pollution prevention in an aluminium grinding facility

Pollution Prevention in an Aluminum Grinding Facility by N. Rajagopalan, V.M. Boddu, S. Mishra, and D. Kraybill Illinois Waste Management & Research Center, Champaign, Ill.

H ighly polished aluminum disks are utilized as substrate in memory storage devices. The

polishing step uses considerable volumes of grinding coolant and alkaline cleaner as processing aids. The even- tual disposal of the clenaers and coolants can be troublesome and expensive due to their high BOD levels. Compa- nies often pretreat such effluents to mandated levels or face punitive sur- charges. In some instances the fluids may even be shipped off-site, an extremely expensive option.

In this article the effectiveness of source reduction and in-process recycling in reducing both the BOD levels in wastewater as well as wastewater volume discharges in one manufacturing facility is presented.

BACKGROUND

The coolants, cleaners, and rinsewaters were identified as the major con- tributors to the effluent volume and BOD, loading (Table I). The daily average volume of wastewater discharged to the sewer was about 35,500 gallons per day. The company was, therefore, identified as a large volume user of sanitary district facilities. The daily average BOD, in the combined plant effluent was 4,100 mg/L (BOD2,, = 6.030 mg/L) in excess of the discharge limit of 200 mg/L BOD,.

Table 1. Estimated Daily Average Flow Rates of Various Process Streams and Their BOD Loads

Process Stream

Chemical etch rinsewater

Coolant A Coolant B Alkaline cleaner Rinsewaters Total (estimated) As per sanitary

records

Volume BOD20 fwd 0x74

2,000 65a

2,500 25.000a 10,500 12,000”

4,000 3,235a 16,500 Ob 35,500 5,680 33,000 6,300

;Laboratory analysis Assumed

Reduction in BOD was warranted through process alternatives, recycle schemes, or waste treatment. Reduc- tion in effluent discharge volume was also pursued as a desirable objective. This was due to the possibility of obtaining variations in permissible levels of BOD in the wastewater discharge to the sanitary sewer if the facility could be reclassified as a small volume user (<20,000 gpd); therefore, the subsequent investigations focused on identifying processes and procedures that would lead to reducing the BOD levels of the waste stream and reducing the volume of wastewater discharged to the sewer to below 20,000 gpd.

APPROACH

The main goals of the study were to examine various process alternatives to reduce the BOD,(, to values closer to the regulatory l&its and to reduce the volume of wastewater discharged to the sewer.

Large capital investment and dedi- cated personel are required for optimal operation of end-of-pipe treatment technologies such as biological treatment to reduce BOD. The potential for upset of the biological process due to the pres- ence of biocides in the coolants was also a cause for concern. Other options, such as chemical treatment, were not success- ful in reducing the BOD of the effluent. End-of-pipe treatment technologies by themselves would also not reduce wastewater volume: therefore, it was decided that an in-depth investigation of the manufacturing process was required to see if volume and strength of the waste could be reduced at the source through material substitution and in-process recycling.

MATERIAL SUBSTITUTION

Coolants were the major contributor to the high BOD of the wastewater and were targeted as the streams most likely to benefit from material substi-

tution. The facility, working closely with the coolant supplier, identified a substitute coolant (Coolant C) after rigorous evaluation of several different coolants. It was estimated that a 70% reduction in BOD loads would result by substitution of coolants A & B with a lower BOD coolant C that was func- tionally equivalent (Table 11).

IN-PROCESS RECYCLING

The technical and economic feasi- bility of in-process recycling of coolants, cleaners, and associated rinsewaters was then evaluated to estimate maximum achievable reductions in BOD and wastewater volume. Based on previous experience and the technical literature, the problem was defined as the need to selectively separate the contaminants from the active ingredi- ents of the cleaner and coolants. The contaminant-free cleaner and coolant would then be recycled back to the process, reducing both effluent discharge volume and BOD loads. Sev- eral technologies may be appropriate for such separations. These include conventional filtration (bag, cartridge, etc.), membrane filtration, hydro- clones, centrifugation. and fioccula- tion. Choice and selection of appropriate technologies was intluenced by two criteria-their suitability for the spe- cific targeted streams and ease of use.

