Municipal Membrane Plants:
Growth, Trends, and Concentrate Disposal Practices
and Issues in the United States
Michael Mickley
Mickley & Associates
Boulder, Colorado, USA
Abstract
Population growth and resultant community and industrial development in the United
States have increased the demand for potable water. Membrane desalting technologies
(reverse osmosis – both brackish (BRO) and seawater (SRO), nanofiltration (NF), and
electrodialysis/electrodialysis reversal (ED/EDR)) are the technologies of choice to provide
new sources for potable water through treatment of lower quality resources (brackish and
saline waters). At the same time decrease of the quality of surface water and groundwater
has given rise to increased treatment requirements of the Safe Drinking Water Act
Amendments. These requirements include higher removal levels for disinfection byproducts,
synthetic organic compounds, viruses, microorganisms, and turbidity. Low-pressure, nondesalting membrane processes (ultrafiltration (UF) and microfiltration (MF)) are increasingly
used to meet many of these needs.
Statistics developed in a recently completed study for the Bureau of Reclamation
(Mickley, 2001) document the growth and trends in the use of membrane technology to
provide water and wastewater treatment in the U.S. Statistics are also provided on the
disposal of concentrate (from desalting plants) and backwash (from UF and MF plants).
Current and emerging issues and environmental concerns associated with concentrate (and
backwash) disposal are discussed. These issues include:
Challenge of finding a suitable disposal option
Perceptions of membrane technology and concentrate
Major ion toxicity (which applies to desalting membrane processes where groundwater is
the source)
Receiving water effects (which applies to SRO such as at the new Tampa Bay, Florida
seawater desalination plant)
Disposal of high level of cysts (which applies to backwash from MF and UF membrane
processes)
Need for additional treatment of membrane concentrate/backwash prior to disposal (due to
elevated levels of various contaminants)
GROWTH IN THE MEMBRANE WATER AND WASTEWATER INDUSTRIES
Table 1 is a tabulation of operating membrane plants by 2-year period and by membrane
technology. It was developed based on data from a 1992 survey (Mickley et al. 1993), a
129
more recent 1999 survey (Mickley, 2001), estimates of the number and types of plants not
contacted in the surveys for the years up to 2000, and estimates of the membrane plants built
during 2000. This and all such plant tallies have minimum size cutoffs that influence the
numbers of plants listed. For this tabulation a size cutoff of 25,000 gpd (94.63 m3/d) was
used for both desalting plants and low-pressure UF and MF plants. This cutoff eliminates
most smaller plants that serve truck stops, mobile home parks, hospitals, campgrounds, and
the like.
In the final four columns of Table 1 the number of desalting and MF/UF plants is tallied
for each two-year period along with the total number of plants and the cumulative number of
plants.
Most of the plants are water treatment plants (WTPs). There are an estimated 22
wastewater treatment plants (WWTPs) employing membrane technology in the U.S.
Figure 1 shows the cumulative number of plants by two year period beginning in 1971.
The total number of all types of plants, the number of MF/UF plants, and the number of
desalting plants are shown separately.
Several important statistics and events are evident from Table 1 and Figure 1. These
include:
Most (163, or 74%) of the 221 desalting plants are brackish water RO plants with few (9,
or 4%) seawater RO plants and about an equal number of NF and ED/EDR plants (26, or
12% and 23, or 10% respectively).
The earliest plants built in the 1970’s were brackish RO and ED/EDR plants with the first
NF plant coming on line in 1987.
The number of desalting plants being built per period has been in double digits since 1989.
MF plants begin appearing in large numbers in 1994 and these numbers have steadily if
not dramatically increased in each two-year period since that time.
UF plants first appear in large numbers in the year 2000.
If these trends continue, the number of MF/UF plants will outnumber the desalting plants
in the period 2001/2002, or soon thereafter.
DESALTING PLANT TRENDS
Other findings about desalting plants from the surveys include:
As shown in Table 2, most of the early desalting plants were in Florida, with 61% of the
plants operating in 1992 being in Florida; however, since 1992 only about 26% of the new
plants are located in Florida.
Table 2. Location of desalting plants
Plant Type
Florida
California
Other States
Total
<1993
61%
9
30
100%
1993 through 1999
26%
15
59
100%
130
Table 1. Estimated Number of Membrane` WTPs and WWTPs in the 50 States of the U.S.
