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Achieving extremely low effluent phosphorus in wastewater treatment
A combination of discharges during critical stream conditions creates new challenges for wastewater dischargers. The most challenging combination occurs when a restrictive total maximum daily load (TMDL) is combined with inadequate stream conditions that provide little or no effluent dilution. In this situation, in-stream phosphorus concentration targets for restoration of water quality must be met end-of-pipe in the treatment plant effluent and may be at, or below, the limits of treatment technology.
The city of Las Vegas Water Pollution Control Facility, the city of Coeur d’Alene, Idaho, and the Spokane County, Wash., Wastewater Treatment Plant all have evaluated alternatives to achieve effluent TP that is approaching the limits of technology. There remains considerable debate as to the limit of technology; some suggest as high as 0.05 mg/L, while others suggest below 0.01 mg/L. Further, it is important that effluent permits be written with recognition that these extremely low concentrations cannot be treated as maximum values because daily variations in treatment performance are such that only median or mean limits over long periods are achievable. This article reviews the current technologies that can be applied to achieve extremely low effluent phosphorus concentrations.
First, the theoretical limits of the lowest phosphorus that can be obtained for different removal mechanisms are discussed. Then, for each current available treatment technology alternative, a brief description of the process and representative pilot or full-scale performance data is presented.
Phosphorus is present in wastewater primarily in two forms: organic phosphorus and orthophosphate. Other chemical species (chemical precipitants, etc.) can be present, usually in smaller quantities. TP is typically the sum of organic and orthophosphate.
Phosphorus can be removed via biological, chemical, or combined biological and chemical processes. Biological phosphorus removal relies on the function of a specific group of microorganisms that are capable of taking up phosphorus as intracellular storage, and the phosphorus is removed from the liquid by sludge wasting. Full-scale performance data indicate that effluent TP less than 1 mg/L is reliably achieved with biological systems. The lowest effluent TP observed at biological wastewater treatment facilities ranges from 0.1 to 0.3 mg/L. Orthophosphate concentrations as low as 0.02 mg/L have been measured, below which the phosphorus concentration becomes limiting for biological reaction. Because of the limiting phosphorus level in biological systems and the susceptibility of biological processes to disturbance from process and environmental condition changes, chemical phosphorus removal is typically necessary to achieve even lower phosphorus levels and stable performance.
Chemical phosphorus removal is brought about by the addition of salts of multivalent metal ions to form precipitates of sparingly soluble phosphate. The commonly used chemicals are aluminum (Al(III)), ferric iron (Fe(III)) and calcium (Ca(II)). The chemistry of phosphorus precipitation with iron or aluminum is quite complex due to the formation of various metal phosphorus complexes and metal hydroxyl complexes, as well as adsorption of phosphorus onto the precipitates. Depending on the dose ratio, either metal phosphate precipitation alone occurs, or both metal hydroxide and metal phosphate precipitation occurs. The lowest soluble phosphorus level that can be obtained is determined by the solubility of phosphorus, which depends on the dose ratio and pH. At pH around 7, the lowest residual soluble phosphorus concentration observed is 0.01 to 0.02 mg/L for aluminum and 0.04 to 0.05 mg/L for Fe(III). Residual soluble phosphorus less than the solubility limit has been observed as a result of adsorption of phosphorus onto the precipitates or onto adsorbents added. With calcium and at high pH, residual soluble phosphorus less than 0.006 mg/L can be obtained.
Several treatment plants have successfully achieved low effluent phosphorus limits with a combination of biological or primary chemical phosphorus removal followed by tertiary polishing. These include the city of Las Vegas using direct filtration as a tertiary polishing step to achieve 0.17 mg/L TP and Clean Water Services in Oregon achieving 0.07 mg/L TP with a tertiary sedimentation/filtration process.
Effluent TP consists of particulate phosphorus and soluble phosphorus. To achieve effluent TP concentrations close to the soluble limit, particulate phosphorus has to be completely removed. Solids and liquid separation processes that provide near complete removal of particulates (total suspended solids and colloids) are therefore required. These processes include deep-bed dual-stage filtration, microfiltration or membrane biological reactor (MBR) technology. For treatment goals of effluent TP even lower than the solubility limits, adsorption using adsorbent media or reverse osmosis (RO) will be required.
The treatment technologies that have been successful in achieving very low effluent TP at pilot and/or full-scale applications that are commercially available include: dual-stage Parkson Dynasand filtration, Blue Water BluePro technology, microfiltration and MBR processes, and microfiltration followed by RO.
