HomeMy WebLinkAbout02212024 PACAB Work Session Laydown - Jaffa Lai z 1z1 J24 IP'k 4
Public Works Director
Doug Schoessler
CMRs excerpts from periods ending November 30, 2022s from January 13, February 3& 18, March 18
and April 1,2023
Y Wastewater and Lagoon overview study:
The third-party study was presented by R & M Engineering at the October 25 Council
Meeting. , Lc 1 I i-ow w � � (`'"
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• -The City was successfu ugh Senator Murkowski and others in recently getting
approved for$2,000,000 o unding for sludge removal on the City's sewer lagoons. This
project is tentatively scheduled for 2025 depending on sludge buildup in the lagoons.There
is also a 20%match on those funds that will have to be paid.
• -We are currently working on other grant or loan funding to replace the liner at that same
time.The liner replacement will also require the use of a helicopter to lift out the biodomes
Tc and reinstall them on the new liner.
• -This will also be the time to make any upgrades to the air systems and blower components
since all "in-lagoon" systems will have to be removed to install a new liner. Because the
lagoon will have to be drained(one side at a time). We can capitalize on construction costs
and efforts by installing new system upgrades at the same time.The new permits also added
rl a disinfection requirement to be completed in the next 5 years.
011 ' • -We are working on the funding and coordination of the many components and the
planning efforts in the next few years will help make a successful project.
CMR excerpt from Period ending November 30, 2022
Wastewater and Lagoon overview study:
This study is complete. The third-party study was presented by R & M Engineering at
the October 25 Council Meeting. Recommendations are as follows:
pi,
C�- 1. Using a multi-function portable probe, perform summertime DO, pH and
7)(p,j,4
temperature measurement profiles in grid pattern from both lagoons during a
period of non-rainy conditions and at least 3 days after a significant rain or high
user population event. Testing profiles are being logged for all
measurements. We are finishing data collection once we can get 3 days of no
rain. It's been raining a lot and we still have to perform our other testing and
�� routine work tasks.
/_) d� 2. During the same effort, from at least four separate locations in each lagoon
aeration zone (front and back zones of both basins), collect composite samples Tw3
of BOD, TSS, carbonaceous BOD (CBOD), soluble carbonaceous BOD (SCBOD),
ammonia, total Kjeldahl nitrogen (TKN), alkalinity, nitrites nitrates Testing
profiles are being logged for all measurements. We are finishing data collection
once we can get 3 days of no rain. It's been raining a lot and we still have to
perform our other testing and routine work tasks. k 74°
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,
PACAB HARBOR
• We have been working on picking up the winter sand on both sides of the bay.
> The 50-ton and 330-ton Travelift have increased; fishing vessels are getting ready for the
summer season.
➢ We are training several new harborworkers on the maintaining and operating the Travelifts.
• We are working on the SMIC yard drainage to keep water flowing into the ditches.
• We have opened G, K and L-float; contractor is working on chlorinating the system for our
DEC permit.
> We are still working with the public with COVID protocol in mind while continuing to do boat
lifts and public contact in the Seward Harbor office.
➢ This spring we will continue the cleanup at the old Raibow dump area,just a few more
items to deal with and we can lease the property out again.
➢ The harbor office will be open seven days a week starting May 7, 2023.
`r The North East harbor launch ramp is completed, the contractor finished on 12-1-2022, we
have received the as-built drawings and will closing out this project with ADF&G.
> We are currently working with the Director from the Pacific Northwest & Alaska Maritime
Administration U.S.DOT on finding a suitable grant for the washdown pad project.
• We are waiting on funding for the 5-ton crane on I-Dock, Corp Permit has been approved
> We have sent the Coast Guard the resolution and the Purchase and sale agreement for
them to look over. (Have not received any response as of tod )
> The City Manager, Vice Mayor and I had a very productive trip to Washington DC.
Public Works Director
Doug Schoessler
3. Consider the eventual supplementing or replacement of the existing coarse
bubble aeration system with a fine-bubble aeration system. The initial estimate
came in over$600,000 and included draining the lagoon and adding more
options than what we needed to accomplish this goal. We are working with R&M
and the contractor on scope.
