Spray painting plant containing apparatus for purifying contaminated air

In a plant for purifying contaminated air, the contaminants are transferred from the air to a liquid flowing through a spray booth through which the contaminated air also flows in intimate contact with the liquid. The liquid is then collected in a container, from which it is recirculated to the spray booth. The closed liquid circulation circuit in flow communication with the spray booth is connected to a biological cleaning step in which the contaminants dissolved and/or suspended in the liquid are decomposed and then separated. The biological cleaning step comprises a bioreactor in which the main portion of the bio-degradation occurs, and a separator, for separation of the biosludge coming from the reactor. At least part of the contaminated liquid passing through the spray booth is continuously transferred to the biological cleaning stage from which substantially the same quantity of the cleaned liquid is recirculated to the spray booth circulation circuit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a plant for purifying contaminated air in 
which the impurities are transferred from the air to a liquid in a spray 
booth through which the contaminated air is caused to pass in intimate 
contact with the liquid which also flows through the same booth, the 
liquid being collected in a container from which it is recirculated to the 
spray booth. 
2. Description of the Prior Art 
The utilization of a spray booth is well known, for instance, in spray 
painting, the object to be painted is placed so that the excess paint will 
accompany an air stream through the booth. In the booth, the excess paint 
is brought into intimate contact with a liquid, wherein pigment and 
solvent contained in the paint are absorbed by said liquid, thus purifying 
the remaining air. 
The liquid flowing through the spray booth is usually water which 
circulates in a closed system. Since the hydrocarbon-based solvents 
present in paints have limited solubilities in water, a saturation limit 
will very soon be reached. As a result thereof, the hydrocarbon 
concentration in the air which has passed through the spray booth will be 
unacceptably high. It has long been recognized that the exposure of people 
to this atmosphere in a room where spray painting is in progress may 
represent a health hazard. 
In the United States for instance, hydrocarbon emission regulations have 
been imposed by federal, state and local governments. Inter alia, these 
emission regulations require a hydrocarbon emission reduction of 
approximately 80% in painting with specific laquer topcoat paints. This 
goal may be reached by combining different measures to obtain the desired 
effect. Such measures include the use of other less harmful paints and the 
introduction of changes in the painting process. The changes include 
utilizing electrostatic painting and adding hydrocarbon emission abatement 
systems after the painting station. 
Large and expensive abatement systems have been installed to purify the 
solvent-contaminated air coming out from the spray booth. These systems 
work by passing the air through a purifying plant containing activated 
carbon or some kind of scrubber in which the solvents from the spray booth 
are washed out. 
According to another type of air purification, the solvent-contaminated air 
is passed through an incineration oven or some other kind of heating 
arrangement where the poisonous airborne hydrocarbon compounds are burnt. 
In the prior art there are examples of plants for purifying air, such as 
the one described in the German Patent DE-B No. 25 47 675. The plant in 
this patent employs a scrubber where the circulating scrubbing liquid is 
exposed to a biological purification stage. However, using such a plant 
for purifying the exhaust air coming out from a scrubber, would be 
extremely costly, since the plant must be dimensioned for the greater 
quantities of air which flow through a spray booth. In practice therefore, 
the problem has not been solved in this way due to the high costs 
involved. 
Consequently, it has hitherto not been possible to solve both the 
economical and the practical problems which are associated with the 
emission of hydrocarbons contained in spray booth air. Moreover, those 
systems which have presented practical solutions to the problem, have not 
met the minimum emission regulations which the authorities have imposed 
and which already need be met by the middle of this decade. 
Accordingly, there still remains the need for a system to purify 
contaminated air flowing in intimate contact with a liquid, which system 
meets minimum emission regulations while being practical and economically 
feasible. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a plant 
which presents a practical solution to purifying spray booth air and meets 
the demands for a good working environment as well as being economical 
enough to be useful in the manufacture of painted products at a price 
which is commercially viable. 
This object is substantially realized according to the present invention by 
providing a biological purification stage in which dissolved and/or 
suspended contaminations in a liquid may be biodegraded or broken down and 
are then separated. The biological purification stage comprises, at least 
one bioreactor in which the main part of the biological bio-degradation 
takes place, and a separator means for the separation of the biosludge 
coming out of the bioreactor. 
According to the invention, a spray booth included in a paint spraying 
plant is, in addition, utilized for the collection of paint particles and 
for the absorption of solvents which are effected by continuous cleaning 
of the circulating liquid. As a result, the liquid is kept under its 
saturation limit for these solvents all the time. 
