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1 <br />'.J <br />1 <br />1 <br />1 <br />1 <br />I <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />BMP C252: High pH Neutralization Using CO2 <br />Purpose <br />When pH levels in stormwater rise above 8.5 it is necessary to lower the pH levels to the <br />acceptable range of 6.5 to 8.5, this process is called pH neutralization. pH neutralization <br />involves the use of solid or compressed carbon dioxide gas in water requiring neut- <br />ralization. Neutralized stormwater may be discharged to surface waters under the Gen- <br />eral Construction NPDES permit. <br />Neutralized process water such as concrete truck wash -out, hydro -demolition, or saw -cut- <br />ting slurry must be managed to prevent discharge to surface waters. Any stormwater con- <br />taminated during concrete work is considered process wastewater and must not be <br />discharged to surface waters. <br />Reason for pH Neutralization: <br />A pH level range of 6.5 to 8.5 is typical for most natural watercourses, and this neutral <br />pH is required for the survival of aquatic organisms. Should the pH rise or drop out of this <br />range, fish and other aquatic organisms may become stressed and may die. <br />Calcium hardness can contribute to high pH values and cause toxicity that is associated <br />with high pH conditions. A high level of calcium hardness in waters of the state is not <br />allowed. <br />The water quality standard for pH in Washington State is in the range of 6.5 to 8.5. <br />Ground water standard for calcium and other dissolved solids in Washington State is <br />less than 500 mg/I. <br />Conditions of Use <br />Causes of High pH: <br />High pH at construction sites is most commonly caused by the contact of stormwater with <br />poured or recycled concrete, cement, mortars, and other Portland cement or lime con- <br />taining construction materials. (See BMP C151: Concrete Handling (p.313) for more <br />information on concrete handling procedures). The principal caustic agent in cement is <br />calcium hydroxide (free lime). <br />Advantages of CO2 Sparging: <br />Rapidly neutralizes high pH water. <br />• Cost effective and safer to handle than acid compounds. <br />• CO2 is self -buffering. It is difficult to overdose and create harmfully low pH levels. <br />• Material is readily available. <br />2014 Stormwater Management Manual for Western Washington <br />Volume 11 - Chapter 4 - Page 409 <br />