4. Consider replacing the existing blowers with new, more-efficient units having a
higher aeration output to increase the capacity of the existing aeration system
111 for improved treatment of greater-than-average conditions. R&M will continue
after aeration improvement for pricing on replacing the large system blowers
with newer high efficiency blowers.
5. Consider modifying the influent and effluent header piping to provide additional
inlets and outlets serving each basin. Per engineering update 3/25/2022: "If it's
determined to provide worthwhile benefit, this work could be accomplished the
next time the lagoon is dewat red. From past performance, it is apparent that '
the lagoon would provide sufficient treatment without this improvement, as the
large size of the lagoon and the use of the coarse-bubble aeration produces a lot
of residence time. This concept should be regarded as an enhancement and not
an imperative improvement."
6. Continue implementing a regular sludge removal program. Scheduled for
( 2025/2026. Consider the procurement of a floating dredge unit to reduce th•
need for dewatering the lagoon basins for this effort. Per engineering update
3/25/2022: "I'm still researching this with folks in the wastewater lagoon industry
(still awaiting some inputs), but I'm coming to the conclusion that the City would
probably be better off contracting out the sludge removal work, rather than
doing this work themselves. For the size and depth of Seward's lagoon, a
considerable degree of equipment and temporary set-up would be needed to do
this work, and it would not appear to be good use of the City's resources to
invest in this equipment for use on an occasional basis (say every 5 to 10
years)".
7. The use of sodium nitrate could continue to be used by the City as a relatively
quick and temporary measure for addressing odors. This method does not
appear to be a good permanent solution for proactively mitigating or preventing
odors. Agreed, when the above aeration improvements are approved and
installed, this use will be reduced.
9
CRW Technical Memorandum
ENGINEERING GROUP LLC
Date: October 6, 2021
To: Doug Schoessler, DPW Director, City of Seward
Thru: Morgan Welch, PE, R&M Consulting, Inc.
From: Jon Hermon, PE—CRW Engineering Group, LLC j����
Project: City of Seward WWTF Review
Project (CRW#77101.00)
Subject: Review Summary
Background
The City of Seward has operated its wastewater treatment lagoon at Lowell Point since 1980. Since
1993,the lagoon has featured the two-cell configuration with which the City currently operates. Using
this configuration, the City has been,with some particular exceptions, meeting its discharge permit
requirements. The lagoons were drained to remove sludge in 2015 and then again in 2020 to repair
aeration line breaks. Each time shortly thereafter, water quality and/or odor problems ensued. In the
case of the 2020 event,the odor issue was exacerbated because one cell could not be refilled until the
following spring after repair of the line breaks. The City has since addressed the odor issues with the
filling and resumed aeration of the restored cell,the use of sodium nitrate and sludge-reducing bacteria
(SRB).
Lagoon Configuration and Operation
The Lowell Point wastewater treatment facility(WWTF)treats wastewater generated by the entire city.
The 2021 population of Seward is estimated to be about 2,850. However,the City has been using
round-number estimates of 3,000 residents and 1,000 non-residents as the average daily number of
users throughout the year. The user population tends to increase during the summer season,with an
reported peak of around 20,000 users during the July 4th weekend. The City's sewer collection system
conveys the regional wastewater to Pump Station 3,which is located at the beginning of Lowell Point
Road on the southwest corner of town. From this lift station,the wastewater is pumped approximately
2 miles south to the lagoon system at Lowell Point. After treatment, the water is conveyed from the
lagoon system northward, about% mile back up the road, to be discharged into Resurrection Bay
through an ocean outfall.