Having generally described the invention, a more complete understanding can 
be obtained by reference to the drawings which are provided herein for 
purposes of illustration only, and are not intended to be limited unless 
otherwise specified.

Other objects, advantages and features of the present invention will become 
apparent to those skilled in the art from the following discussion. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In a suitable embodiment of a plant according to the present invention at 
least a portion of the contaminated liquid from the spray booth is 
channeled via the biological purification stage, from which substantially 
the same portion of cleaned liquid is returned to the closed circulation 
of the liquid through the spray booth. 
In another embodiment of the present invention, the container and the 
separator may form one unit which is included in both the spray booth 
liquid circulation circuit and the biological purification stage. 
A further embodiment of the plant according to the invention is intended 
for use in spray painting booths, where the biological purification stage 
is intended for cleaning water in which different substances such as 
solvents, binding agents and colouring pigments are dissolved or 
suspended. 
The plant circuit according to FIG. 1 includes a spray booth (1) intended 
for use in spray painting objects in a way which is described more in 
detail in connection with FIG. 2. Fans (not shown) or other air 
pressurizing means generate an air stream through the spray booth, causing 
the air (2) contaminated with paint particles to enter the upper part (3) 
of the spray booth. It is here that the air is brought into intimate 
contact with a liquid (4) which also flows through the spray booth. The 
liquid is preferably water. The thus contaminated water is then collected 
in a container (5), which is arranged in the lower portion of the spray 
booth, and the water in the container is then recirculated to the upper 
part (3) of the spray booth via a conduit by a pressure generating means, 
such as pump (6). 
In accordance with the invention, part of the contaminated water 
circulating through the spray booth is transferred to a biological 
purification stage. The water is thus, first taken via a conduit (7) to a 
stabilization tank (8) where nutrients are added. As is known in the art, 
these nutrients may comprise nitrogen and phosphorous compounds, such as 
ammonium hydroxide (NH.sub.4 OH) and phosphoric acid. The nutrients are 
supplied batchwise to the stabilization tank and in proportion to the 
paint concentration in the air. The tank is kept filled and the excess 
liquid is returned to the container (5) in the spray booth through a 
conduit (9) connected with the container. A minor amount of liquid departs 
from the tank via a conduit (10) connected to a sand filter (11). 
The water passes from top to bottom through the filter and the particles 
suspended therein are separated from the solvent-laden water in the sand 
filter which preferably comprises a pressure chamber filled with sand of 
suitable grain size. Two filters are preferably used for alternate 
connection into the circuit, so that one filter can be in operation, while 
the other is cleaned by means of water and pressurized air, which are 
caused to flow backwards through the filter. 
From the filter, the liquid flows into a bioreactor, preferably in the form 
of a biotower. This tower contains a filler material on which 
microorganisms are grown. The total efficiency of the tower may be 
substantially increased by moderating pH-value and temperature, as well as 
the concentration of the solvents and nutrients. The changes in these 
influencial factors may be effected in a separation unit placed after the 
biotower in the closed liquid purification circuit. Other reactors could 
be used instead of a biotower i.e., as known in the art a fluidized bed or 
an activated sludge tank can be employed. 
The separation unit is represented in the form of a microflotation unit 
(13) receiving water from the bottom of the tower via a conduit (39) and 
delivering the treated water by continuously recirculation via a conduit 
(14) to the top of the tower. In the microflotation unit (13) the dead 
microorganisms are separated from the water coming from the biotower and 
the cleaned water is sent back to the container (5) via a conduit (15). 
The separated biosludge then flows from the microflotation unit (13), 
through a further conduit (16), to another bioreactor (17), to which the 
paint sludge separated in the container (5) is also supplied through a 
separate conduit (18). 
The bioreactor (17) is in this case an activated sludge tank, which means 
that air must be supplied to the tank for accomplishing aerobic 
degradation. The conduit (18) carrying the paint sludge from the container 
(5) to the bioreactor (17), is connected to a surface separator (19) in 
the container (5), and this separator can also be combined with a bottom 
separator (not shown) from which the paint sludge is supplied through a 
conduit (20) to the bioreactor (17). The residual sludge is finally 
discharged from the bioreactor to a recovery plant. The bioreactor (17) 
can naturally also take the form of a passive sludge bed such as a 
digestion tank in which the final degradation of organic material occurs 
anaerobically. 
