In 2010, the state of Wisconsin passed legislation known as the “Phosphorus Rule,” which changed standards for phosphorus in surface waters. As a result, Wisconsin wastewater treatment facilities (WWTFs) have been preparing for increasingly stringent effluent requirements for total phosphorus (TP) over the last 5-10 years.
Importantly, these standards changed effluent limits at WWTFs from technology-based limits to significantly more rigorous water quality-based limits. Before the Phosphorus Rule, the typical TP effluent concentration was a monthly average of 1.0 mg/L TP. The new rules can require an effluent TP concentration as low as 0.075 mg/L on a six-month average basis.
phosphorus filtration & compliance with the “Phosphorus Rule”
Adding phosphorus filtration to your WWTF can help your municipality significantly reduce its TP effluent concentration and comply with the updated TP effluent requirements. We’ll look at the various types of phosphorus filters available, how they work, and what the advantages and disadvantages are for each system, so you can make the right choice for your WWTF.
TYPES OF PHOSPHORUS FILTERS
There are two main groupings of phosphorus filters: precipitation-based filters and adsorption type filters. Backwash, the wastewater used in cleaning the filter, is typically routed to the WWTF headworks. For all filters, phosphorus is ultimately removed through the biosolids which are typically applied to farm fields for beneficial re-use.
Precipitation-based filters convert soluble reactive phosphorus to solids using a chemical coagulant and polymer immediately upstream of the filter with chemical precipitation. The coagulant is normally an iron or aluminum metal salt, which reacts with the phosphorus in the wastewater. The phosphorus-containing solids are then removed with a cloth, membrane, or other media.
With adsorptive filters, a chemical is introduced to the wastewater to coat a media (e.g., fine-grained sand). The coated media adsorbs phosphorus from the wastewater as it flows upward through the sand bed. With this type of filter, solids are not formed with chemical precipitation and there is no physical separation barrier.
Precipitation-Based Filters
There are several varieties of precipitation-based filters, including:
Cloth or Membrane Disc Filters
After the chemical treatment of the coagulant and polymer with controlled mixing, wastewater flows through a rotating disc assembly with a submerged cloth or membrane.
The wastewater then flows through the filter into an effluent pipe in the center of the disc.
When the headloss across the filter reaches a preset level, a backwash sequence is initiated.
During the backwash, the rotating discs are cleaned with spray water or a vacuum pump with a shoe that rubs against the filter media.
Aqua Aerobics Cloth Filter
Advantages:
High level of phosphorus removal, lower headloss through filter
Potential Disadvantages:
Higher dosage of chemical coagulant required
Precise mixing and chemical treatment required immediately upstream of filter
Compressible Media Filters
These filters separate phosphorus-containing solids from the wastewater with a compressible media.
Wastewater flows from the bottom of the filter up through the media, and when loss through the media reaches a preset level, a backwash cycle is initiated.
The backwash starts with a metal plate that decompresses the media, then wastewater flows through the media and scours the solids. This process also uses air to agitate the media and remove solids during the backwash cycle.
The backwash water containing the solids collects at the top of the filter, and a backwash pump conveys the solids to the WWTF headworks.
Normal operation allows parallel flow through two filters. When one filter is backwashed, the forward flow through the filter is reduced and a greater flow rate is introduced to the other filter in operation.
Schreiber Compressible Media Filter
Advantages:
Lower headloss through the filter
Lower capital cost
Potential Disadvantages:
Lower level of phosphorus removal
Equipment has a large vertical footprint
Drum Filters
Drum filters separate coagulated phosphorus-containing solids, which are formed with a coagulant and polymer with fine media around a drum.
The water flows from the outside of the filter to the inside.
When the filter is partially clogged and the water level on the outside of the drum increases to a pre-set level, a backwash cycle is initiated and cleans the drum.
A back wash pump provides suction pressure against the outside of the drum with a shoe. The drum then rotates against the stationary shoe, cleaning the drum.
Nexcom Drum Filter
Advantage:
Filter can be designed for low flow rates for smaller communities
Potential Disadvantages:
Lower level of treatment
Precise mixing and chemical treatment required immediately upstream of filter
Adsorptive Filters
In contrast to the variety of precipitation-based filters available, there is really only one main type of adsorptive filter: a continuous backwash filter.
Continuous Backwash Filters
Phosphorus is removed using the principle of adsorption where phosphates accumulate on sand particles.
With this technology, wastewater flows up through a bed of sand. The sand is coated with hydric ferric oxide (HFO) through the addition of a ferric metal salt immediately upstream of the filter, but after the secondary clarifiers.
As the wastewater passes through the HFO-coated sand, phosphate particles from the wastewater are adsorbed to the surface of the sand particles.
The bed is continuously regenerated by an airlift pump, which lifts sand particles from the bottom of the filter up through a center column to a central baffle assembly.
The baffle assembly creates turbulence, which removes the phosphates from the sand.
The difference in specific gravity also separates the phosphates from the sand at the top of the filter.
The phosphates, which still contain usable ferric, can then be re-introduced to the WWTF to lessen the amount of chemicals from the first removal process.
The sand is routed to the top of the filter bed and is regenerated with HFO the ferric coagulant.
BluePro© Continuous Backwash Filter Configuration
Advantages:
High level of phosphorus removal
Lower dosage of chemical coagulant required
Potential Disadvantage:
Need more space for hydraulics through the filter (harder to fit within an existing hydraulic profile)
Learn More
Every WWTF has its own unique needs, and phosphorus filtration is not a one-size-fits-all solution to lowering your TP effluent limits. To find the right phosphorus filtration for your community, please contact one of our experts today!
About the Author
David W. Arnott, P.E.
Team Leader/Senior Project Manager
Dave has extensive experience in the planning, design, and construction project management of wastewater treatment facility renovation and upgrade projects. Areas of expertise include treatment processes, hydraulics, mechanical equipment, and wastewater disinfection. He also has experience in sanitary and water pumping station design and sanitary sewer wet weather facilities.
