and Reid, 1966; Craggs, 2005; Middl

and Reid, 1966; Craggs, 2005; Middlebrooks and Pano, 1983;
Middlebrooks et al. 1982; Pano and Middlebrooks, 1982).
This nitrogen loss may be due to algal uptake or bacterial
action. It is likely that both mechanisms contribute to the
overall total nitrogen reduction. Another factor contributing
to the reduction of total nitrogen is the removal of gaseous
NH
3 under favorable environmental conditions. Regardless
of the specific removal mechanism involved, NH3 removal
in facultative wastewater ponds has been observed at levels
greater than 90%, with the major removal occurring in the
primary cell of a multicell pond system (Crites et al., 2006;
Middlebrooks et al., 1982; Shilton, 2005; USEPA, 2011).
21.8.1.7.2 Phosphorus
Phosphorus (P) is most often the growth-limiting nutrient in
aquatic environments. Municipal wastewater in the United
States is normally enriched in phosphorus even though
restrictions on phosphorus-containing compounds in laundry
detergents in some states have resulted in reduced concentrations since the 1970s. As of 1999, 27 states and the District of
Columbia had passed laws prohibiting the manufacture and
use of laundry detergents containing phosphorus. However,
phosphate (PO43–) content limits in automatic dishwashing
detergents and other household cleaning agents containing
phosphorus remain unchanged in most states. With a contribution of approximately 15%, the concentration of phosphorus from wastewater treatment plants is still adequate to
promote growth in aquatic organisms.
In aquatic environments, phosphorus occurs in three
forms: (1) particulate phosphorus, (2) soluble organic phosphorus, and (3) inorganic phosphorus. Inorganic phosphorus,
primarily in the form of orthophosphate (OP(OR)3), is readily utilized by aquatic organisms. Some organisms may store
excess phosphorus as polyphosphate. At the same time, some
PO
4
3– is continuously lost to sediments, where it is locked up
in insoluble precipitates (Craggs, 2005; Crites et al., 2006;
Lynch and Poole, 1979). Phosphorus removal in ponds occurs
via physical mechanisms such as adsorption, coagulation,
and precipitation. The uptake of phosphorus by organisms in
metabolic function as well as for storage can also contribute to
its removal. Removal in wastewater ponds has been reported
to range from 30 to 95% (Assenzo and Reid, 1966; Crites et
al., 2006; Pearson, 2005). Algae discharged in the final effluent may introduce organic phosphorus to receiving waters.
Excessive algal “afterblooms” observed in waters receiving
effluents have, in some cases, been attributed to nitrogen and
phosphorus compounds remaining in the treated wastewater.
21.8.1.7.3 Sulfur
Sulfur (S) is a required nutrient for microorganisms, and it is
usually present in sufficient concentration in natural waters.
Because sulfur is rarely limiting, its removal from wastewater is usually not considered necessary. Ecologically, sulfur
compounds such as hydrogen sulfide (H2S) and sulfuric acid
(H2SO4) are toxic, while the oxidation of certain sulfur compounds is an important energy source for some aquatic bacteria (Lynch and Poole, 1979; Pearson, 2005).