Combined Process of Ferrate Preoxidation and Biological Activated

Chapter 28

Combined Process of Ferrate Preoxidation and Biological Activated Carbon Filtration for Upgrading Water Quality *

Downloaded by UNIV OF OTTAWA on March 16, 2013 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch028

Jun Ma , Chunjuan Li, Yingjie Zhang, and Ran Ju School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, People's Republic of China Corresponding author: email: [email protected]; fax: 8645182368074 *

The preoxidation of polluted surface water with ferrate was conducted with respect to its impact on the following biofiltration. It was found that preoxidation with ferrate promoted the biodegradation of organics with substantial reductions of chemical oxygen demand (COD ), UV absorbance. It was also found that the removal of NH -N in biological activated carbon (BAC) process was also substantially improved as compared with the cases without ferrate preoxidation and with ozone preoxidation. In addition, the experiments were conducted related to the effect of potassium ferrate oxidation of raw water of Songhua River on its changes of molecular weight distribution in order to investigate further the enhancement of ferrate preoxidation on the removal of organics. The results indicated that the concentration of organics with molecular weight (MW) of 10k-100k and less than 0.5k were substantially increased after the raw water was coagulated with ferrate preoxidation, which suggested that these oxidation products are readily removed by subsequent biofiltration. Mn

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Introduction One of the principal methods for enhancing the biodegradation is preoxidation, which is generally aimed at destroying the structure of organics and then the formation of some products readily removed by the following biological treatment. Preoxidation partially oxidizes the dissolved organic 446

© 2008 American Chemical Society

In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

447 carbon (DOC), which results in an increase of assimilable organic carbon (AOC) and a decrease of organic micro-pollutants. The partially oxidized DOC is better biodegraded in BAC process than that in GAC process (The oxidant is immediately used before granular activated carbon column and no biocoenosis is detected on the column). Preoxidation processes have been widely studied in drinking water treatment to remove pollutants and to improve the biodegradability of organic constituents (7-6). Ozonation is the most commonly applied process for preoxidation. The 0 /BAC combination proved to be very efficient in reducing DOC because of the biodegradability enhanced upon ozonation (7-72). However, ozone oxidation has the disadvantages of high cost, inconvenient operation and maintenance. For example, ozone can reduce the levels of THMs and halo acetic acids (HAAs), but it can form the potent carcinogenic bromate ion by reacting with bromide present in water (75,14). Ferrate is environmental friendly water treatment agent that can meet these new challenges confronted in the water industry. Ferrate has been proved in previous studies as a selective and effective oxidant in degrading various synthetic and natural organic pollutants, inactivating microorganisms, and coagulating colloidal particles and removing heavy metals (75-20). More importantly, it is an environmental friendly treatment chemical, which does not produce any harmful by-products in water treatment process. In this paper, the potential role of Fe(VI) as preoxidants to enhance the following biological process in drinking water treatment was investigated through long-term experiment. The performance of ferrate was compared with ozone preoxidation in terms of removing COD , UV54-absorbance, ammonianitrogen. And the variation of molecular size of raw water induced by ferrate preoxidation was analyzed in a batch study.


3

Mn

2

Experiments Materials and Equipment Potassium ferrate (K Fe0 ) of high purity (98 % plus) was prepared by the method described by Thompson et al. (27). Potassium ferrate solution was prepared by dissolving potassium ferrate solid in distilled water just before use in order to minimize the loss of ferrate. Ozone was produced with an ozone generator (Tongli XFZ-58I, Tsinghua) that used dried oxygen. Distilled water was continuously bubbled with gaseous ozone. The aqueous ozone was monitored with ultraviolet spectrometer at A,=258nm (e=3000 M'cm")(22) until a constant concentration was reached (i.e. saturated ozone containing solution). This ozone containing solution was prepared just before addition. Aluminum Sulfate (A1 (S0 ) 18H 0, Tianjin chemical Inc., Tianjin, China) is selected as the coagulant. A polluted waterfromdown stream part of Songhua River was selected in this study. A summary of the raw water quality is shown in Table I. 2

4

1

2

4

3

l

2


448 Table I. Characteristics of the raw water from Songhua River Parameters PH

Turbidity (NTU) UV -abs (cm") COD (mg L") 1

254

1

Mn

+

Ammonia-nitrogen (NH -N)


