Abstract

Simultaneous application of solar photo-Fenton and ozonation (SPFO) for the efficient treatment of real wastewaters was studied. Four different industrial effluents were selected for the study: landfill leachate, pharmaceutical effluent and two textile wastewaters, in order to demonstrate the effectiveness and versatility of the proposed technology. SPFO performance was compared with individual processes (either solar photo-Fenton or ozonation), as well as the hybrid Fenton and ozonation treatment. In highly polluted wastewaters, combined strategies led to higher organic matter removal than O3 and photo-Fenton processes applied individually. Solar light favoured catalyst regeneration, allowing removal efficiencies up to 67% of chemical oxygen demand (COD) and 62% of total organic carbon (TOC) (in the case of textile wastewaters) using an initial concentration of only 10 mg Fe2+ L−1. The reduction of catalyst consumption, along with the absence of sludge production (since Fe2+ removal from the effluent is not required), led to a significant decrease in operational costs (up to 1.22 € kg−1 COD removed) when combined Fenton and ozonation was applied under solar light. SPFO results in a versatile, effective and economically efficient technology, thus postulating as a promising alternative for reducing the organic load of highly polluted industrial effluents prior to biological treatment.

INTRODUCTION

In the last decades, the increase of industrial activity has caused important contamination issues in water, soil and air. Industrial wastewaters present very heterogeneous characteristics in terms of physical properties (temperature, pH, conductivity, etc.), chemical composition (organic matter, inorganic compounds, metals, etc.), presence of pathogens, toxicity and biodegradability, thus representing a challenge to define proper technologies for their adequate treatment and potential reuse. Among highly polluted industrial effluents, landfill leachates (especially those coming from mature – more than 10 years – landfills) are characterised by high ammonium content, low biochemical oxygen demand (BOD) to chemical oxygen demand (COD) ratio (BOD/COD < 0.1) and high fraction of refractory large molecules (humic acids) (Cortez et al. 2010). Pharmaceutical wastewaters also present structurally complex organic chemicals coming from the synthesis process (therefore highly dependent on the industry), while textile wastewaters' most problematic compounds are mainly attributable to the extensive use of various dyestuffs and chemical additives (such as polyvinyl alcohol, surfactants, etc.) (Bautista et al. 2008). In the case of this kind of highly polluted industrial wastewater, biological treatments (which are usually characterised by low economical costs) can have limited success due to their low biodegradability and/or high toxicity, so harder technologies such as advanced oxidation processes (AOPs) are increasingly being applied as pre-treatment. AOPs are based on the generation of highly oxidising agents like hydroxyl radicals (•OH) for unselective attack on organic matter. The most common AOPs used in wastewater treatment are ozone-based technologies, Fenton reaction and photo-assisted processes.

Fenton's process is based on the catalytic decomposition of H2O2 in the presence of Fe2+ at acidic pH to generate •OH. Fenton's reagent has been extensively used, since it does not require an external energy source for activating the H2O2 decomposition, involves short treatment times and uses easy-handling reagents. Many studies in the literature have reported the efficiency of Fenton's process for treating industrial effluents and landfill leachates (Bautista et al. 2008). However, its efficiency depends on temperature, pH, H2O2 concentration and Fe3+ reduction to regenerate the Fe2+. Fenton oxidation presents its maximum catalytic activity at pH around 2.8–3.0. At pH < 2.8, Fe2+ complexes react more slowly and H2O2 is solvated to form stable oxonium ion [H3O2]+ (Babuponnusami & Muthukumar 2014). At pH > 3, Fe3+ starts to precipitate, mainly as Fe(OH)3, reducing the process efficiency. In addition, the formation of Fe2+ complexes at high pH values decreases the available catalyst concentration (Babuponnusami & Muthukumar 2014). The efficiency of Fenton's process can be improved through the use of irradiation in the UV-visible spectrum (including solar light for cost reduction), which leads to the regeneration of the catalyst without H2O2 consumption and generating extra •OH. The photo-Fenton process has shown high efficiencies in the treatment of diverse effluents such as landfill leachate and wastewater bearing dyes or pharmaceuticals (Pouran et al. 2015).

Oxidation of organic compounds through ozonation can occur directly by ozone, •OH or the combination of both. Direct ozone oxidation is favoured under acidic conditions, whereas higher pH values lead to the generation of oxidant species such as •OH, positively influencing the efficiency of the treatment (Muthukumar et al. 2004). Ozonation has been successfully employed for treating wastewaters containing different contaminants, like dyes (Tehrani-Bagha et al. 2010), pharmaceuticals (Rosal et al. 2010) and landfill leachate (Cortez et al. 2010). The efficiency of the process can be improved through its combination with UV light (Amat et al. 2005) and H2O2 (Peroxone reaction) (Hübner et al. 2015).

