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Effect of dissolved oxygen in reclaimed wastewater transformation during transportation. Case study: Tenerife, Spain
~
Pergamon
War. Sci. Tech. Vol. 37, No. I, pp. 123-130,1998. @ 1998IAwQ. Published by Elsevier Science LId Printed in Great Britain.
PH: S0273-1223(97)0076I-O
0273-1223/98 $19'00 + 0-00
EFFECT OF DISSOLVED OXYGEN IN RECLAIMED WASTEWATER TRANSFORMATION DURING TRANSPORTATION. CASE STUDY: TENERIFE, SPAIN S. Delgado*, M. Alvarez*, E. Aguiar** and L. E. Rodrfguez-G6mez* • Department of Chemical Engineering, Faculty of Chemistry, University ofLa Laguna, Avda. Astrofisico Fco, Sanchez SIN. 38200 La Laguna. Teneri/e. Spain •• BALTEN (Organismo Autonomo Local de Balsas de Teneri/e) (Tenerife Water and Wastewater Reservoirs Agency), Poligono de San Jeronimo, 83/2 La Orotava; Teneri/e. Spain
ABSTRACf During treated wastewater transport, phenomena related to the presence or absence of dissolved oxygen (DO) have been observed. In Tenerife reclaimed urban wastewater (RUWW) transport from the cap ital, Santa Cruz to the south of the island for agriculture reuse was studied. Anoxic conditions and H~ generation were noted during transport. As RUWW has an electrical conductivity (EC) of about 1600 ltS/cm. BAtTEN decided to add fresh water (FW) with low EC. saturated or over saturated in DO at an intermediate po int in the pipe to reduce the conductivity of water supplied to agriculture. also thus reducing the organic load and consequently H2S generation. However, injection of DO over-saturated fresh water brought on the appearance of nitnfication and later demtrification. A study was carried out of H ~ generation. nitrification and den itrification during RUWW transport with and without added FW. @ 1998 IAWQ. Published by Elsevier Sc ience Ltd
KEYWORDS Denitrification; fresh water ; gravity pipe; nitrification; reclaimed wastewater; reuse; sulfide generation. NOMENCLATURE
= = = =
COD f chemical oxygen demand for filtered sample COD t total chemical oxygen demand DO dissolved oxygen EC electric conductivity FW =fresh water MWW = mixed wastewater (RUWW+FW) mV millivolts ORP = oxidat ion-reduction potential
=
123
124
S. DELGADO et al.
RUWW = reclaimed urban wastewater SS =suspended solids INTRODUCTION Wastewater, whether treated or not, may undergo physico-chemical alterations during its long-distance transport through closed pipes. Most frequently the processes are related to OD consumption, giving rise to the appearance of anoxic and/or anaerobic conditions, usually along with sulfide generation due to the reduction of sulfate. Sulfide generation is one of the most noisome phenomena ocurring during wastewater transportation (Thistlethwayte, 1972; Boon and Lister, 1975; Pomeroy and Parkhurst, 1977; HvitvedJacobsen et al., 1988;Nielsen et al., 1992; Boon, 1995). The agricultural reuse of reclaimed urban wastewater becomes a necessity in places lacking water resources, like the Canary Islands. These are a group of Spanish islands situated in the Atlantic Ocean, off the northwest coast of Africa. They have a population of more than 1,600,000 inhabitants, and a dry subtropical climate tempered by the ocean, with an average annualtemperature at sea level of 21°C. The water demand in the island of Tenerife (population, tourism, industry and agriculture) has far exceeded the island's water resources. In order to make available more water than these provide, an ambitious project was put into operation in 1993, to reuse reclaimed wastewater in crop irrigation on the south coast of the island (especially banana plantations). This reuse is usually distant from urban areas where most of it is obtained, as is the case in Tenerife, so it usuallyhas to be transported through long pipes. RUWW has an EC of about 1600 JlS/cm, which is considered high for this kind of crop. In order to reduce the conductivity of water supplied to agriculture, BALTEN decided to add fresh water with low EC at a point of the pipe which is near the FW source. As FW is in many cases over-saturated in DO, oxidation phenomena such as H2S inhibition and nitrification appear in the MWWpipe. OBJECTIVES The aim of this work is to study the influence of injectionof DO over-saturated FW into a RUWWtransport pipe, and the effect this has on the reactions involving DO occurring during transport, e.g. sulfide generation, DO consumption (appearance of anoxicconditions) nitrification and denitrification. SYSTEMDESCRIPTION The system under study is part of the Reclaimed Wastewater Reuse System in Tenerife. A schematic representation of this system can be seen in Fig. I. Domestic wastewater from an area of more than 300,000 inhabitants is reclaimed in the Wastewater TreatmentPlant (WTP) of Santa Cruz de Tenerife. Reclamation consists of pretreatment, a primary treatmentand an activatedsludge system. The effluent from the WTP is transported by gravity to a PumpingStation,from where it is pumpedto a GravityTransportation Reservoir (GTR).A filled gravity pipe transports this waterfrom the GTR to the point of reuse. The pipe is cast iron with a concrete inside coating, being 0.6 m in diameter and 61 km long. It works permanently, day and night, with an averageRUWWflow of 500 m 31h. The quantityof RUWWtransported to the South of the island depends on the time of year. At a distanceof 10 km from the inlet of this pipe at GTR there is an injectionof fresh water (FW) over-saturated in oxygen at a constant flow of 122 m31h. FW injection takes place at a point where anoxic conditions tend to appear. This filled gravity pipe was the part of the Reuse Systemused for this research.
