1.46 BMP C252 - Treating and Disposing of High pH Water
When pH levels in stormwater rise above 8.5 it is necessary to lower the pH levels to the acceptable range of 6.5 to 8.5 prior to discharge to surface or groundwater. A pH level range of 6.5 to 8.5 is typical for most natural waterbodies and is ideal for the survival of aquatic organisms. Fish and other aquatic organisms may become stressed and die if the pH drops out of this range.
The water quality standard for pH in Washington State is in the range of 6.5 to 8.5. Stormwater with pH levels exceeding water quality standards may be either neutralized on site or disposed of to a wastewater system or concrete batch plant with pH neutralization capabilities.
Neutralized process water such as concrete truck wash-out, hydro-demolition, or saw-cutting slurry must be managed to prevent discharge to surface waters. Any stormwater contaminated during concrete work is considered process wastewater and must not be discharged to waters of the State or stormwater collection systems.
The process used for neutralizing and/or disposing of high pH stormwater must be documented in the Construction Stormwater Pollution Prevention Plan.
High pH at construction sites is most commonly caused by the contact of stormwater with poured or recycled concrete, cement, mortars, and other Portland cement or lime-containing construction materials. (See BMP C151 - Concrete Handling for more information on concrete handling procedures). The principal caustic agent in cement is calcium hydroxide (free lime).
Calcium hardness can contribute to high pH values and cause toxicity that is associated with high pH conditions. A high level of calcium hardness in waters of the state is not allowed. Groundwater standard for calcium and other dissolved solids in Washington State is less than 500 mg/l.
1.46.3 Treating High pH Stormwater by Carbon Dioxide Sparging
Rapidly neutralizes high pH water.
Cost effective and safer to handle than acid compounds.
CO2 is self-buffering. It is difficult to overdose and create harmfully low pH levels.
Material is readily available.
The Chemical Process of CO2 Sparging
When carbon dioxide (CO2) is added to water (H2O), carbonic acid (H2CO3) is formed which can further dissociate into a proton (H+) and a bicarbonate anion (HCO3-) as shown below:
CO2 + H2O ßà H2CO3 ßà H+ + HCO3-
The free proton is a weak acid that can lower the pH.
Water temperature has an effect on the reaction as well. The colder the water temperature is the slower the reaction occurs and the warmer the water temperature is the quicker the reaction occurs. Most construction applications in Washington State have water temperatures in the 50°F or higher range so the reaction is almost simultaneous.
Treatment Procedures of CO2 Sparging
High pH water may be treated using continuous treatment, continuous discharge systems. These manufactured systems continuously monitor influent and effluent pH to ensure that pH values are within an acceptable range before being discharged. All systems must have fail safe automatic shut off switches in the event that pH is not within the acceptable discharge range. Only trained operators may operate manufactured systems. System manufacturers often provide trained operators or training on their devices.
The following procedure may be used when not using a continuous discharge system:
Prior to treatment, the appropriate jurisdiction should be notified in accordance with the regulations set by the jurisdiction.
Every effort should be made to isolate the potential high pH water in order to treat it separately from other stormwater onsite.
Water should be stored in an acceptable storage facility, detention pond, or containment cell prior to treatment.
Transfer water to be treated for pH to the pH treatment structure. Ensure that the pH treatment structure size is sufficient to hold the amount of water that is to be treated. Do not fill the pH treatment structure completely, allow at least 2 feet of freeboard.
The operator samples the water within the pH treatment structure for pH and notes the clarity of the water. As a rule of thumb, less CO2 is necessary for clearer water. The results of the samples and water clarity observations should be recorded.
In the pH adjustment structure, add CO2 until the pH falls in the range of 6.9-7.1. Adjusting pH to within 0.2 pH units of receiving water (background pH) is recommended. It is unlikely that pH can be adjusted to within 0.2 pH units using dry ice. Compressed carbon dioxide gas should be introduced to the water using a carbon dioxide diffuser located near the bottom of the tank, this will allow carbon dioxide to bubble up through the water and diffuse more evenly.
Slowly discharge the water making sure water does not get stirred up in the process. Release about 80% of the water from the pH treatment structure leaving any sludge behind.
Discharge treated water through a pond or into the stormwater system.
Excess sludge needs to be disposed of properly as concrete waste. If several batches of water are undergoing pH treatment, sludge can be left in treatment structure for the next batch treatment. Dispose of sludge when it fills 50% of tank volume.
Dispose of sludge per applicable local, state, and federal regulations.
Treating High pH Stormwater using Food Grade Vinegar
Food grade vinegar that meets FDA standards may be used to neutralize high pH water. Food grade vinegar is only 4% to 18% acetic acid. The remainder is composed of water. Food grade vinegar may be used if dosed just enough to lower pH sufficiently. Use a treatment process as described above for CO2 sparging, but add food grade vinegar instead of CO2.
This treatment option for high pH stormwater does not apply to anything but food grade vinegar. Acetic acid is not vinegar. Any other product or waste containing acetic acid must be approved by the Washington State Department of Ecology.
Disposal of High pH Stormwater
Discharges to the wastewater system are only allowed if approved per BMP C254 - Discharge to Wastewater System.
Only permitted facilities may accept high pH water.
Contact the facility to ensure they can accept high pH water
All equipment should be handled in accordance with OSHA rules and regulations.
Follow manufacturer guidelines for materials handling.
A diagram of the monitoring and treatment equipment
A description of the pumping rates and capacity the treatment equipment is capable of treating.
Each operator should keep a written record of the following:
Amount of CO2 needed to adjust water to a pH range of 6.9 - 7.1
Discharge location and description
A copy of this record should be given to the client/contractor who should retain the record for three years.