organic mat-ter content, the presence of chelating agents and foreign ions, for example calcium (Ca) and potassium (K).

Recovered struvite can be used as a fertilizer because it has similar properties to conventional fertilizers (Ahmed et al., 2006; Taruya et al., 2000) Various authors estimate that the market price of struvite is between €188 and €763 per ton (Molinos-Senante et al., 2011). The positive economical feasibility of stru-vite formation from different waste was demonstrated in many studies (Durrant et al., 1999; Molinos-Senante et al., 2011; Shu et al., 2006). For example, it was indicated that the recovery of P from waste streams has potential to recover more than 90% of dissolved P from digester supernatant as struvite. It was shown

that this recovery is technically feasible and economically bene-ficial. Recovering 1 kg of struvite from a wastewater treatment plant will minimize sludge handling and disposal, and reduce the operating cost by AUD$1.133 (Shu et al., 2006).

There is a growing interest in N and P recycling not just because they are non-renewable resources but also because the recovery of these nutrients from wastewater also leads to signifi-cant improvements in the environment. Therefore, economical feasibility analyses should consider both economical and envi-ronmental benefits obtained. Molinos-Senante et al. (2011) developed a methodology and an empirical application where both type of impacts are considered and quantified. They empha-sized that when the economic feasibility of a P recovery project should not be assessed only on the basis of the market value cre-ated, but also on the basis of the increase in the availability of a non-renewable resource and reduction of environmental impacts..

Typical chemicals used to add Mg and PO4-P ions were reviewed for several studies and listed in Uludag-Demirer et al. (2005). Especially for Mg supplementation, there are several alternatives, such as magnesium oxide (MgO), magnesium hydroxide [Mg(OH)2] and magnesium chloride hexahydrate (MgCl2.6H2O). The effect of Mg source on the removal of NH4-N has been investigated by Celen and Turker (2001). They reported that the use of MgO provided lower NH4-N recovery than that of MgCl2.6H2O. Both MgO and Mg(OH)2 are poorly soluble in water; hence, a fine particle size and vigorous mixing of the reac-tion solution are required when using MgO or Mg(OH)2 for stru-vite precipitation. Therefore, MgCl2.6H2O is preferred as the Mg source. A number of chemicals were also used for P supplementa-tion. Brionne et al. (1994) tested dipotassium hydrogen phos-phate, phosphoric acid (H3PO4) and potassium dihydrogen phosphate, and used H3PO4 because it is a low-cost option. Schulze-Rettmer (1991) proposed H3PO4, and Zdybiewska and Kula (1991) reported better precipitation results when using H3PO4 with magnesium chloride than using it with MgO. Some other researchers used disodium hydrogen phosphate heptahy-drate as the P source (Altınbas et al., 2002; Uludag-Demirer et al., 2005). Using potassium phosphates as the P source will increase the K concentration in the solution, which may interfere with the precipitation of NH4

+ struvite and lead to the formation of potassium struvite instead. Therefore, the use of potassium phosphates is not preferred. However, using H3PO4 will lower the solution pH, which will increase the use of the buffering agent. However, H3PO4 is usually preferred owing to its lower cost compared with the other P salts. The main criteria for selection of the Mg and P sources can be listed as the cost of the chemicals, the presence of counter-ions, such as Ca and K, the solubility of the salts and their effects on solution pH. To minimize fresh rea-gent use and associated chemical costs, internal recycling of Mg and P is possible by thermal or alkali decomposition of the recov-ered struvite (He et al., 2007).

Although H+ is not directly involved in struvite formation reaction (equation (1)), struvite precipitation is highly dependent on pH, which is owing to changes in the activities of both NH4

+

(1)

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794 Waste Management & Research 31(8)

and PO43- ions. The minimum solubility pH for struvite precipita-

tion reported in different studies ranges from 8.0 to 10.7, which may differ owing to changing ionic strength and the composition of wastewaters (Doyle and Parsons, 2002). Struvite precipitation is also influenced by chemical composition of the wastewater (organic matter, presence of chelating agents, ionic strength), the degree of super saturation, temperature and the presence of for-eign ions, such as Ca and K (Doyle and Parsons, 2002; Uludag-Demirer, 2008).

Struvite precipitation has been applied to various types of anaerobically-treated animal wastes, such as calf manure (Schuiling and Andrade, 1999), cattle manure (Zeng and Li, 2006) and poultry manure (Yetilmezsoy and Sapci-Zengin, 2009). However, most of the studies found in the literature focused on the removal/recovery of soluble N and P in the liquid phase effluent of the wastes. Yet the dissolution/release of the bound nutrients from the solid phase is imperative in order to recover the maximum amount of nutrients, particularly P, from wastes. A number of studies focused on the extraction of the bound nutrients from different types of wastes, such as poultry litter (Szogi et al., 2008), pig wastewater (Daumer et al., 2009) and chicken manure incineration ash (Kaikake et al., 2009). To date, there is only one published study on the recovery of nutri-ents from the solid phase of anaerobically-digested poultry manure (Yilmazel and Demirer, 2011) where complete (99.6–100%) and partial (3.3–65.6%) recoveries of dissolved PO4-P and NH4-N were reported respectively. The objective of this study is to apply struvite precipitation to both the liquid and the solid phase effluent of a full-scale biogas plant utilizing poultry manure and maize silage for the dual purpose of N and P recov-ery and production of a slow-release fertilizer, struvite. For this purpose, the liquid and the solid phases, obtained via centrifuga-tion, were treated separately, and acid dissolution was performed prior to struvite precipitation in the solid phase of the sample. The effects of Mg:N:P molar ratio, Mg:P molar ratio and P source on the recovery efficiencies were investigated.

