Page 49 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

4 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

1.3 UNIT PROCESSES
BOX 1.1 CONTEXT AND FIT
This book presents the topic of water treatment in terms of
A particular type of situation is the context for a design
unit processes, which are discussed in the following chapters.
and has to do with water quality, treatment objectives,
For each unit process considered, principles and practices are
operation capabilities, financing, etc. The form is the unit
explained. The unit processes approach is common to the field
process=technology adopted. The fit is the relationship
of chemical engineering and has been assimilated by the field
between the context and the form.An appropriate fitis
of environmental engineering.
desired. In other words, the process=technology selected
must be appropriate for the context. These ideas are
central to design.
1.3.1 DEFINITIONS
As an example, a slow sand filter may be appropriate
technology for a small community, since materials are A water treatment unit process is definedasanengineered
available locally, operation is ‘‘passive,’’ i.e., not requir- system to effect certain intended state changes for the water.
ing operator skill, and parts are simple and easily Examples include screening, gravity settling, coagulation,
repaired locally. On the other hand, slow sand may flocculation, filtration, gas transfer, ion exchange, adsorp-
not be appropriate for a large community as the labor tion, membrane separations, biological treatment, disinfec-
requirement is much more than for a rapid rate plant. tion, oxidation, and chemical precipitation (Sanks, 1978;
For the latter, automation is feasible and parts and Letterman, 1999). These unit processes are the topics of this
skilled labor are likely to be readily available. book. Some of the definitions are as follows:
. Screening. The retention of a substance by a screen
that has a mesh size smaller than the substance to be
In some cases, the paradigm for thinking has been along retained.
the lines of treatment for specific contaminants, mostly those . Gravity settling. A particle falling under the influ-
that are regulated, or perhaps those that pose an industry ence of gravity is called sedimentation.
problem. For example, halogenated hydrocarbons have been . Coagulation. The charge neutralization of a nega-
regulated in drinking water in 1978 starting with tri-halo- tively charged colloid, usually by chemical means,
methanes. Nutrients in wastewater have been a treatment such as the use of alum or a ferric compound.
focus since the early 1970s as a means to reduce algae blooms . Flocculation. A unit process that promotes collisions
in ambient waters. Heavy metals, an industrial waste problem, between particles that attach to each other upon
have also been regulated since the 1970s. There are books as contact, growing in size to increase settling velocity.
well as articles dedicated to these topics. . Filtration. The convection of a water stream through
In the 1960s, some in the academic community began to a porous media with the intent to retain suspended
adopt the unit operations=unit processes approach in organiz- particles within the media.
ing the knowledge of water treatment. This approach was . Gas transfer. The transport of gas between the dis-
inspired by two books, Unit Operations of Sanitary Engineer- solved phase in water and a gas phase.
ing (Rich, 1961) and Unit Processes of Sanitary Engineering . Ion exchange. The exchange of benign ions (such as
(Rich, 1963), and reinforced by persons pursuing graduate Na ) bonded to sites within an ion-exchange mater-
þ
degrees who had taken courses in chemical engineering. ial (such as a zeolite mineral or a synthetic resin)
In comparing the three approaches, the first is limited in intended to be displaced by an ion targeted for
perspective to the problem area at hand (e.g., drinking water removal (such as Ca ) that has a stronger bonding
2þ
treatment, municipal wastewater treatment, industrial waste- force.
water treatment, and hazardous wastes treatment). Screening, . Adsorption. The attachment of a molecule to an
e.g., as a bar screen in wastewater treatment is viewed as a means adsorption site provided by an internal surface of
to protect pumps from items that might cause clogging. In water an adsorbent material. Activated carbon is the best-
treatment, screening may be a microscreen to remove algae. As a known adsorbent for an engineered system, although
unit process, screening is a means to retain objects and particles virtually any solid material can provide adsorption
for whatever purpose. The difference is in the viewpoint. The sites.
first two are by nature empirical and specific, while the third is . Membrane separation. The four types of membrane
adaptable to the purpose without the constraints of tradition. processes are as follows:
With the first two approaches, one lacks the broad perspective 1. Microfiltration (removes colloids and bacteria)
that screening may be applied to any problem area. 2. Ultrafiltration (removes viruses)
In this book, we use the unit processes approach. The unit 3. Nano-filtration (removes large molecular weight
process approach includes operative principles, traditions, organic molecules and some ions)
practices, empirical methods, technology forms, and the spec- 4. Hyper-filtration, more commonly called ‘‘reverse
trum of applications. osmosis’’ (removes molecules and ions)