Connect
the dots to generate the calibration curve. Label, sign and date
the graph.
Chapter 11 Phosphorus return to Table of Contents
Wastewater is relatively rich in phosphorous compounds, that is, domestic wastewater; industrial wastewater may not have any. Phosphorus is essential to the growth of organisms. The discharge of wastewater containing phosphorus may stimulate nuisance algal growths. Phosphorus bans or limitations in synthetic detergents or changes in detergent formulas by the manufacturers have served to reduce the overloading.
Prior to 1940 phosphorous in wastewater was not a big problem. Normal domestic wastewater only contained around 1 to 5 mg/L phosphorous, generally in the form of organic phosphorous or orthophosphate. However with the advent of more scientifically designed detergents for washing clothes and dishes, the amount of phosphorous in wastewater climbed rapidly. Phosphorous compounds are added to these formulations as “builders” to sequester hard water producing metal ions like calcium and magnesium, and improve the cleaning ability of the detergent by stabilizing the dirt particles in solution. Most of these added phosphorous compounds are condensed or polyphosphates.
Chemical-physical removal of phosphorous from wastewater is possible only when the phosphorous is in the orthophosphate form. Although the organic and condensed phosphates can be easily converted into orthophosphate by treating them with strong, hot oxidizing acid conditions, this is not practical on a multi-million gallon per day scale. Fortunately, most biological treatment processes perform the conversion of the organic and condensed phosphates to orthophosphate.
Most orthophosphate salts are not soluble in water and phosphorous reduction is achieved by forming an insoluble salt. The most common methods are to form the insoluble calcium, aluminum or iron phosphates and let the salt particles get caught in a floc and settle to produce a sludge.
Dissolution of lime (CaO, calcium oxide) in water produces calcium ions and hydroxyl ions. The hydroxyl ions serve to raise the pH to around 11 SU. The calcium ions connect with the orthophosphate ions already in solution to form calcium hydroxyphosphate (hydroxyapatite). At pH 11 SU or higher, hydroxyapatite is not soluble and forms a sludge on the bottom of the settling tank along with any calcium carbonate formed by the reaction of calcium ions with carbonate (alkalinity) ions. After the orthophosphate is removed, the water is acidified by carbon dioxide to lower the pH to a value more suitable for discharge to receiving waters. The first stage carbonation can be halted at pH 9.3 SU to allow all the remaining calcium carbonate to precipitate. However, as this is a very finely divided solid, polymer can be added to increase the particle size and speed the settling. The second stage of carbonation reduces the pH to the final desired value of 7 to 8 SU.
Aluminum hydroxyphosphate is formed from alum treatment of the orthophosphate containing wastewater. Alum (AlSO4, aluminum sulfate) reacts with alkalinity to form the floc of aluminum hydroxide. The aluminum ions combine with the orthophosphate and alkalinity to form the solid aluminum hydroxyphosphate. The pH for successful phosphate removal by alum treatment lies in a very narrow range around 6 SU and two molecules of alum are required for every one of phosphate removed. At pH below 5 or above 7 SU, the removal of phosphate is either incomplete or a fine solid forms which has poor settling qualities.
Orthophosphate removal as iron hydroxyphosphate is achieved from ferric chloride or ferric sulfate treatment. The iron ions in the solution combine with alkalinity and phosphate to form the insoluble ferric hydroxyphosphate and the floc ferric hydroxide which helps in the precipitation. The ideal pH for this to occur is around 6 SU. Most of the sludge from this removal is moved into an anaerobic digester where reduction of ferric iron to ferrous iron occurs. This does not affect the phosphorous removal as ferrous phosphate (vivianite), also insoluble.
A problem with phosphate which is not really applicable as preferred method of phosphate removal occurs in supernatant recycle/removal lines from anaerobic digesters. Magnesium ions, ammonium ions and phosphate ions can all exist in solution if the pH is low, around 6 SU or so. However in the recycle pipelines, the pump suction can release carbon dioxide from the supernatant, especially at pipe elbows. When the carbon dioxide is removed, the pH goes up to around 10 SU and magnesium ammonium phosphate (struvite) deposits in the pipe and on screens. Severe cases can result in plugged pipes and major maintenance headaches.
Phosphorus, in addition to nitrogen, is a nutrient which can result in eutrophication of receiving streams and lakes. The NPDES parameter is total phosphorous which means that all forms of phosphorus, of which there are many, must be converted to orthophosphate prior to analysis. The approved conversion procedure is the persulfate digestion method. Of the analysis procedures listed in Standard Methods, only the ascorbic acid method is approved. This is a colorimetric procedure and a calibration curve must be established prior to any analysis of samples. Standard Methods specifies a six point calibration along with a blank. The calibration standards and the blank must be taken through the entire digestion procedure. A full set of quality controls must be run with every batch to include a calibration standard diluted to 50.0 mL with sample gives a matrix spike true value of 1.00 mg/L.
