TM 5-814-3/AFM 88-11, Volume III
CHARACTERISTICS OF WASTEWATER TREATMENT PLANTS
AND GENERAL PLANT LAYOUT
9-1. Types of plants.
Wastewater treatment plants and processes have been classified as preliminary, primary, secondary, and
advanced in chapter 3. A detailed outline of each process is provided in following chapters.
9-2. Elements of advanced wastewater treatment.
Advanced wastewater treatment encompasses several individual unit operations, used separately or in
combin-ation with other processes, to achieve very high overall treatment efficiencies. These processes
employ physical, chemical and biological treatment methods. The objective of advanced wastewater treatment
is to improve the removal of suspended solids, organic matter; dissolved solids, and nutrients. The design
details for advanced treatment unit operations are presented in chapter 15.
a. Polishing ponds. Polishing ponds are used to obtain increased organic and suspended solids removal
efficiencies up to 20 percent from existing treatment. Treatment by polishing ponds can be aerobic (the
biological activity is predominantly aerobic), or facultative (a combination of aerobic and anaerobic biological
activity). Polishing ponds are also utilized to allow dissipation of chlorine residual to make discharge
compatible with shellfish.
b. Post-aeration. Post-aeration applies when a certain effluent dissolved oxygen level must be
maintained Post-aeration can be achieved by diffused aeration, mechanical aeration, or cascade aeration.
c. Microstraining. Microstraining is an effective effluent polishing device for removal of additional
suspended solids and associated biochemical oxygen demand. The process consists of physical straining of
solids through a screen with continuous backwashing, utilizing a rotating drum to support the screen. Static
screens are also used in particular applications.
d. Filtration. Filtration is an effective method for achieving additional suspended solids and biochemical
oxygen demand removal following conventional treatment processes. Filtration is also very effective as a part
of phosphorus removal systems. Filtration can be applied directly to secondary effluents with or without sedi-
mentation and pretreatment by chemical addition.
e. Adsorption with activated carbon. The primary function of carbon adsorption as a sewage
treatment process is the removal of dissolved organics. This process can be applied as advanced treatment
to adsorb non-biodegradable organics, or as a secondary treatment replacing conventional biological
treatment. However, certain organics with small or highly polar molecules (e.g., methanol, formic acid, and
sugars) are not removable by carbon adsorption.
f. Phosphorus removal. The basic phosphorus removal process consists of conversion of
polyphosphates to soluble forms and then to insoluble forms, and subsequent separation of the insoluble
phosphorus forms from the wastewater accomplished through chemical percipitation using lime or mineral
additives such as alum or ferric chloride.. The process basically involves chemical addition, mixing,
flocculation, and sedimentation.
g. Nitrogen removal
. Methods for removing nitrogen from
wastewater include air stripping, biological
treatment, and breakpoint chlorination. Biological nitrification-denitrification appears to be the most practical
alternative in most applications at this time. It involves the biological oxidation of ammonia to nitrate
followed by anaerobic denitrifaction, with nitrogen leaving as nitrogen gas. Nitrification can be accomplished
as a single stage combined with the activated sludge process or as a separate stage. Denitrification is a
separate operation and can be of "suspended growth" or "attached growth" configuration. In this stage,
nitrate is reduced to carbon dioxide, water and nitrogen gas following addition of methanol to provide the
carbon source. Suspended-growth denitrification is an activated sludge-type operation with mixing but
without aeration; attached-growth denitrification is a packed column process with attached biological growth
on the packing media.