Sequential “electrochemical-peroxidation – electro- electro-Fenton” process for the treatment of anaerobic
MATERIALS AND METHODS Sample collection and characteristics
The sample came from a poultry farm in Singapore, which generates approximately 150 tons per day of anaerobic sludge; this sludge requiring proper attention before disposal. It contains very high Chemical Oxygen Demand (COD) and total suspended solid (TSS), which is far from reaching the National Environmental Agency (NEA) sewer discharge standards. The samples were collected and stored in polypropylene containers at 4 oC. The chemical characterization of the raw sample is presented in Table 1.
Table 1. Chemical composition of the anaerobic sludge with the NEA permissive discharge promote hydrolysis of the cell polymeric substances. All the experiments consisted of conducted in a batch glass electrolytic reactor with 0.5 L capacity, powered by a HAMEG 7042-5 power supply (Germany). For the EC and ECP trials, both anode and cathode were pure Fe plates with dimensions 5 x 6.5 x 0.5 cm. The gap between the electrodes was fixed at 4 cm. The working volume was 400 mL and the experiments were conducted at room temperature with constant stirring at 500 rpm. For ECP experiments, H2O2 (30% w/w) was manually added at a fixed time interval of 10 minutes for the first hour and 15 minutes for the second hour. The pH of the treated sample was adjusted to 9 at the end of treatment in order to promote the precipitation of Fe(OH)3. The treated effluent was filtered with vacuum to separate the filtrate from the solid sludge. The sludge was dried in an oven at 55oC overnight for further analysis. 175 mL of the filtrate were recovered and acidified to pH 3 before EF treatment. For EF, a BDD anode (15 x 2.5 x 0.1 cm) and a carbon brush cathode were utilized.
Analytical methods
COD, phosphate (P-PO4), nitrite (N-NO2), nitrate (N-NO3) and ammonium (N-NH4) concentrations were determined with colorimetric standard methods using HACH vials tests.
Total Organic Carbon (TOC) and Total Nitrogen (TN) were determined using a Shimadzu TOC-V CSH analyser. Biochemical oxygen demand at 5 days (BOD5) was determined by respirometric method using an OxiTop® control system (WTW GmbH, Germany). TSS content was measured using a standard method (American Public Health Association, 1999).
The efficiency of treatment was analysed based on the respective TSS, COD and TOC removal efficiencies. For the ECP experiments, the samples were analysed after they settled down for one hour. The pH was measured with a VWR pHenomenal MU 6100H pH-meter.
For most EC-based experiments, the pH of the sample increased from 5 to 8. Determination of iron was carried out by inductively coupled plasma optical emission spectrometry (ICP-OES, PerkinElmer, America).
Cost calculation
For feasibility evaluation, an economic analysis was made taking into consideration the costs associated to the chemical consumption, energy consumption, and cost of sludge disposal.
The energy consumption was calculated based on Eq. 5, where Ecell is the applied voltage (V), I is the current (A), t is the experiment duration (h) and Vs is the sample volume (L).
Energy consumption (kWh L-1) = (EcellIt/1000Vs) (5)
In accordance with Singapore Power (SP) 2017 tariffs, the energy price was estimated at S$
0.16 kWh. For the chemicals, H2SO4 (96% w/w) was evaluated at S$ 0.2 kg-1and KOH at S$
0.2 kg-1. The electrolyte K2SO4 was estimated at S$ 0.2 kg-1, while H2O2 (30% w/w) accounted for S$ 0.4/kg. Disposal costs for dehydrated sludge, including transportation and charges for waste disposal, were evaluated at S$ 0.1 kg-1 of wet dehydrated sludge by assuming that these residues were not considered hazardous material (National Environment Agency SG). The total cost (S$ L-1sample) was calculated according to Eq. 6:
Total Cost S$ L-1sludge = (6)
RESULTS AND DISCUSSION
Conditioning and stabilization: preliminary tests
The primary objective of the present part was to investigate the best strategy for sludge conditioning, aiming at disrupting the floc structure of the sludge. For this purpose, EC with Fe anode was used as the basis for the treatment. Three experimental strategies were explored:
i) EC with both iron electrodes, ii) EF using a graphite cathode and iii) ECP with iron electrodes and external addition of H2O2. The electrolyses were performed at 500 mA (corresponding to 15.4 mA cm-2) and the efficiency was evaluated in terms of TSS, COD and TOC removal and results are presented in Fig. 1.
