minimize use of
hazardous chemicals in the water and wastewater treatment processes
The plant was to be
equipped with a 1,512,000 kcal/hr (500 ton) average load cooling tower system
for HVAC and process cooling with fresh water supplied by the City of
Chandler, AZ. Data on the Chandler water supply, which is obtained from a mix
of local well, Salt River Project, and central Arizona Project waters, showed
that chemistry of the supply water was variable, but could be described on
average as being hard with high alkalinity and dissolved solids. Table 1
notes some relevant average parameters for this water.
| Table 1: City of Chandler, Typical Water Analysis |
| pH su |
7.9 |
| total alkalinity mg/l |
132 |
| calcium mg/l |
138 |
| sulfate mg/l |
73 |
| magnesium mg/l |
67 |
| chloride mg/l |
136 |
| sodium mg/l |
90 |
| silicate mg/l |
8 |
| dissolved solids mg/l |
460 |
| fluoride mg/l |
0.5 |
METHODOLOGY
Typical practice in
similar projects would have the client retain a consulting engineer, who would
then deal with several different designers/suppliers for each water or
wastewater system in the new plant. Thus one supplier would be responsible
for the high quality water system, another for the wastewater system, a HVAC
contractor for the cooling system, and yet another vender for the cooling
water chemistry program. This approach will not result in an overall optimum
system design due to the difficulty in integrating the differing technologies
of multiple suppliers.
A major management
innovation on this project was placement of the design and supply of the
entire plant water and wastewater system with a single supplier. By
proceeding in this fashion, a completely integrated design resulted which
could exploit new technology in one area, such as high dissolved solids
treatment chemistry in cooling towers, to reuse a wastewater stream from
another area, the primary reverse osmosis (RO) unit.
High Quality Water
Traditional design of an
18 megohm-cm water supply system would utilize carbon filtration, pH
adjustment, RO, separate bed deionization, and mixed bed deionization to
achieve the required quality. Such a system would, however, produce a
substantial wastewater flow, consisting of carbon filter backwash, RO reject,
and deionizer regenerate; and utilize hazardous acid and caustic solutions in
its operation.
The wastewater produced
by such a treatment train would be very high in hardness and/or dissolved
solids, and thus not easily recycled or reused. Since it was obvious that a
traditional design would not satisfy our client's objectives, we reviewed the
basis for design and looked for alternative means of obtaining the
same result.
Carbon filters are
commonly used to remove oxidizers, mainly chlorine, from city water sources to
protect downstream units, such as ion exchange resin, from oxidation. Such
filters must be routinely backwashed, which generates a wastewater stream, and
they are rather costly to install and maintain. An acceptable alternative is
to merely inject a small amount of a reducing agent, sodium metabisulfite
solution, into the inlet water to chemically destroy any oxidizers present.
This process produces no wastewater and adds a minimal amount of dissolved
solids to the input water.
Acid is utilized for pH
adjustment of RO feedwater in order to prevent scale formation in the unit due
to concentration of salts in the reject stream. Softening of the inlet water
to the RO unit accomplishes the same thing, prevention of scale formation, and
permits both higher RO recoveries and reuse of the RO reject water as cooling
tower makeup.
A separate bed deionizer
serves to further reduce the level of dissolved solids in the water prior to
final polishing using mixed bed units. Replacement of the separate bed
deionizer unit with a second stage of RO treatment is technically feasible and
eliminates production of very high dissolved solids regenerate wastewater.
Reject from the secondary RO unit is of a quality to be reused as a component
of the feedwater to the first, or primary, RO unit.
Both the primary and
secondary RO units are designed to be capable of operating on inlet waters
that vary from 100% soft water makeup to the designed 95% wastewater
recovery. The major change in unit operation when switching from one water
source to the other is the operating pressure of the primary RO. Operation on 100%
soft water requires a pressure of 12 kgscm (172 psi), while operation using
95% recycle results in an operating pressure of 19.4 kgscm (134 psi).
Operation with 95% recycle also lowers
the dissolved solids content of most of the water streams in the system.