4

Maximum

Minimum

Average

7.5 144.00 0.260 13.30

7.0 1.85 0.093 3.33 0.19

7.1 23.8 0.147 7.42 0.72

2.30

Experiment Procedures Dynamic Experiments

The effectiveness of micro-pollutants removal was studied by dynamic experiments in these processes, such as chemical preoxidation, coagulation and precipitation, filtration and BAC process. Potassium ferrate and ozone were selected as the preoxidant. The dosage of preoxidants was lmg L" . Preoxidation was conducted for 10 min at 300 rpm. The dosage of Aluminum Sulfate was 50 mg L* . The parameters such as turbidity, chemical oxygen demand (COD ), UV -absorbance and NH/-N were measured according to the standard methods(Water and Wastewater Monitoring and Analysis Method, the Fourth Edition, China, 2002). 1

1

Mn

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Batch Test

In order to have a deeper understanding of the mechanism of ferrate preoxidation to enhance the biological process for treating the polluted surface water, a batch study of preoxidation of raw water was further conducted. The experiment was carried out with a magnetic stirrer. 1 mg L" of potassium ferrate was added and mixed at a speed of 300 rpm for 10 min. Then, all the samples were coagulated by adding Aluminum Sulfate, followed by filtration through a 0.45 urn filter to separate any particulate matter. These samples were collected andfractionatedfor the molecular weight distribution using a stirred ultrafiltration cells (Amicon; model 8400). The samples were subsequently measured with a TOC (total organic carbon) analyzer (TOC-VCPH, Shimadzu). 1

Results and Discussion Dynamic Experiments The variation of parameters in different processes with ferrate is shown in Table I. It can be seen that turbidity, COD , UV -absorbance and NH -N +

Mn

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4

449 substantially decreased by 99.4%, 82.9%, 74.9%, 65.3%, respectively, after BAC, with ferrate preoxidation. Furthermore, BAC outlet values of turbidity and NH -N absolutely meet drinking water standards. It must be noted that NH -N concentration is not decreased, but rather increased after preoxidation and coagulation with ferrate. This phenomenon can be explained by the fact that organics containing nitrogen in raw water were partly oxidized to inorganic nitrogen by ferrate preoxidation. In the experiments, it was found that preoxidation with a low dose of potassium ferrate (lmg L") could promote the degradation of organics in the water, with substantial reduction of COD , UV 5 -absorbance and NH -N in BAC reactor as compared to the case without ferrate preoxidation. There were an additional average removal 5%, 12%and 47%, respectively with ferrate preoxidation compared to the case without ferrate preoxidation in BAC process. Obviously, the biodegradability of organics in raw water was improved by ferrate preoxidation. This might be due to the enhanced nitrification of BAC by oxidation products with ferrate and the transformation of higher molecular weight compounds into lower molecular weight compounds induced by ferrate, which could be readily removed by BAC process. Previous studies indicated that higher biological activity in GAC filter was achieved with preozonation of humic or fulvic solutions as compared with the case without ozone preoxidation +

+

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Mn

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4

(23,241

The slightly higher removal of COD and UV -absorbance was observed with ferrate preoxidation than that with ozone, as shown in Figure 1. It is necessary to note the higher removal of NH -N with ferrate than with ozone, in Figure 1. Such information could be seenfromFigure 2. The amount of NH -N removed by GAC with ferrate was up to 57% on average. Ferrate/GAC was always effective in reducing NH -N, when NH -N values of raw water were highly differential (0.19-4.12mg L" ). Major factors to effect the nitrification process were dissolved oxygen (DO), organic load, temperature and ammonia concentration (25). Nitrification was a biological nitrogen removal process, nitrosifying-bacteria (e.g. Nitrosomonas) oxidized ammonia to nitrite and nitrate. In this process, which required a reduced nitrogen species, an increasing in NH -N after ferrate preoxidation was an un-omitted factor (see Table II). On the other hand, due to the inhibition of heterotrophic bacteria on nitrification, the competing for DO was of more concern in the system. In our study, organic load was lower and DO was enough under aerobic condition. Therefore, heterotrophic bacteria had little competitiveness over nitrifying bacteria with both ferrate and ozone preoxidation. In contrast to ozone, ferrate/BAC resulted in an additional decreasing of 36% in NH -N. Under the same performance (DO, temperature, hydraulic load, etc), this additional decrease might be due to trace amount of iron existed, which was possibly favorable for Nitrosomonas bacteria and Nitrobacter bacteria. Unfortunately, the detailed study related to this was not performed deeply in this paper and will be done in the future work. The results of the long-term tests were presented to estimate the effect of both ferrate and ozone pretreatment on GAC running time. The data allowed us Mn

