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ORANGE COUNTY SANITATION DISTRICT
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SP-125-16
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ORANGE COUNTY SANITATION DISTRICT
SP-125-16 BIOGAS SCRUBBER EVALUATION
TECHNICAL MEMORANDUM
BIOGAS SCRUBBER FEASIBILITY STUDY
TABLE OF CONTENTS
Paae No.
1.0 BACKGROUND.......................................................................................................1
2.0 PURPOSE...............................................................................................................1
3.0 EVALUATION DATA................................................................................................2
4.1 MASS BALANCES.................................................................................................3
4.2 Biogas..........................................................................................................3
4.3 Process Water..............................................................................................6
5.1 STUDIES...............................................................................................................6
5.2 Gas Compressors.........................................................................................6
5.3 Cogeneration Engines..................................................................................7
6.0 CONCEPTUAL DESIGN..........................................................................................7
7.1 SIMPLE PAYBACK COST EVALUATION..............................................................9
7.2 Payback.....................................................................................................12
8.1 LARGE-SCALE TESTING....................................................................................16
8.2 Location .....................................................................................................16
8.3 Schematic...................................................................................................16
8.4 Testing Plan ...............................................................................................16
8.5 Construction Cost Estimate.........................................................................19
9.0 CONCLUSIONS.....................................................................................................19
LIST OF TABLES
Table 1 OCSD Plant Data............................................................................................2
Table 2 Pilot Results: December 11, 2013 ...................................................................3
Table 3 Mass Balance: Biogas.....................................................................................5
Table 4 Change In Biogas Quantity..............................................................................6
Table 5 Feasibility-Level Construction Cost Estimate ...................................................9
Table 6 Payback Cost- No Change to CEPT.............................................................12
Table 7 Payback Cost-Alternate CEPT Chemical..................................................... 13
Table 8 Sensitivity Analysis: Increased Recirculation -No Change to CEPT...............15
Table Sensitivity Analysis: Increased Recirculation-Alternate CEPT Chemical.......15
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LIST OF FIGURES
Figure 1 Carollo Biogas Scrubber Pilot..........................................................................4
Figure 2 Full-Size Conceptual Layout...........................................................................8
Figure 3 Biogas Scrubbing System Schematic............................................................10
Figure 4 Future Site Location......................................................................................11
Figure 5 Large-Scale Pilot Schematic.........................................................................17
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Technical Memorandum
BIOGAS SCRUBBER FEASIBILITY STUDY
1.0 BACKGROUND
The Orange County Sanitation District(OCSD)utilizes digester gas as a fuel source to
offset utility costs for the operation of both Plant No. 1 and Plant No. 2. One of the biggest
challenges with the use of biogas is the removal of contaminants. These contaminants are
not only harmful to equipment, but are also regulated by authorities. Currently, OCSD is
embarking on extensive biogas cleaning in order to provide a fuel quality commensurate
with the needs of their existing cogeneration engines, which are being modified to meet
new South Coast Air Quality Management District standards.
The Carollo Engineers, Inc. (Carollo)Biogas Scrubber takes advantage of the treatment
plants' abundant supply of water to provide a convenient and effective method for cleaning
biogas. A simple venturi is used, in combination with a gas/water separator,to force
contaminants into solution and produce a high-quality biogas. Hydrogen sulfide(H2S),
carbon dioxide(CO2), and siloxanes can all be removed by this system, and, due to the
relatively low solubility of methane in water, very little methane is lost during scrubbing.
The Carollo Biogas Scrubber was pilot tested for proof-of-concept at OCSD's Plant No. 1 on
June 12, 2013, and again on December 11, 2013.The results were excellent for removal of
the aforementioned contaminants,with potential for significant benefits to OCSD's
cogeneration program using internal combustion engines.
2.0 PURPOSE
Based on successful pilot testing, OCSD decided to move forward with this second phase of
evaluation of the Carollo Biogas Scrubber. The overall goal for this Phase 2 evaluation is to
determine the feasibility of implementation of the Carollo Biogas Scrubber at Plant No. 1
and Plant No. 2. This phase included work in the following areas:
• Data Collection.
