ENHANCED COAGULATION DOESN'T HAVE TO BREAK THE BANK: YOU CAN PRODUCE HIGHER QUALITY WATER AND CUT YOUR BUDGET

摘要

The City of Houston has tested many coagulants over the past several years in anticipation of thenecessity for enhanced coagulation required by the D/DBP Rule. Many coagulants have shown promisingresults, but one coagulant in particular has been able to enhance water quality and provide a higher levelof TOC removal. These benefits have been realized without the increased costs traditionally associatedwith enhanced coagulation processes.Previous enhanced coagulation studies confirmed that depressing the pH of the raw water was aneffective way to enhance TOC removal. In these earlier studies, the reduced coagulation pH was achievedthrough one of two methods, either through acid addition, or through the addition of excess coagulant (aless effective method for adjusting the pH). Enhanced coagulation may solve the problem of removingTOC, but it creates other problems that must be dealt with. Operators implementing enhanced coagulationmeasures must be careful not to start a cycle of reactionary chemical adjustments, or they may find theirchemical cost spiraling out of control. The more coagulant added, the more lime or caustic needed tostabilize the finished water. As the alum dosage is increased, so does the amount of sludge produced. Andwhere post filtration pH adjustment is needed, significant pH adjustments can result in post precipitationcausing an increase in the finished water turbidity. All of these problems are significantly attenuated whenusing an acidified alum product.Plant scale tests using acidified alum have shown that the overall TOC removal could beincreased by 15%, or more, compared to traditional alum treatment. In plant scale studies it was foundthat the alum dosage, (acid alum vs. standard alum, on a dry basis) could be reduced significantly, asmuch as 40% depending on raw water conditions. Water treated with acidified alum produced a moredense and settleable floc than water treated with a comparable amount of standard alum. Less sludge wasproduced while achieving a higher quality finished water.The initial plant trial was conducted during mid and late summer, a time that historically has themost difficult to remove TOC and the highest potential for DBP formation. The total DBPs formed duringthe trial were significantly lower than in previous years, even though only half of the plant was usingacidified alum. Perhaps the most significant, and unexpected, benefit to using an acidified alum productwas a marked improvement in the settled water filterability. The filterability improved to the point thatthe Phase IV Partnership goals for filter ripening following backwash are now a realistic and anachievable goal. During the trial it was found that the coagulation pH using acidified alum was onlyslightly lower than that of the coagulation pH for the process using standard alum. No significant increasewas necessary in the lime or caustic feed to achieved stability pH, however it was found that applying thelime further into the process yielded an even greater TOC reduction.Using acidified alum will not require additional feed equipment, but will require feed equipmentthat is compatible with an acid based product. Acidified alum is more aggressive than standard alum andrequires more care in handling. Equipment that meets the design specifications for standard alum willprobably not require modifications, but an evaluation of the existing feed system is recommended toinsure the equipment is in good working order and that it is indeed compatible with the more aggressiveacid based product. Evaluation of the data collected during the plant trial suggests that an acidified alumproduct can be used to achieve higher quality water at a reduced overall operating cost. The City ofHouston is continuing to evaluate the benefits of using an acidified alum product. As the trials continueand the benefits are quantified, the City expects that acidified alum will replace standard alum in the Cityof Houston's treatment facilities.