The lagoon features two aerated basins, each having a water volume of approximately 18 million gallons
and a water surface area of about 3 acres. When at their maximum operating volumes, each basin has
water depths ranging from 20 to 22 feet. The basins are separated by a bisecting concrete wall,to the
top of which a floating baffle curtain is attached. Water flows through each basin from influent and to
effluent pipes located at each end. Although the yard piping configuration allows the use of one or two
basins, and a possible reverse flow condition, the aeration system is currently arranged such that
wastewater needs to flow from the north basin to the south basin. The first (north) basin is aerated
with a combination of 69 static tubes and 40 Biodomes, with 58 static tubes grouped relatively close
together for aerating about 25%of the influent end of the first basin. The Biodomes aerate the back end
of the first cell with the remaining 11 static tubes. The second basin is aerated with just 6 static tubes,
with the assistance of two surface aerators, as needed.
According to the Lowell Point WWTF O&M Plan, the facility uses two 40 HP blowers to supply air to the
static tube aeration system. Each blower is capable of conveying 450 SCFM for a total maximum output
Anchorage Office:3940 Arctic Blvd. Suite 300,Anchorage,AK 99503 I (907)562-3252 fax(907)561-2273
Palmer Office:808 S. Bailey St.Suite 104, Palmer,AK 99645 I (907) 707-1352 www.crweng.com
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October 6,2021
City of Seward WWTF Review
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of 900 SCFM. A diesel-driven back-up blower can convey the same 900 SCFM output. The Biodomes are
supplied with air by a 30 HP blower,each with a minimum of 2 SCFM. Assuming that the static tube
aeration system is supplied by both blowers operating at full speed, and that the Biodomes are supplied
with the foregoing flow rate,the estimated air supply rates to each aeration system are summarized in
Table 1 below:
Table 1—Estimated Aeration Capability
System No.of Diffusers Unit Rate(SCFM) Total Rate(SCFM)
Static Tubes 75 12.0 900
Biodomes 40 2.0 80
Total -- -- 980
The City is permitted to treat a maximum of 880,000 GPD(0.88 MGD). Other permitted effluent
requirements are summarized in Table 2 below. BOD=biochemical oxygen demand;TSS=total
suspended solids; DO=dissolved oxygen. On a scale of 0 to 14, pH refers to the degree of acidity(less
than 7.0) or alkalinity(greater than 7.0)of the water,with 7.0 being neutral.
Table 2—Current Discharge Permit Requirements for Various Parameters
( Parameter Units Monthly
Average Weekly
Average Range
\
BOD (July-Oct) mg/L 45 65 --
BOD% Removal % 65 -- --
BOD (Nov-June) mg/L 30 45 --
BOD%Removal % 85 -- --
TSS mg/L 30 60 --
TSS% Removal % 85 -- --
pH -- -- -- 6.5—8.5
DO mg/L -- -- 6—17
11 Fecal Coliforms FC/100 mL 25,000 35,000 50,000
(daily max)
The City is also required to monitor effluent levels of other parameters such as,total ammonia,
enterococci bacteria and carbonaceous BOD(CBOD).
Based on Discharge Monitoring Reports (DMRs)shared by the City from Sept 2019 through mid-May
2021,the combined variability of various monitored parameters are summarized below in Table 3:
L.
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City of Seward WWTF Review
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Table 3-Variability of Currently DMR Parameters
Influent Effluent
Value BOD TSS Temp pH BOD TSS FC Ammonia DO
mg/L mg/L °C -- mg/L mg/L FC/100 mg/L mg/L
mL
Avg 159 320 10.0 7.1 10.0 7.0 13,484 13.1 9.9
Max 370 658 20.0 8.0 43.0 24.0 101,000 34.0 14.8
Min 21.1 79.3 0.5 6.9 3.5 1.7 667 1.1 2.3
Std Dev 84.0 149 6.5 0.1 7.3 4.8 18,016 9.4 2.3
(±)
"Greater
than 243 469 -- -- 17.3 11.7 31,500 22.5 --
Avg."
The City measures only effluent flow rates,the variability of which are summarized in Table 4 below,
along with the associated hydraulic retention time in each of the two basins.