Effect of dissolved oxygen in reclaimed wastewater
RUWWzonc
...
I!IT.bloro Oravily
Transportation
Reservoir
~
MWWzonc
...
.....
t
125
- -
-
-
•
Fresh water injection
I
e: Sampling points aloDg the pipe
San .sid,:"
ReservOIr
I
Pipe leDgth: 61 ~m
Figure I. Schematic representationof the system under study.
The experimental part of this study was carried out between November 1994 and October 1996. FW injection started in July 1995. Sampling was done both before and after injection, of RUWW, FW and MWW (treated + fresh). The samples were taken at different sites along the pipe (Fig. 1). Temperature, DO, pH and ORP were measured in situ. Samples taken for analysis were refrigerated immediately and transported to the laboratory, where each was characterized by measuring SS, COOt' COD f (sample filtered with a filter pore size of 0.45 urn), Color, EC, S042-, N-NOf, N-N0 2-, N-NH 4+ and S2- concentration. ANALYTICAL METHODS All parameters were determined according to Standard Methods. Table I. RUWW and FW characteristics Parameter
km=O
km=lO°
FW
Temp. (0C)
22.7 7.8 3.3 416 12.3
24.6 7.7 0.4
23.4 8.4
pH DO (mg/l) ORP (mY) SS (mg/I)
-4
11.7
COOl (mg/I)
72
64
COD, (mg/I)
55 124 0.1 1660 124 0.5 0.3 39.2
49 117 0.5 1648 132 0.4 0.4 38.7
Sol (mg/I) S2- (mg/I) EC (J.tS/cm) Color (Pt-Co) N-NO; (mg/l) N-NO; (mg/I) N-NH. + (mgll)
over-saturated
401 0.3 4.5 4.8 0.5 0.0 209 3.9 1.3
0.0 0.0
km-O, RUWW entry into pipe km -10, pointjustbefore FW injection into RUWW .. average residence time 6 hours
126
S. DELGADO et al.
RESULTS ANDDISCUSSION Water characteristics In Table 1 average concentrations of the typical parameters for both RUWW (at km=O entering pipe and FW injection point (km=10), and for FW. Behayjor of system and effect of FW jnjection When RUWW is transported 61 km through the system without FW addition, S2. and DO conce ntration and ORP conditions are as Fig. 2 (Test period Nov. 94 - Jun. 95). Each point on the graph corresponds to the average value of 17 measurements carried out during the above study period. Anoxic conditions in the pipe (DO below 0.5 mg/l and ORP less than -100mV (Boon, 1995» are reached in a residence time of the order of 6 h, corresponding to a distance of about 10 km, as observed in Fig. 2. From then on, sulfide generation begins and continues practically constant along the whole pipe. It therefore seems the appropiate point for injection of DO over-saturated fresh water. This FW injection produces an alteration in the system as expressed in Fig. 3, showing the average behavior of the MWW system along the pipe, during the injection period: July 95 - October 96. Each point corresponds to the average value of the 18 measurements performed.
10
600 -ORP 1'00 400
8
200
~6
0
.! 0
Q
4 -200
2
0
,
10
15
20
2'
30
3'
40
0
5
10
15
20
25
30
35
40
10
.
