This article was downloaded by: [University of California, San Francisco]On: 05 October 2014, At: 03:21Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Journal of Environmental Science and Health, Part B:Pesticides, Food Contaminants, and Agricultural WastesPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/lesb20

Microwave treatment and struvite recovery potentialof dairy manureAsif Qureshi a , Kwang Victor Lo a & Ping Huang Liao aa Department of Civil Engineering , University of British Columbia , Vancouver, BC, CanadaPublished online: 24 Apr 2008.

To cite this article: Asif Qureshi , Kwang Victor Lo & Ping Huang Liao (2008) Microwave treatment and struvite recoverypotential of dairy manure, Journal of Environmental Science and Health, Part B: Pesticides, Food Contaminants, andAgricultural Wastes, 43:4, 350-357, DOI: 10.1080/03601230801941709

To link to this article: http://dx.doi.org/10.1080/03601230801941709

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information. Taylor and Francis shall not be liable forany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Journal of Environmental Science and Health Part B (2008) 43, 350–357Copyright C© Taylor & Francis Group, LLCISSN: 0360-1234 (Print); 1532-4109 (Online)DOI: 10.1080/03601230801941709

Microwave treatment and struvite recovery potentialof dairy manure

ASIF QURESHI, KWANG VICTOR LO and PING HUANG LIAO

Department of Civil Engineering, University of British Columbia, Vancouver, BC, Canada

Microwave digestion of liquid dairy manure was tested for the release of nutrients, such as orthophosphates, ammonia-nitrogen,magnesium, calcium and potassium, both with and without the aid of an oxidizing agent (hydrogen peroxide). The orthophosphateto total phosphorus ratio of the manure increased from 21% to greater than 80% with 5 minutes of microwave treatment. More than36% of total chemical oxygen demand (t-COD) of the manure was reduced when microwave digestion was assisted with peroxideaddition. In addition, the volatile fatty acids (VFAs) distribution shifted to simpler chain acids (acetic acid in particular) with anincrease in operating temperature. In the second part of the study, digested manure with increased soluble phosphate was tested forthe recovery of struvite (magnesium ammonium phosphate) at different pH. It was found that up to 90% of orthophosphate canbe removed from the solution. Overall, it was concluded that the oxidizing agent-assisted microwave digestion process can be usedupstream of anaerobic digestion, following which the anaerobically digested manure can be used for struvite recovery. Thus, thismicrowave digestion process presents the potential for enhanced efficiencies in both manure digestion and struvite recovery.

Keywords: Dairy manure; microwave digestion; nutrients release; struvite recovery.

Introduction

Microwave technology has been recognized for its abilityto enhance the release of nutrients from complex solutionmatrices in a very short time (order of a few minutes).[1]

This ability is very useful in a nutrients recovery scheme,where the main hurdle is obtaining nutrients in solubleforms. Application of microwave technology to the treat-ment of dairy manure could be more challenging, due toits complex nature. Dairy manure has low soluble phos-phorus (orthophosphate) to total phosphorus ratio, andhas a high amount of suspended solids (SS). Most of thephosphorus is tied up in the solids portion, and up to 65%of the phosphorus is an insoluble organic form.[2] Anaer-obic digestion processes for the treatment of dairy manureare slow (hydraulic retention times in the order of 10 to40 days) and their effectiveness depends on a number ofoperational and environmental factors. A fast and effec-tive physical-chemical digestion process, such as microwavedigestion, can be of much assistance to nutrient recoverytechnologies.

Address correspondence to Dr. K. Victor Lo, Department of CivilEngineering, University of British Columbia, Vancouver, BC V6T1Z4, Canada; E-mail: kvlo@civil.ubc.caReceived July 13, 2007.