Materials and methodsSample preparation and characteristics

The digester sample was taken from a full-scale biogas plant, co-digesting poultry manure and maize silage at a waste mixing ratio of 0.25:0.75 located in Lower Bavaria, Germany. The biogas plant features an overall digester volume of 2487 m3, divided into two process stages (primary and secondary digesters). The vol-ume and the hydraulic retention time (HRT) of the primary digester are 396 m3 and 13 days respectively. The overall HRT of the plant is 78 days, and it is operated at mesophilic conditions. The sample was collected from the effluent of the primary anaer-obic digester, preserved by freezing during transportation and kept frozen until phase separation. Preserving the sample by freezing did not change any of its chemical properties. Phase separation was achieved by centrifugation (RC6; Sorvall Instruments Dupont, Osterode, Germany) for 15 mins at 17,710

× g and sieving through a screen of 0.56 mm mesh size. The liq-uid phase was directly used in precipitation experiments, whereas the solid phase was subjected to an acidic phosphorus dissolution process before the precipitation experiments. Both the liquid and solid phase samples were kept refrigerated at 4oC until used (Table 1).

Experimental set-up and procedures

Precipitation experiments. Four consecutive steps were fol-lowed in the precipitation experiments: addition of chemicals, mixing, settling and filtration. The experiments were conducted in continuously-stirred batch reactors at room temperature (21–22oC). Each reactor contained 150 ml of sample and was con-tinuously mixed with a magnetic stirrer (MR Hei-Mix L; Heidolph, Schwabach, Germany). During the experiments mix-ing speed was kept constant at 250 rpm. In all experimental runs there was no external addition of NH4-N to the reactors. The total (initial + added) molar concentration ratios of Mg and PO4-P were raised via external addition . After adding the Mg-containing chemical (MgCl2.6H2O), the PO4-P-containing chemical [H3PO4 or sodium dihydrogen phosphate dehydrate (NaH2PO4.2H2O)] was added to the reactor, where necessary. The required amounts of Mg and PO4-P ions to be added were calculated considering initial concentrations of these ions in the sample and were raised accordingly to obtain the desired total molar concentration ratio of Mg:N:P or Mg:P in the reactor. In the experiments MgCl2.6H2O was used in its solid form as the source of Mg. NaH2PO4.2H2O in its solid form or 75% H3PO4 (v/v) solution was used as the P source. The pH adjustments were made using 20% sodium hydroxide (NaOH) (v/v) solution, and the pH was adjusted to 8.5 in each experiment. As the vol-ume of NaOH added was very small, the dilution effect was neg-ligible. All chemicals used in the experiments were analytical grade. After the pH of the solution became constant at the desired level with a variation of ± 0.01 pH units, 30 mins of mixing was applied, and at the end of the mixing period the reactor content was allowed to settle for 60 mins. At the end of settling period the reactor content was filtered through a folded filter with a pore size range of 4–12 μm (MN615; Macherey-Nagel, Duren, Germany), and the filtrate was analyzed for its PO4-P, NH4-N and metal content. The precipitates were dried at 30 ± 2oC over-night. After drying, the precipitate was separated manually from the filter paper and kept at room temperature (21–22oC) until X-ray diffraction (XRD) analysis.

Acidic phosphorus dissolution process

Phosphorus dissolution was achieved in four steps: dilution, acid addition, mixing and solid–liquid separation. The solid phase was diluted by distilled water until the total solids (TS) concen-tration was 5% (17.5% initially, Table 1), which was necessary for continuous mixing in the reactor. Then, the pH of the diluted sample was adjusted to 2.0 by the addition of 20% hydrochloric acid (HCl) (v/v). The acidic mixture was mixed at 250 rpm using

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the jar test apparatus (MSR12/180; Geppert Rührtechnik, Dreieich, Germany) for 2 h. After the dissolution, phase separa-tion was performed by centrifugation (RC6; Sorvall Instruments) at 17,710 × g for 15 mins and sieving through a screen of 0.56 mm mesh size. The solid phase was discarded and the filtrate (P-enriched liquid phase) was characterized and kept refrigerated at 4oC until used (Table 2).

Liquid phase experiments

As given in equation (1), struvite is formed in a theoretical Mg:N:P molar ratio of 1:1:1. However, NH4-N was in excess in the liquid phase sample and the initial Mg:N:P molar ratio was 1:645.5:4.5 (Table 1). Hence, amendments of both Mg and PO4-P were necessary to fulfill the stoichiometric requirements for stru-vite formation and to lower the high initial concentration of NH4-N (3907 mg l-1, Table 1). Although a Mg:P molar ratio of 1:1 is enough for struvite crystallization, higher molar concentration of Mg can increase N and P removal from wastewaters (Adnan et al., 2004; Rahaman et al., 2008). In order to observe the effect of molar concentration ratio of the struvite-forming ions, three different Mg:N:P ratios (1:1:1, 1.3:1:1 and 1.5:1:1) were tested

(Table 3). For comparison, two different P-containing chemicals (H3PO4 and NaH2PO4.2H2O) were used. The experimental set-up is depicted in Table 3.

