Full Description

The City of Houston has tested many coagulants over the past several years in anticipation of the necessity for enhanced coagulation required by the D/DBP Rule. Many coagulants have shown promising results, but one coagulant in particular has been able to enhance water quality and provide a higher level of TOC removal. These benefits have been realized without the increased costs traditionally associated with enhanced coagulation processes. Plant scale tests using acidified alum have shown that the overall TOC removal could be increased by 15%, or more, compared to traditional alum treatment. In plant scale studies it was found that the alum dosage, (acid alum vs. standard alum, on a dry basis) could be reduced significantly, as much as 40% depending on raw water conditions. Water treated with acidified alum produced a more dense and settleable floc than water treated with a comparable amount of standard alum. Less sludge was produced while achieving a higher quality finished water. The initial plant trial was conducted during mid and late summer, a time that historically has been the most difficult to remove TOC and the highest potential for DBP formation. The total DBPs formed during the trial were significantly lower than in previous years, even though only half of the plant was using acidified alum. Perhaps the most significant, and unexpected, benefit to using an acidified alum product was a marked improvement in the settled water filterability. The filterability improved to the point that the Phase IV Partnership goals for filter ripening following backwash are now a realistic and an achievable goal. During the trial it was found that the coagulation pH using acidified alum was only slightly lower than that of the coagulation pH for the process using standard alum. No significant increase was necessary in the lime or caustic feed to achieved stability pH, however it was found that applying the lime further into the process yielded an even greater TOC reduction. Using acidified alum will not require additional feed equipment, but will require feed equipment that is compatible with an acid based product. Acidified alum is more aggressive than standard alum and requires more care in handling. Equipment that meets the design specifications for standard alum will probably not require modifications, but an evaluation of the existing feed system is recommended to insure the equipment is in good working order and that it is compatible with the more aggressive acid based product. Evaluation of the data collected during the plant trial suggests that an acidified alum product can be used to achieve higher quality water at a reduced overall operating cost. The City of Houston is continuing to evaluate the benefits of using an acidified alum product. As the trials continue and the benefits are quantified, the City expects that acidified alum will replace standard alum in the City of Houston's treatment facilities. Includes figures.

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Edition: Vol. - No. Published: 11/01/2002 Number of Pages: 12File Size: 1 file , 280 KB