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Dan region sewage reclamation project
Water Research Pergamon Press 1972. Vol. 6, pp. 351-356. Printed in Great Britain
DAN REGION SEWAGE RECLAMATION PROJECT M. KREMER Mekoroth Water Co. Ltd., POB 20 128, 9, Lincoln Street, Tel Aviv, Israel
1. I N T R O D U C T I O N
THE PURPOSEof the Dan Region Sewage Reclamation Project, Israel, is to solve the problem of sewage disposal and to reuse the reclaimed water of seven towns of the Greater Tel Aviv area. These have a population of 750,000. For this purpose they are organized in the Dan Region Association of Towns. Up until 1970 most of the untreated sewage from these towns was discharged into the Mediterranean Sea. Studies have shown that reclamation of the sewage and its re-use would be cheaper than satisfactory discharge of sewage into the sea, with or even without treatment. Moreover, reclamation could provide water for unrestricted re-use and at a lower cost than most other water projects proposed for development in Israel. The Dan Region Sewage Reclamation Project will operate as follows: Sewage will be collected by means of a coastal trunk sewer and delivered, with the assistance of two pumping stations, to stabilization ponds situated in the sand dunes to the south of the Dan Region. The sewage will then be treated in ponds with a recirculated ratio 1 : 1"5 (sewage:effluent). The BOD load on the ponds will be 23 g m -2 day -1 (200 lb acre -1 day-l). The stabilization pond effluent will undergo lime treatment in an upflow solid contact clarifier for the removal of algae, suspended solids, phosphates, viruses and bacteria at pH = 11.0. The lime treated effluent will flow from the clarifier to detention ponds for ammonia stripping and pH balancing. The effluent will then be pumped to spreading basins in the sand dunes and recharged into the local aquifer. Recovery wells, drilled around the spreading basins at appropriate distances, will pump the reclaimed water from the aquifer and deliver it to the water supply system for general use. The Project is being constructed in stages. The coastal trunk sewer and some of the stabilization ponds have already'been completed and 10 × 106 m 3 yr- 1 (million m 3 per annum) is being treated at present. The second stage of the project is under construction and in June 1972 the total annual quantity of sewage undergoing treatment will reach 30 × 106 m 3 yr -1. The plant is designed to meet the projected requirements of the 'nineties when, according to forecasts, the population of the Dan Region will number about one and a quarter million persons, and the raw sewage will amount to 110-120 × 106 m3 yr -I. 2. E X I S T I N G SYSTEM
In January 1970, the first stage of the project, consisting of a system of anaerobic facultative ponds was put into operation. During the running-in phase the residents of the locality raised objections to the plant because of the odours given off by the anaerobic ponds. A method had to be found to put an end to this nuisance. 351
352
M. KgEM~R
It was decided to follow the example of the Cape Town (S. Africa) plant and install an additional system for the recirculation of effluent. This installation which has produced excellent results in Cape Town is proving equally effective at the Dan Region plant. The ponds, which previously were intended for anaerobic process, now operate R a w sewage flow Sewage concentration (B.O.D.) Ponds area Organic load Ponds volume Recirculation ratio Detention time (sewage) Detention time (sewage and effluent)
Q C A L V R Tt T2
= = = = = = = =
27,000 m a day - t 350 mg I - t 400,000 m ' 23 g m 2 d a y - t 670,000 m a 1 : 1.5 25 days 10 days
as aerobic facultative recirculated ponds, were put into operation in June 1970 and since then the plant has not caused objectionable odour or other nuisances. The ponds in operation at present are Ponds A, B, C, D and E (see FIG. 1) and are the first stage of the Dan Sewage Reclamation Project. A diagram of the ponds showing their operational system is given in FIG. 2. 3. F O L L O W - U P
AND
CONTROL
The effluent from the plant is planned to be recharged into the regional aquifer, after undergoing lime treatment. Special attention must therefore be paid to follow-up and control of the quality of the sewage and the effluent. Decisions on the future development of the plant will depend on how successfully it operates. The plant absorbs 27,000 m 3 day -1 (7.0 × 106 g day -1) of sewage and this amount will be trebled within a year. Quality control is carried out from two standpoints: (1) to check the efficiency of the system and (2) to detect potential environmental hazards or nuisances. / // . . . . . .