Microwave heating is a dielectric heating process. Heatis generated through the realignment of dipoles with os-cillating electric field. The main advantages of microwaveheating are: 1) the uniformity of heating throughout theobject being heated; 2) precise control of the processtemperature and heating time; and 3) a microwave en-hanced heating treatment can be completed in a muchshorter period than that of the conventional conductionheating.[1]

In our previous study, microwave technology was ap-plied to the sludge from an enhanced biological phospho-rus removal (EBPR) wastewater treatment plant, to addressthe need for a fast solubalization process.[3] Approximately100–180 mg L−1 of orthophosphate was released into thesolution with a heating time of 5 minutes, resulting in anincrease of the orthophosphate to total phosphorus ratiofrom 0 to 0.5. Magnesium (Mg) and potassium (K) releasesby different amounts were also observed, although a cleartrend was lacking. However, it was clear that microwavedigestion was very effective in releasing orthophosphatesfrom digester sludge.

After successful application to municipal sludges, mi-crowave digestion technology was extended to treat liquiddairy manure. [4] Dairy manure samples were acidified withH2SO4 (1:50 vol/vol), heated at 170◦C and a heating time of5 minutes; more than 80% of total phosphorus (TP) was re-leased into the solution.[4] An addition of hydrogen peroxide

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an F

ranc

isco

] at

03:

21 0

5 O

ctob

er 2

014

Struvite recovery potential of dairy manure 351

(H2O2) further enhanced the treatment, resulting in a low-ered heating temperature at 120◦C and increased TP releaseup to 85%. While orthophosphate release from dairy ma-nure appeared feasible with microwave digestion, the disin-tegration of organic matter, the release potentials of ammo-nia (NH4-N) and metals, such as magnesium, potassium,and calcium were not investigated.[4]

Thus, the focus of this preliminary research was to inves-tigate the solubilization potential of microwave treatmentwhen applied to dairy manure. The primary target nutrientwas phosphorus, so that the supernatant of the processedmanure could be directly used for the recovery of a phos-phorus fertilizer in the form of struvite (magnesium am-monium phosphate) or K-struvite (magnesium potassiumphosphate). The disintegration of organic matters and thesolubilization of nutrients and metals were also assessed,by subjecting the manure to: 1) microwave treatment (MW)only; 2) the microwave enhanced advanced oxidation pro-cess (MW/H2O2-AOP); and 3) H2O2 oxidation process atambient temperature.

Materials and methods

Experimental design and analysis

Microwave apparatus and treatmentsA closed-vessel microwave digestion system (Ethos TC Di-gestion Lab station 5000, Milestone Inc., U.S.A.) was usedin this study. At a wave length of 2450 MHz, the system’smaximum delivered microwave power is1000 W. Equippedwith a real-time temperature control device, the system hasthe capacity of accommodating up to 12 vessels (each witha volume of 100 mL) in a single run, at operating tempera-tures and pressures up to 220◦C and 3,000 kPa, respectively.

The aim of this experiment was to find the near opti-mum temperature for nutrient release and the extent of re-lease. The main variable studied was the operating tempera-ture, with and without H2O2 addition. An experiment withH2O2 addition was also conducted at ambient temperatureto serve as a reference.

An amount of 0.5 mL of H2SO4 was added to 24.5 mL ofliquid manure (i.e. 1:50 vol/vol) to assist in solubilizationand digestion. Subsequently, based on results from our pre-vious study, four microwave heating temperatures (60, 80,120 and 170◦C) were selected and tested.[4] Each run wascarried out with four replicates. All samples were heatedfor five minutes at the selected heating temperature. Var-ious ramp times (based on a rate of increase of 20◦C perminute) were needed to reach the desired heating tempera-tures.

To study the effects of H2O2 addition, 1 mL of H2O2(1:25 vol/vol) was added to a mixture of 0.5 mL H2SO4and 23.5 mL liquid manure. The effects of heating werecompared with the process performance at ambient temper-

ature. Since foaming was expected for this mixture, initialtests were done on 10 mL of samples with the same ratiosof H2O2 and H2SO4 added.

Dairy manure was obtained from the Dairy Educa-tion and Research Center, University of British Columbia(UBC), in Agassiz, B.C., Canada. About 500 liters of theliquid fraction after a liquid-solids separation process waspumped from the storage pit on-site into a container. Liquiddairy manure was transported by a truck and then stored at4◦C in a cold storage room in the Department of Civil En-gineering, University of British Columbia, over the courseof this study.

