Evaluation of
Ecological Nitrogen Removal Mechanisms
From Freshwater Tributaries Feeding Bournes, Green
and
Great Ponds
By
Harvesting Wetlands Plants and Microbial
Denitrification in
Constructed Plant/Rock Filters
April 17, 2002
Prepared by
David Del Porto, Principle Investigator
Sustainable Strategies
50 Beharrell Street
Concord, Massachusetts
01742
Prepared for
Ashumet Plume Citizens committee
The Town of Falmouth, Massachusetts
59 Town Hall Square
Falmouth, Massachusetts, 02540
Table of Contents
Removal Rates and Area requirements3
2. Mechanisms for Nitrogen Removal3
2.2. Microbial
Nitrification and Denitrification5
3. Hypothesis
for plant/rock filters (p/rf)8
3.1. Nitrogen
Uptake by Plants8
3.3. Heterotrophic
Denitrification9
3.3.1. Passive Carbon
Production from Natural Self-Organization9
3.4. Autotrophic
Denitrification9
Option 2: Self Organizing with Alkalinity Addition. 10
Option 3: Addition of Mineral Carbon11
Option 4: All Mineral Nutrients Provided. 11
4.2. Full-Scale
Site Identification Issues:11
4.2.2 Consideration
of site hydrology flooding, etc11
4.2.3 Consideration
of Existing and Proposed Land Use11
4.2.4 Construction
Constraints Diverting the River During Construction12
5.0
Evaluation of proposed system12
5.2. Reliability
Considerations (drawbacks)13
5.4.1. Sale
of Harvested Plants to Offset the Cost of Harvesting14
5.4.2. Initial
Cost Estimates for Full-scale Design Implementation14
5.4.3. Cost
of Maintenance for each P/RF(not including material replacement) 15
5.4.4. Cost/Benefit
Analysis of Proposed System15
5.5.4. Multi-functional
uses16
System Design and Bio-Chemistry17
The Town of Falmouth, Massachusetts is currently evaluating
the use of constructed wetlands to mitigate nitrogen loading to three salt-water
ponds: Great, Green and Bourne. A
long-term monitoring program of the ponds conducted by Falmouth Pond Watchers
found evidence of eutrophication and showed that additional nutrient loading to
the ponds would result in significant ecological degradation .(Johnson,
2000).
Sustainable Strategies has been contracted to develop a conceptual design for a new wetland approach to denitrifying tributaries to three ponds that have been determined to be eutrophic due to septic system drainage and lawn fertilization. The four primary systems to be developed are:
1. Nitrification of ammonia
2. Nitrogen removal by harvesting wetland plants
3. Subsurface carbon and possibly sulfur-contributing rock filter
4. Re-aeration to restore dissolved oxygen levels
Land use areas in the upper three-pond watershed include
approximately 2% cranberry bogs, 6% fresh ponds and streams, 75% natural fields
and woods, and 12% developed area. The
lower watershed areas include approximately 2% fresh ponds and streams, 3%
vegetated wetlands, 15% salt ponds, 49% natural fields and woods, and 32%
developed areas..(reference?)
The tributary stream flow to the three ponds is largely controlled by bog operators, who close or open culverts as necessary for cranberry cultivation. Based on USGS data, the average tributary flows are approximately 9 million gallons per day (MGD) into Great Pond, 2.0 MGD into Green pond, and 1.8 MGD into Bourne pond. (Ashumet Plum Citizens Committee report)
The pond watersheds are subject to non-point source nitrogen
loading in low concentrations.
Approximately 65% of the nitrogen loading to the ponds is from septic
systems in the lower watershed (south of route 28), and an additional 26% is
contributed from lawn fertilizer .(Johnson,
2000)..
The nitrogen contribution from the upper watershed makes up most of **% of the
total nitrogen loading to each of the ponds.
Nitrogen contribution from the Ashumet plume is expected to increase
nitrogen loadings by 4% to Great Pond, 8% to Green Pond, and 6% to Bourne Pond.
Additional nitrogen will be contributed by future land development.