Lab analysis expects that the sample not pick up extras from the apparatus used in the testing. Experience in testing for phosphorous has shown a need to take extra care. Here are some suggested Lab techniques for the glassware used for Phosphate analysis:
First:
dedicate glassware for only phosphorous testing,
Then keep separate, dedicated work areas,
Control any contact [even air],
Dedicated cleaning tools, brushes,
Thorough mechanical cleaning,
Hot HCl acid soaking,
Rinse with proven DI water.
Total
Phosphorus Checklist (4500-P B, E SM18)
Reagents
___ 1. Phenolphthalein indicator solution: Dissolve 0.5 g phenolphthalein disodium salt in reagent water and dilute to 100 mL.1
___ 2. 10.8 N Sulfuric acid: add 30 mL concentrated sulfuric acid to 60 mL reagent water. Allow to cool, then dilute to 100 mL.
___ 3. Ammonium persulfate (NH4)2S2O8 or potassium persulfate K2S2O8.
___ 4. 1 N sodium hydroxide: Dissolve 4.0 g sodium hydroxide (NaOH) pellets in 50 mL reagent water. After cooling, dilute to 100 mL with reagent water.
___ 5. 5 N sulfuric acid: Add 7.0 mL concentrated sulfuric acid (H2SO4) to 40 mL reagent water. After cooling dilute to 50 mL.
___ 6. Potassium antimonyl tartrate solution: Dissolve 0.1372 grams potassium antimonyl tartrate K(SbO)C4H4O6 w 2H2O in 40 mL reagent water and dilute to 50 mL.
___ 7. Ammonium molybdate solution: Dissolve 2.0 g ammonium molybdate (NH4)6Mo7O24 w 4H2O in 50.0 mL reagent water.
___ 8. 0.1 M ascorbic acid: Dissolve 1.76 g ascorbic acid C5H8O6 in reagent water and dilute to 100 mL.
___ 9. Stock phosphate standard: Dry potassium phosphate monobasic (KH2PO4) in a 104 ºC for at least 1 hour, cool in a desiccator. Dissolve 219.5 mg anhydrous potassium phosphate monobasic in reagent water and dilute to 1000 mL. Each mL of this solution contains 50.0 lg P.
___ 10. Working phosphate standard: Dilute 10.00 mL of the stock phosphate standard to 200.0 mL. 1.00 mL of this solution contains 2.50 lg Phosphorus.
___ 11. Wash and set out fourteen (14) 125 mL Erlenmeyer flasks or 150 mL beakers.
___ 12. Calibration standards: Dilute accurate volumes of the working standard to 50.00 mL as follows:
mL Working Std
P mg/L
50.00
2.500
40.00
2.000
30.00
1.500
20.00
1.000
10.00
0.500
5.00
0.250
3.00
0.150
1.00
0.050
0.50
0.025
0.00
reagent
blank
then transfer to the Erlenmeyer flasks.
___ 13. Add 50.0 mL well-mixed sample to the flasks marked
sample and duplicate. Prepare the matrix spiked samples by adding 1.00 mL of the stock phosphate standard to a 50.0 mL volumetric flask then diluting to volume with the sample. Transfer the matrix spike and matrix spike duplicate to Erlenmeyer flasks.
___ 14. Add 2 to 3 drops phenolphthalein solution to each Erlenmeyer
flask or beaker and mix. Mark the level of the volume on side of the flask or beaker with a pen. Add just enough sulfuric acid solution to discharge any pink color.
___ 15. Add 1.00 mL 10.8 N Sulfuric acid to each flask or
beaker and mix.
___ 16. Add either 0.40 g Ammonium persulfate (NH4)2S2O8 or
0.50 g potassium persulfate K2S2O8 to each beaker or flask.
___ 17. Put the beakers or flasks on a hot plate and heat with gentle
boiling for 40 minutes.
___ 18. Allow to cool, add 2 to 3 drops phenolphthalein solution
and neutralize to a faint pink color with sodium hydroxide solution (about 10 mL).
___ 19. Dilute with reagent water to the 50.0 mL marked line.
___ 20. Make up the combined reagent: Combine 50 mL 5 N sulfuric
acid and 5 mL potassium antimonyl tartrate solution with mixing. Add 15 mL ammonium molybdate solution and mix. Add 30 mL 0.1 M ascorbic acid and mix. The combined reagent must be used within 4 hours.
___ 21. Add 8.00 mL combined reagent to each flask and mix.
___ 22. Set the colorimeter at 880 nm and use the reagent
blank to set zero absorbance.
___ 23. After 10 minutes but before 30 minutes, read and record
the absorbance of each calibration standard.
___ 24. Using graph paper, plot absorbance against concentration.
Connect
the dots to generate the calibration curve. Label, sign and date
the graph.
This sample calibration curve has a lot more than 6 points on it. It should also come out linear at least up to 1.5 mg/L.
___ 25. Calculate the %R and the RPD for the batch and update
the control charts.
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