Note: Descriptions are shown in the official language in which they were submitted.
There has beell a growirlg concern on the part of naturalists,
industrialists, federal, sta-te, and local legislative bodies, and the general
population about controlling industrial pollution of the enviro~ment.
Pollution of air and surface waters by direct emission of wastes rom indus-
trial plants into the air or into lakes and streams has been of concern for
many years. More recently~ pollution of underground waters as well as of air
and surface waters as a result of storing solid and liquid wastes at land
disposal sites has been recognized as a serious problem. Federal and state
statutes, municipal ordinances3 and regulations issued by agencies such as the
EPA increasing the stringency of pollution standards are almost continuously
being enacted and promulgated. While the ideal standard from the standpoint of
pro*ection of the environment is ~ero discharge oE pollutants, for many areas
of technology, it is widely believed at present that this standard is not
practical, and standards have been accordingly established at levels which are
considered feasible.
The present methods o~ treating industrial wastewater effluents
include ion exchange, reverse osmosis, e~aporation, filtration, and chemical
destruction of contaminants. The use of these processes, which are discussed
in more detail below, has been unsuccessful in economically reaching the
2~ ultimate goal of ~ero discharge of liquid effluent.
In ion exchange processes the effluent i5 passed through a bed of
solid ion exchange resins. A reversible chemical reaction takes place between
the ion exchange resins and the effluent by means of which the harmful ions
contained in the effluent are interchanged with non-polluting ions from the
ion exchange resins. The purified effluent can then be discharged or returned
to the process which generates the effluent. In time, the ion exchange
resins become contaminated and must be decontaminated and regenerated by back-
7~
washing. In the process o-f backwashing the ion exchange resins, wastewater is
generated which is more highly contaminated than the original wastewater and
which must be disposed of by some method. Also, the method is expensive and
the ion exchange resins have to be replaced periodically.
Reverse osmosis is effective in some cases but is limited in the
types of applications to which it may be applied because calcium salts deposit
in the semi-permeable membranes and most industrial processes include a lime
treatment which introduces additional calcium to furtller Eoul the membranes.
Moreover, chromic acid and high pH cyanide baths attack and destroy the
membranes.
In evaporation processes the effluent passes through one or more
evaporator units which concentrate it for further handling. An example of one
such evaporation process is disclosed in the United States Patent No.
3,973~987, issued August 10, 1976, to Hewitt et al. Evaporation processes have
the disadvantages that the evaporator units are relatively expensive and use
considerable amounts of energy. As energy becomes less abundant and more
expensive, it will be even more difficult to justi~y using this method than it
is now. ~oreover, if the efflueTIt contains cyanide at relatively high concen-
trations and sufficiently high hydrogen ion concentrations, carry-over o~
cyanide into the "purified" water is a problem.
Chemical destruction methods are perhaps the most com~lon, and
lend themselves to both continuous and batch type operations and can be used
on small or large volumes of ef1uent. Most toxic contaminants are reduced
to an acceptable level but some, such as cadmium, cause problems whereby the
present and anticipated pollution control standards cannot be met. Substantially
zero discharge can be accomplished or a short period of time by chemical
destruct methods by reclrculating the treated effluent. However, soluble
-- 2 --
salts build up in the treated e~-~luent, and consequently the treated effluent
can be recycled only relatively few times. At some point, it is necessary to
dump the recirculated effluent in which soluble contaminants have built up to
a high concentration.
~ iltration methods have long been used to separate contaminants
from industrial wastewater, but dissolved contaminants must first be precipit-
ated to remoye them. Use of chemical precipitants can introduce additional con-
taminants into the "purified" water. Consequently, this method is not only
limited to precipitable contaminan-ts, but can also be slow, costly, and/or
partially self-defeating.
It is accordingly one object of this invention to provide a pro-
cess and system for economically removin~ impurities from industrial waste-
water.
It is another object to provide an economically feasible proce~s
for substantially ~ero discharge of contaminants in liquid effluent.
It is another object to m;ni~;ze the energy cost of removing
impurities from industrial waste~ater.
It is another object to proYide a process and system for recycling
effluent within an industrial process $or extended periods of time.
In accordance with this i.nyention, there is provided a process
for purifying wastewater from an industrial process comprising introducing
said wastewater into a steam boiler, heating said wastewater within said
boiler to produce a steam component and a liquid component concentrated in
impurities, removing steam rom said boiler and using it for an industrial
purpose such as for heat or a source of energy to provide mechanical motion,
and removing from the boiler at least a portion of the liquid component con-
taining a high concentration of dissolYed salts and possibly precipitated solids.
~ ~3~'7~
The condensed steam is recycled to the boiler or for use in the industrial
process.
~ n the preferred method of carrying out this invention suspended
solids, and soluble and/or insoluble organics such as oil are removed from the
wastewater before it is introduced into the boiler.
The use of a steam boiler to concentrate impurities and concomitant-
ly produce steam for use as an energy source meets the objects set forth above.
Unexpectedly, industrial wastewater can be used as a feedwater to a steam
boiler safely and without harm to the boiler. This flies in the face of conven-
tional wisdom on feedwater quality requirements for a boiler, which teaches
that boiler Eeedwater should be as pure as possible to prevent corrosion and
scale formation in the boiler. Scale formation not only reduces the rate of
heat transfer and increases the amount of fuel required, but it may cause hot
spots resulting in burnout of the heat transfer surface.
Tlle process described herein is ~ery adaptable to existing equip-
mcnt in most industrial operations, being usable with existing steam boilers in
the plant. ~ery little extra energy is required by the boiler to produce steam
from the contaminated industrial wastewater as compared to the usually very pure
boiler feedwater. Thus, the process and sys-tem are relatively economical and
energy efficien-t, and adds only a few percent to the cost of energy consumed
in an industrial plant. SigniEicant economies also result from recycling
the treated wastewater to the industrial process which produces the wastewater.
