Removal and recovery of nutrients as struvite from anaerobic digestion residues of poultry manure
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Removal and recovery of nutrients as struvite fromanaerobic digestion residues of poultry manureY.D. Yilmazel a & G.N. Demirer aa Department of Environmental Engineering, Middle East Technical University, 06531 Ankara,TurkeyPublished online: 13 Jun 2011.
To cite this article: Y.D. Yilmazel & G.N. Demirer (2011) Removal and recovery of nutrients as struvite from anaerobicdigestion residues of poultry manure, Environmental Technology, 32:7, 783-794, DOI: 10.1080/09593330.2010.512925
To link to this article: http://dx.doi.org/10.1080/09593330.2010.512925
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Vol. 32, No. 7, May 2011, 783794
ISSN 0959-3330 print/ISSN 1479-487X online 2011 Taylor & FrancisDOI: 10.1080/09593330.2010.512925http://www.informaworld.com
Removal and recovery of nutrients as struvite from anaerobic digestion residues of poultry manure
Y.D. Yilmazel and G.N. Demirer*
Department of Environmental Engineering, Middle East Technical University, 06531 Ankara, Turkey
Taylor and Francis
Received 11 January 2010; Accepted 30 July 2010
The removal and the recovery of nutrients, namely nitrogen (N) and phosphorus (P) from anaerobically digested andsolidliquid separated manure effluents via struvite precipitation were investigated. Both the liquid and the solidphases of the poultry manure digester effluent were subjected to struvite precipitation experiments. The Mg:N:Pmolar ratio of 1:1:1 in the liquid phase resulted in an average NH
-N removal efficiency of 86.4%, which increasedto 97.4% by adjusting the Mg:N:P ratio to 1.5:1:1. The acidic phosphorus-dissolution process was applied to the solidphase of the effluent to obtain a phosphorus-enriched solution. Nutrient recovery experiments with NaOH as thebuffering reagent were conducted with and without addition of external chemicals (Mg and P sources) to evaluate theinfluence of the Mg:N:P molar ratio, the Mg:P molar ratio and pH. All the experiments depicted complete PO
-P(99.6100.0%) and partial NH
-N (3.365.6%) recoveries from the phosphorus-enriched solution.
anaerobic digestion; poultry manure; nutrient recovery; phosphorus dissolution; struvite
The poultry and the livestock industries are growingrapidly and this has led to large quantities of animalwaste production. Atuanya and Aigbirior  reportedthat the poultry production is the fastest growingcottage industry, and estimated the annual solid wastegeneration from poultry farms at the level of millionsof tons. Gungor-Demirci and Demirer  reported thatthe production of cattle and poultry manure in Turkeywas approximately 20 million tons of dry matter in2000. Direct land application of manure is the mostpreferred method of utilization, but it is not alwaysfeasible. Because nutrients in manure are not necessar-ily present in the same proportion needed by the crops,if applied at a rate higher than plant uptake, there is agreat risk of nutrient leaching and run-off resulting ineutrophication of surface waters . Moreover, thehigh prevalence of pathogenic microorganisms in freshpoultry poses a potential threat to human healththrough contamination of water bodies from untreatedpoultry wastewater . If the land to be used for directapplication is distant or the location is sensitive toodour, some type of manure treatment may be required.Some of the currently used options for the managementof poultry manure such as landfilling and incinerationlead to the loss of nutrients as well as environmentalproblems .
Anaerobic digestion (AD) is an established technol-ogy to convert animal waste into profitable by-productsas well as to reduce the relevant air, water and soilpollution problems [1,2,6,7]. However, because ADremoves mainly carbon, additional processes to removenitrogen (N) and phosphorus (P) should also be used tomeet the stringent effluent criteria. Moreover, there is ashift from the removal to the recovery of nutrients as aresult of increasing concerns regarding limited naturalresources and the importance given to the sustainabletreatment technologies.
