Closing the water cycle in the agro-industrial sector by reusing treated wastewater for irrigation

Accepted Manuscript Closing the water cycle in the agro-industrial sector by reusing treated wastewater for irrigation

Pompilio Vergine, Carlo Salerno, Angela Libutti, Luciano Beneduce, Giuseppe Gatta, Giovanni Berardi, Alfieri Pollice PII:

S0959-6526(17)31408-7

DOI:

10.1016/j.jclepro.2017.06.239

Reference:

JCLP 9989

To appear in:

Journal of Cleaner Production

Received Date:

24 March 2017

Revised Date:

26 June 2017

Accepted Date:

29 June 2017

Please cite this article as: Pompilio Vergine, Carlo Salerno, Angela Libutti, Luciano Beneduce, Giuseppe Gatta, Giovanni Berardi, Alfieri Pollice, Closing the water cycle in the agro-industrial sector by reusing treated wastewater for irrigation, Journal of Cleaner Production (2017), doi: 10.1016/j.jclepro.2017.06.239

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Closing the water cycle in the agro-industrial sector by reusing treated wastewater for

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irrigation

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Pompilio Verginea, Carlo Salernoa, Angela Libuttib, Luciano Beneduceb, Giuseppe Gattab,

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Giovanni Berardia, Alfieri Pollicea

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aIRSA

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bDepartment

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Napoli, 25 - 71121 Foggia Italy

CNR, Viale F. De Blasio, 5 - 70132 Bari, Italy of Science of Agriculture, Food and Environment, University of Foggia, via

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email addresses:

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Pompilio Vergine, [email protected]

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Carlo Salerno, [email protected]

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Angela Libutti, [email protected]

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Luciano Beneduce, [email protected]

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Giuseppe Gatta, [email protected]

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Giovanni Berardi, [email protected]

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Alfieri Pollice, [email protected] (corresponding author)

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Abstract

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Reuse of treated wastewater for crop irrigation can contribute to mitigate water stress,

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especially in Mediterranean countries. The use of reclaimed municipal wastewater for this

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purpose was demonstrated by numerous studies and full-scale installations. On the other hand,

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reuse of industrial effluents in irrigation is uncommon and the knowledge in this field is limited.

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This work aims at assessing the suitability of agro-industrial effluent reuse for irrigation. In the

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case study presented, a full-scale tertiary treatment based on membrane ultrafiltration and UV

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disinfection was tested at an agro-industrial site in Apulia (Italy). The wastewater treatment

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plant processed the stream produced at a vegetable canning factory, and the treated effluents

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were used for field scale irrigation tests. The variability of wastewater quality and its effects on

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treatment process performances and reclaimed water quality were investigated. An economic

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evaluation of the full scale tertiary treatment was also performed. The results showed that the

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adopted technologies effectively removed suspended solids and the faecal indicator Escherichia

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coli below the local standards for reuse in irrigation. Furthermore, the use of treated agro-

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industrial wastewater had no inhibitory effects on the growth of tomato and broccoli, neither

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resulted in any faecal contamination of crops. In general, the present study shows that reuse of

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treated wastewater for irrigation is a suitable practice to close the water cycle in the agro-

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industrial sector. This is very important in areas where the sustainability of agriculture and

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transformation activities depends on the water available for irrigation. This practice also avoids

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the discharge of pollutants into water bodies, reducing the environmental impacts of agro-

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industrial productions.

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Keywords: agri-food industry; crops irrigation; membrane ultrafiltration; water reuse.

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1. Introduction

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Reuse of treated municipal wastewater is an established practice in many countries, and also

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untreated streams are commonly used in several developing countries lacking collection and

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sanitation services (Capra and Scicolone, 2007). The increasing needs for food and irrigation

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water due to the expanding world population make reuse crucial. Worldwide, about 7.1 billion

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m3/year (5% of treated wastewater and 0.18% of water consumption) are reused mainly for

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irrigation (about 50%) and industrial purposes (about 20%) (GWI, 2009). These figures show

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that despite the increased environmental awareness and the understanding that water is a limited

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resource in many regions of the planet, relevant improvements are still needed to achieve

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sustainable water utilization practices. These aspects were highlighted in the definition of the

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Sustainable Development Goals, and specifically in SDG 6 where water reuse is clearly

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included as a practice requiring specific attention (UN, 2015).

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Wastewater treatment and reuse offer important environmental and economic advantages. As

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half of the global water bodies are seriously contaminated, wastewater treatment and reuse

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promote environmental security by alleviating the pollution of freshwater resources, while

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providing more water for irrigation (Corcoran et al., 2010). The advantages of this practice are

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twofold. First, it represents a continuous and stable supply especially during peak water demand

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periods. Furthermore it also allows the recovery of nutrients, resulting in a reduction of

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chemical fertilizer inputs, contributing in the medium term to decrease the nutrient

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concentrations in natural water bodies (Tran et al., 2016).

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Most full and demo scale activities on treated wastewater reuse in irrigation are based on the

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adoption of municipal wastewater as a source (Pedrero et al., 2010). In this case, the approach

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is normally based on the upgrade of existing wastewater treatment plants (WWTP) with the

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introduction of tertiary treatments. Several pilot studies and full scale installations have shown

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that a number of different technologies are suitable for producing reclaimed municipal effluents

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complying with the standards for reuse (Norton-Brandão et al., 2013). In particular, membrane

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filtration followed by disinfection was shown to be a reliable and effective technology for this

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purpose (Pollice et al., 2004).

