Here we discuss the first iteration of a model that examines the case for using oysters as a tool to add incremental capacity for removing nitrogen from waste water. It was presented at the 7th Annual Massachusetts Water Resources Conference. The Massachusetts Oyster Project is a non-profit active in oyster restoration. www.massoyster.org

1. massoyster.org
The Massachusetts Oyster Project For Clean WaterThe Massachusetts Oyster Project For Clean Water
Restoring oysters to their naturalRestoring oysters to their natural
range to improve harbor waterrange to improve harbor water
qualityquality
Exploration of the Cost-Exploration of the Cost-
Effectiveness of Using Oysters forEffectiveness of Using Oysters for
Nitrogen Removal FromNitrogen Removal From
Wastewater Relative to the CostWastewater Relative to the Cost
of Constructing Incrementalof Constructing Incremental
Waste Water Treatment PlantWaste Water Treatment Plant
CapacityCapacity

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Nitrogen WasteNitrogen Waste
 Average person excretes 12 pounds per year.Average person excretes 12 pounds per year.
 It also can be in high concentrations inIt also can be in high concentrations in
agricultural run-off.agricultural run-off.
 Excessive release to the environment can leadExcessive release to the environment can lead
to a build up of nutrients, called eutrophication.to a build up of nutrients, called eutrophication.
eutrophication can in turn encourage theeutrophication can in turn encourage the
overgrowth of weeds, algae, and cyanobacteriaovergrowth of weeds, algae, and cyanobacteria
(blue-green algae) in an algal bloom.(blue-green algae) in an algal bloom.
 The decomposition of the algae by bacteria canThe decomposition of the algae by bacteria can
use up so much of the oxygen that the areause up so much of the oxygen that the area
effectively dies.effectively dies.

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Oyster BenefitsOyster Benefits
 Filtering water in tidal estuariesFiltering water in tidal estuaries
 30 gallons per oyster per day30 gallons per oyster per day
 Feeding on phytoplankton and siltFeeding on phytoplankton and silt
 Improves water clarityImproves water clarity
 Offsetting impact from sewer overflows and other manmadeOffsetting impact from sewer overflows and other manmade
pollutantspollutants
 Capturing nitrogen, algae and bacteriaCapturing nitrogen, algae and bacteria
 Serve as keystone species in the benthic environmentServe as keystone species in the benthic environment
Crassostrea Virginica

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Oysters Also Sequester NitrogenOysters Also Sequester Nitrogen
 Oysters are 1.4% nitrogen by weight accordingOysters are 1.4% nitrogen by weight according
to Shumway World Aquaculture 2003.to Shumway World Aquaculture 2003.
 Shells are stable and last for centuries in situ.Shells are stable and last for centuries in situ.
 Harvested shellfish remove the shells from theHarvested shellfish remove the shells from the
ecosystem- permanently shifting the nitrogenecosystem- permanently shifting the nitrogen
burden.burden.

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Oysters Offer Other AdvantagesOysters Offer Other Advantages
 Oysters are passive by nature as they doOysters are passive by nature as they do
not migrate or travel.not migrate or travel.
 Oysters require minimal on-siteOysters require minimal on-site
maintenance.maintenance.
 Reproducing populations of oystersReproducing populations of oysters
become a frees self-sustaining nitrogenbecome a frees self-sustaining nitrogen
sequestration system.sequestration system.

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Oysters Are More Active in WarmOysters Are More Active in Warm
Weather When The Burden Is HigherWeather When The Burden Is Higher
Particulate Carbon
Source: MWRA

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Cost of RemovalCost of Removal
 Waste water treatment plants are builtWaste water treatment plants are built
with a variety of goalswith a variety of goals
 Reducing suspended solidsReducing suspended solids
 Reducing total oxygen burdenReducing total oxygen burden
 Reducing nitrogen is just one part of theReducing nitrogen is just one part of the
puzzle.puzzle.
 Adding incremental treatment forAdding incremental treatment for
additional nitrogen removal can beadditional nitrogen removal can be
expensive.expensive.

