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.

Table 1: Summary of treatment outcomes. Note the reduction in secondary clarifier effluent color levels following Ozonation
Figure 4: Graphical Representation of Results. Color is reported as Platinum Cobalt color Equivalents, mg/l. note enhancement to color removal in biological basin due to ozone recycle

Figure 5: savings enabled by ozone decolorization