Water systems of varying sizes that are affected with arsenic
contamination are looking to implement treatment solutions to meet more
stringent standards for arsenic of 10 ppb in drinking water. However,
before a full-scale treatment system can be installed, its performance
must sometimes be verified through laboratory or field pilot testing.
Adsorption using metal oxide media is one of the most promising commercially
available technologies for arsenic removal from ground water sources.
Adsorption is best defined as a continuous flow process conducted at
a loading rate of about 7 gpm/ft² downward through a pressurized
adsorber containing a fixed bed of media. In addition to loading rate,
the other key process parameter of adsorption is empty bed contact time,
which dictates the amount of water resident within the bed of media
required to effect complete arsenic adsorption. The process is simple
and straightforward, offering selective removal of arsenic while having
little to no impact on other water quality parameters.
Metal oxide media such as activated aluminia, ferric oxide, titanium
oxide and zirconium oxide are available for adsorptive arsenic removal
treatment. These media typically tend to have a high capacity, resulting
in long run lengths before before becoming exhausted. Therefore, verifying
the performance of a particular adsorption system can take months when
laboratory and field pilot tests are employed. In addition, testing
of this nature can be costly and the long run times require the use
of large volumes of water. However, the development of rapid small-scale
column testing (RSSCT) has helped to substantially resolve the limitations
of laboratory and field pilot testing.
An RSSCT is a mini‑column test developed to rapidly determine
an adsorptive media’s use rates, bed life, viability and reliability
for use in designing full-scale adsorption systems. Originally developed
to evaluate activated carbon for water treatment, it has been successfully
applied to metal oxide media for arsenic adsorption. The method uses
adsorption theory to develop scaling relationships which allow the performance
of small columns to be scaled up to predict performance and aid in the
design of a full-scale treatment operation system. The RSSCT procedure
is a less expensive testing option compared to traditional methods of
field pilot work.
Conducted
in a laboratory and following a specific protocol, the RSSCT is a dynamic
bench scale test that accelerates the adsorption cycle by scaling down
conventional testing hardware. The test design ratios the full-scale
system media mean particle diameter (MPD) to the mini‑column MPD,
i.e., MPD (full scale)/MPD (mini‑column). Selectivity, capacity
and adsorption kinetics are the three principal factors affecting the
performance of an adsorbent media.
An RSSCT is conducted using actual water from the well or project
site and is pumped through a small column that has similar kinetics
of the full-scale system. The results can be obtained in a fraction
of the time, compared with on‑site pilot testing, and easily scaled
up to simulate full-scale operation with reliability. The RSSCT process
typically takes one month to complete.
The RSSCT columns are 1.1 cm diameter glass columns approximately
30.5 cm in length with Teflon end caps. The 3.2 mm diameter influent
and effluent tubing, also made of Teflon, is connected to the column
end caps using PVC pressure fittings. Glass wool is typically packed
at the top and the bottom of the column to support the media. The free
space of the column has been filled with 5 mm diameter borosilicate
glass beads. Before adjusting the media length and the flow rate, the
columns will be back washed to remove the fines. The back-washing procedure
consists of connecting the columns in an up-flow configuration and passing
ultrapure water through the column until the water runs clear. The typical
flow rate for the mini-column back washing is in the range of 10-20
ml/min and results in bed expansion of approximately 40 percent.
Conducting an RSSCT can take as little as one-fourth the time of traditional
field tests. With concern over arsenic contamination growing worldwide
and the implementation of arsenic treatment solutions accelerating,
such time savings are likely to prove attractive for water systems.
For more information on RSSCT, contact Rich Dennis, Separation
Products manager for Severn Trent Services at rdennis@severntrentservices.com.
For more information, email info@severntrentservices.com