The new maximum contaminant level (MCL) of 10 parts per
billion (ppb) for arsenic presents tremendous opportunity for the water treatment
industry, though for many dealers, making decisions on specific treatment
options remains confusing at best. A wide range of technologies, some new and
some more traditional, is being marketed and applied for arsenic treatment.
Each of these technologies has specific properties impacting its suitability
for any particular scale of application.
The breadth of the arsenic issue and the growing need for a
diverse solution set have prompted some manufacturers to explore the boundaries
of available technologies and, in some cases, extend their application into
other platforms. Ferric oxide-based adsorbents, for example, have been applied
in municipal systems for arsenic treatment for several years. Over the past two
years, the technology has extended to point-of-entry and point-of-use
applications and most recently has been proven effective in a carbon block
While rare, the ability of a single water treatment
technology to perform effectively across many treatment platforms is not
unique. Activated carbon is perhaps the most obvious example of a technology
that has proven successful across a very wide range of application scales.
Modification and engineering are, of course, required to adapt a technology to
a specific scale/platform, but the engineering effort required to transition an
adsorbent-based technology into other platforms often is much smaller than for
From a technical design point-of-view, adsorbents are
reasonably easy to apply as fewer factors typically impact adsorbent system
performance than with other technologies. When applying adsorbents to drinking
water treatment at any scale, only a handful of factors generally are
of the particular adsorbent for the contaminant of interest
area of the adsorbent
of the water
drop and occluding species
From a pragmatic, end-user?s point of view, however,
most of the above are taken care of by the manufacturer?s
characterization and usage recommendations for the adsorption media. Consumer
or operator concerns essentially can be limited to capacity/lifetime, changeout
and disposal of the media.
Primarily due to their simplicity of application and
operation, adsorption media have been more widely accepted and applied than
other technologies. Chemical additions, feed pumps and real-time monitoring
generally are not required, making operation and maintenance comparatively
Expanding the application of a technology to other treatment
platforms requires more than just easy application. The economics involved must
be viable, as well. For example, technology costs extend far beyond, the
cost/capacity ratio of media.
When considering new products or applications, the practical
costs of operation and maintenance, costs of data accumulation, testing and
research and development, and marketing costs must be factored in to yield a
realistic cost per gallon to the end user. Because of the systems?
simplicity, operations and maintenance costs of adsorbent-based systems often
can be kept lower than systems based on other technologies. Selection of
appropriate adsorption media, which exhibit high capacity, good selectivity and
few pretreatment concerns, further increases cost effectiveness. The existing
knowledgebase around the application of adsorption to drinking water treatment
is extensive, and because the process is fairly well understood and the
application mechanics are reasonably simple, product development costs often
are lower than with other technologies. In some cases such as with granular
ferric oxide, the technology has been applied for years at the municipal level,
giving prospective point-of-entry or point-of-use users not only a high comfort
level, but a large existing base of fundamental performance data. With
technologies where such history does not exist, larger amounts of fundamental
research must be directed at understanding the basic technology or material in
addition to research into platform-specific performance.
An existing water treatment history on other platforms also
may mean that a product has third-party testing such as NSF 61
certification?a tremendous benefit both from a user acceptance and a
development cost point of view.
So far, it is concluded that the success of a treatment
technology extending across many platforms is reliant on ease of application
and cost effectiveness of the solution. There exist some practical limitations,
of course. For example, it is doubtful, for example, that even if technically
possible, arsenic removal with lime-softening on a point-of-use platform would
ever prove commercially viable.
Table 1 lists most of the commercially available treatment
technologies for arsenic removal and outlines characteristics of each that
allow suitability for various treatment platforms. Of the arsenic removal
technologies listed, granular ferric oxide demonstrates one of the widest
applicability profiles. A brief review of the application of the technology to
each treatment platform, beginning with large municipal systems and
highlighting particular strengths/ weaknesses of the technology which make it
suitable for each application now will be given and will use granular ferric oxide
as an example.
Granular ferric oxide first was commercially applied to
arsenic treatment in municipal water systems in the United Kingdom. Picture 1
shows an example of a 6.0 mgd municipal ferric oxide-based treatment system.
