Environmental Engineering Reference
In-Depth Information
common occurrence from agricultural and urban storm water runoff. The concentration of
phosphorus is much lower in nonpoint sources and more dificult to capture.
Phosphate ions are the common form of phosphorus found in water. This is a central
phosphorus atom surrounded by four oxygen atoms and carries three negative charges
in the form of electrons, so it is written as PO 3− . These negative charges have to be coun-
terbalanced by three positive charges, such as sodium. Phosphate ions can combine with
trivalent iron or aluminum cations in acidic soils to form insoluble precipitates.
Removal of phosphorus from wastewater is challenging. Unlike nitrogen, which can be
microbiologically converted to nitrogen gas that escapes passively into the atmosphere,
phosphorus is not volatile. Treatment options include biological or chemical/physical sys-
tems. Biological phosphorus removal entails luxury microbial uptake in an anaerobic/
aerobic sequencing system. The result is excess accumulation in the biomass, beyond nutri-
tional requirements, and removal when the excess biosolids are removed. Phosphorus can
also be precipitated with an iron or aluminum salt, and compounds such as struvite can be
produced by combining phosphorus with magnesium and ammonium. As with biological
systems, a precipitated phosphorus complex must be physically removed from the waste-
water by sedimentation or iltration.
More eficient and economical treatment systems are needed to recover phosphorus from
wastewater. This is where “nanotechnology” shows great promise as an enabler for water
puriication to supplement traditional water puriication methods. Better solutions need to
be lower in overall cost, durable, and more effective for the removal of contaminants from
water. As described in this chapter, nanotechnology shows exceptional performance for
phosphorus removal and can provide faster removal in more compact, lower-cost systems.
A novel composite is described, consisting of a highly porous, iron-based ceramic contain-
ing high concentrations of nanomaterials that are effective in sorbing phosphorus, which
can be subsequently removed to allow reuse of the media and economic recovery of the
phosphorus for other applications.
31.2 Phosphorous Removal Technologies
Phosphorous from domestic, industrial, and agricultural wastewater is typically removed
by chemical, sorption, or biological approaches, and combinations are sometimes used.
Soluble phosphorous can be removed by precipitation with metal salts or sorption of
ions onto a media. Salts of iron, aluminum, calcium, and magnesium are typically used as
precipitating agents. These salts dissolve in water and chemically react to form phosphates,
which can be coagulated and separated through settling and/or the use of ilters. Speciic,
commonly used metal salts include different forms of alum (aluminum sulfate), sodium
aluminate, ferric chloride, ferric sulfate, ferrous sulfate, and ferrous chloride (Bachand,
2003; Panasiuk, 2010; Camm, 2011).
Alternatively, dissolved phosphorous can be removed through “sorption” by a media,
which refers to the combined processes of adsorption and precipitation of soluble phos-
phate. Sorption processes can be categorized into electrostatic forces, physical forces, and
chemical bonding (Minton, 2002). Ion exchange is a form of electrostatic forces. As the
solution passes through the media, a pollutant ion (preferred ion) may be replaced with
an ion from the media (less preferred ion). The media is exhausted when all the least pre-
ferred ions are exchanged by preferred ions (Minton, 2002). During adsorption, matter
Search WWH ::




Custom Search