• Mass Balances.
• Engine and Compressor Studies.
• Identification of Discharge Water Treatment.
• Conceptual Design.
• Site Visit.
• Payback Cost Evaluation.
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• Sensitivity Study.
• Large-Scale Testing Plan.
This report summarizes the results of the work performed in Phase 2 as well as
recommended next steps for OCSD.
3.0 EVALUATION DATA
OCSD provided information found in Table 1 in order to complete the feasibility study in
Phase 2.
Table 7 OCSD Plant Data
SP-125-16 Biogas Scrubber Evaluation
Orange County Sanitation District
Information Item Plant No. t Plant No.2 Data Source
1. Quantity of digester gas 2,420,000 old(low)to 3,633,000 ofd SP-14101
available to be treated (cfd) (high)per plant
2. Gas Quality:
• Methane(%) 62 62 Jeff Brown
• Carbon Dioxide (%) 36.5 36.5 Jeff Brown
• Hydrogen Sulfide(ppm):
— Current Operation(�) 30 30 Jeff Brown
— No 112S Control with 3,000 3,000 Jeff Brown and Rudy Kilian
Ferric Chloride(3)
• Total Siloxanes (ppm) 7.514t 7.514t SP-141
3. Ferric Chloride Dosage(gallons/month):
• Headworks:Total for CEPT approx. 116,000 approx. 97,500 Typical for 2013-Current
Headworks-Solely for H2S None None Jeff Brown
Control
• Digesters approx. 17,50015I None Typical for 2013-Current
4. Femic Chloride Cost($/dry ton) $540161 $540161
5. O&M Costs for Activated $19,000(7) $19,000i7)
Carbon($/exchange)
6. Power Cost($/kilowatt-hour) $0.087 $0.087 Jeff Brown
7. O&M Cost for Discharge Water Based on the cost of pumping from
Treatment PEPS to AS Plant
Notes:
(1) The range of available gas was determined by averaging high and low values for the projected gas production
in the year 2020 from SP-141.
(2) Current operation:adding ferric chloride to the digesters as needed with no change to current CEPT dosing.
(3) There is no recorded history of uncontrolled H2S at OCSD available.This concentration is typical for facilities
that do not control H2S with similar processes.
(4) Actual siloxane concentrations at OCSD have been observed at much lower levels than the value used from
SP-141.
(5) Ferric chloride used at digesters was recently reduced for Plant No. 1,and may be eliminated in the future.
(6) Cost of ferric chloride ranges from$502.00 to$579.00 per dry ton among the suppliers contracted to OCSD.
(7) This cost is all-inclusive for a single carbon change-out. Change-out is required for every 184 to 200 million
cubic feet of biogas cleaned.Approximately six changes per plant will be required each year by 2020.
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4.0 MASS BALANCES
The Carollo Biogas Scrubber pilot unit consists of a gas/water contactor with a gas release
valve and a venturi. The scrubber uses the Venturi to pull a vacuum and draw biogas into
the system.Through pressure and water contact, contaminants that are highly soluble in
water leave the biogas with the discharge water stream, and scrubbed biogas is produced
with minimal remaining contaminants. See Figure 1 for a photo of the Carollo Biogas
Scrubber during pilot testing at OCSD.
During pilot testing, samples of biogas were taken at the inlet and outlet of the Carollo
Biogas Scrubber and tested at AtmAA Inc. and at OCSD's Plant No. 1 lab facility. The
results from this pilot showed that this system not only removes H2S and CO2, but also
significantly removes siloxanes from biogas. It was also discovered that some nitrogen and
oxygen, contained within the plant water used, left solution and was added to the scrubbed
biogas. The average results from the December 11, 2013, pilot testing are shown below in
Table 2.