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According to a 2004 study by GannetAccording to a 2004 study by Gannet
Fleming the incremental cost per poundFleming the incremental cost per pound
removed ranged up to $30.29removed ranged up to $30.29

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To look at the cost of NitrogenTo look at the cost of Nitrogen
removal via oysters we built a modelremoval via oysters we built a model
using certain assumptionsusing certain assumptions
 Oyster spat 1 mm in size were purchasedOyster spat 1 mm in size were purchased
for $0.01 eachfor $0.01 each
 Volunteer labor was available from non-Volunteer labor was available from non-
profits such as the Mass Oyster Projectprofits such as the Mass Oyster Project
 Oyster shells contain 1.4% nitrogen byOyster shells contain 1.4% nitrogen by
weight.weight.
 We ignored those shells that died beforeWe ignored those shells that died before
one year, nitrogen in the meat, and anyone year, nitrogen in the meat, and any
sequestration from reproduction of thesequestration from reproduction of the
stockstock
 We also assumed that shells from oystersWe also assumed that shells from oysters
that died in year 2 died at a size of onethat died in year 2 died at a size of one
alive for 365 days.alive for 365 days.
 We left out the amortization of the cost ofWe left out the amortization of the cost of
a $12,000 upweller as well as electricitya $12,000 upweller as well as electricity
and insurance pending actual experience.and insurance pending actual experience.

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Our modelOur model
Variables to be filled
Hard Numbers
Output
Cost per pound of nitrogen removal at treatment facility $16.55 Cost for a pound of Nitrogen Removal $16.55
Cost per Oyster $0.01 Cost per Oyster $0.01 Year 1 Year 2 Year 3
Oysters Bought 1655 Oysters bought 1655
Percentage Nitrogen 1.4% Percentage surviving year one 45%
Oysters Surviving Year 1 745
One Year Oyster Shell Weight 0.044
Oysters dying in year 2 148.95
Two Year Oyster Shell Weight (lbs) 0.088 Shell weight (lbs) each 0.044
Shell pounds 6.5
Three Year Oyster Shell Weight (lbs) 0.118 Nitrogen Percentage 1.40%
Pounds of Nitrogen in Shell 0.09
Percentage Surviving Year 2 80%
Oyster Survival Year 1 45% Oysters Surviving Year 2 596
Number 745 Oysters dying in year 3 179
Oyster Survival Year 2 80% Shell weight (lbs) each 0.0875
Number 596 Shell Pounds 15.6
Oyster Survival Year 3 70% Nitrogen Percentage 1.40%
Number 417 Pounds of Nitrogen in Shell 0.22
Finalist Survivals 25%
Percentage Surviving Year 3 70%
Oysters Surviving Year 3 417
Shell weight (lbs) each 0.12
Increase in nitrogen removal 0.0% Shell Pounds 49.3
Nitrogen Percentage 1.40%
Pounds of Nitrogen in Shell 0.7
Total Pounds of Nitrogen 1.00
Increase in Efficiency 0.0%

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DrawbacksDrawbacks
 Oysters can be a tempting hazard andOysters can be a tempting hazard and
efforts must be made to ensure that theyefforts must be made to ensure that they
are not harvested- particularly if they areare not harvested- particularly if they are
downstream from waste treatment plantsdownstream from waste treatment plants
that provide incomplete treatment.that provide incomplete treatment.
 Oysters are hardy and will grow in manyOysters are hardy and will grow in many
places, but establishing reproducingplaces, but establishing reproducing
colonies is challenging.colonies is challenging.

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AdvantagesAdvantages
 While not appropriate as a first line treatmentWhile not appropriate as a first line treatment
oysters can provide a vehicle for augmentingoysters can provide a vehicle for augmenting
nitrogen removal without adding bricks andnitrogen removal without adding bricks and
mortar, which could be important in constrainedmortar, which could be important in constrained
locations.locations.
 Oysters can be placed at low cost and offerOysters can be placed at low cost and offer
recurring benefits.recurring benefits.
 Since many sites have high pollution levels thisSince many sites have high pollution levels this
will not allow harvesting creating much neededwill not allow harvesting creating much needed
sanctuary populations.sanctuary populations.
 With over 200 species inhabiting an oyster reefWith over 200 species inhabiting an oyster reef
we are enhancing biodiversity, creating shelter forwe are enhancing biodiversity, creating shelter for
creatures low on the food chain such as crabscreatures low on the food chain such as crabs
and shrimp as well as feedstock for threatenedand shrimp as well as feedstock for threatened
species, such as flounder.species, such as flounder.

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ConclusionsConclusions
 This preliminary analysis shows that placementThis preliminary analysis shows that placement
of oysters may offer a cost-effective way toof oysters may offer a cost-effective way to
augment conventional waste treatment inaugment conventional waste treatment in
reducing nitrogen load.reducing nitrogen load.
 Further examination and refinement of theFurther examination and refinement of the
model based on experience will providemodel based on experience will provide
additional value.additional value.
 There are a host of ancillary environmentalThere are a host of ancillary environmental
benefits that may make oysters an attractivebenefits that may make oysters an attractive
option as waste treatment facilities seek tooption as waste treatment facilities seek to
enhance their public image.enhance their public image.