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Control of biological iron removal from drinking water using oxidation-reduction potential
w.... Sci. T«II.
~ Pergamon PIT: S0273-1223(98)OO573-3
Vol. 38, No.6, pp. 121-128, 1998.
IAWQ
C 1998 PIIbJished by Elsevier Science LId. Printed in Great Britain. All righll reserved 0273-1223198 $19'00 + 0-00
CONTROL OF BIOLOGICAL IRON REMOVAL FROM DRINKING WATER USING OXIDATION-REDUCTION POTENTIAL Catherine V. Tremblay*. Andre Beaubien**. Philippe Charles*** and James A. Nicell* • Department o/Civil Engineering and Applied Mechanics, McGill University, 817 Sherbrooke St. W.• Montria~ Quebec, Canada, H3A 2K6 ** Gamma Innovation, 7995 Rhone, Brossard, Quebec, Cantula, J4X 2K7 **. Centre International de Recherche sur l'£au et l'Environnement, Lyonnaise des £aux, 38, rue du Prisident Wilson, 78230 Le Pecq, France
ABSTRACf Biological removal of iron to produce drinking water was established in a pilot plant treating raw water with a pH of 5.7. The objective was to evaluate the use of oxidation-reduction potential (ORP) as a control tool and detennine its relationship to dissolved oxygen (00) and residual iron concentration in filtered water from an operating biological filter. Results showed that above a low minimum value of DO, residual iron concentration and ORP were not affected by varying the 00 level. A non-linear regression was established 10 correlate total residual iron concentration to ORP with an R2 of 0.8848. This correlation can be used to predict iron concentration when ORP is in the range 300 to 470 mY. Below this range, total residual iron is greater than or equal to 3 mgll and above, total residual iron is len than the French regulation Iimil of 0.2 mgll. Pilot plant operating conditions were implemented in the primary filter of an industrial plant in France, improving iron elimination and doubling the length of the filtration cycle. ~ 1998 Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Dissolved oxygen; drinking water treatment; iron bacteria; iron removal; ORP control.
INTRODUCflON Groundwater is often mildly acidic and devoid of dissolved oxygen (reducing). Thus. ferrous iron (Fe 2+) is usually present, either in dissolved mineral fonn or associated with various mineral or organic chelating agents. French regulation indicates maximum admissible concentrations of 0.2 mg/l for iron. However, in current practice. iron is usually treated to a maximum concentration of 0.1 mgll to avoid problems such as corrosion and obstructions in the water distribution network over the long tenn. The World Health Organization indicates a maximum level of iron allowable in drinking water of 0.3 mgll (Degrtmont, 1991).
121
122
C. V. TREMBLAYetal.
Biological treatment of iron makes use of specific sessile micro-organisms "iron bacteria" which instigate, directly or indirectly, the oxidation of reduced ferrous ions in raw water, and also act to retain the precipitates in a biological sand filter. When biological treatment can be achieved, it offers several advantages (no chemical oxidants, flocculants or coagulants needed; reduced overall space requirements; higher filtration rate; increased filter capacity) over the conventional physical-chemical processes that eliminate iron through chemical oxidation and sand filtration (Mouchet, 1992). The use of oxidation-reduction potential (ORP or redox potential) to control biological systems has been previously studied for application in wastewater treatment systems (Charpentier et al., 1987; Wareham et 01., 1993). Similarly in drinking water treatment, ORP is of interest because it is related to bacterial activity which mediates aquatic redox reactions (e.g., involving iron). Not only can the ORP-time relationship be correlated with biological activity (Wareham et al., 1993), it also has an advantage over the traditional dissolved oxygen indicator used for aerobic systems in that ORP is still significant when DO levels approach zero (Charpentier et al., 1987). ORP, in conjunction with pH, is also of great importance in identifying the area of iron bacteria activity, as studied by Baas-Becking and colleagues (Wolfe, 1964) and shown in Figure 1 (Mouchet, 1992). The lower limit is important and corresponds to rH 14, where rH represents the negative logarithm of the hydrogen ion concentration producing the existing redox conditions (Degr6mont, 1991). The upper limit is also indicated but seems only to be of importance when the pH exceeds 7.5, at which point biological action competes with physical-chemical oxidation (Mouchet, 1992).
=
It should be noted that the biological region straddles the theoretical boundary between the region of ferrous ion stability and ferric hydroxide precipitation. This is related to the fact that iron bacteria relying on iron oxidation as a source of energy are gradient organisms, signifying that they are not likely to develop under strongly reducing or strongly oxidizing conditions, but rather at the point where they have a source of both ferrous ions and air (Wolfe, 1964). 900
I
Fe(OH)3
800
Physical-Chemical
X
oxidation of iron
700 600
S' ! i:l
SOD 400
I
~
~~""~4,
~~12:
Fe 2 +
300 200 100
/
Competition
h)'s~~t'c~enmical and biological oxidation
~
BiOlOgiCal' oxidation ~ 'Ofiron
"i..
:'-
o+-----...--=-~....-~ '-0 15.15 6.0 6.15 7.0 7.15 8.0
pH
Figure 1. Iron bacteria activity as a function of ORP (Eh) and pH (from Mouchet, 1992).
The objective of this work is to investigate the effect of varying the dissolved oxygen concentration in a biological filter for iron removal. The measurement of ORP in the filtered water is evaluated as a control tool to regulate air injection and as an indicator of residual iron concentration.