Year
BRO
SRO
NF
EDR
MF
UF
DESALTING
MF/UF
< 1971
1971/72
1973/74
1975/76
1977/78
1979/80
1981/82
1983/84
1985/86
1987/88
1989/90
1991/92
1993/94
1995/96
1997/98
1999/00
0
4
8
8
10
5
9
14
3
4
16
18
17
9
11
27
0
0
0
0
1
1
0
0
0
1
0
3
0
0
1
2
0
0
0
0
0
0
0
0
0
3
1
8
3
2
3
6
1
0
4
0
1
0
0
0
3
0
3
3
2
2
2
2
0
0
0
0
0
0
0
0
0
0
0
1
14
14
43
63
0
0
0
0
0
0
0
0
0
0
0
0
2
0
2
12
1
4
12
8
12
6
9
14
6
8
20
32
22
13
17
37
0
0
0
0
0
0
0
0
0
0
0
1
16
14
45
75
Totals
163
9
26
23
135
16
221
151
Total Cumulative
total
1
1
4
5
12
17
8
25
12
37
6
43
9
52
14
66
6
72
8
80
20
100
33
133
38
171
27
198
62
260
112
372
372
Comments:
1. Only plants greater than 25,000 gpd (94.63 m3/d) were considered
2. The tabulation includes an estimated 22 WWTPs
3. The tabulation is a combination of hard data (from the 1992 and 1999 surveys conducted by Mickley & Associates)
4. The tabulation also contains estimates - mostly for the 1999/00 time frame
Where:
BRO = brackish reverse osmosis
SRO = seawater reverse osmosis
NF = nanofiltration
EDR = electrodialysis/electrodialysis reversal
MF = microfiltration
UF = ultrafiltration
131
400
350
Cumulative Number of Plants
300
All Plants
All Plants
250
All Plants
200
150
Desalting
Plants
100
Desalting Plants
50
MF/UF
Plants
MF/UF Plants
0
<
19
71
71
19
2
/7
73
19
4
/7
1
5/
97
76
77
19
8
/7
79
19
0
/8
1
82
1/
8
9
1
84
3/
8
9
85
19
6
/8
87
19
8
/8
1
9/
98
90
91
19
2
/9
93
19
4
/9
Two-Year Period
Figure 1. Cumulative Number of Utility Membrane Plants by Two-Year Period
132
1
96
5/
9
9
1
98
7/
9
9
99
19
0
/0
Although not evident from the statistics presented here, there are now 20 states (as
opposed to 13 in 1992) that have desalting plants. Fifteen states have low-pressure
systems.
The size of both desalting and MF/UF plants has increased dramatically in recent years.
Table 3 shows the increase in size of desalting plants from 1993 through 1999.
Table 3. Desalting Plant Size by Year of Startup
Size range in MGD (m3/d)
Year
<0.3
(<1136)
1993
1994
1995
1996
1997
1998
1999
2
1
3
2
2
1
0
11
0.3 - < 1
1-<3
3-6
>6
2
1
1
0
3
1
3
11
4
1
0
2
2
4
3
16
2
1
1
0
0
3
8
15
1
0
3
1
1
0
4
10
(1136-<3785) (3785-<11355) *11355-22710) (>22710)
From 1993 through 1997, 27% of the desalting plants (10 of 36) built were of size 3 MGD
(11,355 m3/d) or greater. In 1998 and 1999 the percentage doubled to 56% (15 of 27).
Desalting plant types built before and after 1992 are quite similar, as shown in Table 4.
Table 4. Type of desalting plants being built
Plant Type
BRO
SRO
ED/EDR
NF
Total
<1993
73%
5
11
11
100%
1993 through 1999
72%
2
15
11
100%
MF and UF Plant Trends
At the time of the 1992 survey there were only 1 MF system and no UF systems operating
in water and wastewater treatment plants. At the end of 1999 there were still only 7 UF
plants. There were, however, a total of 100 MF plants identified (operating as of 1999) that
were of size 25,000 gpd (94.63 m3/d) or greater.
The distribution of these plants is very different from that of desalting plants, with almost
one third of the plants in California and one sixth in Virginia. Only one UF plant is located
in Florida, in sharp contrast to membrane desalting plants.
133
Prior to 1998 almost all of the MF plants used Memcor technology. Table 5 shows
installations of Memcor MF membrane systems and demonstrates the growth in plant size
within the last 5 years. Prior to 1996 only 1 of 19 plants was of size greater than 1 MGD
(3,785 m3/d). Since 1996, however, 25 of the next 66 plants built were larger than 1 MGD
(3,785 m3/d).