Thus, in most cases, the treated water is sufficiently pure to use as all or
a part of the make-up water required for the industrial process. In addition,
valuable chemicals may be recovered from the aqueous phase containing high
concentrations of salts formed in the steam generating unit. The concentrated
aqueous phase formed in the steam generating unit which may contain precipit-
- 4 -
ates may be further concentrated by driving off nontoxic constitllents so that
there will only be a small amount of toxic constituents remaining which can be
easily and safely disposed of. ~letal salts in the form of substan-tially pure
crystals can be obtained by evaporating water from the concentrated aqueous
phase removed from the boiler.
For a better understanding of the invention, reference is made to
the following descriptiDn of a preferred embodiment thereof, taken in conjur.c-
tion with the figures oE the accompanying drawing in which:
Figure 1 is a schematic diagram of a system in accordance with this
invention and which illustrates a process also in accordance therewith;
Figure 2 is a sectional view of a fire-tube boiler usable with a
system in accordance with the present invention;
Figure 3 is a schema*ic diagram showing modifications to the system
shown in Figure 1.
In carrying out this in~ention, liquid wastewater from an
industrial process is introduced into a steam boiler to produce steam and a
liquid component enriched in impurities. The feed to the boiler may be any
industrial waste and this invention is not restricted to but is particularly
useful for those industrial wastes containing a high concentration of heavy
metal salts. The invention will accordingly be described in detail with
particular reference to metal processing, including metal surface finishing,
metal plating, pickling and similar processes wherein the wastewater contains
high concentrations of heavy metal salts.
~aterials which may be present in the wastewater include but are
not limited to salts of one or more of the elements including aluminum, cobalt,
copper, nickel, cadmium, zinc, chromium, gold, silver, antimony, lead, rhodium,
iridium, palladiul~, molybden~m, iron, tin, arsenic, ~arium, boron, calcium,
lithium~ magnesium, manganese, mercury, potassium, sodium and -titaniwm. The
anions which may be present include but are not limited to fluoride, chloride,
sulfate, nitrate, cyanate and cyanide. Carbon powder may also be present.
The concentration of impurities in the feedwater may be at any
level; however~ for most efficient operation of an industrial plant, it will
range from a level where the impurity concentration is almost too high to be
useful in the industrial process to its saturation point under ambient con-
ditions~ While a feed containing solids could be introduced into the boiler,
in the preferred operating mode solids are separated from the liquid, such as
by settling, and the clari-fied liquid is fed to the boiler. Such a clarified
liquid will normally be saturated with at least the component forming the
solids at ambient temperatures, e.g. about 10 to about 30C. Typically, the
wastewater fed to the boiler will contain from about 200 ppm by weight to
about 10,000 ppm by weight, and usually about 2,000 ppm by weight of dissolved
salts, but amounts of dissolved salts as high as 30,000 ppm and higher may be
present.
In the preferred ~ethod of carrying out this invention the feed to
the boiler is wastewater which at least in part has been sub~ected to repeated
precipitation of heavy metals with suitable precipi-tating agents. Lime and
sodium hydroxide are suitable precipitating agents and the selection of one or
the other, or even another agent will depend on the specific conditions under
which the process Is carried out. The use of lime has the advantage that the
concentration of dissolved salts will increaseata relatively slow rate in
view of the low solubility oE calcium salts, especially the carbonate. Lime
has the disadvantage that it maintains the concentration of calcium at a level
at which some scale may form in the boiler.
On the other hand, the use of sodium hydroxide as a precipitating
-- 6 --
0~
agent permits t}-e invention to be c~rried out virtually free of sca]e-forming
materials. ~lowever, in view of the high solubility of sodium compounds the
concentration of dissolved salts increases more rapidly than when lime is used.
The boiler can readily keep the concentration of the sodium salts at an accep-
table level; however, during periods in which the boiler may not be needed to
produce energy, as in the summertime when -the boiler is not used to produce
steam for heat, lime may be the chemical of choice, at least during the off-
period for the boiler.
~ any wastewaters, especially those from metal plating processes,
contain water-miscible and/or water-immiscible organic compounds and/or
materials such as acetone, rust-preventive oils, organic brighteners, such as
benzaldehyde, chlorinated solvents such as perchloroethylene and trichloro-
ethylene, and organic-metallic compowlds. I~ these organic impurities are
introduced into the boiler they steam distill and appear in the condensate,
and thus to some extent contaminate the purified water. In the preferred
method of carrying out this invention the organic impurities are removed from
the wastewater before it is introduced into the boiler. In the preferred
method of removing organic impurities from wastewater before it is in~roduced
into the boiler the wastewater i5 contacted with a water-immisicible organic
compolmd in which the organi.c impurities are soluble, thereby stripping the
organic impurities from the water. Oil is the pre-ferred organic stripping
agent since it may~already be in the sys;tem, for example as a result of its
use as a rust-preventing agent, and advantage can be taken of its water--immisci-
bility, density, and miscibility with organic impurities. Surprisingly,
organics soluble in water, such as brighteners as exemplified by benzaldehyde,
as well as the water-immiscible ones are removed by the oil. In the preferred
method of removing organic impurities wastewater containing the organic
-- 7 --
impurities is passed through a layer of oil floating on a body oE wastewater
as is discussed in detail below in the discussion of Fi~ure 3.
This method of removing organic compounds from wastewater may be
used with purification methods other than those involving use of a boiler to
separate inorganic impurities, such as reverse osmosis, and is not limited to
the use of a boiler to concentrate impurities.
In order to reduce corrosion in the boiler the feed should be ad-
justed to a pH so that the pll of the wastewater in the boiler is in the range
of 8 to lO. Suitable compounds to control pH level and to minimize scale
development are discussed in more detail below.
The boiler may be operated at any pres;sure for which the boiler
was des-igned and the operating pressure wlll usually be determined by the nature
of the steam usage. ~or ins~ance, if steam is used for heating the boiler may
operate below about 15 psig, while if steam is used to supply mechanical energy
the pressure may be as high as that for which the boiler was designed.
