Environmental Engineering Reference
In-Depth Information
In natural water and wastewaters, phosphorus occurs mostly as dissolved
orthophosphates and polyphosphates, and organically bound phosphorous. Changes
between these forms occur continuously due to decomposition and synthesis of
organically bound forms and oxidized inorganic forms, at different pH values.
Phosphorus in water bodies' sediments can be mobilized by bacteria and released to the
water column. Sources of phosphorus include domestic wastewater, industrial effluents,
detergents and fertilizers that could be washed out with the runoff from agricultural land.
In most natural surface waters, phosphorus ranges from 0.005 to 0.020 mg PO 4 -P/l
(Chapman 1998).
2.2.3 Inorganic constituents - metals
“Heavy metals” is a generally collective term applying to the group of metals and
metalloids with an atomic density greater than 6 g/cm 3 . Although it is only a loosely
defined term, it is widely applied to elements such as Cd, Cr, Cu, Pb, Zn, Ni and Hg,
which are commonly associated with pollution and toxicity problems. In contrast, the
ability of a water body to support aquatic life, as well as its suitability for other uses
depends on many trace elements. Some metals e.g., Mn, Zn, and Cu present in trace
concentrations, are important for the physiological functions of living tissue and regulate
many biological processes. However, even small amounts of Hg and Cd can be toxic to
both humans and aquatic life. The absence of Fe and Mn in some priority pollutants' lists
results from their frequent classification as major ions (Chapman 1998). The effect of
these metals on aquatic organisms depends upon the bioavailabilty of the metals to
organisms. Bioavailability is influenced by water hardness, pH, life cycle (age) of
organisms and water temperature. Water hardness diminishes the potential of dissolved
metals to cause toxicity to aquatic life. Softer waters tend to make the metals even more
toxic. In surface waters, at normal pH and redox conditions, most trace elements are
readily adsorbed onto particulate matter. Consequently, the actual dissolved element
concentrations are very low and the monitoring of trace metals on a routine basis is
difficult due to the contamination of the sample from air, particularly lead. The toxicity of
metals in water also depends on the degree of oxidation of a given metal ion together
with the forms in which it occurs. For instance, the maximum allowable concentration of
Cr +6 in USA is 0.001 mg/l whereas for Cr +3 it is 0.5 mg/l (Chapman 1998). As a rule, the
ionic form of a metal is the most toxic form. However, toxicity is reduced if the ions are
bound into complexes with, for example, natural organic matter such as fulvic and humic
acids.
Considering the toxicity of different water quality constituents, normally the criteria
used to limit their concentrations in order to prevent health or environmental hazards, are
determined based on experimental or epidemiological studies. The last are long and
costly investigations and in many cases the results are unreliable. Experimental studies
are usually carried on animals whose reactions are known to be similar to the humans'.
The results obtained are then extrapolated to conform to human conditions by means of
models. The effect of toxic constituents on human health and aquatic life could be
expressed in various ways. The following definitions (after Degremont 1991) could be
useful in order to understand the effect of different pollutant constituents and
corresponding criteria for their regulation:
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