Table 4-Variability of DMR Effluent Flowrates
Value Effluent Flow Rate Avg. Hydraulic
Residence Time
(per basin)
(MGD) (days)
Average 0.453 39.7
Maximum 1.20 15.0
Minimum 0.240 75.0
Standard Deviation (±) 0.161 29 to 62
"Greater than Average" 0.614 29
Influent BOD and TSS loadings are calculated from the concentrations and flowrate variability reflected
in the tables above. Since only effluent levels have been measured,estimated influent ammonia
loadings are calculated based on typical concentration values for domestic wastewater(avg. 25 mg/L;
max 35 mg/L; min 15 mg/L; std dev 4 mg/L). From these values, oxygen requirements in lb/day are
estimated and shown in Table 5 below. These values are averaged over the volume of the entire lagoon.
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City of Seward WWTF Review
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Table 5—Variability of Oxygen Requirements
BOD TSS Ammonia-N Total Total
Winter Summer
Value Loading 02 Loading 02 Est. 02 Loading 02 Req'd
Req'd Req'd Loading Req'd
lb/day lb/day lb/day lb/day lb/day lb/day lb/day lb/day
Avg 601 841 1,208 846 95 435 1,687 1,904
Max 3,703 5,184 6,585 4,610 350 1,611 9,794 10,639
Min 42 59 159 111 30 138 170 231
Std Dev 113 158 200 140 5 25 298 318
(±)
"Greater
than 714 999 1409 986 100 459 1985 2444
Average"
To obtain the estimated oxygen requirements need to biologically stabilize BOD,TSS and ammonia,the
following factors are used:
• 1.4 lb of 02 per lb of BOD. This factor is a conventional amount used in wastewater treatment
design.
• 0.7 lb of 02 per lb of TSS. This factor(assumed to be 50%of BOD 02 requirement) is
conservatively used to estimate the additional oxygen requirement associated with TSS(such as
algae) and sludge aging. This factor is not always used in oxygen calculations, but is used in this
study as a consideration for treating additional oxygen demand generated as a result of long
detention times in the lagoon cells(which favor algal growth and sludge aging).
• 4.6 lb of 02 per lb of ammonia-nitrogen. This factor is used to estimate the oxygen needed for
nitrification,to convert ammonia (NH3)to nitrates(NO3). Nitrification is assumed to
substantially occur only during the summer months when water temperatures are warmer. Use
of this oxygen requirement amounts to the difference between the summer and winter total
oxygen requirements shown in the table above.
It needs to be noted that the total maximum and minimum oxygen requirements indicated in Table 5
above are extreme values,assuming that the maximum and minimum of the BOD,TSS and ammonia
parameters occur on the same day. This type of occurrence is unlikely and shouldn't be used in the
evaluation of the capabilities of the treatment process. Instead,the average values are reviewed with
the standard deviation amounts added or subtracted. As a simplified way to characterize the variability
of these parameters,the standard deviation calculation is used to provide a sense of how dispersed
1 measurements within a data group are. Measurements falling within one standard deviation from the
average are assumed to occur approximately 2/3rds of the data group timeframe. Relative to the
maximum and minimum parameter values,these measurements therefore have a greater probability of
occurring concurrently. Therefore,"greater-than-average"values of flow,TSS, BOD,fecal coliforms (FC)
and ammonia are calculated as conservative amounts by adding the standard deviation to the average
and summarized in the above table for oxygen requirements. "Less-than-average" conditions are
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October 6,2021
City of Seward WWTF Review
Review Summary
reflected in flow, TSS, BOD, FC and ammonia loadings less than the average conditions summarized
above.
Aeration Requirements
The Lowell Point WWTF is assumed to have been originally designed as a "partial-mix" treatment
system, meaning that oxygen is provided only for biological treatment and not for also keeping solids
suspended (as would be the case in a "complete-mix" system). With a partial-mix system, solids will
settle and accumulate at the bottoms of lagoon cells, where further treatment slowly occurs by
anaerobic decomposition. In this manner, the lagoon system also functions facultatively, wherein both
aerobic and anaerobic processes provide biological treatment. The first cell functions more aerobically
than the second cell,which has substantially less aeration and functions more like a facultative lagoon.
