Sea water desalination by reverse osmosis. Recent experience

Sea water desalination by reverse osmosis. Recent experience

Desalination, 14 (1974) 21-31 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands SEA WATER DESALINATION BY REVERSE OSM...

659KB Sizes 0 Downloads 183 Views

Desalination, 14 (1974) 21-31

© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

SEA WATER DESALINATION BY REVERSE OSMOSIS RECENT EXPERIENCE* P. TREILLE

AND J . M .

ROVEL

Socidre Degrcn,onr. Suresnes (France)

(Received November 12, 1973)

SUMMARY

The sea water desalination plant at Houat, in operation since July 1971, has a nominal capacity of 50 m'/day of water containing less than 250 mg/I of chlorides. The description of the installation reveals the importance of the pretreatments required for proper operation and membrane conservation . The twostage treatment has permitted suitable adaptation to the variation in production which is a function of the season . Operating costs are analysed . Optimum plant operation resulting in increased production of desalinated water as a consequence of higher conversion rates is discussed . Since the 1970 Symposium in Dubrovnik, a desalination plant employing reverse osmosis has been built and put into operation on the Isle of Houat, located off the Quiberon peninsula on the French Atlantic coast . The initial contract specified that the plant must produce 50 m 3/day of a water containing less than 250 mg/I of chloride ions at a sea water temperature of 18`C . The problem to be resolved is thus clearly more difficult than that encountered for the Isle of Cavallo, mentioned during the last Symposium, for which a single-stage treatment was found to be sufficient . The standard analysis of the sea water at Houat is as follows : pH 8, Resistivity 30 ohms x cm, Suspended matter content 4 .8 mg/1, TAC 2 .8 mequiv/l, TH 120 mequiv/1, TM 100 mequiv/l, SO, -- 1 .53 g/l, CI - 19.2 g/l . PUMPING STATION

Since the sea in this region is subject to strong tides, the resulting water movements cause a relatively high turbidity, and considerable amounts of algae * Presented at the Fourth International Symposium on Fresh Water from the Sea, Heidelberg, September 9-14, 1973 .

22

P . TREILLE AND I . M. ROVEL

are washed into shore . Therefore, when the project was elaborated, it was decided that construction and maintenance of the sea water intake structure could be facilitated if the sea water was pumped only at high tide and was then stored in an intermediary reservoir from which the desalination plant would be supplied continuously . Such intake structure accessibility permits easy removal of the algae and sand which are carried in during storms . The sea water is pumped to a 100 m' reservoir through a polyethylene pipeline (inside diameter 90 mm) by two horizontal spindle, centrifugal type immersed pumps under a head of 24 m . The unit capacity of these pumps is 42 m 3/h. and each one is equipped with a 4 .4. kW power electric motor supplied with three-phase 380 V current . The operation of the pumps is regulated by a high level controller located in this reservoir. In order to avoid the proliferation of living organisms in this circuit, high doses of chlorine gas are injected into the water during a discontinuous prechlorination treatment effected near the sea water intake structure (10 g/m' for 5 minutes injection every 10 minutes). The chlorine supply is stored in two cylinders, each containing 30 kg of chlorine ; the solenoid valve controlling injection is regulated by a volumetric meter placed on the sea water pipe . CLARIFICATION

The desalination plant is supplied by gravity from the reservoir with a continuous flow of sea water at a rate of 12 m 3/hr. Upon reaching the plant, the water first passes into a flocculation tank equipped with a stirrer . The flow of sea water into this tank triggers the injection of a 15 g/m 3 dose of aluminium sulphate in the form of a 10 % solution . The water remains in the tank for approximately 12 minutes, thus permitting the colloids to group together in the form of sufficiently large floc . The flocculated water then passes into an open sand filter employing sand with a grading of 0 .95 mm . Since the filtering surface is 2 .5 m 2, the nominal output of 12 m 3 /hr is filtered at a speed of 4.8 M 3 /M 2 -hr. The filter is washed approximately every two days as a function of the increase in head loss . The washing cycle is as follows : - Drainage of the upper part of the filter ; - Unclogging by means of water returned upwards at a low speed (6 to 8 m/hr for 5 minutes) ; - Washing with water at a low speed and scour air for 10 minutes ; the scour air flow is 140 m'/hr, while the water speed is the same as in the preceding stage ; - Rinsing with a rapid flow of water for 5 minutes ; the water speed reaches 20 m/hr. The washing cycle lasts 20 minutes, and approximately 8 m 3 of sea water are consumed .