Table II. Achievable Reduction in Effluent BOD Through Coolant Substitutiona

Process Sfream

Prior to substitution: Coolant A Coolant B

After substitution: Coolant C

Reduction in BOD discharge compared to baseline in Table I

Estimated BOD,, (mg/L) In effluentd

Volume BOD,, @NJ iw4

2,500 25,000 10,500 12,000

13,000 3.700 71%

1,724

aAssumes no cllanges to other operations

18 0 Copyright Elsevier Science Inc. METAL FINISHING l NOVEMBER 1998

High LOW

LOW

Figure 1. A two-dimensional mapping of the quality of water in process inputs and outputs provides a framework for deter- mining cascade rinse opportunities.

CONCEPTUAL APPROACH TO RINSEWATER RECYCLING

Figure I provides the framework used for exploring recycling options for rinsewater from various processes. The approach is fairly simple in that the quality of water required for various processes and that available from various processes are mapped to determine cascade rinse opportunities. In Figure I arrows going into a process indicate an input and arrows pointing away from a process represent an output. The diagonal represents the inter- section of identical quality levels on both axes. As an example, point a represents the requirement of high-quality water as input to Process A (e.g., critical rinsing). The output from Process A, represented by point b, is lower in quality than the input and is. therefore. below the diagonal. Point c represents the quality of water required as input to Process B. Note that point c is to the right of point b, indicating that the effluent from Process A is of higher quality than that required in Process B. This suggests that the effluent from Process A can be used as input to Pro- cess B. Process C represents a water treatment technolog,y, such as filtration, where output water quality is bet- ter than the process input (point 1‘ is above the diagonal). The implementation of such identified opportunities would depend on a detailed cost-benefit analysis. One issue that needs thought while adopting such an approach is the choice of an appropriate metric 10 measure quatity. In some

Figure 2. The solid lines indicate work flow and water usage in one grinding cell. The dashed lines show proposed operational changes to conserve water and reduce BOD discharge through recycling the alkaline cleaner and cascading rinsewater.

cases this might be something as simple as total dissolved solids (TDS) or total organic carbon (TOC) while oth- ers might require a composite metric.

ALKALINE CLEANER AND ASSOCIATED RINSEWATER RECYCLING

Figure 2 is a schematic of the alkaline cleaning steps being followed at the facility. An important observation was that the alkaline cleaner (2% concentration) wa\ being continuously overflowed to maintain a low level of suspended solids concentration. It was suggested that the alkaline cleaner be filtered and recycled at least once before being discharged. Membrane filtration (micro/ultrafiltration) had the potential to produce a particulate-free cleaner filtrate but would have been an expensive option in this instance. Fur- thermore, membrane filtration of the alkaline cleaner would have resulted in loss of nonionic surfactants (known from previous experience with the cleaner) and required additional atten- tion to solution chemistry maintenance. Filtration through a S-urn cartridge filter was considered adequate to control the level of suspended solids in the alkaline cleaner and allow it to be recycled. Tt was also recommended that the final rinsewater, which was of extremely high quality, could be re- used within the process as make-up as shown by the dashed lines in Figure 2.

COOLANT AND ASSOCIATED RINSEWATER RECYCLING

Figure 3 is a schematic of coolant and rinsewater flows from the grinding operation. Surlace finish resulting

20 METAL FINISHING l NOVEMBER 1998

Figure 3. The solid lines indicate work flow and water usage in one grinding cell. The dashed line shows proposed operational changes to conserve water and reduce BOD discharge through recycling coolant and cascading rinsewater.

from the grinding process was sensi- tive to the contaminant particle size. Reuse of the coolant was limited by both the level of contaminant particles as well as their size distribution. The facility at the time of assessment was recirculating the coolant after succes- sive filtration through S-urn and I-urn cartridge filters. The size of the residual alumina particles in the coolant was, therefore, found to be predomi- nantly in the submicron range. Floccu- lation and filtration to remove the alumina particles was technically feasible: however, addition of an external chemical to the coolant fluid would have necessitated detailed investigation of the compatibility of the flocculant with the coolant as well as the substrates. Moreover, such process changes would have involved obtaining approval from the facilities’ customers, a lengthy, cumbersome process. Under these cir- cumstances membrane filtration (mi- crofiltratinn) was considered an appropriate choice for producing a particulate-free coolant. Bench and pilot trials ‘were initiated to verify the utility of microfiltration for extending the coolant life. A ceramic membrane was chosen for evaluation based on factors such as separation efficiency, chemical stability, and ease of cleaning. Extensive pilot tests were conducted with Coolant C.