Oxidation-reduction potential
123
METHODS AND MATERIALS The site of an industrial scale drinking water treatment plant was selected for the study. Located in a seaside town in the southwest comer of France. the plant operates primarily during the summer months. producing drinking water at a rate of about 1000 m 3 per day. Two biological filters operate in series to carry out sequential iron and manganese removal. The study was subdivided into three phases. the first of which (June-July 1996) focused on evaluation of primary filter operation (iron removal). Since operating conditions at the facility could not be altered during the peak summer months. a pilot plant was constructed to carry out the proposed trials during the second phase (Aug.-Sept.-Oct. 1996). In the final phase. the primary filter was studied anew based on results from the pilot plant (Oct. 1996). PJjmarv filter The filter bed measures I m in depth. has a surface area of 3.14 m and consists of sand with an effective size of 1.18 nun. In total. the reactor stands 3 m in height. Parameters of the raw water before any treatment include: no dissolved oxygen. low organic loading. an average total iron concentration of 3.8 mg/!. of which 90 to 100% was dissolved; and an average total manganese concentration of 0.04 mg/! which was 100% dissolved. Raw water pH and ORP were approximately 5.7 and 290 mV. respectively. Before entering the primary filter. the raw water was injected with sodium hydroxide solution and air to increase pH to within the range of 6.5-7.0 and to increase the DO content. The superficial velocity of filtration was approximately
30 rnIh. The gauge pressure which was measured at the head of the primary filter varied from approximately 470 to 500 kPa during a filtration cycle. A backwash of the primary filter using raw water and air scour was carried out after every 500 m3 of filtration. fjlotplant Figure 2 shows the schematic of the pilot plant constructed in August 1996. Sand of 1.35 mm effective size filled the polyethylene column to aim depth. In total. the pilot plant measured 3 m in height, with an inside bed surface area of 0.005 m . All valves and nozzles were of PVC construction.
c
r------.---t~A~~9 6 2
3
4
I-Plant raw water tapped 2-Proccu water to pilot 3-Backwash water to pilot 4-Air i'liection to process S-Air lJ1ioction to backwash 6-Biological pilot plant 7-pH, Eh, DO probe bath
8-To data acquisition 9-To drain
To
plant
8
!
BE;::;:3, Figwe 2. Biological iron removal pilot planL
9
124
A sampling port on the raw water line of the plant (before the caustic and air injection) was tapped to supply raw water to the pilot plant. An automatic valve was used to halt flow to the pilot when the plant was not in operation. Compressed air from the plant was used for in-line air injection to the pilot plant. Air and water flow for both normal operation and backwashing were controlled using rotameters. Initial gauge pressure was regulated to 50 kPa, superficial velocity to 45 mIh (a filtration rate of 225 lib) and air injection equivalent to 0.6 mgll DO in the flItered water. Raw water and air scour were used to perform a backwash (valves A and D closed; valves B and C open) when the pressure increased above approximately 100 kPa or filter saturation was perceived. Analytical methods During the studies of both the plant primary filter and the pilot plant, values of pH, ORP and DO were measured continuously in a probe bath at the exit of the filter. Measurement of pH and ORP were achieved with two Polymetron sensors from Zellweger Analytical, the ORP probe having an internal Ag/AgCI reference electrolyte solution and a platinum electrode. The DO probe was the Oxi 323-B by wrw. The pH and DO probes were calibrated frequently and the ORP probe verified periodically with buffer solution. The redox probe platinum ring was polished daily during the trials. All sensors were connected to a data acquisition system through a central processor equipped with a 10 channel input/output (110) multiplexer card. Analyses of total and dissolved iron were performed with the DR2000 spectrophotometer from HACH. All methods used were documented in the accompanying instrument manual (HACH, 1992). All reagents were furnished by HACH. The differentiation between total and dissolved iron was achieved by filtering samples on individual disposable 0.45 mm membranes by Millipore. Samples with concentrations exceeding the upper concentration limit for the method were diluted I: 10 with demineralized water. Aeration-ORP trials The normal operating conditions were first established in the pilot plant. Then the air injection to the raw water was varied over several days, and with each adjustment, the pH, ORP, DO, total residual iron and dissolved iron in the filtered water were recorded until stabilization, usually over a period of several hours. RESULTS AND DISCUSSION Primaxy filler operation Analyses on the raw water, pre-filtered primary water (raw water plus sodium hydroxide solution and air), and primary filtered water were performed throughout June and July 1996. The mean values are summarized in Table I. Table I. Water treatment plant, June and July 1996 Water quality parameters Total iron (mg/L) Dissolved iron (mg/L) pH ORP (mV) Dissolved oxygen (mg/L)
Raw
3.80:i: 0.19 3.26:i: 0.14 5.73:i: 0.03 288 7 O.oo:i: 0.00
=
=
Mean values Std. Dev. Pre-filtered primary Primary filtered
4.82:i: 0.38· 1.26 0.04 7.30 0.12 157 23 1.06 0.09
= =
= =
0.339:i: 0.277 0.130 0.078 6.91= 0.06 349 60 0.40:i: 0.84
=
=
• Samples ofpre-filtered primary water showed notably higher levels of total iron than the raw water. It Is thought !bat the aarnpling point was too close to the injec:tion o(sodium hydroxide solution, not allowing (or luftkient mixing of aarnple.