8
;a •
6
.!0
'"0
-400
0
e-
~
~
-600
IIIi
4
2
0 Residence time(b) Figure2. Averagevariation In DO. S2- and ORP for RUWW (Nov. 94-Jun.9').
Effect of dissolvedoxygen in reclaimed wastewater
127
When FW is injected, the DO concentration in the water rises from less than 0.5 mgll (anoxic conditions) to more than 2 mgll . As expected, the ORP also increases to oxidizing values . As seen in Fig. 3. DO is consumed very quickly, likewise ORP descend s to even negative values.
If Figs 2 and 3 are compared, it can be seen that FW injection raises the wastewater ORP thus reducing sulfide generation. In general, the organic matter content of the wastewater can be reduced by 20%. This exerts a determining influence over sulfide generation, taking into account the typical sulfide formation equations (Thistlethwayte, 1972; Boon and Lister, 1975; Pomeroy and Parkhurst, 1977; Hvitved-Jacobsen et al., 1988; Boon, 1995). The DO reduction is much more marked in MWW than RUWW. This drop may be related with the nitrification phenomena observed in the MWW . to
600
.ORP "00 -400
8
200
..
.::'"
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!.
.!
0
0
Q
0
4
·200 2
-400
0 0
10
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8
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~
6
.
4
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~
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,
-600
10
"
20
25
30
3'
40
"
20
25
30
3'
40
I I I I I
I I I
2
0 0
10
Residencelime(h)
Figure 3. Average variation in DO. S2. and ORP during the FW injection period (July 95-October 1996).
Table 2. Nitrification rates Date (1996)
23/5
12/6
18/6
2/7
16/7
18/7
22/7
+ N-N02Vm'it g~-N03' + N-N02')/m'it
0.03
0.16
0.12
0.22
0.03
0.06
0.12
0.30
0.42
0.40
0 .40
0.07
0.24
g(N-N03'
formed (2) (I) Nitrification in RUWW section(from kIn-O to kIn-10).
(2) Nitrification in MWW section (after FW injection).
128
S. DELGADO tt 01.
NjtrificatjQn-denjtrificatjQn process In Fig. 4 the average variations in N-N03', N-NOf and DO are depicted for the period between January and October 1996. Each point corresponds to the average Qf 14 measurements. The tWQ sections of the pipe transporting RUWW and MWW respectively, can be seen. At km=O and km=lO, starting points of each, there are the necessary conditions for nitrification to take place: presence of HCOf and DO, high N-NH 4+, low organic matterlNH 4+ ratio, pH 7-8 and biological film on the inner surface of the pipe (Balmelle et al., 1992; Henze et al., 1995). However as seen in Fig. 4, in the RUWW the observed nitrification is in fact much less notable than in MWW. Table 2 shows the nitrification levels for RUWW and MWW.Figure 3. Average variation in DO, S2. and ORP during the FW injection period (JuI.95-Oct.96).
6.,---r--------------., RUWW
MWW
~ 4
.!
8
3
2
10
15
20
25
30
35
40
35
40
• NII"I' • Nlttlll
• ~6 .... 4 2 0 10
0
15
20
25
30
Residence limo(b)
Figure 4. Average variationin DO. nitrate and nitrileduring the FW injectionperiod.
Table 3. Denitriflcation rates DIll (1996)
713
2513
2314
1615
2315
12/6
1816
2n
18n
2217
8110
,{N-NO,Jlm"b
0.01S
0.032
0.009
0.006
D.OII
0.019
0.019
0.027
0.002
D.OIl
0.012
COlllUmed
Ql
(3) Denitrification rale for MWW _tioll.
The nitrification rates for the biological film adhering to the inside of the pipe wall has been considered since the biomass concentration in the biological film is about 100 times greater than that of suspended solids, using a density of 40 kg SSV/m 3 of biofilm (Zhang and Bishop, 1994; Henze et al., 1995), and an average pipe biofilm thickness of 3 mm. Nitrification is higher in MWW than in RUWW. This may be due to the DO concentration in MWW being greater. It should be taken into account that the first MWW sampling point is at 1 km from the FW injection.