Today, more than 500,000 people are supplied water from municipal systems using
granular ferric oxide for arsenic removal. Granular ferric oxide (GFO) should
not be confused with other ferric-based products. GFO is a dry material and has
a ferric oxide assay of 88 percent. This material has not shown itself prone to
iron bacteria growth or severe handling and pressure drop complaints due to
excessive fines. The capacity of this material versus ferric treated adsorbents
has shown significant performance differences.
As Table 1 indicates, ferric oxide also has been
successfully applied to small community water systems. Because of their smaller
scale and different flow requirements, small community adsorption systems
require faster adsorption kinetics than do larger municipal systems, as bed
contact times typically are shorter. The rapid kinetics of granular ferric
oxide suit these requirements reasonably well. Unlike larger municipal systems,
many small systems have no form of disinfection and, therefore, preoxidation of
As (III) to As (V) may be considered impractical in small community systems.
Therefore, the ability to remove both species with a single treatment step is
more critical. While pH adjustment certainly is possible at the small water
system level, it is preferable to utilize an arsenic treatment technology that
does not require the added complexity and cost of pH adjustment. The broad
range of pH over which ferric oxide is effective (typically 5.5?8.5)
makes it an attractive solution for the small community water system platform.
Picture 2 shows a typical ferric oxide-based arsenic adsorption system for
small water system applications.
As a technology is extended into the
whole-house/point-of-entry platform the number factors affecting suitability
are further increased. Picture 3 shows a typical point-of-entry application of
granular ferric oxide. Application of the media on a point-of-entry platform
typically mandates even faster kinetics as available space for adsorption beds
is considerably reduced. Perhaps even more importantly, the successful
point-of-entry application requires that operations and maintenance by the
homeowner be minimal. Point-of-entry technologies that require regeneration
with chemicals or pH adjustment using acids or caustics present an unattractive
level of complexity to the homeowner and increase the probability of
operational problems. Media disposal also becomes even more important at the
point-of-entry level. Adsorption media that bind contaminants tightly and pass
the EPA?s TCLP test generally are disposed of in a sanitary solid waste
landfill, eliminating the costs and complexity to the homeowner of dealing with
hazardous material disposal.
From a design standpoint, point-of-use platforms introduce
an even larger set of challenges to an arsenic removal technology. Rapid
kinetics and high capacity are absolutely critical at this scale. Dry, granular
adsorbents often can be adapted to point-of-use applications relatively easily
by modifying particle size. Decreasing particle size has the effect of speeding
up adsorption kinetics and increasing capacity by increasing available surface
area for adsorption. Trade-offs must be made, of course, as smaller particle
sizes typically produce increased pressure drops. An adsorption medium also
must demonstrate good handling characteristics as automated cartridge
production typically requires a dry, flowable material. If a material breaks
down easily on handling, it may generate too many fine particles and cause
issues with plugging and pressure drop and may even result in small particles
of media in the cartridge effluent. At the point-of-use scale, pretreatment
ability is minimal and pH adjustment is impractical, so a technology either
must be capable of ?standing on its own? or working in conjunction
with other, conventional point-of-use technologies. Picture 4 shows examples of
granular ferric oxide point-of-use devices.
Recently, ferric oxide technology was extended into the
carbon block platform. Incorporating an adsorption technology into a carbon
block requires a large amount of development effort. In addition to the general
point-of-use criteria discussed above, an adsorption medium must exhibit
physical/chemical characteristics enabling it to be compounded into block form
with other media. Formulating a carbon block often results in the obscuration
of a portion of the surface area of the medium, thereby further increasing the
requirements for adsorption capacity and very rapid kinetics. Picture 5 shows
recently developed carbon block products incorporating granular ferric oxide to
achieve an arsenic claim.
There certainly is no requirement that a treatment
technology be applicable to a wide range of platforms. Many treatment
technologies perform well at specific scales or under specific conditions. Very
few are capable of performing on all treatment platforms. Those that are
capable of doing so typically are those that are capable of handling a broad
range of water parameters; are easy to use, handle and process; demonstrate
high capacities; have rapid adsorption kinetics; and can be engineered into
economically viable solutions.