Oxidation-reduction potential
l~
Results show that 71 to 75% of total iron was precipitating after the addition of caustic and air (pre-filtered primary water), but before even entering the filter. This suggested that a significant amount of physical• chemical oxidation was taking place before the filter. Furthermore, microscopic analysis of sludge from the backwash revealed none of the typical iron bacteria (Mouchet, 1992), such as Gallionella or Leptothrix. However, the operating conditions were clearly in the region of iron bacteria activity. Thus it was postulated that iron bacteria which are difficult to detect by direct examination (Hanert, 1981) were perhaps present, such as those from the Siderocapsaceae family. Therefore, the plant was operating in the transition area between biological and physical-chemical regimes. Because the operating conditions of the facilities could not be altered during the peak summer months, the pilot plant study was undertaken in order to verify biological treatment feasibility and if successful, to complete the aeration-ORP trials on a pilot scale. filot plant startuP After 16 days of operation under seemingly favourable conditions for biological growth, the pilot plant was still not eliminating any iron. Because the raw water pH (5.7) was slightly lower than recommended (Mouchet, 1992), a second trial to instigate bacterial growth was attempted by temporarily supplying the pilot plant with pre-filtered water from the plant (see Table I for chemical parameters). After filtering this water for approximately 20 hours, the pilot filter visibly contained a considerable amount of red precipitate and the gauge pressure in the column had increased to 80 kPa. The filter was regenerated and the supply line changed back to the raw water. The pilot plant was put back into operation with a reduced f10wrate of ISO 1Ih, or a superficial velocity of 30 mIh, and column pressure regulated to 50 kPa. The evolution of total residual iron and ORP in the filtered water from the pilot plant, shown in Figure 3, took place over 122 hours of operation from Sept. 9 to Oct. 7, 1996. It is evident from Figure 3 that a correlation exists between ORP and total residual iron (essentially dissolved form). This aspect is further discussed with results from the aeration-ORP trials below. 600 500
.
100
!- ORP A Total residual iron
3,5
I
• I- • '·i t' ::, -. -, A
AA A
3 2,5
- S -; ....
1,5
1 -~
I
A
Ao\
e~
2
• .g
!
0
. t
•
0,5'" 8
AA
o +r.....................,,....................."f'""T'....,.....,ara.a....,..,.................'T""I"....................+ 0 07109196
14/09196
21109196
28/09196
05/10196
12110196
Date
Figure 3. Start-up of biological iron elimination on pilot scale.
final study of the primarv filter Once the plant was no longer producing water for the water distribution network, it was possible to stop the injection of caustic soda (Oct. 19?6) in an attempt to develop the same performance as on the pilot plant. In addition, the length of the filtranon cycle was doubled from 500 m 3 to 1000 m 3, and the duration of the backwash shortened from 5 min. to 3 min. (Oct. 1996). These changes in operation were implemented primarily due to visual inspection which showed that the backwash was not preserving enough of the bacterial mass to accomplish sufficient iron removal.
126
C. V. TREMBLAY daL
After approximately 169 hours of filter operation (3 cycles of 500 m3 and 10 cycles of 1000 m 3 with a DO concentration of 1.0 mgll, primary filtered water had a consistent total residual iron concentration of 0.025 mgll (std.dev.: 0.007), as reflected by an ORP of 508±9 mY. Savings in backwash water and improved reliability were realized. Figure 4 illustrates the evolution of ORP and total residual iron from the primary filter and is similar to the results from the pilot plant shown in Figure 3. Samples of sludge collected in September and October from the pilot plant did not reveal the most common iron bacteria, Gallionella or Leptothrix, nor iron bacteria of the genus Siderocapsa. On-going study of the site is attempting to identify the micro-organisms present in the primary filter. Biological treatment can be postulated as several performance factors attested to biological and not physical-chemical oxidation. Firstly, the pH and ORP of the raw water were clearly in the region of iron bacteria activity on Figure 1. Secondly, the velocity of filtration was typical for biological iron removal (30 mIh), whereas physical-chemical processes are usually restricted to a maximum rate of 15 mIh (Mouchet, 1992). Also, the effective size of sand in the pilot plant and primary filter was 1.35 mm and 1.18 mm, respectively, whereas physical• chemical oxidation requires sand between 0.5 and I rom in size (Degremont. 1991). Lastly, iron retained for one cycle of filtration on the primary filter corresponded with maximum capacity possible in physical• chemical treatment, approximately 1.2 kglm 2 (for a I m bed depth). Since the filter pressure drop was minimal during a cycle (10 to 20 Pa) and iron content in the filtered water was consistently low (0.03 mgll) up to automatic filter regeneration, it seemed likely that the length of filtration cycle could be further 2 increased to achieve retention levels typical to biological treatment [Specific filter capacity =2 to 5 kglm (Mouchet, 1992)]. 550 500
~
!
~
0
450
.•,
I.ORP A_Total residual iron I
t
400
i
t •!
AS; ..
• ••
"
f •
2
Ii
1,5 1
e~
- ..... El ;;
•• :sI.g
E~ e O,S~ ~ '!