Table 2 summarizes some
relevant parameters for the RO units while operating on 100% soft feedwater
and 95% recycle.
| TABLE 2: Primary and Secondary RO Unit Data |
| Parameter |
Primary RO |
Secondary RO |
| |
soft |
recycle |
soft |
recycle |
| Operating Pressure kgscm |
12 |
9.4 |
10.9 |
10.9 |
| Stages |
2 |
|
1 |
|
| Stage 1 |
129.5 |
|
129.5 |
|
| Stage 2 |
51.8 |
|
|
|
| Type Membrane |
TFC |
|
TFC |
|
| Feed 1pm (gpm) |
112 (29.6) |
|
84 (22.2) |
|
| Reject 1pm (gpm) |
28 (7.4) |
|
8.4 (2.22) |
|
| Permeate 1pm (gpm) |
84 (22.2) |
|
75.7 (20.0) |
|
| % Recovery |
75 |
|
90 |
|
| Feed DS * mg/l |
540 |
123 |
16.3 |
4.9 |
| Reject DS * mg/l |
2187 |
488 |
189 |
44.6 |
| Permeate DS * mg/l |
16.3 |
4.9 |
1.3 |
1.9 |
* Dissolved Solids
Complete elimination of
sulfuric acid and sodium hydroxide use in the plant is achieved via use of
exchange mixed bed deionizers, where the regeneration of the mixed bed
exchange tank is done off-site. Use of such exchange tanks is possible as the
effluent from the secondary reverse osmosis unit has a dissolved solids
content of 1.9 mg/l. With this level of dissolved solids about 1,326 kl
(360,000 gallons) of water can be polished by a typical 0.11 cu m (4 cubic
foot) capacity mixed bed exchange tank, giving about twelve days operation
between tank exchanges.
Figure 1 is a schematic
block diagram of the final design for the high quality water portion of the
project.
Wastewater
The high quality water
produced by the proceeding train is used as cooling and rinse water during the
diamond saw cutting of individual integrated circuits from the large discs on
which they are fabricated. Rinse water is used as supplied, while cooling
water is often pH adjusted by introduction of carbon dioxide gas with a small
amount of non-ionic surfactant, less than 5 mg/l, added for improved surface
wetting. Both rinse and cooling water use is one pass to minimize solids
retention on the integrated circuits. A wastewater sample
obtained from a similar operating plant gave the following results upon
analysis.
| Table 3: Wastewater Sample Results |
| turbidity (ntu) |
155 |
| silicon (mg/1) |
7.0 |
| suspended solids (mg/1) |
15 |
While most integrated
circuits produced today do not have significant levels of heavy metals
present, resulting in basically heavy metal free cutting wastewaters, some
high performance semiconductors are based upon gallium arsenide, which when
processed contaminate the water with arsenic. A recent development by IBM,
replacing the aluminum circuit paths in integrated circuits with copper, will
result in a significant amount of copper being added to the wastewater when
such products are processed. Since it is likely that heavy metals will be
present in future process wastewaters, the design of the wastewater treatment
system had to provide for their removal.
Based upon our research
and operating experience with such systems, we selected a wastewater treatment
train consisting of an inclined plate clarifier followed by microfiltration.
The inclined plate clarifier would initially serve as the primary solids
removal device prior to the microfilter and serve to recycle the reject from
the microfilter. In the event that heavy metals are introduced into the
products being packaged, the clarifier unit is designed for use of
precipitation chemistry for their removal. A proprietary chemistry based upon
modified polyaluminum compounds, which minimize addition of dissolved salts to
the treated wastewater, has been devised for this process step. Several
operating systems have demonstrated the ability of this chemistry to remove
arsenic, lead, and copper from various wastewaters, including those produced
by gallium arsenide crystal production.
Inclined plate clarifier design was based upon a flow
rate of 9.5 lpm/sq meter (0.25 gpm per square foot) of projected plate surface
area. Careful attention was given to the areas of mix tank geometry (square
dimensions), mixing energy ((0.2 kw/1000 1 (1 hp/1000 gallons capacity)),
plate pack influent entry, and effluent weir placement and geometry. Glazed
flat fiberglass plates with plastic fasteners and PVC spacers were specified
to minimize corrosion and deposition problems in the plate pack itself. The
three specified mix tanks, clarifier, and clear well were produced as a single
unit. Chemical feed systems were specified as retrofits to be installed if
needed in the future.
The microfilter was
designed to use a new PVDF membrane, series J, developed by Desalination
Systems of Vista, CA. This membrane has been used in a spiral wrap
configuration in several microfilter units for removal of suspended solids
from waters and wastewaters with excellent results. Due to its composition,
it is very resistant to fouling and chemical degradation. Based on past
performance, a microfilter unit constructed with a total of seven Desal 10 cm
(4”) dia. X 102 cm (40”) length series spiral wrap membrane assemblies was
specified to provide a maximum permeate flow of 75.7 lpm (20 gpm) at an
operating pressure not to exceed 7 kgscm (10 psi). This unit has a total
active membrane surface area of 62.4 sq m (672 sq ft) with a particle size
cutoff of 0.4 micron.