Effect of dissolved oxygen in reclaimed wastewater
129
Taking into account the high DO consumption rate in this area, the initial DO concentration may be deduced to be higher than 4-5 mgll. As soon as DO has disappeared from the medium, and anoxic conditions have been reached, a denitrification process was observed (Fig. 4) from nearly all the samples. In the RUWW this process was very weak (Fig. 4), while in MWW it is much greater, reaching complete denitrification in many cases. This may be due to the ratio organic matterlN-NOf being much lower in the RUWW than in the MWW. The denitrification levels registered are shown in Table 3. It takes place from the time the necessary conditions for it are reached: high organic matterlN-NOf ratio and absence of DO (Abeling and Seyfried, 1992; Henze et al., 1995). Fig. 4 indicates that in MMW the denitritification phenomenon (nitrite consumption) is total in most cases, while complete nitrate consumption does not usually occur. A relationship has been sought between nitrification, denitrification and ORP. Denitrification has been observed to be much more efficient the more reductive the ORP is, as was expected (Lie and Welander, 1994). However, denitrification processes with positive ORP values have been registered . In any case, if a relation is tentatively established between DO concentration and the ORP value, one comes to the conclusion that there is a wide range of ORP values for DO concentrations less than 0.5 mgll. Although DO is totally consumed, the ORP value depends on the biological reactions occurring in the wastewater. Therefore, the greater the organic matter concentration in the wastewater, the lower will be the ORP value attained after a long residence time. The introduction of FW into the RUWW gives rise to the appearance of a nitrification-<1enitrification process after which the nitrogen content in the wastewater is seen to be reduced at the end of the pipe. However, the reduction in nitrogen during MWW transport, probably due to these processes is relatively minor, less than 10%. Consequently the EC of wastewater supplied to agriculture has been reduced by 20% with respect to RUWW, and more important: the risk of sulfide generation in the system has been diminished. CONCLUSIONS In the system under study, RUWW transport gives rise to H 2S generation after a 6-10 hour residence time by which anoxic conditions are reached. When DO over-saturated FW is injected at the starting point, a dilution effect is produced accompanied by oxidation phenomena. Thus, H 2S formation is inhibited and a nitrification-<1enitrification process appears in the pipe. ACKNOWLEDGEMENTS This work was financially supported by a cooperative agreement between BALTEN and the Dpt. of Chemical Engineering of the University of La Laguna, and by a grant from the Autonomous Government of the Canary Islands. REFERENCES Abeling , U. and Seyfried, C. F. (1992) . Anaerobic-aerob ic treatment of high-strength ammonium wastewater> nitrogen removal via nitrite . Wat. Sci. Tech., 26{5/6), 1007·1015 . Balmelle, B., Nguyen, K. M., Capdeville, B., Cornier, J. C. and Deguin, A. (1992) . Study of factors controlling nitrite build-up in biological processes for water nitrification . Wat. Sci. Tech., 26{5/6), 1017·1025 . Boon A. G. (\995). Septicity in sewers : causes, consequences and containment. Wat. Sci. Tech. 31(7), 237· 253. Boon A. G. and Lister A. R. (1975) . Format ion of sulph ide in rising main sewers and its prevention by injection of oxygen. Prog. Wat. Tech. 7(2), 289-300. Henze, M., Harremoes, P., la Cour Jansen, J. and Arvin, E. (1995). Wasterwater Treatment. Biological and Chemical Processes. Springer- Verlag .
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Hvitved-Iacobsen T.• Juette P. H., Nielsen P. H. and Jensen N. Aa (1988). Hydrogen sulphide control in municipalsewers. In Hahn, H.H. and Klute, R. (Eds), Pretreat. in Chern. Wat. and Wast. Treat., Proceedings of the 3rd International Gothenburg Symposium, Gothenburg, Sweden. Springer Verlag, 239-247. Lie, E. and Welander, T. (1994). Influence of dissolved oxygen and oxidation-reduction potential on the denitrification rate of activated sludge. Wat. Sci. Tech.• 30(6), 91-100. Nielsen. P. H., Raunkjaer, K.• Norsker, N. H., Jensen, N. Aa. and Hvitved-Jacobsen, T. (1992). Transformations of wastewater in sewer systems - A review. Wat. Sci. Tech., 25(6),17-31. Pomeroy. R. D. and Parkhurst. I. D. (1977). The forecasting of sulfide build-up rates in sewers. Prog. Wat. Tech. 9, 621-628. Thistlethwayte, D. K. B. (ed.) (1972). Thecontrololsulfides in sewerage systems. Butterworths, Sydney. Zhang. T. C. and Bishop, P. L. (1994). Density, porosity. and pore structure of biofilms. Wat. Res.• 28(11), 2267-2277.