Although both Fenton-based processes and ozonation can be very effective in the removal of organic pollutants, they present certain drawbacks which have hindered their application for treating industrial effluents at industrial scale: (i) the high requirement of reagents (Fenton's process) or electricity (ozonation); (ii) the application of high pressure and temperature for increasing the efficiency; (iii) the use of UV lamps for carrying out photochemical processes. Therefore, it is necessary to develop innovative and efficient technologies that, strengthening the advantages of these processes, allow at the same time overcoming their weaknesses. In this context, the simultaneous application of AOPs (e.g. H2O2/UV, O3/UV, O3/H2O2, UV/H2O2/O3, UV/H2O2/TiO2, ultrasounds/Fenton, UV/Fenton, electrolysis/Fenton) can increase the oxidation rate by promoting the generation of reactive species, usually leading to higher efficiencies than using the single processes successively (Comninellis et al. 2008).

The present paper is focused on developing the simultaneous combination of solar photo-Fenton and ozonation (SPFO) processes for the treatment of different real wastewaters. SPFO is expected to generate synergistic effects, increasing the process efficiency by direct ozonation, hydroxyl radical generation favoured at basic pH and/or decomposition of O3 to •OH in the presence of Fe2+ species.

Until today, there is almost no information in the literature about the combination of both technologies in the same process, and related works are focused on the removal of specific compounds in synthetic aqueous solutions, including pesticides (Farre et al. 2007) and aromatic acids like p-coumaric acid (Monteagudo et al. 2005), terephthalic acid (Thiruvenkatachari et al. 2007) and p-hydroxybenzoic acid (De Heredia et al. 2001). Additionally, they use UV lamps instead of solar light. Recently, the proposed technology has been studied at pilot scale for the removal of emerging compounds spiked in a secondary effluent from a municipal wastewater treatment plant (Quiñones et al. 2015). In this context, the present work is a pioneer in the application of simultaneous SPFO processes to real industrial effluents. The SPFO technology is compared with SPFO and combined Fenton-ozonation for the treatment of four different industrial wastewaters. From the results obtained at laboratory scale, an economical analysis in terms of operational costs is carried out for the two most effective technologies, in order to select the optimal treatment for each wastewater.

METHODS

Studied wastewaters

Different industrial effluents were selected: landfill leachate (WW1), pharmaceutical wastewater (WW2) and textile industry wastewater before and after a biological treatment (WW3a and WW3b, respectively). Table 1 shows the main physicochemical characteristics of selected effluents. Landfill leachate presented high conductivity (36.3 mS cm−1), high organic matter load (8,200 mg COD L−1) and elevated nitrogen content as ammonium (3,370 mg total nitrogen (TN) L−1). The pharmaceutical wastewater was a mixture of the effluent from washing the reactor used for omeprazole formulation and the entire plant wastewaters, presenting a high content of organic solvents (18,300 mg COD L−1). WW3a was the influent from a wastewater treatment plant in Igualada (Spain) which receives a mixture of urban wastewaters and effluents from several textile industries in the area (presenting 2,210 mg COD L−1). This wastewater is currently treated in a biological sequencing batch reactor (SBR), although its effluent (WW3b, with 320 mg COD L−1) needs further treatment during contamination peaks to fulfil discharge limits in terms of colour. All studied wastewaters need to be properly treated (aiming at decreasing COD, TN or colour) in order to fulfil discharge limits into sewer (Table 1).

Table 1

Physicochemical characterisation of the industrial wastewaters and discharge limits into sewer (DL) according to regional legislation (Catalan Decree 130/2003 of 13 May for regulation of public sanitation)

  WW1 WW2 WW3a WW3b DL 
pH 8.7 9.2 7.7 7.6 6–10 
Chemical oxygen demand (COD) (mg L−18,200 18,300 2,120 320 1,500 
Total organic carbon (TOC) (mg L−12,770 5,000 640 110 – 
Total nitrogen (TN) (mg L−13,370 160 151 25 90 (N-org + N-NH4+)
22.6 (N-NO3
Conductivity (mS cm−136.3 4.7 8.1 8.7 6.0 
Turbidity (NTU) 181 45.1 73.5 18.8 – 
Total dissolved Fe (mg L−13.83 <1 9.28 1.90 10 
Colour Perceptible (diluted 1:30) Negligible (diluted 1:30) Perceptible (diluted 1:30) Perceptible (diluted 1:30) Negligible (diluted 1:30) 
  WW1 WW2 WW3a WW3b DL 
pH 8.7 9.2 7.7 7.6 6–10 
Chemical oxygen demand (COD) (mg L−18,200 18,300 2,120 320 1,500 
Total organic carbon (TOC) (mg L−12,770 5,000 640 110 – 
Total nitrogen (TN) (mg L−13,370 160 151 25 90 (N-org + N-NH4+)
22.6 (N-NO3
Conductivity (mS cm−136.3 4.7 8.1 8.7 6.0 
Turbidity (NTU) 181 45.1 73.5 18.8 – 
Total dissolved Fe (mg L−13.83 <1 9.28 1.90 10 
Colour Perceptible (diluted 1:30) Negligible (diluted 1:30) Perceptible (diluted 1:30) Perceptible (diluted 1:30) Negligible (diluted 1:30) 