Table 5. Size of Memcor MF plants by year in MGD (m3/d)
Year
1991
1992
1993
1994
1995
1996
1997
1998
1999
<0.3
(<1136)
1
0
2
9
4
6
10
8
3
43
0.3<1
1-3
>3
total
0
0
0
2
0
0
3
6
5
16
0
0
0
0
0
2
4
4
5
15
0
0
0
1
0
0
2
4
4
11
1
1.2
0
0
2
2.4
12 14.1
4
4.6
8
9.4
19 22.4
22 25.9
17 20.0
85 100.0
(1136-<3785) (3785-<11355) (>11355)
%
Differences Between Concentrate and Backwash
Backwash is considerably different from desalting concentrate in that the volume of waste
is significantly less for a given sized plant and backwash does not contain concentrated salts.
The TDS level of the backwash is close to that of the feedwater. Backwash, however, may
contain high levels of microorganisms such as giardia and cryptosporidium as well as high
levels of organics and particulates.
In general, the environmental concerns associated with concentrate disposal include the
effects of salinity, dissolved salts, and of contaminants (natural or otherwise) that might be
present in the source water. For backwash, however, the concerns are mostly with
contaminants (viruses, microorganisms, organics).
Because of the much smaller size of the backwash there are frequently more disposal
options available at a given plant site.
Concentrate and Backwash Disposal Trends
In general, the disposal options most frequently used for concentrate and backwash
disposal include surface water discharge, disposal to sewer, disposal by land application,
disposal by evaporation pond, and disposal to deep well. Rarely are more than one or two
options available at a given plant site and those available are not necessarily permittable (due
to environmental concerns).
134
Surface water discharge is the disposal to any receiving water. This includes oceans,
rivers, canals, ditches, etc. Availability of this potential option is location-dependent.
Suitability of this option depends on the salinity and water quality of the
concentrate/backwash and the volume (or flow) and quality of the receiving water.
Discharge to sewer means discharge to the front-end of a WWTP. Concerns include the
volume of concentrate relative to the capacity of the WWTP and the effect of heavy metals
on the WWTP operation.
Disposal by land application is frequently limited by the salinity of the concentrate. For
this reason land applications most frequently involve combination of the concentrate with
WWTP effluent as dilution water. The specific options may include percolation ponds,
drainage fields, and spray irrigation. These applications may be considered to serve water
reuse or surficial aquifer recharge purposes. The options are also limited by the land
availability, size of the concentrate, ability of the soil to uptake water, and climate (the option
needs to be available year round).
Disposal by evaporation pond is dependent on climate, land availability, and the net
evaporation rate.
Disposal to deep well requires the presence of a certain underground geological structure,
namely a confined lower water quality aquifer that is isolated from overlying fresh water
aquifers. Such structures are relatively rare.
The design and cost factors involved with these disposal options are discussed at length in
a recent Bureau of Reclamation report entitled Membrane Concentrate Disposal: Practices
and Regulation (Mickley, 2001). The project deliverables were provided in CD format and
included:
a Microsoft Access database of information on 150 membrane plants
a 253-page report discussing the plant survey, federal and state-by-state regulations
affecting concentrate disposal, design considerations for the disposal options, capital and
operating costs of the disposal options, worksheets for calculating preliminary level capital
costs, and closed form equations for preliminary level cost estimates
Microsoft Excel calculation pages for preliminary level capital costs for several disposal
options
A front-end menu to guide the user through the CD content options.
In the future all these items may be available by download from the Bureau of
Reclamation website.
The concentrate disposal methods used by the desalting plants have changed somewhat
since 1992 as shown in Table 6. While the percentage of plants disposing to surface
waters has remained about the same, more plants are disposing concentrate to the sewer.
135
Table 6. Concentrate disposal means being used
Disposal option
Surface discharge
Discharge to sewer
Deep well
Evaporation pond
Spray irrigation
Total
1992
48%
23
12
6
12
100%
1999 (post-1992 data)
45%
42
9
2
2
100%
It should be noted that disposal of concentrate to deep well takes place only in Florida.