Fig. 1. Removal efficiency of preliminary experiments. Experimental conditions: V = 400 mL, pH = 5, Na2SO4 = 0.1 M, i= 500 mA, 2 h-treatment. For EC, two iron plates were used as electrodes. For EF, iron anode and graphite cathode. For ECP (iron electrodes), H2O2 was progressively added during the treatment.
The results showed limited TSS, COD and TOC removal efficiencies by EC or EF, while the best performance was achieved by means of ECP (68, 74 and 69% of TSS, COD and TOC removal efficiency, respectively). These results can be explained in terms of the complexity of the sludge sample (high content of organic matter and suspended solids, and low filterability).
In the case of EC, the coagulation process promoted by the electrochemical release of
Fe2+/Fe3+ ions was not efficient enough to disrupt the polymeric material and induce proper coagulation. For EF (Fe anode and graphite cathode), two issues were encountered: first, the electrochemical production of H2O2 on the surface of graphite was not sufficient for the constant production of •OH via the Fenton’s reaction (Eq. 3) and second, the rate of the Fenton’s reaction was slow at the operating pH of 5. It is well known that the Fenton’s reaction has an optimal operating value of 3 according to Fe3+ ions speciation in solution (Pignatello et al. 2006). One EF experiment was performed at pH 3 (results not shown) and the removal efficiencies were not significantly improved. Although many studies have demonstrated that EF is very efficient in degrading organic matter (Moreira et al. 2017;
Brillas & Martínez-Huitle 2015), its direct application to a highly contaminated anaerobic sludge waste in this investigation did not give satisfactory results.
The external addition of H2O2 in ECP (1:5 [Fe2+]/[H2O2] ratio, calculated from the theoretical amount of iron released from the anode according to Faraday’s law, m = ItMw/nF) significantly enhanced the removal efficiencies, dramatically improving EC performance. In fact, H2O2 promoted the Fenton’s reaction for the production of •OH, which contributed to the oxidation of the organic material, ultimately provoking disintegration of the sludge flocs, releasing free water and exposing cellular material to further oxidative attack. In this way sludge conditioning and stabilization was effectuated by a combined coagulation/Fenton’s oxidation process. The control experiment with H2O2 addition without application of external current evidenced the low oxidative power of H2O2 in the absence of Fe2+ ions: TSS, COD and TOC were only removed by 32%, 11% and 18%, respectively. The inset panel in Fig. 1 depicts the visual appearance of the samples throughout the experimental period.
ECP optimization
Taking into account the preliminary experiments, ECP was selected as the conditioning and stabilizing method. Accordingly, the process was optimized with relation to the main parameters affecting its performance, namely: pH, current and [Fe2+]/[H2O2] ratio.
Current density is a key operating parameter affecting the performance of electrochemical processes since it controls the rate of the electrochemical reactions taking place. In the case of EC and related processes, it is particularly important because, according to the Faraday’s law, current is directly linked to the production of Fe2+ ions, whose precipitation as Fe(OH)3
determine the coagulation efficiency. Furthermore, when H2O2 is used (ECP), the amount of dissolved Fe2+also determines the chemical production of •OH (Eq. 3). Thus, the performance of ECP was investigated at different current values (100 mA, 500 mA and 1000 mA) with initial pH 5 and [Fe2+]/[H2O2] ratio of 1:5. Results are presented in Fig. 2, showing that the removal efficiency increased as the current increased from 100 mA to 500 mA, with the latter value giving the best TSS, COD and TOC removal efficiencies (68%. 74% and 69%
respectively). This behaviour can be explained in terms of the amount of Fe2+/Fe3+ ions produced during the electrolysis, which increased with current density. Thus, an increase in both coagulant dose and Fe2+/Fe3+ free ions, resulted in a better coagulation/oxidation ECP process. On the contrary, the application of higher current values (1000 mA) did not help further increase the removal efficiency, which can be ascribed to: i) H2O2 ‘s higher tendency to oxidize to O2 at high current as shown in reaction 7 and 8 and most importantly ii) an excess of Fe2+ ions that promoted the competitive waste reactions 9 and 10, consuming •OH.
Moreover, the rise in current density (and in the applied potential consequently) entails heat generation and higher energy consumption, thus, the current density shouldn’t be increased to very high values to prevent such adverse effects (the optimal value was 500 mA).