It has unexpectedly been discovered that the presence oE cyanide
in a low concentration within the boiler has a beneficial effect in helping
preyent corrosion. While the mechanism is not known, it is believed that the
cyanide acts as a scayenger for oxidizing agents ~Yhich may be present. Con-
sequently, in the preferred method of carrying out this process a low concen-
tration of cyanide is maintained within the boiler.
In order to keep the cyanide from distilling over with the steam~
it is necessary to keep the pH of the wastewater in the alkaline range. At a
pH of 6 all cyanide will distill over. At a higher pH the amount distilling
over will depend upon the concentration of the cyanide. ~or example, at a
cyanide concentration o~ 200 ppm by w-eight some cyanide wlll distill at a pH
of 7.5, while none will distill at a pH ~reater than 8Ø At a concentration
'f7~
o-f 2000 ppm some cyanide will distiLl ove-r at a pH of 8 while none will distill
at a pll oE 9. The cyanide concentration is preferably kept below 2000 ppm,
since in higher concentrations cyanide may attack portions of the boiler as,
for example, welds. Ihe most preferred cyanide concentration is within the
range of from about 1 ppm to about 200 ppm.
Many plating wastes contain cyanide, and it may be necessary to
treat the wastewater to prevent the cyanide from building up within the boiler
to too high a level. Cyanide may be maintained at a proper level within the
boiler by adding calcium hypochlorite to oxidize it to carbon dioxide and
nitrogen. However, it has unexpectedly been discovered that oxidation of cyan-
ide with H202 results in a significant reduction of solids being carried over
with the steam. ~or example, when cyanide was destroyed with calcium hypo-
chlorite, solids were present in the condensed steam at a level of about 40 ppm
by weight. When the oxidizing agent was H2Q2 the concentration of solids in
the condensed steam was reduced to about 4 ppm by weight. The concentration
oE solids in the condensate in a system having a steam trap was reduced even
further, to about 1 ppm by using stainless steel or CP~C pipe after the steam
trap. The use of oxidizers in a boiler containing steam is believed to the
contrary to usual boiler operating practice.
In the preferred ~ethod of carrying out this process chromate is
present in a concentration of at least about S ppm by weight and preferably in
a concentration of at least about 10 ppm. Concentrations of about 10 ppm to
about 2000 ppm are most typical, but the concentration may range to 5000 ppm
or higher. Chromate helps reduce corrosion o:E metal within the boiler, and it
may be added if it is not already present in the wastewater fed to the boiler.
It has unexpectedly been folmd that for use in treating wastewater
from a ~etal plating process where the heavy metals comprise cadmium, copper,
_ ~ _
nickel, tin, zinc, chromium and iron, the amount of scale -formed ~Jithin the
boiler is negligible and dissolved solids can accumulate to a level greater
than about 40 percent by weight at the temperatures within the boiler. The
absence of a significant amount of materials having an inverted solubility
curve, such as calcium and magnesium sal~s, may explain the low incidence of
scale information. In the preferred method of carrying out this invention the
concentration of ~a is maintained at a level below about 200 ppm by weight,
more preferably below about 100 ppm by weight, and in the most preferred me~hod
CA is maintained ~elow about 10 ppm. The total concentration of material
having an inverted solu~ility curve is preferably kept below about 300 ppm and
most preferably ~elow about 20 ppm by weight.
The concentration of metal salts in the boiler, dissolved plus
precipitated, may suitably range from a level slightly higher than that of the
wastewater feed to a level of about 55 percent by weight. Inasmuch as zero
liquid effluent from the industrial plant is the ultima-te goal, and a necessary
adjunct to that goal is the productlon of the impurities in the form of solids,
in the preferred method of carrying out this invention the salts are concen-
trated in the steam boiler to as high a level as possible consonant with safe
and efficient operation of the boiler. The preferred concentration of solids,
dissolved and precipitated, is in the range of about 5 percent to about 30
percent by weight and the most preferred concentration is from about 10 percent
to about 20 percent ~y weight. Tae concentrated solids are removed as by
blowdown when the dcsired concentration is reached, and the re~oval may be
either batch or continuous~.
In this process of treating wastewater and recycling the substan-
tially pure water produced in accordance with this invention, the industrial
process requires little or no make-up water from other sources. This feature
~ lQ -
has the advantages -~hat not only is the cost of make-up water drastically
reduced, but calci~lm does not acc~nulate in the system as it would if continu-
ously added in make-up water. In order to keep calcium and magnesium concen-
trations low, make-up water is pre~erably softened or it may consist of rain-
water which has been captured and added.
The invention may be embodied in various forms, and the embodiments
shown in the drawings are only provided to illustrate the invention.
Referring to Figure 1 of the drawing, an industrial process which
produces contaminated wastewater ef~luent is generally designated as 10. The
industrial process may include just about any industrial process which pro-
duces contaminated wastewater effluent, but the process according to the present
invention will be described with particular reference to metal processing,
including metal surface finishing, metal plating, pickling and the like. It is
to be understood that the present invention has application to a wide range of
other industrial processes providing an e$~1uent with a relatively high concen-
tration o impurities and the reference to metal processing is merely by way
o example.
The wastewater eE1uent from the lndustrial process 10 preferably
eikher does not contain extremely large amounts of corrosive chemicals or it
contains corrosion-resisting chemicals. The ~aste~ater from a chrome plating
process, wherein the effluent contains chromium ions which aid in protecting
boiler tubes Erom corrosion, is an example o:E e~1uent containing corrosion-
protecting chemicals.
