Total Phosphate in Wastewater Information:

Properties:

Phosphate is an anion that contains one phosphorous atom and four oxygen atoms and carries a negative three charge. It has a chemical formula of [PO4]3- and is derived from phosphoric acid (H3PO4). Phosphate is also referred to as orthophosphate and can be present in inorganic or organic compounds (organic compounds contain carbon and inorganic compounds do not). Total phosphate refers to both inorganic and organic phosphate concentrations.

Sources:

Sources of phosphate include human and animal waste, industrial chemicals and detergents, and agricultural run-off.1

Importance in Wastewater:

Phosphate is abundant in wastewater and should be removed before treated effluent is discharged. There are two main methods for phosphate removal: chemical removal and biological removal. Chemical treatments are generally more common and easier to maintain while biological treatments may produce less sludge and cost less in the long run.1

Health/Environmental Concerns:

In nature, phosphorous is considered a limiting nutrient for plant growth; this means that plant growth is restricted until sufficient phosphorous is available for uptake. When large quantities of phosphate reach waterways, algae species take up the nutrient and bloom. These algae blooms can strip waters of Dissolved Oxygen (DO) and suffocate fish, aquatic animals, and plant species. This process is referred to as eutrophication. Additionally, some species of algae release toxins that are harmful to animals and humans. Due to environmental implications of high phosphate levels in treated effluent, it is important that phosphorous is monitored and removed during wastewater treatment processes.

Action:

As mentioned, phosphate removal strategies can be separated into two main categories: chemical methods and biological methods. When phosphate is dissolved in wastewater, it cannot be separated out by filtration; coagulants and flocculants must first be added to form insoluble phosphate-containing compounds that precipitate out of solution.2 Typically, metal salts like ferric sulphate, Fe2(SO4)3, or aluminium sulphate, Al2(SO4)3, are used as coagulants and flocculants for phosphate removal. Once these compounds are added, phosphate-containing compounds will begin to form solid flocs that will either settle or rise. Subsequent filtration and separation allows for successful phosphate removal. Some disadvantages to chemical methods are the production of excess sludge waste and the recurring costs of coagulants and flocculants.3 Additionally, some chemicals affect other water quality parameters (such as pH) and require subsequent treatment before discharge.3

An alternative to physio-chemical phosphate removal is biological removal. Rather than using chemicals to precipitate out phosphate, biological removal uses microorganisms called phosphorus accumulating organisms (PAOs) that can store phosphate at 5-30% of their dry-weight biomass.3 Biological phosphate removal may be advantageous where low sludge production is preferable and in the long-term reduction of operating costs.3 However, certain aerobic and anaerobic cycling is required for phosphorous uptake and usage by PAOs, and continuous monitoring is required to ensure wastewater parameters such as Biological Oxygen Demand (BOD), pH, and DO are conducive to microorganism activity and survival.

1 Bunce, J. T.; Ndam, E.; Ofiteru, I. D.; Moore, A.; Graham, D. W. A Review of Phosphorus Removal Technologies and Their Applicability to Small-Scale Domestic Wastewater Treatment Systems. Frontiers in Environmental Science 2018, 6, 8. https://doi.org/10.3389/fenvs.2018.00008.

2 Treatment Solutions: Phosphorous Removal from Wastewater. Lenntech, Netherlands. Accessed 16 Mar. 2020.

3 Phosphorus Treatment and Removal Technologies. Municipal Division, Wastewater Program, Minnesota Pollution Control Agency: MN, United States. 02 Jun. 2006.