Chemical analysis

The processed manure (effluent) was tested for solubleCOD, total COD, orthophosphates (PO4-P), ammonia-nitrogen (NH4-N), magnesium, potassium, calcium,volatile fatty acids (VFAs, for H2O2 assisted digestion),total phosphorus (TP), total Kjeldahl nitrogen (TKN),total solids (TS) and total volatile solids (VS) content,suspended solids (SS) and volatile suspended solids (VSS)according to the Standard Methods.[5] All analyses, exceptCOD and TS, were performed by flow injection (LachatQuick-Chem 8000 Automated Ion Analyzer, Lachat Instru-ments, U.S.A.). Metal analysis was performed with a VarianAtomic Absorption spectrometer.

A Hewlett Packard 5890 Series II gas chromatographequipped with a flame ionization detector (FID) was usedto measure volatile fatty acids (VFA). Volatile separationwas accomplished with an HP FFAP (free fatty acid phase)column (0.25 m × 0.31 mm with 0.52 μm film thickness).The injection temperature was set at 175◦C and the FID at250◦C. Helium gas was used as the carrier at a head pressureof 69 kPa.

Struvite recovery

The volume of samples obtained after the microwave di-gestion was not enough to suitably test struvite recoveryfrom liquid manure. Therefore, based on the results of runswith peroxide addition at ambient temperatures (discussedin a later section), an experiment was conducted in whichH2SO4 and H2O2 were added in the previously mentionedratios to about 12 L of liquid manure. Subsequently, themixture was allowed to react for 3 days. Following 6 hoursof settling, 8 L of the supernatant was collected.

For struvite recovery, 250 mL each of the supernatant wastaken in 6 beakers and stirred at 20–40 rpm for 24 hours.The initial operating pH values were at 7.0, 8.0, 8.5, 9.0,10 and 11, by adding sodium hydroxide solution. After areaction time of 12 hours, the final pH values were 6.43,7.49, 8.39, 8.49, 8.67, and 9.11. The supernatant was ana-lyzed for PO4-P, TP, NH4-N, TKN, Mg, K, Ca, total andsoluble COD, total suspended solids (TSS) and VSS. Allvalues were measured in duplicates.

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an F

ranc

isco

] at

03:

21 0

5 O

ctob

er 2

014

352 Qureshi et al.

Results and discussions

Liquid dairy manure

Chemical analyses were conducted on liquid dairy manuresamples (each with three replicates) to determine the initialconcentrations of interest. The average values and standarddeviations are: 187 ± 4 mg L−1 of TP, 38.5 ± 0.5 mg L−1

of PO4-P, 489 ± 31.2 mg L−1 of NH4-N, 23.6 ± 0 g L−1 oftotal COD, 7.6 ± 0.11 g L−1 of soluble COD, 553 mg L−1

of K, 373 mg L−1 of Ca, and 150 mg L−1 of Mg.

Microwave treatment of dairy manure

The initial TP for the manure was 187 ± 4 mg L−1. Afterthe microwave (MW) digestion, the orthophosphate con-centration increased from ∼ 40 mg L−1 before treatment, toa high value of ∼160 mg L−1 (Fig. 1). The PO4-P/TP ratioincreased from 0.21 to 0.86, indicating that after microwavetreatment, most of the phosphorus was in orthophosphateform. The increase in PO4-P is similar to that reported byPan et al.[4] For the MW/H2O2-AOP, the PO4-P content ofthe manure also increased to about 80% of TP at a heat-ing time of 5 minutes (Fig. 1). It should be noted that thevalue at 20◦C (Fig. 1) corresponded to oxidation by hydro-gen peroxide at ambient temperature without microwavetreatment. There was a significant increase in PO4-P con-centration, however, it required a reaction time of 6 hours.Figure 1 also shows that the soluble PO4-P concentrationin the manure did not change appreciably, when subject tomicrowave treatment at 60 and 80◦C. This was similar to theresults obtained from the previous studies.[3,6] The PO4-Pcontents at temperature 80, 120 and 170◦C were similar forboth types of microwave treatment (i.e. with and withouthydrogen peroxide). It is possible that more PO4-P solubi-

