By Jordi Batlle and Josep Lluis Bisbal
Coagulants can be adapted to different industrial processes in order to improve the efficiency of chemical treatment. The two cases presented here, one from textiles and one from the chemical industry, both illustrate the need for chemicals suitable for each process and situation.
The use of water in industry—though decreasing with time—is still quite high, especially in sectors such as pulp and paper, textiles and tannery. Effluents coming from the mills become highly polluted and need to be treated in order to improve quality parameters such as turbidity, chemical oxygen demand (COD) and total phosphorus content. Moreover, more restrictive demands on the final quality of the effluents have led to additional steps to meet environmental legislation, such as reusing process water; replacing dirty processes with clean technologies; combining different treatment methods, i.e. chemical and biological; and improving existing methods.
The chemical way
Chemical treatment has been a technology widely used for this purpose, as has biological treatment. The former, however, has several advantages over other alternatives—high flexibility, easy to maintain, simple infrastructure and lower sensitivity to changes.
This treatment involves the precipitation of pollutants in a process of coagulation and flocculation. Metallic salts—usually iron and aluminium salts—are mainly used to coagulate particles, while high molecular weight polymers such as polyacrylamides are responsible for the subsequent flocculation.
Metallic salts may, however, have some disadvantages that are magnified when high doses need to be applied. These include an increase in sludge volume and production, a higher alkalinity consumption, as well as a rather high residual metal content in the treated water.
One company with experience in the field of water treatment has been developing new coagulants to overcome or minimize such difficulties while improving the efficiency of the chemical treatment process. This has resulted in a broad range of products offering tailor-made products.
Two cases in which aluminum salts have been successfully replaced by new, tailor-made coagulants, are presented in the following.
In this first example, chemical treatment was applied using aluminum sulfate as a coagulant. During the last two years, however, an increase in production has caused operational problems in the sewage water treatment with reduced efficiency as a result.
The dose of 3-4 kilograms per cubic meter (kg/m3) decreased the pH to below 5, and an addition of sodium hydroxide was necessary to restore the pH value, which in turn increased the treatment cost significantly. Other effects were a reduction of the floc size and a decrease in floc settle-ability, despite an anionic polymer addition, with important quantities of flocs being washed away during peak periods. Obviously, there was huge sludge production and the de-watering capacity was often exceeded.
The perfect blend
The target was to obtain a cost-effective coagulant, which allowed normal plant operation under present conditions. Laboratory tests were performed with several coagulants—ferric salts, PAX-XL—with negative results. Blends of inorganic salts and organic polymers were achieved with a modified aluminum-based coagulant, which also contains iron and cationic organic polymers. The coagulant was then tested in full-scale trials, which gave the following main conclusions:
- Up to 80 percent dose reduction compared to aluminum sulfate,
- Low influence on pH; no NaOH addition needed,
- Improved floc sedimentation,
- Better turbidity and color removal,
- Fifty percent reduction in sludge produced,
- Sludge de-waterability decreased and FeCl3 had to be used as a sludge conditioner, and
- Cost-efficient solution.
This second example refers to a chemical industrial company with a fermentation step as the main reaction process, and a flow of 100 cubic meters per day (m3/d). Flocculation takes place after this step. The supernatant—or clear liquid overlying settled or precipitated material—is extracted with an organic solvent to obtain the final product. The sludge is then de-watered in a centrifuge, re-suspended in water and re-flocculated. This second sludge is de-natured again either in a centrifuge or in a filter press and finally disposed in a landfill. Any supernatants and lixiviates—or soluble constituents—resulting from these steps are also extracted to increase the yield of the process.
The former treatment consisted of the successive addition of 70 kilograms per cubic meter (kg/m3) of PAX-18 (liquid polyaluminum chloride, 16.8 percent Al203) as a coagulant, 31 kg/m3 of NaOH to overcome the dramatic fall in pH and reach an optimal value of 8.7, and 0.75 kg/m3 of an anionic polyacrylamide as a flocculant. The weak points of such a treatment were the high amount of coagulant needed, which necessitated use of sodium hydroxide to reach the optimal working pH; a high residual metal content in the supernatant, and a high financial cost. Moreover, fluctuations in the pH—as a consequence of the successive addition of PAX and NaOH—could also have a negative impact on the yield of the final product.
Hitting the mark
There were clearly defined aims when trying to improve the flocculation process—the need to minimize impact on the pH and residual metal content, and increase the overall coagulant efficiency. Any potential change, however, shouldn’t interfere with the rest of the process, namely the extraction step or the de-waterability of the produced sludge.
Laboratory trials with various coagulants in the flocculation showed a good performance from blends of inorganic coagulants and organic polymers. Analyses of floc formation and settle-ability, impact on pH, turbidity of the treated effluent, de-waterability of the sludge and the lack of influence in the extraction step, pointed out the good efficiency of UPAX-33, an aluminum-based development coagulant, which also contains cationic organic polymers.
Successive full-scale trials with the aforementioned coagulant led to successful results with significantly lower doses. Therefore, good flocculation performance was achieved with 12-15 kg/m3 of UPAX-33 as a coagulant, 4.4 kg/m3 of NaOH, and 0.38 kg/m3 of anionic PAM as flocculant. The decrease in pH was, in this case, much lower than in the former treatment, which improved the yield of the final product. A drastic residual aluminum reduction was also detected (over 76 percent). This tailor-made treatment turned out to reduce the cost by up to 15 percent without considerably affecting other stages such as sludge de-watering and organic extraction.
About the authors
Jordi Batlle and Josep Lluis Bisbal work for Kemira Kemwater. This article was reprinted with permission of Steve Minett, who heads Minett Media of Cambridge, England, an independent editorial production and placement agency, covering companies such as ITT Industries, Waterlink, Kemira Kemwater and Alfa Laval AB. This article refers to 14-stage vertical turbine pumps from Goulds Pumps, an ITT Industries subsidiary. For more information, contact: +44 1954 230 250, +44 1954 232 019 (fax), email: email@example.com or website: www.minettmedia.co.uk