Wastewater purification
at Lake Tahoe
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saving a beautiful lake
A new man-made lake, bright blue and of
sparkling clarity, has been created in the
Diamond Valley area of Alpine County, Cali-
fornia, across Luther Pass in the High Sier-
ras from one of America's leading scenic
and recreational resources—Lake Tahoe.
The new lake is unique in several respects.
Its contents are purified wastewater pro-
duced by a new wastewater purification
plant which has been described as the most
advanced water reclamation facility in opera-
tion anywhere in the world. While the im-
pounded reclaimed wastewater is being used
for recreation and irrigation, the primary
reason for the creation of the new lake is
preservation of Lake Tahoe.
World-famous for its beauty and clarity,
Lake Tahoe has a surface area of 192 square
miles. It is unusually deep, ranging down
to more than 1600 feet, and it contains 122
million acre feet of water, enough to cover
the entire state of California to a depth of 14
inches. It is rated as one of the three clear-
est lakes in the world, along with Crater Lake
in Oregon and Lake Baikal in Russia.
Along with great scenic beauty, favorable
location and climate have made Lake Tahoe
a year-around recreation center. In the South
Tahoe area, the number of annual vacation
visitors has increased nearly tenfold since
1950, and in that same period year-around
residency has increased from 2,500 to al-
most 40,000.
This mushrooming growth pf resort and
recreational developments around the south
shore created a serious threat to Lake
Tahoe. Drainage of sewage effluent into the
lake could greatly accelerate the process of
aging, called eutrophication, through which
lakes eventually die. Organic material from
the surrounding area seeps into the water
and enriches it with nutrients which stim-
ulate the growth of algae. As the algae
growth increases, the water becomes turbid,
and the lake eventually becomes a meadow.
Eutrophication takes thousands of years by
nature, but is greatly accelerated when nu-
trient-rich sewage effluents are allowed to
drain into the lake.
To prevent this, the South Tahoe Public
Utility District in 1961 started on a long-
range program to develop a treatment proc-
ess more advanced than conventional sec-
ondary treatment to handle waste disposal
problems in the region. This resulted in the
opening in 1965 of a new 2.5 mgd tertiary
treatment plant. Growth in the region had
outstripped the capacity of both the new
tertiary plant and the existing secondary
treatment facility. More capacity had to be
added.
The present plant will process 7.5 million
gallons of wastewater daily, sufficient to
serve a city of 100,000 population. From
primary treatment through sludge incinera-
tor, the plant cost about $6 million to design
and construct, part of which was met by
federal grant funds, including a $1 million
research and development grant from the
Federal Water Pollution Control Administra-
tion.
The new plant produces water that is
crystal clear, colorless and odorless, and
practically free of trouble-causing phos-
phates and nitrogen. The reclaimed waste-
water is so free from pollutants that it would
be welcome almost anywhere except in the
Tahoe basin. The laws of both California
and Nevada require the export of all waste-
water from the Tahoe basin by 1970. To
meet this, the purified water is lifted 1,235
feet to an elevation of 7,735 feet at the Luther
Pass summit, and transported through a 27-
mile pipeline to the reservoir created by a
rockfill dam on a dry tributary of Indian
Creek.
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TO DISCHARGE
SECONDARY
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MULTIPLE
HEARTH
INCINERATION
FURNACE
RECALCINING
FURNACE
the Tahoe water
reclamation process
The Tahoe process of purification and nutrient re-
moval begins where conventional waste treatment
systems end. Effluent from the secondary clarifier
receives chemical clarification by the addition of lime
as a coagulant. This removes the suspended matter
and most of the phosphates. The high pH (11.5-12)
effluent from the chemical clarifier is pumped to a
lath-packed stripping tower which removes the
ammonia-nitrogen. As it leaves the tower, the water
is recarbonated to reduce the pH, using carbon di-
oxide recovered from furnace stack gases. Filtration
on mixed media separation beds filters out remaining
turbidity, phosphates and calcium carbonate. Final
polishing is provided by granular activated carbon,
which removes color, odor and almost all of the re-
maining organic material.