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Reuse of treated industrial wastewater in irrigation is rarely adopted, due to the potential

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hazard of non-biodegradable compounds that may be present in these streams, depending on

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their origin. However, treated agro-industrial effluents may be considered for reuse, due to

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relatively steady composition (depending on the industrial processes) and more limited

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microbiological contamination with respect to municipal sewage (Isosaari et al., 2010). This

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practice has some clear advantages when vegetable processing companies also grow the crops at

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the same site. In this case, the quality of wastewater resulting from the industrial processes is

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normally known, and can be partially controlled. Moreover, custom tailored reuse practices can

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promote nutrient recovery according to specific crop needs (e.g. phenological stages).

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From a regulatory standpoint, the European Commission has recently approved new

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guidelines aimed at the harmonization of water reuse practices across EU countries (European

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Commission, 2016). A technical document currently under review by the European Commission

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(that will possibly serve as a basis for a new directive on treated wastewater reuse) includes

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agro-industrial wastewater as a possible source, as specified in Annex III of the Directive

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91/271/EEC (European Commission, 1991).

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Recovery of agro-industrial effluents becomes especially relevant in Southern European

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countries, where the economy is strongly based on irrigated agriculture. In these areas the water

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requirements for the agro-industrial sector account for up to 80% of the total water needs, and

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crops cultivation is often carried out under water deficiency or with unsustainable exploitation

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of water resources (EEA, 2012). Apulia (South-Eastern Italy), is one of the European regions

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most heavily affected by water shortage (Xiloyannis et al., 2002). Here, groundwater resources

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were overexploited during the past decades to fulfil the high water requirements of agriculture.

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This has caused a progressive groundwater salinization due to seawater intrusion into the water

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table (Polemio, 2016). Therefore the assessment of non-conventional water sources has become

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critical for alleviating the stress on natural resources, sustain agriculture and contribute to the

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overall regional development. In this context reuse of agro-industrial effluents may play a

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relevant role.

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This work reports some results of a long term demonstration activity aimed at assessing the

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suitability of reusing treated agro-industrial effluents for the irrigation of horticultures. A full-

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scale tertiary treatment based on membrane ultrafiltration and UV disinfection was tested at an

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agro-industrial site in Apulia. The variability of wastewater quality and its effects on treatment

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process performances and reclaimed water quality were investigated. Costs and savings, in

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terms of water and recovered nutrients, were also estimated. Finally, the fate of faecal pollution

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indicators and the possible salt accumulation in soil were studied in order to provide a

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comprehensive review of opportunities and drawbacks involved in this reuse practice. The

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results provide a basis for further investigations on treated agro-industrial wastewater reuse in

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regions heavily affected by water scarcity. The possibility of closing the water cycle in this

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water-intensive industrial sector has the double advantage of limiting the related water stress

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(and overall environmental impacts), and supporting irrigated agriculture.

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2. Materials and methods

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2.1 Site description

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The demonstration activities were carried out within the premises of Fiordelisi s.r.l., an agro-

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food company located in Apulia (Southern Italy). The company’s business includes growing,

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processing, packaging, and marketing preserved ready-to-eat horticultures (tomato, eggplant,

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zucchini, pepper, broccoli, etc.). The main products of Fiordelisi are oil preserves and dried

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vegetables.

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All the water used by Fiordelisi is pumped from groundwater, with a limited availability for

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both industrial processes (about 15 m3/h) and irrigation (about 70 m3/h). The wastewater

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produced in the factory is mainly originated by vegetable processing, cleaning of floors and

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machinery, and a small fraction from the toilets (5-10 %). Before upgrading the plant for reuse,

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wastewater was treated through a dedicated WWTP, and the effluent was discharged into a

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small water body nearby (canal). The WWTP was based on a conventional activated sludge

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process arranged according to a pre-denitrification design. Aim of this scheme was to remove

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mainly the organic pollution (measured as Chemical Oxygen Demand, COD) and the nutrients

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(nitrogen and phosphorus) to concentrations complying with the local discharge standards. In

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recent years, due to growing industrial production, the water consumption for processing and

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crop irrigation significantly increased. During the warm season the irrigation requirements

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reached a value close to the maximum flow rate available at the well. Therefore, in 2012 the

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company decided to consider the possibility of reusing part of the reclaimed wastewater for

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irrigating its own fields. For this purpose, a full scale tertiary treatment system was

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commissioned and built, and open field irrigation tests were carried out. Figure 1 reports a

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scheme of the WWTP and its integration with the production factory upstream and the test field

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downstream.

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The results presented here describe the WWTP operation for a period of approximately 18

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months (October 2014 - March 2016) and the field tests carried out for the cultivation of two

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crops in succession, tomato (spring-summer 2015) and broccoli (fall-winter 2015-2016).

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2.2 Wastewater treatment plant Figure 1 shows the three main steps of Fiordelisi’s WWTP: 

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Primary treatments: screening (0.5 mm); oil and grease removal by gravity separation; equalization (270 m3); pH adjustment, by dosing sodium hydroxide.



Secondary treatments: activated sludge process, composed of anoxic (130 m3) and

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aerobic (520 m3) tanks (in Figure 1, the anoxic tank is the zone of the activated sludge

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process where no air is supplied); secondary sedimentation, chemically assisted by

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adding aluminium poly-chloride.

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

Tertiary treatments: sand filtration, membrane ultrafiltration, and UV radiation.

The volumes of produced wastewater fluctuated considerably during the day according to the

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different production processes carried out at the factory. As a result, despite the equalization

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tank, the influent flow rate to the secondary treatments varied between 0 and 30 m3/h. The

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filtration unit, composed of a sand filter and 8 modules of ultrafiltration membranes, was

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designed for a flow rate of 12 m3/h. The secondary settled effluent exceeding this maximum

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flow was discharged.

