Micronutrient Addition and Its

Impact on WWTP Performance

Ronald W. Ballard, Joan A. Giltner, and James K. Cable
CH2M HILL, Atlanta, GA 30303
Richard D. Langford
Hoechst Celanese Corporation, Narrows, VA

The Hoechst Celanese facility in Narrows, Virginia, treats process wastewater in

an aerobic biological wastewater treatment plant ( WWTP). Historically, the
WWTP has experienced high effluent TSS levels. When the facility's National
Pollutant Discharge Elimination System (NPDES) permit is modified, more
stringent effluent TSS limits will be incorporated based on the OCPSF effluent
guidelines, Without significantly improved effluent quality, effluent filtration will
be required. At the historical effluent TSS levels experienced by the WWTP,
conventional filters will not provide the reliability needed for permit compliance.
In 1990, influent sampling indicated that the poor effluent quality could be the
result of nutrient limitations. Nitrogen was identified as a limiting macronutrient,
but effluent quality did not substantially improve when only nitrogen additions
were increased. After a program was implemented to add micronutrients
concurrently, effluent quality significantly improved. It was concluded that
micro- and macro-nutrient additions coupled with effluent filtration will be
required to meet the future OCPSF TSS limits.

BACKGROUND

Hoechst Celanese Corporation produces industrial cellulose Hoechst Celanese's current and anticipated future NPDES
acetate (CA) resin, filament, and tow at its plant in Narrows, permit limits are summarized in Table 1. The anticipated future
Virginia. Process wastewater, consisting primarily of acetic NPDES permit limits for maximum monthly average BOD and
acid, is treated onsite prior to discharge to the New River. As TSS are approximately 19 and 15 percent, respectively, of the
shown in Figure 1 , the existing wastewater treatment plant current NPDES permit limits.
(WWTP) includes screening, equalization, biological treatment
(activated sludge), and secondary clarification. Sanitary waste-
water is pretreated in an Imhoff tank and combined with the
process wastewater for treatment. Waste sludge from the treat-
ment process is thickened and aerobically digested prior to
being pumped to onsite dewatering ponds. The National Pol-
lutant Discharge Elimination System (NPDES) Permit under
which the WWTP is operating expires in April 1992. The Vir-
ginia Water Control Board is currently modifying this permit
to incorporate the final effluent guidelines for the Organic
Chemicals, Plastics, and Synthetic Fibers (OCPSF) subcate-
gory which were published in November 1987. These guidelines
establish stringent limits for biochemical oxygen demand
(BOD), total suspended solids (TSS), and priority pollutants. FIGURE 1. Existing WWTP

80 February, 1992 Environmental Progress (Vol. 11, No. 1)

 Table 1 Current and Anticipated Future NPDES Permit Limits
NPDES Permit Limits ( k d d )
Current Anticipated OCPSF OCPSF/Current
Maximum Maximum Maximum
Monthly Daily Monthly Daily Monthly Daily
Parameter Average Maximum Average Maximum Average Maximum
BOD 973 1877 184 378 0.19 0.20
TSS 2023 3170 298 950 0.15 0.30

As shown in the first half of Figure 2, the WWTP historically

experienced high effluent TSS. The sludge did not settle well
and the cloudy effluent contained dispersed floc. This situation
has made compliance with current NPDES permit limits dif-
ficult. When the more stringent solids limits are finalized, the
WWTP will not be able to maintain permit compliance without
operational and/or unit process improvements.
A WWTP improvement program is currently under way
which includes adding, among other things, effluent filtration.
However, at the historical effluent TSS levels, the operability
of the filters was in question. It is considered to be imperative I-A*-89 lo-Ocl-89 IP-DecC-89 27-Feb-W 8-Mw-W 17-Jul-4) 26-W-W 5-k-W