[ i---
-
\
/
\'\ ". (
Recirculation
pump
FIo. 1. D a n Region sewage project 30 × 106 m a yr -~ stage (1972).
D a n Region Sewage Reclamation Project
-F
Checkpoint Check point Check point Check point No2 No3 No 4 No 5
Check point No I
O
Q Raw sewage
353
Pond 'I'RI A -FB
O
m
I
l
Effluent tolime treatment
R'I'5 O FIG. 2. Operational data.
3.1 EJ~ciency of the system An important conclusion which can be drawn after one year's operation is that the main treatment of the sewage occurs in the first pond, as shown by the decrease in the concentration of organic matter and the removal of nutrient material by 80 per cent (see FIGs. 3 and 4). Moreover, in the first pond the main treatment takes place in the region of the raw sewage inlet; the load per unit area there is identical to that found in typical anaerobic ponds, while at the same time there is no evidence of the creation of objectionable odours or other nuisances. The direct cause of this is obviously the recirculation of the effluent which is done at a ratio of 1 : 1.5. The recirculated effluent carries with it to the first pond a high concentration of dissolved oxygen; the rate of saturation is 100-200 per cent during daylight hours and about 30-50 per cent during the night. The recirculated effluent also has high concentrations of micro-organisms and algae which arc believed to prevent the creation of anaerobic conditions in the upper layer of the pond and even to increase the growth of algae and the supply of dissolved oxygen in the ponds. I000
C.O.D. removal
SO0 800
400
B.O.D.
removal
Z.
200
,-;
SO0
u
400
30•
o
a)
7OO 600
300
200
I OC
lO0
I
I
C. point C p o i n t Nol No2 ROW sewoge
I
i C. point NO3
C, point No4
C.poinf No5 Effluent
[
C. p o l n t Cpoint Nol NOZ Row mowo ge
F i e . 3. B U D , C O D removal.
I C.point NO3
I C.point NO4
I C. point NO5 Effluent
354
M.K.RSMER 300
250
200
tS0
I00
T. S S . 50
V. S S . C poLnt No I Raw sewage
C, point No 2
C. po,nl NO 3
C point NO 4
C. pmn~ No 5 Effluent
FIG. 4. Suspendedsolids removal. Studies are now being made on the efficiency of the system, the optimal recirculation ratio, the influence of climatic factors and the effect of stirring the water in the ponds, with the aim of increasing the organic load to above 23 g m -2 day B.O.D., which is the normal load at present. Table 1 shows the average results of analysis of the sewage at various points in the system during January-April 1971. 3.2 Location and prevention of environmental nuisances The principal potential nuisances for the local population, who live several kilometers from the plant, are odours and mosquitoes. In the operational regime described above permanent aerobic conditions are ensured throughout the pond system, as shown by the high dissolved oxygen concentration and the low concentration of sulphides, volatile acids and other malodorous substances. Very sensitive control instruments have been installed in the surrounding settlements. These detect H2S in the air at a very early stage. The problem of mosquito control is easily dealt with by antimalaria spraying from helicopters every 12 days from May to October.
