Journal of Environmental Management 79 (2006) 215–220 www.elsevier.com/locate/jenvman The application of potassium ferrate for sewage treatment Jia-Qian Jianga,*, Alex Panagoulopoulosa, Mike Bauerb, Pete Pearceb b a School of Engineering (C5), University of Surrey, Guildford, Surrey GU2 7XH, UK Thames Water Limited, Spencer House, Manor Farm Road, Reading, Berkshire RG2 0JN, UK Received 6 July 2004; revised 7 May 2005; accepted 20 June 2005 Available online 22 September 2005 Abstract The comparative performance of potassium ferrate(VI), ferric sulphate and aluminium sulphate for the removal of turbidity, chemical oxygen demand (COD), colour (as Vis400-abs) and bacteria in sewage treatment was evaluated. For coagulation and disinfection of sewage, potassium ferrate(VI) can remove more organic contaminants, COD and bacteria in comparison with the other two coagulants for the same doses used. Also, potassium ferrate(VI) produces less sludge volume and removes more contaminants, which should make subsequent sludge treatment easier. q 2005 Elsevier Ltd. All rights reserved. Keywords: Coagulation; Disinfection; Potassium ferrate(VI); Sewage treatment; Sludge production 1. Introduction Environmental regulation and public health concerns require that sewage collected from communities must be treated to the given standards before it is returned to surface waters or reused. Sewage contains a range of contaminants, which are either suspended or dissolved. Based on their characteristics, the contaminants in the sewage can be divided into four major groups, namely, total solids (e.g. suspended and colloidal solids), inorganic constituents (phosphorus, nitrates and heavy metals), organic constituents (e.g. proteins, carbohydrates and oils and fats) and microorganisms (e.g. bacteria, fungi, protozoa and viruses), of which about two thirds are organic constituents. Most sewage treatment works consist of screens, grid and fat removal tanks, primary settlers, activated sludge processes or biofiltrators and secondary clarifiers. Primary treatment aims to enhance the removal of colloidal particles, organic constituents and harmful microorganisms and leads to less remaining particles and organic contaminants; this is favourable to the subsequent biological and physicochemical treatment processes. * Corresponding author. Tel.: C44 1483 686609; fax: C44 1483 450984. E-mail address: [email protected] (J.-Q. Jiang). 0301-4797/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2005.06.009 Coagulation and oxidation/disinfection are two important unit processes for water and wastewater treatment. Coagulation destabilizes colloidal contaminants and transfers small particles into large aggregates and adsorbs dissolved organic materials onto the aggregates, which can then be removed by sedimentation and filtration. Disinfection is designed to kill the harmful organisms (e.g. bacteria and viruses) and oxidation is used to degrade various organic contaminants. A wide range of coagulants and oxidants/disinfectants can be used for water and wastewater treatment. The most common coagulants used include ferric sulphate, aluminium sulphate, and ferric chloride, and the oxidants/disinfectants used are chlorine, sodium hypochlorite, chlorine dioxide, and ozone (Jiang and Lloyd, 2002). An efficient water treatment chemical reagent should ideally be able to disinfect microorganisms, partially degrade and oxidise the organic and inorganic contaminants, and remove colloidal/suspended particulate materials and heavy metals. A potential chemical reagent that meets these criteria is a ferrate(VI) salt (e.g. potassium ferrate), which is a very strong oxidant; under acidic conditions, the redox potential of ferrate(VI) ions is strongest among all oxidants/ disinfectants used for water and wastewater treatment. It is also a coagulant; during the oxidation/disinfection process, ferrate(VI) ions are reduced to Fe(III) ions or ferric hydroxide, and this simultaneously generates a coagulant in a single dosing and mixing unit process (Jiang and Lloyd, 216 J.