Crystallization of N and P in the form of magnesiumammonium phosphate hexahydrate (MgNH
O,struvite or MAP) is one of the possible techniquesused to remove and recover nutrients from wastewater. Struvite is a valuable fertilizer since it releasesnutrients slowly and has non-burning features owing toits low solubility in water. Struvite formation isobserved at the stoichiometric ratio of 1:1:1 of the ionscomposing struvite according to the following reaction[10,12]:
The success of MAP precipitation depends on twomajor factors: Mg:N:P molar ratio and the pH of the
*Corresponding author. Email: firstname.lastname@example.org
Mg NH PO H O
MgNH H O
+ + -+ + + fi
Y.D. Yilmazel and G.N. Demirer
solution . In a given solution, struvite can form andprecipitate if the product of Mg
ionactivities exceed the ion-activity product at equilibriumor the thermodynamic solubility product . AlthoughH
is not directly involved in the reaction, struviteprecipitation is highly dependent on pH. This is simplydue to changes in the activities of both NH
.Theoretically, solubility of struvite decreases as pHincreases up to a pH level of 9.0. At a pH level above9.0, struvite becomes more soluble as a result ofdecreasing and increasing ion activities of ammonia andphosphate, respectively . However, the pH of mini-mum solubility of struvite may differ owing to changingionic strength and composition of wastewaters. Theminimum solubility pH for struvite precipitationreported by a number of researchers displays a range ofvalues, from 8.0 to 10.7 [10,12]. Struvite precipitation isalso influenced by chemical composition of the waste-water (organic matter, presence of chelating agents,ionic strength), the degree of supersaturation, tempera-ture and the presence of foreign ions such as calcium[810,14].
The struvite precipitation technique has beenapplied to various wastes, such as swine waste ,dairy manure , calf manure , landfill leachate, semiconductor wastewater , slaughterhousewastewaters  and anaerobic digester sidestreams. However, to the best of the authors knowledge,there is only one published work, by Yetilmezsoy andSapci-Zengin , on struvite precipitation from anaer-obically digested poultry manure. In their study, threecombinations of chemicals were tested at different pHlevels, and the performance of the struvite precipitationprocess was evaluated by measuring the remainingchemical oxygen demand (COD), colour and ammoniaconcentrations .
Therefore, in order to fill the gap in the literature,the current study aimed to recover N and P from bothphases of the anaerobically digested and solidliquidseparated effluents of a full-scale poultry manure diges-tion plant. Most of the studies in the literature focusedon the removal/recovery of the readily available nutri-ents in the wastewaters [8,13,16,17]. However, thedissolution of nutrient from the solid phase is the onlyway to recover the highest amount of nutrients, particu-larly phosphorus, from animal wastes. Recently anumber researchers focused on the extraction of thenutrients present in the solid phase of the wastewatersludge [23,24], poultry litter  and piggery wastewa-ter . This study illustrates the feasibility of therecovery of nutrients from the solid phase of anaerobi-cally digested poultry manure by the adoption of anovel phosphorus dissolution process, thereby obtain-ing a phosphorus-enriched liquid phase.
Materials and methods
The wastewater sample was collected from the effluentof a full-scale biogas plant operated at mesophilicconditions. It digests poultry manure generated from asmall poultry farm housing 30,000 laying hens, locatedin Forchtenberg, Germany. The sample was kept refrig-erated at 4
C until used. Owing to the high solidscontent, the effluent sample was subjected to solidliquid separation, and both phases of the effluent werecharacterized (Table 1).
Experimental set-up and procedures
The experimental study was performed at the laborato-ries of the Department of Wastewater Technology(AWT) at the University of Stuttgart, Germany.
Struvite precipitation experiments
The solidliquid separation of the anaerobic digestereffluent was achieved by centrifugation (RC6, SorvallInstruments DuPont, Osterode, Germany) for 15minutes at 17,710
and sieving through a screen of0.56 mm (0.022 in) mesh size. The liquid phase wasused directly for struvite precipitation experiments,where the solid phase was subjected to the phosphorusdissolution process before the struvite precipitationexperiments.