rng I-i
Fag 1-1 mg 1-1 as CI mg 1-1 as A.B.S. rag 1- 1 units in mm 3
Dissolved oxygen (morning)
Dissolved oxygen (evening) Chlorides Synthetic detergents Fats Algae--total count
N N H2S Acetic Acid
P
CaCO~ SO4
nag 1- t m~g 1-1 mg 1- ~ rag 1-1 mg 1-1 as rag 1-1 as l-I as mg 1-1 mg as mg 1-1 as mg 1-1 as nag 1-1 as
°C
Unit
Temperature pH B.O.D. (5 day, 20°C) C.O.D. Total suspended solids Volatile suspended solids Alkalinity Sulphate Phosphate Nitrogen (Kjeldahal) Nitrogen (ammonia) Sulphide Volatile acids
Analysis
78 55 0.1 130 0 0 130 20 180 0
II
20 8.3 350 800 280 60 470 45
Check point 1 raw sewage
30 17 0-2 70 2 5 110 8 10 2000
5
17 7.8 50 220 80 15 270 38
Check point 2
TABLE 1. ANALYSIS OF RAW SEWAGE AND EFFLUENT AT VARIOUS POINTS IN SYSTEM (SEE FIG.
2),
28 17 0-1 75 5 8 115 8 8 2500
5
17 8"0 25 195 60 5 270 35
Check point 3
Results
28 17 0.1 65 6 15 120 7 7 3000
5
17 8"2 25 180 50 5 265 35
Check point 4
J A N . - A P R I L 1971 (AVERAGES)
29 16 0 20 6 20 120 7 4 3700
5
17 8"3 30 170 50 2 230 35
Check point 5 effluent
tA
.o.
~"
8--
e~
~o"
m
356
M. KRel~S
4. SUMMARY AND CONCLUSIONS In plants for the reclamation of wastes which treat raw sewages in areas similar to that of Israel (sub-tropical), good results can be expected from a system operating on the basis of faculative recirculated aerobic ponds. The method is also cheaper than with plants using mechanical equipment where both the investment and running costs are high. An outstanding advantage of this system is the fact that the long detention time of the sewage permits accurate control and a quality check before the e~uent reaches the stage of tertiary treatment and recharge to the groundwater. It is intended to operate this system with a BOD load of not more than 23 g m - 2 day -1. Where the effect of stirring the liquid in the ponds may be observed, this is recommended in order to prevent stratification in the ponds. This can encourage the growth of algae with an accompanying increase in the organic load to the ponds of 20-30 per cent.
DAN REGION SEWAGE RECLAMATION PROJECT M. KREMER Mekoroth Water Co. Ltd., POB 20 128, 9, Lincoln Street, Tel Aviv, Israel
1. I N T R O D U C T I O N
THE PURPOSEof the Dan Region Sewage Reclamation Project, Israel, is to solve the problem of sewage disposal and to reuse the reclaimed water of seven towns of the Greater Tel Aviv area. These have a population of 750,000. For this purpose they are organized in the Dan Region Association of Towns. Up until 1970 most of the untreated sewage from these towns was discharged into the Mediterranean Sea. Studies have shown that reclamation of the sewage and its re-use would be cheaper than satisfactory discharge of sewage into the sea, with or even without treatment. Moreover, reclamation could provide water for unrestricted re-use and at a lower cost than most other water projects proposed for development in Israel. The Dan Region Sewage Reclamation Project will operate as follows: Sewage will be collected by means of a coastal trunk sewer and delivered, with the assistance of two pumping stations, to stabilization ponds situated in the sand dunes to the south of the Dan Region. The sewage will then be treated in ponds with a recirculated ratio 1 : 1"5 (sewage:effluent). The BOD load on the ponds will be 23 g m -2 day -1 (200 lb acre -1 day-l). The stabilization pond effluent will undergo lime treatment in an upflow solid contact clarifier for the removal of algae, suspended solids, phosphates, viruses and bacteria at pH = 11.0. The lime treated effluent will flow from the clarifier to detention ponds for ammonia stripping and pH balancing. The effluent will then be pumped to spreading basins in the sand dunes and recharged into the local aquifer. Recovery wells, drilled around the spreading basins at appropriate distances, will pump the reclaimed water from the aquifer and deliver it to the water supply system for general use. The Project is being constructed in stages. The coastal trunk sewer and some of the stabilization ponds have already'been completed and 10 × 106 m 3 yr- 1 (million m 3 per annum) is being treated at present. The second stage of the project is under construction and in June 1972 the total annual quantity of sewage undergoing treatment will reach 30 × 106 m 3 yr -1. The plant is designed to meet the projected requirements of the 'nineties when, according to forecasts, the population of the Dan Region will number about one and a quarter million persons, and the raw sewage will amount to 110-120 × 106 m3 yr -I. 2. E X I S T I N G SYSTEM