-Q. Jiang et al. / Journal of Environmental Management 79 (2006) 215–220 Table 1 Characteristics of sewage (January–March 2003) Due to the variation in sampling date and time and flow rate, the characteristics of the sewage varied and this can be seen in Table 1. Parameter Range pH Turbidity (NTU) Total suspended solids (mg LK1) UV254-abs (cmK1) Colour as Vis-abs at 400 nm (cmK1) Total COD (mg LK1) Soluble CODa (mg LK1) Total Coliform (per 100 mL) Faecal Coliform (per 100 mL) 7.25–7.88 29.4–73.3 97–303 0.168–0.316 0.011–0.041 353–527 154–194 4!108–2.2!109 3.3!108–2!109 a Samples were filtered with 0.45 mm filter before the COD measurement. 2002). More importantly, ferrate(VI) is also an environmental friendly treatment chemical, which will not produce any harmful by-products in the treatment process. Previous studies have shown that potassium ferrate (VI) can perform superiorly in degrading various synthetic and natural organic pollutants (Bartzatt and Carr, 1986; Sharma and Bielski, 1991; Bielski et al., 1994; Norcross et al., 1997; Gulyas, 1997; White and Franklin, 1998; Sharma, 2002; Jiang and Wang, 2003a), inactivating harmful microorganisms (Murmann and Robinson, 1974; Gilbert et al., 1976; Schink and Waite, 1980; Kazama, 1995; Jiang and Wang, 2003b), and coagulating colloidal particles and heavy metals (Bartzatt et al., 1992; Jiang et al., 2001; Jiang, 2003). This paper explores the potential of using potassium ferrate(VI) for sewage treatment. The performance of potassium ferrate(VI) was compared with conventional coagulants in terms of reducing the turbidity and total and dissolved COD, the inactivation of bacteria, and the comparative sludge production. 2. Sewage Sewage samples were collected from a Sewage Treatment Works of Thames Water, and obtained from the plant influent prior to the primary grit chamber. Samples were collected in high-density polyethylene (HDPE) jerricans, which were washed and rinsed with tap water before use. 3. Experimental procedures for coagulation and disinfection Comparative performance in treating sewage with potassium ferrate (FR) and two traditional coagulants, ferric sulphate (FS) and aluminium sulphate (AS) was evaluated by a standard jar test procedure. The FR was prepared in the laboratory following a modified chlorination procedure (Thompson et al., 1951), in which a strong alkaline hypochlorite solution was made by the reaction of a given amount of KMnO4, HCl and KOH, and then reacted with ferric nitrate to form the FR. AS and FS were provided by Merck Ltd, UK. The coagulation pH was adjusted to either 5 or 7 in the use of FR but was not adjusted when using AS and FS. The final pH varied from 6.75 to 7.48 depending on the coagulant doses. The selected coagulant doses ranged from 0.07 to 0.56 mmol LK1 as either Al or Fe and this is shown in Table 2. Blank samples were used for all jar tests conducted, where no coagulant was added, but the sample pH was adjusted if it was necessary. Upon the coagulant being dosed, the sewage sample was rapidly mixed at a speed of 400 rpm for a period of one minute and then allowed to flocculate at a speed of 35 rpm for a period of 20 min. After slow mixing finished, the samples were allowed to settle for a period of 60 min. At the end of sedimentation, 100 mL of supernatant was withdrawn for the measurement of various quality parameters, including turbidity, colour (as Vis400-abs), total and dissolved COD and total and faecal coliforms. The removal percentages were calculated based on the parameters of the blank samples (no coagulant addition, only pH adjustment). The pH was adjusted with the addition of either sulphuric acid or sodium hydroxide. The procedures for measurement of water quality parameters followed the Standard Methods (AWWA et al., 1992). Table 2 Chemical doses for sewage coagulation AS FS FR (mg Al LK1) (mmol Al LK1) (mg Fe LK1) (mmol Fe LK1) (mg Fe LK1) (mmol Fe LK1) 4 8 12 15 0.15 0.30 0.44 0.56 6 10 16 22 28 0.11 0.18 0.29 0.39 0.50 4 7 10 15 18 22 26 30 0.07 0.13 0.18 0.27 0.32 0.39 0.46 0.54 J.