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The pressurized sand filter had a surface of 2.1 m2 and the volume of the filtering material was

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1.4 m3. The ultrafiltration membrane modules were Kristal 600ER (Hyflux), composed of

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hollow fiber polyethersulfone membranes with a nominal pore size of 0.05 µm and a surface of

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60 m2 per module. The membranes were operated in the out-in cross-flow recirculation mode.

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Periodical backwashing of both the sand filter (15 min duration every 8 h of operation) and the

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membrane modules (30 sec duration every 45 min) was performed using the membrane

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permeate. The membranes were also periodically cleaned using chemical agents, according to

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the procedures suggested by the manufacturer: during 45 min, clean water at 40°C added with

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NaOH up to pH 11; during the next 15 min, addition of 100 mg NaClO/L.

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“On demand” UV disinfection was operated in-line with irrigation, i.e. the UV system was

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switched on simultaneously with the irrigation pumps, in order to save energy and comply with

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the temporary effects of this specific type of disinfection (DNA repair, Poepping et al. 2014).

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The mercury-vapor lamps (6 lamps, 200 W each) provided a UV dose ranging between 110 and

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150 Wh/m3, depending on the flow rate used for irrigation.

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Figure 1. The water reuse scheme, including wastewater production, treatment processes, and

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test field arrangement. Tertiary treatments are in a grey background, and sampling points are

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identified.

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2.3 Field tests

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Irrigation tests were performed in an open field of about 5000 m2, located within the premises

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of Fiordelisi. Tomato and broccoli were cultivated under a structure covered with an anti-hail

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net. The cultivation periods were April-September and October-March for tomato and broccoli,

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respectively.

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Three types of water were used for crops irrigation: two types of reclaimed wastewater, with

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different degrees of treatment, and the well water conventionally used by Fiordelisi. They will

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be referred to as SW (secondary treated wastewater), TW (tertiary treated wastewater), and GW

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(groundwater). Figure 1 shows the treatment processes that each reclaimed wastewater

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underwent and the scheme of the test field, which was arranged according to a complete

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randomized block design, with three replications (plots) for each water source. Before being

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used for irrigation, all the three water sources were stored into 10 m3 tanks for a period between

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1 and 7 days, depending on the irrigation needs. On-demand UV disinfection was operated upon

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irrigation with the TW.

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Drip irrigation was used for both crops, with a single drip line placed between each couple of

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plants rows. During the tomato cycle, the drip lines were placed under a black plastic mulching

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film. Irrigation was performed when the soil moisture in the effective root zone (0-50 cm) was

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depleted to the threshold value of 40%. At each irrigation, the soil water content was increased

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to field capacity with a water volume varying from 100 to 300 m3/ha, depending on the crop

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growth stage.

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Fertilization, pest and weed control were performed according to local management practices.

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Fertilization provided about 200 kg N/ha, 250 kg P/ha, and 150 kg K/ha to tomato and about

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100 kg N/ha, 150 kg P/ha, and 70 kg K/ha to broccoli. Pre-transplanting fertilization was

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applied to soil by supplying a solid fertilizer, composed of nitrogen and phosphorus (in both

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organic and inorganic forms). Throughout both crop cycles, a weekly fertirrigation was

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performed according to common practices, by supplying ammonium nitrate (replaced by

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ammonium sulphate in the last month of cultivation), ammonium phosphate, and potassium

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nitrate (replaced by potassium sulphate in the last month of cultivation).

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2.4 Monitoring and analyses

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The water samples identified in Figure 1 were collected monthly over the whole experimental

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period and analysed for the main physical-chemical parameters and for faecal indicators.

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Besides sampling the water sources used for irrigation (SW, TW, and GW), the following

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samples were also collected to monitor the performance of the WWTP and tertiary treatment:

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influent wastewater after equalization (WW); outlet of the sand filter (SF); membrane permeate

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(UF); membrane permeate after 1-7 days of storage in a tank (ST). SW represents the treated

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wastewater that was sent to discharge before the introduction of the tertiary treatment. Soil

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samples were taken from the high density root zone (0-30 cm) of plots irrigated with GW, SW,

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and TW by using a soil auger. They were then air-dried, passed through a 2 mm sieve and

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analysed for the main physical-chemical parameters and for faecal indicators. Soil samples were

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collected before transplanting the crops and monthly during crop cultivation. Plant samples

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were collected during each crop cycle at the same time of soil sampling, whereas tomato fruits

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and broccoli heads were collected at harvesting. Plants and edible parts of the crops were

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analysed for faecal indicators. The corresponding marketable yields were also measured.

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Electrical conductivity (EC) and pH were measured in all water and soil samples. Analyses of

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Chemical Oxygen Demand (COD), Total Suspended Solids (TSS), Total Nitrogen (TN),

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ammonia, nitrite, nitrate, Total Phosphorus (TP), potassium, anionic surfactants, and total oil

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and grease, were performed in water samples according to Standard Methods (APHA, 2005).

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Analyses of organic matter (OM) and nitrate were performed in soil samples according to

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official Italian methods (MAF, 1992). The faecal contamination indicator Escherichia coli was

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enumerated in water samples through the Colilert®-18 (IDEXX Laboratories Inc.), in soil and on

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crops through the spread plate method, executed on TBX agar (Oxoid) with an incubation at

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44°C for 24 h. Faecal coliforms were enumerated in water samples through the membrane

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filtration method, with 0.45 µm nitrocellulose membranes (Whatman), C-EC agar (Biolife), and

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incubation at 44 °C for 24 h, in soil and on crops samples through the spread plate method, with

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C-EC agar and incubation at 37°C for 48 h. Salmonella spp. were monitored in water samples

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according to ISO 19250:2013, in soil and crops samples according to ISO 6579:2002.