FIG. 2 shows an alternative embodiment of a spray painting plant where a 
container (5) and a separator (13) according to FIG. 1 are arranged in a 
single unit (21) which is consequently part of the spray booth liquid 
circulation circuit and the biological purification stage. The 
contaminated liquid from the spray booth (1) is supplied through a conduit 
(22) directly to the separator, which is in the form of a microflotation 
unit (21) in this embodiment, as well. The same quantity of liquid taken 
from the spray booth through this conduit (22) is tapped out from the 
microflotation unit (21) and recirculated to the spray booth (1) through a 
conduit (23) by means of a pump (6). The microflotation unit (21) and the 
biotower (12) have a separate liquid circulation circuit, similar to that 
described in connection with FIG. 1, via two conduits (14) and (39) 
connected to both, the liquid thus being taken from the separator (21) to 
the top of the biotower (12) and from its bottom back to the separator. 
The nutrients which are necessary for the biological purification, are 
supplied in this embodiment to the liquid which is recirculated via 
conduit (14) to the top of the biotower. The separator (or microflotation 
unit 21) is consequently given here the double function of separating the 
impurities suspended in the spray booth liquid, and also the biosludge 
coming from the biotower. The biosludge is then transferred via conduit 
(16) to the bioreactor section. In this plant there are two bioreactors 
(24 and 25) connected in series, both being of the activated sludge type. 
The first reactor (Bioreactor 1) is connected to the microflotation unit 
(21) and to the second bioreactor (Bioreactor 2) via separate conduits. 
The second bioreactor (B2) is connected to the first bioreactor (B2) and 
to a sedimentation tank (26). Water and sludge from the second reactor 
(25) are supplied to a sedimentation tank (26), from which cleaned water 
is recirculated to the first reactor (24) via a conduit (27). 
FIG. 3 shows an embodiment of a spray booth for spray painting, having a 
container for collecting the spray booth liquid which circulates between 
the spray booth and the container. The object to be painted (29) is placed 
in the upper portion of the spray booth (28). The air contaiminated with 
paint is caused to flow through a venturi (30) in which this air and the 
liquid are intimately mixed so that the paint particles and the solvent 
are transferred from the air to the liquid. 
The liquid, in this case water, is circulated from the container (31) 
through the conduit (32) and through the venturi (30). After the venturi, 
the liquid now laden with paint particles and solvent is separated from 
the air in a dewatering section (33) from which the liquid is caused to 
flow back to the container (31), while the air departs through an exhaust 
duct (34). The paint sludge in the container (31) floats on the surface 
where it can easily be removed by means of a skimming device as is 
indicated in FIG. 1. Gaseous solvent is prevented from being entrained in 
the air stream from the spray booth by means of a water trap (35). 
FIGS. 4A and 4B show the biotower (36) in two sideviews, one partly 
sectioned. The tower contains a filling material acting as the matrix for 
bacterial growth and action. The tower is placed on a settling tank (37) 
and the filling material is supported by the stainless steel grid (38) 
arranged in the tank cover. Water is supplied to the top of the biotower 
through a conduit (14), where it is freely distributed over the packing. 
During the downward migration of the water the solvent is absorbed and 
metabolized in the layer of microorganisms covering the packing material. 
The water is then collected in the tank (37) and is supplied to the 
microflotation unit through a conduit (39). Air is supplied through a 
lower duct (40) and is drawn through the tower from bottom to top in 
counter flow to the water flow and led away through an upper duct (41). 
The microflotation unit is shown in FIG. 5 from above and in a sectioned 
sideview in FIG. 6. In the microflotation unit dead organisms are 
separated from the water coming from the tank (37) of the biotower (36). A 
minor portion of the cleaned water is pressurized by means of a 
high-pressure pump (42), and is then saturated with compressed air in a 
pressure tank (43). At this high pressure the water is capable of 
dissolving 80-100 ml of air per liter of water. This pressurized water is 
then expanded in a tank (44) to which the overflow conduit (39) from the 
biotower is also connected. This process is performed such that the air 
dispersion is transported through a conduit (45) which also serves to 
distribute air and introduce it into the lower portion of the expansion 
tank (44), to which the biosludge from the biotower is supplied via the 
conduit (39). A baffle plate (46) will force the incoming flow of fluid 
upwards, in order to mix it with the pressurized water expanding in the 
container. 