SEA WATER DESALINATION BY REVERSE OSMOSIS

23

A 16 m 3 buffer tank located downstream of the filter allows the storage of a certain quantity of filtered water and ensures a continuous supply to the desalination unit, even when the filter is being washed . An injection of dilute hydrochloric acid : - destroys a portion of the bicarbonate : this reduces the risk of scale formation ; - maintains the pH at a value close to 6 : this is beneficial for conservation of the selective properties of the cellulose acetate membranes since the hydrolysis rate is reduced . The 200 g/l acid solution is injected by a dosin„ pump at a rate of 2 .5 1/hr (i.e. for 12 m 3 of sea water) into a 1000 tter r tank in which the level is kept constant by means of float valve . A 12 m'/hr capacity pump ensures he mixing of the water-acid solution, while an automatic regulation device measures the pH and controls the injection of acid : the latter device causes the entire plant to stop in case of defective pretreatmentAfter passing through this series of preliminary pre-treatments, the water flows by gravity to the osmosis unit . REVERSE OSMOSIS UNIT

Since the contract conditions concerning the treated water analysis were rather severe, even though the rhythm of plant operation was not (and is not yet) well established, we believed it prudent, when the project was drawn up, to divide the treatment into two stages . The Isle of Houat is essentially a tourist resort, and the desalination plant must supply the potable water required during the tourist season . Consequently, the osmosis unit will function at only reduced capacity or intermittently for a good part of the year : experience in maintaining membrane efficiency under such operating conditions is still rather limited . Furthermore, the Isle is at the terminal of the electric power distribution network, and numerous power failures interrupt operation . The corresponding stops and starts also have a negative influence on plant efficiency . As a function of alt these conditions, we have adopted the following general arrangement for the osmosis unit (Fig . 1) : The daily production of the 1st stage is 60 m3 of water with an average chloride ion content of approximately 1 g/l . The production of the first modules (approximately 20 m 3/day)-i.e. those which are swept the first by the sea water . thus under the best hydraulic conditions and with the minimum increase in salinity-is sent directly to the treated water tank. The CI - ion content is approximately 400 mg/l . The remainder of the first stage production (approximately 40 m 3 /day) is re-treated in a second stage which produces 30 m'/day of water with a low chloride content (less than 100 mg/1) .

24

P. TREILLE AND J . M . ROVEL

Fig. I . Isle of Houat-reverse osmosis unit .

The mixture of the two productions in the treated water tank results in a chloride salinity of less than 250 mgfl . This particular arrangement both reduces the number of modules required in the first stage (since 40 m'/day can be rejected and sent through the second stage), and limits the volume re-treated in the second stage to a strict minimum since 20 m 3/day produced by the first stage can be used directly . The perfection of highly selective membranes which stand up well under use permitted such an arrangement to be installed : this set-up is more economical than re-treating the entire first stage production in the second stage . Overall conversion is 17%, The modules utilized are the M P 36-18 type manufactured by Rhone-Poulenc, and have a useful surface of 2 .8 m 2 . They are fitted with highly selective tubular membranes of the cellulose acetate type described during the Dubrovnik Symposium in 1970 . These modules are enclosed in a plastic envelope . - The first stage consists of 96 modules separated into 6 rows of 16, and offers a total membrane surface of 270 m 2 . The rows are grouped together in parallel by pairs . and an electric pump supplies each group with 4 m 3/hour under 70 bars . The rotation speed is 1400 rpm . The pumps are of the screw type with the rotor and shaft in stainless steel, stator in synthetic rubber . The power of the electric motors is 20 kW ; this is necessary for starting the screw pumps . Auxiliary pumping equipment includes : - stainless steel safety valve rated at 80 bars ;

