Wastewater Lagoon Phosphorus Removal Methods

Three ways phosphorus can leave a wastewater lagoon without intervention are described

Pathways of Wastewater Lagoon Phosphorus Removal

Phosphorus is removed from wastewater through either biological or chemical methods. While some phosphorus is removed naturally through regular lagoon biological treatment, it is difficult to meet any restrictive effluent standard with natural processes alone. Three ways phosphorus can leave a wastewater lagoon without intervention are:

• Through uptake by Phosphorus Accumulating Organisms (PAOs) like bacteria and algae.
• Through primary sedimentation, as inorganic phosphorus complexes sink to the bottom of the lagoon. As much as 15 percent of phosphorus is removed as settleable solids.
• Out with the effluent.

Regular lagoon treatment, especially with longer retention times, will remove some phosphorus, but not enough to meet effluent standards. Phosphorus removal, like ammonia removal, is temperature sensitive: According to Nutrient Removal in a Cold-Region Wastewater Stabilization Pond: Importance of Ammonia Volatilization,  Journal of Environmental Engineering  (April 2006), nearly 50 percent of influent phosphorus was lost from the water column during the summer-fall treatment period, but only about 35 percent was removed during the winter-spring period. Effluent can be stored through the winter and discharged when nutrient levels are the lowest to take advantage of nutrient removal that occurs during the warmer temperatures. This the most cost-effective method if sufficient lagoon capacity is available, although it will likely not remove enough phosphorus to meet the lower effluent limits that regulating bodies will be demanding in the future.

In order to meet very low phosphorus effluent levels of less than 1 mg/L or even lower, additional treatment is necessary.

Chemical Precipitation of Lagoon Phosphorus

In its Wastewater Phosphorus Control and Reduction Initiative, the Minnesota Environmental Science and Review Board (MESERB) states: “Chemical precipitation is the only feasible process for phosphorus removal for lagoon systems.” (Enhanced biological phosphorus removal [EBPR] is based on a suspended growth activated sludge process.)

Unlike nitrogen, phosphorus does not have a gaseous form, so the only way it is removed is through sedimentation or precipitation—conversion to a solid that either settles in the sludge blanket or is removed through filtration. Soluble wastewater phosphorus can be coagulated with the addition of a metal salt like aluminum sulfate (alum); an iron salt like ferric chloride; or lime. Lime is now rarely used, due to the difficulty of handling and storing it, as well as its propensity to create sludge. According to the EPA’s  Nutrient Control Design Manual , lime sludge can reach up to 0.5 percent of the volume of wastewater treated, the most of any chemical removal process.

The following chart, adapted from the EPA’s  Nutrient Control Design Manual , highlights the pros and cons of the most popular metal salts used for the precipitation of phosphorus in wastewater.

PhosBox™ Lagoon Phosphorus Removal

Triplepoint has considered all the available science in developing its PhosBox lagoon phosphorus removal process, to optimize phosphorus removal while controlling costs.

PhosBox is installed as a final step before the discharge point to maximize the precipitation of phosphorus without interfering with lagoon treatment.
Sensors monitor phosphorus levels and automatically dose the precise amount of metal salts needed for coagulation, mixing them in the influent chamber. The cloth disk filtration system captures the coagulated phosphorus and removes it from the effluent stream to a storage lagoon; the clean water is discharged.

Source: TriplePoint Water

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