Table 2 Pilot Results: December 11,2013
SP-125-16 Biogas Scrubber Evaluation
Orange County Sanitation District
Component Inlet Gas Quality0) Outlet Gas Quality0) Removal
Methane(CH4)(4) 61.7% 58.6% 4.9%
Carbon Dioxide(CO2)(4) 36.5% 2.2% 94.1%
Hydrogen Sulfide(H2S)t5l 7.7 ppm 0.00(2) 100%
Siloxanestsl 3.3 ppm 0.09 ppm 97.3%
Nitrogen(N2)(4) 1.2% 30.0% -
Oxygen(02)(4) 0.3% 8.7% -
Notes:
(1) Values based on average of samples analyzed from pilot test by AtmAA and OCSD.
(2) H2S levels were below the detection limit of testing equipment for both OCSD and AtmAA.
(3) Nitrogen and oxygen gained during the scrubbing pilot came from dissolved air leaving plant
water and transferring into the biogas.This impact may change based on the water supply used
for biogas scrubbing and the number of effective cycles used in the scrubbing process.
(4) Values reported are percent by volume.
(5) Values reported are parts per million by mass.
4.1 Biogas
The pilot testing used a single pass of water through the Carollo Biogas Scrubber to
simplify operation, which eliminates the concern of saturating the process water with
contaminants and halting the driving force of contaminant removal. Operating a full-scale
system on a single pass would require a significant amount of plant water and discharge
water to be sent downstream for treatment. For the current study, recirculation of water is
considered in order to minimize the plant water requirements and flow of water requiring
treatment.
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Feed Gas Collection
Discharge Gas Collection
Gas Separating Valve _
Gas Contactor S Feed Gas Collection
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CAROLLO BIOGAS
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FIGURE
ORANGE COUNTY
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The biggest obstacle when recirculating water is that, as concentrations of contaminants
approach their solubility limits,the driving force to remove contaminants decreases.
Currently, sufficient data does not exist to determine the solubility limits of siloxanes and
their impact on the driving force in the removal of all biogas contaminants. CO2 is expected
to be the controlling contaminant due to its relatively low solubility compared to 1-12S and far
greater concentrations in biogas than both H2S at 3,000 ppm and siloxanes at 7.5 ppm.
Based on CO2solubility, potential recirculation rates of two and four effective cycles were
used to evaluate potential payback for this process at OCSD, because these rates may
bracket the amount of recirculation possible for effective CO2 removal. This needs to be
determined through additional pilot testing.
Table 3 shows the inlet biogas quality and projected outlet biogas quality based on the pilot
test results and both effective recirculation cycles of process water. Not only will water
recirculation reduce the flow of water to treatment, but it may minimize the amount of
nitrogen and oxygen added to the scrubbed biogas as it is depleted further from the process
water with each cycle. Similarly, methane removal (loss)from the gas will be reduced
with water recirculation as the water becomes saturated with the less soluble
methane.The ratio of nitrogen and oxygen added to, and the methane removed from,the
scrubbed biogas is assumed to be reduced from the concentrations measured during pilot
testing based on the number of effective cycles. Further pilot testing is required to
determine the extent recirculation can be used while maintaining the driving force required
to remove contaminants and the impact on methane, nitrogen, and oxygen recirculation will
have.
Table 3 Mass Balance: Biogas
SP-125-16 Biogas Scrubber Evaluation
Orange County Sanitation District
No. of Cycles - - 2 2 4 4
Inlet Gas
Inlet Gas Mass Outlet Gas Outlet Gas Outlet Gas Outlet Gas
Component %Volume (moles/f 3) %Volume (moles/f 3)t2t %Volume (moles/f 3)t2t
Methane(CH4) 62.00 0.73 80.3 0.71 87.2 0.72
Carbon Dioxide 36.50 0.43 2.9 0.03 3.12 0.03
(CO2)
Hydrogen 0.30 0.0036 0.0 0.0 0.0 0.0
Sulfide(H2S)
Siloxanes Negligible 7.5 ppm Negligible 0.225 ppm Negligible 0.225 ppm
Nitrogen(N2) 0.00 - 15.00) 0.12 7.5 0.05
Oxygen (02) 0.00 - 4.40t 0.04 2.2 0.02
Notes:
(1) Nitrogen and oxygen gained during the scrubbing pilot came from dissolved air leaving plant water
and transferring into the biogas.This impact may change based on the water supply used for biogas
scrubbing and the number of effective cycles used in the scrubbing process.