In January 1970, the first stage of the project, consisting of a system of anaerobic facultative ponds was put into operation. During the running-in phase the residents of the locality raised objections to the plant because of the odours given off by the anaerobic ponds. A method had to be found to put an end to this nuisance. 351
352
M. KgEM~R
It was decided to follow the example of the Cape Town (S. Africa) plant and install an additional system for the recirculation of effluent. This installation which has produced excellent results in Cape Town is proving equally effective at the Dan Region plant. The ponds, which previously were intended for anaerobic process, now operate R a w sewage flow Sewage concentration (B.O.D.) Ponds area Organic load Ponds volume Recirculation ratio Detention time (sewage) Detention time (sewage and effluent)
Q C A L V R Tt T2
= = = = = = = =
27,000 m a day - t 350 mg I - t 400,000 m ' 23 g m 2 d a y - t 670,000 m a 1 : 1.5 25 days 10 days
as aerobic facultative recirculated ponds, were put into operation in June 1970 and since then the plant has not caused objectionable odour or other nuisances. The ponds in operation at present are Ponds A, B, C, D and E (see FIG. 1) and are the first stage of the Dan Sewage Reclamation Project. A diagram of the ponds showing their operational system is given in FIG. 2. 3. F O L L O W - U P
AND
CONTROL
The effluent from the plant is planned to be recharged into the regional aquifer, after undergoing lime treatment. Special attention must therefore be paid to follow-up and control of the quality of the sewage and the effluent. Decisions on the future development of the plant will depend on how successfully it operates. The plant absorbs 27,000 m 3 day -1 (7.0 × 106 g day -1) of sewage and this amount will be trebled within a year. Quality control is carried out from two standpoints: (1) to check the efficiency of the system and (2) to detect potential environmental hazards or nuisances. / // . . . . . .
[ i---
-
\
/
\'\ ". (
Recirculation
pump
FIo. 1. D a n Region sewage project 30 × 106 m a yr -~ stage (1972).
D a n Region Sewage Reclamation Project
-F
Checkpoint Check point Check point Check point No2 No3 No 4 No 5
Check point No I
O
Q Raw sewage
353
Pond 'I'RI A -FB
O
m
I
l
Effluent tolime treatment
R'I'5 O FIG. 2. Operational data.
3.1 EJ~ciency of the system An important conclusion which can be drawn after one year's operation is that the main treatment of the sewage occurs in the first pond, as shown by the decrease in the concentration of organic matter and the removal of nutrient material by 80 per cent (see FIGs. 3 and 4). Moreover, in the first pond the main treatment takes place in the region of the raw sewage inlet; the load per unit area there is identical to that found in typical anaerobic ponds, while at the same time there is no evidence of the creation of objectionable odours or other nuisances. The direct cause of this is obviously the recirculation of the effluent which is done at a ratio of 1 : 1.5. The recirculated effluent carries with it to the first pond a high concentration of dissolved oxygen; the rate of saturation is 100-200 per cent during daylight hours and about 30-50 per cent during the night. The recirculated effluent also has high concentrations of micro-organisms and algae which arc believed to prevent the creation of anaerobic conditions in the upper layer of the pond and even to increase the growth of algae and the supply of dissolved oxygen in the ponds. I000
C.O.D. removal
SO0 800
400
B.O.D.
removal
Z.