Due to the pressure release, microscopic air bubbles are formed which 
become attached to the suspended solids causing them to float to the 
surface. The solids, which are dead microorganisms, can be removed by 
means of a skimmer or a scraping device (47), as shown in FIG. 6, 
comprising an endless chain (48) with two scraping members (49), for 
removing the pollutants or the biosludge floating on the surface. The 
biosludge is transferred to a smaller tank (50) located next to the main 
tank (44), while the cleaned water is recirculated both to the top of the 
biotower and to the circulation system passing through the spray booth, as 
indicated in FIGS. 1 and 2. The biosludge is then transferred to the 
bioreactor section via a conduit (16), for final degradation. 
In order to further illustrate the present invention the following examples 
are presented. The specific conditions and proportions in these examples 
are presented as being typical only and should not be construed to limit 
the scope of the present invention unduly. 
EXAMPLES 
Example 1 
In a plant according to FIG. 1, water at 80 l/min and air at 0.5 m.sup.3 
/sec are caused to flow through a spray booth which is shown in detail in 
FIG. 3. 
A paint killer TEXO LP 781 is introduced into the water circulating through 
the spray booth 1 in such quantity that the pH-value is about 8. The paint 
killer makes the paint non-sticky in the water so that it does not stick 
to the apparatus. The effect of the paint killer decreases at a pH below 
8. 
The water in the spray booth assumes approximately the temperature of wet 
air, in this case about 12.degree.-15.degree. C. The pressure drop over 
the spray booth venturi is about 130-140 mm water column. 
A paint according to Table 1 is sprayed in the booth at a rate of 2-2.5 
kg/h for 5-7 hours daily. Of the total amount of solvents present in the 
paint about two thirds by weight are separated such that about one third 
is absorbed in the spray booth water and another third is trapped in the 
paint sludge. The paint sludge is removed as required, by skimming the 
surface in the tank as indicated in FIG. 1. A partial flow of about 5 
l/minute of spray booth water is supplied to the biotower (12) via the 
stabilization tank (8) and the sand filter (11). Nutrients, substantially 
nitrogen in the form of ammonium hydroxide and phosphorous in the form of 
phosphoric acid are added in the stabilization tank (8). Ammonium 
hydroxide is added to the extent of 60 ml per kg sprayed paint and 
phosphoric acid to the extent of 4 ml per kg sprayed paint. 
The biotower (36)is shown in detail in FIGS. 4A and 4B and has a filler of 
the type known as "Norton Actifil 90.RTM.". A biomass consisting of a 
bacterial culture grows on the filler. This culture was taken from a 
municipal sewage plant and the species of microorganisms grown on the 
filler are shown in Table 2. The microorganisms collected from the sewage 
plant have to become acclimatized to their new environment before any 
considerable biodegradation can occur. The water was distributed over the 
filler to an amount of about 50-55 liters/min and was then collected at 
the bottom of the tower together with the biosludge removed from the 
filler material. Ambient air was sucked through the tower from the bottom 
to the top to oxygenate the water and the microorganisms. The solvents 
absorbed in the spray booth water are degraded in the biotower. The 
degradation releases heat which makes the temperature in the biotower 
water some degrees higher than that in the spray booth. A suitable 
pH-value in the biotower is between 6-9, which means that in this 
experiment the spray booth water had a correct pH. If this were not the 
case the pH-value could have been adjusted by adding sulfuric acid or 
caustic soda. 
The water with biosludge from the biotower is supplied to a microflotation 
unit (13) of the type shown in FIGS. 5 and 6, where the biosludge is 
separated after the water is pumped to the top of the tower (12). The 
microflotation unit is of the "dissolved air" type, i.e. a minor portion 
of the cleaned water is pressurized in a tank to 4-6 bar by means of 
compressed air. The pressurized water is mixed with non-pressurized water 
coming into the flotation plant. The sudden pressure decrease of the 
previously pressurized water creates microscopic air bubbles which adhere 
to the suspended solids, such as dead microorganisms, which then float to 
the surface where they are skimmed off at intervals and collected in a 
special tank included in the plant. 
The same amount of water, 5 liters/min, which is supplied to the biotower 
(12) is also fed back to the spray booth (1) from the microflotation unit 
(13). 
The water which evaporates from the spray booth as well as from the 
biotower is replenished by a daily addition of about 120-130 liters of 
water. The water system contains about 4000 liters in total, with the 
spray booth containing about 800 liters. 
During spray painting the concentration of solvent in the water will 
increase and after the spraying operation it will decrease to its initial 
values, as a result of the biological degradation in the bioreactor. 
The solvent concentration is most easily measured with a TOC-instrument 
(TOC=Total Organic Carbon) and in this case a Beckman 915 TOC is used. 