A A
A
A
A
B
A
A 350 +-r................,....,I"'"'T'".,....,.....................,I"'"'T'"..,a.;~ .........,........,....,.......~O
30/09196
07/10196
14/10196
21/10196
28/10196
Date Figure 4. Primary filter start-up without pH adjustment
Aeratjon-ORP trials on pilot Once biological activity was established in the pilot filter, it was possible to carry out aeration trials in early October. Because the column gauge pressure was only slightly elevated (by 20 kPa), the trials were initiated without first regenerating the filter. Figure 5 shows the evolution of DO, total residual iron and ORP over the 7 runs. Despite the large variations in DO, the removal of iron was practically constant, as was the ORP, with the exception of run 2 where air injection was completely halted. At the very end of the trials, reduced iron concentration increased suddenly to 0.33 mgll, accompanied by a decrease in ORP to 390 mY. The filter had achieved saturation and a backwash was initiated. Thus the dramatic drop in ORP signaled the need for filter regeneration. The trials revealed that an optimum aeration corresponds with a low level of dissolved oxygen (0.2-0.6 mgll). A comparison of runs 1 and 2 shows the value of using ORP in contrast to DO to evaluate process
Oxidation-reduction potential
127
reliability. In both cases, the DO levels are similar, 0.20 and 0.16 mg/l, respectively. However, the ORP is much higher in run I, corresponding to excellent removal of iron. It should also be noted that analyses indicated residual iron in the f1Itered water to be essentially in dissolved form. 6 Run
2
3
4
5
6
7
700
......- Total residual iro~-0- DO -.- ORP J
600 500 400
~ .....
300 ~ 200
0
100
o ~~ID:::::;::=~.-,_-..~----.....~-.. .-+ 0 07/10196
08/10196
09/10196
10/10196
11110/96
12/10/96
Date Figure S. Results from the aeration-CRP trials at pilot scale.
Several authors attest to the fact that physical-chemical oxidation of iron cannot take place at an appreciable rate when the pH is below neutral (Ehrlich, 1990; Mouchet. 1992). The implication of this statement for the present research is twofold. Firstly, it supports the presumption of biological action. Secondly, because the pH of the raw water studied was low, it would be theoretically possible to further increase the air injection without greatly influencing physical-chemical oxidation. Thus, the dissolved oxygen content could be increased to saturation level and there would be little to no competition for ferrous ions between biological and physical-chemical oxidation. Conversely, in biological processes where the raw water has a pH in the neutral range, increasing the DO level would possibly provoke some physical-chemical oxidation. Thus, evaluating the effect of DO and control of air injection may be of greater importance for such facilities. As with the pilot plant start-up. it is possible to see a correlation between ORP and iron concentration in Figure S. The data from the aeration-ORP trials was combined with the data from start-up of the pilot plant (Figure 3) to demonstrate the relationship between ORP and total residual iron as shown in Figure 6. A non• linear regression was determined, which yields a correlation coeffficient (R 2) of 0.8848.
600 SSO 500
>'
4S0
!
400
~
ORP - {S2S.603 + 574.893·TRI}/{1 + 2.157·TRI} R1 -O.8848 I• Start-up data :
I
:4 Trials data
•
I
350 300 2S0
•
200 +----+----+----o+----+------t 2 o S 3 4 Total reslduallroa coaceatratJoa (m&!L) Figure 6. Correlation between ORP and total residual iron during start-up and trials on pilotlcale.
128
C. V. TREMBLAY etaL
The regression model may be applied within upper and lower limits to predict total residual iron concentrations for a given ORP measurement. The model appears to become ineffective at ORP levels greater than approximately 470 mY, where experimental results showed iron concentrations to be consistently below regulation limits. In addition, for values of ORP below approximately 300 mY, total residual iron levels were found to be consistently greater than or equal to 3 mgll. Thus, for the present application of biological elimination of iron from drinking water, the above equation can be used to predict total residual iron concentration for ORP levels between 300 and 470 mY.
where TRI: ORP: a: b:
c:
total residual iron concentration in filtered water (mgll) standard oxidation-reduction potential of filtered water (mY) constant, 525.603 mV constant, 574.893 IImg omV constant, 2.157 IImg. CONCLUSION
This study has shown that the application of ORP to regulate air injection at the drinking water treatm~nt plant studied is not useful since biological iron removal is not influenced by variations in 00 concentration above the minimum requirement. A good correlation between ORP and total residual iron was determined, having an R2 value of 0.8848. From this relationship total residual iron concentration can be predicted when the ORP is between 300 and 470 mY. Below this range, total residual iron is equal to or greater than 3 mgll, and above this range total residual iron is below the French regulation limit of 0.2 mgll.
REFERENCES Charpentier. J., Ftorentz, M. and David, G. (1987). Oxidation-reduction potential (ORP) regulation: A way to optimize pollution removal and energy savings in the low load activated sludge process. Wat Sci. Tech., 19(3-4),645-655. . Degn!mont (1991). Water Treatment Handbook. F. BernI! and Y. Richard (cds.), vol I and 2, 6th cdn, Lavoisier Publishing, Pans. Ehrlich. H. L. (1990). Geomicrobiology, 2nd cdn, Marcel Dekker Inc., New York. HACH Company Ltd. (1992). HACH DR2000 Procedures Manual, french cdn, Loveland, USA. Haner!, H. H. (1981). The Genus Siderocapsa (and other iron- or manganese-oxidizing eubacteria). In: The Prokaryotes: A Handbook on habitats, isolation, and identification of bacteria, M. P. Starr et al. (cds), vol 1. Springer-Verlag, New York, pp. 1049-1059. Mouchet, P. (1992). From conventional to biological removal of iron and manganese in France. J. AWWA., 84(4), 158-167. . Wareham, D. G.• Hall, K.l. and Mavinic. D. S. (1993). Real-time control of wastewater treatment systems using ORP. Wat. SCI. Tech., 28(11-12), 273-282. . d Wolfe. R. S. (1964). Iron and Manganese Bacteria. In: Principles and Applications in Aquatic Microbiology, H. Heukelekian an N. C. Dondero (cds). John Wiley & Sons, Inc.. New York, 82-97.
