As phosphorus discharge limits are becoming tighter, chemical phosphorus removals are practiced more frequently along with biological nutrient removal (BNR). However, the mechanism of chemical phosphorus removal is poorly understood due to the complex nature of the thermodynamics and the kinetics involving in the reaction.
Many different forms of phosphorus species exist in wastewater. Orthophosphate (PO43-) is the most abundant species in wastewater in general, which reacts with inorganic coagulants such as iron, aluminum, calcium, etc. to form insoluble precipitates. Polyphosphates are oligomers of orthophosphate that can somewhat react with coagulants due to the opposite charges they have. Organic phosphorus mainly exists as phospholipids that are a component of cell membranes as they can form lipid bilayers, but they do not react with coagulants directly. However, once wastewater is treated biologically, vast majority of non-orthophosphates except those bound with refractory organic chemicals, e.g. pesticides, herbicides, etc., end up with orthophosphates by biological degradation.
Many mathematical models have been developed to explain the equilibrium of the chemical reactions among metal ions, phosphorus, hydroxyl ions, etc. as functions of temperature, pH, ionic strength, etc. Unfortunately, none of them appear to fit with the observations in the field due to the following reasons.
- Unidentified ionic species and microorganisms in water make the equilibrium far more complicated than those in lab conditions.
- The kinetic competition between metal hydroxide formation and metal phosphate formation makes the equilibrium (or thermodynamics) based predictions invalid. If fact, as explained in mechanism section, inorganic coagulant forms metal hydroxides immediately after contacting water at near neutral pH. Subsequently hydroxide ligands in the metal hydroxide are exchanged by orthophosphates, but this is a very slow reaction that is affected by mixing, temperature, ionic strength, pH, etc.
Various inorganic coagulants can be used to remove phosphorus in biological wastewater treatment as summarized in Table 1. Alum and ferric chloride are the most commonly used coagulants in MBR process because they can reduce phosphorus concentration in effluent down to 0.005-0.04 mg/L (Takács, 2006). In addition, aluminum and iron can improve membrane performance by not only reducing soluble microbial products (SMP) concentration, but also forming larger floc that has less impact on membrane fouling (Lee, 2001). As can be seen in the table, iron requirement is around twice the aluminum requirement in terms of chemical dosage to remove same amount of phosphorus. Therefore, overall costs of iron based coagulants are comparable with aluminum based ones although iron based coagulants are less expensive in general.
Sodium aluminate and polyaluminum chloride (PAC) can be used instead of other aluminum salts, but may not be effective as other aluminum salts because aluminum ions are pre-occupied by hydroxyl ligands that act as a kinetic barrier when aluminum reacts with orthophosphate as explained in “mechanism” section. Lime is not commonly used in MBR mainly because it needs high pH such as 9 or above to reduce free phosphorus level sufficiently low. If alum and ferric chlorides decrease pH excessively, lime can be combined to supply alkalinity and help scavenge orthophosphate.
Table 1. Chemicals used to remove phosphorus in biological wastewater treatment
|Chemical / Trade name||Chemical Equation in solids||Molecular weight (g/mol)||Equivalent ratio|
g Metal /g P
|Precipitates||Appearance as aqueous solution|
|Aluminium Sulfate (Alum)||Al2(SO4)3.14H2O||594.37||0.871||Alx(PO4)y(OH)3x-3y||Transparent|
|Polyaluminium Chloride (PAC)||Al2Cl(OH)5||174.45||0.871||Transparent|
|Ferric Sulfate||Fe2(SO4)3.5H2O||489.96||1.803||Fex(PO4)y(OH)3x-3y||Reddish Brown|
|Ferric Chloride||FeCl3.6H2O||270.30||1.803||Reddish Brown|
|Pickle Liquor||Fe2+ and Fe3+||–||–||Reddish Brown|
|Calcium hydroxide (Lime)||Ca(OH)2||74.09||1.941 /|
|Ca3(PO4)2 / Ca5(OH)(PO4)3||Sold as powder|
© Seong Hoon Yoon