Fig. 1. Effect of microwave treatment on PO4 release (error bars represent 1 standard deviation).

lization could be achieved at heating temperatures higherthan 170◦C; although it would certainly require more powerinputs. These findings also confirmed results from our pre-vious studies[3,7] that the three most significant factors formaximizing orthophosphate solubilisation were, in the or-der from most to least significant: (i) microwave heatingtemperature, (ii) the combined effect of microwave heatingtemperature and hydrogen peroxide addition, and (iii) sul-phuric acid addition. Microwave heating temperature wasfound to be the dominating factor with respect to maximiz-ing orthophosphate yield from the sewage sludge. The addi-tion of hydrogen peroxide in the MW/H2O2-AOP seemedto facilitate the disintegration of organic matters, and theoxidation processes.[7.8]

Concentrations of the soluble ammonia (NH4-N) inthe liquid manure increased with increasing temperature.With or without hydrogen peroxide addition, the trendswere almost identical (Fig. 2). Hydrogen peroxide addi-tion (MW/H2O2-AOP) resulted in s slightly higher solubleammonia concentration. The soluble NH4-N content ofthe microwave-treated manure at higher operating temper-atures (∼980 mg L−1) was much more than that of manuretreated with H2O2 alone at 20◦C. In the previous study,[7] thesoluble ammonia concentrations from sewage sludge werealso affected by three important factors, in the order ofsignificance: (i) hydrogen peroxide concentration, (ii) heat-ing temperature, and (iii) sulphuric acid addition. However,there was no dominating factor with respect to maximizingsoluble ammonia. In this study, it seemed that microwaveheating affected soluble ammonia release more than hydro-gen peroxide concentration.

While total COD (t-COD) remained the same for themicrowave only experiment (Fig. 3), soluble COD (s-COD)increased more than twofold (from ∼4000 to ∼9800 mgL−1). This suggests that microwave treatment helped in

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an F

ranc

isco

] at

03:

21 0

5 O

ctob

er 2

014

Struvite recovery potential of dairy manure 353

Fig. 2. Effect of microwave treatment on NH4-N release (error bars represent 1 standard deviation).

solubilizing the complex organics as well. To investigate thisfurther, VFAs were also analyzed in the next set of H2O2assisted digestion experiments.

The effect of MW/H2O2-AOP was more pronounced onthe COD profiles as compared to the microwave only pro-cess (Fig. 3). The s-COD of the manure increased with anincrease of temperature till 80◦C, while the s-COD valuesdecreased with an increase of microwave temperature. Thet-COD also remained very steady until 80◦C. As tempera-ture increased further, the t-COD decreased from ∼26,000mg L−1 to about 18,500 mg L−1 at 120◦C, and ∼16,000 mgL−1 170◦C Compared to the initial t-COD value, there wasnearly a 36% decrease in the t-COD value, but, there wasno significant change in the s-COD value. It was possiblethat solubilization and destruction took place simultane-ously; resulting in a total reduction of t-COD, with a con-

Fig. 3. Effect of microwave treatment on total and soluble chemical oxygen demand (CODs) (error bars represent 1 standard deviation).

comitant solubilization of complex, previously recalcitrantorganics. Overall, while the total organic pollution poten-tial was reduced, the soluble content remained nearly thesame.

The MW/H2O2-AOP was proposed to occur by two ma-jor reaction processes, similar to the wet-air oxidation.[8]

The first process involves the breakdown of large particu-late organic matters into smaller and more soluble organiccomponents, whereas, the second process involves furtheroxidation or gasification of the resulting organic products.The resulting oxidation products can be VFAs and/or CO2depending on the reaction conditions.[9] There is an advan-tage to operate the microwave enhanced advanced oxida-tion process (MW-AOP) process at a higher temperature;it would produce CO2 (a final oxidation product), and re-duce organics in the solution. This would also benefit the

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an F

ranc

isco

] at

03:

