Orthophosphate in Wastewater

Ortho-Phosphate in WastewaterPhosphorus, as soluble orthophosphate, is a critical nutrient in all biological processes, including activated sludge treatment. It is utilized by bacteria in making energy (ATP molecules) and in creating phospholipid bilayers, their cell structure. This procedure is to determine the ortho-phosphate content of wastewater samples from various points in the system.

Orthophosphate (oPO4) Is determined colorimetrically by reacting ammonium molybdate in an acid medium with dilute solutions of phosphorus to form a phospho-molybdate complex.   This complex is reduced to an intensely blue-colored complex by ascorbic acid.   The amount of light absorbed by the sample is proportional to the phosphorus concentration. For this procedure, the Hach powder pillows for Method 8048 will be used to simplify the analysis.

For the purpose of nutrient addition and control, a residual of 1-2 mg/L should be present in the supernatant of the mixed liquor.

The sample (5 mL) is filtered into a centrifuge tube (if there are high amounts of solids, they are centrifuged down) and diluted with 5 mL of deionized water (1:1 dilution). The diluted sample is used to zero the instrument. The content of one PhosVer 3 phosphate powder pillow is added to the sample which is then capped, shaken, and allowed to react for two minutes. The sample is then read on spectrophotometer in mg/L of P.

In activated sludge, the two primary symptoms of nutrient deficiency are excessive filaments and excessive polysaccharide or slime. Nutrient deficiency may also cause the production of viscous foam, and impact the jelly-like consistency of the activated sludge which will interfere with the sludge compaction (this is referred to as slime bulking).

Excessive filaments and excessive polysaccharide are lesser problems in aerated stabilization basins (ASB’s) but are still indicators of nutrient deficiencies. In aerated stabilization basins, insufficient nutrient availability will lead to poor biochemical oxygen demand (BOD) removal due to the inability of the bacteria to divide and create more workers. This will result in linear biochemical oxygen demand (BOD) removal, rather than exponential or logarithmic removal as expected under optimum conditions.

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