The wastewater eEfluent from process 10 is conducted by means of
conduit 12 through ~al~e 13 directly into a steam boiler 1~. Steam boiler 1
may be of any conventional construction and may include, for example, fire-
tube, water-tube or package-type boilers. In steam hoiler 1~, the ~astewater
-- 11 ~
e:E:Eluent is heated to produce a steam component thereby concentrating the
impurities in the boiler in the aqueous phase. Although the impurities are
concentrated to a level exceeding their solubility at ambient temperature, they
may either remain in solution at the temperature within the boiler, or they
may precipitate~
Standard boiler compounds may be introduced into the wa.stewater
effluent before it enters boiler 1~ to inhibit or minimize the build-up of
scale and to reduce corrosion in the boiler. Alternatively, the boiler com-
pounds can be added to the boiler. ~here the boiler compounds are added direct-
ly to boiler 1~, they are i.ntroduced through entry duct 16 by means of pump 18
since these boilers are pressurized vessels. In using these compounds i~ is
desirable to adjust the pH of the wastewater within the boiler to be within a
range of about 8 to la.
Suitable boiler compounds are well known to those skilled in the
art. The choice and appropriate a~ount oE a proper boiler compound or compounds
may easily be determined by mere routine experimentation, taking into considera-
tion the type of wastewater ef1uent. Suitable boiler compounds include,
for example, sodium phosphate, soda ash, am~nonia, volatile amines, such as
morpholine ,md cyclohexylamine, chelating agents, such as EDTA, and polyacryl-
amides of the type made according to United States Patent 3,~63,730, issued
August 26, 196.~, to Booth et al.
~hile the heat transer sur$aces within the boiler are not harmed
by the wastewater, it has unexpectedly been found. that the conventional level
controls made of brass may corrode and it is preferred to use level controls
made of stainless steel.
The concentrated i~puritie$, which may~contain p~ecipitates, will
accumulate in the boiler 1~ and may ~or.m a slud~e which can be removed by
- 12 ~
blowdown through condu;t 20 and conventional blowdown valve 21. A combination
of sluclge and seale may accumulate and can be removed by blowdown valve 20
and~or scraper devices.
~ rom boiler 1~, the steam component is conducted through conduit
22 as working steam used for any industrial purpose as indicated at 24, such as
heating a plant or heat exchanger or for driving turbines. As the steam is
used for the industrial purpose lt condenses forming relatively pure water
which is conveyed through the conduit 26 to a condensate return tank 28.
The condensed water can be selectively pumped from condensate
return tank 28 directly to boiler 14 by pump 35 through conduit 31 and valve 33
when there is insufficient untreated or pretreated wastewater effluent entering
the boiler. Preferably, valve 33 is controlled by a standard water level
sensing means in the water tank.
Alternatively, the water may be conducted from condensate return
tank 28 through conduit 30 to a storage tank 32. The water from storage tank
32 is conducted through conduit 34, valve 36, pump 38 and check valve 43 bac~
to the original industrial process to be used therein. Where pump 38 is
necessary, an accumulator device 45 should be used to compensate for any surge
in line pressure resulting from the starting of the pump and otherwise help
to maintain uniformity of pressure. The accumulator device may be any standard
device incorporating a piston, diaphragm, or bello~s. Pump 38 may be unneces-
sary where gravity feed may transfer ~ater from storage tank 32 to industrial
process la. Another variation would be to let the condensate go directly to
the industrial process 10 or to the process from the condensate return tank 28.
~any industrial processes yield ~:astewater effluents containing
insoluble materials. In this case, a process according to the present in-
vention can include some pretreatment o$ the ~aste~ater effluent. One such
~ 13 -
process is described with reference again to Figure 1 of the drawing.
Wastewater effluent containing dissolved ions and solids produced
by industrial process 10 is conducted through conduits ~0, 42 and 44 and valves
46 and 48 into settllng tanks 50 and 52. Of course, depending upon the system,
any number of settling tanks may be used.
For the purpose of illustration, assume that the wastewater effluent
contains 1,000 ppm suspended and dissolved solids. Suitable flocculents or
precipitating agents, such as lime, are added to the wastewater effluent in
settling tanks 50 and 52. Lime is useful as an agent to remove calcium or
magnesium present as bicarbonates forming insoluble carbonates as illustrated
by the equation:
Ca(011)2 ~ Ca(llC03~2 -~ 2CaC03 ~ ~ 2~12Q
~lowever, the concentration o~ calcium is preferably minimized as by using an-
other agent such as sodium hydro~ide instead of lime.
The tanks 50 and 52 are preferably used alternately, that is, one
tank is filled then the other, so that the process is a ~atch type process. A
continuous system can also be used if desired. A~ter a period of time, the
effluent separates into two components, a relatively clear component 54 and 56
containing only dissolved solids, such as sodium and potassium chlorides,
nitrates, sulfates, etc. in a concentration o~ about l,OQ0 ppm, and a sludge
or precipitated component 58 and 60, having a concentration of solids of about
2-5%.
In many instances, the wa$tewater e~fluent may be recycled back to
the industrial process for use therein after the suspended solids are removed.
Thus, the component ~ and 56 containing the dissolved solids is removed through
cond~its 62 and 6~ and through valYes 66 and 68 from settling tanks 50 and
52, respectively. Conduits 62 and 64 are connected to tanks 50 and 52,
-- 1~ -
respectively, at a point above the allticipated level of sludge 58 and 60 so
that only components 5~ and 56 are removed. The component containing the dis-
solved solids is then conveyed through conduit 70 to storage ta-nk 72.
The le~el of li.quid in tank 72 can be raised and the concentration
of dissolved solids therein diluted by adding water fr~m condensate return
tank 28. Water is selectively conveyed from tank 28 to tank 72 through con-
duit 37, valve 3~ and pump 41. The pump and valve can be controlled by level
sensing devices and concentration sensing devices known to those ski.lled in the
ar~. The liquid containing the dissolved solids in tank 72 is recycled through
conduit 74, valve 78, pump 82, check Yalve 87 and conduit 86 back to industrial
process 10. Of course, where gravity feed is available, pump 82 is ~mnecessary.
