Page 786 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Biological Reactors 741

stage sequence first with ‘‘acid-former’’ bacteria; the reaction
Substrate concentration, S (mg/L)
0 50 100 150 200 250 300 350 products from this first stage are organic acids. In the second
0 stage, the organic acids react with methane bacteria to form
methane and carbon dioxide gases. These same anaerobic reac-
tions can occur with almost any organic substrate, including in
2
the natural environment. Anaerobic treatment, as distinguished
from ‘‘digestion’’ of municipal solids, is used normally only for
4 high-strength wastewaters such as meatpacking wastes, but it
Depth Z (m) 6 is discussed mostly in the context of anaerobic sludge diges-
has been advocated for broader applications.
The anaerobic process is the topic of this major section and

solids to a ‘‘stabilized’’ humus material, which can be easily
8 tion. Its purpose is to transform noxious and infectious organic
dewatered and disposed to a landfill or to agricultural land.
Although anaerobic digestion—sludge drying—disposal
10
remains the most common approach in handling organic
solids, several other methods and combinations have emerged
12 since the early 1960s, summarized in Table 23.7 (see also,
McCarty, 1966). The permissible combinations of flow
FIGURE CD23.8 Trickling-filter model output.
sequences are indicated; ocean disposal has not been permit-
ted since about 1990. The processing and disposal of organic
23.3.2.2 Trickling-Filter Spreadsheet Model solids is a major issue and land disposal has emerged a
Table CD23.6 is an excerpt from a spreadsheet trickling-filter frequent alternative, subject to stringent regulations (see for
model. The spreadsheet is formatted to identify variables and to example, USEPA CFR Parts 257, 403, 503, 1993).
discern their respective effects on output through a linked plot.
An example of a plot copied from the spreadsheet is shown as
Figure CD23.8, which shows the characteristic decline of 23.4.1 EVOLUTION OF SEPARATE SLUDGE DIGESTION
substrate concentration with depth. The finite-difference The problem of sludge disposal was recognized early in the
model, as opposed to a lumped-coefficient model, accounts history of wastewater treatment. It was usual practice, prior to
for the effect of decreasing S on variables that affect S. the advent and widespread practice of using separate sludge
digestion tanks, to associate sludge digestion with primary
settling. Thus the primary settling tanks during the early
23.4 ANAEROBIC REACTORS
period of wastewater treatment, before and around the turn
Anaerobic means ‘‘without oxygen.’’ The anaerobic process is of the century, were designed also to hold large amounts of
used most commonly in municipal wastewater treatment to the accumulated organic solids for sludge digestion. These
‘‘stabilize’’ settled organic solids from clarifiers. In this context were called single-story septic tanks, as illustrated schematic-
it is called ‘‘anaerobic digestion.’’ The solids react in a two- ally in Figure 23.9.

TABLE 23.7
Permissible Flow Sequences of the Organic Solids Stream
Initial Processes Interm. Final Disposal
Grind, Sludge Garden
Anaerobic Vacuum Heat Wet Heat, Drying and Agricultural Land- Ocean
Origin Destination Digestion Filtration Treatment Combustion Bag Beds Lawn Land fill Dumping
Raw material Primary sludge x x x x x
Secondary sludge x x x x
Initial Anaerobic x x
processes digestion
Vacuum filtration x x
Intermediate Heat treatment x x x
processes Wet combustion
Grind heat bag x
Sludge drying x
beds x x x