Based on the recent DMR data summarized above and using approximate calculations and depending on
flow and wastewater conditions, it appears that the biological oxygen requirements can exceed the
aeration capacity of the WWTF (980 SCFM—all blowers operating). If TSS and sludge are considered as
oxygen demand generators, then aeration requirements more than double the amount of air needed
without considering TSS. Ammonia is considered only for summertime conditions, when water
temperatures are warmer for facilitating nitrification.
For purposes of these calculations, the population of Seward is projected to increase by 4.5% in the next
10 years and 9% in the next 20 years. This would add approximately 130 to 260 persons, respectively,
as year-round users. At 125 GCPD water usage per capita,this growth would add 0.016 to 0.032 MGD to
the average influent flow and approximately 22 to 44 lbs of BOD to the daily average.
These calculations are summarized in Table 6 below for both the current timeframe and Year 2041 for
projected, future average conditions. Conditions that appear to exceed the facility's aeration capability
are shaded in orange/beige.
Table 6—Aeration Requirements for Various Wastewater Condition Scenarios
Flow BOD TSS Ammonia-N Aeration
Condition Scenario Requirement
MGD mg/L mg/L mg/L SCFM
0.453 159 -- -- 408
0.453 159 -- 25 634
Average - -
0.453 159 320 -- 839
0.453 159 320 25 1066
0.614 243 -- -- 875
0.614 243 -- 29 1,232
Greater than Average
0.614 243 469 -- 1,746
0.614 243 469 29 2,103
Greater than Average,w/ 0.614 243 469 -- 992
fine bubble aeration 0.614 243 469 29 1,195
0
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October 6, 2021
City of Seward WWTF Review
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Flow BOD TSS Ammonia-N Aeration
Condition Scenario Requirement
MGD mg/L mg/L mg/L SCFM
0.88 159 -- -- 792
Average Conditions at 0.88 159 -- 25 1232
Max Flowrate 0.88 159 320 -- 1631
0.88 159 320 25 2070
0.485 173 -- -- 436
Projected Average 0.485 173 -- 25 679
Conditions-Year 2041 0.485 173 349 -- 985
0.485 173 349 25 1227
General conclusions drawn from these results are as follows:
• With exception to the case of treating BOD,TSS and ammonia concurrently,when the lagoon
system is operating in lower-than-average to average conditions, it appears to have sufficient
aeration capability(i.e.at 980 SCFM—all blowers operating).
• When treating for BOD,TSS and ammonia concurrently in average conditions,the lagoon system
\ appears to have insufficient aeration capability.
• When treating only for BOD (such as during the winter season) and assuming no TSS-generated
oxygen demand,the lagoon system appears to have sufficient aeration capability in greater-
than-average conditions.
• When treating for BOD,TSS and ammonia concurrently at greater-than-average conditions, or
combinations thereof,the lagoon system appears to have insufficient aeration capability.
• Using typical, "rule-of-thumb" performance values in the aeration calculation,the use of fine-
bubble aeration appears to be nearly twice as efficient as the coarse bubble aeration for the
same treatment conditions. For greater-than-average conditions,the fine-bubble aeration
requirement is nearly met within the air flow capabilities of the WWTF.
• Except for the case where only the treatment of BOD is considered (i.e.,wintertime operation
without regard to any TSS and sludge-imposed oxygen demands),the treatment facility does not
appear to have sufficient aeration capabilities for when flowrates approach the maximum
permitted flow rate of 0.88 MGD, at average wastewater conditions.
• Accounting for projected user population growth for the next 20 years,average wastewater
conditions increase a minor amount and thereby impose a minor additional oxygen demand on
the aeration system.
With regard to meeting the DO discharge permit requirement for the mixing zone,the lagoon effluent
receives a substantial amount of aeration as it freefalls several feet into the discharge piping of the flow
control structure. This arrangement and continuing open channel aeration within the discharge piping
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October 6,2021
City of Seward WWTF Review
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largely mitigates any effects in the mixing zone from deficient DO in the wastewater lagoon. Based on
the DMR data,the City has for the most part been meeting the DO permit requirement.
