Orthophosphate Issue in SWRO Plant

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We have a high level of orthophosphate in seawater and we use currently a coagulant.

Are there other alternatives? The actual pretreatment is not efficient. 

Thanks in advance. 

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3 Answers

  1. Dear Jalil Bourras

    The total phosphate applied shall not exceed 10 mg/L as PO4 . Public water systems (PWSs) commonly add phosphates to the drinking water as a corrosion inhibitor to prevent the leaching of lead and copper from pipes and fixtures.Inorganic phosphates (e.g., phosphoric acid, zinc phosphate, and sodium phosphate) are added to the water to create orthophosphate, which forms a protective coating of insoluble mineral scale on the inside of service lines and household plumbing. Phosphate removal is currently achieved largely by chemical precipitation, which is expensive and causes an increase of sludge volume by up to 40%. An alternative is the biological phosphate removal (BPR).

    The words  orthophosphate  and  polyphosphate  contain the word "phosphate," these two chemical compounds serve radically different water treatment purposes. A public utility system's failure to understand the significant differences between these two treatment compounds could result in serious water-quality problems and possible MCL violations. An incorrect selection of phosphate chemical blends by a public utility system could even create serious public health problems.

    Phosphorous removal processes

    The removal of phosphorous from wastewater involves the incorporation of phosphate into TSS and the subsequent removal from these solids. Phosphorous can be incorporated into either biological solids (e.g. micro organisms) or chemical precipitates.

    Phosphate precipitation

    Chemical precipitation is used to remove the inorganic forms of phosphate by the addition of a coagulant and a mixing of wastewater and coagulant. The multivalent metal ions most commonly used arecalcium, aluminium and iron.

    Calcium:

    it is usually added in the form of lime Ca(OH)2. It reacts with the natural alkalinity in the wastewater to produce calcium carbonate, which is primarily responsible for enhancing SS removal.

    Ca(HCO3)2 + Ca(OH)2 à 2CaCO3 ↓+ 2H2O

    As the pH value of the wastewater increases beyond about 10, excess calcium ions will then react with the phosphate, to precipitate in hydroxylapatite:

    10 Ca2+ + 6 PO43- + 2 OH- ↔ Ca10(PO4)*6(OH)2 ↓

    Because the reaction is between the lime and the alkalinity of the wastewater, the quantity required will be, in general, independent of the amount of phosphate present. It will depend primarily on the alkalinity of the wastewater. The lime dose required can be approximated at 1.5 times the alkalinity as CaCO3. Neutralisation may be required to reduce pH before subsequent treatment or disposal. Recarbonation with carbon dioxide (CO2) is used to lower the pH value.

    Aluminium and Iron:

    Alum or hydrated aluminium sulphate is widely used precipitating phosphates and aluminium phosphates (AlPO4). The basic reaction is:

    Al3+ + HnPO4 3-n  ↔ AlPO4 + nH+

    This reaction is deceptively simple and must be considered in light of the many competing reactions and their associated equilibrium constants and the effects of alkalinity, pH, trace elements found in wastewater. The dosage rate required is a function of the phosphorous removal required. The efficiency of coagulation falls as the concentration of phosphorous decreases. In practice, an 80-

    90% removal rate is achieved at coagulant dosage rates between 50 and 200 mg/l. Dosages are generally established on the basis of bench-scale tests and occasionally by full-scale tests, especially if polymers are used. Aluminium coagulants can adversely affect the microbial population in activated sludge, especially protozoa and rotifers, at dosage rates higher than 150 mg/l. However this does not affect much either BOD or TSS removal, as the clarification function of protozoa and rotifers is largely compensated by the enhanced removal of SS by chemical precipitation.

    Ferric chloride or sulphate and ferrous sulphate also known as copperas, are all widely used for phosphorous removal, although the actual reactions are not fully understood. The basic reaction is:

    Fe3+ + HnPO4 3-n  ↔ FePO4 + nH+

    Ferric ions combine to form ferric phosphate. They react slowly with the natural alkalinity and so a coagulant aid, such as lime, is normally add to raise the pH in order to enhance the coagulation.

    Strategies

    The main phosphate removal processes are (see picture below):

    1. Treatment of raw/primary wastewater

    2. Treatment of final effluent of biological plants (postprecipitation)

    Treatment contemporary to the secondary biologic reaction (co-precipitation).

  2. Dear Jalil,

    Please have a look to the Hach’s RTC-P System optimizes chemical phosphorus removal by adjusting chemical dosing in real time through the continuous measurement of phosphate concentration and flow, allowing you to maintain consistent effluent phosphorus values and enhance your chemical phosphorus control system for unprecedented chemical savings.

    https://www.hach.com/asset-get.download.jsa?id=52040745453

    Best of luck... and do not forget to look at the TOC(COD) monitoring solutions, in name of the B3500 BioTector units to control and monitor the SWRO systems!

    - Robert