MICROFILTRATION PILOT STUDIES

A pilot membrane module (Enviro- Pure Solutions, Wheaton, III.) equipped with a ceramic membrane (U.S. Filter. Warrendale, Pa.) ofo. I-urn pore size was

used for the on-site pilot study. The initial stages of the study focused on obtaining information on membrane productivity, propensity to foul, membrane cleaning, and maximum achievable coolant volume recovery for the spent coolant. The later stages of the study focused exclusively on coolant recyclability, BOD characteristics of the concentrate, and applicable process schemes to achieve overall BOD reduction.

A discussion of the pilot experi- ments is facilitated by definition of the following terms:

Fllr.dP,.otllrL,ti~,it~: A measure of the volumetric tlow rate of filtered coolant obtained per unit area of membrane in unit time. In this report flux is reported in units of L/m*/hr 01 LMH.

Feed: The solution that is fed to the membrane system for separation. Spent coolant contaminated with aluminum fines was the feed in this instance.

Foulirq: Refers to the lowering of membrane flux mainly due to particle deposition onto the membrane and chemical interaction between the fluid being processed and the membrane surface.

Per.nleuteiFiltl.ute: The solution that passes through the membrane.

Conc~er~tt~crtelRctcntcrtel~en~b~ut~e Residue: The solution that is en- riched in the contaminants after a clean permeate has been separated from the feed.

Membrane flux is the single most important parameter that substantially impacts equipment cost. A low-membrane flux results in a larger system leading to higher equipment and operating costs. A very low membrane flux can render a technically feasible separation economically unviable. Both fouling and membrane productivity/ flux are to some extent controlled by operating conditions; therefore, the im- pact of operating conditions on the membrane productivity and fouling was also investigated.

Parameters under operator control are normally temperature, pressure. and use of a back-pulsing device for periodic cleaning of the membrane pores. In the plant study conducted, temperature was fixed at ambient conditions for safety reasons. Pressure was

varied between two levels of 30 and 60 psi. These were chosen to determine if the rate of fouling could be reduced by lowering the operating pressure. Back- pulsing was used in preliminary trials to determine its effectiveness. For the pilot unit tested, the back-pulsing mechanism was not very effective. Subsequently, the permeate was throt- tled deliberately to impose a back pressure on the membrane downstream side to slow clown the build-up of a cake layer on the membrane surface. The consequent loss of productivity was offset by increased cumulative volume of filtrate produced over a longer period of operation without need for cleaning.

The reusability of filtered Coolant C (permeate) was tested by plant person- nel in a coolant test bed. Characteris- tics, such as grind rate, gross surface defects, and surface finish, are illustra- tive of some of the tests used to eval- uate the functionality of the recycled coolant. The coolant effectiveness was evaluated both with and without any additional fortification with fresh Coolant C.

Feed, permeate, and retentate sam- ples were collected for analysis from the above tests to determine surfactant loss, suspended solids loading, and BOD values.

The pilot system was cleaned with a solution containing about 2% by volume nitric acid between the various tests for about 30 minutes at room temperature. Water flux was measured to monitor if the system was clean after the acid wash. In general no major problems were noticed in restoring the membrane flux after contact with the coolant. In one or two instances an odor reminiscent of a sulfide was de- tected and was probably due to the breakdown of an unkown sulfur com- pound in the coolant formulation. Thoroughly flushing the system with water to remove all coolant in the system prior to acid wash eliminated this odor and is recommended as a safety precaution.