SEA WATER DESALINATION BY REVERSE OSMOSIS

25

- oil-bath, anti-vibration pressure gauges for pressure control ; - by-pass valve equipped with a limit switch permitting a gradual pressure increase during starting up . Each row of modules is connected to the corresponding pump by means of a flexible high pressure hose . The end of each row is fitted with a regulating valve which allows the desired pressure to be maintained . In addition, a pressostat permits detection of any leakage in the circuit . Its operation, in case of an incident which results in a pressure drop, causes the corresponding pump to cease operation . No stand-by pumps were provided . Should one pump break down, the two corresponding rows of modules are connected manually to the distribution collector of the other two pumps ; the consequences of such a breakdown are minimized, and only show up as slight variations in plant performance . Although the 16 modules of each row are mounted in series, the purified water production is divided between two collectors : the production of the first 5 modules of each row flows into one collector, and is sent directly to the treated water cistern . The remaining I I modules produce the water necessary for operation of the second stage : this flow is directed to a small buffer tank which is also connected to the treated water cistern at its lower part . The second stage consists of a high pressure screw-type pump, capacity 1600 1/hour under 60 bars, and 24 modules supplied in series . These modules are identical to those of the first stage . One stand-by pump was installed . In case of a breakdown, the row of modules can be hooked up to the pump in service . The brine produced in the second stage with a salinity lower than 10 g/l is clearly less salty than the pre-treated sea water . Consequently, it is returned to the pre-treatment tank supplying the first stage pumps . A purified water recycling device has been installed so that all traces of salt on the membranes can be eliminated and that rinsing with pure water is still possible when the plant is stopped . This device can also be utilized to fill the reverse osmosis modules with a conditioning solution which prevents membrane deterioration during prolonged shut-downs . COMPLEMENTARY TREATMENT

Before being sent to the water tower on the Isle, the water is injected with carbon dioxide, stored in 20 kg cylinders under 60 bars, at a rate of 44 grams per m 3 , then passes through a neutralite filter (mixture of calibrated grains of calcium carbonate and magnesium carbonate of constant solubility) . This filter has a section of 1 .250 m 2 and contains approximately 2000 1 of neutralite . The neutralite is consumed at a rate of 90 .5 g per m 3 of treated water.



P. TREILLE AND I . st. ROVEL

26

I 2 3 4 -

Fig . 2 . Isle of Houat-sea water desalination plant . Output : 50 m'jday . 12 - Scour air blower CO2 room 5 - Electrical panel 13 - Treated water pumps Chorine room 6 - Electric meters 14 - Osmosis pumps 1st stage Preparation unit for 7 - Sand filter 8 - Acid injection tank 15 - Osmosis pumps 2nd stage flocculation reagents Neutralite filter 9 - Filtered sea water pump 16 - Modules 17 - Osmosis electrical panel 10 - Pumps supplying the (reminera)ization) chlorinator with water 18 - Anti-hammer device 11 - Wash water pumps

Fig. 3. Module installation .

SEA WATER DESALINATION BY REVERSE OSMOSIS

27

Fig . 4 . Pumping system .

As a result of this operation, a certain quantity of bicarbonate is reintroduced into the water (approximately 2 milliequivalents per liter) and renders the water more pleasant for drinking . Finally, a sterilization device injects a dilute sodium hypochlorite solution (containing 1 .5 g of active chlorine per liter) from a 500 1 tank at a rate of 6 liters per m3 of water ; the chlorine dose so injected into the desalinated water is close to 1 g/m3 . The bacteriologically pure water produced by the reverse osmosis unit thus remains sterile in the water tower and in the distribution network .

GENERAL LAYOUT

With the exception of the sea water intake and the large tanks, the various treatment units are centralized in a building whose basic dimensions are : 15 m length, 7 .5 in width and 6 m height . Fig . 2 shows the arrangement of the various units in this building, Fig . 3 depicts the module installation, and Fig . 4 illustrates the pumping system .

OPERATION RESULTS

When tested at the factory before being shipped out, the modules installed at the Isle of Houat had the following average operational characteristics : Efficiency : 97 .3 % (minimum value 96 io) Production : 660 liters/day, i.e. 235 liters/day per m2 of membrane This measurement was effected with a 35 g/l sodium chloride solution under 60 bars at 20°C and a flow rate of 1500 1/h at the module inlet .

Fig . 5. Isle of Houat--evolution of production . The installation began producing desalinated water on July 20th, 1971, only after the pressure had been increased gradually over one hundred hours to ensure proper membrane compaction . The output reached 55 m 3/day of desalinated water with a 230 mg/l chloride content with a sea water temperature of 22°C, and under an average pressure of 70 bars : the contract specified 50 m 3 /day at 18°C and 250 mg/l of chloride . From the beginning of operation until January 31st, 1973, the installation produced 6000 m 3 of desalinated water : this production was spread over two



