Civil Engineering Reference
In-Depth Information
and only a small fraction is released to the sewer system. Little research has been
conducted on the effects of washwater recycle on treatment plant efficiency and fin-
ished water quality in iron and manganese removal plants. To minimize any potential
adverse impacts of recycle, it is recommended that recycle flow be continuous and
that recycle flow be kept to below 10 percent of the total flow.
If a utility is considering adding washwater recycle to their facility, the backwash
tank or basin should be covered and protected.
Process Control
On-line finished water turbidity or color instruments provide process monitoring and
control in an iron and manganese treatment plant. To determine the levels of these
metals, the operator takes regular grab samples that are either analyzed in the field or
submitted to a commercial lab. To evaluate and control oxidant dosage, some plants
use on-line chlorine analyzers or redox potential probes. Once the chemical dosage is
set by manually adjusting the stroke length on the chemical feed pump, the feed pump
speed is varied to match the flow through the treatment facility as measured by the
raw water meter. This control system is commonly referred to as flow pacing the feed
pumps. The head loss or solids accumulation in the filter medium is monitored by
pressure sensors on the inlet and outlet of the filter. When the pressure loss across the
filter reaches a preset level, it will automatically trigger the backwash cycle using
motor-controlled valves. A series of alarms monitor critical process set points and have
the ability to shut down the plant if conditions warrant. The modern iron and man-
ganese removal plant is usually unattended for significant periods of time. Program-
mable logic controller (PLC) monitoring tools and the associated alarm systems can
alert the operator of plant problems from remote locations through the SCADA system
via computer modem linkup.
An iron and manganese system treating a groundwater source has only minor var-
iations in raw-water quality. Therefore, once fine-tuned and trimmed, the system will
operate satisfactorily with only minor process adjustments by the operator. If the treat-
ment plant processes water from several wells, minor adjustments in the process may
be required as wells come on or off line. Seasonal variations in water quality may also
result in required process changes. The operator must be careful not to get complacent
in the operation of the system because problems can develop that would be difficult
and expensive to correct.
Conversion to Biological Processes
In Europe, many conventional physical chemical iron and manganese removal plants
are converting to the biological process. These plants are making this change to take
advantage of the features of the biological processes, to reduce operations and main-
tenance costs, to expand plant capacity within the same plant footprint, and in many
cases to improve finished water quality. To date, North American plants have not
shown a similar conversion trend because of an unfamiliarity with the biological pro-
cess, an acceptance of lower-quality water, a lack of desire to commit the monetary
resources required to make the conversion, and a lack of desire to abandon the current
system of treatment. However, the situation is likely to change as utilities and engineers
become more familiar with the biological process, as the public demands higher-quality
water and consents to pay for it, and as additional capacity requirements conflict with
the lack of available space for expansion.
