Continuous Cultivation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 637

Dichromate may be used as an oxidizing agent, and by a redox balance, the amount of
oxygen required to oxidize organic compounds can be calculated. This method is faster
(order of 3 h), easier, and less expensive than BOD measurements.
The TOC content of wastewater samples can be determined by using a TOC analyzer.
After proper dilutions, samples are injected into a high-temperature (680 e 1,350 C) furnace
and all organic carbon compounds are oxidized to CO 2 , which is measured by an infrared
analyzer. To determine the TOC content, wastewater samples should be acidified to remove
inorganic carbon compounds (mainly carbonates). The total carbon content of wastewater
can be determined before and after acidification, and the difference is inorganic carbon
content.
The nitrogen content of wastewater samples is usually measured by total Kjeldahl
nitrogen determination. Other key nutrient concentrations, such as phosphate, sulfur, and
toxic compounds, should be determined before waste streams are treated.
The concentration of biomass in a waste treatment system using suspended cells is
measured as combustible solids at 600 C (CS600) or some referred to as the mixed-liquor
volatile suspended solids (MLVSS). Basically wastewater of known volume is filtered and
the collected solids dried and weighed to give mixed-liquor suspended solids. This material
is then “volatilized” by burning in air at 600 C. The weight of the remaining noncombustible
inorganic material is the “fixed” solids. The difference between the original mass prior to
combustion and the fixed solids is the combustible or “volatile” portion at 600 C. The
CS600 or MLVSS is assumed to be primarily microbes, although carbonaceous particles are
included in the measurement. A typical waste treatment operation employing biological
treatment includes the following steps:

(1) Primary treatment includes the removal of coarse solids and suspended matter
(screening, sedimentation, and filtration) and conditioning of the wastewater stream by
pH adjustment and nutrient additions (e.g. PO 3 4 and NH 4 ).

(2) Secondary treatment is the major step in biological treatment; it includes biological
oxidation or anaerobic treatment of soluble and insoluble organic compounds. Organic
compounds are oxidized to CO 2 and H 2 O by organisms under aerobic conditions.
Unoxidized organic compounds and solids from aerobic treatment (e.g. cell wall material,
lipids d fats) are decomposed to a mixture of CH 4 ,CO 2 , and H 2 S under anaerobic
conditions. A sludge of undecomposed material must be purged from either system.
(3) Tertiary treatment includes the removal of the remaining inorganic compounds
(phosphate, sulfate, and ammonium) and other refractory organic compounds by one or
more physical separation methods, such as carbon adsorption, deep-bed filtration, and in
some cases membrane-based techniques, such as reverse osmosis or electrodialysis.

Biological wastewater treatment usually employs a mixed culture of organisms whose
makeup varies with the nature of the waste. Biological treatment may be aerobic or anaer-
obic. The major aerobic processes (or reactor types) used in wastewater treatment are 1) acti-
vated sludge, 2) trickling filter, 3) rotating biological contractors, and 4) oxidation ponds.
A schematic of a typical wastewater treatment flow sheet is shown in Fig. 12.12 . Activated
sludge processes include a well-agitated and aerated continuous-flow reactor and a settling
tank. Depending on the physical design of the tank and how feed is introduced into the
tank, it may approximate either a PFR or CSTR. A long narrow tank with single feed