Environmental Engineering Reference
In-Depth Information
10.6.5 Thermal Treatment
Thermal treatment (or wet air oxidation) subjects sludge to a high
temperature and pressure in a closed reactor vessel. The high tempera-
ture and pressure rupture the cell walls of any microorganisms present
in the solids, causing chemical oxidation of the organic matter. This pro-
cess substantially improves dewatering and reduces the volume of mate-
rial for disposal. It also produces a very high strength waste, which must
be returned to the wastewater treatment system for further treatment.
10.6.6 Chlorine oxidation
Chlorine oxidation also occurs in a closed vessel. In this process,
chlorine (100 to 1000 mg/L) is mixed with a recycled solids flow. The
recycled flow and process residual flow are mixed in the reactor. The sol-
ids and water are separated after leaving the reactor vessel. The water is
returned to the wastewater treatment system, and the treated solids are
dewatered for disposal. The main advantage of chlorine oxidation is that
it can be operated intermittently. The main disadvantage is production
of extremely low pH and high chlorine content in the supernatant.
10.6.7 stabilization operation and Performance
Depending on the stabilization process employed, the operational
components vary. In general, operations include pumping, observations,
sampling and testing, process control calculations, maintenance, and
housekeeping. Performance of the stabilization process will also vary
with the type of process used. Generally, stabilization processes can
produce a 40 to 60% reduction of both volatile matter (organic content)
and moisture.
10.7 sludge deWaTering
Digested sludge removed from the digester is still mostly liquid.
Sludge dewatering is used to reduce volume by removing the water to
permit easy handling and economical reuse or disposal. Dewatering
processes include sand drying beds, vacuum filters, centrifuges, filter
presses (belt and plate), and incineration.
10.7.1 sand drying beds
Drying beds have been used successfully for years to dewater sludge.
Composed of a sand bed (consisting of a gravel base, underdrains, and
8 to 12 inches of filter-grade sand), drying beds include an inlet pipe,
splash pad containment walls, and a system to return filtrate (water) for
treatment. In some cases, the sand beds are covered to protect drying
solids from the elements. In operation, solids are pumped to the sand
bed and allowed to dry by first draining off excess water through the
sand and then by evaporation. This is the simplest and least expensive
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