Where the pump is used, accumulator device 89 is also used for maintaining
uniformity of pressure.
The recycling o~ the ~astewater effluent component containing only
dissolved salts aids in greatly reducing the amount of water necessar~ from
primary sources, such as the municipal water system, thus conserving water, a
valuable natural resource. In addition, many of the dissolved chemicals con-
tained in the component containing the dissolved salts are heneficial for the
industrial purpose. Thus, there may be a two-way cost savings. Typically,
the component containing the dissolved salts may be recyled for a long period
of time, such as, for example, one year. ~he recycled component will eventual-
ly contain too large a concentration of dissolved salts to be useful in *he
industrial proc0ss. ~t tha-t time, it is introduced into boiler 14 through
conduit 76, valve 80, pump 84 and conduit 8%. Boiler compounds are not
necessa.ry, but in the pref0rred method of carrying out this invention they
are added to the pretreated, recycled component be~ore it is introduced into
the boiler. The boiler produces steam for an industrial use during which the
- 15 -
'7~
steam is condensed and the resulting water is recycled to industrial process
10 and/or boiler 1~ as set forth hereinabove.
Preferably a portion of the component containing the dissolved salts
in tank 72 is continuously rec~cled to the industrial process while a smaller
portion is being conveyed continuously to the boiler. In this manner, the
industrial process receiyes a recycled component containing dissolved salts
and a substantially pure component ~hich has gone through the steam and conden-
sation cycle as set ~orth hereinbe~ore. Processes that require high quality
water can receive condensate continuously and this method can eliminate ion
exchange Ullits.
Sludge 58 and 60 in tanks 50 and 52 may be pumped through condui-ts
90, 92, 9~ and through valves 94 and 96 by a pump 98 to a concentrator tank
100. The sludge 58 and 60 from settling tanks 50 and 52 may typically have a
concentration of about 2-5% solids. The sludge is trans~erred to concentrator
tank 100 and a~ter standing oyernight produces a relatively clear component 102
containing dissolved salts and a concentrated sludge component 104 which may
over a period of time build to 15% solids content. Component 102 is recycled
to tank 50 through conduit 108 and valve lQ~ ~or recycling to industrial process
10 and~or to be con~eyed -to boiler 14 as described hereinbe~ore. ~h~TI concen-
2Q trated sludge 10~ becomes too concentrated or builds up to a predetermined level
in tank 100, it is~ discharged -through concluit 106 and valve 107.
Concentrated sludge la4 and any sludge or scale ~ormed in boiler
14 may be concentrated ~urther by any suitable process. The more concentrated
sludge and scale is reduced to a very small volume and may be readily dis-
carded, or recycled to metal processors.
The energy in the boiler stack gases may be used to concentrate
sludge by heat exchange between the hot gases which are the combustion products
- 16 -
o-f the boiler fuel and the sludge, and a process wherein the sludgo fr~n the
boiler is introduced into the boiler stack and water is removed from the sludge
by evaporation is highly energy efficient. The water content can thus readily
be reduced to less than about 2 percent by weight.
In practicing this invention it has been found that the nature of
the industrial wastewater Eed to -the boiler 14 may cause a foaming problem in
the boiler and the liquid would then haye a tendency to surge up and down there-
in. As a result, the hoiler 14 may shut down and/or discharge the wastewater
instead of steam from the boiler. It is thus an important aspect of this
invention to provide headroom when necessary at the top of the boiler to
accommodate the foam and to relieye the prohlem of discharging water ins-cead of
steam. For most boilers it is belieyed that there should be provided at least
about one -foot of space between the top of the wastewater and the top of the
boiler 14. I~ necessary a conventional ~loat switch can b& provided at the
desired water level in the boiler 1~ and can operate a valve at the inlet to the
boiler to prevent over-loading the boiler with wastewater.
~ith reference to I~igure 2, boiler 10a represents a standald fire-
tube boi]er ~or use with this inyention. In this boiler heat travels from hot
combustion gases within the tubes through the tube walls to water within the
boiler's water tank. The direction of temperature drop across the tube wall
i5 from the combustion gases to the ~astewater. The transfer of heat is
represented by ~he equation
Q - ~IA ~T
where
Q is the amount of heat ~ransferred per unit time,
A i5 the area of the surface through ~hich th& heat is transferred,
is the oyerall heat trans$er coefficient, and
- 17 -
AT is the difference in -temperature between the fluid being
heated and the hot combustion gases.
The QT for steam boilers is high relative to that of evaporators
where the heat required for eyaporation usuaily is supplied by condensing
steam~ and consequently the heat transfer area and thus the size of the boiler
can be much smaller than that of an evaporator having an equivalent capacity
for converting water to steam. An additional disadvantage of an evaporator
which increases the cost of a syste~ using an evaporator is that it needs a
source of energy, which in most cases is a steam boiler.
Roiler lOa includes outer side walls 12a and l~a, outer bottom
wall 16a and outer top wall 18a which may be integral or contiguous with water
tank top wall 20a. In addition to top wall 20a, tank 12a comprises bottom
wall 22a and side ~alls 24a and 26a. No novelty is claimed in the precise
construction of the boiler or the water -tank. The drawing is merely representa-
tive of standard fire-tube boilers in which the present invention is operable.
~ater is pumped into tank 12a through conduit 12 and valve 1~ which
may be control]ed by a standard water level detector associated with tank 12a.
Conduit 12 is also provided with check valve lla. ~hen the boiler is ~Jsed to
purify industrial waste~ater, conduit 12 is connected to a source of industrial
wastewater such as industrial process 10 shown in Pigure 1. The wastewater
may be conveyed directly to boiler lOa or may be pretreated in accordance with
the process previously described or any other desired process. The water is
introduced into the tank to a level 27a just above the uppermost row of ~oiler
tubes 50a so as to allow space in the tank for steam 29a. Steam produced by
the boiler exits through conduit 22 and its flow is controlled by any conven-
tional valve, not shown. The boiler may include any conven~ional blowdown
valve and conduit, not shown~ and any conventional valved inlet port, not shown,
= 18 -
for the addition o~ standard boiler compounds to minimize scalc bwild-up and
corrosion.