21 0

5 O

ctob

er 2

014

354 Qureshi et al.

Fig. 4. Effect of advanced oxidation process (AOP)-microwave digestion on volatile fatty acids (VFAs) (expressed as mg/L acetic acid;Ac = acetic acid, Pr = propionic acid; error bars represent 1 standard deviation).

subsequent struvite precipitation process. Waste strengthaffects struvite solubility: in the presence of organics in thewastewater, struvite solubility is increased, and therefore isless likely to crystallize.[10]

Without hydrogen peroxide, microwave digestion wasjust a thermal destruction process; very little oxidationproducts such as VFA and CO2 were expected. The t-CODin the microwave digested manure remained more or lessthe same as in the starting manure.

The VFA profiles (Fig. 4) confirm this reasoning. Therewas a slight increase in the total VFAs until 80◦C, whichthen decreased similarly to that of the t-COD profiles. Thissuggested that at higher temperatures, CO2 was generated

0

200

400

600

800

1000

1200

0 20 40 60 80 100 120 140 160 180

T (oC)

K,

Mg

an

d C

a (m

gL

-1)

K: MW Mg: MW

Ca: MW K: MW/H2O2-AOP

Mg: MW/H2O2-AOP Ca: MW/H2O2-AOP

Initial Value

Fig. 5. Effect of microwave treatment on K, Mg and Ca (error bars represent 1 standard deviation).

as the end product, while at lower operating temperatures,VFAs were the end product. Interestingly, the acetic acidcontent increased from about 50% to >85% of the totalVFA content. There was no iso-butyric acid left in the sam-ple after 120◦C and 170◦C treatment. Thus, the MW-AOPtreatment helped in the formation of simpler fatty acids.

These simpler chain organics are very important foranaerobic digestion and methane production. According tothese preliminary results, if the MW/H2O2-AOP setup wereto be used upstream of anaerobic digestion, the efficiencyof digestion would increase. Considering the breakdownof these VFAs, it is also conceivable that this MW/H2O2-AOP would help in breaking down other complex organics

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an F

ranc

isco

] at

03:

21 0

5 O

ctob

er 2

014

Struvite recovery potential of dairy manure 355

0

20

40

60

80

100

120

140

160

6 6.25 6.5 6.75 7 7.25 7.5 7.75 8 8.25 8.5 8.75 9 9.25

pH

PO

4-P

(m

gL

-1)

0

100

200

300

400

500

600

700

800

900

NH

4-N

(m

gL

-1)

PO4-P NH4-N

Initial Value

Fig. 6. Change in PO4-P and NH4-N content of treated supernatant with pH.

as well. This could further increase the conversion of thesebroken-down complex organics to VFAs in an anaerobicdigester.

When liquid manure was subjected to microwave diges-tion only, the release of metals such as potassium, magne-sium and calcium into solution was also observed (Fig. 5).When subjected to the MW/H2O2-AOP treatment, solubleK concentration increased with temperature, but there wasno obvious increase in soluble Mg and Ca concentrations.The metal release trends are similar for both tested oper-ation modes, i.e., with and without H2O2 addition. Theseresults are consistent with those reported by Wong et al.,[3]

where no clear and consistent increase or decrease in metalconcentration was reported after microwave digestion of

0

20

40

60

80

100

120

140

6 6.25 6.5 6.75 7 7.25 7.5 7.75 8 8.25 8.5 8.75 9 9.25

pH

TP

(m

gL

-1)

0

150

300

450

600

750

900

1050

TK

N (

mg

L-1

)

TP TKN

Initial Value

Fig. 7. Change in total phosphorus (TP) and total kjeldahl nitrogen (TKN) content of treated supernatant with pH.

aerobically digested solids from a municipal waste treat-ment plant. More research on this aspect is warranted.