The increase of disposal to sewer appears to be influenced by at least two factors. First,
more plants are being built in states other than Florida and since there are no deep well
disposal options available/allowed in states other than Florida, newer plants in other states
dispose to the sewer more frequently than in Florida. Second, it reflects the decreased
numbers of smaller plants (fewer small plants are being built). Smaller plants frequently
disposed to land applications (evaporation ponds, percolation ponds, spray irrigation, etc.)
and as can be seen these disposal options were used less frequently in the plants built since
1992. The next table reflects these two factors. Table 7 shows disposal method data as a
function of plant size.
Table 7. Concentrate disposal means by size of plant in MGD (m3/d)
Disposal option
<0.3
(<1136)
1992 data
0.3-<1
1-<3
Surface disposal
Disposal to sewer
Deep well
Evaporation pond
Spray irrigation
Total
44%
23
4
10
18
100%
Disposal option
1999 (post-1992 data)
<0.3
0.3-<1
1-<3
Surface disposal
Disposal to sewer
Deep well
Evaporation pond
Spray irrigation
Total
(<1136)
42%
50
0
0
8
100%
>3
(1136-<3785) (3785-<11355) (>11355)
52%
35
4
0
9
100%
53%
18
29
0
0
100%
55%
15
25
0
5
100%
>3
(1136-<3785) (3785-<11355) (>11355)
23%
69
8
0
0
100%
50%
42
0
8
0
100%
56%
19
25
0
0
100%
Roughly similar trends with size are apparent in the 1992 and post-1992 survey results.
Surface disposal appears to be a well used disposal option for all sized plants. While disposal
to sewer is also widely used, its use falls off somewhat with larger sized plants. Deep well
136
disposal is used primarily for larger plants. Table 8 shows the disposal options being used for
backwash from UF and MF plants.
Table 8. Backwash disposal options being used
Disposal option
Surface discharge
Discharge to sewer
Evaporation pond
Percolation pond
Septic tank
Spray irrigation
Total
1992
36%
48
2
7
2
5
100%
Note that unlike concentrate disposal, deep well injection has not been used for
backwash disposal. This is due mainly to the low number of low-pressure membrane
systems in Florida, the only state presently using deep well disposal for concentrates.
Although not reflected in data presented, disposal of backwash from MF and UF plants
does not follow any trend with plant size. This is likely due to the small volumes of
backwash that result in the surface discharge or disposal to sewer options being available
at most sites.
Concentrate/Backwash Disposal Issues
Membrane concentrate and backwash are regulated just like any other industrial
effluent. Federal guidelines set by the USEPA provide the framework within which the
states operate. State and local regulations must be at least as stringent as the Federal
guidelines, but can be and frequently are more stringent. Thus regulatory practices vary
somewhat from state to state. Florida with the largest number of desalting plants in the US
has frequently encountered regulatory challenges and issues before other states. Since
1986 regulation of concentrate disposal to surface waters in Florida has changed
significantly. Beginning then with only a few required monitoring parameters, the
requirements now include many possible chemical-specific parameters, whole effluent
toxicity testing, and sometimes biodiversity studies of the receiving water and even nearby
land and wildlife.
[Whole effluent toxicity tests involve exposing selected organisms to the effluent and
dilutions of it over a period of time and observing acute (lethality) and chronic (growth,
reproduction, etc.) endpoints. These tests give an indication of the quality of the whole
effluent as opposed to the quality indicated by the presence/absence of specific
chemicals.
Biodiversity studies may involve monitoring of plant, micro- and
macroorganism (including macroinvertebrates in the benthic bottom sediments of
receiving water), and fish, at the receiving water location and a control location –
typically upstream of the receiving water.]
137
When water quality standards and other monitoring standards for the concentrate cannot
be met, then the plant is not in compliance with the regulations. Either process changes or
regulatory relief (a mixing zone, a variance, an exemption) may come into play to allow
the plant to operate in compliance.
Early use of toxicity tests showed that concentrate produced from plants using
groundwater sources was typically toxic from low levels of dissolved oxygen and high
levels of hydrogen sulfide. Such plants now routinely aerate/degasify and otherwise treat
concentrate to make it non-toxic for surface discharge. Until the early 1990’s the only
other causes of failed toxicity tests in Florida membrane plants were a case of heavy metal
toxicity due to use of dissimilar pump metals and use of a toxic antiscalant (antiscalant
chemicals are now required to be non-toxic at the levels of use for drinking water plants).
South Florida is fortunate in a sense to have deep well injection as an option for
concentrate disposal. Where surface discharge is not possible, often deep well injection is
possible, although it is an expensive matter.