29

SEA WATER DESALINATION BY REVERSE OSMOSIS

periods of continuous operation at full capacity (the summers of 1971 and 1972during which the Isle's population rose above 2000 inhabitants), and two periods of highly discontinuous operation at reduced capacity (during which the yearround population of 500 inhabitants first utilized the rain water collected in cisterns, as was customary before the plant was installed) (Fig . 5). Between January 31st, 1973, and August 31st, 1973, 4000 additional m 3 were produced . The average working life of the membranes exceeded one year : it was only necessary to fit new membranes to 40 modules in June 1972, and to 20 modules in November 1972 . The possibility of adapting the operating capacity to both water requirements and membrane performances during periods of reduced demand is, of course, a distinct advantage . The proportion of water treated in the second stage can thus be modified to correct excessive salinity of the water produced in the first stage . Membranes whose performances have diminished and are no longer suitable for full-capacity operation may thus still be utilized during periods of reduced production . Nevertheless, the membranes have been subjected to discontinuous operation which is not a priori beneficial : - frequent power failures provoke non-programmed stoppages and difficulties in rinsing the membranes ; - intermittent operation during slack periods requires the injection of a conditioning solution as soon as the shut-down exceeds 48 hours . This is an operating instruction which the operator does not follow strictly ; - in any case, the corresponding variations in pressure inevitably place the membranes under stress. OPERATING COSTS

During the twelve actual months of plant operation between October 1, 1971, and November 30, 1972, the production reached 4650 m 3 . The corresponding expenses per m 3 of product can be broken down as follows: Electricity (including heat :eg) Chemical reagents Membrane replacement (40 modules) Maintenance (other than modules) Personnel Additional expenses (transport, telephone, etc.) Total

5.00 0.90 8.80 4_30 10.80

F .F. F . F. F .F. F .F_ F.F.

3 .20 F .F. 33.00 . .F F

30

P . TREILLE AND J . M. ROVEL

This cost does not take into account the interventions by our Technical Department during the first year of operation of this plant which was considered in some aspects as a pilot unit. It is certain, however, that actual operating costs will drop largely below this first estimate once the plant begins normal operation : the items concerning personnel, maintenance and additional expenses should decrease, since adjustments were still being made during this period, in particular concerning determination of means of operation at reduced capacity .

FUTURE PERSPECTIVES

The general evolution of desalination technology should also lead to a reduction in operating costs : this evolution takes the form of increased performances, and especially of increased production . As an example, the forty modules reinstalled in July 1972, after membrane replacement and improvement of the internai hydraulic system, showed performances highly superior to those of the modules installed in 1971 when placed under the same factory test conditions as described previously : average efficiency is 97.9 and there is no individual efficiency below 97 .4 ; average production reaches 860 liters/day, i.e. a 30 % increase . We thus anticipate both a reduction in the number of modules required to produce the contractual output, and a longer utilization of these same modules : since the initial performances of these modules differ even more from the minimum performances (below which membranes must be replaced), the total number of membranes to be replaced for a given production is reduced . Moreover, since the water demand has risen rapidly due to development of the tourist industry (itself a consequence of the implantation of the desalination plant), the have examined the possibilities of increasing the daily production by utilizing the existing circuits in the most efficient manner. Study showed that simply increasing the number of first and second stage modules, without making any other changes in the sea water circuit, will permit peak operation production of 70 m'/day with a chloride content close to 350 mg/l, and 24 % conversion . The proposed layout is as follows : Three rows of pumps, each including 3 series of 16 modules, constitute the first stage, whose overall production is approximately 80 m'/day . The production is not directly proportional to the number of series of modules, however, since each series is supplied by a sea water flow which is only 2/3 of the normal actual output ; this causes a decrease in the sweeping speed in the interior of the modules, and a greater increase in the final salinity . Utilization of the second stage stand-by pump for peak operation by con-



SEA WATER DESALINATION BY REVERSE OSMOSIS

31

necting it to a new series of modules to treat the water produced by the "downstream" modules of the first stage . This would only require: in the first stage : the installation of 48 modules and the necessary connections ; in the second stage : - either the addition of 12 more modules of the type presently in use ; - or replacing all 20 second-stage modules now in use (which could then be installed elsewhere) with 5 hollow fiber modules . This last solution appears particularly advantageous, for it requires much less space ; moreover, reduction of the service pressure due to the use of hollow fibers compensates for the increase in output, and the electric power consumption does not rise. Since the chemical inertia of the plastic membranes facilitates operation, a membrane working life of several years, even with discontinuous operation, can be anticipated ; this will result in lower operating costs . Under these conditions, the cost is estimated at 15% of the total initial investment for a production increase of 40% . The overall cost of the m3 -day installed will be reduced to 80 % of the initial cost. The only increase in operating costs will be due to eventual replacement of the additional cellulose acetate membranes ; these costs should be reduced by more than 30% .