Water quality data for tributaries to the three ponds
indicate that the majority of nitrogen loading (1.6 mg/l) is in the form of
nitrate or nitrite, with the remainder as
organic and a small fraction of ammonia (<0.05 mg/l).
Constructing a landscaped rock/plant filter (R/PF) with subsurface, horizontal water flow will remove significant amounts of nitrogen (Egan) while providing for a multi-use facility.
1.
Based on ourVaruni's
research, we should be able to remove a
substantial amount of
nitrogen can be removed by harvesting plants alone. T
From what I could gather, the uptake of 143 kgs per acre per year in reflected
a single harvest is possible. The
growing seasonGrowth rates in Falmouth this
part of the country should allow two or three harvests.
Plant biomass is
by nature 80 to 95 percent water.
2. Nitrogen
conversion through both heterotrophic and autotrophic denitrification by microbial
communities living in the R/PF will also yield positive resultsshould also be
possible. The major
limiting factors are available carbon, carbonate, and sulfur, all of which can
be dealt with a simple choice of
media such
as crushed concrete or stone. Crushed concrete
which is made from Portland cement (SIO2, Fe2O2, AL2O3, CaO, MgO and
SO3) and aggregate in combination
with other minerals may well be the ideal substrate. It is a low-cost
recycled waste product as well. Re-oxygenation
will be required. Depending on the
final location of the R/PFreactive beds,
passive oxygenation via water structures should be possible.
harvest.
Experience gained from the R/PF
with constructed wetlands in a Solar Aquatics systems (Strong) demonstrates
that anaerobic
microbial conversion of nitrate rates of 425 kgs per acre is possible in
a 2 m deep R/PF
marsh. When combined with vegetative removal, a net of 570
kgs of
Nitrogen per acre can be removed from the tributary flows.Combining
this with a vegetative removal gives a net of 570 kgs per acre.
While a pilotpreliminary
study would be required to assess the final sizing criteria, a rough estimate
is that for
complete removal by vegetation alone may be more than double the size for that of a combined vegetation removal plus
anaerobic microbial denitrification system. The area estimates are:
3 acres would
be sufficient for Bourne,
11 acres for Green, and 32
acres for Great pond
·
2.2 acres for Bourne pond
·
2.5 acres for Green pond
·
4.0 acres for Great pond
for complete
removal by vegetation alone.
Plants can take up both the nitrate (NO3) and ammonia (NH4) forms of nitrogen. Plant nitrogen use is related to the three factors: supply at the root zone, uptake across the root membrane, and translocation within the plant to sites of nitrogen activity and storage. The rate at which nitrogen is supplied to the plant depends on the concentration of available nutrients at the root zone and on mass flow of nutrients. Nutrient uptake efficiency depends on the genetic characteristics of a plant species and the ability of roots to adjust to quantities of available nitrogen.
The root can take in nutrients through ion exchange passively down a concentration gradient and actively against a concentration gradient. The nitrate concentration in a soil solution can be as high as 300 600 mg/L (Larcher, 1995). At freshwater sites in Massachusetts coastal areas, the nitrate concentration is 0.04 2.28 mg/L (Massachusetts Office of Coastal Zone Management). Therefore, most native wetland plants should be genetically able to access nutrients in the current concentrations at the Falmouth sites (total nitrogen = 0.3 to 0.7 mg/L).
Uptake rate will vary with the growth stage of the plant. Much of the total nutrient uptake occurs when the plant is producing new shoots and has been completed before the rapid increase in biomass begins. Therefore, multiple harvests to encourage shoot growth throughout the season may be ideal for nitrogen recovery.