(2) Per cubic foot of inlet gas volume.
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The high rate of CO2 removal from the biogas reduces the total volume of gas leaving the
scrubber.Table 4 shows the reduction in mass for 1 cubic foot of biogas after being
cleaned as well as the resulting change in total biogas flow.
Table 4 Change In Biogas Quantity
SPA 25-16 Biogas Scrubber Evaluation
Orange County Sanitation District
Unit Mass Flow
Inlet Gas 1.17 moles/ft3 2,100 cfm
Outlet Gas(two cycles) 0.90 moles/ft3 1,631 cfm
Change(two cycles) 23.0% 22.3%
Outlet Gas(four cycles) 0.82 moles/ft3 1,474 cfm
Change(four cycles) 29.7% 29.8%
4.2 Process Water
Assuming plant water is to be used for the process, discharge water will be relatively clean
with very low biochemical oxygen demand (BOD)and will contain only the additions of
sulfides and CO2 from the scrubbed biogas. It is estimated for a full-scale system that only
800 pounds of suedes per day will be added to the total loading of the plant, which
represents a small fraction of total suedes typically found at a wastewater treatment facility.
If CO2 is assumed to stay in solution, this would add 32,000 pounds of CO2 to the process
water per day, but much of the CO2 is expected to leave solution as it is released into the
biogas scrubber water recirculation tank.
There may be other impacts, such as corrosion or scaling within the system, that are
currently unknown.
5.0 STUDIES
The scrubbed biogas will impact downstream equipment such as the gas compressors and
cogeneration engines.
5.1 Gas Compressors
The volumetric flow of biogas out of the Carollo Biogas Scrubber is reduced, largely due to
the high removal of CO2.This translates into a decrease in required compression since a
large volume of inert gas is no longer being compressed with the usable methane.
Currently, raw biogas is sent to the compressors. 112S and siloxanes deteriorate
compressor components and, in turn, produce further wear on the equipment.With the
proposed location of a full-scale system upstream of the compressors, only scrubbed gas
will pass through the compressors;this has potential for significant maintenance savings.
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5.2 Cogeneration Engines
Engine manufacturers confirmed that the projected biogas quality could require changes to
the current cogeneration engine operation. The higher available British thermal unit(BTU),
reduced flow, reduced CO2, and additional oxygen in the scrubbed biogas probably would
require initial adjustments to the engines for proper operation. In addition, periodic changes
may be needed. These changes would be minimized if a consistent Flow and quality of gas
can be provided to the cogeneration engines, but further testing is needed to determine this
consistency under actual operating conditions.
Similarly, engine emissions would be expected to vary from current operations, but the raw
emissions and emission treatment impacts are unknown and should be determined through
additional testing.
6.0 CONCEPTUAL DESIGN
For conceptual design of a full-scale system that recirculates water with two effective
recirculation cycles,the following design criteria were used:
• Average Digester Gas Flow: 2,100 cubic feet per minute (cfm)per plant.
• Total Process Water Flow(including recirculation): 12,400 gallons per minute(gpm).
• Makeup Water Flow/Discharge to Treatment Water Flow: 6,200 gpm.
• Biogas Scrubber Units:
— Venturi Size: 8-inch unit.
— Quantity: 6 duty+ 1 standby.
• Biogas Scrubber Inlet Water Pressure: 70 pounds per square inch gauge(psig).
• Contact Chamber Pressure:4 psig.
• Water Recirculation Pumps:
— Type: Horizontal End Suction Centrifugal.
— Quantity: 2 duty+ 1 standby.
— Flow: 3,100 gpm per pump.
— Horsepower: 170 brake horsepower(hp) per pump.