Pergamon
War. Sci. Tech. Vol. 37, No. I, pp. 123-130,1998. @ 1998IAwQ. Published by Elsevier Science LId Printed in Great Britain.
PH: S0273-1223(97)0076I-O
0273-1223/98 $19'00 + 0-00
EFFECT OF DISSOLVED OXYGEN IN RECLAIMED WASTEWATER TRANSFORMATION DURING TRANSPORTATION. CASE STUDY: TENERIFE, SPAIN S. Delgado*, M. Alvarez*, E. Aguiar** and L. E. Rodrfguez-G6mez* • Department of Chemical Engineering, Faculty of Chemistry, University ofLa Laguna, Avda. Astrofisico Fco, Sanchez SIN. 38200 La Laguna. Teneri/e. Spain •• BALTEN (Organismo Autonomo Local de Balsas de Teneri/e) (Tenerife Water and Wastewater Reservoirs Agency), Poligono de San Jeronimo, 83/2 La Orotava; Teneri/e. Spain
ABSTRACf During treated wastewater transport, phenomena related to the presence or absence of dissolved oxygen (DO) have been observed. In Tenerife reclaimed urban wastewater (RUWW) transport from the cap ital, Santa Cruz to the south of the island for agriculture reuse was studied. Anoxic conditions and H~ generation were noted during transport. As RUWW has an electrical conductivity (EC) of about 1600 ltS/cm. BAtTEN decided to add fresh water (FW) with low EC. saturated or over saturated in DO at an intermediate po int in the pipe to reduce the conductivity of water supplied to agriculture. also thus reducing the organic load and consequently H2S generation. However, injection of DO over-saturated fresh water brought on the appearance of nitnfication and later demtrification. A study was carried out of H ~ generation. nitrification and den itrification during RUWW transport with and without added FW. @ 1998 IAWQ. Published by Elsevier Sc ience Ltd
KEYWORDS Denitrification; fresh water ; gravity pipe; nitrification; reclaimed wastewater; reuse; sulfide generation. NOMENCLATURE
= = = =
COD f chemical oxygen demand for filtered sample COD t total chemical oxygen demand DO dissolved oxygen EC electric conductivity FW =fresh water MWW = mixed wastewater (RUWW+FW) mV millivolts ORP = oxidat ion-reduction potential
=
123
124
S. DELGADO et al.
RUWW = reclaimed urban wastewater SS =suspended solids INTRODUCTION Wastewater, whether treated or not, may undergo physico-chemical alterations during its long-distance transport through closed pipes. Most frequently the processes are related to OD consumption, giving rise to the appearance of anoxic and/or anaerobic conditions, usually along with sulfide generation due to the reduction of sulfate. Sulfide generation is one of the most noisome phenomena ocurring during wastewater transportation (Thistlethwayte, 1972; Boon and Lister, 1975; Pomeroy and Parkhurst, 1977; HvitvedJacobsen et al., 1988;Nielsen et al., 1992; Boon, 1995). The agricultural reuse of reclaimed urban wastewater becomes a necessity in places lacking water resources, like the Canary Islands. These are a group of Spanish islands situated in the Atlantic Ocean, off the northwest coast of Africa. They have a population of more than 1,600,000 inhabitants, and a dry subtropical climate tempered by the ocean, with an average annualtemperature at sea level of 21°C. The water demand in the island of Tenerife (population, tourism, industry and agriculture) has far exceeded the island's water resources. In order to make available more water than these provide, an ambitious project was put into operation in 1993, to reuse reclaimed wastewater in crop irrigation on the south coast of the island (especially banana plantations). This reuse is usually distant from urban areas where most of it is obtained, as is the case in Tenerife, so it usuallyhas to be transported through long pipes. RUWW has an EC of about 1600 JlS/cm, which is considered high for this kind of crop. In order to reduce the conductivity of water supplied to agriculture, BALTEN decided to add fresh water with low EC at a point of the pipe which is near the FW source. As FW is in many cases over-saturated in DO, oxidation phenomena such as H2S inhibition and nitrification appear in the MWWpipe. OBJECTIVES The aim of this work is to study the influence of injectionof DO over-saturated FW into a RUWWtransport pipe, and the effect this has on the reactions involving DO occurring during transport, e.g. sulfide generation, DO consumption (appearance of anoxicconditions) nitrification and denitrification. SYSTEMDESCRIPTION The system under study is part of the Reclaimed Wastewater Reuse System in Tenerife. A schematic representation of this system can be seen in Fig. I. Domestic wastewater from an area of more than 300,000 inhabitants is reclaimed in the Wastewater TreatmentPlant (WTP) of Santa Cruz de Tenerife. Reclamation consists of pretreatment, a primary treatmentand an activatedsludge system. The effluent from the WTP is transported by gravity to a PumpingStation,from where it is pumpedto a GravityTransportation Reservoir (GTR).A filled gravity pipe transports this waterfrom the GTR to the point of reuse. The pipe is cast iron with a concrete inside coating, being 0.6 m in diameter and 61 km long. It works permanently, day and night, with an averageRUWWflow of 500 m 31h. The quantityof RUWWtransported to the South of the island depends on the time of year. At a distanceof 10 km from the inlet of this pipe at GTR there is an injectionof fresh water (FW) over-saturated in oxygen at a constant flow of 122 m31h. FW injection takes place at a point where anoxic conditions tend to appear. This filled gravity pipe was the part of the Reuse Systemused for this research.