DISCUSSION OF PILOT AND ANALYTICAL DATA

Measurement of water flux before and after a membrane separation experiment provides information on maximum flux obtainable at a given pressure and the

7,000

6,000

5,000

E 4,000

kf 3,000 G

0 5 10 15 20 25 JO Transmembrane pressure.psi

Figure 4. Water flux of membrane measured before and after contact with coolant. The membrane was cleaned with acid after processing Coolant C. The data indicate that no irreversible fouling of the membrane occured due to contact with Coolant C.

ability of the membrane to be cleaned. The water tlux measured as a function of pressure is shown in Figure 4 . Data obtained before and after contact with coolant indicate that no irreversible fouling was experienced with Coolant C and that the membrane was easily cleaned with acid washing.

Figure 5 shows the variation of coolant flux with pressure. The flux varies almost linearly with pressure indicating no phenomenon such a compressible cake formation or varying cake resistance at a recirculation rate of 20 to 30 gpm. Coolant C permeate flux is about 345 LMH at an average transmembrane pressure of 40 psi. Minimal fouling with Coolant C was observed at average transmembrane pressures of

100

0 o---,cl 20 30 40 50 60 -70

'Trsnsmembrane pressure,psi

Figure 5. Variation of Coolant C flux with pressure. The linearity of the data suggest that no phenomenon such as compressible cake formation or varying cake resistance was encountered. If such phenomenon had been encountered, higher recirculation flow rates would have been required leading to higher operating costs.


0 L

.

