WATER QUALITY - Water Quality Engineering in Natural Systems - page 15

aquatic vegetation, and clogs the gills of fish. The sedi-
mentation of suspended solids in receiving waters can
cause a buildup of organic matter in the sediments,
leading to an oxygen-demanding sludge deposit. This
sludge deposit can also adversely affect fish populations
by reducing their growth rate and resistance to disease,
preventing the development of eggs and larvae, and
reducing the amount of food available on the bottom of
the water body. land erosion from human activities,
such as mining, construction, logging, and farming, is the
major cause of suspended sediment in surface runoff.
Well-operated municipal wastewater treatment
plants produce effluents with SS values of less than
30 mg/l. However, it is recommended that communities
that discharge treated domestic wastewater into lakes
or pristine streams reduce their SS values to less than
10 mg/l to protect the aquatic life in the receiving water
(Serrano, 1997).

For 1 cm (= 0.01 m) to cover the bottom of the
300 × 100 m lake, the volume of sediment is
0.01(300)(100) = 300 m 3 . Hence, the time required for
300 m 3 of sediment to accumulate is given by

accumulation time for 1 cm of sediment

300
9 38 10
3 20 10

=

.
.

×

− 7

8

= × seconds = 3700 days = 10 . years

It is interesting to note that at this rate of sediment
accumulation, the lake will be filled completely in
approximately 20,000 years.

2.3.4 Nutrients

Nutrients are the essential elements to sustain growth
and life function. Of the approximately 100 elements in
the periodic table, about 30 are constituents of living
things and can be broadly classified as nutrients. Some
of these nutrients are required in relatively large
amounts and are termed macronutrients , whereas others
are needed in only trace quantities and are called micro-
nutrients . Despite the fact that some elements are
required only in trace quantities, their availability
may control the productivity of the entire ecosystem.
Included in this group of potentially limiting elements
are nitrogen and phosphorus. Both nitrogen and phos-
phorus are widely used in fertilizers and phosphorus-
based household detergents, are commonly found in
food-processing wastes and animal and human excre-
ment, and are most responsible for the overenrichment
of nutrients in surface waters. Nutrients in fertilizers
tend to bind to clay and humus particles in soils and are
easily transported to surface waters through erosion
and runoff. Other significant sources of nutrients include
malfunctioning septic tanks and effluents from sewage
treatment plants. Nutrients are considered as pollutants
when their concentrations are sufficient to cause exces-
sive growth of aquatic plants. Excessive plant growth
associated with overenrichment of nutrients causes
oxygen depletion, which causes increased stress on
aquatic organisms, such as fish. In addition to threaten-
ing the viability of aquatic life, excessive amounts of
algae and decaying organic matter cause color, turbidity,
odors, and objectionable tastes that are difficult to
remove and that can greatly reduce the acceptability of
the water body as a source of domestic drinking water.
In most cases, phosphorus is the limiting nutrient in
freshwater aquatic systems, and nitrogen is the limiting
nutrient in estuarine and coastal waters.

EXAMPLE 2.3

An outfall discharges wastewater into a flood-control
lake that is approximately 300 m long, 100 m wide, and
20 m deep. The suspended solids concentration in the
wastewater is 30 mg/l, the wastewater discharge rate is
0.05 m 3 is and the bulk density of the settled solids is
1600 kg/m 3 . Assuming that all of the suspended solids
ultimately settle out in the lake, estimate the time
required for 1 cm of sediment to accumulate at the
bottom of the lake.

Solution

The SS concentration in the wastewater is
30 mg/l = 0.03 kg/m 3 , and the discharge flow rate is
0.05 m 3 /s. under steady-state conditions, the rate at
which suspended solids are discharged into the lake is
equal to the rate of sediment accumulation at the
bottom of the lake and is given by

sediment mass accumulation rate

=
=

( . 0 03 0 05

0 0015

)( . )

. kg/s

Since the bulk density of the settled solids is 1600 kg/m 3 ,
the rate of volume accumulation is

sediment volume accumulation rate

0 0015
1600
9 38 10 7

.

=

2.3.4.1 Nitrogen. Nitrogen stimulates the growth of
algae, and the oxidation of nitrogen species can consume

= . ×

−

m /s

3