Solid (P-enriched liquid) phase experiments

In the phosphorus-enriched liquid phase, the initial molar concen-tration of NH4-N was the highest among the three struvite forming ions, and the initial molar ratio of Mg:N:P was 1:6.3:1.6 (Table 2). The effects of the molar ratio of Mg:N:P and the molar ratio of Mg:P were investigated separately. In the former test both Mg and PO4-P were added, and in the latter test only Mg was added to the reactors. Similar to the liquid phase experiments, three different Mg:N:P ratios (1:1:1, 1.3:1:1 and 1.5:1:1) were tested (Table 3).

To investigate the effect of the molar ratio of Mg:P on the recoveries of PO4-P and NH4-N, three different Mg:P ratios, namely 1:1, 1.3:1 and 1.5:1, were used. For the adjustment of the Mg:P molar ratio, PO4-P concentration in the solution was taken as the basis and only Mg was added externally.

Two control experiments (P7 and P8) were carried out without changing the initial Mg:N:P ratio, i.e. there was no addition of

Table 1. Characteristics of the liquid and solid phase samples.

Parameter Liquid phasea Unit Solid phasea Unit

TS 39 ± 1 g kg-1 175 ± 10 g kg-1

VS 69 ± 1 % of TS 79.6 ± 0.6 % of TSCOD 44,208 ± 658 mg l-1 nd –sCOD 27,366 ± 0 mg l-1 nd –TKN 6173 ± 136 mg l-1 30.1 ± 2.8 mg g-1 dry matterNH4-N 3907 ± 105 mg l-1 nd –PO4-P 60.5 ± 0.7 mg l-1 nd –TP 209 ± 1 mg l-1 16.1 ± 0.2 mg g-1 dry matterAluminium 6.7 ± 0.1 mg l-1 878 ± 101 mg kg-1 dry matterCalcium 441 ± 11 mg l-1 4030 ± 7 mg kg-1 dry matterCadmium < 0.025 mg l-1 < 2.3 mg kg-1 dry matterCobalt 0.2 ± 0.0 mg l-1 3.12 ± 0.07 mg kg-1 dry matterChromium 0.3 ± 0.0 mg l-1 7.76 ± 0.44 mg kg-1 dry matterCopper 3.4 ± 0.0 mg l-1 113 ± 1 mg kg-1 dry matterIron 110 ± 0 mg l-1 4715 ± 2 mg kg-1 dry matterMercuryb 0.008 mg l-1 < 0.078 mg kg-1 dry matterPotassium 5380 ± 96 mg l-1 26,235 ± 32 mg kg-1 dry matterMagnesium (Mg) 10.5 ± 1.8 mg l-1 8018 ± 108 mg kg-1 dry matterNickel 0.3 ± 0.0 mg l-1 5.13 ± 0.74 mg kg-1 dry matterLead 0.3 ± 0.0 mg l-1 < 4.66 mg kg-1 dry matterZinc 15.5 ± 0.1 mg l-1 649 ± 2 mg kg-1 dry matterConductivity 33 mS cm-1 – –pH 7.9 – – –NH4-N 278.9 mM – –PO4-P 2 mM – –Mg 0.4 mM – –Mg:N:P 1:645.5:4.5 – – –

TS: total solids; VS: volatile solids; COD: chemical oxygen demand; sCOD: soluble COD; TKN: total Kjeldahl nitrogen; NH4-N: ammonium nitro-gen; PO4-P: phosphorus phosphate; TP: total phosphorus; nd: not determined.aMean ± SD (n = 2).bHg concentration in the liquid phase depicts the mean concentration of the duplicate analysis.

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external sources of Mg and/or P. To investigate the effect of pH it was adjusted to 8.5 and 9.5, in P7 and P8, respectively, by the addition of NaOH.

Analytical methods

Chemical oxygen demand (COD), soluble COD, TS, volatile sol-ids, total Kjeldahl nitrogen, total phosphorus, NH4-N, PO4-P and

Mg analyses were performed according to the Deutsches Institut für Normung e.V. (DIN) standards (DIN 38402-11, 2006). Metal (Mg, aluminium (Al), Ca, Cd, cobalt, Cr, copper, iron (Fe), Hg, K, Ni, Pb, Zn) analyses, except Mg of the liquid samples, were carried out using optical emission spectrophotometer with induc-tively coupled plasma in the hydrochemistry department of the University of Stuttgart, as described by DIN standards. Mg deter-minations of the liquid samples were carried out by using Cadas 200 Spectrophotometer (Dr. Lange, Dusseldorf, Germany) and LCK 326 reagent sets (Hach Lange, Dusseldorf, Germany).

XRD analysis

The XRD technique was used to determine the compositional structure of the products and to confirm the presence of struvite in the precipitates. The dried precipitate was separated manually from the filter paper and identified by XRD. The XRD analyses of the precipitates collected from experiments L6, P1 and P3 were performed at the Advanced Analysis Laboratory of Istanbul University, Turkey, using the Rigaku D-Max 2000 X-ray diffrac-tometer (Rigaku, Ettlingen, Germany) using Cu Kα radiation. The diffractometer was operated at 40 kV and 20 mA. The data were collected over the two-theta range of 5–70o using a step size of 0.05o, and counts were collected for 1.5 s at each step. The XRD analyses and subsequent identification of the dried precipitates collected from experiments P6 and P8 were performed in the Karlsruhe Research Center, Germany. The XRD analyses of the samples from P6 and P8 were conducted using a Siemens Bruker D5000 X-ray diffractometer (Bruker-AXS, Karlsruhe, Germany) using Cu Kα radiation at the Karlsruhe Research Center. The data were collected over the two-theta range of 13.5–34.5o using a step size of 0.03o, and counts were collected for 10 s at each step.