that solids settleability be improved both now and in the future FIGURE 2. Effluent Quality
to maintain filter performance and consistently comply with
permit requirements. the WWTP routinely contains adequate concentrations of
phosphorus. Therefore, nitrogen was believed to be causing
the solids settleability problem. In nutrient limiting conditions,
OBJECTIVES microorganisms convert influent organic substrate (COD) to
storage products such as extracellular polymer rather than
synthesizing new cells.
The objectives of this study were to identify the cause(s) of To reduce short-circuiting and low DO concentrations in the
poor solids settleability and to develop a program to overcome aeration basin, the rotation of selected aerators was reversed,
these limitations for both current and future NPDES permit while other aerators were relocated. A second DO survey con-
compliance. ducted after these modifications indicated improved distri-
bution of oxygen throughout the basin. A urea storage tank
and metering pump were also installed to replace the manual
WWTP Evaluation method of nitrogen addition historically used at Hoechst Ce-
lanese. These modifications, which were made from late 1988
In 1988, a WWTP evaluation was conducted to assess unit to early 1990, did not significantly improve WWTP perform-
process performance and identify limitations. A chemical ox- ance, and possible micronutrient limitations were identified.
ygen demand (COD) balance around the aeration basin indi- In April 1990, a program was implemented to correct these
cated that aeration capacity was inadequate for peak waste micronutrient deficiencies.
loads. A dissolved oxygen (DO) survey of the aeration basins
also indicated that low DO conditions existed in the basin even
during average waste load conditions, indicating short-cir- MICRONUTRIENT EVALUATION
cuiting and poor mixing. A microscopic examination of the
mixed liquor indicated the presence of significant amounts of Procedures
extracellular polymeric material, which negatively impacts
solids settleability. The examination also indicated that very On three separate occasions, 7-day composite samples of
few filamentous organisms were present. The floc stained Neis- influent, mixed liquor, and effluent samples were collected and
ser-positive and contained many clumps of bacteria which were the mixed liquor samples digested. The filtered influent, di-
strongly Neisser-positive. These features suggest a nutrient de- gested mixed liquor, and filtered effluent samples were ana-
ficiency, likely in nitrogen or phosphorus. The effluent from lyzed for micronutrients. Using the mixed liquor TSS

Table 2 Micronutrient Analysis

Cell Dry Weight (Vo) Comment
Parameter Typical June ’89 Nov. ’89 Jan. ’91
Calcium 0.5a 1.4 0.7 1.6 -
Iron 0.2” 0.24 0.14 0.26 Marginal
Potassium 1 .O” 0.29 <O.lS 0.67 Deficient
Magnesium 0.5” >0.5 >0.5 >0.5 -
Manganese O.Olb NA 0.01 0.01 -
Sodium 1.O” 1.8 0.8 1.1 -
Zinc O.Olb NA 0.01 0.02 -

NA = Not Available
a Gaudy, Anthony F., Jr. and Elizabeth T., Elements of Bioenvironmental Engineering. Engineering Press, Inc., San Jose, California (1988)
Metcalf and Eddy, Inc., Wastewater Engineering: Treatment/Disposal/Reuse. McGraw-Hill Book Company, New York (1979).

Environmental Progress (Vol. 11, No. 1) February, 1992 81

 Table 3 ImDact of WWTP ODeratina Conditions on Effluent TSS
Impact on
Time Period Condition Effluent TSS (mn/l)
February through May 1987 Non-limiting nitrogen conditions 55
August 1989 through March 1990 Routinely nitrogen and potassium-limited 180
April through mid-May 1990 Nitrogen limited, potassium sufficient, added ferric chlo- 60
ride to aid settling
Mid-May through June 1990 Sludge age 20 days 16
Non-limiting nitrogen and potassium conditions
July through December 1990 Routinely nitrogen limited, decreased target sludge age to 59
15 days