Costs are reduced by facilities for reclaiming ma-
terials for reuse. Spent carbon is regenerated in a
six-hearth furnace, developing temperatures up to
1,700 degrees. Lime sludge is thickened in a cen-
trifuge and recalcined in a similar furnace and carbon
dioxide from the stacks is used for recarbonation.
Disposal of the biological sludge from the primary
and secondary treatment units is accomplished by
burning it in a multiple-stage smokeless incinerator.
How advanced purification system starts where conventional waste
treatment ends is shown in this aerial photograph of South Tahoe
plant. (1) Headworks and primary sedimentation basin. (2) Primary
sedimentation basin. (3) Activated sludge aeration tanks. (4) & (5)
Secondary clarifier. (6) Chemical flocculation basin (start of advanced
treatment process). (7) Return sludge pump station. (8) Chemical
clarifier. (9) Nitrogen removal tower. (10) Separation beds and carbon
columns. (11) Backwash decant tank. (12) Solids disposal building,
housing sludge incinerator, lime recalcining furnace, dewatering facil-
ities. (13) Plant effluent pump station. (14) Effluent storage ponds. (15)
Administration building. (16) Sludge digesters (standby use only).
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nitrogen removal
Removal of nitrogen is accomplished in a 50-foot-
high forced ventilation stripping tower, the first of
its kind used in a municipal wastewater plant. Be-
cause of the high pH of the lime-treated water dis-
charging from the chemical clarifier, the ammonia is
present as dissolved gas, rather than ammonium ion
in solution. The tower is packed with treated hem-
lock slats spaced at 11/z inches vertically and 2 inches
horizontally. As the water strikes a slat, droplets
are formed, and the ammonia gas escapes from the
droplet. High air circulation removes the ammonia
gas as it escapes. Air enters through side louvers,
travels horizontally through the packing to the center,
where it is discharged upward through a fan which
has a capacity of 700,000 cubic feet per minute.
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mixed media filtration
Effluent from the nitrogen stripping tower is re-
carbonated in two stages, using compressed carbon
dioxide from furnace stack gases, to reduce the pH
to the desired level in preparation for the next stage
of treatment—filtration. Alum is added to the filter
influent water. There are three pairs of pressure fil-
ter beds, with each pair operating in series as a
unit. The mixed media filters, designed especially
for filtering waste water, consist of coarse coal,
medium sized sand and fine garnet, arranged from
coarse to fine in direction of flow. The filters are
equipped with rotary surface washers. Backwashing
is triggered automatically by head loss or turbidity.
polishing on activated
carbon
Final polishing is provided by adsorption on ac-
tivated carbon. The filtered water flows under pres-
sure to eight carbon columns which operate in
parallel. Each column contains about 22 tons of
8x30 mesh granular activiated carbon. The upflow
system, in which the flow of water is up from the bot-
tom, permits hydraulic withdrawal of spent carbon
at the bottom, the addition of fresh carbon at the top.
This flow pattern provides for the effluent passing
through the most active carbon as it leaves the plant.
The carbon column effluent is colorless, odorless,
low in organics, and of sparkling clarity.
nimiiiiiii iilllini
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solids disposal and materials reclaiming
All solid plant wastes are processed in multiple hearth furnaces. The biological and waste
chemical sludges are incinerated. The lime mud is recalcined and reused in the process. The
spent granular carbon is regenerated and reused.
Biological sludge from the primary and secondary
treatment units and lime sludge from chemical
clarifier are thickened in centrifuges before incin-
eration or recalcination. Phosphates are removed
from the system by feeding about 25 per cent of
the recalcined lime into the primary treatment
tanks, where it is drawn off with the biological
sludge.
Air scrubbers (foreground) cool and wash furnace
gases to remove particulate matter. There is no
odor, smoke or steam plume, and no air pollution.
The solids disposal building houses sludge dewatering and incineration system and lime recalcining furnace. Note absence
of smoke from the furnace stack.
Organic materials removed by the carbon are con-
sumed when the saturated carbon is regenerated
in a multi-hearth furnace operated at 1,680 de-
grees F in a limited oxygen atmosphere. Attrition
loss in full regeneration is about 5 per cent. Carbon
is moved hydraulically in slurry •form.