The results show that for a quantity of about 12-13 kg paint sprayed per 
day, about 6 kg of solvent will be separated, and thereof 3 kg per 24 
hours will be degraded in the bioreactor. 
Example 2 
In a plant according to FIG. 2 the water from the spray booth is fed 
directly into a microflotation plant as that shown in FIGS. 5 and 6. 
Cleaned water from the microflotation unit (21) is recirculated to the 
spray booth (1). Water from the biotower (12) is also supplied to the 
microflotation unit and clean water from the plant is recirculated to the 
top of said biotower. Paint sludge and biosludge are consequently 
separated simultaneously. The sludge is treated in two bioreactors (24) 
and (25) connected in series. 
The 12-13 kg of paint sprayed during a day according to Example 1 yield 
about 4-5 kg of dry substance together with about 3 kg of solvent 
contained in a sludge cake having about 50% water content. This sludge 
cake is transferred to the two bioreactors (24) and (25) which are 
connected in series and are of the activated sludge type. The first 
bioreactor, which the sludge cake enters first, comprises a container with 
a volume of about 300 liters. The container is ventilated with air at a 
pressure of about 1 bar. The ventilation causes mixing and oxygenation. 
The concentration of the paint sludge will be decreased by the addition of 
water of about 5% dry substance. 
Nitrogen in the form of ammonia, and phosphorous in the form of phosphoric 
acid, are also added. This is, of course, not necessary if sufficient 
nutrients are supplied in some other way. 
Sludge and water are transferred to the second reactor via an overflow. The 
second reactor has a volume of about 250 liters and is likewise 
ventilated. Water and sludge from this second reactor are finally led to a 
sedimentation tank (26). A mixture of paint sludge and biosludge, to a 
quantity of about 3.2-3.6 kg per 24 hours with an about 50% water content, 
comes from the bottom of the sedimentation tank. 
Cleaned water from the overflow is recirculated to the first reactor to 
decrease the concentration of inflowing sludge. 
The biodegradation occurs with the aid of the cultivation described in 
Example 1. If the spray booth is connected in accordance with Example 1 no 
further grafting of bacteria is necessary since there are enough bacteria 
in the water. A suitable pH-value is 6-9, which means that the spray booth 
water described in Example 1 maintains the right value. The temperature 
will be about the same as that of the spray booth water. 
The results here show that about 3 kg of solvent contained in the paint 
sludge and part of the paint binder have been broken down. 
TABLE 1 
______________________________________ 
Solvent in 27% LDL-laquer 
Reduced paint system consisting of 80% by volume of 
paint 397 YAJ 615 and 20% by volume of thinner 38228. 
Dry solids content: 32% by weight 
Solubility in 
% by % Density 
water g/100 ml 
Component volume by wt. kg/liter 
at 20.degree. C. 
______________________________________ 
MEK 3 2.8 0.805 27.0 
P-naphtha 10 7.7 0.660 0.01 
VM & P naphtha 
1 0.8 0.70 0.01 
Toluene 6 6.1 0.867 0.05 
Cellulose acetate 
1 1.1 0.975 22.0 
Mineral spirits 
41 36.8 0.77 0.01 
(180-210.degree. C.) 
Solvesso 150 
9 9.4 0.890 0.05 
Butyl cellosolve- 
9 9.5 0.9 1.1 
acetate 
DBE-2 10 12.7 1.086 4.5 
Diethyl phtalate 
10 13.1 1.123 insoluble 
______________________________________ 
TABLE 2 
______________________________________ 
Biotower 
______________________________________ 
Growth on filler material 
Good growth of: Citrobacter freundil 
Serratia species 
Aeromonas hydrofila 
Minor growth of: Streptococcus faecalis 
Yeast fungus 
No thermostabile coliform bacteria or stafylococcus 
discovered. 
Biotower water 
______________________________________ 
Growth of Concentration (No./ml): 
______________________________________ 
Citrobacter freundii 
9 .times. 10.sup.5 
Aeromonas hydrofila 3 .times. 10.sup.5 
Serratia species 
Staphylococcus aureus 
12 
Streptococcus faecalis 
5 
Pseudomonas species 20 
Total number of bacteria at 22.degree. C. 
4 .times. 10.sup.7 
______________________________________ 
Having now fully described the invention, it will be apparent to one of 
ordinary skill in the art that many changes and modifications can be made 
thereto without departing from the spirit or scope of the invention as set 
forth herein.