A study on a paddy field in Japan recommends multiple harvests to recover the most nitrogen. The wetland is planted with Phragmites australis reeds. The study does not mention temperature or supply of nitrogen at the root zone. Old shoots were reaped at the beginning of the observation period. The nitrogen yield from two harvests is 1.5 times greater than the yield from one harvest per year after the beginning of observation. Two harvests, 110 days and 365 days after the beginning, mid-July and March respectively, yielded 45 g N /m2, with 35 g N/m2 from the first harvest and 10 g N/m2 from the second harvest. Three harvests, 100 days, 200 days, and 365 days after the beginning, July, October, and March respectively, yielded 47.5 g N/m2. (Hosoi et al, 1998)
Nitrogen that is taken up by the roots is translocated to sites of nitrogen activity or nitrogen storage. Nitrogen activity involves the synthesis of amino groups for proteins and is located primarily in the roots of woody plants and in the leaves of herbaceous plants. Nitrogen partitioning, the distribution of nitrogen among plant parts, varies with plant type and season.
In woody plants, nitrogen moves into new leaves and shoots in the spring, and then out of the leaves before they drop and into storage sites in the trunk and branches in autumn. Seasonal movement of nitrogen in herbaceous plants occurs as the nitrogenous breakdown products from aging leaves move to the growing shoots or to storage sites in the root. Nitrogen partitioning suggests that harvesting should occur before leaves age and the contained nitrogen is translocated to storage sites.
A study on two perennial grasses, Miscanthus x gigianteus and
Spartina cynosuroides, shows the effects of both high uptake rates in the
spring and translocation to storage sites later in the season. The above-ground Mmiscanthus
crop accumulated its maximum nitrogen content, 25.6 g N/ m2, in mid
July, while the total mass of above-ground dry matter was only 2 kg/ m2. After July, nutrient content in the
above-ground matter declined, and the content in the rhizome increased. In mid- September, when the miscanthus Miscanthus crop had
reached is its maximum biomass yield,
3 kg/m2,
the nitrogen content of the above-ground crop had fallen to 20 g N/ m2. (Beale and Long, 1997)
Nutrient accumulation and partitioning is shown for various
species in Table 1. Below-ground
nitrogen accumulation is not measured in the Phragmites australis study.
Data from the Phragmites australis
nutrient removal study may also be incomparable to data from the bamboo and
miscanthus crop growth studies because the crop studies focused on minimum
nutrient removal to indicate nutrient efficiency This is common for agricultural studies that
seek to optimize the
efficiency of crop production as opposed to, asking, What is the
maximum amount of Nitrogen a plant can uptake?
rather than on maximum nutrient removal (or
something like that
maybe remind the reader what the phragmites study was
about).
Nutrient Accumulation
and Partitioning
|
Species |
Above-ground N (kg/ha) |
Below-ground N (kg/ha) |
Comments |
|
Phragmites australis a |
450 |
- |
two harvests |
|
Phragmites australis a |
475 |
- |
three harvests |
|
P. pubescens (bamboo) b |
227 |
552 |
50 year year-old stand |
|
G. atter (bamboo)b |
131 |
251 |
4 year year-old stand |
|
Miscanthus x
giganteusc |
256 |
100 |
in mid July |
|
Miscanthus x
giganteusc |
200 |
130 |
at peak of biomass in mid September |
a. a Hosoi et al, 1998
b. b Kleinhenz
and Midmore, 2001
c Beale and Long, 1997
The rates of nitrification or the conversion of
ammonia
(NH3 and NH4) to fully oxidized Nitrate (NO3)are
is affected
by a number of variables, but the most important is adequate dissolved oxygen for
the aerobic nitrifying bacteria (Crites 2000). Nitrosomonas spp and Nitrobacter spp are typical denitrifying
bacteria
found in nitrifying environments. High concentrations of ammonia and
nitrite will inhibit nitrification. The BOD5/TKN ratio will affect the
concentration of nitrifying organisms.
More specifically, a BOD greaterless than
50 mg/L will inhibit nitrification (Bruce).
The free organics in preference to NH3???
At
low
dissolved oxygen DO concentrations, oxygen becomes the
limiting nutrient (minimum dissolved oxygen concentration of 1 mg/L required in
the oxic nitrification zone Lampe).
The growth and metabolism of nitrifying bacteria are also affected by
temperature and pH, with an optimal temperature being 30.5° C. with a range
of 8°C - 2406°C--- and pH range of 7.5 to 8.6
(although the bacteria canmay
acclimate to a lower pH). This speaks to a
shallow P/RF in the beginning of the system to provide the dissolved oxygen requirements for the
aerobic nitrifying bacteria.