Issue #1 – Challenges in finding suitable disposal options
General: Relative to industrial plants, membrane WTPs frequently have a greater
challenge in disposing of effluent even though the effluent is to a large degree
concentrated raw water. First, WTPs have less freedom of location, as they need to be
located near population areas. This frequently results in few disposal options being
available. Second, the economics surrounding WTPs are different than those of most
industrial plants where costly effluent treatment prior to disposal can usually be
incorporated into product cost. The cost of concentrate disposal can be significant to the
WTP.
General: The same laws that require improved drinking water quality and lead to the
use of membrane technology also require increased protection of drinking water sources,
making it more difficult to dispose of membrane concentrate and backwash via any
method that may affect surface and groundwater. A ‘conflict’ of sorts may exist between
different divisions of the same state regulatory agency with the drinking water division
advocating the use of membrane technology to solve pressing water volume and quality
challenges, and the permitting and compliance division making membrane concentrate and
backwash disposal difficult.
Specific: In Florida, smaller plants are disappearing due to regionalization, with county
utilities buying up small independent plants. In a good portion of south and central
Florida, where deep injection wells are possible, large plants can usually afford the cost of
deep well injection. Such wells can accommodate, with minimum pH adjustment and
possible treatment for removing hydrogen sulfide, almost any concentrate regardless of
water quality. Medium-sized plants, however, may be too small for deep well injection to
be economical and may be too large for land application approaches such as spray
irrigation and percolation ponds. The limiting factor for the land application options is the
need for sufficient dilution water so that the resultant blend of concentrate and dilution
138
water will be compatible with groundwater. Concentrate disposal is difficult where
disposal to surface water is not possible, for reasons that include issues just discussed.
Issue #2 – Perceptions of membrane technology
One of the contributing factors to the regulatory ‘conflict’ just mentioned is that by
definition membrane concentrate and backwash are classified as industrial wastes. The
public usually regards industrial waste as toxic and hazardous, and thus membrane
concentrate and backwash suffer from inclusion in this group. Opposition to new plants
on the basis of environmental concern associated with concentrate disposal frequently
starts from this perception.
A similar perception problem sometimes occurs with regulators not familiar with
membrane technology as used in water treatment plants. Similar to the public, they are
not aware of the differences between membrane concentrate/backwash and other industrial
wastes.
The State of Florida has enacted legislation to change the name of membrane
concentrate and backwash from industrial waste to ‘drinking water plant by-product.’
The regulation of concentrate and backwash disposal is unchanged; however, the category
change helps with the perception problems.
Another perception that is carried by many is that membrane-produced drinking water
is expensive water. While in general, membrane-produced drinking water may yet be
more expensive than conventionally produced drinking water, the cost of membrane plants
has decreased significantly over the last 10 years and it needs to be realized that the
membrane processing is producing a required higher quality drinking water. This higher
quality is typically not possible from more conventional technology.
The solution to this concern is education of the public, decision makers, legislators, and
regulators.
Issue #3 – Major ion toxicity – a relatively new discovery (brackish desalting
plants)
Major ion toxicity was the focus of a recently completed four-year study (Mickley,
2000) funded by the American Water Works Association Research Foundation
(AwwaRF). In the early 1990’s several Florida plants failed whole effluent toxicity tests
that used the mysid shrimp (mysidopsis bahia) as a test organism. The reason for these
failures could not be determined with any clarity. Some suspicions were raised as to the
cause being common ions (Mickley et al., 1993). In 1995 Florida Department of
Environmental Protection (FDEP) issued testing guidelines for the determination of the
presence of what was called ‘ion imbalance’ toxicity. The AwwaRF project was started in
late 1995 with the objectives of going beyond the FDEP guidelines to:
Develop an understanding of the nature of major ion toxicity
139
Develop protocols for determining both its presence and cause(s)
Develop models to predict if major ion toxicity is likely to occur given water quality
analyses of major ions.
