Disinfection of raw wastewater and activated sludge effluent using Fenton like reagent
© Aslani et al.; licensee BioMed Central. 2014
Received: 19 September 2014
Accepted: 6 December 2014
Published: 14 December 2014
Background and objectives
Water shortage problems have led to find either new water resources or improve wastewater treatment technologies in order to reuse. Due to less performance of previous units in microbial removal, disinfection has become a necessary step in wastewater treatment plants. In the present study performance of hydrogen peroxide (HP) and modified Fenton’s reagent (HP + Cu++) was considered for the disinfection of raw wastewater (RW) and activated sludge effluent (ASE).
Materials and methods
Plastic containers of 10-liter volume each were used for RW and ASE sampling. Microbiological analyses of the RW and ASE were performed in triplicate; before and after the disinfection process. Fecal coliforms were analyzed by the direct (without enrichment) multiple fermentation tube procedure.
The results showed that using HP alone did not have any significant disinfection capability. In RW and ASE, the highest dose used in this study could reduce fecal coliforms (FC)s by only 1.54 and 1.16 log-inactivation, respectively. However, Maximum removal efficiency of modified Fenton in RW and ASE was 4.63 and 3.41 log-inactivation, respectively. The results suggested that Cu++ ions used in combination with H2O2 produced very rigorous synergistic effect, and HP disinfection capacity increased significantly.
Hydrogen peroxide, when applied alone, was not successful in disinfecting of either RW or ASE, and neither the WHO guideline nor the Iranian standard could be met. However, modified Fenton showed very significant disinfection potential and could reduce FCs under World Health Organization (WHO) guideline and Iranian national standard for agricultural irrigation.
Nowadays, most countries have water shortage problems and it is the main reason that scientists are looking for new water resources and also try to develop new technologies to reuse treated and, in some cases, untreated wastewater as a water resource. Using untreated wastewater is not common in developed countries; but, in some poor and developing countries because of the lack of provisions by the authorities, people use untreated wastewater for agricultural irrigation or some other uses such a washing clothes; such usage would be considered as an important source for exposing people to some water-related diseases. According to WHO, wastewaters containing less than 1000 CFU (colony forming unit)/100 ml of fecal coliforms (FC) and no more than 1 helminthes Egg/l can be safely used for irrigation purposes ,.
Different units (i.e. physco-chemical and biological) of wastewater treatment plants attempt to remove pathogenic microorganisms to some extent; however, in the case of microorganism’s removal, there are no specific physco-chemical or biological processes which could provide qualified effluent of category A, defined by World Health Organization. Therefore, final disinfection is an obligatory step in wastewater treatment, especially when final effluents are to be re-used -. Different kinds of disinfectants have been used for many years (e.g. chlorine, chlorine dioxide, ozone, uv irradiation, etc.). Although application of the mentioned disinfectants have been proved to be effective, there are still some practical limitations. It has been proven that using chlorine as a disinfectant can produce harmful disinfection by-products (DBPs); in addition, the problem of storage and safe handling has led recent studies to look for alternative disinfectants ,-. In the last few decades, hydrogen peroxide (HP) has been introduced as an environmentally friendly disinfectant for wastewater disinfection. It has disinfection capability and does not leave any unfavorable environmental effects ,. HP has weak disinfection capability by itself; however, recent research has shown that metallic cations such as silver (Ag+) and iron (Fe++) have a synergistic effect on HP disinfection potential -,,-. The main disinfection mechanism in the removal of microorganisms by HP is free OH• radical production; so, it is conceivable that when HP is combined with other metallic ions, more radicals are produced. Combining HP with ferrous iron such as Fe++ results in an oxidation system which is called Fenton’s reaction ,,,; also, when HP is combined with other ions such as copper (cu++), the system is known as modified Fenton’s or Fenton-like reaction .
The main objective of the present study was to investigate the feasibility of modified Fenton’s reagent for the disinfection of RW and ASE. The results were compared with the effectiveness of HP alone, copper ions alone, and also the previous results that investigated Fenton’s reagent performance in the disinfection of similar test samples under similar conditions. It should be noted that studying raw wastewater disinfection was essential in the present study, since many treatment plants, especially in developing countries, try to bypass the untreated influent in some occasions such as hydraulic overflow during rainy periods and failure of treatment processes, which cause microbial contamination in receiving water bodies.
The materials used in the experimental set up included copper chloride dihydrated, hydrogenperoxide 30% solution, sodium chloride, sodium thiosulfate pentahydrate, and A1-Medium. All the chemicals used in this study were purchased from Merck Company. The necessary stock solutions were made from the above materials and doubled distilled water. The glassware was washed daily and autoclaved at 121°C for 15 min before use.
Sampling and preparing the samples
Plastic 10-lit volume containers were used for RW and ASE sampling. The samples were taken from a municipal wastewater treatment plant in an activated sludge process located in the north of Tehran, Iran, and were immediately transported to the laboratory within an hour, where they were analyzed on a daily basis. The samples were taken from wastewater entering to the plant, and before disinfection unit for RW and ASE, respectively.
Experimental phases and applied doses of reacting substances
CT factor is used for evaluation and comparison of different phases. In this formula, C is the initial concentration of applied material and T is contact time in minute. Therefore, according to other studies, CT is calculated by multiplying the initial used concentration (C) by contact time (T).