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Plant Controls
Control panel for filter plant, carbon columns, is pictured above. All filter operations are
fully automatic. Backwash is initiated by high head loss or high effluent turbidity. The
beds are backwashed, filtered to waste and restored on line automatically. Effluent
quality is measured and recorded.
South Tahoe Public Utility District
Board of Directors: Robert W. Fesler, President; Robert W. Wakeman, Donald H.
Kortes, Edward Hegarty, Gerald G. Ream, Directors; Russell L Gulp, General Manager.
Research
The entire wastewater purification plant, in a sense, is a research and development project.
Original concepts incorporated in the design include a process for economical use ol lime
as a coagulant for solids removal; recovery and recalcination of lime for reuse; a stripping
tower capable of large-scale removal of ammonia nitrogen; large-scale filtration of effluent;
carbon columns for large scale removal of taste, odor, color, ABS from synthetic detergents,
and trace organlcs; regeneration of carbon for reuse.
Major Suppliers
BSP Corporation
Solids handling system, including furnaces for carbon regeneration, sludge incineration,
lime recalcination.
Ca/gon Corp.
Granular activated carbon.
Bird Machine Company
Sludge and spent lime dewataring centrifuges.
Neptune MicroFLOC Corp.
Filter media, control panels and process patents.
Wallace & Tlernan
Gravimetric feeders and lime slakers.
The Mar/ey Company
Nitrogen stripping tower equipment.
Byron Jackson Pump Company
Effluent pumps.
Dorr Oliver, Inc.
Primary, secondary and chemical clarifier; aeration equipment, lime mud thickener,
sludge pumps.
Engineers
Clair A. Hill & Associates — Redding, Calif.
Consulting engineers for the South Tahoe Public Utility District.
Cornell, Howland, Hayes & Merryfield—Corvallis, Oregon
Design of the wastewater reclamation plant.
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plant performance
a new lake
Wastewater undergoes a number of cleaning operations, as illustrated by the
samples taken at successive steps in the process. From left, raw sewage, primary
effluent, activated sludge secondary effluent, chemical clarifier effluent, separation
bed filtrate, and the final product, reclaimed water from the carbon columns. This
process removes all of the suspended solids, color, odor and bacteria, 99.8 per
cent of the BOD, 96 per cent of the COD, 98 per cent of the MBAS, 94 per cent of
the phosphorous and 50 to 98 per cent of the nitrogen.
Quality Parameter
MBAS (Methylene Blue
absorbable substances)
mg/1
COD (Chemical Oxygen
Demand) mg/1
BOD (Biological Oxygen
Demand) mg/1
Turbidity, JTUs
Suspended Solids, mg/1
Phosphorus, mg/1 as P
Ammonia Nitrogen, mg/1
as N
Colitorm Bacteria
MPN/100
Color, units
Odor
Raw
Waste
Water
Influent
4-9
200-500
250-300
100
225
10-15
20-30
15,000,000
High
Odor
Activated
Sludge
Plant
Effluent
0.4
20-60
15-30
20-50
10-20
5.7
25-35
150,000
High
Odor
Water Reclamation Plant
Nitrogen
Removal
Tower
Effluent
0.4
15-25
5-10
2-5
5-10
5.7
5-9
150,000
High
Odor
Separation
Bed
Effluent
0.4
14-20
4
0.4
0.9
0.8
5-9
15
10-30
Odor
Chlorinated
Carbon
Effluent
0.1
10
1
0.2
0.9
0.06
Converted to
Chloramines
under
2.2
Colorless
Odorless
This new 165-acre lake in Alpine County, Califor-
nia, contains only reclaimed sewage wastewater from
the South Lake Tahoe plant. The lake will store over
one billion gallons of water which is released into
Indian Creek during the irrigation season for use on
downstream ranches. The lake has been planted
with rainbow trout, and a minimum water level will be
maintained to permit recreational development.
TECHNOLOGY
TRANSFER
Environmental Protection Agency
Washington, D.C. 20460
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