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The operating pressures at the inlet of the sand filter and at the inlet and outlet of the

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membrane modules were measured by pressure transducers. An electromagnetic flowmeter

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(Pmag, SGM Lektra) was used to measure the permeate flow rate produced by the membranes.

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Turbidity in the influent to the sand filter was measured through a probe (SOLITAX, Hach). All

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these data were recorded every 30 minutes. The power consumption of the filtration unit was

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measured by an electric meter.

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3. Results and discussion 3.1 Characteristics of the agro-industrial wastewater

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The overall volume of wastewater produced by Fiordelisi during the year 2015 was about

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80,000 m3. The wastewater was originated through the industrial processes and, to a smaller

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extent, by cleaning the premises and equipment, and from the toilets (Figure 2). Wastewater

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from the toilets was about 5-10% of the total volume, and enough to generate a relevant faecal

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pollution. Process water was the main source of organic and inorganic pollution. Four steps of

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the production processes generated wastewater streams: vegetable washing, acidification,

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packaging, and pasteurization (Figure 2). The flow rates and the quality characteristics of these

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streams differed considerably. Vegetable washing and bottle pasteurization were the most water

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consuming steps. A small survey was conducted to estimate the pollution levels of wastewater

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coming from the different production processes. The results of the 3 samples analysed suggested

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that the streams from vegetable washing and packaging were the most polluted. Both of them

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were acidic (pH 4-6), had a relevant organic pollution (5-6 gCOD/L), and a high, but variable,

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salinity (EC between 2 and 15 mS/cm). The packaging stream had also a relevant presence of

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oil, visible to bare eye (not measured). On the contrary, the pasteurization step produced a less

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polluted stream (pH 8-9, EC below 1 mS/cm, COD below 0.2 g/L). However, all the process

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water streams were observed to occasionally contain significant organic pollution, depending on

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the type of processed vegetables and the specific conditions applied (temperature, duration,

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amount and type of acids and additives used).

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Figure 2. Origin of the agro-industrial wastewater.

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The overall characteristics of the wastewater generated by Fiordelisi were monitored over the

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year by analysing samples collected downstream the equalization step (sampling point WW in

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Figure 1). On average, this agro-industrial wastewater was observed to have low pH, high

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salinity, and a considerable presence of surfactants and organic fractions, as reported in Figure

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3. The average concentration of total oil and grease was 28±24 mg/L (3 samples analysed),

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suggesting limited effectiveness of the existing de-greasing unit. The EC, pH, and COD had a

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significant variability, that can be related to the variable type and amount of vegetables

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processed over the year. The values of TN, TP, TSS, and faecal contamination of this agro-

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industrial wastewater were similar to those measured in municipal wastewater (Figure 3).

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3.2 Treated wastewater quality

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The results of the monitoring campaign on the different water sources are summarized in

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Figure 3, where the local limits for reuse in agriculture are also reported (Regione Puglia, 2012).

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Considering the median values, the activated sludge process removed 97 % of the COD, 91 %

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of the TN, and 94 % of the TP, and allowed the SW to comply with the standards for reuse for

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these three parameters (Figure 3a-c). The tertiary filtration unit, by retaining the residual

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biomass present in the secondary settled wastewater, provided an additional removal of

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nutrients and organic fraction.

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The secondary treatments removed most of the anionic surfactants contained in the influent

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wastewater (89 %, considering median values). Enhanced biomass sedimentation through the

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dosage of AlCl3 in the secondary settler may have contributed to achieve this high removal of

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surfactants (Aboulhassan et al., 2006). Although the tertiary filtration unit retained also another

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fraction of these compounds, the residual content of surfactants in the TW was still higher than

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the standard limits for reuse (Figure 3d). The removal of surfactants could be further increased

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either by changing the coagulation-flocculation process and improving their sorption onto

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biomass flocs, or by introducing advanced oxidation processes (Ríos et al., 2017). However, a

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more sustainable approach to meet the standard effluent limit of 0.5 mg/L would involve the

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reduction of the detergents used at the factory, that would result in a lower content of surfactants

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in the raw wastewater.

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As expected, the wastewater EC did not change significantly after the secondary and tertiary

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treatments, as neither biological processes nor micro/ultrafiltration affect the water salinity. Its

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value was often close or slightly above the limits for reuse in both SW and TW (Figure 3f). A

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specific desalination process based on reverse osmosis would guarantee the EC decrease below

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the standard limits, but it would also considerably increase the overall treatment cost. A more

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sustainable strategy to reduce the salinity of the reclaimed wastewater would be segregation of

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highly salted wastewater contributions upstream of the WWTP. As a matter of fact, the survey

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conducted to separately evaluate the single industrial streams showed EC values between 0.1

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and 15 mS/cm across the different wastewater sources (Figure 2). Therefore, segregation and

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separate treatment of smaller volumes of salty wastewater would strongly reduce the salinity of

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the remaining streams sent to reclamation and reuse.

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Figure 3. Box plots of the main reuse-related parameters in the influent wastewater (WW) and

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in the three types of water used for irrigation (SW, TW and GW). Dotted lines represent the

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standard limits for reuse.

