Water Adaptation Management and Quality Initiative

Project 26, Final report

Advanced Oxidation Processes for Treatment of Organics in

Recirculated Greenhouse Nutrient Feedwater

a study conducted for

Gryphon Automation, Leamington, ON

Ontario Greenhouse Vegetable Growers, Leamington, ON

OMAF & MRA, Harrow, ON

by

Wei Feng, Ph.D.a, Wenhao Chen B.Sc.b, and Keith Taylor Ph.D.c

under the auspices of

Farm and Food Care Ontario

February 11, 2015

a postdoctoral fellow, Department of Chemistry and Biochemistry, bM.Sc. candidate, Department of Chemistry and Biochemistry, c author for correspondence and Professor, Department of Chemistry and Biochemistry,

University of Windsor, Windsor, ON N9B 3P4. Tel. 519-253-3000, x5031; email:

taylor@uwindsor.ca

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Advanced Oxidation Processes for Treatment of Organics in Recirculated

Greenhouse Nutrient Feedwater

Background

A significant facet of sustainability in the hydroponic greenhouse industry is tied to recycling of

the nutrient feedwater. This water is presented continuously to the roots in a nutrient-rich

formulation and is displaced from the root matrix material in a depleted form. To be recycled,

this feedwater (about 25% of the original volume supplied) must be made back up in beneficial

nutrients, but it must not be allowed to accumulate chemical species which adversely affect plant

growth. Such species could be inorganic ions, such as sodium, chloride and sulfate which

accumulate to become ‘limiters’, or certain organic compounds which either have been applied

to promote crop health or secreted by the roots as plant growth modulators (the classical example

is the auxin, indole acetic acid; others are organic acids and carbohydrates). Several phenolic

acids, such as vanillic acid (VA), caffeic acid (CA) and ferulic acid (FA) have been identified

from the nutrient water in hydroponic culture of tomato by GC/MS [1, 2]. On the other hand,

fungicides, such as Previcur N (propamocarb hydrochloride), are commonly used in greenhouse

pepper, tomato and cucumber culture in order to control pythium root diseases. It is toxic to fish,

aquatic invertebrates, and marine/estuarine organisms if runoff from greenhouses gets to the

lakes. To extend water recirculation in the greenhouse industry, it is important to know the fate

of those compounds in the recirculated greenhouse water; thereby, the efficiency of both water

and nutrient use can be increased.

Advanced oxidation processes

A combined UV-O3 disinfection treatment unit was recently developed by Gryphon Automation

(Leamington, ON) and it has been widely applied in local greenhouse farms. The design uses UV

treatment as the primary disinfection, and ozone treatment as the polishing step with an extended

contact time. Besides disinfection, the dissolved oxygen concentration was increased in the

nutrient water due to the injection of ozone, which is beneficial to plant growth. The current

treatment is not capable of destroying small organic compounds such as plant growth modulators

with any appreciable efficiency, but with minor reconfiguration, it can be converted to various

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advanced oxidation processes (AOPs), which are much more aggressive, having been shown in

hundreds of studies to effectively decompose organic molecules, eventually to CO2 (dubbed

‘mineralization’) if the treatment is prolonged sufficiently. The AOP generates hydroxyl radicals

in situ, which are highly reactive with an oxidation potential of 2.8 V. If ozone can be injected

into water prior to the UV treatment, dissolved ozone absorbs UV radiation and produces H2O2

as an intermediate, which consecutively reacts with UV to form hydroxyl radicals. The two-step

reaction is shown below:

O3 + H2O + hv (λ=254 nm) → H2O2 + O2 (1)

H2O2 + hv →2 HO∙ (2)

Also, hydrogen peroxide is a common source of hydroxyl radical in AOPs. By the addition of

H2O2, the treatment can be converted to UV/H2O2, O3/H2O2 or O3/ UV/H2O2. The reaction of

O3/H2O2 is shown below:

2 O3 + H2O2 → 2 HO∙ + 3 O2 (3)

Objectives

The objectives for this study were:

1. Design and build a pilot-scale unit based on the parameters of Gryphon’s UV-O3

treatment unit;

2. Examine the formation of hydroxyl radicals under various AOPs (UV/H2O2, O3/UV, O3/

UV/H2O2, O3/H2O2) using the pilot unit;

3. Determine the hydroxyl radicals’ scavenging effect in greenhouse water;

4. Investigate the effect of various AOPs on the removal of total organic compounds in

greenhouse water;

5. Determine the concentrations of plant growth modulators and residual fungicides in

greenhouse water;

6. Investigate the effect of various AOPs on the removal of plant growth modulators and

fungicides in greenhouse water.