Experimental procedure

The selected wastewaters were treated using different technologies: (i) SPF; (ii) ozonation; (iii) combined Fenton and ozonation; and (iv) combined SPFO. The optimisation of several experimental conditions (pH, initial concentration of H2O2 and Fe2+, O3 dose and time of treatment) was evaluated for the different studied effluents and technologies. Experiments were carried out at 30 °C, the optimal temperature for the Fenton reaction (Bautista et al. 2008) and beneficial for ozonation, since the efficiency of this process increases with temperature up to 40 °C (Wu et al. 2008). The influence of pH was assessed first, studying the processes with no pH modification, at pH 3 (the optimal value reported for the Fenton treatment of different wastewaters (Bautista et al. 2008)) and 9 (the optimal value reported for ozonation (Muthukumar et al. 2004). H2O2 initial concentration was set at the stoichiometric concentration necessary to oxidise the COD, studying the possibility of reduction to 0.5 of this value, as well as the effect of an increase up to 1.5 of the stoichiometric amount. Fe2+ dose was firstly set at 10 mg L−1 (the discharge limit according to the regional legislation; Catalan Decree 130/2003 of 13 May for regulation of public sanitation), studying also the use of 100 mg L−1 and the molar ratio 1:10 (Fe2+:H2O2), a common dose used for industrial wastewater treatment (Bautista et al. 2008). Ozone dose was varied by using a flow meter, injecting 0.35–3.5 L min−1 of a gas flux with a concentration of ozone around 30 mg m−3, leading to a dose of 0.01, 0.05 and 0.10 g O3 min−1. Finally, the time of treatment was extended from 1 h to 2–3 h under optimal conditions in order to study a possible increase in the removal efficiency.

Chemicals

Hydrogen peroxide (50% w/w, Scharlab) and FeSO4·7H2O (98% purity, Scharlab) were used as Fenton's reagents. For pH acidification and neutralisation, H2SO4 (95–98%, Scharlab) and NaOH pellets (reagent grade, Scharlab) were used. The ozone generator was fed with O2 (99.995% purity, Air Products).

Experimental setup

Experiments were carried out using a 5 L Pyrex® jacketed reactor (main reactor), using different setups according to the studied treatment. During assays involving SPF, wastewater was recirculated (50 L h−1) from the main reactor to a Compound Parabolic Collector (CPC), which consists of one borosilicate tube (32 mm diameter, 1,500 mm length, 1.4 mm thickness) surrounded by a couple of parabolic mirrors, which concentrate solar light on the borosilicate tube. CPCs can concentrate up to 10 suns through its geometric reflective shape, since all UV radiation reaching the collector (both direct as diffuse, regardless the incident direction) is reflected to reactor, lighting the inside part of the tubular reactor. The system is fixed in a metallic platform with an inclination of 42 ° (the latitude of the site). This kind of solar reactor was chosen because of its high efficiency of light concentration reported even under low irradiation intensity (Rodriguez et al. 2004).

Normalised illumination time (t3mw) was defined to standardise the solar irradiation, considering that average solar UV flux on a perfect sunny day was about 3 mW cm−2 (Perez et al. 2006). This parameter was calculated according to Equation (1), defined by Perez-Estrada et al. (2005), where t3mw,n is the normalised illumination time at time ‘n’; t3mw,n-1 is the normalised illumination time at time ‘n-1’; UVG,n is the average incident radiation flux during Δtn (measured using a Lutron YK-35UV radiometer); Vi is the irradiated volume (1.2 L) and Vt the total reaction volume (2 L).  
formula
(1)
In the case of experiments involving ozone, an O3 generator (Hidro GH 15) fed with pure O2 at 1 bar was used. The generated O3-enriched flow (O3 concentration around 30–35 g m−3, measured by a MINI-HICON SMC 900 analyser bypass-connected) was introduced into the main reactor through a ceramic diffuser by using a flow regulator to control the ozone dose entering the reactor. The gas flowing out the reactor was driven to a catalytic ozone destructor (Ingenieria del Ozono S.L., DC-PVC-01) passing through a foam trap to avoid the arrival of liquid to the ozone destructor in case of foam formation.

Analytical methods

Total organic carbon (TOC) and TN were analysed using a TOC-Vcsh apparatus from Shimadzu with a TN-analysis unit. COD was measured using COD digestion vials and a DR 6000™ UV VIS spectrophotometer, both from Hach. Residual H2O2 was removed before COD determination by adding NaHSO3 in excess and oxidising the remaining sulphite with oxygen bubbling. Colour removal was determined following the criteria established in the Catalan Decree 130/2003: visualisation at a 1:30 (v:v) dilution. Turbidity was analysed using a LaMotte 2008 turbidity meter and an ICPMS 7500CX from Agilent was used for measuring Fe concentration.

Data analysis

Reported results were the average values from at least duplicate runs. Standard errors were lower than 10% in all the experimental runs.

RESULTS AND DISCUSSION

Landfill leachate (WW1)

Figure 1 shows the removal efficiencies obtained for the treatment of the landfill leachate by means of SPF (a), ozonation (b), combined Fenton and ozonation (c) and SPFO (d) under different operational conditions detailed in a table within each graph. For the sake of comparison, the normalised illumination time (t3 mW) during the SPF and combined SPFO experiments is also represented. t3mw was calculated measuring irradiation at different times (0, 5, 15, 30 and 60 min) and applying Equation (1) to 60 min of treatment.

