Phosphorus Removal and Recovery from Wastewater using Sorbent Technologies
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Phosphorus (P) is an essential nutrient in fertilizers that are necessary for food production. Wastewater may represent a renewable source of nutrients if methods for recovering P from dilute wastewater streams can be developed. Adsorption, a low cost and efficient process, has the potential to recover P from wastewater as it can transfer contaminants from the liquid to the solid phase for easy separation. This study evaluated fourteen commercial sorbents for potential phosphorus recovery from synthetic wastewater (SWW) using batch testing. Commercially available sorbents (e.g. ion exchange resins (IEX), granular ferric oxide, hybrid IEX and activated alumina) were obtained from several companies and tested for phosphate removal in a 48-hour adsorption test. Seven of the sorbents exhibited substantial phosphate removal were then tested for recovery using acidic (HCl), basic (NaOH), salt (NaCl) and basic salt (NaOH + NaCl) desorption solutions. Sorbents were evaluated with respect to P recovery from the SWW. An IEX sorbent was found to recover the largest fraction at 23 % P from the SWW; while all other sorbents recovered less than 20 % P from the synthetic wastewater. The three top performing sorbents from batch testing were chosen for column testing to investigate their potential for P adsorption and recovery with a specific target of generating a concentrated chemical desorption effluent. Sorbents included two metal oxide sorbents (granular ferric hydroxide and activated alumina) as well as an ion exchange (IEX) resin. After the sorbents were tested for P removal in column tests, chemical desorption solutions were utilized to recover P from the spent sorbents. Recovery from metal oxide sorbents was conducted using basic (NaOH) and acidic (HCl) solutions while recovery from IEX sorbent used salt (NaCl) and basic salt (NaOH + NaCl) solutions in addition to acidic and basic treatments. Sorbents were evaluated on the basis of P adsorption as well as recovery from the sorbent and the initial synthetic wastewater (SWW) stream. The IEX sorbent demonstrated the highest removal of 64 % P from the SWW, while the metal oxide sorbents adsorbed between 23 and 43 % P. Desorption using NaOH was most effective for metal oxide sorbents, which were found to recover 39 % P (granular ferric hydroxide) and 21 % P (activated alumina) from the initial SWW. Sorbent C recovered the largest quantity of P (61%) from SWW with the use of NaCl. Due to its good performance, sorbent C was used to recover P from two wastewater samples. Using NaCl, sorbent C recovered 47 and 15 % of P from secondary and final effluent samples. In addition to a shift in wastewater treatment to P recovery, wastewater treatment is also focusing on producing effluent that meets ultra-low effluent P discharge limits. In order to achieve this goal, non-reactive phosphorus (nRP) must be removed; nRP contains condensed phosphates and organic phosphorus (OP) species that are recalcitrant in secondary wastewater treatment and tend to remain in final effluents. An advanced oxidation process (AOP) which couples TiO₂/UV photolysis with ultrafiltration (UF) to oxidize and remove nRP species was tested. Tests utilizing a mixture of two OP model compounds were conducted to determine the effect of TiO₂/UV photolysis on the model compound removal and to elucidate the mechanisms of phosphorus removal; nRP was removed through adsorption and UV irradiation. The AOP was also tested for P removal from three municipal wastewaters and one automotive industry effluent. In all cases, phosphorus removal was found to occur through filtration, surface complexation onto the TiO₂ and UV oxidation. Total phosphorus removal efficiencies between 90-97 % were observed for the municipal wastewater effluents and 44 % removal was observed in the industrial effluent after treatment using AOP. Conversion of nRP to reactive P (RP) was evident during TiO₂/UV treatment of samples that had high concentrations of nRP; the total amount of phosphate liberated was not quantified due to phosphate binding to TiO₂. In summary, the AOP effectively oxidized nRP to RP, achieving a high level P removal in real wastewater effluents and retaining P on the TiO₂ solids. Investigations into P recovery by TiO₂ nanoparticles revealed that adsorption of P onto TiO₂ was due to a combination of inner sphere complex formation and calcium bridging. Precipitation of calcium phosphate was observed at pH values above 10. Recovery of P from TiO₂ after concentrating of the TiO₂ solids and application of a chemical desorption solution was assessed. Recovery with an NaOH desorption solution was minimal due to calcium phosphate precipitation while recovery using HCl was limited, releasing only 2 % of adsorbed P. Recovery from TiO₂ nanoparticles loaded with calcium phosphate precipitates was also investigated. A recovery of 35 % P was observed from TiO₂ solids via the dissolution of the precipitates.
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Holly Gray (2018). Phosphorus Removal and Recovery from Wastewater using Sorbent Technologies. UWSpace. http://hdl.handle.net/10012/13532