Pilot unit design

A pilot-scale unit was built based on Gryphon’s UV-O3 treatment unit to test the proposed AOPs,

see Figures 1 and 2. The flow rate was 3 L/min, at which the applied UV dosage was 230 mJ/cm2

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at 95% UVT (254 nm), which is commonly used in greenhouses for water disinfection. For

treatments containing ozone, pure oxygen was the feed gas to the ozone generator at 3 psi, and

air flow rate was controlled at 1 L/min. Fine air bubbles were created when water passed through

a venturi injector, which significantly enhanced the mass transfer efficiency of ozone dissolving

into water. Excess air and undissolved ozone were separated from the water stream to minimize

the interference with UV irradiation. Ozone concentrations in feed and vent lines were measured

by the ozone analyzer; dissolved ozone concentration in water was measured by a colorimetric

method using indigo trisulfonate reagent [3]. The procedure was developed based on the

Standard Methods: 4500-Ozone B: indigo colorimetric method [4]. Hydrogen peroxide was

added to the feed tank at 2 mM (68 mg/L) if used.

Figure 1 Schematic of the pilot unit

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Figure 2 Picture of the pilot unit

Greenhouse water

Source waters were collected by Gryphon from two local greenhouse farms (Leamington, ON),

which grow peppers and cucumbers, between July and December 2014. UV absorbance at 254

nm, total organic carbon (TOC) and pH were measured after collection; see Table 1. Samples

were stored at room temperature.

Table 1 Summary of greenhouse water characterization

Plant TOC (mg/L) pH UV A254 %UVT 254*

Green pepper 26.2 6.3 0.57 26.9

Cucumber 9.8 4.6 0.46 34.7

*UVT = UV Transmittance

Hydroxyl radical formation

AOPs rely on in situ generation of hydroxyl radicals. p-Chlorobenzoic acid (pCBA) was selected

as a probe compound, since the disappearance of this compound can be used as an indirect

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measurement of the hydroxyl radical formation [5], since it reacts much more rapidly with

hydroxyl radicals than directly with ozone or H2O2 or UV irradiation, therefore, the interference

from a secondary oxidant is minimized. Various experiments were carried out using different

combinations of 230 mJ/cm2 UV dose, 2 mM H2O2 and 3.9 g/h of ozone, with pCBA added to a

feed tank and the flow recirculated during the treatment. As shown in Figure 3, all four processes

can effectively remove pCBA indicating the presence of hydroxyl radicals during the reaction.

The use of O3 and H2O2 together provided an additional source for generation of hydroxyl

radicals compared with other AOPs, therefore the O3/ UV/H2O2 process gave the highest

removal rate for pCBA. Also, first-order rate constants and half-lives for each AOP in Figure 3

are given in Table 2.

Figure 3 Removal of pCBA under various AOPs

Table 2 First order rate constants and half-lives of pCBA under various AOPs

UV UV/H2O2 O3/UV O3/UV/H2O2 O3/H2O2

k (1/ min) 0.0159 0.304 0.252 0.326 0.306

T1/2 (min) 43.6 2.3 2.7 2.1 2.3

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Greenhouse water ozone demand

Initial ozone demand of greenhouse water was determined by mixing 15 mL of greenhouse water

with 10 mL of ozonated water. The ozonated water was prepared by bubbling ozone into water

with 100 mM acetic acid (pH = 4) to increase the stability of the stock solution [3]. As shown in

Figure 4, half of the ozone demand was taken within 30 seconds of contact time. The pilot unit

has a delay of ~30 s before dissolved ozone reaches the UV reactor at 3 L/min flow; therefore,

sufficient ozone needs to be provided to overcome the demand.

