Prescribed pretreatment for limiting constituents in reclaimed water concentrate

Reclaimed water from the Sub-Regional Operating Group (SROG) cities of Glendale, Mesa, Tempe, Phoenix, and Scottsdale is used for beneficial uses such as turf or crop irrigation, cooling water for power plants, and groundwater recharge/indirect potable reuse. The total dissolved solids (TDS) in the reclaimed water (usually above 1,000 mg/L) as well as the concentrations of individual ions (such as sodium, chloride, hardness, silica) adversely impact the reclaimed water reuse potential. SROG recently funded a brine minimization demonstration project to execute pilot-scale testing on cost-effective concentrate management technologies. The project primarily aims at achieving near Zero-Liquid-Discharge from the City of Scottsdale Water Campus RO concentrate (i.e. target overall recovery ≥ 97-98%) and producing water suitable for various end uses. To overcome the limiting fouling and scaling constituents (organics, calcium sulfate, calcium phosphate, barium sulfate, strontium sulfate, etc.) that cause membrane fouling and limit the overall recovery, a series of brine pretreatment technologies are being investigated at bench and pilot scale, including MIEX, advanced oxidation process (AOP) followed by biological filtration, coagulation, softening, ion exchange, and adsorption processes such as granular activated carbon (GAC). Many of these technologies can reduce the levels of the target inorganic and organic constituents, but are not cost effective at the high concentrations present in the RO concentrate. For example, 40% of total organic carbon (TOC) can be removed (from 40 mg/L to 24 mg/L DOC) using coagulation but this method requires a coagulant dose of 500 mg/L ferric chloride. GAC can remove over 90% of the organics, but the carbon usage and regeneration frequency is cost prohibitive. MIEX resins (chloride form) can remove 40-60% of the brine TOC despite high sulfate concentration and the sodium form can remove hardness. However, resin regeneration, which produces another brine stream that must be handled, still requires further testing. Previous projects conducted at the same facility demonstrated that advanced oxidation processes such as ozone and H₂O₂, UV and H ₂O₂, or UV and TiO₂ can remove 90+% organics, converting organics and micro-pollutants into carbon dioxide. But the energy consumption for addressing organics alone using such technologies is comparable to treating the concentrate using thermal brine concentrators and crystallizers. Ideally biological treatment could provide high organic removal cost effectively. But going through a nitrification-denitrification activated sludge wastewater treatment plant, the organics present in the concentrate are low in biodegradable fractions (40 mg/L TOC, 0 mg/L BDOC). For this project, it is proposed to use low dose advanced oxidants (i.e. ozone and H₂O ₂). This approach would be affordable since the applied doses would be sufficient to break the organic compounds into biodegradable fractions but not high enough to mineralize them. Subsequent, engineered, biological filtration techniques are proposed to remove organic compounds. Bench testing and modeling work started in September 2010 and will be completed by April 2011. One-year pilot testing will start in Summer 2011. Preliminary bench results at Arizona State University indicated that the advanced oxidation process reduced DOC from 40 to around 20 mg/L, which includes 13 mg/L of BDOC. Assuming the biological filter can remove most of the BDOC, the process removed ∼82% organics. The results also indicated that the process could remove hardness and phosphate through forming white precipitations (Calcium Oxalate, Calcium Phosphate), which helps to address the inorganic limiting constituents in the same step. This presentation will focus on the concentrate pretreatment technology testing results, summarizing the performance of various treatment proposed processes in removing each limiting constituent in the brine. 2011