Phosphorus management options in wastewater treatment

Phosphorus is a valuable, limited resource that is essential to life. But in the wrong place — the lakes and streams of our watersheds — too much phosphorus can be an environmental hazard.

According to the U.S. EPA, nutrient impairment prevails in 58% of the nation’s rivers and streams, 45% of our lakes, about two thirds of our coastal areas, and more than one third of our estuaries. As EPA steps up efforts to address this situation, more public and private facilities across the country will be facing the need to reduce the amount of nutrients, including phosphorus, they release into the environment.

Facilities that have never been permitted before may be facing new permits, and those with current permits will likely be facing lower permitted limits.

Removing phosphorus in wastewater

Municipal wastewater typically contains between 4–8 mg/L of total phosphorus. To reach new limits of 0.5 mg/L or lower, treatment system designers have a range of available technologies on which to draw, including chemical removal, biological removal, ballasted clarification, filtration and phosphorus recovery.

Chemical removal: reliable and time-tested

Chemical removal of phosphorus from wastewater is proven and reliable but requires on-going operating expense and generates chemical sludge.

This method primarily uses aluminum, iron coagulants, or lime to form chemical flocs with phosphorus. These flocs are then settled out to remove phosphorus from the wastewater.

This complex, often multipoint process involves competing reactions that can vary with pH, alkalinity, reaction time and mixing intensity.

Phosphorus occurs in several different forms throughout the wastewater process, and understanding its distribution is vital to monitoring and removal. Chemicals are frequently added at both primary and secondary stages, and tertiary treatment is used in plants that have ultra-low phosphorus level requirements.

Below are some dosages of chemicals for removal of measured total suspended solids (TSS) or volatile suspended solids (VSS). These numbers represent typical yields under particular wastewater conditions:

Total Suspended Solids

• 1 mg/L iron dosage = 1.9 mg/L TSS
• 1 mg/L aluminum dosage = 2.9 mg/L TSS

Volatile Suspended Solids

• 1 mg/L iron hydroxide = 0.25 mg/L TSS
• 1 mg/L aluminum hydroxide = 0.35 mg/L TSS
• Affects digester VSS destruction
  
  

Biological phosphorus removal

In conventional biological treatment, phosphorus is removed as a normal part of aerobic biological growth. The microorganisms are then separated from the wastewater by settling or filtration. The biological yield of this process is approximately 0.5 lb per pound of BOD removed, with phosphorus content of about 2% by weight.

Under specific conditions, the process could yield the following:

Under wastewater with:

• 60 g BOD/cap/d, 1.5–2 g P/cap/d, 0.3 m3/cap/d = 200 mg/L BOD, 6.7 mg/L P, 30:1 BOD:P

The assimilative uptake of P would be:

• Yield = 0.5 mg VSS/mg BOD, 2% P, 2 mg/L removed (30–40% removed)
• Effluent P = 3–4 mg/L
  
  

Enhanced biological phosphorus removal

Enhanced Biological Phosphorus Removal (EBPR), in which the reactor processes are optimized for high-efficiency phosphorus removal, can increase the biomass content of waste sludge to 3–6% phosphorus compared to conventional activated sludge systems (normally 1.5–2%).

Under the right conditions (generally, a BOD:P ratio of 30:1 with anaerobic/aerobic cycling of biomass), EBPR can reduce or eliminate the need for chemical precipitation or filtration, achieving effluent phosphorus levels below 1 mg/L or even less than 0.3 mg/L on its own.

EBPR can be achieved in continuous flow systems or batch systems. Continuous processes can be designed with an aerated-anoxic zone with a strong oxygen deficit. In other cases, the addition of an anaerobic selector can be used. A sequencing batch reactor (SBR) can be operated with a static fill step at the beginning of treatment to create a strong reducing environment.

In each case, by sequencing the biomass through a sufficiently anaerobic stage, and with a sufficient food source, prior to the reintroduction of oxygen, it is possible to selectively grow a biomass capable of storing phosphorus at a higher than conventional level. The phosphorus is ultimately removed from the system by settling and removing sludge.

Some considerations when designing for EPBR:

• A small anaerobic selector with mechanical mixer has been used successfully to enhance P removal in two-channel systems. It has also been used in systems requiring both low TN and TP levels to allow the anaerobic and aerated-anoxic environments to be optimized independently.
• In some SBR systems a static fill period can be added as the first treatment step, followed by mixed anoxic, aerated fill and react steps, providing excellent conditions for fermentation, volatile fatty acid uptake and phosphorus release.
• Increasing wasting rates can elevate sludge production rates above normal and increase biological production for best phosphorus removal.
• To minimize the risk of high effluent phosphorus, the sludge retention time (SRT) within the final clarifier should be less than 30 minutes. This is accomplished by using higher recirculation rates and a suction sludge removal device and maintaining a low or zero (less than 1 ft) sludge blanket.
• If discharge permits require phosphorus limits less than 1.0 mg/L, tertiary solids removal (filtration) may be necessary.
• To effectively operate both phosphorus and nitrogen removal processes, a process control system is recommended for maintaining tight control over operating parameters.
• If EBPR is used in a plant with anaerobic digestion, it is usually necessary to chemically treat the digester supernatant to precipitate the soluble phosphorus.
  
  

Clarifier enhancements and tertiary treatment

Ballasted clarification, which uses a high-density inert material such as microsand or magnetite to ballast biological floc or conventional chemical floc, enhances settling rates while substantially reducing costs.

For biological processes, magnetite-based ballasted clarification can achieve a sludge volume index (SVI) of less than 50 and provide up to 300% more treatment capacity in existing tankage. For chemical processes, ballasted clarification infuses magnetite as a weighting agent into traditional chemical floc to efficiently reduce total phosphorus (TP) levels and settle floc up to 30 times faster than conventional treatment.

Disc filtration can enhance the effectiveness of biological and chemical processes by actively filtering phosphorus flocs out of the effluent stream instead of relying on gravity settling. In this process, effluent from the secondary solid-liquid separation process is blended with aluminum or iron coagulants in a rapid mix tank.

The development of chemical phosphorus flocs is then grown into larger floc via polymer dosing and mixing in a flocculation tank before feeding into the disc filter system.

An effective biological treatment process in conjunction with phosphorus removal polishing via a disc filter can usually meet even the most stringent discharge permits.