H2O2 ĺHO2•+ H++ eí (7)
HO2•ĺ O2 + H++ e- (8)
Fe2++ •OH ĺ)H3+ + HO- (9)
Fe3++ H2O2 ĺ)H2++ HO2•+ H+ (10)
Fig. 2. Effect of current on the TSS, COD and TOC removal efficiency at pH 5 and 1:5 [Fe2+]/[H2O2] ratio. ECP with iron plate electrodes. V = 400 mL, Na2SO4 = 0.1 M, 2 h-treatment.
The pH plays a fundamental role in ECP as it affects iron solubility, complexation and redox cycling between Fe2+and Fe3+ ions (Brillas et al.2009; Yahiaoui et al. 2011). To examine the effect of pH on the treatment of anaerobic sludge, a series of experiments with pH ranging from 3 to 8 (with 8 being the initial pH of the sludge sample) were conducted under 500 mA of current and a [Fe2+]/[H2O2] ratio of 1:5. The results (Fig. 4) showed that ECP displayed the best efficiencies under slightly acidic condition (pH 5) with 68%, 74% and 69% of TSS, COD and TOC removal efficiency, respectively. Even though Fenton-based processes generally operate optimally in acidic medium (pH between 2 and 4), during ECP, the efficiency of the Fenton’s reaction is coupled to the coagulation process, whose optimal pH value is found between 5-8 due to Fe(OH)3 formation (Lakshmanan et al. 2009). Accordingly, the performance is governed by a compromise between electrocoagulation and Fenton’s oxidation. In this way, under acid conditions, coagulation efficiency dropped, dramatically impacting ECP performance. On the other hand, at high pH values (pH 8), the rate of the Fenton’s reaction was significantly diminished as ferric hydroxide precipitated, hence hindering the production of •OH. Additionally, at higher pH, the scavenging of •OH by carbonate species (CO32-\HCO3-) is another factor that led to a decrease in the removal efficiency (Pignatello et al. 2006). The optimal pH value of 5 favoured both coagulation and Fenton’s oxidation, ensuring enough Fe2+ ions for H2O2 decomposition via the Fenton’s reaction, while a progressive increase of the pH during the electrolysis assisted the coagulation process.
Fig. 3. Effect of pH on the TSS, COD and TOC removal efficiencies at 500 mA and 1:5 [Fe2+]/[H2O2] ratio. ECP with iron plate electrodes. V = 400 mL, Na2SO4 = 0.1 M, 2 h-treatment.
The concentration of H2O2 plays a crucial role in ECP because •OH concentration is directly dependent upon Fe2+ and H2O2 concentration. In general, the efficiency increases as the [Fe2+]/[H2O2] ratio rises, but the optimal ratio depends on the characteristics of the treated effluent (Pilli et al. 2015; Moussavi & Aghanejad 2014). The effect of H2O2 concentration ([Fe2+]/[H2O2] ratio of 1:1, 1:5 and 1:10) was evaluated under 500 mA of current and pH 5.
According to the results (data not shown), the removal efficiency did not change significantly at these three ratios. This behaviour can be ascribed to an excess of H2O2 concentration: on the one hand, the waste reaction of H2O2with •OH (Eq. 11) was promoted by high contents of H2O2. On the other hand, at a concentration higher than the optimum value, H2O2 self-decomposed to H2O and O2 as shown in Eq. 12, leading to less •OH production. The 1:5 [Fe2+]/[H2O2] ratio was chosen as the optimal value because of the properties of the solid sludge obtained at the end of the 2 h treatment, which presented better filterability and settleability properties as compared to the sludge produced with the 1:1 ratio.
H2O2+•2+ĺ+22•+ H2O (11)
2H2O2 ĺ22 + H2O (12)
After 2 h of ECP treatment under optimal conditions (pH 5, 500 mA of current and [Fe2+]/[H2O2] ratio of 1:5), a sludge cake with good settleability was obtained. The treated sludge was vacuum-filtered and further analysed. Table 2 shows the chemical composition of the dry dewatered sludge. It can be seen that it contains an important amount or organic carbon, P-PO4, N-NH4 and NO3-, which confer it fertilizing properties. As expected, Fe was significantly present in the sludge with a concentration of 70.58 mg L-1.
Table 2. Chemical composition of ECP dry sludge after treatment.