Figure 1

Normalised illumination time (t3 mW, •) and removal of TOC (dark grey), TN (grey) and COD (black) achieved under different operational conditions for the treatment of the landfill leachate (WW1) through SPF (a), ozonation (b), combined Fenton and ozonation (c) and SPFO (d). The striped area corresponds to the % removed by acidification.

Figure 1

Normalised illumination time (t3 mW, •) and removal of TOC (dark grey), TN (grey) and COD (black) achieved under different operational conditions for the treatment of the landfill leachate (WW1) through SPF (a), ozonation (b), combined Fenton and ozonation (c) and SPFO (d). The striped area corresponds to the % removed by acidification.

Almost 70% COD removal and more than 30% TOC mineralisation was achieved after 1 h of SPF treatment (t3 mW = 38 min) at pH 3 using 1.5 of the stoichiometric dose of H2O2 and 10 mg Fe2+ L−1 (E113). However, colour was not eliminated and around 10% of COD and TOC removal was due to the pH adjustment to 3 before the treatment. This effect, only observed for landfill leachate, could be explained by the high content of humic acids, which, according to literature (Wang et al. 2012), can precipitate at pH around 3. The increase of initial Fe2+ concentration did not improve the obtained results, since solar irradiation favours the regeneration of the catalyst, thus reducing the required initial concentration (Hermosilla et al. 2009). Ozonation was inefficient for the treatment of this wastewater as a result; acidification again being the main cause of TOC and COD removal (10% for TOC and COD and 5% for TN). Highest efficiencies (72%, 25% and 83% of TOC, TN and COD removal, respectively) were obtained by combined Fenton and ozonation (E314), though again an important fraction was due to pH acidification (around 10% for TOC and COD and 5% for TN). However, the use of huge quantities of catalyst was required (1:10 molar ratio of Fe2+:H2O2) due to the absence of UV-Vis radiation. The application of this high Fe2+ concentration would require the separation of iron precipitates after final neutralisation of the effluent. In contrast, the SPFO process allowed the removal of around 70% of COD, performing the treatment with a low catalyst concentration (10 mg Fe2+ L−1) and requiring no acidification. This fact is of real interest, since significant quantities of sulphuric acid were needed for decreasing the pH due to the presence of different buffers (mainly carbonate and phosphate), and also a huge amount of foam was formed during acidification. Economical comparison between SPF and SPFO was carried out later on in order to evaluate if it is more economical to acidify or to inject ozone when solar UV/Fenton is applied for treating landfill leachate. The removal of colour only achieved discharge limits (no colour visible after 1:30 vol. dilution) by using SPFO technology under optimal conditions (E413). Further treatment (e.g. through biological processes) would be necessary in order to meet discharge limits in terms of COD, since the extension of the time of treatment up to 3 h did not improve the quality of the obtained effluent.

Pharmaceutical wastewater (WW2)

The results observed for the application of the different treatments to the pharmaceutical wastewater are represented in Figure 2, including the experimental conditions used for each process. Increasing H2O2 dose, Fe2+ initial concentration, pH modification and time of treatment (or normalised t3 mW) did not clearly improve the results obtained by using SPF process. This technology, as well as ozonation alone, was not efficient enough for the treatment of this highly polluted effluent, achieving COD degradations around 30%, and TOC and TN removals under 15% even after 3 h treatment. Combined Fenton and ozonation resulted in higher removal efficiencies, especially when the treatment was extended up to 3 h. The use of 100 mg Fe2+ L−1 allowed improvement of the mineralisation of organic matter (23% TOC removal) during the combined treatment in the absence of solar light (E325). The increase of catalyst dose up to 1:10 molar ratio of Fe2+:H2O2 would lead to massive quantities of iron, so this condition was not studied for WW2. Again, SPFO resulted in being a very effective technology for the treatment of this wastewater, achieving almost 60% COD removal after 2 h (t3 mW = 84 min) under mild conditions (stoichiometric H2O2 dose, pH 3 and 10 mg Fe2+ L−1) (E424). During the processes based on Fenton's reaction, the increase of H2O2 initial concentration did not lead to better results. Moreover, in the case of the combined Fenton and ozonation process, the use of 1.5 of the stoichiometric H2O2 caused a decrease in the TOC mineralisation and TN removal. This fact has been reported previously in the literature (Monteagudo et al. 2005) and it can be explained because, at low concentrations, hydrogen peroxide and ferrous ions act as initiators of •OH but at high concentrations they can act as radical scavengers. Therefore, optimising the dose of reagents is of great importance (Thiruvenkatachari et al. 2007). The effluents resulting from the optimised treatments (combined Fenton and ozonation, E325, and SPFO, E424) should be further treated (e.g. through biological processes) in order to meet discharge limits.

Figure 2

Normalised illumination time (t3 mW, •) and removal of TOC (dark grey), TN (grey) and COD (black) achieved under different operational conditions for the treatment of the pharmaceutical wastewater (WW2) through SPF (a), ozonation (b), combined Fenton and ozonation (c) and SPFO (d).

Figure 2

Normalised illumination time (t3 mW, •) and removal of TOC (dark grey), TN (grey) and COD (black) achieved under different operational conditions for the treatment of the pharmaceutical wastewater (WW2) through SPF (a), ozonation (b), combined Fenton and ozonation (c) and SPFO (d).