Anaerobic hHeterotrophic
denitrification is the most commonly used biological process for nitrate removal in
conventional treatment systems. The
process consists of the conversion of nitrate to nitrogen gas, and will occur
under anoxic conditions with adequate supply of organic carbon. Heterotrophic
bacteria, such as Pseudomonas spp,
obtain energy for growth from conversion of nitrate to nitrogen gas, using
carbon as an electron donor and nitrate as the electron acceptor. Organic
carbon can be supplied by biomass decomposition or as an additive in the form
of methanol, ethanol, acetate or sugar.
The rate of heterotrophic denitrification is affected by the
concentration of nitrate, concentration of carbon, dissolved oxygen,
temperature, and pH (Crites 2000).
Reaction rates are increased with higher nitrate concentration, higher
organic carbon concentration, and lower dissolved oxygen. The optimum temperature is 30.5° C. with a range of
10 - 40range
is° C.***,
and the optimum pH range is 6.5 to 7.5.
The process of anaerobic autotrophic denitrification has an advantage over heterotrophic denitrification in that it does not require the use of external organic carbon sources. This process is the conversion of nitrate to nitrogen gas by autotrophic bacteria that derive energy from inorganic oxidation-reduction reactions with elements such as hydrogen and various reduced-sulfur compounds (H2S, S, S2O32-, S4O62-, SO32-), and use inorganic carbon compounds (CO2, HCO3-) as their carbon source (Lampe, 1996).
The two sulfur-denitrifying bacteria most researched are Thiobacillus denitrificans and Thiomicrospira denitrificans, both of
which are generally found in the natural environment (Lampe 1996, Nahar
2000). Other sulfur denitrifying
bacteria are Thiosphaera pantotropha,
an aerobic denitrifier, and Beggiatoa
alba, a sulfur denitrifier that uses H2S at the
aerobic-anaerobic boundary of ocean or fresh water
(website
reference). Optimal
conditions for the bacteria are pH 7 (range 6-8) and temperature above 75°75
deg. F (range 41-104° deg.F,
5-40°
deg.C) (website
reference)
Autotrophic denitrification will dominate under anoxic
conditions if the carbon source is inorganic and if there is an adequate supply
of sulfur. Many recent studies have
been conducted to evaluate the performance of autotrophic denitrifiers with the
use of sulfur and limestone additives.
(Lampe 1996, Flere 1997, Bezbaruah 2001, Nahar 2000, etcreference
some of the studies referenced in the Lampe paper ).
Lampe and Zhang conducted bench-scale experiments to
evaluate the sulfur-based autotrophic denitrification process. They used both elemental sulfur and
thiosulfate as sulfur sources, and found that thiosulfate was superior because
it dissolved and mixed more readily in water.
Limestone media was used with granular sulfur in Ssulfur-Llimestone
Aautotrophic
Ddenitrification
(SLAD) fixed-bed reactors. The
researchers found that in a fixed-bed reactor, the optimum ratio of sulfur to
limestone for denitrification was 3:1, and that limestone buffering was
critical. The experiment with SLAD
aerobic and anaerobic batch pond systems showed that nitrate reduction was more
efficient in anaerobic batch systems than in aerobic batch systems. The study concluded that nitrate-nitrogen
removal was enhanced with the use of granular sulfur and limestone media and
that sulfur-based systems are feasible for
in situ remediation of nitrate-contaminated surface waters.
Flere and Zhang followed up on the previous study by
evaluating four bench-scale aerobic SLAD ponds for remediation of
nitrate-contaminated surface water. The researchers found that under aerobic
conditions, nitrate removal in the ponds was 85 to 100%. The addition of sodium bicarbonate (Na2HCO3
baking sodacommon
name for this?) succeeded in
raising the pH and enhancing nitrate removal efficiencies in the pond
systems. Sulfate was produced at a rate
of between 40 60 mg/L per 1 mg/L of nitrate reduced, thus making sulfate
production a critical issue in the use of in
situ aerobic SLAD systems. Additional bench reactors simulating SLAD pond
systems under anaerobic conditions showed that sulfate production was lower
than that under aerobic conditions.