Microbiological analyses of the RW and ASE were performed in triplicate before and after the disinfection process according to the doses shown in Table 1. Fecal coliforms were analyzed by direct (without enrichment) multiple fermentation tube procedure (Standard Methods, 9221E-2). The samples were inoculated and incubated on A1 medium and their ability to produce gas and turbidity was determined. For the enumeration of FC bacteria, the samples were incubated at 37°C for 3 h and the tubes were then transferred to a 44.5°C water bath for 19–21 h.
Results and discussion
Physico-chemical and microbiological characteristics of RW and ASE
580 ± 98
65 ± 32
347 ± 26
35 ± 16
769 ± 150
68 ± 20
7.61 ± .34
7.3 ± 14
9.2E + 06
5.5E + 05
Figure 3(b) shows the result of phase 3 of the experiments on ASE. The initial number of FCs in the ASE which was used for disinfection analysis was 5.1E + 05 MPN/100 mL. Different doses of HP and Cu++ were combined and evaluated (see Figure 3(b)), representing that, by combining 80 mg HP/L and 5 mg Cu++/L, it was possible to reduce the number of FCs to below the WHO guideline and Iranian Standard.
According to Figure 4(b), it is obvious that, when HP was applied as the only disinfectant, the number of FCs decreased by a logarithmic order, whereas applying a combination of HP and Cu++ led to the linear removal of FCs. In addition, the results showed that synergistic effect increased by increasing the concentration of HP and Cu++; in other words, synergistic effect increased by lowering HP to Cu++ ratio. Using modified Fenton’s reagent in the CT value of 2550, i.e. 80 mg/L HP plus 5 mg/L Cu++, 3.41 log-inactivation could be achieved, while using HP alone in the CT value of 2400 (related to 80 mg/L HP) only demonstrated 0.64 log-inactivation (see Figure 1(b)). Then, again, it is clear that Cu++ had a very high synergistic effect on HP performance; when combined, the overall efficiency was 3.41 log-inactivation; 1.92 of which was related to the synergistic effect of Cu++ (see Figure 5). In the case of ASE, again it was determined that disinfection performance of modified Fenton was more severe than that of Fenton’s reagent; e.g. according to the previous study , disinfection of ASE by Fenton’s reagent had 2.03 log-inactivation in the CT value of 15000; i.e. combination of 400 mg/L HP and 100 mg/L Fe++, whereas modified Fenton showed 2.37 log-inactivation in the CT value of just 1875 which was related to the combination of 60 mg/L HP and 2.5 mg/L Cu++.
Synergistic effect of Cu++ ions on HP disinfection performance is shown in Figure 5. According to this figure, it is obvious that the combination of 1 and 2 mg/L Cu++ ions with the fixed dose of HP, i.e. 900 mg/L, showed different synergistic effects of 2.41 and 2.66, respectively; then, it is clear that the synergistic effect was increased by increasing Cu++ concentration. On the other hand, it can be observed that the combination of 80 mg/L HP (minimum dose of HP used) and 5 mg/L Cu++ (maximum used dose) had a good synergistic effect, which was enough for FCs removal in ASE. Therefore, as Fenton’s reagent needed the HP doses of 2 to 10 times of Fe++ for the best performance, finding a fair ratio of HP and Cu++ combination seemed necessary. According to the WHO guideline for Cu++ in drinking water, healthy precautions must be taken into account; concentrations of above 5 mg/L imparts color and an undesirable bitter taste to water. Also,, WHO health-based guideline for Cu++ is 2 mg/L . The following studies using HP in combination with different kinds of metallic ions, such as iron, copper, and silver showed that, the same as chlorine and chloramines, it can provide a long-lasting residual biocidal action in the final effluent. Also, according to the previous studies ,,,,, this new commercially stabilized available combination does not produce any disinfecting by-products and is effective in controlling biofilms in water distributing pipes.
In some of the developing countries and also in countries with water shortage problems, using wastewater as a water resource for irrigation can compensate for most of the difficulties. However, its use poses adverse health hazards due to the high number of pathogenic organisms in wastewater. Although the secondary treatment of wastewater can reduce and remove most of the unwanted elements and produce a good-quality effluent in terms of nutritional constituents, to ensure microbiological quality, final disinfection is required. There are many conventional disinfectants such as chlorine, ozone, etc.; but, due to their numerous disadvantages, finding a ’non-conventional’ disinfectant with a long-lasting effect was considered in this research. In this regard, very satisfactory results were obtained by the follow-up treatment using HP in combination with copper.
Hydrogen peroxide, when applied alone, was not efficient in disinfecting either RW or ASE; morever, neither the WHO guideline nor Iranian standard could be met.
However, addition of Cu++ turned out to be very effective, as in RW, the CT value of 27060 reduced FC numbers by 4.63 log-inactivation, whereas using HP alone in the same CT value just showed 1.6 log-inactivation. In the case of ASE, the results were more reasonable and, in the CT value of 2550, 3.41 log-inactivation was observed compared to the lower than 1 log-inactivation in the CT value of 4500 demonstrated for HP alone. These results clearly indicated the synergistic effect of Cu++ on HP performance. Finally, it can be said that, from the stand point of technical feasibility, a combination of HP and Cu++ could reduce the number of FCs below the WHO guideline for irrigation in both RW and ASE. Further, in the case of ASE, it was possible to reduce FCs to as low as the Iranian standard, i.e. 400 MPN/100 mL, which is required for irrigation. However, economical feasibility of the process should be also studied in future works.
This research was supported by Tehran University of Medical Sciences and Health Services grant.
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