Textile wastewater (WW3a)

Figure 3 shows the removal efficiencies obtained under different operational conditions for the treatment of the textile wastewater by means of the four studied processes. The lower organic matter content of this effluent compared with the previous ones allowed high efficiencies to be obtained after just 1 h of treatment, regardless of the pH value. The increase in H2O2 initial concentration during SPF experiments led to a TOC mineralisation around 40% (E13a4), noticeably higher than those obtained using the stoichiometric dose. Similar COD removals were achieved through 1 h ozonation, although this technology was less effective for TOC mineralisation. The use of ozone, alone or in combination with Fenton or photo-Fenton, allowed discharge limits to be met in terms of colour, one of the main problems of the studied wastewater, with an important content of dyes from textile industries. The synergistic effects derived from the combination of both processes were highlighted when treating this wastewater. More than 65% of COD removal was reached using moderate doses of ozone and H2O2 (E33a2), but higher concentrations of these reagents did not lead to better results. As for the other studied wastewaters, increasing the catalyst dose during combined Fenton and ozonation improved the treatment results, in this case resulting in higher TOC elimination (E33a6). SPFO at pH 3 allowed a COD removal efficiency over 65% to be obtained using only 10 mg Fe2+ L−1 (E43a3), a much lower catalyst dose than the one required for the combined Fenton and ozonation treatment without solar irradiation. In addition, the enlargement of the treatment up to 2 h (t3 mW = 84 min) (E43a5) led to removal efficiencies of 67% COD, 26% TN and 62% TOC, reaching discharge limits also in terms of colour.

Figure 3

Normalised illumination time (t3 mW, •) and removal of TOC (dark grey), TN (grey) and COD (black) achieved under different operational conditions for the treatment of the textile wastewater (WW3a) through SPF (a), ozonation (b), combined Fenton and ozonation (c) and SPFO (d).

Figure 3

Normalised illumination time (t3 mW, •) and removal of TOC (dark grey), TN (grey) and COD (black) achieved under different operational conditions for the treatment of the textile wastewater (WW3a) through SPF (a), ozonation (b), combined Fenton and ozonation (c) and SPFO (d).

Pre-treated textile wastewater (WW3b)

The results obtained for the effluent from the biological reactor of the textile wastewater plant are depicted in Figure 4. 10 mg Fe2+ L−1 was always used as the catalyst dose due to the low organic matter content of this wastewater. SPF treatment at pH 3 using 1.5 of the stoichiometric dose of H2O2 and 10 mg Fe2+ L−1 (E13b4) led to very high efficiencies (around 87% removal of COD and TOC). However, the reduction of the H2O2 dose to 300 mg L−1 (0.5 of the stoichiometric amount, E13b5) was enough to meet discharge limits in terms of COD, TOC and colour removal. Ozonation was very effective for colour removal: the use of 0.01 g O3 min−1 at pH 9 (E23b5) would only require 30 min to achieve an uncoloured effluent, whereas 15 min was enough to achieve total colour removal when using 0.10 g O3 min−1 (E23b4) (results not shown). Under these conditions, 55% and 32% of COD and TOC removal were achieved, respectively. The combination of Fenton and ozonation allowed performing the treatment with no pH modification, achieving a dischargeable effluent even at mild conditions (stoichiometric H2O2 amount, 10 mg Fe2+ L−1 and 0.01 g O3 min−1) (E33b4). SPFO was not studied since, from the obtained results, there was no need to intensify the studied treatments.

Figure 4

Normalised illumination time (t3 mW, •) and removal of TOC (dark grey), TN (grey) and COD (black) achieved under different operational conditions for the treatment of the pre-treated textile wastewater (WW3b) through SPF (a), ozonation (b) and combined Fenton and ozonation (c).

Figure 4

Normalised illumination time (t3 mW, •) and removal of TOC (dark grey), TN (grey) and COD (black) achieved under different operational conditions for the treatment of the pre-treated textile wastewater (WW3b) through SPF (a), ozonation (b) and combined Fenton and ozonation (c).

In summary, SPFO represents an efficient alternative to treat highly polluted industrial effluents as a part of a coupled process strategy that would allow the legislation limit to discharge the treated effluents in a sewer to be fulfilled. Other technologies studied in this article, such as SPF and combined Fenton and ozonation, would also be also an efficient treatment alternative. The effluents obtained for landfill leachate and pharmaceutical wastewater (with a starting organic matter content of 8,200 and 18,300 mg COD L−1, respectively) should be further treated (e.g. by means of biological processes) to fulfil the legislation limits both in terms of organic matter (1,500 mg COD L−1) and nitrogen content (112.6 mg TN L−1). For the textile wastewaters, the studied AOP technologies could be used as pre-treatment improving the current SBR process (WW3a) or as post-treatment, improving the characteristics of the SBR effluent, especially in terms of colour (WW3b). In this context, AOPs have been extensively reported as effective processes in combination with biological systems for the treatment of landfill leachates, pharmaceutical wastewaters and textile effluents, among others (Oller et al. 2011). For a better optimisation of the combined processes, the biodegradability of the effluents coming from the AOP should be evaluated (e.g. by means of respirometric methods, as carried out by Sanchis et al. 2014). However, the application of these analytical techniques, as well as the performance of biological processes, were outside the scope of the present project but will be considered in future projects.

