Chemical Removal of Phosphorus
Chemical removal of phosphorus (P) has been utilized for decades. It is easily integrated into any configuration with the addition of facilities for chemical storage and feed. It starts working quickly and is easy to turn down or turn off. Chemical treatment is generally a good choice for Water Resource Recovery Facilities (WRRFs) with one or more of the following characteristics: small flows (<0.5 mgd), extended periods of unattended operation, or discharge limits of 1.0 mg P/L or higher. For other WRRFs, the cost of chemical, chemical handling requirements, increased sludge generation, and fouling of quartz sleeves on UV disinfection lamps may offset the benefits forcing a hard look at biological P removal.
Both chemical and biological P removal methods are effective converting soluble P into particulate P. The concentration of total phosphorus (TP) in treated effluent ultimately depends on the presence and function of settling and filtration steps. A well-functioning clarifier can remove down to 0.5 mg P/L; a conventional sand filter down to 0.1 mg P/L; and a membrane filter down to 0.01 mg P/L.
Chemistry and Mechanism of P Removal
Three classes of chemicals are used for chemical P removal: iron salts, aluminum salts, and lime (calcium). Precipitation of P with lime was one of the earliest chemical treatments. Lime is inexpensive and widely available. However, handling requirements and the high pH required have limited its present usage. Iron and aluminum salts are the most common chemicals used today.
Ferrous or ferric iron may be used. Ferric salts are more effective but ferrous is easily oxidized to ferric in aerated tanks. Waste pickle liquor, a by-product of steel processing, has been used extensively as an inexpensive source of the chemical. In the 1990’s the City of Toledo Bay View Water Reclamation Facility dosed waste pickle liquor, a by-product of steel processing, to the aerated grit tanks where it would be oxidized to ferric for greater effect. Variability in quality and heavy metal content has curtailed usage of pickle liquor in recent years.
Alum is the most common aluminum salt. Sodium aluminate and polyaluminum chloride (PAC) are more expensive but do not depress pH, an advantage for low alkalinity waters.
Understanding Chemical Removal
The understanding of chemical P removal with iron and aluminum salts is undergoing a makeover. The traditional understanding is an equilibrium precipitation model based on formation of metal phosphates which are insoluble and can be captured by settling or filtration. The equilibrium model is convenient from a design standpoint because chemical dosage, sludge generation, and alkalinity consumption are all calculated from balanced chemical formulas. Full-scale plant data showing lower effluent P than predicted and experimental observations revealing that metal phosphate does not precipitate at the pH and P concentrations typically encountered in municipal wastewater treatment demonstrated flaws with the equilibrium model. An alternative model based on formation of a hydrous metal oxide (HMO) precipitate is now coming into acceptance.
HMO Formation. Image Credit: Dr. Sam Jeyanayagam, CH2M Hill
Co-precipitation is the main P removal mechanism in the surface complexation model (SCM). Iron or aluminum salts react with alkalinity in water to form HMO precipitate with variable capacity to adsorb soluble P. Mixing intensity at the point of chemical dosing is a critical factor limiting adsorption. The TP removal for a given dosage is limited by the amount of high-HMO generated at the dosing point.
A high mixing intensity generates a high proportion of HMO with high adsorptive capacity. High-HMO sorbs P instantaneously. Within moments of chemical addition, high-HMO precipitates grow and consolidate into flocs, changing to a form with low adsorptive capacity. A low mixing intensity, on the other hand, generates a higher proportion of HMO with low adsorptive capacity (low-HMO) increasing the chemical dosage required to achieve target P levels. Adsorption of soluble P to low-HMO continues until no adsorption sites remain. This may account for residual removal with return activated sludge, even when chemical feed is stopped. Chemical sludge recirculation has been engineered into the tertiary phosphorus removal system at Coeur d’Alene, Idaho to take advantage of the continued adsorption and reduce total chemical costs.
The growing flocs act like a fishnet to entrap non-settleable colloidal P and drag it down, a minor removal mechanism. Therefore, another benefit of high mixing is that it increases the contact between soluble P and HMO in the initial moments after dosing and thereafter increasing adsorption and entrapment of colloidal P in the growing HMO floc. (Baby Floc Shown in Image. Image Credit: Dr. Vladimir Kitaev, Wilfrid Laurier University)
Chemical Dosage Strategy
Early in my career I came to believe that the most optimal chemical addition strategy was adding chemical upstream from primary settling. My first experience with chemical P removal was as a college intern at Bay View. Pickle liquor was added to aerated grit tanks and P was removed with primary sludge in the primary settling tanks downstream, a strategy known as pre-precipitation. Ferrous was stored in three large, insulated and heat-traced tanks inside a concrete containment. Pumps and a maze of piping and valves were located in an adjacent building.
One of my projects was to map the distribution system and create valve tags (documentation of its construction was lost or never existed). The wisdom, as it was explained to me, was that pre-precipitation resulted in fewer side-reactions and more efficient chemical use. Besides that, addition of chemical to secondary treatment increased the inert content of mixed liquor making it necessary to keep a higher solids inventory and increasing final settling tank loading rates.
Simultaneous precipitation is the most common strategy for WRRFs that do not have primary treatment. Chemical is added to the mixed liquor upstream from final settling tanks and P is removed with waste activated sludge (WAS). The final settling distribution chamber is a common dosing point. One YSI customer doses chemical just upstream of their oxidation ditch rotors.
Some facilities that do have primary treatment may also choose simultaneous precipitation for a more predictable and stable process. Some wastewater P may yet be converted to soluble form, a requirement for chemical P removal, in primary treatment. On the other hand, removing too much P in primary treatment can cause a P deficiency in downstream biological treatment. Using the same ratio as below, for a primary effluent BOD concentration of 100 mg/L, a minimum concentration of 1.0 mg P/L is required to support biological growth.
The Hampton Roads Sanitation District (HRSD) York River Treatment Plant utilized both pre-precipitation and simultaneous precipitation strategies in the mid-2000’s. A baseline amount of ferric chloride was added to primary treatment with additional ferric added to secondary treatment to achieve target effluent concentrations leaving a small amount of soluble P to support biological growth in downstream biological denitrification filters.
A multi-point chemical addition strategy, as in the HRSD example, is required to achieve the lowest effluent TP. The strategy may also include chemical addition to tertiary treatment, known as post-precipitation. Solids would be captured on sand filters or in tertiary clarifiers. Many facilities do not already have tertiary facilities so new construction would be required to implement post-precipitation.
Additional Blog Posts of Interest
The Science of Phosphorus | Blog Series | Blog 1 of 5
The Science of Phosphorus | Blog Series | Blog 2 of 5
The Science of Phosphorus | Blog Series | Blog 3 of 5
The Phosphorus Problem: Wastewater Treatment Options and Process Monitoring Solutions