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In terms of microbiological contamination, according to the local regulation for reuse

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Salmonella spp. must be absent and E. coli must be below 10 CFU/100mL in 80% of the

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samples and always below 100 CFU/100mL. Results of the monitoring campaign showed that

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Salmonella spp. were always absent in all the irrigation water sources, including SW. As for the

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E. coli, the secondary treatment removed about 2 logs of the initial concentration, and the

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ultrafiltration membranes removed 3 logs on average (Figure 3h). A residual presence of E. coli

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in the membrane permeate was observed, although the membrane nominal pore size was 0.05

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µm (i.e. well below the bacterial size). A careful inspection of the P&I of the ultrafiltration unit

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revealed the possibility of a slight contamination of the membranes during

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backwashing/cleaning operation, which was resolved by modifying the piping. In any case, the

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final UV disinfection placed in line with irrigation effectively removed the residual E. coli in the

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final effluent (TW), allowing compliance with the local limits for reuse. This finding suggests

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that residual bacterial contamination may be expected downstream of ultrafiltration membranes,

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due to possible integrity losses or imperfect design. Therefore the presence of a disinfection step

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before effluent supply to irrigation is always advisable. In particular, the effectiveness of

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ultrafiltration in removing turbidity makes UV disinfection especially suitable for application

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downstream of membrane processes.

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The concentration of suspended solids in SW was higher than 10 mg/L in more than 50% of

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the samples (Figure 3g). The sand filter retained a relevant fraction of the suspended solids

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(about 60%, considering median values), but compliance with the standard limit for reuse was

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achieved only in the effluent of the ultrafiltration process. However, increased suspended solid

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concentrations were observed in the tank where the membrane permeate was stored before

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irrigation. This phenomenon was associated to bacterial re-growth, as shown by the prokaryotic

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abundance measured through flow cytometry analyses (results not reported in this paper). On

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the other hand, a relevant die-off of the faecal indicator E. coli was observed in the storage tank

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(Figure 3h). The combination of bacterial re-growth and E. coli die-off was probably caused by

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the ”static” storage, meaning that when the tank was full the membrane permeate was sent

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directly to discharge without passing through the tank. In principle microbial re-growth might

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affect also groundwater, when stored for a sufficient time. This effect should be controlled by

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minimizing the storage time of all water resources to be used in irrigation. Another possible

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strategy for limiting the effects of bacterial re-growth in irrigation with reclaimed wastewater

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could be storing the secondary settled effluent, and applying the tertiary treatments “on

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demand” (i.e. upon irrigation). This strategy would take advantage of the decay of faecal

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bacteria during storage (Cirelli et al., 2009).

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3.3 Operation of the filtration unit

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The filtration unit (sand filter and membrane ultrafiltration) was operated with an influent

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pressure that ranged between 2.0 and 3.0 bar. Part of this pressure was necessary for filtration

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through the sand filter (1.0-2.0 bar), and the residual influent pressure to membranes was

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between 0.8 and 1.5 bar. Membranes were chemically cleaned when the influent pressure was

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close to 1.5 bar. The influent pressure tended to increase and the permeate flow rate to decrease

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between two cleaning events. The average permeate flow rate produced by the 8 membrane

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modules during the 18-month experimental period was 4.2±1.2 m3/h, corresponding to a flux of

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8.8±2.5 L/m2/h, i.e. less than expected for a tertiary ultrafiltration process. Overall, the

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productivity of the filtration unit was affected by the characteristics of the feed water. Figure 4

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displays the membrane flux and the influent turbidity during the first two months of operation

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after the installation of new membrane modules. At higher turbidity values (first and last week

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in Figure 4) the produced permeate flux decreased more rapidly, and more frequent chemical

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cleaning cycles were required.

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Figure 4. Influence of the feed water characteristics on the operation of the filtration unit.

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Vertical lines indicate chemical cleanings.

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The measured membrane flux was always much lower than the value indicated by the

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manufacturer (50 L/m2/h). This was probably due to heavy fouling caused by the presence of

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relevant oil concentrations in the feed water. As a matter of fact, the average concentration of

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total oil and grease in the membrane inlet water was 1.5±0.6 mg/L, against a maximum value of

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1.0 mg/L recommended by the manufacturer. This high concentration of oil reached the tertiary

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treatment due to the limited effectiveness of the primary de-greasing system (based on gravity

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separation). More efficient technologies, such as dissolved air flotation (DAF) applied to the

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secondary settled effluent, could simultaneously optimize the removal of suspended solids and

369

oil, thus improving the performance of tertiary membrane ultrafiltration.

370 371

3.4 Nutrient recovery

372

The fertilization practices adopted in this study included the supply of nitrogen, phosphorus,

373

and potassium as indicated in section 2.3. Nutrients were also given to plants through irrigation,

ACCEPTED MANUSCRIPT 374

in different quantities depending on the characteristics of the water source and the irrigation

375

volumes provided.

376

The three water sources had a very low content of TP, negligible in terms of fertilization

377

potential. The average concentration of potassium in GW was 11.6±1.5 mg/L, much lower than

378

in SW and TW (64.2±9.3 mg/L and 60.3±4.1 mg/L, respectively). On the contrary, the average

379

concentration of TN in GW was 24.3±2.5 mg/L, much higher than the concentrations in SW and

380

TW (4.2±3.6 and 2.6±2.4 mg/L, respectively).

381

Figure 5 displays the amounts of nutrients supplied to the two crops through irrigation. The

382

calculation was done by multiplying the average concentration of each compound in the

383

different water sources by the water volume supplied during the entire cultivation period. The

384

latter was approximately 1000 m3/ha for broccoli and 5000 m3/ha for tomato. The values

385

obtained are compared with the nutrient needs for the specific crops, according to local practices

386

(see section 2.3). The high water requirements for the cultivation of tomato resulted in a

387

relevant recovery of nitrogen and potassium from groundwater and treated wastewater

388

respectively. This suggests that by carefully considering the nutrient content in irrigation water

389

it is possible to limit the supply of these compounds through chemical fertilization, with

390

consequent savings. In particular, the GW has provided more than half of the nitrogen

391

requirements of tomato, while either SW or TW have supplied an amount of potassium that was

392

more than twice the requirement (Figure 5a).