Figure 4 Greenhouse water ozone demand

Hydroxyl radical scavenging effect

One of the major factors affecting AOPs’ performance is the percentage of hydroxyl radicals

scavenged by the background water matrix. The presence of carbonate and bicarbonate in

greenhouse water could significantly reduce the mineralization of organics. The reactions are

shown below,

CO32− + HO ∙ → 𝐶𝑂3

− ∙ + 𝑂𝐻−

𝐻𝐶𝑂3− + HO ∙ → CO3

2− + 𝐻2𝑂

To determine the scavenging effect, pCBA was added to greenhouse pepper water and treated by

the O3/UV process with a recirculated flow, shown in Figure 5. Under the same operating

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conditions, there was 43% less removal of pCBA in pepper water than in tap water after 16

minutes of reaction. First-order analysis showed the presence of carbonates reduced the removal

rate of pCBA by 5-fold, thereby, the half-life time was increased by 5-fold. Thus, scavenging of

hydroxyl radicals is clearly occurring.

Figure 5 Hydroxyl radical scavenging effect by greenhouse water

Treatment of greenhouse water by AOPs

Cucumber water was treated by UV/H2O2, O3/H2O2, O3/UV, and O3/ UV/H2O2 to determine the

overall removal of organic carbon under real application conditions (no recirculation). Samples

were taken before and after the treatment for TOC analysis. Hydrogen peroxide was added to the

feed tank before the treatment at 2 mM, and the applied ozone dose was ~2.7 g/h. The results are

shown in Table 3. The highest percentage of TOC removal was given by the UV/H2O2 process.

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Table 3 Removal of organic carbon from cucumber water by various AOPs

AOPs UV/H2O2 O3/UV O3/H2O2 O3/ UV/H2O2

% TOC

removal 18.2 6.9 2.2 13.8

On the other hand, recirculation increases UV exposure and ozone dose to the water which can

lead to a higher percentage of mineralization. By applying the O3/UV process to pepper water

with a 30 min recirculation, the removal of TOC was increased to 30%, see Figure 6. Because

organic compounds in greenhouse water strongly absorb UV light, the UV irradiation available

for absorption by ozone is reduced. For the O3/UV process, mineralization of small molecules,

such as glyoxal, glyoxylic acid, oxalic acid and formic acid, is expected [6]. Although phenolic

compounds can be easily oxidized by ozone, it is difficult to reach complete mineralization by

O3/UV [7]. However, the treatment increased the UVT of pepper water by almost 30% (Figure

6), which would reduce the required UV dose for disinfection by 2-fold. Thus, less energy would

be required for UV disinfection. A full-scale study on integrated ozone and UV treatment for

water disinfection in a sewage treatment plant showed that a 10% UVT increase, reduced the

cost of UV equipment needed and power requirements by 40% or more; the ozone pre-treatment

also reduced the formation of disinfection by-products [8].

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Figure 6 Change of TOC and UV transmittance with recirculation of greenhouse pepper water

using O3/UV.

Removal of phenolic acids and Previcur N in synthetic wastewater

Vanillic acid (VA), caffeic acid (CA), p-coumaric acid (pCA) and ferulic acid (FA) were

selected as model compounds for plant growth modulators. Including Previcur N, the five

selected compounds were added to tap water in a 9 L feed tank and run with a recirculated flow

under different AOPs. HPLC methods were developed for measuring the target compounds,

Table 4.

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Table 4 HPLC conditions for measuring target compounds in water

Compounds Mobile phase solvents

Ratio of

mobile

phase

Injection

volume (µL)

T

(℃)

Wavelength

(nm)

Pump A Pump B Pump A/B

VA, CA, pCA,

FA* Acetonitrile

0.1%

Formic

acid

20/80 10 40 254, 310

Previcur N** Acetonitrile

5 mM

Trisodium

phosphate

30/70 10 20 210

*Waters symmetry C18 reverse phase column **Waters Xbridge column.

The initial concentrations of the four phenolic acids and Previcur N were 10 mg/L each and 100

mg/L, respectively. Samples were taken from the feed tank at 3-minute intervals during the

experiment. As shown in Figure 7, with first-order analysis parameters in Table 5, partial

removal of all five compounds was achieved by direct UV irradiation (no ozone or hydrogen

peroxide present), and the removals were slightly increased when H2O2 was added with UV. In

fact, hydrogen peroxide at the concentration level used (2 mM) absorbs only a small fraction of

the available UV due to its low extinction coefficient at 254 nm. Batch reactor study was

conducted to measure the H2O2 decomposition rate under UV irradiation with the pilot UV

reactor, the result was 0.05 mM/min. Within less than 30 seconds’ contact time in the UV

reactor, the hydroxyl radicals generated were insufficient to react with the target compounds.

