A WASTE MINIMIZATION PROGRAM

UTILIZING STATISTICAL PROCESS CONTROL CONCEPTS

F. C. Gilmore and M. F. Healy

Mobay Corporation

New Martinsville, West Virginia

ABSTRACT

Mobay Corporation operates a large production facility a t New Martinsville,

W e s t Virginia. The plant has a complete wastewater treatment system which

utilizes activated sludge biological treatment followed by activated carbon

adsorption. For several years, gas chromatography has been used to monitor

specific organic compounds in the wastewater treatment process, in addition

to the standard analytical parameters, such as Total Organic Carbon, Biologi-

cal Oxygen Demand, and Chemical Oxygen Demand, commonly used with such

processes. In order to reduce the levels of organics fed to the treatment

plant, a program was instituted in 1983 in which the concepts of statistical

process control were applied to the reporting of these analytical data. This

focused the attention of the production units on their waste streams, as-

sisted with justification of process modifications, and documented the

improvements achieved. Over a five-year period, specific organics were

reduced by 80 percent, and levels of priority pollutants in the plant efflu-

ent were reduced to generally non-detectable levels.

\

BACKGROUND

Mobay Corporation operates a large production facility a t New Martinsville,

W e s t Virginia. The plant produces primarily polyurethane chemicals and

intermediates. The plant also produces iron oxide pigments and concentrated

hydrochloric and nitric acids. The plant has a complete wastewater treatment

system which utilizes activated sludge biological treatment followed by acti-

vated carbon adsorption. A simplified flow diagram of the wastewater system

is presented in Figure 1. The biological system consists of two activated

sludge units operated in parallel, with sludge recycle from a common sec-

ondary clarifier. The activated carbon system consists of two or three tow-

ers operated upflow.

For several years, gas chromatographic (GC) analyses of the primary streams

a t the wastewater treatment unit have been used to supplement the standard

analytical parameters, such as Total Organic Carbon (TOC), Biological Oxygen

Demand (BOD), and Chemical Oxygen Demand (COD), The GC method used for

these analyses quantifies nine different organics, including isomers. Of

course, numerous other compounds are present, but these nine compounds

.serve to monitor most of the plant processes. The primary sources of each

of these compounds is indicated in Figure 2.

The organic compounds fed to the treatment plant have varying degrees of

treatability by the wastewater treatment system. Some compounds, such a s

aniline, are removed readily by the biological system. Others, such as dini-

trotoluene, are not readily removed biologically, and may even be toxic to

the biota. Some constituents, such as high-molecular-weight polyether

polyols, are not degraded biologically to any significant degree. Most organic

compounds passing from the biological system are adsorbed to some degree

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by the activated carbon system, polar organics being adsorbed less than

non-polar organics. It can be shown that the concentration of constituents

in the effluent wastewater is directly proportional to the feed concentra-

tions, all other things being equal, so reducing the amounts of compounds in

' the feed to the treatment system has a direct effect on the quality of the

effluent, Reducing the levels of toxic compounds in the feed improves the

efficiency of removal of all degradable constituents.

It was recognized that regulatory requirements for priority pollutants in

wastewater effluents were becoming more stringent. In order to improve the

efficiency of the treatment system as well as to improve the overall effluent

quality, a program was implemented in 1982 to improve the degree to which

the analytical monitoring data were reported to the production units. It was

recognized that it was more efficient to remove constituents in the produc-

tion units, where the compounds are more concentrated, where removal pro-

cesses can be tailored to the specific compounds, and where the compounds

can be recovered for recycle.

DISCUSSION

The stream selected for primary reporting was the feed stream to the bio-

logical units (Biox Feed). This stream had been monitored for specific organ-

ics since 1978 in order to better control the biological system. A t the t ime

this program was begun, this was the first wastewater stream whose flow

was' continuously measured. This point also follows equalization, which

reduces variation in the composition. Since only one feed sample per day is

analyzed, the leveling effect of the preceding tanks is desirable, improving

the representativeness of the sample. The disadvantage is that the sample

actually represents an event which occurred at the production unit almost

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two days previously. Each sample is a composite of 3 samples taken at

8-hour intervals. The daily flow variation is less than 10 percent, so flow

proportioning is not necessary.

The gas chromatographic method used for this program was developed pri-

marily for this application. The method had to suffice for the whole range of

concentrations. Preference was given to the range of concentrations in the

feed samples. The sample is adjusted to p H 8 and extracted with chloroform,

using a concentration factor of 50 to 1. A 2.5-meter column packed with

OV-3 methyl-silicone is used, with a flame ionization detector. Bromobenzene

is used as an internal standard. The normal quantitation limits are around

0.1 ppm. In the first few years that this method was used, these sensitivi-

ties sufficed even for the effluent samples. In the past couple of years,

effluent concentrations usually have been below detection limits with this

method. Most of the time, the secondary clarifier effluent concentrations are

at or below these quantitation limits. In order to get reliable quantitation of

the effluent samples, more sophisticated procedures, such as the EPA 600

series GC methods are required. These methods are too time-consuming and

manpower-intensive for our daily monitoring.