-Q. Jiang et al. / Journal of Environmental Management 79 (2006) 215–220 FR (33.8 NTU) AS (31.1NTU) FR (416ppm) FS (33.1 NTU) % Removal of total COD % Removal of turbidity AS (413ppm) FS (410ppm) 40 100 80 60 40 20 0 35 30 25 20 15 10 5 0 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.00 Coagulant dose as Al or Fe (mmol L-1) Results of coagulation and disinfection of sewage are presented and discussed as follows. For all figures presented, the numbers in brackets represent the values of the sewage quality parameters of the blank samples, which underwent the jar test including the pH adjustment but without the coagulant addition. Fig. 1 shows the comparative performance of turbidity removal with three coagulants. In general, AS and FS can remove about 80–85% of turbidity and their performance was similar. However, over the dose range studied, FR can achieve 85–95% turbidity removal; the advantage of using the FR thus appears. Moreover, the superior performance of FR has been demonstrated for colour removal as shown in Fig. 2. FR can remove approximately 50% more colour than AS and FS for the same doses when the chemical doses are greater than 0.18 mmol LK1. This is also the case when comparing the total and dissolved COD removal (Figs. 3 and 4). Percentage removal of total COD with FR was 5– 20% more than that with FS, and 5–28% more than that with AS for the same doses. The overall removal performance for dissolved COD was not as good as that of total COD, but FR still can remove 5–10% more dissolved COD than AS and FS (Fig. 4). The outstanding performance in reduction of COD with ferrate(VI) is consistent with previous studies, where a range AS (0.010) FS (0.010) 100 80 60 40 20 0 0.00 0.20 0.30 0.40 0.50 0.30 0.40 0.50 0.60 of organic contaminants have been shown to be readily oxidised by ferrate (VI). The organic compounds investigated were alcohol (Norcross et al., 1997), carboxylic compounds (Bielski et al., 1994), amino-acids (Sharma and Bielski, 1991), phenol (Rush et al., 1995), organic nitrogen compounds (Carr and Erickson, 1988), aliphatic sulphur (Bartzatt and Carr, 1986), nitrosamines compounds (Read et al., 1998), recalcitrant organics (Gulyas, 1997), thiourea (Sharma et al., 1999), chlorine oxyanions (Carr and Mclaughlin, 1988) and hydrazine compounds (Johnson and Hornstein, 1994). All these organic contaminants could be present in the wastewater, which is a mixture of municipal sewage and industrial effluents. The high oxidation efficiency of ferrate(VI) in reducing COD can be attributed to the high redox potential; under acidic conditions, the redox potential of ferrate (VI) ions is higher than that of molecular ozone, and ferrate(VI) possesses the strongest oxidation capacity among all oxidants/disinfectants realistically applicable to water and wastewater treatment (Jiang and Lloyd, 2002). Due to its high re-dox potential, FR not only oxidises organic matter present in sewage, but also kills bacteria that traditional coagulants lack the power to do. This study demonstrates that at both pH 5 and 7, FR can achieve more than 4Klog10 bacteria inactivation, whilst AS and FS can FR (165ppm) 0.60 AS (164ppm) FS (164ppm) 0.30 0.50 40 35 30 25 20 15 10 5 0 0.00 0.10 0.20 Fig. 3. Comparative total COD removal performance with three coagulants. % Removal of soluble COD 4. Results of coagulation and disinfection FR (0.012) 0.10 Coagulant dose as Al or Fe (mmol L-1) Fig. 1. Comparative SS removal performance with three coagulants. % Removal of colour (as Vis-abs, 400 nm) 217 0.10 0.20 0.40 0.60 Coagulant dose as Al or Fe (mmol L-1) Coagulant dose as Al or Fe (mmol L-1) Fig. 2. Comparative colour removal performance with three coagulants. Fig. 4. Comparative dissolved COD removal performance with three coagulant. 