Table 2. Characteristics of the phosphorus (K)-enriched liquid phase.

Parametera Valuesb

NH4-N (mg l–1) 1484 ± 4PO4-P (mg l–1) 827 ± 3Magnesium (Mg) (mg l–1) 406 ± 1Aluminium (mg l–1) 2.11Calcium (Ca) (mg l–1) 1581Cadmium (mg l–1) < 0.025Copper (mg l–1) 0.284Iron (mg l–1) 122Potassium (mg l–1) 1439Nickel (mg l–1) 0.136Zinc (mg l–1) 15.3NH4-N (mM) 105.9PO4-P (mM) 26.7Mg (mM) 16.7Ca (mM) 39.4Mg:N:P 1:6.3:1.6Ca:Mg 2.36:1

NH4-N: ammonium nitrogen; PO4-P: orthophosphate.aThe Al, Ca, Cd, Cu, Fe, K, Ni, Zn concentrations depict the mean concentration of the duplicate analysis.bMean ± SD.

Table 3. Experimental conditions of struvite precipitation experiments.

Exp. Tested molar ratio

Basis of chemical addition

Adjusted molar ratio

Chemicals added pH

Liquid phase experimentsL1 Mg:N:P NH4-N 1:1:1 H3PO4, MgCl2.6H2O 8.5L2 Mg:N:P NH4-N 1.3:1:1 H3PO4, MgCl2.6H2O 8.5L3 Mg:N:P NH4-N 1.5:1:1 H3PO4, MgCl2.6H2O 8.5L4 Mg:N:P NH4-N 1:1:1 NaH2PO4.2H2O, MgCl2.6H2O 8.5L5 Mg:N:P NH4-N 1.3:1:1 NaH2PO4.2H2O, MgCl2.6H2O 8.5L6 Mg:N:P NH4-N 1.5:1:1 NaH2PO4.2H2O, MgCl2.6H2O 8.5Solid [phosphorus (P)-enriched liquid) phase experimentsP1 Mg:N:P NH4-N 1:1:1 H3PO4, MgCl2.6H2O 8.5P2 Mg:N:P NH4-N 1.3:1:1 H3PO4, MgCl2.6H2O 8.5P3 Mg:N:P NH4-N 1.5:1:1 H3PO4, MgCl2.6H2O 8.5P4 Mg:P PO4-P 1:1 MgCl2.6H2O 8.5P5 Mg:P PO4-P 1.3:1 MgCl2.6H2O 8.5P6 Mg:P PO4-P 1.5:1 MgCl2.6H2O 8.5P7a – – 1:1.6b na 8.5P8a – – 1:1.6b na 9.5

Mg: magnesium; N: nitrogen; NH4-N: ammonium-nitrogen; PO4-P: phosphate-P; H3PO4: phosphoric acid; MgCl2.6H2O: magnesium chloride; NaH2PO4.2H2O: sodium dihydrogen phosphate dihydrate; na: no addition.aP7 and P8 are control experiments.bInitial Mg:P ratio of the P-enriched liquid phase.

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Results and discussion

In the liquid phase experiments, struvite precipitation led to ‘removal and recovery’ of the soluble nutrients from the waste-water. Therefore, the reductions in the concentrations of the ions are referred to as ‘removal’. In the solid phase experiments a pre-treatment step was added to the system in order to make the nutri-ents available for struvite reaction. Therefore, the removal of the nutrients from the P-enriched liquid phase is referred to as ‘recovery’, but not removal.

Struvite precipitation from the liquid phase sample

In order to represent the actual removal of the ions from the wastewater; the removals are reported considering the initial con-centrations of these ions present in the wastewater (Table 1), but not the total (initial + added) concentrations (Figure 1). As depicted in Figure 1, NH4-N removals obtained in these experi-ments ranged between 67.0% and 74.7%, and increasing the molar concentration of Mg did not enhance NH4-N removals.

However, the variation of Mg:N:P from 1:1:1 to 1.3:1:1 led to an increase in the removal of PO4-P, from 90% to 98%, and yet the difference in the removal of PO4-P was minimal by the variation of Mg:N:P from 1.3:1:1 to 1.5:1:1. Other studies have also dem-onstrated that excess Mg has a greater influence on residual PO4-P concentration in comparison to NH4-N (Uludag-Demirer and Othman 2009). Similar results are reported in the literature, indi-cating the observations of higher PO4-P removals from wastewa-ter as the molar concentration ratio of Mg:P is increased (Munch and Barr, 2001). The study of Rahaman et al. (2008) demonstrated that the decrease in the residual PO4-P concentration was minimal when Mg:P ratio was changed from 1.3:1 to 1.6:1, although there was a considerable drop in the residual PO4-P concentration when Mg:P ratio was changed from 1:1 to 1.3:1.