The compounds quantified by this method are listed in Table 1 along with

the normal range of concentrations found in the Biox Feed sample.

For reporting purposes, the MNT isomers are totaled and reported simply a s

MNT. Similarly, the DNT is reported as the total of the isomers. Thus, nine

component values are normally reported.

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TABLE 1

Quantified Wastewater Components

Abbreviation Compound ppm Range

............................................................

Benz Benzene .01 - 120

To1 Toluene .01 - 60

Ani Aniline .01 - 900

MCB Chlorobenzene .01 - 50

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The other analytical methods are standard procedures normally used for

wastewater analyses. These are all described in Standards Methods for the

Examination of Water and Wastewater , 16th Edition, 1985. The primary par-

ameters of interest to this study are Biological Oxygen Demand (BOD) and

Total Organic Carbon (TOC).

The analytical efforts were backed up by a modest QA/QC program. This was

not only to insure consistency in the results but also to insure that the

inevitable questions raised by the production units being monitored could be

answered positively - "Yes! The numbers are real!" Once the production

units realized that they could rely on the analytical results, they found

they could generally correlate them with their operations.

The primary QA/QC procedures consisted of calibration of instruments

at least daily, along with analysis of standards and spiked samples on a

regular basis and careful recording of all analytical data. The data were also

monitored for consistency and reasonableness, and unusual results were

investigated immediately.

The suspected primary source of errors were the flow measurements. W e

found that, in spite of our best efforts, it was extremely difficult to insure

accuracy of the process flow meters at all t imes. While we recognized that

our quantity results could contain significant error at any given t ime, we

reasoned that the errors should average out over t ime, so we just made

every reasonable effort to minimize any known sources of error in flow

measurements.

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Figure 1

Block Flow Diagram - Wastewater Trea tment

MNB

Po lyeth ers Others

Wastewater

A+ Biox Feed 1 I

Biological 1 Oxidation CI arif ica tion

Carbon Adsorption

Final 1 Effluent

To Wastewater 7 Treat men t I

T T Aniline

Methanol

Others I To1 uene

I - I

L

I n o r d e r to maximize t h e efficiency of recording, repor t ing ,

and use of the data, a computer" program was written using Lotus 1-2-3 for

processing the data. Each spreadsheet contained a month's data, consisting

of over 100 measured or calculated parameters daily. The spreadsheet was

programmed, making extensive use of macro routines, to automate most of

the applications of the data. For example, routines were written to generate

Shewart-type control charts for any selected parameter. Routines were also

written to remove zero values which result from calculations involving miss-

ing data values. This is important for proper calculation of statistics, and

current versions of 1-2-3 have no straightforward means for dealing with

such situations. U s e r s can perform all of the programmed features by simply

selecting various menu options.

Wastewater treatment operating data have been reported each morning since

1974 to various individuals, including Production management. The Lotus

spreadsheet now generates and prints this report automatically. The specific

organics monitoring data were simply added to the normal morning report,

and the distribution list was expanded to include operating unit supervision.

A f t e r two years of this monitoring program, it w a s decided to

augment the program by monitoring of several of the individual process

wastewater streams. For purposes of allocating wastewater treatment costs,

28 wastewater streams are monitored semi-monthly, using TOC to measure

organics content. These data were used to select the five streams with the

largest TOC contribution for more extensive monitoring. Automatic samplers

were placed in these five streams to take hourly samples. A composite was

used to generate a daily average value. D a t a from these samplers were

included in the monitoring program.

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The portion of the Morning Report which report these data is similar to that

shown in Table 2. The Upper Control Limits (UCL) are based upon the prior

year statistics and are set at two standard deviations from the mean, after

rejection of obviously abnormal values (two-sigma l imits) . The program

automatically "flags" those components which exceed the UCL. Since these

UCL's are intended as "alert" values, the use of two-sigma l imi ts is pre-

ferred over the three-sigma l imi ts normally used with Shewart charts. It is

recognized that variations in the composition of these waste streams are not

entirely random, so that normal distribution curves do not apply exactly.

The assumption of normal probability is usually acceptable, and obvious

deviations are dealt with by applying judgements based upon experience or

knowledge of the process circumstances.

Meetings were held with members of Production Supervision to explain the

program and enlist their support in responding to values outside the control

limits. Good communications is important to the success of such a program.

The awareness which results from consistent follow-up when excursions

occur tends to reduce the frequency and magnitude of losses.