A conceptual full-scale system designed to treat an average of 2,100 cfm of biogas can be
seen in Figure 2. Six duty biogas scrubbing units are required to provide sufficient treatment
capacity,with a seventh unit in place as a standby.With six 8-inch units at full capacity,
receiving 2,060 gpm each, total digester gas suction capacity is 2,160 standard cubic feet
per minute(scfm). Six 8-inch units are expected to provide sufficient turndown capability to
cover the full range of expected digester gas flows.
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Met Gas
Make-up Water
Recirculation Pump (Typ-3)
Biogas Scrubber
CONCEPTUALLAYOUT
ORANGE COUNTY -_-
DISTRICT
Water from the scrubbers will discharge into an aboveground recirculation tank, which
allows wastewater to passively flow over a weir to a drain as makeup water is added to the
system. Makeup water is introduced downstream of the recirculation pump discharge to
utilize all of the makeup water to remove contaminants from biogas before it is
wasted/drained. A submerged baffle wall provides energy dissipation prior to flow entering
the recirculation pump suction line.
Controls for this system can vary widely depending on the level of automation OCSD
desires. A sample process flow/control diagram is shown in Figure 3 with water and gas
flowmeters and a makeup water flowmeter. Pumping rates could be adjusted with variable
frequency drives(VFDs) based on incoming gas available for treatment, controlled by
digester gas pressure.
After the conference meeting on May 21, 2015, OCSD led a site visit of Plant No. 1, and it
was determined that the best potential location for this system would be at the existing
Dewaterng Building C,which will be demolished upon start-up of the new dewatering
centrifuges for Plant No. 1. See Figure 4 to see this location.
7.0 SIMPLE PAYBACK COST EVALUATION
The basis for the feasibility-level construction cost estimate is as follows:
• There is sufficient plant water(6,200 gpm)available for the process.
• Gravity flow would be used to transport discharge water to the Primary Effluent Pump
Station (PEPS)for pumping to the aeration basins for treatment.
The feasibility-level construction cost estimate for a full-scale system is$3.6 million;
the breakdown of the estimate can be seen in Table 5.
Table 5 Feasibility-Level Construction Cost Estimate
SP-125-16 Biogas Scrubber Evaluation
Orange County Sanitation District
Area Cost(USD)111
Site Work $300,000
Mechanical Piping and Equipment $2,700,000
EI&C $600,000
Total(2) $3,600,000
Notes:
(1) Cost is calculated in 2015 dollars with no escalation.
(2) Total cost does not include engineering,administration, and OCSD project costs.
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Make-Up Water M Venturi
Gas Gas
Scrubber Scrubber
No. 1 No.2
Recirculation Recirculation Recirculation
Pump No.1 Pump No.2 Pump No.3
Recirculation Sump
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7.1 Payback
Payback for each plant was determined by taking into account the potential savings and
operational costs of a full-scale biogas scrubbing system when weighed against the
feasibility-level construction cost estimate. The following create the basis for each facility.
• There would be no cost savings in reducing the amount of ferric chloride used for
chemically enhanced primary treatment(CEPT)at both plants.A change in ferric
chloride dosing would negatively impact effluent quality, i.e., there is no"overdosing"
for CEPT that is being used for H2S control at the digesters.
• Through discussions with OCSD, it has been determined that there would be no cost
savings for eliminating ferric chloride dosing at Plant No. 1 and Plant No. 2 digesters,
because digester dosing has been (or will be)eliminated.
7.1.1 No Chanaes to Current CEPT
Table 6 shows a breakdown of the annual savings and expenditures for the case where
ferric chloride is still used for CEPT at both plants and two effective recirculation cycles are
used. This case is not economically feasible as there is no savings at Plant No. 1 or
Plant No. 2.