Struvite recovery

Struvite precipitation is controlled by pH, supersatura-tion, temperature and the presence of impurities such ascalcium.[11] It can occur only when the concentrations ofMg+2, NH+

4 and PO−34 exceed the solubility product (Ksp)

of struvite. The Ksp is given by the following equation:

Ksp = [Mg+2][NH+4 ]

[PO−3

4

]

The Ksp is also dependent on the pH of the solution. Highstruvite precipitaion potentials, or low struvite solubilities,

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an F

ranc

isco

] at

03:

21 0

5 O

ctob

er 2

014

356 Qureshi et al.

0

50

100

150

200

250

300

350

400

450

500

6 6.25 6.5 6.75 7 7.25 7.5 7.75 8 8.25 8.5 8.75 9 9.25

pH

Mg

an

d C

a (

mg

L-1

)

0

200

400

600

800

1000

1200

1400

K (

mg

L-1

)

Mg Ca K

Initial Value

Fig. 8. Change in Mg, Ca and K content of treated supernatant with pH.

occur in a regime of higher pH. Therefore, struvite forma-tion is enhanced when the pH is between 7 and 11, andthat the optimal pH for formation is between 9 and 9.5. Itshould also be noted that total suspended solids (TSS) in-terfere with the precipitation process at TSS concentrationsabove 1000 mg L−1.[12] Dairy manure slurries contain veryhigh TSS concentrations which are not suitable for the stru-vite formation process. However, the MW/H2O2-AOP wasalso able to reduce total organics and/or suspended solidsin the solution at temperatures above 120◦C. The resultingsupernatant yielded very low suspended solids in the solu-tion. The MW/H2O2-AOP not only solubilized phosphate,ammonia and metals, but also reduced TSS as mentionedearlier. It is a very useful pre-treatment process for the stru-vite precipitation process.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

6 6.25 6.5 6.75 7 7.25 7.5 7.75 8 8.25 8.5 8.75 9 9.25

pH

t-C

OD

(m

gL

-1)

0

5000

10000

15000

20000

25000

30000

35000

40000

VS

(m

gL

-1)

t-COD VS

Initial Value

Fig. 9. Change in total chemical oxygen demand (t-COD) and volatile solids (VS) content of treated supernatant with pH.

The initial pH of the peroxide-treated (at ambient tem-perature) supernatant was 1.4. Addition of alkali to thissupernatant showed a clear decrease in soluble PO4-P con-centrations. With an initial concentration of over 140 mgL−1, more than 90% of the PO4-P was removed from thesolution at pH 9.11 (Fig. 6). This was the highest removalrate achieved in this study. A similar decrease of NH4-Nconcentration with increasing pH was also observed. It ap-pears that pH values >8 were more suitable for the removalof both PO4-P and NH4-N (Fig. 6). Similar trends wereobserved in the TP and TKN variations with pH (Fig. 7).

Decreases in soluble Mg and Ca concentrations were alsoobserved (Fig. 8). However, the nearly constant concen-tration of soluble K at pH 7.49, 8.39, 8.49, 8.69 and 9.11was also observed. Reductions in NH4-N, Mg and Ca, and

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an F

ranc

isco

] at

03:

21 0

5 O

ctob

er 2

014

Struvite recovery potential of dairy manure 357

no significant reduction in K suggest that the phosphoruscompound being formed was magnesium ammonium phos-phate or calcium phosphate, but not magnesium potassiumphosphate.

Another observation was the reductions of t-COD withincreases in pH from 6.43 to 9.11 (Fig. 9). There were sig-nificant t-COD reductions in this pH range when comparedto the initial value of t-COD at close to 9,000 mg L−1. Thismight have happened due to the production of glycerol andcrude soap, caused by the hydrolysis of VFAs by alkali.Similarly, a significant decrease in the VS of the supernatantwas also observed over this range of pH. Both TSS and VSSwere reduced, respectively, from 15,300 mg L−1 and 9,000mg L−1 in the peroxide treated supernatant to ∼2,600 mgL−1 and 280 mg L−1 after the P-recovery experiment (notshown in figure). This might have been caused by solidsincorporation on the settled precipitate. Thus, in additionto the recovery of phosphorus, an AOP process combinedwith struvite recovery further removed the organic contentof the manure, although there might be negative impact onthe quality of the struvite crystal. Further study is needed.

