Chemical oxidation

Many advanced chemical oxidation technologies are based on the production of hydroxyl radicals, possessing an extremely high oxidation potential (+2.73 V); to reduce the temperatures (and hence the pressures) required for wet oxidation, it has long been proposed that organically polluted wastewaters should be treated with Fenton reactant, i.e. hydrogen peroxide in the presence of iron salt. In the Fenton process, the completion of oxidation is dependent on the hydrogen peroxide : organic pollutant ratio (usually 2 : 1 equiv : equiv), on the catalyst : peroxide ratio (ca. 1 : 10 mol : mol) and on pH (between 3 and 4); the rate of oxidation is determined by the initial Fe11 concentration and the temperature. Usually, the amount of added salt is low, less than 20 mM, but there are examples of the Fenton processes running at higher concentrations, up to 60 mM. Comprehensive recent analyses of the Fenton reaction for water purification and recovery have reported that yields and rates can be enhanced by assisting the reaction photochemically or electrochemically. Given sufficient time, a long list of chemicals can be destroyed completely by the Fenton process, apart from, rather surprisingly, some common carboxylic acids such as acetic, maleic and fumaric acid, and acetone which is often found among the effluent of the Fenton treatment - being formed in situ by oxidation of a variety of precursors. The higher recalcitrance to the Fenton treatment of the above compounds sustains the use of biological treatments for the detoxification of OMWW. Bressan et al. (2004) evaluated, on a laboratory scale, the possible advantages, in terms of efficacy and depolluting effectiveness, of a combined and synergic action of a catalytic oxidation treatment in liquid phase by using an iron/hydrogen peroxide system together with advanced microbial biotechnologies. OMWW were oxidized up to 80-90% by stoichiometric amounts of diluted hydrogen peroxide (35%) and in the presence of water soluble iron catalysts, either Fe11 or Fem, at concentrations up to 1% w/w and more, i.e. much larger than those reported for conventional Fenton processes. In the combined action, the mineralization activity of a selected microbial consortium was used to degrade residual volatile and non-volatile organic compounds into CO2 and biomass (Table 20.5). Results suggested that the operational methodology was capable of reducing the potential impact of wastewaters. The chemical process is characterized by the fact that the oxidation is carried out in the presence of large amounts of iron salts, either ferric or ferrous, at over 0.05 mol L-1, and therefore far beyond the amounts usually reported for conventional Fenton processes. Under these conditions, and contrary to common belief, oxidation of strongly polluted wastewater is definitively competitive with the dismutation of hydrogen peroxide, so that almost all of the added hydrogen peroxide is consumed to perform the abatement of COD, with very few organics left in the reaction mixtures. Moreover, because oxidation is very fast, no photochemical or electrochemical assistance is necessary. The marked exothermicity of the process can be controlled by gradually adding the iron salt or the oxidant to the wastewater.

The observed break in the efficiency of the Fenton treatment at iron concentrations between 30 and 50 mM (around 0.1%w/w of Fe) is the most unexpected finding of this investigation, strongly suggesting that different mechanisms are taking place when smaller or larger concentrations of iron are used. Along with the generally accepted mechanism of the Fenton reaction of hydroxyl radicals, in recent years several alternative hypotheses

Table 20.5 COD removal and chemical and microbiological parameters (CFU, total ATP, total phenol, GI) on OMWW after combined chemical and biological treatments (adapted from Bressan et al., 2004)

COD residual

Parameters after both

Added

(COD removal %)

treatments

H2O2 (%)

Chemical Biological

CFU mL-1

Total ATP

Total phenol

GI

treatment treatment

(log)

(ng mL-1)

(%)

(%)

0

Untreated sample

11.4 (0) 11.4 (0)

1.7

3.5

1.24

26.0

0

Untreated sample

11.4 (0) 5.8 (49)

5.48

9.01

0.75

45.5

15

4.90 (59) 4.30 (62)

4.62

8.60

0.44

71.5

30

4.35 (62) 3.25 (71)

5.39

8.72

0.32

78.5

60

2.20 (81) 1.20 (90)

5.24

8.10

0.27

81.0

100

1.70 (85) 1.15 (90)

4.39

8.44

0.21

76.5

CFU, colony forming units; GI, germination index.