Determine if the toxicity is due to the membrane process
Determine whether major ion toxicity is likely to occur in other membrane processes
Document the regulatory and technical options available to deal with the occurrence of
major ion toxicity
Project tasks were undertaken to:
Determine several single ion LC50 values (the lethal concentration at which 50% of the
whole effluent test organisms die) at salinities of 10 and 20 parts per thousand
Determine the cause(s) of toxicity in 9 membrane concentrates for Florida membrane
plants
Document the nature of the toxicity through dilution and salinity adjustment studies
No causes of toxicity other than that due to major ions or fluoride were determined in any
of these concentrates. High calcium levels were indicated in 7 of the 9 concentrates, high
fluoride levels were indicated in four of the 9 concentrates and a deficient (low) level of
potassium was indicated in one concentrate. The characteristics and nature of major ion
toxicity were documented, protocols were developed to determine the specific cause(s) of ion
toxicity, and a method for predicting the occurrence of major ion toxicity was developed
(Mickley, 2000). It was established that this type of toxicity in brackish water membrane
plant concentrate is unlike heavy metal or pesticide toxicity (for example). The toxicity is not
due to the membrane process but due to the common ion makeup of the groundwater source.
It was also established that this type of toxicity is not present in seawater membrane
concentrate.
As a result of these studies (which were partially funded by FDEP) and other studies, the
State of Florida has enacted laws to treat this type of toxicity differently than other types of
toxicity.
The remaining concern is that the level of understanding now existing about major ion
toxicity is not widely known and this type of toxicity may unnecessarily lead to regulatory
challenges at membrane plants in other states.
The solution to this environmental concern is in making research results and the
characteristics of major ion toxicity (Mickley, 2000; API, 1999; FDEP, 1995) more visible.
In this way the occurrence of major ion toxicity need not be a deterrent in the continued
growth of brackish membrane desalting processes.
Issue #4 – Issues with large seawater RO plants – a new application in the U.S.
The Tampa Bay Desal seawater RO plant has now passed all the environmental regulatory
hurdles, and construction has begun, with completion scheduled for December 2002. This
plant will be the largest seawater RO plant in the U.S. and the projected low costs are
attracting feasibility studies in several other coastal cities. The success of this project will
likely result in the construction of several similar plants in the U.S. While the environmental
140
concerns with disposal of concentrate to receiving waters are well known (Mickley, 1996),
there has been little experience in the U.S. with seawater RO concentrate disposal, in general,
and none for plants of this size (23 MGD or 87,055 m3/d). Because the Tampa Bay plant
discharges concentrate to the somewhat enclosed Tampa Bay rather than the open Gulf of
Mexico, this plant has undergone the highest level of disposal permit review and scrutiny of
any industrial waste disposal permit issued by the FDEP. The permit application required the
permittee to conduct detailed studies of the near-field and far-field area of Tampa Bay.
Hillsborough County, in which the plant is being built, conducted their own detailed studies.
The concentrate monitoring requirements are extensive, and include monitoring of the
effluent, internal plant discharge, membrane plant intake water, and several locations in the
receiving water. A biological monitoring program will include, as a minimum, monitoring of
submerged grasses, benthic macroinvertebrates, and fish. Chronic toxicity tests will include
both the mysidopsis bahia and menidia beryllina test species.
The concern here is simply that this is a new disposal situation for the U.S., and it needs to
be carefully monitored and documented. The solution to this environmental concern will
hopefully be the successful operation of the plant.
Issue #5 – Concentrates with spikes of contaminants – an emerging situation
(desalting plants)
In Florida, some concentrates have had spikes of radionuclides due to high naturally
occurring source water levels. These plants dispose concentrate to deep injection wells and
do not have to address the water quality disposal requirements associated with surface water
disposal. A situation of greater concern surrounds membrane processing of surface water and
groundwater contaminated with synthetic chemicals, pesticides, manure runoff, etc. This
situation will produce a concentrate with an elevated level of such contaminants. To the
author’s knowledge this situation has not occurred in the U.S., with the exception of some
high nitrate-containing concentrates. The situation has occurred more frequently in other
countries, including the United Kingdom, where research is ongoing into the treatment of
concentrate for the removal of pesticides and arsenic.
The solution to this challenge will involve the identification of technical and economical
means of treating the concentrate to remove the contaminants. While technical means exist
in most cases, it will be difficult to identify treatments that do not significantly increase the
cost of concentrate disposal.
