Navigating Changing Feedwater Conditions

Reverse Osmosis Eliminates TDS in Boiler Feedwater

Covanta Pasco, Inc. 
is a resource recovery facility 
located in Springhill, Fla., that burns waste (refuse or trash) as fuel
for heating boiler units to produce high pressure steam. The high pressure
steam is used to drive a steam turbine connected to a generator unit for
producing all the electricity required at the plant. Any excess electrical
energy produced is sold back to Florida Power & Light Co. In November 2002,
Covanta replaced the plants carbon media filters and chemically regenerated
two-bed deionization (DI) exchange system with a double-pass reverse osmosis
system, model DELTA-20DP manufactured by Crane Environmental in Venice, Fla.
The existing carbon filters and DI system were used to filter sediment and
reduce the total dissolved solids (TDS) from its on-site well as feedwater for
the boiler units.

The original system had become extremely high maintenance in
both labor and chemical costs per year. By design, the DELTA-20DP from Crane
Environmental does not require chemical regeneration, which in turn decreases
labor, system downtime and maintenance expenses throughout the year. Reverse
osmosis membranes are designed to reject the dissolve solids or ions in
feedwater and can produce consistently low TDS or low conductivity permeate
water, which is ideal for boiler feedwater applications. With the minimum
maintenance, the DELTA-20DP also ensures that Covanta Energy PASCO has a
continuous supply of clean, low TDS feedwater to its boiler units. style="mso-spacerun: yes"> 

In December 2002, Covanta Pasco discovered that the
DELTA-20DP system was displaying higher operating pressures with lower permeate
flow and called Crane Environmental’s Service department for assistance.
After conducting an on-site inspection of the RO system and an additional
analysis on the well water supply, it was determined that fine sediment, water
hardness, silica and dissolved iron levels on the well water supply had
increased, causing fouling of the RO membranes. Crane Environmental Field
Service technicians performed an on-site membrane cleaning and recommended replacing the 5 micron prefilter cartridges with 1 micron nominal pleated cartridges. Since additional evaluation on the operation of the RO unit was recommended, Covanta
Pasco signed a 12-month service agreement with Crane Environmental to perform
on-site monthly performance checks including operational evaluations, water
analysis, equipment maintenance, membrane cleanings, operator training and
on-call technical support.

An operational evaluation on the RO system was conducted during
the first five-months into the service agreement. Crane Environmental then
presented Covanta Pasco with three options for improving the pretreatment of
the feedwater to the RO unit. 

* Inject antiscalant with iron inhibitor to control
the water hardness and reduce iron fouling.

* Install an iron-removal filter with antiscalant

* Install a twin-alternating, metered water softener
unit designed to remove the water hardness and the dissolved iron.

The third option was selected because it offered the most
simple and practical solution. Efficiency is built in to the pretreatment
system solution because it continually provides the RO with softened, iron-free
feedwater at the 40-45 gallons per minute flow rate required. When the
in-service number one softener reaches the programmed metered per gallon
throughput capacity, the system immediately places the standby number two
softener into service, while the number one softener goes off line and performs
automatic regeneration. This design significantly decreases system downtime,
maintenance and only requires clean solar salt for regeneration of the softener
resin beds. In turn, the RO membranes are designed to easily reject sodium
chloride (salt) and the softened water aids in maintaining the membranes in
peak performance.  It was decided
by Covanta Pasco that because of the unexpected change in feedwater conditions,
the addition of the pre-treatment system was the most cost-efficient solution,
and would eliminate the need to go back to continuous and expensive chemical membrane cleanings Before the upgrades to the Covanta system were in place, membrane cleaning was
required approximately every one to two months. Membrane cleanings now are only recommended every six months to one time per year.

Under the Crane Environmental Service Agreement, daily
operation data of the DELTA-20DP is recorded by Covanta Pasco operators and
entered into a computerized data-log program, which easily is e-mailed to the
Crane Environmental Service department for weekly reviews. Should problems
arise throughout the year, Crane Environmental Field Service remains available
for complete technical support.


About the author