concentrations from the samples, the percentage of the cells’ Data from August 1989 through March 1990 indicate that
dry weight for each micronutrient was calculated and compared nitrogen and potassium limitations existed. Iron was probably
to typical values referenced in the literature. Micronutrient marginal as well. The sludge age was 15 days, indicating that
limitations were considered to exist when the measured per- the nutrient requirements were approximately the same as in
centage was less than the literature values. As shown in Table February through May 1987. Because nitrogen was limited as
2, it appeared that the WWTP was deficient in potassium and well as micronutrients, effluent TSS levels were very high at
marginally sufficient in iron. Potassium addition was initiated 180 mg/l.
in April 1990. In April through May 1990, solids settleability improved and
effluent TSS levels began to go down. It was in April that
potassium hydroxide feed facilities were installed and ferric
DISCUSSION OF RESULTS chloride was periodically added to the system. The effluent
TSS levels were at approximately the same level as in February
through May 1987, probably because of a macronutrient lim-
To assess the ability of the micronutrient additions to im- itation-nitrogen.
prove solids settleability, WWTP historical data beginning in In mid-May 1990 the sludge age was increased to 20 days,
1987 were reviewed. Five time periods representative of various which decreased the nutrient requirements, and the nitrogen
operating conditions were identified (see Table 3). Data from addition was increased. The potassium hydroxide feed was
August 1989 through December 1990 are presented in Figures continued; however, the ferric chloride additions were discon-
3, 4, and 5 to illustrate the impacts of various operating con- tinued. From mid-May through June, effluent TSS levels were
ditions. Nitrogen deficiencies are identified to differentiate the very low at 16 mg/l. There were no nitrogen or potassium
impacts of micronutrient deficiencies. Sludge age (SRT) is also limitations and probably no iron limitation. The iron has al-
noted because as SRT increases, nutrient requirements de- ways been marginal in dry cell analysis, but with the previous
crease. month’s additions, there may have been additional iron stored
In February through May 1987, the WWTP operated with in the system.
non-limiting nitrogen conditions and a sludge age of 15 days.
No micronutrient additions were made. The solids exhibited
poor settling characteristics and the effluent TSS concentration
was high at 55 mg/l.

* Marginal I
JQI-l kb-10 Mm-Z May-’ Jm-I0

3
-4m
Jan-l kb-10 Mar-22 May 1 Jun-10

I
AWI rp 10 osi-m NW-?P k-al Jan 1 kblO Morn M q l JUI lo

FIGURE 3. WWTP Historical Trends FIGURE 4. WWTP Historical Trends

August- December 1989 January-June 1990

82 February, 1992 Environmental Progress (Vol. 11, No. 1)

 ------ Sufficient

2-23
Deficient
' ' Marginal
Dewotenng

Thickener

To Landfill Sludge Aerobic

Dewotenng Digestion

FIGURE 6. Future WWTP

JU 1 AWJ 10 SWIM O C I ~ ~ e c 9

2m has adversely impacted its compliance with current NPDES

permit limits. When the OCPSF guidelines are incorporated
into Hoechst Celanese's new NPDES permit, compliance will
be virtually impossible without WWTP improvements.
As shown in Figure 6 , in addition to increased equalization
capacity and aboveground biological treatment facilities, ef-
-4w fluent filters will be constructed to allow the plant to consist-
Jd I Aug I 0 Sop120 Oclu) Dec 9 ently meet the new limits. Without consistently good solids
settleability, filter performance and permit compliance will be
negatively impacted.
This study not only supported the widely accepted impor-
tance of macronutrient supply of nitrogen but also showed the
importance of micronutrient supply. Even with the non-nitro-
gen limiting conditions, deficiencies in potassium or iron re-
sulted in poor solids settleability and increased effluent TSS.
o-- ~- 1 1 1 Control of sludge age can also be used in this regard since an
JUI i AWJ 10 SOPI m OCIM ~ e 9c
increased sludge age results in a decreased nutrient demand.
FIGURE 5. WWTP Historical Trends As shown by the results from mid-May through June, Hoechst
July-December 1990 Celanese can achieve good solids settleability and low effluent
TSS by supplying the activated sludge system with sufficient
In July through December 1990, effluent TSS began to in- quantities of iron and potassium, as well as nitrogen.
crease to an average of 59 mg/l-almost identical to April
through mid-May 1990, when nitrogen was also limited and
micronutrients were not. The sludge age was decreased to ap-
proximately 15 days, which contributed to the nitrogen limi- ACKNOWLEDGMENTS
tation.
Clifford Randall, Ph.D., P.E., has provided WWTP op-
CONCLUSIONS erations assistance at Hoechst Celanese for many years. Por-
tions of the data collected by the WWTP operated at his
The poor solids settleability and high effluent TSS which direction were used in this analysis. Dr. David Jenkins con-
has been historically experienced at Hoechst Celanese's WWTP ducted the microscopic analyses on the sludge.

Environmental Progress (Vol. 11, No. 1) February, 1992 83