Issue #6 – Backwash with high levels of virus, cysts, microorganisms (MF and UF
plants)
As reflected in statistics presented, low-pressure membrane systems (UF and MF) are
increasingly used to meet the higher removal requirements dictated by the Safe Drinking
Water Act Amendments. More specifically, these requirements include increased removal of
141
disinfection by-products, synthetic organic compounds, turbidity, viruses, and
microorganisms. One of the more frequently used applications is to remove giardia and
cryptosporidium cysts that can cause health-related problems. The membrane backwash from
the UF and MF processes has high levels of these cysts. Currently there is no federal water
quality standard restricting the discharge of microorganisms into receiving waters. Clearly
this regulation will come, and there is thus the need for removal or inactivation of the
microorganisms prior to disposal.
UV technology has been successful in inactivating various microorganisms when they are
present at lower levels in waters of low turbidity. Successful solution of this environmental
challenge will include the characterization of backwashes and identification of suitable
technologies to inactivate cysts when present at high levels and in turbid conditions.
Issue #7 – New twists to disposal options
It would be helpful if new and economical concentrate/backwash disposal options were
found. Unfortunately, this has not happened yet. There have been instances where
membrane concentrate has found a local use – such as spraying on roads in arid regions to
keep dust down. There are other disposal solutions of this nature, but they are very few and
are specific to local conditions and needs. One of the most promising developments is what
is called ‘enhanced evaporation.’ The basic idea is to spray concentrate (or another effluent)
into the air to provide large surface areas for evaporation. This may take place over an
evaporation pond to enhance the net evaporation taking place at the pond site. If the air
above the pond is moving, it is possible to significantly increase the amount of evaporation
per area of pond, and so to decrease the size of evaporation ponds required to dispose of a
given volume of effluent. The portion of the spray that is not evaporated falls back to the
pond.
There are at least two groups making commercial systems – one using spray irrigation
equipment and another using modified snow making equipment. The author is aware that the
Bureau of Reclamation has studied some of this technology. The author is working with a
company in Boulder, Colorado that has a unique and highly efficient delivery device to
produce a fine mist above holding ponds. In all these instances the technical and economic
feasibility of the systems are undocumented. And of course, evaporation ponds -- enhanced
or otherwise -- cannot be used everywhere.
Conclusions
There has been dramatic growth in the use of membrane technology to address quantity
and quality problems to meet drinking water needs. Part of this growth has been in the
increased application of desalting membranes (RO, NF, EDR) to treat lower quality water
sources. There are several current and emerging issues or concerns with disposal of desalting
plant concentrate.
142
The issues surrounding major ion toxicity has been successfully dealt with (Mickley,
2000) and it needs not be an ongoing environmental concern. Of ongoing concern, however,
are:
The site-specific challenge of finding a suitable disposal option. Changing regulatory
requirements and the limited economical resources of water treatment plants complicate
this challenge.
The perception of the ‘industrial waste’ nature of membrane concentrate held by much of
the public and by some of the regulatory community.
The continued deterioration of surface water and groundwater quality that will
increasingly result in concentrate with higher levels of contamination. This will make
concentrate disposal more challenging.
The other part of the growth of membrane technology in the U.S. is in relatively new
applications (MF, UF, and seawater RO). Each of these areas has associated environmental
challenges that are likely to be resolved.
Membrane technologies are well suited to meet the water quantity and quality challenges,
and they have a significant history of successful and reliable operation. The environmental
concerns raised here will need to be addressed in a timely manner to support continued
growth of these technologies.
Acknowledgements
U.S. Department of Interior, Bureau of Reclamation, Denver, Colorado for funding of the
project Membrane Concentrate Disposal: Practices and Regulation. Agreement No. 98-FC81-0054.
References
1. API (American Petroleum Institute). 1999. The Toxicity of Common Ions to Freshwater
and Marine Organisms. Washington D.C.: American Petroleum Institute.
2. FDEP, 1995. Protocols for Determining Major-Seawater-Ion Toxicity in MembraneTechnology Water-Treatment Concentrate. Tallahassee, Fla.: FDEP.
3. Mickley, M. 1996. Environmental Considerations for the Disposal of Desalination
Concentrates. The International Desalination & Water Reuse Quarterly, 5(4)
February/March 1996, 56-61.
4. Mickley, M. 2001. Membrane Concentrate Disposal: Practices and Regulation. Denver,
Colo.: Bureau of Reclamation.
5. Mickley, M., 2000. Major Ion toxicity in Membrane Concentrate. Denver, Colo.:
AwwaRF and AWWA
6. Mickley, M., R. Hamilton, L. Gallegos, and J. Truesdall, 1993. Membrane Concentrate
Disposal. Denver, Colo.: AwwaRF and AWWA.
143