CFU, colony forming units; GI, germination index.

have been proposed, pointing to the participation of high valent oxo-iron complexes, as indeed earlier proposed by Bray and Gorin (1932). Parallel reactions can be envisaged also for FeIII, leading to high valent and reactive oxo-iron species. The extreme efficiency of the reaction does not allow us to distinguish whether a radical mechanism is operating, since common hydroxyl scavengers, such as 2-proanol, disappear almost immediately upon oxidation. The Fenton system works effectively with a very unusual iron/ hydrogen peroxide ratio (around 1/100, at 1 : 1 oxidant to COD ratio, equiv : equiv). The chemical treatment based on the described Fenton reaction indicates the possibility of rather effectively abating the polluting load of OMWW, by up to 80-90% in terms of COD. However, the treatment results in the total disappearance of the viable microflora (sterilization effect), probably as a result of the direct action of hydrogen peroxide or of the formation of toxic intermediates. Sub-stoichiometric amounts of the oxidizing reagents (modulated Fenton treatment) lead to partial removal of COD and alter the chemical composition of the OMWW in a more favourable way to the subsequent biological action. Even if longer times (15 days) are necessary, the biological treatment not only allows the attainment of further, even if less significant, demolition of the COD (up to 90%) but finally offers the immediate possibility of overcoming the intrinsic low germinability of the wastewaters. The final value of the germination index (GI) parameter is always >70%, a limit that attests the absence of chemicals capable of inhibiting the germination of seeds. These GI data must be compared not only with those of the samples treated only chemically (around 10% GI) but also with those of the samples treated only biologically, i.e. without a chemical pre-treatment, which even though it is higher (at around 40%) is still unsatisfactory. It is therefore reasonable that the chemical pre-treatment effectively removes important organic compounds that inhibit the biological oxidation.

The main component of the costs of the process is hydrogen peroxide (presently ca. €0.2 per kg of 35% solution, i.e. €0.009 /oxidation equivalent); therefore a 'soft' chemical pre-treatment (60% of the stoichiometric demand) of an average OMWW with COD of 80 000 mg L-1 (10 equiv L-1) requires €54 mc-1, in terms of hydrogen peroxide consumed. The other weak point of the treatment relates to the huge amounts of iron salts necessary, 50-100 mM, i.e. 2-5 g Fe L-1, well beyond the standard accepted for iron in wastewater (2-4 mg L-1). However, it should be noted that a large part of the added iron separates from the solution at the end of the treatment as insoluble FeIII hydroxide, which could be recovered in high yields. This point must not be underestimated, since the addition of the iron salts to the initial OMWW sample always yields clear solutions, probably because of the complexing ability of the very concentrated organics present therein. In the course of the reaction, however, almost all of the organics are destroyed and the small residual amounts of the complexing agents are not able to maintain significant quantities of iron in solution. Fiorentino et al.

(2004) described the application of two oxidation methods by polymer-supported reagents and then the combination of the most effective one with an advanced biological process based on the use of selected bacteria. The use of the immobilized oxidants on the solid phase is an advantageous technique due to the simple handling of the reagents, the opportunity of recycling and the possibility of controlling the reaction and the product yields, reducing the formation of oxidation by-products. In the study, OMWW were collected in southern Italy and subjected first to a chemical oxidative procedure with FeCl3 and then to a biological treatment. The latter was performed in a pilot plant where mixed, commercial, selected bacteria - suitable for polyphenols and lipid degradation - were inoculated. The effect of treatments was assessed by the extent of COD removal, reduction of total phenols and decrease of toxicity - using primary consumers of the aquatic food chain (the rotifer Brachionus calyciflorus and the crustacean Daphnia magna). Results indicated that the chemical oxidation was efficacious in reducing all the parameters analyzed. A further decrease was found by combining chemical and biological treatments (Tables 20.6 and 20.7). Beltran et al. (1999) utilized ozone alone and combined with hydrogen peroxide of UV radiation for chemical OMWW oxidation as a useful pre-treatment before an aerobic biological oxidation step. These processes allowed high COD reductions, nearly complete disappearance of aromatic

Table 20.6 Phenol removal calculated by HPLC

(adapted from Fiorentino et al., 2004)

Compound Removal (%)

Protocatechic acid 20

Hydroxytyrosol 72

Catechol 53

Tyrosol 10

4-Hydroxybenzoic acid 10

Vanillic acid 2

Table 20.7 Percentage removal of the chemically oxidized OMWW in the biological pilot plant after different time periods (days) (adapted from Fiorentino et al., 2004)

Parameter

Starting value

1 day

Mean reduction (%) 2 days 3 days

4 days

COD (mg L-1)

4000

37.5

51.3

60.9

62.5

Total phenol (mg L-1)