Fe TVSS N-NH4 N-NO3- P-PO4 TC
mg L-1 70.58 0.54 7.00 8.00 100 14.67
EF post-treatment
ECP alone was able to achieve TSS, COD and TOC removal efficiencies of 89%, 85% and 67
% respectively. However, the liquid sample still contained a high concentration of COD and TOC. Accordingly, the remaining filtrate after dewatering was further treated by means of EF using a BDD anode and a carbon brush cathode. The aim was to make use of the outstanding EF capacity to mineralize organic pollutants, as stated in many previous studies (Lin et al.
electrolysis was performed at 500 mA (13.3 mA cm-2 with respect to the anode) and pH 3 (Olvera-Vargas, Oturan, et al., 2014; Mousset, Wang, et al., 2016). The Fenton’s reaction was catalysed by the remnant amount of Fe2+/Fe3+ions form the ECP process (7.35 mg L-1). The evolution of COD and TOC during the sequential ECP-EF treatment are presented in Fig. 4.
As expected, during EF the concentration of organic matter decreased with time due to the oxidative attack of •OH formed i) homogeneously through the Fenton’s reaction (Eq. 3) and ii) heterogeneously on the BDD surface according to Eq. 4.
The remaining COD and TOC content after ECP was reduced by 90% and 63%, respectively, following 4 h of EF treatment, reaching a COD content only slightly higher than the Singaporean standard limit for discharge into watercourse and recreation purposes (100 mg L
-1). The BOD5 (120 mg L-1) was below the NEA limit (400 mg L-1). More remarkably, COD was reduced to less than 50 mg L-1 after 6 h of treatment, overpassing the NEA watercourse discharge limit. It is worth mentioning that the solution became crystal clear after only 2 h of EF (cf. inset in Fig. 4).
Fig. 4. Overall COD and TOC evolution during the sequential ECP-EF process. For ECP: iron plate electrodes, V = 400 mL, Na2SO4 = 0.1 M, I= 500 mA, 1:5 [Fe2+]/[H2O2] ratio, pH = 5 and 2 h-treatment. For EF: V = 175 mL, Na2SO4 = 0.1 M, I = 500 mA, pH = 3 and 4 h-treatment. Inset panel: visual evolution of the sludge sample during the overall h-treatment.
The cost for the overall ECP-EF treatment was estimated according to the method described in the materials and methods. The total cost per litre of sludge, including conditioning, ECP, precipitation of iron excess, EF post-treatment and final pH adjustment, was estimated to be S$ 0.053 L-1, about half of the cost of disposal by incineration. It is important to bear in mind that the present strategy contemplates discharge into watercourse and for recreation purposes.
Furthermore, the solid produced after ECP could be sold as a fertilizer. As observed in Fig. 5, the chemical cost represents the largest percentage (77%), while power and sludge management costs occupied the remaining fraction (19% and 4%, respectively). The cost break-down for each stage depicted in Fig. 5b indicates that the intermediate operations, conditioning and filtration, took up to 12% and 9%, respectively.
Fig. 5. Cost breakdown for each stage of the sequential treatment (a) and total operational costs per category (b). The costs per litres of treated sludge.
CONCLUSIONS
A sequential electrochemical treatment was applied to condition and stabilize anaerobic sludge from a poultry farm. The two-step integrated ECP-EF treatment consisted in: i) 2 h-ECP at pH 5, current of 500 mA and [Fe2+]/[H2O2] of 1:5, followed by ii) 6 h-EF (at pH 3 and 500 mA) of the residual filtrate after dewatering. During the ECP stage, the sludge was efficiently destabilized and conditioned by the synergistic effects of electrocoagulation and the Fenton’s reaction, which resulted in production of a dewatered sludge with good settling and filterability properties. The filtrate after dewatering was subjected to EF, during which the excess of organic matter was efficiently oxidized by means of •OH. The produced effluents after 2 h of ECP and 4 h of EF were able to reach the NEA sewer discharge standards in terms of COD (600 and 180 mg L-1, respectively), BOD5 (400 and 130 mg L-1, respectively) and TSS (400 and 50 mg L-1, respectively). Furthermore, following 6 h of EF, the effluent reached the COD watercourse discharge limit of 50 mg L-1 of COD. The total energy consumption was 0.0634kWh L-1sludge, while the estimated total operational cost for the treatment was S$
0.053 L-1 sludge. In conclusion, electrochemical technology is presented as a sustainable potential alternative for sludge remediation through this two-step integrated ECP-EF process.
ACKNOWLEDGMENTS
The authors would like to thank the Singapore Ministry of Education Academic Research Fund Tier 1 through the grant WBS R302000145112.
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