Bezbaruah, Zhang and Stansbury conducted a lab-scale study
of a SLAD system for nitrogen removal from municipal wastewater. An un-planted
wetland R/PF operated
for autotrophic denitrification consisted of an aerated nitrification zone
followed by an anoxic SLAD zone followed by an anaerobic polishing zone for
sulfate removal. The anaerobic
polishing zone achieved about 50% sulfate removal, a rate that could be
improved by providing more organic carbon.
Nahar, Fox and Wass evaluated sulfur-driven autotrophic denitrification in existing, full-scale constructed wetlands. The wetlands all received influent with nitrate-N concentration of less than 10 mg/L and sulfate concentrations typically exceeding 100 mg/L. In field tests of an existing un-planted constructed wetland, the researchers found that denitrification was occurring despite the absence of organic carbon. They also found that the addition of organic carbon resulted in significant sulfate reduction. It appeared that the sulfide stored in natural wetland sediments drove denitrification during periods of low organic carbon. The researchers found that the rate of dissolution of sulfide or other reduced-sulfur compounds from wetland sediments may limit sulfur-driven denitrification; however, without that limiting factor, rapid sulfide oxidation could result in acid production that could harm the aquatic ecosystem
These studies indicate that autotrophic denitrifiers exist
naturally in wetland sediments and that sulfur-driven autotrophic
denitrification is a feasible alternative or supplement to heterotrophic
denitrification. One issue that must be
addressed in the use of SLAD systems for
in situ remediation of nitrate-contaminated surface waters is the
production of acidity and sulfate. Both
of these products have a negative effect on the rate of autotrophic
denitrification and on the aquatic ecosystem.
Another concern with sulfate production is that at high concentrations
(~2000 mg/L), sulfate will react with limestone and precipitate into an
insoluble gypsum sludge, which may clog pore spaces of R/PFwetland
media (CMD).
Limestone (CaCO3) is commonly used to raise alkalinity
and pH in passive treatment of acid mine drainage (AMD), and can be used for
the same purpose in autotrophic denitrification. In addition, the bicarbonate (HCO3-)
produced from limestone dissolution can serve as a carbon source for
autotrophic denitrifiers. Dolomitic and calcitic limestone is
are also commonly used in fertilizers
to supply calcium and alkalinity. The
rate of limestone dissolution increases as the pH decreases, but the reaction
can be reversed if the concentration of calcium is high. More research is needed to determine the optimum
ratio of limestone to sulfur in a subsurface constructed wetlandR/PF. Crushed concrete
which is made from Portland cement (SIO2, Fe2O2, AL2O3, CaO, MgO and
SO3) and aggregate may well be the ideal substrate and
it is a recycled product as well.
Sulfate Rreduction
can occur in an anaerobic environment with sufficient organic carbon. Assimilatory sulfate reduction is the
conversion of sulfate into protein containing sulfur. Sulfate is a common element in fertilizers, in forms including
ammonium sulfate, calcium sulfate (gypsum), potassium sulfate, and magnesium
sulfate (Eepsom
salt).
Dissimilatory sulfate reduction is the formation of sulfide
by sulfate reduction. Heterotrophic
anaerobic bacteria decompose organic carbon compounds using sulfate as a
terminal electron acceptor. The sulfate is converted to sulfide (H2S),
or ionized as HS- and S2-, or precipitated as a
polysulfide, elemental sulfur, or iron sulfides. (Bezbaruah, Zhang) Organic carbon serves as an oxidizable
carbon source in addition to hydrogen.
Sulfate-reducing bacteria are strictly anaerobic. (Zhang et alreference)
More investigation needs to be conducted to determine other
issues related to the use of sulfur, such as the production of hydrogen sulfide
gas (H2S), which is responsible for the rotten-egg odor
associated with sulfur.