Effect of dissolved oxygen in reclaimed wastewater
RUWWzonc
...
I!IT.bloro Oravily
Transportation
Reservoir
~
MWWzonc
...
.....
t
125
- -
-
-
•
Fresh water injection
I
e: Sampling points aloDg the pipe
San .sid,:"
ReservOIr
I
Pipe leDgth: 61 ~m
Figure I. Schematic representationof the system under study.
The experimental part of this study was carried out between November 1994 and October 1996. FW injection started in July 1995. Sampling was done both before and after injection, of RUWW, FW and MWW (treated + fresh). The samples were taken at different sites along the pipe (Fig. 1). Temperature, DO, pH and ORP were measured in situ. Samples taken for analysis were refrigerated immediately and transported to the laboratory, where each was characterized by measuring SS, COOt' COD f (sample filtered with a filter pore size of 0.45 urn), Color, EC, S042-, N-NOf, N-N0 2-, N-NH 4+ and S2- concentration. ANALYTICAL METHODS All parameters were determined according to Standard Methods. Table I. RUWW and FW characteristics Parameter
km=O
km=lO°
FW
Temp. (0C)
22.7 7.8 3.3 416 12.3
24.6 7.7 0.4
23.4 8.4
pH DO (mg/l) ORP (mY) SS (mg/I)
-4
11.7
COOl (mg/I)
72
64
COD, (mg/I)
55 124 0.1 1660 124 0.5 0.3 39.2
49 117 0.5 1648 132 0.4 0.4 38.7
Sol (mg/I) S2- (mg/I) EC (J.tS/cm) Color (Pt-Co) N-NO; (mg/l) N-NO; (mg/I) N-NH. + (mgll)
over-saturated
401 0.3 4.5 4.8 0.5 0.0 209 3.9 1.3
0.0 0.0
km-O, RUWW entry into pipe km -10, pointjustbefore FW injection into RUWW .. average residence time 6 hours
126
S. DELGADO et al.
RESULTS ANDDISCUSSION Water characteristics In Table 1 average concentrations of the typical parameters for both RUWW (at km=O entering pipe and FW injection point (km=10), and for FW. Behayjor of system and effect of FW jnjection When RUWW is transported 61 km through the system without FW addition, S2. and DO conce ntration and ORP conditions are as Fig. 2 (Test period Nov. 94 - Jun. 95). Each point on the graph corresponds to the average value of 17 measurements carried out during the above study period. Anoxic conditions in the pipe (DO below 0.5 mg/l and ORP less than -100mV (Boon, 1995» are reached in a residence time of the order of 6 h, corresponding to a distance of about 10 km, as observed in Fig. 2. From then on, sulfide generation begins and continues practically constant along the whole pipe. It therefore seems the appropiate point for injection of DO over-saturated fresh water. This FW injection produces an alteration in the system as expressed in Fig. 3, showing the average behavior of the MWW system along the pipe, during the injection period: July 95 - October 96. Each point corresponds to the average value of the 18 measurements performed.
10
600 -ORP 1'00 400
8
200
~6
0
.! 0
Q
4 -200
2
0
,
10
15
20
2'
30
3'
40
0
5
10
15
20
25
30
35
40
10
.
