(640b) Color Removal and Biosolids Reduction with Ozone in Textile Plant Wastewater Treatment

Color
Removal and Biosolids Reduction with Ozone in Textile Plant Wastewater
Treatment

Paulo Bon^a, Andre Rambor^a,
Malcolm Fabiyi^b, Karen Connery^b, Richard Novak^b

^aWhite
Martins Gases Industriais Ltda. Sao Paulo, SP, Brazil.

^bPraxair,
Inc. 7000 High Grove Boulevard, Burr Ridge, IL, USA

Introduction

In order to reduce the
costs associated with tertiary physical-chemical color removal (using Aluminum
Sulfate and a combination of polymers as flocculants and coagulants), an ozone
decolorization process was developed at textile wastewater plant which employed
high-purity oxygen activated sludge treatment. The treatment targets for color
and turbidity are ≤200 mg/l PtCo color units and 50 NTU respectively.  The
plant treats 4,200 m³/day of wastewater with an average influent COD
value of 1,650 mg/l, basin MLSS of 2,500 mg/l, and F/M in the range 0.5 ? 0.6
kg COD/kg SS/day.  High levels of COD and color removal are achieved in the
process (Table 1, Figure 1a & 1b).  Substantial cost reduction over the
previous chemical tertiary treatment process was demonstrated, as well as
reduction in waste sludge.

Status

The ozone
decolorization scheme was implemented in 2008 and has operated continuously
since. 70 mg/l of ozone is applied to the secondary clarifier effluent. 65% of
the ozone is consumed in the ozone contactors. The residual gas from the
ozonation process comprising up to 3.5% ozone (w/w) and about 2.5 mTons/day of
recovered pure oxygen are recycled back to the biological basin. The DO level
in the ozonated tertiary effluent is raised to an average of about 15 mg/l. This
type of recycle scheme which leverages the O₂ content in the vent
gas stream for providing a portion of the O₂ demand in the
biological basin and utilizes the residual ozone to achieve incremental color
removal in the biological basin has been implemented at a number of White
Martins operated wastewater facilities for close to a decade.

The recycled O₂-O₃
stream is dissolved into the biological basin using a mechanically agitated
contactor which is able to aspirate the vent gas stream. About 70% of the
oxygen supply to the biological basin is provided using the O₂ in
the vent gas stream.

Methodology

12 kg/hr of ozone at
10% w/w, produced using a Wedeco SMO 800S ozone generator, is dosed to an ozone
contacting system made up of 3 x 7.5 m high cylindrical contacting tanks that
provide an average liquid retention time of about 30 minutes. The specific
energy associated with the generation of ozone at this facility is about 11 kWh/kg
O₃. The gas feed to the generator is pure oxygen from a liquid
oxygen supply source.  All water quality measurements were carried out using
Standard methods. UV-VIS absorbance measurements were taken using a Hach 550
Spectrophotometer which was zeroed at 395 nm. Absorbance measurements for each
sample were done across a wavelength spectrum from 395 to 905 nm.  

Results &
Observations

(a) Effect of Ozonation
on Excess Solids: The use of ozone has led to the complete
elimination of the inert tertiary sludge. Prior to ozonation, the inert sludge
from the physical chemical color removal process contributed about 6-7 tons per
day of the total plant sludge of 10 - 11 wet tons per day. Waste activated
sludge was also reduced by about 53% corresponding to a reduction in sludge
yields from 0.163 to 0.076 kg SS/kg COD removed due to the lysis action of
ozone in the biological basin. The specific ozone consumption associated with
sludge reduction is 0.25 kg O3/kg SS reduced.

 (b) Effect of
Biological and Ozone Treatment on Color: Across both the ozone
contactor and the biological basin, the specific ozone aided color removal is
13.3 kg PtCo/kg O3. UV-VIS absorbance measurements (Figure 3) indicate that the
treatment steps are clearly effective for removing constituents that are
strongly absorbing in the 400-750 nm range, which overlaps with the visible
spectrum. The ozone recycle also enhanced the color removal in the activated
sludge basin (Figure 4). Very little divergence in the absorbance measurements
of the raw vs. treated samples is observed in the 750-905 nm range.

(c) Effect of
Ozonation on Treatment Costs: Ozonation had a
significant impact on treatment costs. The use of ozone eliminated the inert
sludge generated by the previous process. Overall, the operating cost
associated with the tertiary color removal process was reduced by 82% (Figure 5).

(d) Effect of
Ozonation on Biological Treatment Capacity:
The addition of a strong oxidizing agent like ozone to an activated sludge
process elicits concern for the potential impact that it could have on the
vitality of the micro-life (Jarvik et al, 2010). The system's capacity for
wastewater treatment has been retained. No deleterious effect of ozonation on
carbonaceous removal has been observed.

References

1.      Beltran, F. J.
Ozone Reaction Kinetics for Water and Wastewater Systems, Lewis Publishers,
Florida 2004.

2.      Davis, B. The
Chemistry of Color Removal: A Processing Perspective. Proc. Of the South
African Sugar Technol Assoc. Vol 75, 2001

3.      Jarvik, O.,
Kamenev, S., Kasemets, K., & Kamenev, I. Effect of Ozone on Viability of
Activated Sludge Detected by Oxygen Uptake Rate (OUR) and ATP Measurement.
Ozone: Science & Engineering, 32, pg 408-416

4.      Reife, A., &
Freeman, H.S. Environmental Chemistry of Dyes and Pigments. John Wiley &
Sons, Inc, NY, 1996

Figure 1 a (left) shows
the effect of physical chemical process on color removal and sludge generation.
Figure 1 b (right) shows the effect of ozonation on decolorization.

Figure 2: In-Situ Oxygenation
System

Figure
3: Effect of treatment on UV-VIS absorbance. The effect of treatment on
absorbance and light transmittance (i.e., decolorization) in the 400-750nm
range of the visible spectrum can be observed.

Figure
5: savings enabled by ozone decolorization