200
,-;
SO0
u
400
30•
o
a)
7OO 600
300
200
I OC
lO0
I
I
C. point C p o i n t Nol No2 ROW sewoge
I
i C. point NO3
C, point No4
C.poinf No5 Effluent
[
C. p o l n t Cpoint Nol NOZ Row mowo ge
F i e . 3. B U D , C O D removal.
I C.point NO3
I C.point NO4
I C. point NO5 Effluent
354
M.K.RSMER 300
250
200
tS0
I00
T. S S . 50
V. S S . C poLnt No I Raw sewage
C, point No 2
C. po,nl NO 3
C point NO 4
C. pmn~ No 5 Effluent
FIG. 4. Suspendedsolids removal. Studies are now being made on the efficiency of the system, the optimal recirculation ratio, the influence of climatic factors and the effect of stirring the water in the ponds, with the aim of increasing the organic load to above 23 g m -2 day B.O.D., which is the normal load at present. Table 1 shows the average results of analysis of the sewage at various points in the system during January-April 1971. 3.2 Location and prevention of environmental nuisances The principal potential nuisances for the local population, who live several kilometers from the plant, are odours and mosquitoes. In the operational regime described above permanent aerobic conditions are ensured throughout the pond system, as shown by the high dissolved oxygen concentration and the low concentration of sulphides, volatile acids and other malodorous substances. Very sensitive control instruments have been installed in the surrounding settlements. These detect H2S in the air at a very early stage. The problem of mosquito control is easily dealt with by antimalaria spraying from helicopters every 12 days from May to October.
rng I-i
Fag 1-1 mg 1-1 as CI mg 1-1 as A.B.S. rag 1- 1 units in mm 3
Dissolved oxygen (morning)
Dissolved oxygen (evening) Chlorides Synthetic detergents Fats Algae--total count
N N H2S Acetic Acid
P
CaCO~ SO4
nag 1- t m~g 1-1 mg 1- ~ rag 1-1 mg 1-1 as rag 1-1 as l-I as mg 1-1 mg as mg 1-1 as mg 1-1 as nag 1-1 as
°C
Unit
Temperature pH B.O.D. (5 day, 20°C) C.O.D. Total suspended solids Volatile suspended solids Alkalinity Sulphate Phosphate Nitrogen (Kjeldahal) Nitrogen (ammonia) Sulphide Volatile acids
Analysis
78 55 0.1 130 0 0 130 20 180 0
II
20 8.3 350 800 280 60 470 45
Check point 1 raw sewage
30 17 0-2 70 2 5 110 8 10 2000
5
17 7.8 50 220 80 15 270 38
Check point 2
TABLE 1. ANALYSIS OF RAW SEWAGE AND EFFLUENT AT VARIOUS POINTS IN SYSTEM (SEE FIG.
2),
28 17 0-1 75 5 8 115 8 8 2500
5
17 8"0 25 195 60 5 270 35
Check point 3
Results
28 17 0.1 65 6 15 120 7 7 3000
5
17 8"2 25 180 50 5 265 35
Check point 4
J A N . - A P R I L 1971 (AVERAGES)
29 16 0 20 6 20 120 7 4 3700
5
17 8"3 30 170 50 2 230 35
Check point 5 effluent
tA
.o.
~"
8--
e~
~o"
m
356
M. KRel~S
4. SUMMARY AND CONCLUSIONS In plants for the reclamation of wastes which treat raw sewages in areas similar to that of Israel (sub-tropical), good results can be expected from a system operating on the basis of faculative recirculated aerobic ponds. The method is also cheaper than with plants using mechanical equipment where both the investment and running costs are high. An outstanding advantage of this system is the fact that the long detention time of the sewage permits accurate control and a quality check before the e~uent reaches the stage of tertiary treatment and recharge to the groundwater. It is intended to operate this system with a BOD load of not more than 23 g m - 2 day -1. Where the effect of stirring the liquid in the ponds may be observed, this is recommended in order to prevent stratification in the ponds. This can encourage the growth of algae with an accompanying increase in the organic load to the ponds of 20-30 per cent.