218 J.-Q. Jiang et al. / Journal of Environmental Management 79 (2006) 215–220 Table 3 Comparative performance of bacteria inactivation (in log10 terms) a Coagulation pH Total Coliform Faecal Coliform a Simhoff : unsettled suspended solids in the supernatant of Imhoff cone after sedimentation b AS FS FR 6.75–7.48 0.89–1.05 0.96–1 6.75–7.48 0.89–1 1–1.05 5 O4 O4 7 O4 O4 a AS and FS dose required was O0.50 mmol LK1 as either Al or Fe(III). FR achieved O 4 log10 bacteria inactivation at doses !0.27 mmol LK1 as Fe(III). Ssludge deposit : sludge in the bottom of Imhoff cone after coagulation and sedimentation, including sludge from contaminants removed and the coagulant used. b only achieve approximately 1Klog10 bacteria inactivation (Table 3). Chlorine is commonly used as a disinfectant for drinking water and wastewater treatment. Since the discovery of chlorinated by-products (e.g. trihalomethanes), which cause potentially negative health effects in human beings (Rook, 1974), great efforts have been made to either minimise the concentration of natural/synthetic organic compounds prior to disinfection, or remove the chlorinated by-products after disinfection. However, this will greatly increase the overall cost of water treatment. Alternative disinfectants (e.g. bromine, iodine, chlorine dioxide, and ozone) have thus been considered to replace the chlorine. However, they form a range of other by-products, which are also considered to be toxic to some extent to the human population and to aquatic life. For these reasons potassium ferrate(VI) has been investigated as an alternative to chlorine for the disinfection of water and wastewater. The disinfecting properties of ferrate(VI) were first observed by Murmann and Robinson (1974) when they investigated the effectiveness of ferrate(VI) as a disinfectant to kill two pure laboratory cultures of bacteria (Non-recombinant Pseudomonas and Recombinant Pseudomonas). Later, the study conducted by Gilbert et al. (1976) showed that at pH 8.2 and a dose of 6 mg LK1 as Fe, ferrate(VI) could kill 99.9% of Escherichia coli (E. coli) when the contact time was 7 min. Another study using ferrate(VI) to disinfect secondary effluent (Waite, 1979) revealed that 99.9% of total coliforms and 97% of the total viable bacteria could be removed by ferrate(VI) at a dose of 8 mg LK1 as FeO2K 4 . Our previous studies of using ferrate(VI) for drinking water treatment (Jiang et al., 2001; Jiang, 2003; Jiang and Wang, 2003b) have also confirmed that ferrate (VI) has sufficient disinfection capability to kill E. coli. At pH 8.0, at a dose of 6 mg LK1 as Fe(III) and a contact time of 30 min, ferrate(VI) can achieve more than 5–6 log10 inactivation of E. coli. To achieve the same disinfection efficiency for the same conditions, combined chemical doses (chlorine and ferric sulphate) were required; which were 10 mg LK1 chlorine (as Cl) plus 4 mg LK1 ferric sulphate (as Fe). The studies also demonstrated that the disinfecting ability of ferrate(VI) increased markedly if the water pH was below 8.0. Fig. 5. Schematic representation of mass balance after chemical coagulation. 5. Study of sludge production The sludge generated from primary sedimentation and coagulation processes has a large volume and contains a high percentage of water, which requires an extra treatment process to handle. Water industries wish to use a coagulant with good treatment performance and less sludge production and thus ease the sludge treatment process. Comparison of the sludge production is thus useful for the selection of coagulants. Sludge production under the optimum operating conditions was studied by the use of a cone-shaped vessel called an Imhoff cone (Fig. 5). The optimum treatment conditions resulting from the previous studies were selected for the sludge production experiments and these can be seen in Table 4. 