~ Pergamon PIT: S0273-1223(98)OO573-3
Vol. 38, No.6, pp. 121-128, 1998.
IAWQ
C 1998 PIIbJished by Elsevier Science LId. Printed in Great Britain. All righll reserved 0273-1223198 $19'00 + 0-00
CONTROL OF BIOLOGICAL IRON REMOVAL FROM DRINKING WATER USING OXIDATION-REDUCTION POTENTIAL Catherine V. Tremblay*. Andre Beaubien**. Philippe Charles*** and James A. Nicell* • Department o/Civil Engineering and Applied Mechanics, McGill University, 817 Sherbrooke St. W.• Montria~ Quebec, Canada, H3A 2K6 ** Gamma Innovation, 7995 Rhone, Brossard, Quebec, Cantula, J4X 2K7 **. Centre International de Recherche sur l'£au et l'Environnement, Lyonnaise des £aux, 38, rue du Prisident Wilson, 78230 Le Pecq, France
ABSTRACf Biological removal of iron to produce drinking water was established in a pilot plant treating raw water with a pH of 5.7. The objective was to evaluate the use of oxidation-reduction potential (ORP) as a control tool and detennine its relationship to dissolved oxygen (00) and residual iron concentration in filtered water from an operating biological filter. Results showed that above a low minimum value of DO, residual iron concentration and ORP were not affected by varying the 00 level. A non-linear regression was established 10 correlate total residual iron concentration to ORP with an R2 of 0.8848. This correlation can be used to predict iron concentration when ORP is in the range 300 to 470 mY. Below this range, total residual iron is greater than or equal to 3 mgll and above, total residual iron is len than the French regulation Iimil of 0.2 mgll. Pilot plant operating conditions were implemented in the primary filter of an industrial plant in France, improving iron elimination and doubling the length of the filtration cycle. ~ 1998 Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Dissolved oxygen; drinking water treatment; iron bacteria; iron removal; ORP control.
INTRODUCflON Groundwater is often mildly acidic and devoid of dissolved oxygen (reducing). Thus. ferrous iron (Fe 2+) is usually present, either in dissolved mineral fonn or associated with various mineral or organic chelating agents. French regulation indicates maximum admissible concentrations of 0.2 mg/l for iron. However, in current practice. iron is usually treated to a maximum concentration of 0.1 mgll to avoid problems such as corrosion and obstructions in the water distribution network over the long tenn. The World Health Organization indicates a maximum level of iron allowable in drinking water of 0.3 mgll (Degrtmont, 1991).
121
122
C. V. TREMBLAYetal.
Biological treatment of iron makes use of specific sessile micro-organisms "iron bacteria" which instigate, directly or indirectly, the oxidation of reduced ferrous ions in raw water, and also act to retain the precipitates in a biological sand filter. When biological treatment can be achieved, it offers several advantages (no chemical oxidants, flocculants or coagulants needed; reduced overall space requirements; higher filtration rate; increased filter capacity) over the conventional physical-chemical processes that eliminate iron through chemical oxidation and sand filtration (Mouchet, 1992). The use of oxidation-reduction potential (ORP or redox potential) to control biological systems has been previously studied for application in wastewater treatment systems (Charpentier et al., 1987; Wareham et 01., 1993). Similarly in drinking water treatment, ORP is of interest because it is related to bacterial activity which mediates aquatic redox reactions (e.g., involving iron). Not only can the ORP-time relationship be correlated with biological activity (Wareham et al., 1993), it also has an advantage over the traditional dissolved oxygen indicator used for aerobic systems in that ORP is still significant when DO levels approach zero (Charpentier et al., 1987). ORP, in conjunction with pH, is also of great importance in identifying the area of iron bacteria activity, as studied by Baas-Becking and colleagues (Wolfe, 1964) and shown in Figure 1 (Mouchet, 1992). The lower limit is important and corresponds to rH 14, where rH represents the negative logarithm of the hydrogen ion concentration producing the existing redox conditions (Degr6mont, 1991). The upper limit is also indicated but seems only to be of importance when the pH exceeds 7.5, at which point biological action competes with physical-chemical oxidation (Mouchet, 1992).
=
It should be noted that the biological region straddles the theoretical boundary between the region of ferrous ion stability and ferric hydroxide precipitation. This is related to the fact that iron bacteria relying on iron oxidation as a source of energy are gradient organisms, signifying that they are not likely to develop under strongly reducing or strongly oxidizing conditions, but rather at the point where they have a source of both ferrous ions and air (Wolfe, 1964). 900
I
Fe(OH)3
800
Physical-Chemical
X
oxidation of iron
700 600
S' ! i:l
SOD 400
I
~
~~""~4,
~~12:
Fe 2 +
300 200 100
/
Competition
h)'s~~t'c~enmical and biological oxidation
~
BiOlOgiCal' oxidation ~ 'Ofiron
"i..
:'-
o+-----...--=-~....-~ '-0 15.15 6.0 6.15 7.0 7.15 8.0
pH
Figure 1. Iron bacteria activity as a function of ORP (Eh) and pH (from Mouchet, 1992).
The objective of this work is to investigate the effect of varying the dissolved oxygen concentration in a biological filter for iron removal. The measurement of ORP in the filtered water is evaluated as a control tool to regulate air injection and as an indicator of residual iron concentration.