Economic analysis

Table 2 shows the estimation of the operational costs derived from the application of the most effective technologies for the treatment of each studied wastewater under the best operational conditions tested. The estimates are given in the most common approach (per unit volume of wastewater) but values per unit of COD removed are also included for the sake of comparison. Use of reagents (H2O2, FeSO4, H2SO4 and NaOH), solid waste (iron sludge) management, electricity consumption (ozone, agitation, recirculation pump and heating) and effluent post-treatment were considered. pH modification required the use of H2SO4 and NaOH in order to achieve the desired pH value and also for neutralisation (necessary for effluent discharge to sewer). The use of catalyst doses above 10 mg L−1 generated a ferrous sludge when the effluent was neutralised, being necessary in its management as solid waste. For heating costs, the electrical consumption required for increasing the temperature of 1 m3 of water up to 30 °C was calculated. As initial water temperature, 15.6 °C was considered for non solar processes (the average annual water temperature in a wastewater treatment plant), while 22.8 °C was used for solar treatments. In the case of the most polluted wastewaters (WW1, WW2 and WW3a), the obtained effluents did not fulfil the required limits in terms of COD (1,500 mg L−1) and nitrogen content (112.6 mg TN L−1), so a post-treatment would be necessary before discharge. Although a biodegradability assessment would be strongly recommended before applying a biological process, this kind of technology could be proposed as post-treatment, and here its associated costs have been assumed. Current average prices for Spain have been used for post-treatment of the effluents by a biological process (Mas Ortega et al. 2016), as well as for ferrous sludge management previously mentioned (Pliego et al. 2012).

Table 2

Estimated operational costs for the treatment of the studied wastewaters by means of the most effective technologies

  Reagents
 
Solid waste
 
  Electricity consumption
 
  Effluent
 
H2O2 Fe H2SO4 NaOH Fe sludge O3 Agitation Recirc. pump Heat Post-treatment 
Landfill leachate (WW1) 
 SPFO (E413) 
  Quantity (g m−32,500 56 – – – Quantity (kWh m−317 0.067 0.052 8.33 Quantity (m3
  Unit cost (€ kg−10.23 0.33 – – – Unit cost (€ kWh−10.0715 0.028 Unit cost (€ m−30.7 
  Cost (€ m−35.75 0.02 – – – Cost (€ m−31.22 0.005 0.004 0.23 Cost (€ m−30.7 
  Total cost (€ m−37.93 
  Total cost (€ kg−1 COD) 1.45 
 SPF (E113) 
  Quantity (g m−325,000 56 240 30 – Quantity (kWh m−3– 0.067 0.052 8.33 Quantity (m3
  Unit cost (€ kg−10.23 0.33 0.13 0.56 – Unit cost (€ kWh−10.0715 0.028 Unit cost (€ m−30.7 
  Cost (€ m−35.75 0.02 0.03 0.02 – Cost (€ m−3– 0.005 0.004 0.23 Cost (€ m−30.7 
  Total cost (€ m−36.76 
  Total cost (€ kg−1 COD) 1.18 
 Fenton-ozonation (E314) 
  Quantity (g m−325,000 14,386 240 30 10,600 Quantity (kWh m−317 0.067 – 16.66 Quantity (m3
  Unit cost (€ kg−10.23 0.33 0.13 0.56 0.09 Unit cost (€ kWh−10.0715 – 0.028 Unit cost (€ m−30.7 
  Cost (€ m−35.75 4.75 0.03 0.02 0.95 Cost (€ m−31.22 0.005 – 0.46 Cost (€ m−30.7 
  Total cost (€ m−313.98 
  Total cost (€ kg−1 COD) 2.05 
Pharmaceutical wastewater (WW2) 
 SPFO (E424) 
  Quantity (g m−339,000 56 60 10 – Quantity (kWh m−335 0.134 0.104 16.66 Quantity (m3
  Unit cost (€ kg−10.23 0.33 0.13 0.56 – Unit cost (€ kWh−10.0715 0.028 Unit cost (€ m−30.7 
  Cost (€ m−38.97 0.02 0.008 0.006 – Cost (€ m−32.5 0.01 0.007 0.47 Cost (€ m−30.7 
  Total cost (€ m−312.69 
  Total cost (€ kg−1 COD) 1.22 
 Fenton-ozonation (E325) 
  Quantity (g m−339,000 556 60 10 420 Quantity (kWh m−351 0.2 – 49.98 Quantity (m3
  Unit cost (€ kg−10.23 0.33 0.13 0.56 0.09 Unit cost (€ kWh−10.0715 – 0.025 Unit cost (€ m−30.7 
  Cost (€ m−38.97 0.18 0.008 0.006 0.038 Cost (€ m−33.65 0.014 – 1.4 Cost (€ m−30.7 
  Total cost (€ m−314.97 
  Total cost (€ kg−1 COD) 1.24 
Textile wastewater (WW3a) 
 SPFO (E43a5) 
  Quantity (g m−34,800 56 60 10 – Quantity (kWh m−317 0.134 0.104 16.66 Quantity (m3– 
  Unit cost (€ kg−10.23 0.33 0.13 0.56 – Unit cost (€ kWh−10.0715 0.028 Unit cost (€ m−3– 
  Cost (€ m−31.1 0.02 0.008 0.006 – Cost (€ m−31.22 0.01 0.007 0.47 Cost (€ m−3– 
  Total cost (€ m−32.84 
  Total cost (€ kg−1 COD) 2.02 
 Fenton-ozonation (E33a6) 
  Quantity (g m−34,800 2,565 60 10 2,970 Quantity (kWh m−38.5 0.067 – 16.66 Quantity (m3
  Unit cost (€ kg−10.23 0.85 0.13 0.006 0.27 Unit cost (€ kWh−10.0715 – 0.46 Unit cost (€ m−30.7 
  Cost (€ m−31.1 0.85 0.008 0.006 0.27 Cost (€ m−30.61 0.005 – 0.46 Cost (€ m−30.7 
  Total cost (€ m−34.01 
  Total cost (€ kg−1 COD) 2.79 
Pre-treated textile wastewater (WW3b) 
 SPF (E13b5) 
  Quantity (g m−3340 56 60 10 – Quantity (kWh m−3– 0.067 0.052 8.33 Quantity (m3– 
  Unit cost (€ kg−10.23 0.33 0.13 0.56 – Unit cost (€ kWh−1– 0.0715 0.028 Unit cost (€ m−3– 
  Cost (€ m−30.08 0.02 0.008 0.006 – Cost (€ m−3– 0.005 0.004 0.23 Cost (€ m−3– 
  Total cost (€ m−30.35 
  Total cost (€ kg−1 COD) 1.45 
 Fenton-ozonation (E33b4) 
  Quantity (g m−3680 56 – – – Quantity (kWh m−30.067 – 16.66 Quantity (m3– 
  Unit cost (€ kg−10.23 0.33 – – – Unit cost (€ kWh−10.0715 – 0.028 Unit cost (€ m−3– 
  Cost (€ m−30.16 0.02 – – – Cost (€ m−30.36 0.005 – 0.46 Cost (€ m−3– 
  Total cost (€ m−31.01 
  Total cost (€ kg−1 COD) 9.55 
  Reagents
 