Dual-stage sand filtration
The Parkson Dynasand D2 system consists of two continuous self-cleaning filters in a series to achieve high filtration efficiency. The first-stage filter uses large sand grain to provide more solids-handling capacity. The second filter acts as a polishing unit, utilizing smaller sand grain and providing higher filtration efficiency. The third component of the system is the Lamella gravity settler, which treats rejects from both filters. Chemicals are usually dosed before the first filter. In wastewater treatment applications, the manufacturers report effluent TP is typically on the order of 0.01 to 0.05 mg/L.
Blue Water technology
The Blue Water BluePro technology process includes a pre-reactor for chemical mixing and a moving-bed reactive filter filled with iron oxide-coated sand (IOCS). IOCS is an adsorbent media with high affinity for many pollutants. Therefore, the reactive filter provides both filtration and adsorption effects. Average effluent TP of 0.06 mg/L was achieved at a pilot study in Moscow, Idaho, and near 0.02 mg/L was achieved in a pilot study in Coeur d’Alene, Idaho.
Microfiltration & MBR with chemical addition
Microfiltration processes and MBRs both use membranes for solids and liquid separation. The pore size of membranes range from 0.04 to 0.2 micrometers. The process can eliminate nearly all particulate materials, including chemical precipitants, and produce effluent TP close to the solubility limit. Effluent TP of 0.02 to 0.05 mg/L was obtained with MBR processes at full-scale plants. Using microfiltration as tertiary treatment, effluent TP of less than 0.01 mg/L was achieved at influent TP of 0.08 mg/L.
The USFilter Trident HS-1 process is a modification of the original Trident dual media filter, which has been used predominately for the treatment of drinking water. The process includes a tube clarification stage followed by a buoyant adsorption media clarifier and finally mixed media filtration. Multiple chemical feed points are provided for ferric or alum, polymer and chlorine solution. Average effluent TP of approximately 0.02 mg/L was achieved in recent pilot studies.
Microfiltration with membranes followed by RO is currently the most advanced treatment, and it can produce effluent TP below the normal detection limit (0.005 mg/L). The membrane filtration removes nearly all particular phosphorus. RO further removes the soluble phosphate by approximately 95 to 99% to achieve very low effluent phosphorus levels.
A number of developing technologies are capable of producing low effluent TP as evidenced by laboratory and pilot-scale studies. The developing technologies include packed columns filled with adsorbent material such as iron oxide particles, blast furnace slag, zeolite, iron/calcium oxides, crushed limestone and activated aluminum oxide, etc. The high adsorption capacity of these materials enhances adsorption of soluble phosphate ion and may therefore possibly bring effluent soluble TP below solubility limits. In addition, engineered wetlands using additional adsorbent on the system floor with selected micro-plants and microbes, and with controlled flow rates and possible recycle flows, have been shown to produce effluent soluble P<0.01 mg/L. Immobilized bacteria, or immobilized micro-algae on hollow cellulose fibers/beads at lab-scale, produced effluent soluble P<0.02 mg/L. Potential application of these developing technologies to achieve low phosphorus still requires larger scale studies and economical and feasibility evaluations.
Increasingly stringent effluent limits on phosphorus demand non-traditional treatment technologies. There are theoretical limits for the lowest phosphorus that can be achieved with different removal mechanisms. Plants using a combination of biological or chemical phosphorus removal followed by tertiary polishing have successfully achieved low effluent phosphorus levels, such as the city of Las Vegas (0.17 mg/L TP) and Clean Water Services, Oregon (0.07 mg/L TP).
Questions remain about the limits of technology and whether total phosphorus levels as low as 0.05 mg/L, or lower, can be accomplished and sustained at full-scale facilities of significant size. Removal of particulates and colloidal material becomes necessary in order to achieve extremely low phosphorus levels, which requires advanced solids and liquid separation processes such as membrane filtration or multi-stage filtration. Processes that enhance phosphorus adsorption show potential for achieving extremely low phosphorus levels.
The technologies discussed here show promise. Results from bench and pilot studies have shown good results under the well-controlled testing conditions. How reliable the results will be under full-scale, variable loading and environmental conditions (temperature, pH, water chemistry, etc.) still remains to be demonstrated. Site-specific conditions, including existing treatment facilities, wastewater characteristics and water chemistry, are all expected to be significant factors influencing the levels of effluent performance that can be attained.
Further testing and full-scale operating experience with these technologies will determine how reliably extremely low levels of effluent phosphorus can be achieved. It is important that wastewater utilities participate in water quality studies, TMDLs and discharge permit negotiations related to extremely low effluent phosphorus limits with the most complete understanding possible of the available treatment technologies, level of development for full-scale application and realistic effluent performance potential.