| Overview: |
Reverse osmosis (RO) technology
typically uses membrane separation systems to remove inorganic salts from
wastewater. However, reverse osmosis also can be used to treat wastewater
containing certain organic solvents. The RO system uses a semi-permeable
membrane to separate pure water from contaminated liquids. Osmotic theory
asserts that when a contaminated solution is separated from pure water by
a semi-permeable membrane, the higher osmotic pressure of the contaminated
solution will cause the water to diffuse into the contaminated solution.
Water will continue to permeate into the contaminated solution until the
osmotic pressure of the contaminated solution equals that of the pure
water.
RO occurs when an external pressure is exerted
on the contaminated solution. Water will flow in the reverse direction
from the contaminated solution into pure water. RO systems can be used to
separate pure water from contaminated matrices, and can be used in the
treatment of some hazardous wastes. RO results in a concentration of
hazardous chemical constituents separated from pure water by the
membrane.
Ultrafiltration (UF) is a pressure-driven,
membrane filtration process that is used to separate and concentrate
macromolecules and colloids from wastewater. A fluid is placed under
pressure on one side of a perforated membrane of a measured pore size. All
materials smaller than the measured pore size pass through the membrane,
leaving large contaminants concentrated on the feed side of the membrane.
UF is used as a pretreatment step to RO or as a stand-alone process.
Control of pass-through constituents can be achieved using a membrane with
a limiting pore size, or by installing a series of membranes with
successively smaller pores. The UF process cannot separate constituents
from water as effectively as RO. However, the two technologies can be used
in tandem, with UF removing most of the relatively large constituents of a
process stream before RO application selectively removes water from the
remaining mixture. The UF process is applicable for particles in the
molecular range of 0.1-0.01µm, while the RO process is applicable for
particles in the ionic range of less than 0.001µm.
Membrane technology can be used to treat a
variety of wastes, including sanitary landfill leachate containing both
organic and inorganic chemical species, water-soluble oil wastes used in
metal fabricating and manufacturing industries, solvent-water mixtures,
and oil-water mixtures generated during washing operations at metal
fabricating facilities.
Waste feed, process permeate, and rinse water
are potential feed materials to the skid-mounted RO-UF modules consisting
of a tank and a high-pressure feed system. The feed system also consists
of a centrifugal feed pump, a prefilter cartridge housing, and a triplex
plunger pump. The processing units are self-contained and need only
electrical and interconnection process piping to operate.
RO-UF modules have been used in military
applications as part of larger systems, but no facilities have been
identified that use the technology as a standalone process. However,
several commercial applications have been identified and the data can be
easily transferred to military applications.
|
| Compliance Benefit: |
The use of a reverse osmosis
and ultrafiltration wastewater treatment process may help facilities meet
the requirements of waste reduction under RCRA, 40 CFR 262,
Appendix. This technology also may be used to meet effluent standards
stipulated on NPDES permits (40 CFR 122.) Wastewater treatment
units covered under NPDES permits or that provide for pretreatment prior
to discharge into a POTW may not require a hazardous waste treatment
permit.
The compliance benefits listed here are only
meant to be used as a general guideline and are not meant to be strictly
interpreted. Actual compliance benefits will vary depending on the factors
involved (e.g., the amount of workload involved.)
|
| Materials Compatibility: |
Many membranes are susceptible
to attack by free chlorine or other oxidizers in the feed water. Thin film
composite membranes generally can handle 1,000 ppm-hours of oxidizing feed
water before they are destroyed. However, cellulose acetate membranes can
generally handle up to 1-ppm free chlorine continuous in the feed water
without deterioration of the membrane. The membrane process does not
produce an oxidizing waste stream as one can deoxidize the feedwater with
sodium metabisulfite solution prior to the membrane processes to protect
the membranes from chemical attack.
If iron or sulfur are present in the feed water
and the water is in an oxidizing state, one can precipitate iron (III) or
elemental sulfur onto the membrane surfaces which will foul the membranes.
Both compounds are nearly impossible to remove from membranes once they
are deposited because to resolubilize them one would need to clean the
membranes at a pH range that would destroy the membranes.
|
| Safety
and Health: |
Hydrocarbon-bearing material
management raises health concerns. Inhalation of vapors can be dangerous
to human health causing oral, dermal, and ocular effects. Personal
protective equipment is recommended.
Consult your local industrial health
specialist, your local health and safety personnel, and the appropriate
MSDS prior to implementing any of these technologies.
|
| Economic Analysis: |
The capital cost of a typical,
40,000 gallons per day (gpd) membrane treatment unit is approximately
$350,000. For this system, electrical costs are $0.000024 per one gallon
of wastewater treated and maintenance costs are $0.0003 per one gallon of
wastewater treated.
For the treatment of gray water, the capital
cost of a 1,000 to 20,000 gpd RO-UF treatment unit is approximately
$500,000. Electrical and maintenance costs would be similar to the single
membrane treatment unit.
Based on a case study developed by PPG
Industries, Inc., in Cleveland, OH, the following comparison of the
application of RO-UF and traditional disposal was developed. Although the
information is based on a commercial industry application of the
technology, the data can be applied to a military setting.
Assumptions:
- Hazardous waste disposal off-site costs
$1/gallon
- Facility generates 390,000 gallons/year of
wastewater or approximately 1,000 gallons/day
Cost Comparison for Treatment
by RO-UF System vs. Disposal as Hazardous
| |
Traditional
Disposal |
RO-UF
system |
| Capital and
Installation Costs |
$0 |
$450,000 |
| Operational
Costs: |
|
|
| Operation &
Maintenance Costs |
$0 |
$175,000 |
| Hazardous Waste
Disposal Costs |
$390,000 |
$19,800 |
| Total
Costs (not including capital and installation costs) |
$390,000 |
$194,800 |
| Total
Income: |
$0 |
$0 |
| Annual
Benefit: |
-$390,000 |
-$194,800 |
Economic Analysis Summary
Annual Savings for RO-UF: $195,200
Capital
Cost for Diversion Equipment/Process: $450,000
Payback Period for
Investment in Equipment/Process: 2.3 years
Click Here to view an Active
Spreadsheet for this Economic Analysis and Enter Your Own Values. To
return from the Active Spreadsheet, click the reverse arrow
in the Tool Bar.
|