As shown in Table 3, H3PO4 was used as the P source in experiments L1, L2 and L3, whereas NaH2PO4.2H2O was used in experiments L4, L5 and L6; all the other variables were the same for these experiments. A paired t-test was performed to evaluate

the results obtained from the two sets of experiments: the first set is considered as experiments L1, L2 and L3, and the second set is formed by experiments L4, L5 and L6. A paired t-test is used when there is one measurement variable and two nominal varia-bles, where one of the nominal variables has only two values (McDonald, 2009). In this study, residual NH4-N and PO4-P con-centrations are the measurement variables, and the nominal vari-ables are Mg:N:P ratio and P source. Because one of the nominal variables (P source) has only two values (H3PO4 and NaH2PO4.2H2O), a paired t-test is suitable for the evaluation of the results obtained from the liquid phase experiments. Two inde-pendent paired t-tests were performed; the residual NH4-N con-centration was taken as the measurement variable in one test whereas the residual PO4-P concentration was the measurement variable in the other one. The results of the paired t-tests depicted that with 95% confidence, the residual NH4-H and PO4-P concen-trations measured in the first set of experiments (L1, L2 and L3), where H3PO4 was used as the P source, are not different than the residual NH4-H and PO4-P concentrations measured in the sec-ond set of experiments (L4, L5 and L6), where NaH2PO4.2H2O was used as the P source. There is no considerable advantage and/or disadvantage of any P source over each other from nutrient removal point of view. However, the use of different chemicals may affect the composition of the resulting solution, for example salinity (Schulze-Rettmer, 1991).

There are two major removal mechanisms of NH4-N in sys-tems with high concentrations of Mg and PO4-P, namely struvite precipitation and air stripping (Uludag-Demirer and Othman, 2009). The rate of air stripping on the removal of NH4-N removal was not determined in this study. However, the average NH4-N removal via air stripping at a pH level of 8.5 at room temperature (21–22oC) was recorded as 2% in another study, which used the effluent of a bench-scale poultry manure digester (Yilmazel et al., 2011). This suggests that at pH level of 8.5 there is negligi-ble loss of NH4 to the atmosphere. Therefore, high removal effi-ciencies of NH4-N in the experiments (L1–L6) can be considered as an indication of the formation of struvite in the reactors. In order to confirm the presence of struvite XRD analysis was used. The XRD pattern generated from the sample collected from L6

Figure 1. Removal efficiencies of the liquid phase sample experiments by the use of (a) phosphoric acid (H3PO4) and (b) sodium dihydrogen phosphate dihydrate (NaH2PO4.2H2O).

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798 Waste Management & Research 31(8)

matched with the database standard for struvite (i.e. position and intensity of the peaks; Figure 2), identifying the precipitate to be struvite, and no other minerals were detected.

NH4-N and PO4-P recovery from the solid phase sample via struvite precipitation

The usual operation applied in practice for handling solid phase is dumping it to landfill, incineration or land application (Shaffer and Walls, 2005). However, the applications of the first two options leads to the loss of nutrients, whereas the third option leads to some problems associated with health, odor and the envi-ronment. Muller et al. (2004) demonstrated that sludge cells can be dissolved by acidic or alkali treatment at low or ambient tem-peratures. Weidelener et al. (2005) compared the efficiencies of acidic and alkaline dissolution for three different sludge samples and reported higher efficiency in the case of acidic dissolution. In addition, Szogi et al. (2008) successfully applied acidic treatment for the purpose of P extraction from poultry litter. Also, Cohen (2009) reported more than 85% P dissolution when acidic dis-solution was employed for P recovery from the ash of incinerated sewage sludge and animal carcasses. Hence, acidic dissolution was applied in this study.

Acidic treatment leads to the transformation of available phosphates into orthophosphate, which is due to the disintegra-tion of the cell walls and mineralization of the microbial cells (Neyens et al., 2003; Weidelener et al., 2005). Acidic dissolution also releases P from insoluble inorganic phosphate complexes and this further increases the PO4-P concentration in the resulting liquid phase sample (Adnan et al., 2004). Metals which were

normally integrated in organic complex molecules were also released into the liquid phase (Neyens et al., 2003). The charac-teristics of the P-enriched liquid phase are given in Table 2. Metals can be incorporated into the crystal lattice or sorbed into the surface of struvite (Kamnev et al., 1999; Rontentalp et al., 2007). However, only two counter-reacting ions were present in high concentrations, namely Ca and K.

The effect of the Mg:N:P molar ratio

In P1, considerably high recovery efficiencies of N and P (89% and 82% for NH4-N and PO4-P respectively) were achieved (Figure 3). There was a big enhancement in P recovery between Mg:N:P ratios of 1:1:1 and 1.3:1:1; the recovery efficiency jumped from 82% to 99%. However, similar to the liquid phase sample residual PO4-P concentration did not change when Mg:N:P ratio was adjusted to 1.5:1:1.

The Mg:N:P ratio in the residual solution from P1 was 1:27:12 (Mg: 10.3 ± 0.4 mg l-1; NH4-N: 161 ± 5 mg l-1; PO4-P: 151 ± 2 mg l-1), which indicates Mg was the limiting constituent and hence the addition of extra Mg was necessary to increase struvite pre-cipitation in the reactor. Yet, Mg being the limiting constituent in the residual solution in P1 may imply the precipitation of Mg-containing minerals other than struvite. To test this XRD analysis of the precipitate collected from P1 was performed (Figure 4).