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Table II. Variation of parameters in different processes with ferrate preoxidation*


Parameters

Turbidity (NTU) UV -abs(cm )

Preoxidation and coagulation

Raw water

Filtration

BAC

23.80 0.147

0.72

0.13

0.13

0.063

0.060

0.025

Cooking L )

7.42

3.36

2.97

1.86

Ammonia-nitrogen

0.72

0.80

0.66

0.25

1

254

1

1

(mgL ) •Average values of long-term performance

CODMn

l'V, -abs

NII/-N

M

Parameters

Figure 1. The effect of different preoxidants on COD > UV 4-absorbance and NH/-N in BAC reactor Mn

25



451

Figure 2. The effect of different preoxidants on the variation after BAC

Figure 3. The effect of different preoxidants on the COD after BAC process

Mn

+

ofNH -N 4

with running time


452 to see if the BAC running time would be prolonged. As shown in Figure 3, the removal of COD was decreasing as the running time was increasing in three different tests. When the experiments had been conducted over 60 days, the removal of COD was 0%, 15.5% and 19.8% in GAC process, respectively, with none, ferrate and ozone preoxidation. These data indicated that the BAC running time was prolonged by either ferrate or ozone pretreatment. Mn

Mn


Batch Study As shown in Figure 4, ferrate preoxidation caused an additional removal of about 13% and an absolute removal of 27% after coagulation, which were mainly due to structural transformations of waters. The decrease in organics with molecular size of 100k-0.45u.rn and 0.5k-10k was observed. However, it was also noted that the organics with molecular size of 10k-100k and less than 0.5k were increased with ferrate preoxidation during coagulation. The decrease of both 100k-0.45u.rn and 0.5k-10k as well as the increase of less than 0.5k indicated that some high molecular weight organic substances were broken into smaller ones during oxidation. It was more interesting to note that the molecular size of 10k-100k was increased, which might be the result of polymerization induced by oxidation. It has been realized by some researchers that NOM affected the adsorption of trace organic compounds not only by directly competing for adsorption sites but also by blocking carbon pores (26,27). The decrease of the organic concentration of molecular size of 100k-0.45u,m reduced the pore blockage, so heterotrophic bacteria and nitrifying bacteria obtained enough space to propagate. In the GAC filter, the filtration mainly reduced the amount of intermediate and low MW (28). The decrease of 0.5k-10k helped BAC reactor prolonged the running time. Molecular size less than 0.5k having lower molecular weight, such as aldehydes and carboxylic acids, could be easily removed by using a BAC process (29). In general, larger molecules were microbially resistant, whereas smaller molecules were microbially labilefractionsof DOC, and ferrate preoxidation produced more available molecules for BAC process and reduced non-available ones.

Conclusion Preoxidation of organics with ferrate produced lower molecular weight compounds that could serve as substrate for microorganisms. In spit of the highly differential of NH/-N concentration in raw water, its removal rate in BAC with ferrate preoxidation was up to 57% on average. It was the most costeffective way to use biological nitrification with ferrate preoxidation to reduce ammonia load. Compared with preozonation, ferrate preoxidation has the



453

MVV ofihe samples

Figure 4. The change ofTOC in terms of different MWDOM with Al (S0 ) of 50 mg L' andferrate oflmg L' 1

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advantages of low cost, easy operation and maintenance. Ferrate preoxidation might be an economical method to enhance the conventional water treatment process on occasions with limited funds for capital investment. Combined process of ferrate preoxidation and BAC could efficiently purify drinking water.

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Combined Process of Ferrate Preoxidation and Biological Activated

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