Solid waste
 
  Electricity consumption
 
  Effluent
 
H2O2 Fe H2SO4 NaOH Fe sludge O3 Agitation Recirc. pump Heat Post-treatment 
Landfill leachate (WW1) 
 SPFO (E413) 
  Quantity (g m−32,500 56 – – – Quantity (kWh m−317 0.067 0.052 8.33 Quantity (m3
  Unit cost (€ kg−10.23 0.33 – – – Unit cost (€ kWh−10.0715 0.028 Unit cost (€ m−30.7 
  Cost (€ m−35.75 0.02 – – – Cost (€ m−31.22 0.005 0.004 0.23 Cost (€ m−30.7 
  Total cost (€ m−37.93 
  Total cost (€ kg−1 COD) 1.45 
 SPF (E113) 
  Quantity (g m−325,000 56 240 30 – Quantity (kWh m−3– 0.067 0.052 8.33 Quantity (m3
  Unit cost (€ kg−10.23 0.33 0.13 0.56 – Unit cost (€ kWh−10.0715 0.028 Unit cost (€ m−30.7 
  Cost (€ m−35.75 0.02 0.03 0.02 – Cost (€ m−3– 0.005 0.004 0.23 Cost (€ m−30.7 
  Total cost (€ m−36.76 
  Total cost (€ kg−1 COD) 1.18 
 Fenton-ozonation (E314) 
  Quantity (g m−325,000 14,386 240 30 10,600 Quantity (kWh m−317 0.067 – 16.66 Quantity (m3
  Unit cost (€ kg−10.23 0.33 0.13 0.56 0.09 Unit cost (€ kWh−10.0715 – 0.028 Unit cost (€ m−30.7 
  Cost (€ m−35.75 4.75 0.03 0.02 0.95 Cost (€ m−31.22 0.005 – 0.46 Cost (€ m−30.7 
  Total cost (€ m−313.98 
  Total cost (€ kg−1 COD) 2.05 
Pharmaceutical wastewater (WW2) 
 SPFO (E424) 
  Quantity (g m−339,000 56 60 10 – Quantity (kWh m−335 0.134 0.104 16.66 Quantity (m3
  Unit cost (€ kg−10.23 0.33 0.13 0.56 – Unit cost (€ kWh−10.0715 0.028 Unit cost (€ m−30.7 
  Cost (€ m−38.97 0.02 0.008 0.006 – Cost (€ m−32.5 0.01 0.007 0.47 Cost (€ m−30.7 
  Total cost (€ m−312.69 
  Total cost (€ kg−1 COD) 1.22 
 Fenton-ozonation (E325) 
  Quantity (g m−339,000 556 60 10 420 Quantity (kWh m−351 0.2 – 49.98 Quantity (m3
  Unit cost (€ kg−10.23 0.33 0.13 0.56 0.09 Unit cost (€ kWh−10.0715 – 0.025 Unit cost (€ m−30.7 
  Cost (€ m−38.97 0.18 0.008 0.006 0.038 Cost (€ m−33.65 0.014 – 1.4 Cost (€ m−30.7 
  Total cost (€ m−314.97 
  Total cost (€ kg−1 COD) 1.24 
Textile wastewater (WW3a) 
 SPFO (E43a5) 
  Quantity (g m−34,800 56 60 10 – Quantity (kWh m−317 0.134 0.104 16.66 Quantity (m3– 
  Unit cost (€ kg−10.23 0.33 0.13 0.56 – Unit cost (€ kWh−10.0715 0.028 Unit cost (€ m−3– 
  Cost (€ m−31.1 0.02 0.008 0.006 – Cost (€ m−31.22 0.01 0.007 0.47 Cost (€ m−3– 
  Total cost (€ m−32.84 
  Total cost (€ kg−1 COD) 2.02 
 Fenton-ozonation (E33a6) 
  Quantity (g m−34,800 2,565 60 10 2,970 Quantity (kWh m−38.5 0.067 – 16.66 Quantity (m3
  Unit cost (€ kg−10.23 0.85 0.13 0.006 0.27 Unit cost (€ kWh−10.0715 – 0.46 Unit cost (€ m−30.7 
  Cost (€ m−31.1 0.85 0.008 0.006 0.27 Cost (€ m−30.61 0.005 – 0.46 Cost (€ m−30.7 
  Total cost (€ m−34.01 
  Total cost (€ kg−1 COD) 2.79 
Pre-treated textile wastewater (WW3b) 
 SPF (E13b5) 
  Quantity (g m−3340 56 60 10 – Quantity (kWh m−3– 0.067 0.052 8.33 Quantity (m3– 
  Unit cost (€ kg−10.23 0.33 0.13 0.56 – Unit cost (€ kWh−1– 0.0715 0.028 Unit cost (€ m−3– 
  Cost (€ m−30.08 0.02 0.008 0.006 – Cost (€ m−3– 0.005 0.004 0.23 Cost (€ m−3– 
  Total cost (€ m−30.35 
  Total cost (€ kg−1 COD) 1.45 
 Fenton-ozonation (E33b4) 
  Quantity (g m−3680 56 – – – Quantity (kWh m−30.067 – 16.66 Quantity (m3– 
  Unit cost (€ kg−10.23 0.33 – – – Unit cost (€ kWh−10.0715 – 0.028 Unit cost (€ m−3– 
  Cost (€ m−30.16 0.02 – – – Cost (€ m−30.36 0.005 – 0.46 Cost (€ m−3– 
  Total cost (€ m−31.01 
  Total cost (€ kg−1 COD) 9.55 

