Environmental Engineering Reference
In-Depth Information
(adsorbate such as molecules, ions, particles, polymers, colloids) dispersed in solution
accumulates on the surface of an adsorbent (Kasprzyk-Hordern, 2004). Adsorption may
occur through weaker physical sorption processes or through a stronger chemisorption
where bonds are formed between the adsorbate and adsorbent. Physical forces are due to
van der Waal interactions as partial charges of the adsorbate are attracted to the electro-
static charges of the adsorbent (Minton, 2002). Chemisorption is a much stronger interac-
tion but requires an available sorption site on the adsorbent. Once chemical bonds form,
desorption becomes dificult. In general, physical adsorption has rapid kinetics, followed
by a slower diffusion of adsorbed matter into the matrix of the adsorbent (Kim et al.,
2008).
Phosphorus-sorbing materials typically contain appreciable concentrations of alumi-
num (Al), iron (Fe), lanthanum (La), calcium (Ca), or magnesium (Mg). Sorption media
can be naturally occurring materials, by-products of industrial processes or engineered
products.
31.2.1 Chemical Precipitation
Chemical precipitation is widely used for removal of phosphorus. In wastewater treat-
ment, the chemical may be added before primary or secondary treatment or as part of
a tertiary treatment process. The effects of phosphorus and calcium concentrations, pH,
temperature, and ionic strength on theoretical removal are thermodynamically modeled
(Song et al., 2002); however, further studies inferred these processes to be more complex
than theoretically predicted. A major concern in all precipitation processes is formation of
excess sludge that needs to be removed or processed, which leads to additional costs for
treatments. Sludge formation is most pronounced when lime (CaOH
2
) is used for precipita-
tion. Predictions concerning the best precipitation practices also vary widely in different
studies (Takacs, 2006; Neethling and Gu, 2006).
The required dosage level of the precipitating chemical is related to the concentration of
the soluble phosphorous. For example, if the target concentrations are >2 mg/L, a dose of
1.0 mol of aluminum or iron per mole of phosphorus is suficient. For lower phosphorus
concentrations (0.3-1.0 mg/L), the dose must be in the range of 1.2-4.0 mol of aluminum or
iron per mole of phosphorus. The pH value is also an important factor for eficient removal
of phosphorus, when using alum or other salts, as the solubility of their precipitates vary
with pH. For alum, phosphorus removal is most eficient in a pH range 5-7 and for ferric
salts it is 6.5-7.5. It should be emphasized that the impact of different competing reactions
occurs and needs to be taken into account. Therefore, the amount of metal salts needed
has to be determined practically in each case rather than simply basing calculations on
the theoretical chemical reaction with phosphorus (Tchobanoglous and Burton, 1991). If
removal involves lime, care is needed to ensure reaction between excess calcium ions and
phosphate (Tchobanoglous et al., 2002). While lime can be an effective agent for phospho-
rus removal, its application is slightly diminished because of the high volume of produced
sludge.
31.2.2 Sorption of Phosphorous
A wide variety of materials containing calcium, aluminum, iron, magnesium, and lantha-
num have also been used for the sorption of phosphorus. Key media characteristics are
composition, surface area, porosity, hydraulic conductivity, and cost.