Burner 28a may he any suitable, conventional burner of the type
used in boilers, s~ch as a gas burner, oil burner, coal burner or a combination
thereof. Heat from burner 28a travels through chamber 30a between ~he outer
boiler walls and the water tank walls. The heat is then routed by baffle 32a
through fire tubes 34a, 36a and 38a into a chamber 42a. Chamber ~2a is defined
by boiler outer wall ]4a, tank wall 26a and baffles ~Oa and ~a. ~rom there,
the heat progresses through fire -tu~es ~6a, 48a and 50a into chamber 54a
bounded by boiler outer wall 12a, tank wall 2~a and curved baffle 52a. In its
path through the boiler, hot gases transer their heat through the fire tubes
to the water and then are e~hausted through -flue 56a.
Attached to tank l~a is scale receptacle 58a for receiving any
scale scraped fro~ tank 19a. Suitable conventional gasket material or sealing
means may be used to prevent water fro~ leaking out of tank 19a or scale
receptacle 58a.
~ccording to a prefexred embodiment which is particularly helpful
when the wastewater contains water-immiscible organic liquid contaminants,
especially those having a higher specific gravity than the wastewater, ~ith or
without undissolved metal salts, the wastewater is vigorously agitated, prior
to passage of the waste~ater through the oil layer, such as by injecting bubbles
of a gas ~including gas mixtures~, for example oxygen, nitrogen, carbon dioxide
and preferably air. Por example the liquid may be agitated in a first body of
liquid and be passed through the oil layer in a second, separate body of
liquid. ~he operations of agitating liquid in the first body of liquid and
transEerring liquid from the first to the second body of liquid may each occur
continuously or nonconti.nuously, and during certain periods these operations
~ 1~) ~
may occur simultaneously~ alternately or in other time relationships. Pre-
ferably the liquid which is passed ~hrough the oil layer is withdrawn from an
upper portion or the surface of the first body of liquid in a first vessel and
is transferred from that vessel to a second vessel which contains the second
body of liquid and in which the oil layer constitutes at least the upper
portion of the second body of liquid. ~oreover, it i.s desirable that the first
body of liquid exhibit a gradient with respect to the mass of precipitated
metal salts per unit volume of liquid which is positive ~i~h increased depth
of liquid. Thus, there are more suspended and~or settled metal salts so~ids
in a lower portion of the first body or vessel as compared to its upper portion.
The desired gradient may be produced in any convenient way, such as for ex-
ample by providing less vigorous agitation in the upper portion of the first
body or vessel and~or by discontinuing the agitation operation during at least
a portion o:E ~he time when the transferring operation is being conducted.
According to a particularly preerred embodiment the liquid is transferred only
after a period of reduced or no agitation su$$icient to cause appreciable or
substantial settling o suspended solids, and this is preferably but not
necessarily combined with withdra~ing liquid from onlr the surface of the
first body.
~0 ~egardless of the point o withdrawal and sequence of agitating a.nd
transferring, the transferred liquid is caus-ed to flow into and through at least
a portion of the thickness of the oil layer ~hile su~ficiently restricting
agitation of the oil layer or ~aintaining it substantiallr intact. Accll-îng
the oil layer has a lowex speciic gravity than any other liquid which may be
present in the second body, which is usuall~ the case, organics from the firsl
body of liquid ~e~en thcse which may~be hqaYier than water~ beco~e dispersed or
disso]ved in the oil layer o-~ the second body of liquid, while the aqueous
_ 2Q ^
portion of the first body Eorms or passes into a lower aqueous layer in the
second body. One convenient and preferred -techni~ue for effecting transfer is
to withdraw liquid from the last-mentioned aqueous layer and to propel such
withdrawn liquid into contact with an upper portion or the surface of the first
body of liquid in the direction of a dam or weir over which the li~uid at the
surface of the first body is thus caused to flow. The li~uid overflow may
then be passed downwaTdly, pre$erably along a downward-directed surface upon
which it flows, to the oil layer.
A system of recycling wastewater from a metal plating process
showing portions of the system of ~igure 1 with modifications is given in
Figure 3. In this system, the wastewater from the industrial process is added
through line 40 and yalves 46 or 48 to either tank 5Q or tank 52 where it is
treated as by adding a precipitating agent. The resulting mixture is agitated
by air introduced rom an air source (not shownl through line 124 or line 125.
A~ in ~igure 1~ tanks 50 and 52 are used alternately, i.e , when
one tank is full and the wastewater therein is ready to be treated the other
tank is empty and ready to rece~ve wastewater from the industrial process. Oil
and other organic compounds often found in wastewater, such as chlorinated
solvents and brighteners, are removed Erom the wastewater hy passing che waste-
water through oil layer 130 within recirculation tank 136. This is accomplished
by adding enough liquid tv tanks 50 or 52 either from line 40 or from recircula-
tion tank 136 through lines 128 or 12~ to cause wastewater to over10w into
recirculation tank 136.
The oil layer 130 is effective in re~oving from ~astewater organic
compounds soluble in the oil including water-soluble organics such as brighteners
as well as wat~r-immiscible compounds, and is effective in remoying emulsiied
particles which ~ould be difficult to separate from water by differences in
- 21 -
L'~
specific gravity.
I~ an oil layer does not form within one or two cycles after start-
ing the process, enough oil sllould be added to form layer 130 from about 1/4
inch to about 3 inches thick. The oil layer is pre~erably maintained at a
thickness of from about 1 to about 2 inches. While thicknesses greater than
these may be used, there appears to be no advantage to thicker layers. The
te~l "oil" refers to lighter petroleum fractions ordinarily used for rust
preventive purposes or for lubrication such as oils designated as S~E No. 30.