Oxidation-reduction potential
123
METHODS AND MATERIALS The site of an industrial scale drinking water treatment plant was selected for the study. Located in a seaside town in the southwest comer of France. the plant operates primarily during the summer months. producing drinking water at a rate of about 1000 m 3 per day. Two biological filters operate in series to carry out sequential iron and manganese removal. The study was subdivided into three phases. the first of which (June-July 1996) focused on evaluation of primary filter operation (iron removal). Since operating conditions at the facility could not be altered during the peak summer months. a pilot plant was constructed to carry out the proposed trials during the second phase (Aug.-Sept.-Oct. 1996). In the final phase. the primary filter was studied anew based on results from the pilot plant (Oct. 1996). PJjmarv filter The filter bed measures I m in depth. has a surface area of 3.14 m and consists of sand with an effective size of 1.18 nun. In total. the reactor stands 3 m in height. Parameters of the raw water before any treatment include: no dissolved oxygen. low organic loading. an average total iron concentration of 3.8 mg/!. of which 90 to 100% was dissolved; and an average total manganese concentration of 0.04 mg/! which was 100% dissolved. Raw water pH and ORP were approximately 5.7 and 290 mV. respectively. Before entering the primary filter. the raw water was injected with sodium hydroxide solution and air to increase pH to within the range of 6.5-7.0 and to increase the DO content. The superficial velocity of filtration was approximately
30 rnIh. The gauge pressure which was measured at the head of the primary filter varied from approximately 470 to 500 kPa during a filtration cycle. A backwash of the primary filter using raw water and air scour was carried out after every 500 m3 of filtration. fjlotplant Figure 2 shows the schematic of the pilot plant constructed in August 1996. Sand of 1.35 mm effective size filled the polyethylene column to aim depth. In total. the pilot plant measured 3 m in height, with an inside bed surface area of 0.005 m . All valves and nozzles were of PVC construction.
c
r------.---t~A~~9 6 2
3
4
I-Plant raw water tapped 2-Proccu water to pilot 3-Backwash water to pilot 4-Air i'liection to process S-Air lJ1ioction to backwash 6-Biological pilot plant 7-pH, Eh, DO probe bath
8-To data acquisition 9-To drain
To
plant
8
!
BE;::;:3, Figwe 2. Biological iron removal pilot planL
9
124
A sampling port on the raw water line of the plant (before the caustic and air injection) was tapped to supply raw water to the pilot plant. An automatic valve was used to halt flow to the pilot when the plant was not in operation. Compressed air from the plant was used for in-line air injection to the pilot plant. Air and water flow for both normal operation and backwashing were controlled using rotameters. Initial gauge pressure was regulated to 50 kPa, superficial velocity to 45 mIh (a filtration rate of 225 lib) and air injection equivalent to 0.6 mgll DO in the flItered water. Raw water and air scour were used to perform a backwash (valves A and D closed; valves B and C open) when the pressure increased above approximately 100 kPa or filter saturation was perceived. Analytical methods During the studies of both the plant primary filter and the pilot plant, values of pH, ORP and DO were measured continuously in a probe bath at the exit of the filter. Measurement of pH and ORP were achieved with two Polymetron sensors from Zellweger Analytical, the ORP probe having an internal Ag/AgCI reference electrolyte solution and a platinum electrode. The DO probe was the Oxi 323-B by wrw. The pH and DO probes were calibrated frequently and the ORP probe verified periodically with buffer solution. The redox probe platinum ring was polished daily during the trials. All sensors were connected to a data acquisition system through a central processor equipped with a 10 channel input/output (110) multiplexer card. Analyses of total and dissolved iron were performed with the DR2000 spectrophotometer from HACH. All methods used were documented in the accompanying instrument manual (HACH, 1992). All reagents were furnished by HACH. The differentiation between total and dissolved iron was achieved by filtering samples on individual disposable 0.45 mm membranes by Millipore. Samples with concentrations exceeding the upper concentration limit for the method were diluted I: 10 with demineralized water. Aeration-ORP trials The normal operating conditions were first established in the pilot plant. Then the air injection to the raw water was varied over several days, and with each adjustment, the pH, ORP, DO, total residual iron and dissolved iron in the filtered water were recorded until stabilization, usually over a period of several hours. RESULTS AND DISCUSSION Primaxy filler operation Analyses on the raw water, pre-filtered primary water (raw water plus sodium hydroxide solution and air), and primary filtered water were performed throughout June and July 1996. The mean values are summarized in Table I. Table I. Water treatment plant, June and July 1996 Water quality parameters Total iron (mg/L) Dissolved iron (mg/L) pH ORP (mV) Dissolved oxygen (mg/L)
Raw
3.80:i: 0.19 3.26:i: 0.14 5.73:i: 0.03 288 7 O.oo:i: 0.00
=
=
Mean values Std. Dev. Pre-filtered primary Primary filtered
4.82:i: 0.38· 1.26 0.04 7.30 0.12 157 23 1.06 0.09
= =
= =
0.339:i: 0.277 0.130 0.078 6.91= 0.06 349 60 0.40:i: 0.84
=
=
• Samples ofpre-filtered primary water showed notably higher levels of total iron than the raw water. It Is thought !bat the aarnpling point was too close to the injec:tion o(sodium hydroxide solution, not allowing (or luftkient mixing of aarnple.