393

ACCEPTED MANUSCRIPT

394 395

Figure 5. Nutrient recovery for each irrigation water source (GW, SW, and TW). Nitrogen,

396

phosphorus, and potassium supplied through irrigation are compared to those provided

397

according to local practices.

398 399

It is worth highlighting that the concentration of TN was much lower in treated wastewater

400

than in GW. Moreover, TN in both SW and TW was mostly composed of organic nitrogen,

401

whereas in GW it was present only as nitrate, much more readily available to plants. This was

402

due to two factors that are typical of the agro-industrial sector. The first is over-fertilization of

403

soil, which is common in intensively cultivated areas. This practice causes nitrogen leaching

404

and accumulation in groundwater, representing a serious environmental issue (Bouraouia and

405

Grizzetti, 2014). The second is the very high COD/N ratio in the raw wastewater, typical in

406

agro-industrial effluents, which was the reason of the low nitrogen content in treated

407

wastewater. Indeed, most of the wastewater nutrients were removed during the activated sludge

408

process to fulfil the metabolic needs of the microorganisms in charge of organic fraction

409

biodegradation. On the contrary, the requirement of potassium for microbial metabolism is very

410

low, so its removal within the WWTP was negligible. In order to enhance the recovery of

411

nitrogen from wastewater having a high COD/N ratio, anaerobic processes could be applied

ACCEPTED MANUSCRIPT 412

instead of the conventional activated sludge process. This option would also have the advantage

413

of limited sludge production and possible energy recovery.

414

In planning the fertilization of warm-season crops, farmers should take into account the

415

nutrients contained in the irrigation water. This would have the double advantage of favouring

416

savings on chemical fertilizers and promoting progressive mitigation of the groundwater

417

pollution due to agriculture.

418

Besides nutrient recovery, another advantage of treated wastewater reuse in irrigation lies in

419

the avoidance of effluent discharge in the environment. In particular, nutrient recovery from

420

agro-industrial effluents has the double benefit of favouring crop growth while limiting

421

discharge into water bodies (with consequent risk of eutrophication). This is particularly

422

important in nitrate-sensitive areas.

423 424

3.5 Effects on soil and crops yield

425

In terms of physical and chemical characteristics, the main differences among the three

426

irrigation water sources were related to salinity, nitrate, organic fraction, and to a minor extent

427

pH. The effects of these water characteristics on the corresponding parameters measured in the

428

first 30 cm of soil were investigated, and the results are shown in Figure 6.

429

During the experimental period, the pH in soil varied approximately between 8 and 9 in all the

430

plots, with no apparent correlation with the pH in the corresponding irrigation water sources

431

(Figure 6a).

432

In terms of organic fraction, the average COD values in SW and TW were 42±24 and 24±8

433

mg/L, respectively, both much higher than the values observed in GW (4±2 mg/L). These

434

differences did not result in a significant accumulation of organic matter (OM) in soil over the

435

experimental period (Figure 6b), confirming the previously observed slow OM accumulation in

436

soil (Friedel et al., 2000). Indeed, an increase of soil OM due to irrigation with treated

437

wastewater was only observed in the long term (Bationo et al., 2007).

ACCEPTED MANUSCRIPT 438

Also the concentration of nitrate in soil did not appear to be influenced by the type of water

439

used for irrigation, but it was higher and more variable during the cultivation of tomato than

440

during the cultivation of broccoli (Figure 6c). With respect to the tomato cycle, a high amount

441

of nitrogen was provided through fertirrigation during the dryer summer season. This may have

442

caused temporary nitrate accumulation in soil.

443

444 445

Figure 6. Evolution of soil characteristics (first 30 cm). Comparison among the plots irrigated

446

with three different water sources.

447 448 449

The EC was the soil parameter more clearly influenced by the water source used for irrigation, although temporarily. In plots irrigated with SW and TW, the EC in soil increased during the

ACCEPTED MANUSCRIPT 450

cultivation of tomato, suggesting salt accumulation (Figure 6d). The high water requirement for

451

the cultivation of tomato (5000 m3/ha) has caused a corresponding supply of salt through

452

irrigation with treated wastewater between April and September. This, together with the

453

concurrent lack of rain, has favoured the accumulation of salt in soil. These conditions have

454

changed completely during the cultivation of broccoli, both in terms of irrigation requirements

455

and rainfall, allowing the leaching of salt from the topsoil. Therefore the previous EC increase

456

was fully recovered, and no relevant differences were observed at the end of the winter period

457

among the plots irrigated with the three water sources and, for each water source, with respect to

458

the previous year. These results are in agreement with the findings of Morugán-Coronado and

459

co-authors (2011), who reported temporary salt accumulation due to irrigation with treated

460

wastewater, but no effects on the EC of soil after two years of field tests. However, in order to

461

prevent any possible long term effects, alternation of irrigation water sources having different

462

salinity levels is always advisable.

463

In terms of crops yield, all the marketable tomato fruits and broccoli heads cultivated in the

464

test field (three plots for each type of irrigation water) were harvested and weighed. The

465

corresponding results are reported in Table 1. The differences among the crops irrigated with

466

the three water sources were very small (between 1 and 7 %) for both tomato and broccoli,

467

indicating that the irrigation with treated agro-industrial wastewater had no negative effects on

468

productivity. However, while the yield of broccoli was about the same for the three water

469

sources, the yield of tomato was slightly higher in plots irrigated with GW (5 % and 7 % more

470

than in TW an SW plots, respectively). Therefore the characteristics of the irrigation water had

471

no significant influence on the productivity of broccoli, whereas the higher content of nitrates in

472

groundwater slightly enhanced the productivity of tomato. Of course these effects were also

473

influenced by the different irrigation volumes applied to the two crops according to seasonal

474

differences. Recent studies carried out by the authors showed that nitrate supply through

475

irrigation has positive effects on crop yields. In particular, irrigation with treated wastewater

476

containing nitrogen mainly as nitrate enhanced the yield of lettuce by 50% (Vergine et al.,

ACCEPTED MANUSCRIPT 477

2017), whereas irrigation with treated wastewater containing nitrogen mainly as ammonia

478

enhanced the yield of artichoke by about 20% (Gatta et al., 2016).