With this background, we can now present some of the measured data. Fig-

ure 3 shows the average amount of Identified Organics (ID) and Total

Organic Carbon (TOC) in the feed stream to the Biological system (Biox Feed)

for the years 1978 through 1986. The amounts are given in thousands of

pounds per day. The figure shows that loadings during the 1978-1979 t i m e

frame averaged about 6000 lbs/day TOC and about 3800 lbs/day Identified

Organics. During this s a m e period, the average Biological Oxygen Demand

(BOD) was about 6800 lbs/day BOD. In 1980, a new production unit started

up, increasing flows by about 10% and initially doubling the aniline loadings.

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.

Table 2

Environmental Morning Report - Production Monitoring Section

10. Monitor Points: I--Biox Feed - Amt/day-I--- Area Streams Amounts in Lbs/day !Organic Amount UCL : Area Amount

21 30 :Dept I 143 Dept V-B ------ UCL-:Toluene 134 150 IDept I1 564 Flow MGD 0.69 : MCB 45 55 :Dept I11 1008 TSS :Aniline 566 600 :Dept IV 146

Am t 161 300 :DCB 113 # 75 :Dept V-A 39 ,~~~~~~~~~~~~~~~~~~~~~ 0-Tolu 18 25 :Dept V-B 425

- _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ I I Benzene

: MNB 83 200 : EQ Flow: 3 GPM(Avg):MNT 27 60 : RB Flow: 3 GPM(Avg):DNT 33 150 : ................................................................

# - Upper Control Limit Exceeded - Look for cause.

TC - - : uc L 450 780

2450 585 225 590

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The special SPC-based reporting program was initiated in the fourth quarter

of 1982. Percentage reduction data are presented in Table 3, using 1981 a s

the baseline year. In 1981, the amount of aniline in the plant effluent was

around 100 Ibs/day; in 1986 it was down to almost 1 Ib/day. Today, aniline

is usually not detectable in the plant effluent.

Figure 4 shows the TOC in the Biox feed, Secondary Effluent and Final Plant

Effluent for the years 1980-1986. Figure 5 shows the amounts of Identified

Organics in the Biox feed and in the two effluent streams. Figure 6 shows

the BOD values for these three streams. These figures show that the quality

of the effluent streams are directly affected by the quantity of organics in

the feed. The improved quality of all three streams is readily apparent.

Figure 7 shows the relative amounts of nitrobenzene (MNB), nitrotoluene

(MNT) and dinitrotoluene (DNT) in the Biox feed from 1978 through 1986. As

a result of the emphasis this program provided, the production unit respon-

sible for these components was able to make process improvements which

accomplished the reductions indicated. This involved installation of an

extraction column and the piping necessary to return these pollutants to the

production process. Yields were only slightly improved, but the effect on

the plant effluent quality was dramatic. DNT is known to be toxic to biologi-

cal systems, so reduction in the quantity of this component in the process

wastewater improves the overall efficiency of the activated sludge system.

In addition to the obvious environmental improvements, there are potentially

significant cost savings. The eighty percent reduction in Total Identified

Organics 'shown in Table 3 represents over $250,000 per year savings, if

these materials are valued at roughly 25 cents per pound.

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Aniline

Benzene

Toluene

Chlorobenzene

Dichlorobenze

o-Toluidine

Nitrobenzene

Nitrotoluene

Dinitrotoluene

71%

-80%

- 6%

80%

46%

76%

87%

98%

94%

Total ID Org 80%

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Such savings generally are more than sufficient to offset the installa-

tion costs of equipment necessary to improve control of the generating pro-

cess or to recover potential wastes a t the production unit and return them

to the process. In some instances, it may take 5 years or more to recover

the equipment installation costs, but this is offset by the environmental

improvements which result.

The graphs presented also indicate that the improvements noted did not

occur immediately. It has taken several years for the program to produce

dramatic results. The improvements, however, are definite. Figure 8 shows

the first set of UCL values established in 1982 compared with the most

recent values, based on 1986 data. Except for toluene and DCB, all the UCL’s

decreased significantly. DCB has usually been well-controlled, so a change is

not necessarily expected. The higher toluene value is probably the result of

additional processes which use this material.

CONCLUSION

We have shown the waste reductions achieved by a wastewater monitoring pro-

g r a m based upon statistical process control concepts, conducted at an indus-

trial production facility. By daily monitoring amounts of specific organic

compounds in the process wastewater and reporting the results against stat-

istically-derived control l imi t s , the production units were able to take

note of unusual discharges and make correlations with their process condi-

tions. It was possible to justify the installation of equipment and imple-

mentation of various improved control measures which resulted in significant

reductions in amounts of pollutants discharged.

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