Oxidation-reduction potential
l~
Results show that 71 to 75% of total iron was precipitating after the addition of caustic and air (pre-filtered primary water), but before even entering the filter. This suggested that a significant amount of physical• chemical oxidation was taking place before the filter. Furthermore, microscopic analysis of sludge from the backwash revealed none of the typical iron bacteria (Mouchet, 1992), such as Gallionella or Leptothrix. However, the operating conditions were clearly in the region of iron bacteria activity. Thus it was postulated that iron bacteria which are difficult to detect by direct examination (Hanert, 1981) were perhaps present, such as those from the Siderocapsaceae family. Therefore, the plant was operating in the transition area between biological and physical-chemical regimes. Because the operating conditions of the facilities could not be altered during the peak summer months, the pilot plant study was undertaken in order to verify biological treatment feasibility and if successful, to complete the aeration-ORP trials on a pilot scale. filot plant startuP After 16 days of operation under seemingly favourable conditions for biological growth, the pilot plant was still not eliminating any iron. Because the raw water pH (5.7) was slightly lower than recommended (Mouchet, 1992), a second trial to instigate bacterial growth was attempted by temporarily supplying the pilot plant with pre-filtered water from the plant (see Table I for chemical parameters). After filtering this water for approximately 20 hours, the pilot filter visibly contained a considerable amount of red precipitate and the gauge pressure in the column had increased to 80 kPa. The filter was regenerated and the supply line changed back to the raw water. The pilot plant was put back into operation with a reduced f10wrate of ISO 1Ih, or a superficial velocity of 30 mIh, and column pressure regulated to 50 kPa. The evolution of total residual iron and ORP in the filtered water from the pilot plant, shown in Figure 3, took place over 122 hours of operation from Sept. 9 to Oct. 7, 1996. It is evident from Figure 3 that a correlation exists between ORP and total residual iron (essentially dissolved form). This aspect is further discussed with results from the aeration-ORP trials below. 600 500
.
100
!- ORP A Total residual iron
3,5
I
• I- • '·i t' ::, -. -, A
AA A
3 2,5
- S -; ....
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I
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e~
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!
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. t
•
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o +r.....................,,....................."f'""T'....,.....,ara.a....,..,.................'T""I"....................+ 0 07109196
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12110196
Date
Figure 3. Start-up of biological iron elimination on pilot scale.
final study of the primarv filter Once the plant was no longer producing water for the water distribution network, it was possible to stop the injection of caustic soda (Oct. 19?6) in an attempt to develop the same performance as on the pilot plant. In addition, the length of the filtranon cycle was doubled from 500 m 3 to 1000 m 3, and the duration of the backwash shortened from 5 min. to 3 min. (Oct. 1996). These changes in operation were implemented primarily due to visual inspection which showed that the backwash was not preserving enough of the bacterial mass to accomplish sufficient iron removal.
126
C. V. TREMBLAY daL
After approximately 169 hours of filter operation (3 cycles of 500 m3 and 10 cycles of 1000 m 3 with a DO concentration of 1.0 mgll, primary filtered water had a consistent total residual iron concentration of 0.025 mgll (std.dev.: 0.007), as reflected by an ORP of 508±9 mY. Savings in backwash water and improved reliability were realized. Figure 4 illustrates the evolution of ORP and total residual iron from the primary filter and is similar to the results from the pilot plant shown in Figure 3. Samples of sludge collected in September and October from the pilot plant did not reveal the most common iron bacteria, Gallionella or Leptothrix, nor iron bacteria of the genus Siderocapsa. On-going study of the site is attempting to identify the micro-organisms present in the primary filter. Biological treatment can be postulated as several performance factors attested to biological and not physical-chemical oxidation. Firstly, the pH and ORP of the raw water were clearly in the region of iron bacteria activity on Figure 1. Secondly, the velocity of filtration was typical for biological iron removal (30 mIh), whereas physical-chemical processes are usually restricted to a maximum rate of 15 mIh (Mouchet, 1992). Also, the effective size of sand in the pilot plant and primary filter was 1.35 mm and 1.18 mm, respectively, whereas physical• chemical oxidation requires sand between 0.5 and I rom in size (Degremont. 1991). Lastly, iron retained for one cycle of filtration on the primary filter corresponded with maximum capacity possible in physical• chemical treatment, approximately 1.2 kglm 2 (for a I m bed depth). Since the filter pressure drop was minimal during a cycle (10 to 20 Pa) and iron content in the filtered water was consistently low (0.03 mgll) up to automatic filter regeneration, it seemed likely that the length of filtration cycle could be further 2 increased to achieve retention levels typical to biological treatment [Specific filter capacity =2 to 5 kglm (Mouchet, 1992)]. 550 500
~
!
~
0
450
.•,
I.ORP A_Total residual iron I
t
400
i
t •!
AS; ..