The results confirmed the presence of newberyite together with struvite. Also, XRD analysis of the precipitate collected from P3 was performed and presence of struvite was confirmed; no other mineral was detected (data not shown). However, one

Figure 2. X-ray diffraction patterns of the precipitate collected from L6 [the d-spacings of the strong lines of struvite are 4.25, 5.60, 5.90, 2.92, 2.69, 2.66, 4.14 and 2.80. Retrieved from powder diffraction (PDF) card 71-2089].

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Yilmazel and Demirer 799

limitation of pattern preparation is the nature of the material as a diffractor of X-rays (Klug and Alexander, 1967). In the literature it is stated that many crystalline substances give such sharp pow-der patterns that they are detectable when present to the extent patterns of 1–2 %, or less, in a mixture (Bunn, 2007; Klug and Alexander 1967) Other materials give such poor patterns that, although they can be readily identified when alone, they may not be detected when present in a mixture even to the extent of 50% (Klug and Alexander, 1967). In this study, XRD analysis was only used for the confirmation of the presence of struvite in the

precipitates and the minimum detection limit for the samples col-lected from the experiments was not defined.

NH4-N recovery efficiencies were recorded as 89%, 91% and 92% in P1, P2 and P3 respectively (Figure 3). Similar to the liq-uid phase sample the effect of Mg:N:P ratio on NH4-N recovery efficiency was minimal; however, a higher average recovery was achieved (90% from the P-enriched liquid phase and 70% from the liquid phase).

The impact of the variation of the molar ratio of Mg:P from 1:1 to 1.5:1 on the recovery of NH4-N was negligible (15.8–17.2

Figure 4. X-ray diffraction patterns of the precipitate collected from P1 [the d-spacings of the strong lines of struvite are 4.25, 5.60, 5.90, 2.92, 2.69, 2.66, 4.14 and 2.80. Retrieved from powder diffraction (PDF) card 71-2089. The d-spacings of the strong lines of newberyite are 5.94, 4.71, 3.46, 3.04, 5.34, 3.09, 4.50 and 4.14. Retrieved from PDF card 72-0023].

Figure 3. Recovery efficiencies of the solid phase magnesium:nitrogen:phosphorus molar ratio experiments. NH4-N: ammonium-nitrogen; PO4-P: phosphate phosphorus.

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800 Waste Management & Research 31(8)

%) and the recovery efficiency of PO4-P remained constant at around 99.8% (Table 4).

Ca concentration in the P-enriched liquid phase sample was 1581 mg l-1 (Table 2), and Ca concentration in the residual solu-tion in P4 and P6 was measured as 549 mg l-1 and 528 mg l-1 respectively (more than 65% Ca removal via precipitation). Based on these results, it may be speculated that there was forma-tion of Ca-containing minerals, for example hydroxylapatite (HAP) or calcite in the reactor together with struvite as impuri-ties. XRD analysis confirmed that the precipitant from P6 was predominantly struvite (Figure 5). The Ca:Mg molar concentra-tion ratio is the main factor for controlling the precipitate compo-sition. In the literature it is stated that the purity of the precipitate decreases when Ca:Mg ratio is higher than 1:1 (Burns et al., 2003). As given in Table 2, the Ca:Mg ratio was 2.36:1 in the P-enriched liquid phase, yet this ratio was adjusted to 1:1 in P6.

The results of P7 indicated that the addition of Mg is required

to attain residual concentrations of PO4-P lower than 7 mg l-1 at a

pH level of 8.5 (Table 4). Increasing the pH from 8.5 to 9.5 low-

ered the residual PO4-P concentration to 0.9 mg l-1, yet, in this

case, Ca concentration in the residual was measured as 375 mg l-1

(76% removal via precipitation). This indicates a higher amount

of precipitation of calcium phosphates in the reactor, which may

be owing to the high pH (9.5). Numerous researchers have dem-

onstrated that the optimum pH for the precipitation of HAP is

above 9.5, whereas effective struvite precipitation occurs at a pH

of 8.0 and above (Burns et al., 2003). The XRD patterns of the

precipitate collected from P8 confirmed the presence of struvite;

however, there is an apparent broad hump near 30o and this can

be attributed to the presence of amorphous calcium phosphates

(Figure 6).

Low residual concentrations of Al, Fe and Zn (Table 4) indi-

cated their precipitation together with struvite. The presence of

Al, Fe and Zn may lead to formation of other minerals, such as

berlinite, iron phosphate, and zinc ammonium phosphate hexahy-

drate. Yet precipitation of these minerals were not a concern

owing to the very low initial concentrations in the phosphorus-

enriched liquid phase (initial molar concentration ratios of Al:P,

Fe:P and Zn:P were 0.003:1, 0.08:1 and 0.009:1 respectively).

One of the possible species to be formed under these condi-

tions could be potassium struvite (KMP). Initially, Mg was limit-

ing in the P-enriched liquid phase sample (Mg:N:P ratio:

1:6.3:1.6; Table 2); however, external addition of Mg may lead to

the formation of KMP. Residual concentrations of K were meas-

ured as 984 mg l-1 and 965 mg l-1 in P4 and P6 respectively

(around 32% removal via precipitation.) In the literature, it is

stated that KMP could precipitate instead of NH4 struvite, only in

the case of low NH4-N concentrations (Schuiling and Andrade,

1999). Nevertheless, depending on the initial K concentration,

the co-precipitation of KMP and NH4 struvite is possible,

although, in smaller amounts, even with NH4-N concentrations

as high as 2000 mg l-1 (Zeng and Li, 2006).The comparison and evaluation of the experiments with the

highest recovery efficiencies are given in Table 5. Struvite Tabl

e 4.