Increasing H2O2 concentration and UV dose could increase the efficiency of UV/H2O2, but the

residual H2O2 and energy input would not be desirable for greenhouse farm operation. For AOPs

involving ozone, all compounds were effectively removed in less than 20 min. It is known that

ozone reacts with parent phenol and selectively with o-, m- and p- substituted phenolic

compounds, therefore the direct reaction between target compounds and ozone could be

dominant [9]. Also, the cleavage of the carbon-carbon double bond in CA, pCA and FA is likely

the major initial reaction in the removal of those compounds.

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Figure 7 Removal of target compounds under various AOPs.

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Table 5 First-order rate constants and half-lives of target compounds under various AOPs

k (T1/2)

1/min (min)

Vanillic

acid Caffeic acid p-Coumaric acid Ferulic acid Previcur N

UV 0.010(69) 0.013(53) < 0.027* <0.012* <0.0037*

UV/H2O2 0.0095(73) 0.046(15) 0.027(26) 0.012(58) 0.0037(187)

O3/UV 0.21(3.3) 0.25(2.8) 0.26 (2.7) 0.25(2.8) 0.048(14)

O3/UV/H2O2 0.24(2.9) 0.21(3.3) 0.28(2.5) 0.28(2.5) 0.056(12)

O3/H2O2 0.23(3.0) 0.28(2.5) 0.27(2.6) 0.29(2.4) 0.053(13)

* these entries could not be fitted, values given are by comparison with UV/H2O2 below them

Phenolic acids and Previcur N in greenhouse water

The phenolic acids in greenhouse water elsewhere were reported to be in the range of 0.3 – 1.6

µg/L [1]. To quantify those compounds in greenhouse water studied here, a solid-phase

extraction (SPE) procedure for the isolation and pre-concentration of the target compounds was

developed. A Waters Oasis HLB (6 mL, 200 mg) column was installed on a vacuum manifold

and preconditioned with 5 mL of methanol and 5 mL of water. Sample pH was adjusted to 3 for

the isolation. After pre-conditioning, 300 mL of sample was loaded to the column at a rate of 10

mL/min, and then the column was allowed to dry for 5 minutes with vacuum. A pre-elution with

5 mL of 50% methanol at 2 mL/min was conducted to remove interfering compounds, and then

the target compounds were eluted with 2 mL methanol at 1 mL/min. The eluate was evaporated

to dryness under a slow air stream in a 40 ℃ water bath. The dry residue was reconstituted to 1

mL with methanol. The recovery rates of VA, CA, pCA and FA, at 10 µg/L, were 88.2%, 102%,

109% and 106%, respectively. With 300-fold concentration, the limit of quantification for all

four compounds by HPLC/UV was 0.33 µg/L. By applying the above method on greenhouse

pepper water, the HPLC/UV chromatogram in Figure 8 (b) was obtained.

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Similarly, residual concentration of Previcur N was measured by concentrating cucumber water

150-fold using a similar procedure as above. However, sample pH was adjusted to 11.5 because

of the nature of the compound. The addition of sodium hydroxide caused precipitation of metal

ions from the solution. Therefore, a control test was conducted to examine the effect on Previcur

N recovery with hydroxide precipitation and filtration using a 0.45 µm membrane filter. Overall,

the recovery of Previcur N at 5 mg/L was determined to be 105%. With 150-fold concentration,

the limit of quantification of Previcur N by HPLC/UV was 64 µg/L.

As shown in Figure 8 (a) for Previcur N, (b) for the phenolic acids, no significant peaks were

detected at the desired retention times for the five compounds, which indicated that the residual

concentrations, if any, were lower than the detection limit. However, seasonal variation in

greenhouse operation should also be taken into consideration. The greenhouse water samples

were collected in mid-December when there was not sufficient sunshine for the growth of the

plant. To further confirm the presence of all target compounds, more samples are required in

different seasons.

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Figure 8 HPLC chromatograms of 4 phenolic compounds and Previcur N in greenhouse water.

Treatment of phenolic acids and Previcur N in greenhouse water by O3/UV

In greenhouse operation, untreated and treated water are commonly stored in two reservoirs.