By use of the air agitation system the water-immiscible organics
which are heavier than water, such as chlorinated solvents as exemplified by
perchloroethylene and trichloroethylene, are prevented from accumulating in
the sludge by being dispersed throughout tanks 5Q and 52 and thus overflow into
recirculation tank 136. The sludge for recycle to metal processors is t~lUs
relatively free of organic compounds. In using the air agitation system to
disperse the hea~y organics throughout tanks 5Q and 52 the agitation is prefer-
ably intermittent to permit solids to setlle while wastewater overflows into
tank 136.
Operation of the alr agitation system and recirculation pl~p keeps
the concentration of total or~anic carbon in the condensed steam at a relative-
ly low level. If hea~y organic5, such ax the chlorinated solvents, are not
present, recirculatiQn alone will keep the concentration of total organic carbon
in the steam condensate at a negligibly~l~w leyel.
The pre5ence o~ a heaYy organic impurity, such a~ trichloroethylene
and perchloroeth~lene, in the wastewater may re~uire controlling the composition
of oil layer 130 to maintain a density less than that of the wastewater. The
density may be reduced, if necessary, by adding additional oil to the layer,
either with or without a step of removing a portion of the material from layer
- 22 -
130. The specific gravity of the oil layer is preferably maintained below
about 0.~.
The clearJ oil-free water 135 from the recirculation tank 136 is
moved by pump 126 or 127 through conduits 128 or 129 into tank 50 or 52 through
spray heads (not shown~. This recirculating water serves to provide water to
tank 50 or 52 to float the oil and other organics into the recirculation tank,
or to flush sludge 58 or oO from tanks 50 or 52 when they are being emptied.
Clear liquid from tanks 50 to 52 is pumped into s~orage tank 72 through line
70.
The treated wastewater from tank 72 may be recycled as is shown in
Figure l.
The process according to the present invention provides for sub-
stantially zero contaminated wastewater efflu~nt discharge. The wastewater
effluent is treated in accordance with -the present invention and need not ever
leave the system. The only contaminants which leave the system are in the form
of oil ~organics), highly concentrated sludge and/or scale which are easier to
dispose of than large amounts of dilute liquid efEluents, certain of which can
be dried thus further concentrating them and placing them in a form 5Ui cable
for processing by metal manuEacturers.
The process will now be illustrated using the following specific,
nonlimiting examples:
Example l
Approximately 500 gallons o-f recycled wastewater efluent were
obtained from the wash and rinse baths of an electroplating process. The
recycled wastewater effluent, which had been used in the baths for about one
year, contained heavy metals, such as cadmium~ copper, nickel, tin, zinc and
iron. In addition, it contained cyanide, hexavalent chrome, oil, alka~ine
- 23 -
cleaner and various acids.
The cyanide was destroyed by normal chlorination. The hexavalent
chrome was partially reduced by a hydrosulfite and the oil removed continuously
with an oil separator. The heavy metals were precipitated with excess lime
and polyamine flocculents. After this treatment, a sludge component and a clear
component remained. The pH of the clear component was adjusted to approximate-
ly 8 and it was pumped to a reservoir -for use in the electroplating process
as needed. The clear component was recycled once or twice each week and after
about a year the water became unusable due to a build-up of dissolved solids
and interference with the plating operation. The dissolved solids, in a
concentration of about 8,500 mg/l, seemed to consist mostly of sodium sulfate,
sodium chloride and sodium nitrate. Other cations, such as potassium, calcium,
magnesium and ammonia were present, but no e~forts were made to determine exact
amounts. Organic ma-terials, such as wetting agents, were also present.
The recycled component containing the large concentration of dis-
solved solids was then introduced into a small laboratory boiler for testing
to see if the boiler would separate the contaminants from the steam and not
damage the boiler.
The steam produced by the boiler at about 15 p.s.i.g. was condensed
and the water condensate was relatively clean. It contained some ammonia and
iron and had a p~l of 8.8. The sludge produced in the boiler was soft and oo~ed
out of a control valve ~corresponding to a typical blowdown valve) and the
experiment progressed. The boiler contained an average of about 4 gallons of
the component containing the high concentration of dissolved solids as the 500
gallons of wastewater were passed through the boiler. When the boiler was dis-
assembled, some hard scale was found and removed.
~xample 2
A 55 gallon drum of chrome waste was obtained from another plant
that processed copper and copper alloys. The chrome was re~uced to the tri-
valen-t state and the sludge represented ahout 50% of the solution by volume.
The p}l o the solution was adjusted to 8, the solution was agitated and allowed
to stand for about 20 minutes. The sludge was still about 50% by volume and
remained so af-ter leaving the sludge stand overnight. Solids by weight of the
sludge were about 5~.
The waste was then introduced into the laboratory boiler. There
was concern that the sludge would be difficult to concentrate in the boiler
because of its voluminous nature. However, this did not prove to be the case.
The greensludge did not show up in the boiler sightglass or enter the steam
port Difficulties, however, were encountered in other aspects. Even though
the pH was adjusted and maintained at about 8~ the steam had a pH of 2.4 and
badly corroded the boiler steam lines. Excess sodium sulfite was present,
yielding corrosive sulfur dioxide and sulfurous acid. Hexavalent chrome was
added to remove the excess sodium sulfite and the experiment repeated. When
approximately 5 mg~l of hexayalent chrome was maintained in the boiler, no
further sulfur dioxide was carried over with the steam and the pH of the conden-
sate was about 8. There appeared to be no further corrosion of the boiler
system.
Example ~
~nother experiment was carried out with the same chrome wastewater
as used in Example 2. The conditions in the boiler were 5-10 ppm of hexavalant
chrome and the pH ranged rom about 8 to about 10. Morpholine was added to the
boiler to adjust the pH o the steam so that as the steam entered the conden-
sate tcmk, the -~H wa$ between 7.5 and 8.5. The steam pressure was about 15
p.s.i.g.