.

~~~. 0 100 200 300 400 500 600 700

Permeate volume recovered, Liters

Figure 6. Coolant C flux as a function of coolant volume recovered. The profile of the flux decrease is very typical of such processes and reflects the effect of multiple mechanisms such as increase in feed vis- cosity and cake formation. increases in flux at various points, such as close to 500 L, are due to dilution of the concentrated residue with additional contaminated coolant.

60 and 30 psi indicating no pressure- dependent fouling in that range.

Figure 6 shows results obtained with continuous permeate recovery. a situa- tion that would simulate actual operating conditions. The recovery data were collected in a batch-feed and continuous-bleed mode. The operation was initiated with a tank (60 gallons) filled with spent coolant. The solution was then filtered through the membrane. The feed tank was filled with additional fresh feed when the level of the concentrated feed in the tank dropped to about 20 gallons. This mode of add- ing feed intermittently to the tank was required as the feed tank had a limited capacity. Figure 6 shows flux values normalized to 40 psi. .4 total of 1X5 gallons was processed with a permeate recovery of about 17 I gallons (-92% recovery). The flux for the most part was well above 1 SO LMH. The average flux would, therefore, be expected to be between 150-350 LMH for this recovery ratio.

Filtered Coolant Quality The filtered coolant quality was. as

expected, optically clear with no suspended solids. The coolant could be recycled several times after filtration. After three cycles of use, the filtered coolant needed to be fortified with 1 c/o fresh coolant concentrate to make-up for loss of surfactant additives.

BOD Reduction The membrane filtrale and concen-

trate were analyzed for BOD,,. The

Table Ill. Projected Volume and BOD Reductions Through Coolant Recycling, Cleaner Recycling, and Rinsewater Cascading

Process Stream

Coolant C Alkaline cleaner Rinsewater Etching water Volume of wastewater Residual from membrane filtration BOD of wastewater to sewer if membrane concentrate hauled SOD of wastewater if membrane concentrate sewered

Volume (gpd)

Before Affer Process Process

Changes Changes

13,000 2,500 4,000 2,000

16,500 11,150 2,000 2,000

35,500 17,650 410

- - - -

B% Ov4

After Process

Changes

3,700 3,235

0 85 -

22,000 900

1,380

BOD,,, of the concentrate and the permeate stream from coolant C were 22,000 mg/L and 2,600 mg/L respec- tively. The corresponding BOD,, rejection is about 88%. Even though the BOD levels of the concentrate are high, the BOD mass loading to the sewer would be reduced by well over 50% due to the small volume of the concentrate.

Associated Rinsewater Recycling

The rinsewater following grinding was of good quality and could be used for recycling to a subsequent process as shown in Figure 3.

PUlTlNG IT ALL TOGETHER

Table III provides an overview of the expected reductions in wastewater volumes and BOD by implementing the recycling schemes proposed in ear- lier sections. Other options are available and need to be studied further for additional reductions in BOD. Tables IV and V provide estimated cost-benefit savings that could result if the proposed changes were carried out. Costs have been estimated primarily for membrane filtration equipment as piping modifications and cartridge filtration equipment associated engineer-

Table IV. Estimated Savings in Chemical Costs Through Adopting Recycling

Sfream Annual Saving ($)

Coolanta 260,325 Alkaline cleaner 97,440 RO-DI water 8,025 Total 365,790

aAccounts for both coolant substitution and recycling. Calculations include make-up coolant concentrate for fortification (at 1% by volume) of coolant after filtration.

ing costs were not expected to be very significant. The results of the cost-benefit analysis show that considerable fi- nancial incentives exist to adopt the proposed changes. Estimated annual net savings are $23 1.390. The incentives are expected to be increased even more by further reformulating the coolant.

IMPLICATIONS OF HIGH BOD REJECTION FOR ECONOMIC VIABILITY

A higher BOD level in the concentrate is counterproductive because it merely serves to concentrate the problem, albeit in a small volume. This residue would need further treatment or hauling. These options add further to the cost. Two approaches can be adopted in countering this problem: first, isolate the source of the BOD and look for ways to work around it; sec-

Table V. Estimated Capital and Operating Costs Associated with Operation of Membrane System

kern Annual Cost ($1

Capital Cost: Skid-mounted ceramic microfiltra- 300.000

tion system (vendor estimates) Operating Costs:

Electricity 6.750 Maintenance (at 3% of equipment 9,000

costs) Cleaning chemicals 750 Labor 31,200 Total direct operation costs 47,700 Interesta (at 10%) 30,000 Admlnistrative costs 7,500

Total direct and intlirect costs 65,200 Optional residue disposal costs 49,200 Total operating Costs with residue 134,400

disposal

aAssumes equipment bought with borrowed funds. Estimated annual net savmgs for all recycling efforts is $231,390

METAL FINISHING l NOVEMBER 1998 23

ondly, search for a way to reduce concentrate treatment and disposal costs.

An attempt to isolate the source of the BOD was made by conducting extensive laboratory analyses. All indica- tions were that one surfactant in the coolant composition was the primary contributor to the high BOD. Discus- sions with the manufacturer indicate that it is possible to replace this surfactant by increased levels of other surfactants without affecting coolant function. If this can be done, the BOD of the membrane residue stream can be substantially reduced.

The search for a cost-effective treatment method led again to considering flocculation of the concentrate. The critical difference between this approach and using tlocculation on the entire coolant volume is mainly one of compatibility and cost. While no data were collected to support our premise, we speculate that flocculation and subsequent sepamtion of the alumina would allow the supernatant/filtrate to be mixed with the membrane permeate without adverse effects. This is mainly

based on dilution of any residual tloc- culant to very low levels as to be in- significant. The precipitated mass would be a very small fraction of the concentrate and could be disposed of economically.

Clearly, reformulation of Coolant C to eliminate the problematic surfactant is most desirable from the standpoint of operational efficiency. The feasibil- ity of such an approach is also clearly supported by the fact that even though the troublesome surfactant was being stripped during filtering (leading to residual BOD) the function of the coolant was not greatly compromised.

CONCLUSIONS

This study focused on identifying pollution prevention opportunities in an aluminum grinding facility. The primary goals were the reduction of wastewater volume and BOD discharge to the sewer. The synthesis of several activities that include material

substitution. in-process recycling of process streams. and the use of appropriate technologies has the potential to reduce wastewater volume by 50% and the BOD?,, levels in the wastewater to 900 to I.380 mg/L from an initial level of 6.030 mg/L.

This study also highlights that a de- liberate approach to minimizing pollut- ant generation at the source and im- proving inherent process efficiencies can be profitable.

Acknowledgements The authors thank Teresa Chow of

the Waste Management and Research Center. Champaign, Ill., for carrying out the surfactant analysis, and Oakite Chemicals (Berkeley Heights, N.J.) for providing information on coolant formulation. The authors also thank the U.S. EPA Region 5 for making available part of the funding to carry out this study under the Technology lden- tification Sr Implementation Partner- ship grant. MF

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