Conclusions

Overall, microwave digestion, with and without advancedoxidation, yielded more soluble phosphates in the solution.An increase of soluble phosphate was also observed by us-ing an addition of hydrogen peroxide alone, even with-out microwave digestion. NH4-N releases were similar forboth microwave experiments, with or without hydrogen per-oxides. The MW/H2O2-AOP conducted at temperaturesabove 120◦C resulted in reduced total COD concentrations.While soluble COD remained nearly the same, there wasup to ∼36% decrease in the total COD of manure at higherheating temperatures. Heating to 120 and 170◦C also re-sulted in the breakdown of longer chain fatty acids intoshorter chain fatty acids. Up to 90% of the PO4-P wasremoved from the solution for struvite formation experi-ment. A decrease in COD of the supernatant was also ob-served. By combining peroxide addition with microwavetreatment, these efficiencies were further enhanced, e.g.,there was nearly a 36% decrease in the t-COD value.

The MW/H2O2-AOP can be a promising process forincreasing the efficiencies of both anaerobic digestion(through production of smaller chain VFAs) and nutrient

recovery, when applied to dairy manure. It would be use-ful to have the MW/H2O2-AOP followed by an anaerobicdigestion and a struvite recovery processes to reduce thepollution loading from animal waste, with more recoveryof organic content and nutrients.

Acknowledgments

The authors wish to acknowledge research funding bythe Natural Science and Engineering Research Council(NSERC) of Canada.

References

[1] Liao, P.H.; Wong, W.T.; Lo, K.V. Release of phosphorus from sewagesludge using microwave technology. Journal of Environmental En-gineering and Science 2005, 4, 77–81.

[2] Barnett, G.M. Phosphorus forms in animal manure. BioresourceTechnology 1994, 49, 139–147.

[3] Wong, W.T.; Chan, W.I.; Liao, P.H.; Lo, K.V. A hydrogen peroxide/microwave advanced oxidation process for sewage sludge treatment.Journal of Environmental Science and Health, Part A, 2006, 41,2623–2633.

[4] Pan, S.; Lo, K.V.; Liao, P.H.; Schreier, H. Microwave pre-treatmentfor enhancement of phosphorus release from dairy manure. Journalof Environmental Science and Health, Part B, 2006, 41, 451–458.

[5] Am. Public Health Assoc., Standard Methods for the Examinationof Water and Wastewater; 20th Ed. Am. Public Health Association,Washington, D.C. 1998.

[6] Chan, W.T.; Wong, W.T.; Liao, P.H.; Lo, K.V. Sewage sludge nutrientsolubilisation using a single stage microwave treatment. J. Environ-mental Science and Health, Part A, 2006, 42(1), 59–63.

[7] Wong, W.T.; Lo, K.V.; Liao, P.H. Factors affecting nutrient solubil-isation from sewage sludge using microwave-enhanced microwaveadvanced oxidation. Journal of Environmental Science and Health,PartA, 2007, 42 (6), 825–829.

[8] Liao, P.H.; Lo, K.V.; Chan, W.I.; Wong, W.T. Sludge reduction andvolatile fatty acid recovery using microwave advanced oxidation pro-cess. J. Environmental Science and Health, Part A, 2007, 42 (5),633–639.

[9] Shanableh, A.; Shimizu, Y. Treatment of sewage using hydrother-mal oxidation technology application challenge. Water Science andTechnology 2000, 41, 85–92.

[10] Schulze-Rettmer, R. The simultaneous chemical precipitation ofammonia and phosphate in the form of magnesium-ammonium-phosphate, Water Science and Technology 1991, 23, 659–667.

[11] Bouropoulos N.; Koutsoukos P.G. Spontaneous precipitation ofstruvite from aqueous solutions. J. Crystal Growth 2000, 213, 381–388.

[12] Schuiling, R.D.; Andrade, A. Recovery of struvite from calf manure.Journal of Environmental Technology 1999, 20, 765–768.

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an F

ranc

isco

] at

03:

21 0

5 O

ctob

er 2

014