Table 6 Payback Cost- No Change to CEPT
SP-125-16 Biogas Scrubber Evaluation
Orange County Sanitation District
Plant No. 1 Plant No. 2
Annual Savings
Ferric Chloride at Digesters $0 $0
Iron Removed from Solids $0 $0
Carbon $106,400 $106,400
Compressor Energy(22.3%)(�) $155,900 $81,900
Compressor O&M (22.3%)(3) $117,100 $46,800
Annual Costs
Plant Water Pumping -$274,100 -$274,100
Recirculation Water Pumping -$191,800 -$191,800
PEPS Pumping -$35,600 -$35,600
Methane Loss(4) -$122,100 -$122,100
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Table 6 Payback Cost- No Change to CEPT
SPA 25-16 Biogas Scrubber Evaluation
Orange County Sanitation District
Plant No. 1 Plant No. 2
Yearly Savings -$244,200 -$388,500
Simple Paybackl'I No Payback No Payback
Notes:
(1) Based on feasibility-level construction cost estimate. Does not include engineering,
administration, and OCSD project costs.
(2) The electrical cost of operating the gas compressors is reduced by 22.3%of the reported
annual cost from OCSD due to the reduction in gas volume needing compression.
(3) O&M savings for gas compression is anticipated due to the removal of H2S and siloxanes
from the digester gas prior to compression.The impact of this benefit is not definitively known,
so the benefit was calculated as a percent reduction in reported annual O&M costs equal to
the improved compressor efficiency(22.3%).
(4) Based on the loss of power that would have been produced by CenGen with the methane
removed by the scrubbing system.This cost assumes a 33% engine efficiency.
7.1.2 Alternate Chemical Used for CEPT
This case takes into account that,with the implementation of this biogas scrubbing system,
control of H2S with CEPT is no longer needed. This would allow OCSD to move to an
alternate chemical. This case looks at the annual savings required in order to produce a
10-year payback for Plant No. 1, by changing to an alternate chemical for CEPT. If annual
CEPT costs are reduced by$600,000 at Plant No. 1, there is approximately a 10-year
payback for a full-scale biogas scrubbing system. For the same reduction at Plant No. 2,
there are savings, but no reasonable payback. Table 7 summarizes the payback cost
associated with this case.
Changing to an alternate chemical for CEPT may provide potential savings but may also
negatively impact other processes benefitting from the current use of ferric chloride, such
as volatile solids reduction and dewaterability of solids. Careful consideration should be
given to any changes in current CEPT operations as it will influence more than just H2S
control.
Table 7 Payback Cost-Alternate CEPT Chemical
SP-125-16 Biogas Scrubber Evaluation
Orange County Sanitation District
Plant No. 1 Annual Plant No. 2 Annual
Savings Savings
Annual Savings
Ferric Chloride at Digesters $0 $0
Carbon $106,400 $106,400
Compressor Energy(22.3%)('I $165,900 $81,900
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Table 7 Payback Cost-Alternate CEPT Chemical
SP-125-16 Biogas Scrubber Evaluation
Orange County Sanitation District
Plant No. 1 Annual Plant No. 2 Annual
Savings Savings
Compressor O&M(22.3%)t�1 $117,100 $46,800
Reduced CEPT Cost(3) $600,000 $600,000
Annual Costs
Plant Water Pumping -$274,100 -$274,100
Recirculation Water Pumping -$191,800 -$191,800
Discharge Water Treatment -$35,600 -$35,600
Methane Loss -$122,100 -$122,100
Yearly Savings $355,800 $211,500
Simple Payback(4) 10.1 years 17.0 years
Notes:
(1) The electrical cost of operating the gas compressors is reduced by 22.3%of the reported
annual cost from OCSD due to the reduction in gas volume needing compression.
(2) O&M savings for gas compression is anticipated due to the removal of H2S and siloxanes
from the digester gas prior to compression.The impact of this benefit is unknown.The benefit
was calculated as a percent reduction in reported annual O&M costs equal to the improved
compressor efficiency(22.3%).
(3) This cost is based on the net required annual savings needed from changing CEPT chemicals
in order to produce reasonable payback at Plant No. 1. $600,000 is approximately 10% of the
total chemical cost of ferric chloride for 2015 at Plant No. 1. It is 20% of the total ferric chloride
cost at Plant No. 2.
(4) Based on feasibility-level construction cost estimate. Does not include engineering,
administration,and OCSD project costs.
