May 5, 2015 -- A research group led by Rolf Halden -- professor at the School of Sustainable Engineering and the Built Environment and director of the Center for Environmental Security, Arizona State University -- recently published a study that examined methods for recovering phosphorus from wastewater using mathematical modeling.
Wastewater treatment plants (WWTPs) in many cities are currently implementing methods to extract phosphorus before discharging wastewater into the environment. There are two main types of phosphorus recovery methods: chemical and biological. "WWTPs represent ground-zero for addressing the problem of global phosphorus depletion," said Halden.
In the chemical method, WWTPs treat phosphorus dissolved in wastewater. The phosphorus then falls out of solution for easier removal. In the biological method, bacteria introduced into the water collect the phosphorus into removable sludge. A variation includes enhanced biological phosphorus removal (EBPR). This method selectively encourages bacteria that can accumulate phosphorus.
Choosing a method is complicated. "The region's water quality, size of the treatment plant, and economic considerations play a role in the selection," explained Arjun Venkatesan, the study's lead author. The study focused on a combination approach. First, EBPR concentrated phosphorus in sludge. Next, chemical treatment helped phosphorus fall out to form struvite, a usable phosphate mineral. The study showed that a typical WWTP could reclaim approximately 490 tons of phosphorus in the form of struvite each year.
Conventional methods remove only 40 to 50 percent of phosphorus, according to Venkatesan. The secondary treatment of sludge employed by EBPR "achieves an additional 35 percent mass reduction, for a total of about 90 percent removal," he said. EBPR helpfully avoids additional chemicals and reduces sludge production. Both these factors lower the cost of operation -- a key consideration for WWTPs with limited budgets.
Reclaimed phosphorus pays off for the environment with less mining for phosphorus and improved surface water health. Phosphorus recovered as struvite can also generate income. The team estimates that the WWTP used in their case study could generate $150,000 in annual revenue from this two-pronged approach. A plant with existing EBFR facilities can recoup the initial expenses in as little as three years.
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