Discussion
While the foregoing results indicate shortfalls in aeration capacity in various scenarios,they do not
necessarily indicate that the facility's permit requirements will not be achieved, or that odors will ensue.
Flowrates and organic levels entering the plant are constantly varying, and short-term surges can be
readily attenuated in the large water volume of the two basins. Permit violations and odors become
more likely when surges are prolonged (such as in a large, extended rainfall event), when overloading
wastewater conditions happen concurrently(such as when high BOD, ammonia and algae levels occur at
the same time during the summer), when contaminant conditions become chronic (such as the build-up
of aging sludge) or if a prolonged loss or insufficiency of aeration occurred. Nevertheless,an aeration
system that is capable of treating both short-term and long-term wastewater conditions will be an
essential part of mitigating or avoiding permit and odor issues arising from significant events like the
foregoing examples.
The user o ulation rowth 's ntici aced to be slow for the foreseeable future, and flowrates and
organic loading rates are expected to increase a relatively minor amount during this timeframe.
However, as these rates slowly approach the aeration capacity of the WWTF, overloading conditions will
be experienced more frequently. The results summarized in ble_6nd their associated conclusions
reflect this potential. While the lagoon currently operates mostly around average conditions,for a
significant degree of frequency, it approaches or exceeds greater-than-average conditions, and
therefore approaches oxygen demands that tend to exceed the aeration capacity of the facility. These
wastewater conditions would impact the aeration capacity as discussed below: 0
• The influent biochemical oxygen demand (BODs) is a constant condition and always present in
various concentrations. The WWTF is configured to provide most of its aeration treatment
capability in the first cell,and particularly in the front 25%of first cell's water volume. This
approach intends to largely stabilize, mix and disperse influent wastewater as it enters the
lagoon, leaving a lower oxygen demand as the wastewater migrates to the back end and into
the second cell. During periods of high loading, it is suspected that the dissolved oxygen
provided by the aeration system in this front zone is substantially consumed, leaving only low
DO concentrations to enter the second lagoon cell. If sufficient BOD remains in the water
entering the second basin,the aeration system of this cell could be overloaded a result. If
prolonged,this condition would tend to produce anaerobic conditions in the wastewater and
associated septicity and odors.
• TSS and sludge-related oxygen demands depend on the presence and concentration of BOD-
producing TSS constituents (such as algae) and aging sludge. At high concentrations, algae can
add significant amounts of BOD, although algae can produce oxygen as well. When algae die,
interior substances disperse into the water and add to BOD. As previously mentioned, aging
sludge also feeds soluble BOD and nutrients back into the water. The degree that BOD is
augmented by these sources is dependent on how much is present in the on system. The
more BOD produced by these interior sources, the less effective the aeration system will be in
treating influent contaminants. These conditions would tend to be more prevalent during
summertime (when nitrification and denitrification is more active), at night time (when algae is
undergoing respiration) and as sludge accumulates in the basins over the course of years.
0
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October 6, 2021
City of Seward WWTF Review
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• Ammonia is normally present in raw wastewater and therefore always presents a potential
oxygen demand. Nitrifying bacteria require oxygen and warm water temperatures to actively
convert ammonia to nitrates in the nitrification process. During the winter, ammonia reduction
typically becomes negligible. Further,with low DO levels, the nitrification process is
substantially inhibited. To counter this condition, the Biodomes are designed to provide
nitrification to some extent, the degree of which is uncertain for this study. Based on the
measured ammonia levels in the effluent, it is presumed that only a limited amount of
nitrification is provided by the Biodomes, or that ammonia is being fed back into the wastewater
from aging sludge in the second basin. Nevertheless,with sufficient water temperatures and DO
levels, the nitrification process will also impose a oxygen demand that must be addressed by the
aeration system, as well as a demand on alkalinity. If the City is required in the future to
minimize ammonia levels in its mixing zone, then the size of aeration system will need to be V�
reviewed and modified as needed. ( ¶ --01NtK Iiti
While aeration provides a critical part in the biological treatment and mixing of wastewater, other
processes are also involved in meeting the permit objectives. The relatively large water volume of each
lagoon cell produces long detention times and accommodates slower biological treatment rates and a
substantial amount of settling. It is likely that important contributions to the overall BOD and TSS
removal at the WWTF is achieve through the settling of solids and anaerobically in the sludge blanket.
