Environmental Engineering Reference
In-Depth Information
pH, surfaces are negatively charged due to the increased prevalence of hydroxide ions
(OH
−
) in solution. Conversely, at a lower pH, positively charged surfaces are formed as
more hydrogen ions (H
+
) are in solution (Figure 31.2). Engineered media are designed to
offer high positive charge at the pH of interest to effectively bind anionic phosphates.
31.2.3.2 Calcium-Rich Media
At a higher pH, phosphate removal with a calcium-rich media occurs by precipitation of
calcium phosphate (Ugurlu and Salman, 1998). Precipitates were originally amorphous
calcium phosphate as shown in x-ray diffraction studies (Johansson and Gustafsson, 2000).
They also found that precipitation was the primary mechanism removing phosphate from
solution, while calcium and phosphorus speciation data ruled out formation of amorphous
calcium phosphates. Instead, evidence indicates that precipitation to hydroxyapatite is the
primary mechanism. Hydroxyapatite has low solubility and may tightly bind phosphorus.
Regardless of the exact mechanism, Johansson and Gustafsson (2000) concluded that the
strongest phosphorus removal is obtained in substrates from which calcium easily leaches
into supersaturate solutions and for substrates that already contain seeds for hydroxy-
apatite or other apatite to enhance precipitation. The rate of calcium desorption may be
a primary factor in sorption experiments that show there is an initial rapid removal of
phosphorus followed by a lower but more sustained removal rate. The initial high removal
rate is attributed to rapid desorption of calcium from the substrate leading to precipitation
(Cheung and Venkitachalam, 2000). For calcium-rich substrates, the calcium content and its
form are the primary factors for phosphorus sorption capacity (Brix et al., 2001; Johannson,
1999), and it was found that amorphous forms of calcium outperformed crystalline forms.
31.2.4 Biological Treatment to Remove Phosphorus
Phosphorus removal from wastewater has long been achieved through biological
assimilation-incorporation as an essential element in biomass, particularly through growth
of photosynthetic organisms (plants, algae, and some bacteria, such as cyanobacteria).
Traditionally, this was achieved through treatment ponds containing planktonic or
attached algae, rooted plants, or even loating plants (e.g., water hyacinths, duckweed).
The net biomass is then removed before it decays and releases phosphorus in water (Strom,
2006c).
Enhanced biological phosphorus removal (EBPR) is a widely used method to decrease
phosphorus in full-scale wastewater treatment plants (Sedlak, 1991). It is of great interest
because it is possible to reach low concentrations (1.0-0.1 mg/L) and can also result in
minimal sludge production and moderate operational cost (Strom, 2006a, b, c). Compared
with chemical precipitation, the main advantage of EBPR is the absence of metal ions from
coagulant in the sludge. While removal of biological oxygen demand (BOD), nitrogen, and
phosphorus can all be achieved in a single system, it can be challenging to achieve very
low concentrations of both total N and P in such systems.
The EBPR microbiology was reviewed by Mino et al. (1998), Mulkerrins et al. (2003), and
Strom (2006c). In brief, phosphate-accumulating organisms (PAOs) store polyphosphate as
an energy reserve in intracellular granules. Under anaerobic conditions, in the presence
of fermentation products, PAOs release orthophosphate, utilizing the energy to accumu-
late simple organics and store them as polyhydroxyalkanoates. Under aerobic conditions,
the PAOs then grow using the stored organic material to take up orthophosphate and
store it as polyphosphate. Thus, PAOs, although strictly aerobic, are selected by having an
