and emptying of ponds should be preceded
by a detailed evaluation of the wastewater
with a forecast of a phase of controlled
microbial stabilization of the sludge, before
its deposition in agricultural areas.
Adjustment of the quotation stage can be
difficult because of the lack of integration
between production personnel and the oper-
ators of the flotation equipment. One of the
most frequent problems is the discard of the
tanks and hot water in short intervals of
time, causing an abrupt elevation of the vol-
ume, temperature, and pollutants going to
the flotation phase and causing problems for
the biological process, hindering the purifi-
cation and elevating the presence of pollu-
tants.
Secondary Treatment
Treatment through biological (or bacter-
ial) degradation of dissolved organic matter
through biological oxidation is the most
common technique for secondary treatment.
However, secondary treatment can range
from the use of lagoons to sophisticated acti-
vated sludge processes and may also include
chemical treatment to remove phosphorous
and nitrogen or to aid in the flocculation of
solids.
Most lagoons are earthen basins that con-
tain a mixture of water and waste. The mix-
ture in the lagoon is removed continuously
without emptying the lagoon (Safley et al.,
1993). The design of most lagoons is similar.
A dike or berm usually surrounds a lagoon
as a lip of the basin that prevents spills and
overflows. The depth of an impoundment
(lagoon) depends on the volume of waste to
be handled, with increased depth necessary
to contain unforeseeable events, such as
weather.
To accommodate such unforeseeable envi-
ronmental changes, there is usually a storm
event space left free of water. This is usually
the amount of precipitation determined to
have accumulated in 24 hours during the
worst storm in the previous 100 years or the
amount of precipitation from the wettest
month in 25 years. Additional space reserved
for safety measures includes wind set-up and
wave run-up spaces to prevent overflows.
Circular or square lagoons enhance mixing
and are usually less expensive to construct. If
rectangular lagoons are used, a length:width
ratio of 3:1 or less is recommended. Narrow
areas isolated from the main body of water
should be avoided because they may encour-
age mosquito proliferation. Although most
lagoons are approximately 3 m deep, a greater
depth requires less land, enhances mixing, and
minimizes odors.
Lagoons must be sealed to prevent seep-
age that causes groundwater contamination.
A lagoon can be sealed with hard-packed
clay soil or with an industrial liner. A lagoon
is considered sealed in most states if its lower
boundaries (bottom and sides) have a maxi-
mum hydraulic conductivity of 10−^7 cm/sec
(Safley et al., 1993). A minimum of 30 cm of
clay seal on the bottom and sides is required
for most locations, but local ordinances may
vary in their regulations. As lagoon depth
increases, a thicker seal is required. Soil type,
depth to water table, and depth to bedrock
should be considered when locating a
lagoon.
Although primary treatment removes
screenable and readily settleable solid mate-
rial, dissolved solids remain. The primary
purpose of secondary treatment is to con-
tinue the removal of organic matter and to
produce an effluent low in BOD and sus-
pended solids. Microorganisms, most fre-
quently involved in biological oxidation of
existing solids are those that naturally occur
in water and soil environments. Microbial
flora involved in biological oxidations can
assimilate some of the dissolved solids and
convert them into terminal oxidation prod-
ucts, such as carbon dioxide and water, or