SPFO resulted in being the optimal process in terms of operational costs (€ kg−1 COD removed) when compared with combined Fenton and ozonation for treating effluents coming from pharmaceutical and textile industries. In the case of landfill leachate, SPF at pH 3 allowed the reduction of operational costs in comparison to SPFO without modifying the pH value. This indicates that, for this specific landfill leachate, acidification would be more economic than applying ozone when solar UV/Fenton was performed; however it is worth saying that an important part of the organic matter removal during the SPF process was due to pH adjustment. Other studies have reported the convenience of using SPFO, attending to operating costs, resulting in this being even cheaper than SPF alone (Quiñones et al. 2015). For the pre-treated textile effluent, since SPFO technology was not studied for this wastewater, SPF was compared to the combination of Fenton and ozonation, the former being the optimal treatment, with an associated cost of only 0.35 € m−3. The cost of treating textile effluents (in terms of €/m3) was much lower than for the other effluents, mainly due to the differences in organic load (COD), which led to lower reagent doses and treatment times.

CONCLUSIONS

Highly polluted wastewaters like landfill leachate and effluents from the pharmaceutical and textile industries required intensified processes to achieve an effective treatment. Combination of Fenton and ozonation processes led to high COD removal, TOC mineralisation and colour abatement, although the use of huge doses of catalyst were needed. The technology proposed in the present work, SPFO, which combines SPF and ozonation, allowed high removal efficiencies in terms of COD, TOC and colour using Fe2+ concentrations to be obtained, which could be discharged to sewer according to the local legislation. In addition, economic analysis comparing the most effective technologies revealed that SPFO process results in being the optimal technology in terms of operational costs for treating effluents coming from pharmaceutical and textile industries. Further treatment (e.g. through biological processes) would be necessary in order to reduce the organic matter content in the case of the most polluted wastewaters. When looking for a tertiary treatment of textile wastewaters, SPF reached a COD, TOC and colour that would discharge limits to be met using small reagent doses and requiring low operational costs. Therefore, solar strategies such as SPFO or SPF would represent a good technical and economic alternative to other AOPs (such as Fenton or Fenton/ozone treatments) applied to industrial effluents in combination with other treatments such as biological processes.

ACKNOWLEDGEMENTS

This work was supported by the Spanish Ministry of Economy and Competitiveness (MEC), Centre for the Development of Industrial Technology (CDTI) and Norwegian Embassy through the Project FentO3 (ES02-0060) in the EEA Grants programme.

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