7.1.3 Sensitivity Analysis-Additional Recirculation Cycles
Both payback cost cases above were reevaluated for the case where the effective number
of recirculation cycles could be doubled to four for the biogas scrubbing system. The basis
for this sensitivity analysis is:
• CO2 will be allowed to reach theoretical saturation, but release to atmosphere in the
recirculation tank would be expected to reduce the concentration. This was observed
during pilot testing, but the amount of CO2 leaving solution is currently unknown.
• CO2 concentration in water flowing into the scrubbing units is assumed not to exceed
50 percent of saturation. Makeup water flow would then be reduced to 3,100 gpm as
would the associated operations and maintenance(O&M)cost of pumping makeup
water.
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• Recirculation pumps would be increased in capacity to 4,650 gpm to maintain
required total process flow of 12,400 gpm.
• Recirculation pump motors would increase to 255 brake hp to accommodate
additional flow.
Table 8 compares the reevaluated payback period for each plant with no changes to CEPT.
Table 8 Sensitivity Analysis: Increased Recirculation -No Change to CEPT
SP-125-16 Biogas Scrubber Evaluation
Orange County Sanitation District
Payback Case Plant No. 1 Annual Savings Plant No. 2 Annual Savings
Payback(two cycles)(') No Payback No Payback
Payback(four cycles)(2) No Payback No Payback
Notes:
(1) Based on a feasibility-level construction cost of$3.6 million.
(2) Based on a feasibility-level construction cost of$3.8 million.
Table 9 compares the reevaluated payback period for each plant with an alternate chemical
used for CEPT that would reduce annual cost by$600,000.
Table Sensitivity Analysis: Increased Recirculation-AlternateCEPT
Chemical
SP-125-16 Biogas Scrubber Evaluation
Orange County Sanitation District
Payback Case Plant No. 1 Annual Savings Plant No. 2 Annual Savings
Payback(two cycles)(') 10.1 years 17.0 years
Payback(four cycles)(2) 6.7 years 10.1 years
Notes:
(1) Based on a feasibility-level construction cost of$3.6 million.
(2) Based on a feasibility-level construction cost of$3.8 million.
Through doubling the effective recirculation cycles, the payback periods for both plants
could be reduced by several years, if$600,000 can be saved annually on CEPT-related
costs by using an alternative chemical.This value,though high, may merit further
evaluation to determine the feasibility of this savings. If savings potential exists,four
recirculation cycles may allow for reasonable savings and payback at both plants.
7.1.4 Sensitivity Analysis-Larger Biogas Scrubbing Units
Payback cost cases were reevaluated for the case where 4 duty+ 1 standby 12-inch
scrubbing units are used instead of the 6 duty+ 1 standby 8-inch units for the biogas
scrubbing system. The manufacturer estimates the cost of a single 12-inch unit to be more
than double the cost of an 8-inch unit. Since five 12-inch scrubbing units are required, the
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additional capital cost of the biogas scrubbing units would outweigh the savings in reduced
electrical, instrumentation, and control (EI&C)for this alternative. Furthermore,to provide
adequate suction to treat 2100 cfm, the total water flow through the system would also
increase, going from 12,400 gpm to 17,600 gpm. This additional flow further adds cost to
O&M and capital investment to provide recirculation pumping. For these reasons, it is not
viable to consider 12-inch units for this system.
8.0 LARGE-SCALE TESTING
One goal of Phase 2 of evaluation for the Carollo Biogas Scrubber is to determine if further
pilot testing of the equipment is merited.This larger-scale testing would incorporate a larger
pilot unit than previously tested at OCSD, more controls, and testing the process to
determine operational boundaries.
8.1 Location
After a site walk of Plant No. 1 on May 21, 2015, and subsequent discussion, it was
determined that the best location for pilot testing would be the basement of the Gas
Compressor Building.This location has ample room for the pilot without inhibiting
operations staff.This location also conveniently provides all utility connections required for
pilot testing without requiring extensive additional piping.