479 480 481

Table 1. Crops marketable yield. For each water source, average values at the three corresponding irrigated plots. Irrigation water

Tomato yield (t/ha)

Broccoli yield (t/ha)

GW

85.7±1.0

7.6±0.7

SW

80.3±0.6

7.5±0.3

TW

81.6±1.2

7.3±0.4

482 483

3.6 Fate of the faecal indicators

484

The indicators of faecal contamination Escherichia coli, faecal coliforms, and Salmonella spp.

485

were monitored over the whole experimental period in the three water sources used for

486

irrigation, and in soil and crops at harvesting time. The non-disinfected SW displayed a

487

considerably higher content of E. coli compared to GW and TW (Figures 3h, 7a, and 7b). This

488

caused a significant presence of E. coli in the soil (sampled upon irrigation), but it did not result

489

in any relevant contamination of plants or fruits, as shown in Figures 7a and 7b. In the whole

490

experiment, E. coli was never detected in fruits and only once in plants irrigated with

491

conventional water (GW) (Figure 7a). The results regarding faecal coliforms in the different

492

matrixes confirmed those observed in terms of E. coli (Figures 7c and 7d). Although the SW

493

had a much higher faecal pollution than the other two water sources (at least 2-3 logs, on

494

average), the irrigated soils and crops had similar content of faecal coliforms (differences were

495

lower than 1 log). The drip irrigation method used in this study has probably contributed to this

496

result, avoiding the direct contact between water and crops. Salmonella spp. were never

497

detected in any of the samples analysed.

498

ACCEPTED MANUSCRIPT

499 500

Figure 7. Fate of the faecal indicators during the cultivation of tomato and broccoli. Average E.

501

coli and faecal coliform concentrations in the three irrigation water sources and in the

502

corresponding irrigated soils and crops.

503 504

The results presented in Figure 7 show that the presence of the faecal indicators in plants and

505

fruits is scarcely dependent on their concentration in the irrigation water. These findings are in

506

agreement with previous studies suggesting that irrigation with reclaimed effluents with residual

507

faecal contamination does not necessarily imply contamination of the corresponding crops

508

(Palese et al., 2009). Moreover, the isolation of E. coli in one sample of a plant irrigated with

509

well water (having no E. coli), suggests that external contamination sources may have a role in

510

these evaluations (Figure 7a). Indeed, relevant concentrations of E. Coli in soil irrigated with

511

water having no faecal pollution were observed by the authors in previous studies (Vergine et

ACCEPTED MANUSCRIPT 512

al., 2015). Moreover, Forslund and co-authors (2012) found concentration of E. coli up to 4.8 x

513

105 CFU/g in samples of soil irrigated with water having low faecal pollution (maximum E. coli

514

concentration of 20 CFU/mL). The importance of the external environment - typically wildlife -

515

as a source of faecal contamination should be further investigated, as suggested by Langholz

516

and Jay-Russell (2013).

517 518

3.7 Economic evaluation

519

The cost for treating the secondary effluent to produce reclaimed wastewater suitable for

520

irrigation was determined considering the investment cost of the tertiary treatment, the energy

521

consumption and chemicals, and the required labour cost. The membrane productivity was

522

assumed to be constant and equal to the average value measured during the entire experimental

523

period (4.2 m3/h).

524

The power consumption for the filtration unit, including sand filter and ultrafiltration

525

membranes, was measured by an electric meter. The average energy consumption of the

526

filtration process was 0.86 kWh per m3 of treated water. The backwashing streams were sent

527

back to the biological process, so their treatment required additional energy. This was accounted

528

for, and a total cost of 0.68 kWh was assumed for processing 1 m3 of backwashing water

529

through the secondary treatments, as reported in Table 2. Considering also the energy

530

consumption for UV radiation, the overall energy requirement of the tertiary treatment was 1.68

531

kWh/m3 (Table 2). This corresponds to about 0.20 €/m3 (considering a price of 0.12 €/kWh for

532

electricity).

533 534

Table 2. Energy requirements of the tertiary treatments. Operation

Energy consumption (kWh/m3)

Filtration (sand filter plus membranes)

0.86

Sand filter backwashing

0.57

Membranes backwashing

0.11

Membrane chemical cleaning

0.01

UV radiation

0.13

ACCEPTED MANUSCRIPT 535 536

The investment cost for the tertiary treatment was 125,000 € (Table 3). This included pumps,

537

piping, 8 membrane modules, whose price was 2,500 € each, and 6 UV lamps, whose price can

538

be estimated as 150 € each. Considering a lifespan of 7 years for membranes and UV lamps and

539

20 years for the rest of the equipment, the investment cost per unit of treated water was

540

calculated to be 0.23 €/m3.

541 542

Table 3. Investment costs of the tertiary treatments. Equipment

Capital cost (€)

Life span expected (y)

Sand filter, piping and pumps of the filtration unit

95,000

20

Ultrafiltration membranes (8 modules)

20,000

7

UV system (lamps excluded)

9,100

20

UV lamps (6)

900

7

543 544

An additional cost of one hour of labour per day was also considered, and accounted for

545

approximately 0.18 €/m3. The cost of the reagents used for the periodical membrane chemical

546

cleaning was lower than 0.01 €/m3, so it was neglected.

