US Geological Society develops filter to remove phosphorus from wastewater

Video: A look at USGS' low cost phosphorus removal water filter.

Dietary phosphorus that animals and humans consume is excreted into wastewater, where it can accumulate and feed cyanobacteria, spawning harmful algal blooms. These can cause thick mats to accumulate on the water's surface and can deplete water oxygen concentrations, creating hypoxic dead zones responsible for fish and shellfish deaths. 

“Harmful algal blooms are a worldwide problem,” says Philip Sibrell, USGS Engineer and lead designer of the filter. “Since phosphorus is one of the main drivers for freshwater blooms, being able to remove it from wastewater has some real promise for being able to reduce the ecological impacts arising from harmful algal blooms.”

Researchers have been working on the filter for several years at the USGS Leetown Science Center and the team is now ready to put it into public use. Sibrell believes the filter system could support a small town of maybe around 1000 people, and is looking for partners to help make this a reality and help scale up the technology. 

A simple design

USGS scientists began researching mine drainage ochre as a possible agent to remove phosphorus from water in 1999. The first prototype was built in 2012. USGS says that since the early days of the research, phosphorus usage around the world has steadily increased as more nutrient filled fertilizers are being used for home gardens, lawn care, golf courses, and farm fields. While current wastewater phosphorus removal systems are effective at removing the nutrient from water, they are expensive.

The USGS filter is constructed from large tanks that each hold around 600 lbs of granular mine drainage ochre. It uses a gravity flow process. The phosphorus infused water is piped to the top of the tank, it is then allowed to pass through the porous ochre media, and the filtered water is released out the bottom of the tank.    

The simple design, and the fact it uses mine drainage ochre as a filtering agent, keeps costs down. Every component of the water treatment system was purchased as commercial, off-the-shelf items. 

The filter also addresses the problem of acid mine drainage. This occurs when certain minerals in the ground – like pyrite or other sulfide minerals – are disturbed by mining, digging and excavating, exposing them to air and moisture. When this happens, it causes a chemical reaction in the sulfide minerals that creates sulfuric acid and a variety of dissolved metals. This acidic, heavy metals-laced water eventually makes it into local watersheds, altering water pH and introducing new contaminants to ecosystems. Because of these heavy metals, not all mine drainage ochres are suitable for use as a phosphorus filtering agent because some have the potential to contaminate the treated water. Therefore, the ochres used in the USGS phosphorus filter have been sourced from coal mine drainages, which usually results in lower levels of toxic metals like copper, lead and arsenic. Water treated by these coal mine drainage ochres has been laboratory tested and scientists have confirmed that heavy metals are not present in the filtered water in detectable amounts.

Phosphorus recovery

In addition to providing a use for the acid mine drainage byproduct, the system also has the potential to be another source of phosphorus. During the filtering process, once the mine drainage ochre becomes saturated with phosphorus, the phosphorus can be stripped off of the ochre, regenerating the media to be reused again, and recapturing the phosphorus to be recycled.

“The world relies on high phosphorus fertilizers for food production, and many deposits of this vital mineral are becoming depleted,” Sibrell says. “Because of that, the ability to recover the phosphorus from the ochre is a big deal, allowing us to close the recycling loop by reusing both the ochre and phosphorus again.”