:~L~ t~
Tile chrome sludge did not interfere with the normal boiler con-
ditions. The condensate showed the presence of morpholine and a pH of about
8. No noticeable corrosion could be detected in the boiler or in the steam
lines. The concentrated trivalent chrome was removed from the boiler through
the control valve a-t about 60 percent dissolved salts and solids. No hard
scale formed on the inside of the boiler.
A portion of the impurities removed from the boiler were further
concentrated by placing the sludge on a cloth which was placed on a steam table.
More water was driven off and the solids were concentrated to about 97 percent
by weight. The sludge was dark green in color and hard. It was crumbly and
easily separated from the cloth.
Another portion of the 6~ percent solids removed from the boiler
was placed in the exhaust stack for the boiler, ~here water was driven off and
the solids concentrated to about ~8 percent.
Example 4
Sludge from the boiler ~ormed as in Example 3 and consisting of
60 percent solids were pumped from the boiler to a stainless steel conveyor
designed to carry the sludge into the exhaust stack of the boiler. The exhaust
gases, which were at a temperature rom about 350F ~o 45~F, further concen-
trated the solids.
Example 5
The following example was conducted in the laboratory as part of
an economic study. l`t is be]ieved that the economlc savings set forth below
would be achieved.
Conditions at another plant were observed and samples of waste-
water effluent taken. Particular emphasis was placed on the economics of this
plant which illustrated the energy savings associated according to the present
- 26 -
~ ~ ~r~d ~
invention. ApprQximately 750,000 pounds of steam were generated daily in the
winter for heating and proccssing in -the plant. In the summcr, about 200,000
pounds of steam were used daily. The water discharge varied between 100,000
and 140,000 gallons per day throughout the year. About 75 percent of the water
was used in the electroplating department. Even though the water was cleaned
by chemical treatment before being discharged to a stream, it was not considered
clean enough for recycling to the plating department. Samples o~ the chemical-
ly treated wastewater effluent were run through the laboratory boiler and the
water condensed ~rom the steam produced by the boiler provided to be of high
quality and satisfactory for the plating operation.
If the wastewater effluent from the plating operation only were
run through a separate evaporator, the additional cost for energy would be in
excess of $2,000 per day, which would double the budget cost. However, if the
wastewater effluent were introduced into the existing plant boiler, the energy
costs would be only slightly increased. For illustration, Gn a cold ~inter
day, 765,000 pounds of steam were generated and 583,440 pounds (78,000 gallons)
of steam were used in the plating area. By passing all of the wastewater
through the existing boiler in accordance with the present invention, there
would be more than enough water for daily usage. A further advantage would
be that the condensate would be warm (75-100F), which would facilitate rinsing
in the plating operation.
~ince the steam had to be generated anyway, the only additional
costs would be hea-ting the condensate return water more than previously done. It
was estimated that 10 percent additional energy would be required for this
purpose, but this would be offset by less blowdown, so that the net energy loss
would be only 4 to 5 percent.
In the summer, not enough steam would be generated to process the
~ 27 ~
q~
water according -to the present inyention ~ach cycle. A determination would
have to be made as to W~iC}I process in the plant was most critical and would
require the pure high quality condensate produced in accordance with the pre-
sent invention. The rest of the operation would use recycled water from normal
chemical destruc-t methods. The net reslllt would be a substantially closed
loop system and substantially no water would ever leave the system in liquid
form containing pollutants, except in the concentrated sludge from the boiler.
Example 6
Seyeral 55 gallon drums of water effluent were collected from a
plant before waste treatment procedures were carried out on the waste effluent.
The cost of chemicals at this plant was very high for reducing hexavalent
chromium and precipitating heavy metals. The only pretreatment before passing
the waste through the boiler was to adjust the pH to 9 and add polyamines to
prevent the scale formed from sticking to the boiler plate. Hexavalent
chromium was maintained in the boiler. The water which was condensed from the
steam produced by the boiler was of excellent quality, bu-t the scale did adhere
somewhat and mechanical scraping was necessary.
E~ample 7
Trichloroethylene was introduced into the recycled water in the
system of Figure 3 to determine whether it would appear in the condensate if
it were introduced into the boiler and to determine whether it would be removed
by passing the recycled water through an oil layer. Trichloroethylene was
selected since it has been found in ground waters and is considered to be a
carcinogen.
~hen trichloroethylene wa$ a component of the wastewater introduced
into the boiler, the condensed steam ~as blue in color. The reason for the
color is not known~ but it made for easy visual detection of the presence of
- 28 -
trichloroe-thylene. The air agitation system and the recirculation pumps were
then turned on ~or about 30 n~inutes eorcing the water over the overflow through
a 3 inch thick layer of oil. 'I'he wastewater was permitted to settle overnight
and clear water was drawn lnto the boiler. The condensate was clear indicating
the absence of significant amounts of trichloroethylene.
Example 8
The experiment of E~ample 7 was repeated without air agitation.
The condensed steam from the boiler was blue, indicating the presence of tri-
chloroethylene in the condensate.
~ample 9
Drawing oils and organic plating brighteners, but without any
chlorinated solvents, were introduced into the recycled water introduced into
the boiler. No blue color appeared in the condensate; however, analysis showed
a significant concentration of total organic carbon in the condensate.
The present invention may be embodied in other specific forms
without departing rom the spirit or essential attributes thereof. For example,
the industrial wastes may be from sources other than metal processes, as for
example chemical processes~ biological processes, mining industries, or
pharmaceutical industries. The pressure at which the boiler is operated is
determined by its capability and the use to which the steam is put. Pressures
as high as 150 p.s.i. may be desirable for processing waste Erom the pharma-
ceutical industry or for biological wastes to ensure destruction o~ all viruses
and thermophiles. ~ccordingly, reference should be made to the appended claims,
rather than to the foregoing specification as indicating the scope of the
invention.
- 2~ -