• ••
"
f •
2
Ii
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e~
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•• :sI.g
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A A
A
A
A
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A 350 +-r................,....,I"'"'T'".,....,.....................,I"'"'T'"..,a.;~ .........,........,....,.......~O
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Date Figure 4. Primary filter start-up without pH adjustment
Aeratjon-ORP trials on pilot Once biological activity was established in the pilot filter, it was possible to carry out aeration trials in early October. Because the column gauge pressure was only slightly elevated (by 20 kPa), the trials were initiated without first regenerating the filter. Figure 5 shows the evolution of DO, total residual iron and ORP over the 7 runs. Despite the large variations in DO, the removal of iron was practically constant, as was the ORP, with the exception of run 2 where air injection was completely halted. At the very end of the trials, reduced iron concentration increased suddenly to 0.33 mgll, accompanied by a decrease in ORP to 390 mY. The filter had achieved saturation and a backwash was initiated. Thus the dramatic drop in ORP signaled the need for filter regeneration. The trials revealed that an optimum aeration corresponds with a low level of dissolved oxygen (0.2-0.6 mgll). A comparison of runs 1 and 2 shows the value of using ORP in contrast to DO to evaluate process
Oxidation-reduction potential
127
reliability. In both cases, the DO levels are similar, 0.20 and 0.16 mg/l, respectively. However, the ORP is much higher in run I, corresponding to excellent removal of iron. It should also be noted that analyses indicated residual iron in the f1Itered water to be essentially in dissolved form. 6 Run
2
3
4
5
6
7
700
......- Total residual iro~-0- DO -.- ORP J
600 500 400
~ .....
300 ~ 200
0
100
o ~~ID:::::;::=~.-,_-..~----.....~-.. .-+ 0 07/10196
08/10196
09/10196
10/10196
11110/96
12/10/96
Date Figure S. Results from the aeration-CRP trials at pilot scale.
Several authors attest to the fact that physical-chemical oxidation of iron cannot take place at an appreciable rate when the pH is below neutral (Ehrlich, 1990; Mouchet. 1992). The implication of this statement for the present research is twofold. Firstly, it supports the presumption of biological action. Secondly, because the pH of the raw water studied was low, it would be theoretically possible to further increase the air injection without greatly influencing physical-chemical oxidation. Thus, the dissolved oxygen content could be increased to saturation level and there would be little to no competition for ferrous ions between biological and physical-chemical oxidation. Conversely, in biological processes where the raw water has a pH in the neutral range, increasing the DO level would possibly provoke some physical-chemical oxidation. Thus, evaluating the effect of DO and control of air injection may be of greater importance for such facilities. As with the pilot plant start-up. it is possible to see a correlation between ORP and iron concentration in Figure S. The data from the aeration-ORP trials was combined with the data from start-up of the pilot plant (Figure 3) to demonstrate the relationship between ORP and total residual iron as shown in Figure 6. A non• linear regression was determined, which yields a correlation coeffficient (R 2) of 0.8848.
600 SSO 500
>'
4S0
!
400
~
ORP - {S2S.603 + 574.893·TRI}/{1 + 2.157·TRI} R1 -O.8848 I• Start-up data :
I
:4 Trials data
•
I
350 300 2S0
•
200 +----+----+----o+----+------t 2 o S 3 4 Total reslduallroa coaceatratJoa (m&!L) Figure 6. Correlation between ORP and total residual iron during start-up and trials on pilotlcale.
128
C. V. TREMBLAY etaL
The regression model may be applied within upper and lower limits to predict total residual iron concentrations for a given ORP measurement. The model appears to become ineffective at ORP levels greater than approximately 470 mY, where experimental results showed iron concentrations to be consistently below regulation limits. In addition, for values of ORP below approximately 300 mY, total residual iron levels were found to be consistently greater than or equal to 3 mgll. Thus, for the present application of biological elimination of iron from drinking water, the above equation can be used to predict total residual iron concentration for ORP levels between 300 and 470 mY.
where TRI: ORP: a: b:
c:
total residual iron concentration in filtered water (mgll) standard oxidation-reduction potential of filtered water (mY) constant, 525.603 mV constant, 574.893 IImg omV constant, 2.157 IImg. CONCLUSION
This study has shown that the application of ORP to regulate air injection at the drinking water treatm~nt plant studied is not useful since biological iron removal is not influenced by variations in 00 concentration above the minimum requirement. A good correlation between ORP and total residual iron was determined, having an R2 value of 0.8848. From this relationship total residual iron concentration can be predicted when the ORP is between 300 and 470 mY. Below this range, total residual iron is equal to or greater than 3 mgll, and above this range total residual iron is below the French regulation limit of 0.2 mgll.
REFERENCES Charpentier. J., Ftorentz, M. and David, G. (1987). Oxidation-reduction potential (ORP) regulation: A way to optimize pollution removal and energy savings in the low load activated sludge process. Wat Sci. Tech., 19(3-4),645-655. . Degn!mont (1991). Water Treatment Handbook. F. BernI! and Y. Richard (cds.), vol I and 2, 6th cdn, Lavoisier Publishing, Pans. Ehrlich. H. L. (1990). Geomicrobiology, 2nd cdn, Marcel Dekker Inc., New York. HACH Company Ltd. (1992). HACH DR2000 Procedures Manual, french cdn, Loveland, USA. Haner!, H. H. (1981). The Genus Siderocapsa (and other iron- or manganese-oxidizing eubacteria). In: The Prokaryotes: A Handbook on habitats, isolation, and identification of bacteria, M. P. Starr et al. (cds), vol 1. Springer-Verlag, New York, pp. 1049-1059. Mouchet, P. (1992). From conventional to biological removal of iron and manganese in France. J. AWWA., 84(4), 158-167. . Wareham, D. G.• Hall, K.l. and Mavinic. D. S. (1993). Real-time control of wastewater treatment systems using ORP. Wat. SCI. Tech., 28(11-12), 273-282. . d Wolfe. R. S. (1964). Iron and Manganese Bacteria. In: Principles and Applications in Aquatic Microbiology, H. Heukelekian an N. C. Dondero (cds). John Wiley & Sons, Inc.. New York, 82-97.