Figure 2 is a schematic
block diagram of the final design for the wastewater portion of the project.
Cooling Tower
With the fresh makeup
water available, the cooling tower in this plant would typically be operated
at three cycles of concentration with anti-scalant chemistry. Such operation
would result in generation of 25,121 lpd (6,637 gpd) of blowdown to be
discharged and require 75,363 lpd (19,911 gpd) of fresh makeup water.
However, cooling towers
using softened makeup water can be operated with no intentional blowdown,
reducing the amount of water needed, and wastewater generated, by the plant.
Basically, the cycles of concentration in the cooling tower system are
increased to the point where windage from the cooling tower equals the
blowdown rate. At this point zero blowdown is achieved. Typically, the zero
blowdown point is reached between 10 to 20 cycles of concentration in standard
cooling towers.
Operation of the client's cooling towers at 20 cycles,
using secondary RO reject with no wastewater recycle operation as 100% makeup,
could result in worst case dissolved solids values as high as 37,560 mg/l.
Normally, cooling towers are not operated with softened makeup water at such
high dissolved solids values due to the extreme corrosivity of the water. We
have found that by application of specific water treatment chemistry, the
corrosivity of such cooling waters can be controlled to acceptable levels.
Bypass filtration, using cartridge type filters with cleanable elements, is
also required to prevent buildup of suspended solids in the cooling tower with
subsequent underdeposit corrosion.
Application of this new
technology to operate cooling towers with soft water makeup and dissolved
solids levels of up to 40,000 mg/l permits reuse of the primary RO reject as
cooling tower makeup, with supplemental makeup provided by the water softener
as needed. Reuse of the RO reject and operation of the cooling tower with
zero blowdown substantially decreases the plant wastewater discharge and use
of fresh water.
Specific equipment
provided for zero discharge operation of the cooling tower consisted of a
makeup proportional chemical feed system, to ensure proper inhibitor dosage,
and a cartridge type bypass filter sized for one turnover per day.
Figure 3 is a schematic
block diagram of the final design for the cooling tower portion of the
project.
RESULTS
Returning to the
objectives set by the client at the start of this project the following
results were obtained:
A system has
been designed to provide 75.7 lpm (20 gpm) of 18 megohm-cm resistivity water
using Chandler city water as source.
Fresh water use has been minimized by reuse of primary RO reject as cooling tower makeup,
recycle of secondary RO reject and treated wastewater, and zero blowdown of
the plant cooling tower. At the design high quality water use rate of 75.7
lpm (20 gpm), with 95% recovery of wastewater, water softener production will
amount to 55,742 lpd (14,727 gpd) broken out as 45 '833 lpd (12,109 gpd) for
primary RO feedwater and 9,909 lpd (2,618 gpd) for additional cooling tower
makeup. Input fresh water will thus total 57,884 lpd (15,293 gpd), with the
majority of it, 50,242 lpd (13,274 gpd), ultimately consumed by the cooling
tower system.
Wastewater
discharged from the cooling tower has been reduced from 25,121 lpd (6,637 gpd)
to 0 lpd (0 gpd), while fresh water makeup requirements have been reduced from
75,363 lpd (19,911 gpd) to 9,909 lpd (2,618 gpd) via reuse of the primary RO
reject as makeup in place of fresh water.
Discharge of
wastewater has been substantially reduced. The only wastewater discharged from
the entire system is the softener regeneration wastewater at a calculated
volume of 38.4 1/1000 1 (38.4 gal per 1000 gal) of soft water produced. Based
on this data, the total daily process wastewater discharge from the system as
designed amounts to 2,142 lpd (566 gpd).
Hazardous
chemical usage has been essentially eliminated as to treatment of water and
wastewater. In place of sulfuric acid and sodium hydroxide, both hazardous
chemicals, a solution of sodium metabisulfite and common salt has been used.
CONCLUSION
The superior systems
integration possible with a single designer/supplier permits new concepts in
water and wastewater treatment, and cooling tower treatment chemistry, to be
easily applied to design of entire plant water systems. This integrated
approach results in superior designs that provide substantial decreases in
both fresh input water and process wastewater discharges.
