Desalination, 41 (1982) 209-214 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
EFFECT OF RECOVERY ON DESALINATION WATER BY REVERSE OSMOSIS*
OF THE BLACK SEA
AND B. BONEV
of Water Technology,
The effect of fresh water recovery was studied during Black Sea water RO desalination with cellulose acetate tubular membranes. Two different schemes of brine pretreatment before RO were investigated. It was shown that ultrafiltration can be applied as an effective alternative method for Black Sea water pretreatment. Up to 30% recovery, the product flux as well as the salt and the specific ionic rejection are all fairly stable. Special experiments showed that in a stable turbulent regime Ca ions removal from Black Sea water before RO with tubular membranes hold out no technological advantages.
INTRODUCTION In order to determine the needed degree of water pretreatment and the effect of recovery on the main technological parameters during Black Sea water desalination by reverse osmosis, experiments were carried out with a pilot RO plant consisting of pretreatment and RO units with tubular cellulose acetate membranes. The RO system was installed at the Black Sea shore near the Burgas region. Black Sea water salinity in the Burgas region is 16,000-18,000 mg/l. The Black Sea water intake was located 100 m away from the shore at a depth of 2 m.
EXPERIMENTAL Two different kinds of pretreatment units were constructed with the purpose of ensuring sufficiently complete removal of turbidity components such *Paper presented at 7th International Congress CHISA, Prague, 1981. OOll-9164/S2/0000-0000/S2.6001982
AND B. BONEV
as colloidal particles, bacteria and microorganisms found in Black Sea water. In this way fouling on the surfaces of the RO tubular CA membranes can be prevented. The RO feed water quality requirements were as follows  : Turbidity - below 2.0 mg/l, FI - below 7, COD (KMn04) - below 2.5 mg O2 /I, residual chlorine - below 1.0 mg C!lz/I, pH 3-6. The pretreatment systems tested were comprised of the following processes: a) Sea water - chlorination - sedimentation - in-line coagulation - filtration on up-flow multi-media filter - polishing sand filtration and dechlorination [l] . b) Sea water - sand filtration and ultrafiltration_ The flow diagram and equipment specifications of these units are shown in Figs. 1 and 2 and Table I. Multimedia filter
Fig_ l_ Black Sea water pretreatment unit flow diagram -
iFa t .
Fig. 2. Black Sea water pretreatment unit flow diagram -
OF BLACK SEA WATER
TABLE I SPECIFICATION
OF MAIN TREATMENT
First system (Fig. 1) Reservoir-sedimentator Type:
Section volume: Residence time:
Type: Size: Liquid velocity : Column media from bottom:
two section 1.5 m3 2 h
upflow pressured filter i.d. 225 mm, H = 3,000 mm 10 m/h gravel 30-40 mm, gravel lo-15 mm, 2-3 mm, sand sand l-2 mm, 80 X 80 X 50mm
H H H H
= = = =
100 mm 200 mm 300 mm 1,550 mm
Type: Size: Liquid velocity : Column media:
downflow pressurized filter i-d. 225 mm, H = 3,000 mm 10 m/h sand below 1 mm, H = 1,300 mm
Second system (Fig. 2) Sand filter Type:
Size: Liquid velocity: Column media:
downflow pressurized filter i-d. 225 mm, H = 2,000 mm 10 m/h sand 0.8-1.35 mm, H = 1,000 mm
tubular tubular CA Paterson Candy membranes, type T5/A, i-d. 12.5 mm, UK
The RO unit (Fig. 3) is a skid-mounted package type with built-in tanks, heater, sea water feed pump, safety filter, high pressure pump (inlet flow up to 800 l/h and operating pressure up to 6.0 mPa), tubular module with Paterson Candy CA membranes, type T2/15 W (i-d. 12.5 mm) and control instrumentation.
AND B_ BONEV
Fig. 3. Black Sea water RO unit flow diagram.
RESULTS I) Pretreatment
Two pretreatment systems were in operation simultaneously for a half year and provided pretreated water of the needed quality despite the fact that turbidity of raw Black Sea water undergoes seasonal variation_ Data shown in Table II were obtained when the first plant was operated for a year (August-August 1980) by adding a coagulant (FeCl, ; 3-10 mg/l and sodium hypochloride (3-6 mg/l as Cl, ) and the second plant for a half year (March- August 1980) - ultrafiltration at operating pressure 1.0 mPa and water recovery 90%.
Our previous results [l] for Black Sea water desalination by RO were given at recovery about 5%. In the present paper by means of reject circulation at operating pressure 5.0 mPa, water temperature 20°C inlet flow 800 l/h (Re = 22,500) the recovery was varied from 5 to 60%. The results ob-
TABLE II PRETREATED
BLACK SEA WATER ANALYSIS
Pretreated Black Sea water system Item
Turbidity, mg/I COD (KMn04), mg 02/1 FI
1.5-60 1.8-3.5 Not measurable
< 2.0 < 2.0 < 6.0
0 < 1.5 < 6.0
OF BLACK SEA WATER
tained are presented in Fig. 4. When recovery increases from 5 to 60% the product flux decreases respectively from 0.97 to 0.77 m3/m2 d, i.e. by about 20%. The salt and specific ionic rejections decrease too. From the experimental results (Fig. 4) it can be seen that up to about 30% recovery product flux, salt and specific ionic rejections are very fairl~~stable. Experiments with pre-softened Black Sea water also were carried out. The content of Ca ions was decreased from 12 meq/l to 1 meq/l by means of Na cation exchange technique 121. The results obtained are presented in Fig. 5. As can be seen from Figs. 4 and 5, the product flux and desalination as well as the salt and specific ionic rejections are roughly equal in experiments with natural and pre-softened Black Sea water. * On the basis of the above experimental results it can be concluded that at a stable turbulent regime (Re = 22,500) the concentration polarization is almost eliminated, i.e. Ca ion removal from Black Sea water before RO desalination with tubular membranes is not necessary. Cation rejection 0”
rejection Cl-. SO:-
Fig. 4. Product flux, saIt rejection and specific ionic rejection as a function of recovery (natural Black Sea water).
*Even with presoftened Black Sea water desalination some decreasing of salt and ionic rejections in comparison with natural Black Sea water desalination can be seen. It is due to the fact that in presoftened water most of the Ca ions and part of the Mg ions are changed with Na ions towards which the membranes used are with lower selectivity.
214 Product flux (m’lm’ld) .Q Q
AND B. BONEV
Cation rejection Na’ (%I
Salt rejection (551
Fig. 5. Product flux, saIt rejection and specific ionic rejection as a function of recovery (presoftened Biack Sea water)_
CONCLUSIONS 1) The possibility of two different kinds of pretreatment systems for Black Sea water pretreatment before RO desalination with tubular CA membranes was determined experimentally. It was proved that ultrafiltration can be applied as an alternative method for Black Sea water pretreatment. 2) The effect of recovery on product flux and desalinated water qality was studied_ Up to 30% recovery product flux, salt and specific ionic rejections are very fairly stable. 3) At stable turbulent regime (Re = 22,500) Ca ion removal from Black Sea water before RO desalination with tubular membranes hold out no technological advantages.
REFERENCES 1. 2.
I. Dobrevsky, V. Mavrov, B. Elonev, R. Vasilev and A. Zvezdos, Proc. 7th Intern. Symp. on Fresh Water from the Sea, 2 (1980) 239-248. f. Dobrevsky, V. Mavrov. 2. Zvezdov, S. Prodanova and V. Petrov, Proc. 7th Intern. Symp. on Fresh Water from the Sea, 2 (1980) 25-30.