8
;a •
6
.!0
'"0
-400
0
e-
~
~
-600
IIIi
4
2
0 Residence time(b) Figure2. Averagevariation In DO. S2- and ORP for RUWW (Nov. 94-Jun.9').
Effect of dissolvedoxygen in reclaimed wastewater
127
When FW is injected, the DO concentration in the water rises from less than 0.5 mgll (anoxic conditions) to more than 2 mgll . As expected, the ORP also increases to oxidizing values . As seen in Fig. 3. DO is consumed very quickly, likewise ORP descend s to even negative values.
If Figs 2 and 3 are compared, it can be seen that FW injection raises the wastewater ORP thus reducing sulfide generation. In general, the organic matter content of the wastewater can be reduced by 20%. This exerts a determining influence over sulfide generation, taking into account the typical sulfide formation equations (Thistlethwayte, 1972; Boon and Lister, 1975; Pomeroy and Parkhurst, 1977; Hvitved-Jacobsen et al., 1988; Boon, 1995). The DO reduction is much more marked in MWW than RUWW. This drop may be related with the nitrification phenomena observed in the MWW . to
600
.ORP "00 -400
8
200
..
.::'"
E' 6
!.
.!
0
0
Q
0
4
·200 2
-400
0 0
10
I
8
I I I I I
~
6
.
4
.!
~
'"
,
-600
10
"
20
25
30
3'
40
"
20
25
30
3'
40
I I I I I
I I I
2
0 0
10
Residencelime(h)
Figure 3. Average variation in DO. S2. and ORP during the FW injection period (July 95-October 1996).
Table 2. Nitrification rates Date (1996)
23/5
12/6
18/6
2/7
16/7
18/7
22/7
+ N-N02Vm'it g~-N03' + N-N02')/m'it
0.03
0.16
0.12
0.22
0.03
0.06
0.12
0.30
0.42
0.40
0 .40
0.07
0.24
g(N-N03'
formed (2) (I) Nitrification in RUWW section(from kIn-O to kIn-10).
(2) Nitrification in MWW section (after FW injection).
128
S. DELGADO tt 01.
NjtrificatjQn-denjtrificatjQn process In Fig. 4 the average variations in N-N03', N-NOf and DO are depicted for the period between January and October 1996. Each point corresponds to the average Qf 14 measurements. The tWQ sections of the pipe transporting RUWW and MWW respectively, can be seen. At km=O and km=lO, starting points of each, there are the necessary conditions for nitrification to take place: presence of HCOf and DO, high N-NH 4+, low organic matterlNH 4+ ratio, pH 7-8 and biological film on the inner surface of the pipe (Balmelle et al., 1992; Henze et al., 1995). However as seen in Fig. 4, in the RUWW the observed nitrification is in fact much less notable than in MWW. Table 2 shows the nitrification levels for RUWW and MWW.Figure 3. Average variation in DO, S2. and ORP during the FW injection period (JuI.95-Oct.96).
6.,---r--------------., RUWW
MWW
~ 4
.!
8
3
2
10
15
20
25
30
35
40
35
40
• NII"I' • Nlttlll
• ~6 .... 4 2 0 10
0
15
20
25
30
Residence limo(b)
Figure 4. Average variationin DO. nitrate and nitrileduring the FW injectionperiod.
Table 3. Denitriflcation rates DIll (1996)
713
2513
2314
1615
2315
12/6
1816
2n
18n
2217
8110
,{N-NO,Jlm"b
0.01S
0.032
0.009
0.006
D.OII
0.019
0.019
0.027
0.002
D.OIl
0.012
COlllUmed
Ql
(3) Denitrification rale for MWW _tioll.
The nitrification rates for the biological film adhering to the inside of the pipe wall has been considered since the biomass concentration in the biological film is about 100 times greater than that of suspended solids, using a density of 40 kg SSV/m 3 of biofilm (Zhang and Bishop, 1994; Henze et al., 1995), and an average pipe biofilm thickness of 3 mm. Nitrification is higher in MWW than in RUWW. This may be due to the DO concentration in MWW being greater. It should be taken into account that the first MWW sampling point is at 1 km from the FW injection.
