Environmental Engineering Reference
In-Depth Information
32.4.1.3 treatment of other organics
Synthetic organic materials including solvents, paints, pharmaceuticals, pesticides, coking
products, and so forth can be very difficult to treat. Treatment methods are often specific to the
material being treated. Methods include advanced oxidation processing, distillation, adsorption,
vitrification, incineration, chemical immobilization, and landfill disposal. Some materials such as
some detergents may be capable of biological degradation and in such cases, a modified form of
wastewater treatment can be used.
32.4.1.4 treatment of toxic materials
Toxic materials, including many organic materials, metals (such as zinc, silver, cadmium, thallium
etc.) acids, alkalis, and nonmetallic elements (such as arsenic or selenium), are generally resistant to
biological processes unless very dilute. Metals can often be precipitated out by changing the pH or
by treatment with other chemicals. Many, however, are resistant to treatment or mitigation and may
require concentration followed by landfilling or recycling. Dissolved organics can be incinerated
within the wastewater by advanced oxidation processes.
Wastewater originated at farms can be treated with different systems. Simple systems are
anaerobic lagoons (Ritter et al. 1984; Oleszkiewicz 1985; Boiran et al. 1996), where organic
compounds could be effectively treated by a pretreatment system. The problem with lagoons is bad
construction, especially the lagoon bottom. Such lagoons may influence groundwater because of
percolation of wastewater through soil.
Another very frequently used wastewater system is the UASB reactor (Kalyuzhnyi et al. 1998;
Correa et al. 2003; Garcia et al. 2008; Mahmoud 2008; Yetilmezsoy and Sakar 2008, which is
useful for strong wastewaters. During the treatment at anaerobic conditions, we can obtain biogas
that contains up to 70% methane gas.
Many wastewaters, especially from pig farms, are treated in SBR systems (Bernet 2000; Ra
2000; Tilche 2001; Kishida et al. 2003; Obaja et al. 2003, 2005; Ndegwa 2004; Zhang et al. 2006).
32.4.2 u
pflow
a
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ludgE
B
lankEt
UASB technology (Figure 32.12), normally referred to as the UASB reactor, is a form of anaerobic
digester that is used in the treatment of highly polluted wastewater.
The UASB reactor is a methanogenic (methane-producing) digester that evolved from the
anaerobic digester. A similar but variant technology to UASB is the expanded granular sludge bed
(EGSB) digester.
UASB uses an anaerobic process while forming a blanket of granular sludge that suspends in the
tank. Wastewater flows upwards through the blanket and is processed (degraded) by the anaerobic
microorganisms. The upward flow combined with the settling action of gravity suspends the blanket
with the aid of flocculants. The blanket begins to reach maturity at around three months. Small
sludge granules begin to form, whose surface area is covered in aggregations of bacteria. In the
absence of any support matrix, the flow condition creates a selective environment in which only
those microorganisms, capable of attaching to each other, survive and proliferate. Eventually the
aggregates form into dense compact biofilms referred to as “granules.”
Biogas with a high concentration of methane is produced as a byproduct, and this may be captured
and used as an energy source to generate electricity for export and to cover its own running power.
The technology needs constant monitoring when put into use to ensure that the sludge blanket is
maintained and not washed out (thereby losing the effect). The heat produced as a byproduct of
electricity generation can be reused to heat the digestion tanks.
The blanketing of the sludge enables a dual solid and hydraulic (liquid) retention time in the
digesters. Solids requiring a high degree of digestion can remain in the reactors for periods up to