547

The costs for fertilization was about 800 €/ha for the cultivation of tomato and 400 €/ha for

548

the cultivation of broccoli. Fertilization costs were divided as follows: 40% for potassium, 30%

549

for nitrogen, and 30% for phosphorus. The nutrients born by the different irrigation water types

550

were in addition to those supplied through chemical fertilization. The potential savings on

551

chemical fertilizers were evaluated considering the percentage of nutrients supplied with water

552

with respect to the fertilization needs (Figure 5). For the cultivation of tomato, the potential

553

savings were estimated to be 240 €/ha and 280 €/ha for GW and TW, respectively. For broccoli,

554

the corresponding savings would be about one half of those estimated for tomato. Since the

555

potential savings on nutrients related to the two water sources were similar, their effect on the

556

comparative economic evaluation was neglected.

557 558

Therefore, the overall cost of the tertiary treatment was 0.61 €/m3, higher than those normally calculated for municipal wastewater reclamation (0.35 €/m3, Ruiz-Rosa et al., 2016). This was

ACCEPTED MANUSCRIPT 559

due to the relatively small size of the plant and its sub-optimal performance in terms of

560

productivity. Indeed, the membrane flux was much lower than the value indicated by the

561

manufacturer (probably due to the presence of oil in the treated stream), and this was estimated

562

to cause a 30% increase in the treatment cost, approximately. In the specific case of the

563

company Fiordelisi, this cost was compared only to the cost of pumping groundwater from the

564

(relatively shallow) water table, which was about 0.10 €/m3. As a matter of fact Fiordelisi did

565

not pay any fees (taxes, permits) for effluent discharge to the local canal, as long as compliance

566

with the standards was assured. Moreover, groundwater for irrigation was pumped from a

567

proprietary well, thus Fiordelisi had no billing costs from external water companies/utilities

568

except the electric supply. Of course this situation is uncommon, and most local farmers pay

569

fees for irrigation water (0.12÷0.24 €/m3 with a three-tiered pricing structure, Arborea et al.,

570

2017). Some agro-industries also pay for pre-treated effluent discharge to the sewer according to

571

local tariffs. Therefore the economic evaluation would be more favourable towards effluent

572

reuse under normal conditions. These may be defined by tertiary treatment costs lower than

573

those measured at Fiordelisi (higher efficiencies and economy of scale), higher price of

574

conventional sources, and savings on effluent disposal cost.

575 576

4. Conclusions

577

The full scale WWTP of an agri-food industry (Fiordelisi s.r.l.) was monitored for 1.5 years in

578

order to assess its performance. The plant was equipped with a tertiary treatment (ultrafiltration

579

and UV disinfection) for effluent reuse in agriculture. Field irrigation tests were carried out over

580

two seasonal crops (tomato in summer, and broccoli in winter) with two types of effluents and

581

with the conventional well water.

582

The results of the present study show that reuse of treated wastewater for irrigation is a

583

suitable practice to close the water cycle in the agro-industrial sector. In areas where intensive

584

agriculture and transformation activities are present, the sustainability of the agri-food sector

585

depends on water availability. Farms with limited access to water for irrigation could profit of

ACCEPTED MANUSCRIPT 586

treated effluents from wastewater reclamation, decreasing the stress on conventional resources

587

especially during the dryer seasons. In the specific case reported, the recovery for reuse of the

588

whole wastewater yearly produced allowed the irrigation of 13 hectares cultivated with tomato

589

and broccoli in succession. The nutrient contribution of the treated effluents to fertilization was

590

only relevant in terms of potassium (very important for tomato), while nitrogen and phosphorus

591

concentrations were comparable or lower than those measured in the well water.

592

However, the variable and sometimes challenging characteristics of the raw wastewater

593

require careful management strategies. In the specific case reported, the high salinity of the raw

594

wastewater resulted in a temporary increase of salinity in the irrigated soil during the dry

595

season, even though this effect was completely recovered during the following rainy season,

596

when the soil characteristics were restored. Moreover, the occurrence of high concentrations of

597

oily wastewater and the limited effectiveness of the de-greasing system installed as a pre-

598

treatment negatively affected the performance of the filtration system. These aspects suggest

599

that pre-treatment can be a key aspect and may heavily influence the whole wastewater

600

treatment performance.

601

Furthermore, in the treatment of industrial streams, the WWTP operation should interact and

602

communicate with the production processes, in order to allow possible segregation of specific

603

streams, or other strategies for maintaining a constant quality of the produced effluents. In this

604

sense, wastewater treatment could be seen as a production line, and the influent streams could

605

be considered the raw material to be processed. Indeed, by avoiding the discharge of polluted

606

wastewater into water bodies and by reducing the fresh water requirement for irrigation, the

607

reuse of treated wastewater makes the entire industrial production cleaner. In the specific case

608

presented, this practice could save about 80,000 m3 of groundwater, and avoid the discharge of

609

3,000 kg of COD and 280 kg of nitrogen every year.

610 611

AKNOWLEDGEMENTS

ACCEPTED MANUSCRIPT 612 613

The results reported were obtained as partial fulfilment of the EC collaborative project “Demoware” (FP7 ENV Water Inno-Demo-1 contract n. 619040).

614 615

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http://oldwww.unibas.it/utenti/sofo/Frutticoltura%202002.pdf (accessed 28.04.17).

ACCEPTED MANUSCRIPT Highlights 1. Irrigation with treated agro-industrial wastewater is a suitable practice 2. Treated wastewater had no inhibitory effects on the growth of tomato and broccoli 3. There were no negative effects on soil salinity and microbial safety of crops 4. Closing the water cycle in the agro-industry allowed the irrigation of 13 ha 5. Segregation and separate treatment of heavily polluted fractions may be considered