Res

ults

of t

he s

olid

pha

se m

agne

sium

(Mg)

:pho

spho

rus

(P) m

olar

rat

io e

xper

imen

ts.

Exp.

Mg:

PC

alci

um

(Ca)

:Mg

NH

4-N

PO

4-P

Res

idua

l con

cent

ratio

n,a m

g l-1

Rec

over

y ef

ficie

ncy

(%)

Rec

over

y ef

ficie

ncy

(%)

NH

4-N

PO

4-P

Mg

Alu

min

ium

Ca

Iron

Pot

assi

umZi

nc

P4

1:1

1.5:

117

.299

.812

29 ±

25

1.50

± 0

.00

311

± 14

0.1

± 0.

054

9 ±

40.

2 ±

0.0

984

± 10

0.3

± 0.

0P

51.

3:1

1.1:

116

.499

.812

41 ±

41.

30 ±

0.0

041

7 ±

4nd

ndnd

ndnd

P6

1.5:

11:

115

.899

.912

50 ±

10

1.20

± 0

.00

448

± 12

0.1

± 0.

052

8 ±

20.

3 ±

0.0

965

± 7

0.24

± 0

.0P

71:

1.6

2.36

:115

.799

.212

51 ±

16.

83 ±

0.0

019

7 ±

6nd

ndnd

ndnd

P8

1:1.

62.

36:1

13.4

99.9

1286

± 5

0.90

± 0

.00

215

± 5

0.21

375

0.49

1421

0.38

NH

4-N

: am

mon

ium

-nitr

ogen

; PO

4-P

: pho

spha

te p

hosp

horo

us; n

d: n

ot d

eter

min

ed.

a Mea

n ±

SD (n

= 2

).

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Yilmazel and Demirer 801

precipitation can be used for different purposes and according to requirements. For instance, if the objective is to use the treated water for irrigation, the NH4-N concentration should be below 50 mg l-1, as specified in the Technical Bulletin of Water Pollution

Control Regulation of Turkey (MoEF, 1991). Moreover, depend-ing on the location of the plant if it is combined with the treat-ment plant of a poultry processing unit, more stringent criteria need to be met. For example, the effluent discharge criteria

01-077-2303 (C) - Struvite - MgNH4PO4(H2O)6 - Y: 72.99 % - d x by: 1. - WL: 1.54056 - Orthorhombic - a 6.95500 - b 6.14200 - c 11.21800 - alpha 90.000 -beta 90.000 - gamma 90.000 - Primitive - Pmn21 (31) - 2 - 479.206 - I/Ic

#112 090209-Fr1S1-M3 - File: SM_05.raw - Type: 2Th/Th locked - Start: 13.500 ° - End: 34.500 ° - Step: 0.030 ° - Step time: 10. s - Temp.: 25 °C (Room) -Time Started: 17 s - 2-Theta: 13.500 ° - Theta: 6.750 ° - Phi: 0.00 ° - Aux1:

Lin

(cou

nts)

0

100

200

300

400

500

600

700

800

900

1000

1100

2-Theta - Scale13.414 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Figure 5. X-ray diffraction patterns of the precipitate collected from P6.

01-077-2303 (C) - Struvite - MgNH4PO4(H2O)6 - Y: 19.91 % - d x by: 1. - WL: 1.54056 - Orthorhombic - a 6.95500 - b 6.14200 - c 11.21800 - alpha 90.000 -beta 90.000 - gamma 90.000 - Primitive - Pmn21 (31) - 2 - 479.206 - I/Ic

#22 090209-Fr1S1-M4 - File: SM_08.raw - Type: 2Th/Th locked - Start: 13.364 ° - End: 34.369 ° - Step: 0.030 ° - Step time: 10. s - Temp.: 25 °C (Room) -Time Started: 17 s - 2-Theta: 13.364 ° - Theta: 6.750 ° - Phi: 0.00 ° - Aux1: 0.

Lin

(cou

nts)

0102030405060708090

100110120130140150160170180190200210220230240250260270280290300

2-Theta - Scale13.414 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Figure 6. X-ray diffraction patterns of the precipitate collected from P8.

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802 Waste Management & Research 31(8)

specified by the World Bank Group for poultry processing is 10 mg l-1 total N and 2 mg l-1 for total P (IFC, 2007). Similar criteria required by the Water Pollution Control Regulation of Turkey are 15 mg l-1 for NH4-N and 2 mg l-1 for PO4-P (MoEF, 2004).