1 L of sewage was placed in the Imhoff cone, then the coagulant was added at the given optimum dose as shown in Table 4 The optimum conditions for sludge production experiments Coagulant Optimum dose as Al(III) or Fe(III) (mg L Blank AS FS FRlow FRhigh ) K1 0 10 20 15 20 (mmol L K1 0 0.37 0.36 0.27 0.36 pH ) 7 7 7 7 7 Table 5 Wastewater sludge volumes Coagulant Dose as Al(III) or Fe(III) (mmol LK1) Sludge production (in mL) Blank AS FS FRlow FRhigh 0 0.37 0.36 0.27 0.36 20 40 40 30 35 J.-Q. Jiang et al. / Journal of Environmental Management 79 (2006) 215–220 219 Table 6 Mass balance of sewage sludge Coagulant and dosagea AS (0.37) FS (0.36) FRlow (0.27) FRhigh (0.37) a SS in raw sewage SS in supernatant (Imhoff) Sludge deposit (Imhoff) Coagulant produced sludge Contaminants removed (g) (g) (mL) (g) (mL) (g) (g LK1 sewage) 0.382 0.382 0.382 0.382 0.0364 0.0204 0.0335 0.0285 960 960 970 965 0.47 0.49 0.51 0.54 40 40 30 35 0.035 0.042 0.032 0.042 0.017 0.046 0.063 0.088 Dosages were presented in brackets, as Al or Fe, mmol LK1. Table 4, and the mixture was rapidly mixed at a speed of 400 rpm for one min and then allowed to flocculate at a speed of 35 rpm for a period of 20 min. After fast and slow mixing, the solution was allowed to settle for a period of 60 min. The sludge volume produced was measured and is shown in Table 5. It can be seen that a slightly lower volume of sludge was produced with FR at a similar dose compared with AS and FS. The total suspended solids (SS) in both sludge deposit and supernatant in the Imhoff cone were measured following the standard methods (AWWA et al., 1992), and the results were used to calculate the mass balance of sludge production. The components considered to contribute to the sludge production are the contaminants in the raw sewage and the solids produced from the coagulant. Based on the mass balance, the contaminants removed for a given 1 L sewage sample can be calculated using Eq. (1): ½Removed contaminants Z SSludge deposit KSRaw KSImhoff KScoagulant (1) where: SSludgedeposit is the mass of the sludge in the bottom of Imhoff cone after coagulation and sedimentation, SRaw is the mass of suspended solids in raw sewage, SImhoff is the mass of suspended solids remained in the supernatant after sedimentation, and SCoagulant is the mass of the sludge produced by adding the coagulant. Table 7 Comparative performance of coagulants at optimum dose pH Optimum dose as Al(III) or Fe(III) (mmol LK1) Turbidity removal (%) Colour (Vis400-abs) removal (%) Total COD removal (%) Dissolved COD removal (%) Bacteria inactivation (in log10 terms)a a AS FS 6.75–7.48 0.37 6.75–7.48 0.36 80 50 6 4 1 86 50 16 7 1.05 FR The right part of Eq. (1) can be determined by the measurement of suspended solids in various samples together with a theoretical calculation to determine the sludge produced from the coagulant used. In terms of the previous studies (Montgomery, 1985), 1 g of Al produces 3.46 g of dry aluminium hydroxide and 1 g of Fe(III) produces 2.1 g of ferric hydroxide. Using the established ratio and the coagulant dosages, the coagulant sludge can be estimated. Thus, the left part of Eq. (1), the net contaminants removed from 1 L of sewage, can be determined. The study results can be seen in Table 6. It is evident that FR can remove more contaminants (e.g. organic materials as COD or colour) and produce less wet sludge than FS and AS at an equivalent dose; this should then make the handling of the resulting sludge easier. 6. Conclusions In sewage treatment, potassium ferrate(VI) can remove 50% more colour (Vis400-abs) and 30% more COD, and inactivate 3-log10 more bacteria in comparison with AS and FS at the same or even smaller doses (Table 7). In addition, using ferrate(VI) produced less sludge volume, which should then make sludge treatment easier. Acknowledgements The authors thank the UK Engineering and Physical Science Research Council (EPSRC) and Thames Water that provided a CASE studentship for A. Panagoulopoulos. 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