Fact Sheet - pH Adjustment and Clarification
One of the most common approaches used to remove metals from produced water is to raise the pH, add a coagulant chemical to promote solids formation, and then use clarification to remove the resulting metals solids. In some instances, this is the only form of treatment used, while in other instances it serves as a pretreatment step before moving to a more advanced form of treatment.
Several commercial treatment facilities in Pennsylvania that accept produced water and frac flowback water use pH adjustment, coagulation, and clarification to remove metals from wastewater before discharging the water to local rivers or to a municipal wastewater treatment plant. Veil (2010) includes descriptions of site visits to four of these facilities. The photos in this fact sheet were taken at one of the four Pennsylvania facilities. The untreated water is delivered to the facility by tank truck and is unloaded into a receiving area where free oil can be skimmed and heavy solids can settle. The water next moves into a storage tank where blending of water from several truckloads occurs.
From here, the water is sent to a treatment tank. Several additional steps may occur in the treatment tank (agitation, aeration, and pH adjustment with lime or sodium sulfate [Na2SO4] to facilitate the removal of dissolved barium and other metals.
Coagulants may be added to aid in settling; then the water flows to clarifiers to settle. Compare the rust-colored water in the treatment tank photo to the clear water in the clarifier photo. The clarified water is discharged under a permit issued by the Pennsylvania Department of Environmental Protection.
The sludge from the clarifiers is dewatered if needed. Several of the facilities use a filter press to produce a fairly dry sludge cake. Sludge is often taken to a local landfill for disposal, although at one of the facilities it is disposed of in a deep coal mine to help neutralize acid mine drainage.
The facilities do a good job at removing metals, but the treatment processes used do little to reduce the TDS levels. For those facilities, the Pennsylvania Department of Environmental Protection has assessed the impact of TDS discharges on the receiving water body and established effluent limits that protect aquatic organisms.
The CoMag process incorporates additional elements to enhance the clarification process (Denney 2009; Steinke et al 2010). A coagulant is added to aid precipitation. The water flows through baffled, mixed reaction tanks for coagulation, flocculation. At this point, fine particles of magnetite, a form of iron-ore, are added to increase the density of the floc and serve as ballast. The hydraulic retention time (HRT) in the clarifier is shorter than conventional clarification because the ballasted flocs settle 20 to 40 times faster. This allows the clarifier to be smaller than a conventional unit. The magnetic ballast is recovered from the waste solids stream and recycled.
Steinke et al. (2010) present performance data from treating Marcellus Shale flowback water. The CoMag system showed very good removal of iron, manganese, turbidity, strontium, and barium.
Denney,D., 2009, “Produced Water Treatment,” Journal of Petroleum Technology, March, pp. 22-23.
Steinke, T., W. Emmers, C. Cothren, and R. Francis, 2010, “Water Recycling in the Oil and Gas Fields,” presented at the Texas A&M Water Issues and Technologies Short Course, College Station, TX, August 9-10.
Veil, J.A., 2010, “Water Management Technologies Used by Marcellus Shale Gas Producers,” ANL/EVS/R-10/3, prepared for the U.S. Department of Energy, National Energy Technology Laboratory, July, 59 pp. Available at http://www.ead.anl.gov/pub/dsp_detail.cfm?PubID=2537.