16.2

14.8

25.0

46.1

60.2

Toxic unit Brachionus

27.3

5.6

19.4

22.2

30.6

calyciflorus

Toxic unit Daphnia magna

28.2

9.8

21.4

26.8

33.0

content and colour, and moderate total carbon reductions. Other studies demonstrated the utility of photo-Fenton pre-treatment for recalcitrant wastewaters. Most of the chemical processes have proved their worth in the elimination of toxic compounds. On the other hand, these kinds of treatments have been shown to be expensive when compared with biological ones. Furthermore, the biological treatments are, at present, the most compatible with the environment. Thus, Pinto et al. (2003) reported the removal of low molecular mass phenols from OMWW using microalgae, and, in a recent work, OMWW were subjected to biological degradations with aerobes and facultative aerobic bacteria without strict anaerobes. However, it is important to develop efficient chemical pre-treatment processes for biologically recalcitrant compounds, processes that reduce the toxicity, increase the biodegradability of the substances and lead the pre-treated wastewaters to a biological treatment. Fernández-Bolaños et al. (2002) explored the possibility of obtaining hydroxytyrosol in high yield from two-phase olive waste and carried out a series of hydrothermal treatments. Usually when a lignocellulosic material is treated with water or steam to temperatures in the range of 160-240 °C, an autohydrolysis process occurs. Depending on the conditions used, there is a depolymerization and a breaking of the lignin-carbohydrate bonds, resulting in the solubilization of lignin fragments of low molecular mass. As a consequence of such treatment, the solid olive by-product was partially solubilized. Because the hydroxytyrosol is usually part of other molecules such as oleuropein, demethyloleuropein, verbascoside and hydroxytyrosol glucosides the authors defined the experimental conditions that gave the maximum concentrations of free hydroxy-tyrosol and also of other raw compounds (hemicellulose, cellulose, residual oil). The main operational variables governing the autohydrolysis process (temperature and the speed of the reaction) were varied. The effects of certain acidic and basic catalysts were also evaluated. Autohydrolysis plays an important role in this hydrothermal pre-treatment of alperujo, leading to liquor with a pH in the range of 2-5. Nevertheless, because the hydroxy-tyrosol, an orthodiphenol with important nutritional properties, seems to be strongly bound to the solid phase of alperujo, a severe hydrolytic treatment is required for its isolation. The recovery of other organic compounds present in the hydrolysate would help to reduce both the costs and the energy requirements of the process. Fernández-Bolaños et al. (2004) characterized three different samples of alperujo and subjected them to a hydrothermal treatment with and without acid catalyst. The main soluble compounds after the hydrolysis were represented by monosaccharides xylose, arabinose and glucose; and by the oligosaccharide mannitol; and by the products of sugar destruction. Oligosaccharides were separated by size exclusion chromatography. It was possible to obtain highly purified man-nitol by applying a simple purification method. Therefore, in suitable hydrothermal operational conditions, different mixtures of soluble oli-gosaccharides might be obtained.

564 Handbook of waste management and co-product recovery 20.5.6 Composting

Lua and Guo (1999) studied the feasibility of obtaining compost and biogas from the wastewater of the palm oil production process. In addition to the empty fruit bunches, 1-2.5 tonnes of wastewater with a COD of 50-65 g L-1 are released per tonne of palm oil (Ibrahim et al., 1984; Ny et al., 1985). The energy content of the wastewater could be utilized if the wastewater was digested in biogas reactors. A thermophilic digestion process is the most feasible procedure, due to the high initial temperature of the effluents (70-80 °C) and the high ambient temperature in the tropics, which prevents rapid cooling to the mesophilic temperature range (Ibrahim et al., 1984). Anaerobic digestion in the thermophilic temperature range favours the bioavailability of oil by emulsification of palm oil residues. Since the composition and concentration of the wastewater of palm oil mills is fairly constant, the main focus for maintaining a stable digestion process lies with the temperature of the wastewater. If palm oil mill effluent (POME) is digested in fixed-bed anaerobic digestors at 46 °C and a space loading of 6.5 kg COD m-3 per day, equivalent to a hydraulic retention time of 3.5 days, 92% of the COD is degraded to biogas (Siller et al., 1998). The specific gas yield is 1.25 m3 of biogas per kilogramme of oil, the volumetric gas yield in the reactor is 2.9 m3 m-3 per day and the methane content is 72%. Wastewater from olive oil or sunflower oil could be stabilized with a similar efficiency, provided that any inhibiting phenolic compounds in the waste-water of cold-pressed oil are destroyed by pre-treatment (e.g. Rivas et al., 2001). For anaerobic digestion, carrier-mediated reactor systems, such as polyurethane foam-bed reactors (Rozzi et al., 1989), have been successfully applied.

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