A number of struvite precipitation plants have so far been built and operated for municipal wastewaters. However, high concentrations of struvite component ions found in digested ani-mal manure, if recovered, could supply more struvite per unit wastewater volume than when recovered from municipal waste-water. Also, because the technique does not necessarily involve a sophisticated apparatus struvite recovery can become a common practice in agricultural farms. For example, a struvite crystallizer can be easily added to farms with lagoons and digesters. Batch treatment in vessel has difficulty in recovering product; hence, fluidized bed crystallizers should be preferred. The struvite prod-uct is usually relatively easy to dry. Handling the treated liquid may be an issue for farms with a single-stage process unit. However, if the farm has two lagoons in series, the crystallizer could be installed to treat liquid from one lagoon and discharge into the second one. In the case of a biogas plant studied herein, a crystallizer can be installed to treat the effluent of the primary digester and discharge into the secondary digester. A farm recov-ered struvite may have several advantages over industrially- supplied fertilizers because locally-produced struvite might be recognized by the local farmers for its purity and reliable availability.

Conclusions

The present study investigated, for the first time in the literature, the potential for the removal and recovery of NH4-N and PO4-P from both the liquid and the solid phase of a full-scale biogas plant co-digesting poultry manure and maize silage. Both NH4-N and PO4-P removal were achieved by adding necessary amounts of Mg and P to the liquid phase struvite precipitation experi-ments. The P removal efficiency increased with an increase in Mg concentration (Mg:N:P ratios tested 1:1:1–1:5:1:1). The increase in P removal was higher when Mg:P ratio was changed from 1:1 to 1.3:1, yet the P removal efficiency only increased slightly when Mg:P ratio was increased beyond 1.3:1. No statisti-cally significant difference occurred in nutrient removal when the experiments were performed with H3PO4 or NaH2PO4.2H2O. Acidic dissolution was successfully applied to release the bound nutrients from the solid phase digester effluent. Struvite precipi-tation led to more than 90% recovery of both NH4-N and PO4-P from the P-enriched liquid phase sample.

The selection of the chemicals (Mg and/or P) to be added and Mg:P ratio are critical for nutrient recovery from P-enriched liq-uid phase sample. The selection should be based on the down-stream discharge criteria and product purity requirements. To assess the possible agricultural use of the recovered struvite, the content must be analyzed for impurities and heavy metals, and compared with the legal limits. Heavy metals were precipitated together with struvite. However, unlike sewage sludge there is no need for concern regarding heavy metal presence in the Ta

ble

5. C

ompa

riso

n an

d ev

alua

tion

of th

e so

lid p

hase

exp

erim

ents

with

hig

hest

rec

over

y ef

ficie

ncie

s.

AD

eff

luen

t co

mpo

sitio

nEx

p.M

olar

rat

io o

f m

agne

sium

(M

g):n

itrog

en

(N):p

hosp

horu

s (P

)

Res

idua

l con

cent

ratio

n,a m

g l–1

Adv

anta

ges

Dis

adva

ntag

esP

ropo

sed

appl

icat

ion

NH

4-N

PO

4-P

Sim

ulat

ing

stru

vite

st

oich

iom

etry

with

ex

cess

Mg

P3

1.5:

1:1

114

± 0

(92.

3%)

6.43

± 0

.08

(99.

2%)

Hig

h re

cove

ries

of P

O4-

P a

nd

NH

4-N

, pur

ity o

f str

uvite

is h

igh,

ne

arly

four

tim

es h

ighe

r am

ount

of

prec

ipita

te w

as o

btai

ned

Add

ition

of P

and

ex

cess

Mg

For

the

reco

veri

es

of b

oth

NH

4-N

and

P

O4-

P

Ori

gina

l with

ad

ditio

n of

ext

ra M

gP

61.

5:3:

112

50 ±

10

(15.

8%)

1.20

± 0

.00

(99.

9%)

No

addi

tion

of P

sou

rce,

ver

y hi

gh

reco

very

of P

O4-

P, s

truv

ite p

urity

is

high

er in

com

pari

son

with

P8

Add

ition

of

Mg,

hig

h co

ncen

trat

ion

of

NH

4-N

in e

fflu

ent

Onl

y fo

r th

e re

cove

ry o

f PO

4-P

Ori

gina

l with

pH

in

crea

se to

9.5

P8

1:6.

3:1.

612

86 ±

5

(13.

4%)

0.90

± 0

.00

(99.

9%)

No

addi

tion

of M

g an

d/or

P, v

ery

high

rec

over

y of

PO

4-P

Pur

ity o

f str

uvite

is

poo

r, h

igh

conc

entr

atio

n of

N

H4-

N in

eff

luen

t

Onl

y fo

r th

e re

cove

ry o

f PO

4-P

AD

: ana

erob

ic d

iges

ter;

NH

4-N

: am

mon

ium

nitr

ogen

; PO

4-P

: pho

spha

te p

hosp

horu

s.a R

ecov

ery

effic

ienc

ies

are

give

n in

par

enth

eses

.

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Yilmazel and Demirer 803

recovered struvite owing to very low initial concentrations of the heavy metals in the digested poultry manure residues. Yet, in order to fully evaluate fertilizer value of the recovered struvite, pot trial tests and column tests should be performed to assess the plant availability and the slow-release properties respectively.

AcknowledgementsWe express sincere appreciation to the members of the Institute for Sanitary Engineering, Water Quality and Solid Waste Management (ISWA) University of Stuttgart, Germany for the technical support and to Dr.-Ing. Mathias Effenberger for providing the biogas plant samples.

FundingThis study was funded by the Scientific and Technological Council of Turkey through (Grant Number 107Y231) and the BMBF (Germany) through Intensified Cooperation (IntenC): Promotion of German-Turkish Higher Education Research Program.

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