However, these other mechanisms can eventually become detrimental. As sludge accumulations
increase,the tendency for solids carry-over also increases, and the effluent water quality diminishes as a
result. Other issues can result from sludge accumulations. As the sludge blanket ages, soluble BOD and
nutrients are dispersed back into the water above, which imposes an additional oxygen demand on the
treatment system. Denitrification can also occur in aging sludge blankets, which releases nitrogen gas
and causes sludge to float to the water surface. Further, once or twice a year,the turning over of
temperature-stratified water can cause sludge and associated by-products to rise to the water surface,
causing strong odors. The accumulated sludge therefore needs to be reduced or physically removed
from the basins, dewatered and disposed of on a regular basis. Doing so will appreciably complement
the benefits of aeration, and in avoiding or mitigating acute and chronic events that can arise.
Conclusions and Recommendations:
The Lowell Point WWTF appears to be operating largely within its overall treatment capacity and will
likely do so for the foreseeable future. However, it appears that the potential exists for its treatment
capacity to be overloaded on a relatively frequent basis, when flow rates and organic loading rates are
high, and when the oxygen demand is increased by nitrification and TSS-related sources. The City is
encouraged to evaluate this potential in a more comprehensive effort. As this review is a relatively high-
level evaluation,further considerations and assessments are recommended below to provide a more
accurate understanding of lagoon conditions:
• Using a multi-function portable probe, perform summertime DO, pH and temperature
measurement profiles in grid pattern from both lagoons during a period of non-rainy conditions
and at least 3 days after a significant rain or high user population event. Collect and record
measurements from at least the upper quarter depth, half depth and 3/4 depth of the
wastewater. Ideally, measurements would also be taken from just above the sludge blanket and
at a couple feet below the water surface. This effort will provide a good snapshot of the health
of the lagoon wastewater when conditions will tend to be most taxing on the aeration system.
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October 6,2021
City of Seward WWTF Review
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0
• During the same effort,from at least four separate locations in each lagoon aeration zone (front
and back zones of both basins), collect composite samples of BOD,TSS, carbonaceous BOD
(CBOD), soluble carbonaceous BOD (SCBOD), ammonia, total Kjeldahl nitrogen (TKN), alkalinity,
nitrites+nitrates. These tests will help discern the extent of nitrification and TSS-related oxygen
demand sources.
A- • Consider the eventual supplementing or replacement of the existing coarse bubble aeration
system with a fine-bubble aeration system. With the ability to more efficiently oxygenate water,
especially with the large water depth of the lagoons, subsurface fine-bubble aeration could
sufficiently treat greater-than-average wastewater conditions for nearly the same energy
requirements as the existing aeration system. Improved oxygenation will also reduce the
tendency for odors, including the twice-yearly water turnover events during spring and fall
seasons. _
• Consider lacing the existing blowers with new, more-efficient units having a higher aeration
output to increase the capacity oTthe existing aeration system for improved treatment of
greater-than-average conditions. Operate the blowers with variable frequency drives based on
continuous DO measurements to improve power consumption.
• Consider modifying the influent and effluent header piping to provide additional inlets and
outlets serving each basin. Doing so would improve the mixing,dispersal and collection of
wastewater to and from the lagoon basins, and reduce short-circuiting. The relatively large
water volumes of the two basins tends to mitigate the effects of short-circuiting.
• Continue implementing a regular sludge removal program. Consider the procurement of a 0
-0` floating dredge unit to reduce the need for dewatering the lagoon basins for this effort.
• The use of sodium nitrate could continue to be used by the City as a relatively quick and
temporary measure for addressing odors. This method does not appear to be a good
permanent solution for proactively mitigating or preventing odors.
END
Attachments:
• Hand calculations
• Drawings used in calculations
• Spreadsheet calculations
0
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