Four consecutive steps were followed in the struviteprecipitation experiments: (1) addition of chemicals, (2)mixing, (3) settling, (4) filtration. The struvite precipi-tation experiments were conducted in continuouslystirred batch reactors at room temperature (2122
C).Each struvite reactor contained 150 mL of the sample,and was mixed using a magnetic stirrer (MR Hei-Mix L,Heidolph, Schwabach, Germany). During the experi-ments mixing intensity was kept constant at 250 rpm. Inall experimental runs, there was no addition of ammonianitrogen (NH
-N). The total (initial + added) molarconcentration ratios of the ions were varied during theexperiments, and the concentrations of Mg and ortho-phosphate (PO
-P) were raised via addition of theseions externally. After adding the Mg-containing chemi-cal, the PO
-P-containing chemical was added to thereactor where necessary. The required amounts of Mgand PO
-P were calculated considering initial concen-trations of these ions in the wastewater, and were raisedaccordingly to obtain the desired total molar concentra-tion ratio of Mg:N:P in the reactor. In the experimentsMgCl
O was used in its solid form as the Mgsource and 75% H
(v/v) was used as the P source.The pH adjustments were made using 20% NaOH (v/v)
solution. Since the volume of NaOH added was verysmall, the dilution effect was negligible. All chemicalsused in the experiments were analytical grade. After thepH of the solution became constant at the desired levelwith a variation of
0.01 pH units, 30 minutes ofmixing was applied, and at the end of the mixing periodthe reactor content was allowed to settle down for 60minutes. After the struvite formation reaction wascompleted, the reactor content was filtered through afolded filter of pore size range 412
m (MN615,Macherey-Nagel, Duren, Germany) and the filtrate wasanalyzed for its PO
-N and metal content. Theprecipitates were dried in a constant temperature roomat 30
C overnight. After drying, the precipitate wasseparated manually from the filter paper and kept atroom temperature (2122
C) until analysed by X-raydiffraction (XRD).
Phosphorus dissolution process
The solid phase obtained after the solidliquid separationof the biogas plant effluent was subjected to thephosphorus dissolution process prior to the struvite
precipitation experiments. Phosphorus dissolution wasachieved in four steps: (1) dilution, (2) acidic dissolution,(3) mixing and (4) solidliquid separation (Figure 1). Thesolid phase was diluted by distilled water until the totalsolids (TS) concentration became 10%, which allowedcontinuous mixing in the reactor. Then, the pH of thediluted waste was adjusted to 2.0 by the addition of 20%HCl (v/v). The acidic mixture was mixed at 250 rpm byusing the jar test apparatus (MSR12/180, GeppertRuhrtechnik, Dreieich, Germany) for two hours. Afterthe dissolution, separation of the phosphorus-enrichedliquid phase from the remaining solid phase was achievedby centrifugation (RC6, Sorvall Instruments DuPont,Osterode, Germany) at 17,710
for 15 minutes andsieving through a screen of 0.56 mm (0.022 in) mesh size.
Figure 1. Shematic representation of the phosphorus dissolution process.
Ammonium-nitrogen was in excess in the AD effluentand additions of Mg and PO
-P from external sourceswere necessary for struvite precipitation. This isbecause the molar ratio of struvite-forming ions(Mg:N:P ratio) should be at least equal for intentional
Table 1. Characterization of the solidliquid separated effluent.
Parameter Liquid phase
Unit Solid phase
0.0 g kg
4 g kg
1.9 % of TS 31.0
0.6 % of TSCOD 14,516
639 mg L
22 mg L
nd TKN 5838
12 mg L
0.1 mg g
117 mg L
0 mg L
nd TP 287
1 mg L
0.2 mg g
dry matterAl 1.39
0.03 mg L
35 mg kg
dry matterCa 78.6
6.90 mg L
257 mg kg
dry matterCd < 0.025 mg L
< 2.8 mg kg
dry matterCo 0.07
0.00 mg L
< 1.7 mg kg
dry matterCr 0.08
0.01 mg L
0.0 mg kg
dry matterCu 0.46
0.00 mg L
0.6 mg kg
dry matterFe 7.24
0.08 mg L
5 mg kg
786 Y.D. Yilmazel and G.N. Demirer
struvite precipitation. However, at any given pH level,any increase in the Mg:N:P...