Effect of dissolved oxygen in reclaimed wastewater
129
Taking into account the high DO consumption rate in this area, the initial DO concentration may be deduced to be higher than 4-5 mgll. As soon as DO has disappeared from the medium, and anoxic conditions have been reached, a denitrification process was observed (Fig. 4) from nearly all the samples. In the RUWW this process was very weak (Fig. 4), while in MWW it is much greater, reaching complete denitrification in many cases. This may be due to the ratio organic matterlN-NOf being much lower in the RUWW than in the MWW. The denitrification levels registered are shown in Table 3. It takes place from the time the necessary conditions for it are reached: high organic matterlN-NOf ratio and absence of DO (Abeling and Seyfried, 1992; Henze et al., 1995). Fig. 4 indicates that in MMW the denitritification phenomenon (nitrite consumption) is total in most cases, while complete nitrate consumption does not usually occur. A relationship has been sought between nitrification, denitrification and ORP. Denitrification has been observed to be much more efficient the more reductive the ORP is, as was expected (Lie and Welander, 1994). However, denitrification processes with positive ORP values have been registered . In any case, if a relation is tentatively established between DO concentration and the ORP value, one comes to the conclusion that there is a wide range of ORP values for DO concentrations less than 0.5 mgll. Although DO is totally consumed, the ORP value depends on the biological reactions occurring in the wastewater. Therefore, the greater the organic matter concentration in the wastewater, the lower will be the ORP value attained after a long residence time. The introduction of FW into the RUWW gives rise to the appearance of a nitrification-<1enitrification process after which the nitrogen content in the wastewater is seen to be reduced at the end of the pipe. However, the reduction in nitrogen during MWW transport, probably due to these processes is relatively minor, less than 10%. Consequently the EC of wastewater supplied to agriculture has been reduced by 20% with respect to RUWW, and more important: the risk of sulfide generation in the system has been diminished. CONCLUSIONS In the system under study, RUWW transport gives rise to H 2S generation after a 6-10 hour residence time by which anoxic conditions are reached. When DO over-saturated FW is injected at the starting point, a dilution effect is produced accompanied by oxidation phenomena. Thus, H 2S formation is inhibited and a nitrification-<1enitrification process appears in the pipe. ACKNOWLEDGEMENTS This work was financially supported by a cooperative agreement between BALTEN and the Dpt. of Chemical Engineering of the University of La Laguna, and by a grant from the Autonomous Government of the Canary Islands. REFERENCES Abeling , U. and Seyfried, C. F. (1992) . Anaerobic-aerob ic treatment of high-strength ammonium wastewater> nitrogen removal via nitrite . Wat. Sci. Tech., 26{5/6), 1007·1015 . Balmelle, B., Nguyen, K. M., Capdeville, B., Cornier, J. C. and Deguin, A. (1992) . Study of factors controlling nitrite build-up in biological processes for water nitrification . Wat. Sci. Tech., 26{5/6), 1017·1025 . Boon A. G. (\995). Septicity in sewers : causes, consequences and containment. Wat. Sci. Tech. 31(7), 237· 253. Boon A. G. and Lister A. R. (1975) . Format ion of sulph ide in rising main sewers and its prevention by injection of oxygen. Prog. Wat. Tech. 7(2), 289-300. Henze, M., Harremoes, P., la Cour Jansen, J. and Arvin, E. (1995). Wasterwater Treatment. Biological and Chemical Processes. Springer- Verlag .
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Hvitved-Iacobsen T.• Juette P. H., Nielsen P. H. and Jensen N. Aa (1988). Hydrogen sulphide control in municipalsewers. In Hahn, H.H. and Klute, R. (Eds), Pretreat. in Chern. Wat. and Wast. Treat., Proceedings of the 3rd International Gothenburg Symposium, Gothenburg, Sweden. Springer Verlag, 239-247. Lie, E. and Welander, T. (1994). Influence of dissolved oxygen and oxidation-reduction potential on the denitrification rate of activated sludge. Wat. Sci. Tech.• 30(6), 91-100. Nielsen. P. H., Raunkjaer, K.• Norsker, N. H., Jensen, N. Aa. and Hvitved-Jacobsen, T. (1992). Transformations of wastewater in sewer systems - A review. Wat. Sci. Tech., 25(6),17-31. Pomeroy. R. D. and Parkhurst. I. D. (1977). The forecasting of sulfide build-up rates in sewers. Prog. Wat. Tech. 9, 621-628. Thistlethwayte, D. K. B. (ed.) (1972). Thecontrololsulfides in sewerage systems. Butterworths, Sydney. Zhang. T. C. and Bishop, P. L. (1994). Density, porosity. and pore structure of biofilms. Wat. Res.• 28(11), 2267-2277.