Pilot testing would require short trips within the basement to collect samples and log data.
Long-term stays within the Gas Compressor Building would not be required. Two fans will
be used during pilot testing—one near the equipment and one near the basement door.
These fans will provide continual mixing of air, preventing buildup Of CO2 near the pilot that
may occur as CO2 leaves solution.
8.2 Schematic
Figure 5 provides a sample pilot schematic, offering manual control of water recirculation,
gas flow, and outlet gas pressure. Flow and pressure gauges are provided to allow
complete recordkeeping of the process conditions.
8.3 Testing Plan
Testing will consist of three stages, each stage lasting 2 weeks. During testing, the
following matrix elements will be recorded and evaluated:
• Water and Gas:
— CO2.
— Siloxanes.
— Oxygen.
— Nitrogen.
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Sample Line
Raw Digester Gas Sample
FI PI
Scrubbed
Digester Gas
FI
Gas Relief
Valve
FI
Make-Up Water X Venturi
a PI
Gas
FI Scrubber
PI
Recirculation Tank Recirculation
Pump
LARGE-SCALE
Process Blowdown PILOT SCHEMATIC
to Treatment -
A FIGURE
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• Gas Only:
— Methane.
— H2S.
• Optimal Flow through Venturi, including turndown.
• Optimal Pressure in Contact Chamber.
The following equipment will be required:
• One 2-inch venturi biogas scrubber.
• One 500-gallon water recirculation tank.
• One water recirculation pump.
• Instrumentation per Figure 5.
• Equipment skid.
8.3.1 First Staae - Sinale Pass
• Run the system with a single pass of water scrubbing the gas, i.e., no water
recirculation.
• After start-up,the system shall be allowed to run continuously for an hour at stable
conditions prior to sampling.
• Take six inlet and outlet samples of gas and water at six evenly spaced intervals each
day.
• Record water and gas pressure and flow into and out of the scrubber at time of
sampling.
8.3.2 Second Staae-Two Effective Water Recirculation Cycles
• Set makeup water flow and outlet water flow to allow two effective recirculation cycles
in the system.
• Once proper flow is achieved, the system shall be allowed to run continuously for an
hour at stable conditions prior to sampling.
• Take six inlet and outlet samples of gas and water at six evenly spaced intervals each
day.
• Record water and gas pressure and flow into and out of the scrubber at time of
sampling.
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8.3.3 Third Staae - Maximum Effective Water Recirculation Cycles
• Reduce makeup water flow and outlet water flow to 100 gpm less than that required
for recirculation at two effective cycles. Continue to reduce by 100 gpm at each
sampling interval until 10 effective recirculation cycles are reached.
• Once proper flow is achieved, the system shall be allowed to run continuously for an
hour at stable conditions prior to sampling.
• Take six inlet and outlet samples of gas and water at six evenly spaced intervals each
day,for two days, at each flow setting.
• Record water and gas pressure and flow into and out of the scrubber at time of
sampling.
• Repeat the above process and reduce makeup water and outlet water flow by an
additional 100 gpm. Repeat until the loop reaches 10 effective water recirculation
cycles.
8.4 Construction Cost Estimate
The feasibility-level construction cost estimate for the pilot is$350,000, which includes a
$50,000 allowance for instrumentation.
9.0 CONCLUSIONS
If OCSD continues to use ferric chloride for CEPT, reasonable payback is not expected for
either plant. Reasonable savings are not expected for the construction of a biogas
scrubbing system with two effective recirculation cycles, unless an alternative chemical can
be used for CEPT with an associated annual savings of$600,000 at Plant No. 1.
Reasonable savings at Plant No. 2 are not expected in this case. If an alternate chemical is
used and recirculation is doubled, more reasonable payback may be possible for both
plants. Initial findings show higher potential for reasonable payback at Plant No. 1, which
merits consideration of potential savings by changing chemicals used for CEPT at
Plant No. 1. If potential savings exist with an alternate CEPT chemical, further pilot testing
at Plant No. 1 should be performed in order to better define the operational limits and
consequences of the technology.
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