Water pumps from one reservoir through to the disinfection unit to another reservoir with a

limited contact time. In order to determine the removal of trace level plant growth modulators

and residual fungicide by AOP under the above conditions, the four phenolic acids (10 µg/L) and

Previcur N(2 mg/L) were spiked into greenhouse water, and two separate runs were conducted.

The O3/UV process was selected since it is the most convenient combination for Gryphon’s

current set up in local greenhouse farms. Samples were taken before and after the treatment and

concentrated by SPE. Dissolved ozone concentrations measured from sampling port S1 were

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0.32 and 0.42 mg/L for the two runs; and there was no residual ozone measured at sampling port

S2. As shown in Table 6, four phenolic compounds have reached >90 % removal, the remaining

concentrations were lower than 1 µg/L. Thus, the greenhouse water ‘matrix’ has no detrimental

effect on the AOP’s ability to remove these phenolics; this statement includes the hydroxyl-

radical scavenging effect of carbonates in the water, discussed earlier in this report. In contrast,

the removal of Previcur N was relatively insignificant, which could be beneficial for the

operation, since it is a fungicide added to promote plant health. However, more experiments are

required to determine optimum residual concentrations for various classes of compounds as a

function of recirculation of the water.

Table 6 Treatment of phenolic acids and Previcur N in greenhouse water*

Water source Compounds Before (µg/L) After (µg/L) % Removal

Pepper

Vanillic acid 10.25 0.48 95.3

Caffeic acid 8.27 0.67 91.8

p-Coumaric acid 8.19 0.62 92.4

Ferulic acid 7.06 0.60 91.5

Cucumber Previcur N 2010 1695 15

* Single-pass treatment

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Summary

Experiments on greenhouse pepper and cucumber water found that due to the presence of a high

concentration of organic carbon, those samples have a high UV absorbance at 254 nm, hence a

low UV transmittance. Attempts on the removal of the organic carbon were conducted by

various AOPs at pilot-scale, only partial removals were achieved under the limited contact time

of single-pass operation. However, by increasing contact time through recirculation of the water

with the O3/UV process, the removal of TOC was increased to 27% with a 28% increase in UVT.

Furthermore, four phenolic acids and Previcur N were selected as model compounds of plant

growth modulators and crop improvement products found in the greenhouse water. Successful

treatment of the phenolic acids was achieved in the greenhouse water matrix, over 90% removal

of them at trace levels was achieved using the O3/UV process, while the removal of Previcur N

was relatively insignificant.

Recommendations

Based on the understanding of greenhouse water gained in this study, it is believed that an

integrated ozone and UV process would be ideal for greenhouse industry. Although, with limited

time and resources, only a preliminary level of work has been completed, solid results suggest,

firstly, that pre-ozonation dramatically improves UVT in greenhouse water, therefore, it would

reduce the size of the required UV system, hence lowering capital and maintenance costs.

Secondly, ozone as a strong oxidant can easily react with many organic and inorganic

compounds, such as metals, alkenes, alkynes and phenolic acids. The half-lives of the selected

model compounds were around three minutes when ozone was used. Thirdly, it is already known

that the dissolved oxygen concentration is increased in the nutrient water due to the injection of

ozone, which helps with the plant growth. Fourthly, the residual ozone reacts with UV irradiation

to form hydroxyl radicals to further react with the remaining organic compounds. Also, it avoids

the exposure of workers from the toxic gas. Future work should first determine the relationship

between ozone dose and increase of UVT of greenhouse water. By conducting a disinfection test,

various combinations of ozone dose and UV dose can be determined. Furthermore, the capital

and operating costs can be determined for the selected combinations, to establish a baseline for

future work. The second stage would be to quantify the effect of the O3/UV process on the

removal of organic carbon in general and specifically on more plant growth modulators and crop

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improvement products than the five model compounds studied here. The effect of the O3/UV

process would be increased with more ozone provided, however, the balance between cost and

efficiency needs to be determined.

Acknowledgements

Don Murney and Nelson Rafael of Gryphon Automation are thanked for their participation in

this project as the industrial partner. Dr. Justine Taylor, Energy and Environment Coordinator

with OGVG, and Shalin Khosla, Greenhouse Vegetable Specialist with OMAF & MRA, are

thanked for valuable contributions in the formulation and execution of the project.

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