Although not part
of this offering, yet another natural removal mechanism is fish farming in controlled
areas utilizing filter feeders, i.e. silver catfish, to capture remaining nutrients
that may enter the pond.
Certain bi-valve shellfish also can remove a significant amount of
nutrients as well. This could be part
of the commercial
economic
benefit of the new construction that would assist in building support for the
project.
Plants
incorporate nitrogen directly in to themselves equally in above ground and below ground
biomass. During each harvest, in July and in October (, which allows for 100 days of shoot growth before
harvests), maximal maximum removal of nitrogen will occur. While harvesting after the initial growth of shoots
will remove more nitrogen than harvesting when the plant has accumulated
maximal seasonal biomass both are viable removal vectors as if plants are not harvested before
the leaves age, nitrogen is moved to root structures and cannot easily be harvested.
Plants will continue to remove nitrogen during the dormant season as while
there is little visiable growth, there is substantial creation of root
mass at this time.
In order to convert ammonia to
nitrate (nitrification) there needs to be free metabolic oxygen n availalble. The interface between saturaated media and dry media is
sufficiently turbulent to entrain sufficient oxygen for this purpose. The limiting factor here may well be
alkalinity. If there is not sufficient
available carbonate, the reaction will stop.
This can be rectitified by the addition of nutrient, such as sodium
carbonate, or the addition of limestone (approximately 5% of the total
media). Given the normal pH range of the watershed, this should be
adequate.
· Turbulent
front-works will entrain oxygen into flow.
· Shallow
rock filter
· If
alkalinity and pH are not within the optimum range for nitrification (subject
to tributary water quality data), limestone will be used as a portion of the
wetland substrate to increase alkalinity and pH.
Below the upper
moisture interface, the R/PFrock filter will be predominantly anaerobic. Here bacteria will utilize the available
nitrate, both from the influent stream and the converted ammonia, as its oxygen supply to
facilitate metabolizing respiration of organic carbon during
respiration..
As
the plant roots mature in the planted R/PF, there will be
surface sloughing of waste cells, abandoned roots, root production of exudates
(carbohydrates,
organic acids, vitamins and other substances essential for life) and the trickle down of
surface detrictus
as a source of organic carbon. While this may take some
period . Again from the experience of the Solar Aquatic
facility
in Weston, of time (3 to 5 years is required) beforeto the R/PF develope develops into into a self-sustaining system. If immediate results are called for, then , imdeiate results can be achieved via the addition
of soluble carbon such as methanol or acetate solutions can be added.
· Anaerobic
zone
deeper rock filter? Or just lower
portion of rock filter?
· Decomposition
of plant roots may provide adequate organic carbon for heterotrophic
bacteria.
· If
necessary, additional organic carbon can be provided by using coal as a wetland
substrate. (subject to further research)
In
this same zone, additional bacteria will use alternate compounds as theire food source and aid in the
denitrification systems. Any aviailable sulfur will can servie as a sulfur source for autotrophic
denitrification. Determining To determine the full extent of this pathway will require additional research, which
is planned for the piloting stage. There are
similar alalinityalkalinity/pH control requirements
as to heterotrophic denitrification which again can be met by the addition of
limestone to the media.
A
second R/PF wetland may be desired for final removal of sulfate that was produced from autotrophic
denitrification. This coulde be an unplanted bioreactor following the initial
planted zone. Piloting of the initial
designs will reveal the need for this unit process.
· Anaerobic
polishing zone following the sulfur/limestone zone, with sufficient organic
carbon, will reduce sulfate that was
produced from autotrophic denitrification.
· This zone
may further denitrify via heterotrophic denitrification.
· Plants take
up sulfate, so polishing zone could possibly be planted. Decaying roots could provide the organic
carbon for sulfate reduction and heterotrophic denitrification.
Massachusetts
regulations and proper care of
the environment require that the system discharge maintaincontane a DO (dissolved
oxygen) concentration of 7.0 mg/L to maintain healthy conditions for the pond
environs. This can be accomplished by
either a turbulent zone located immediately after the constructed wetlandsR/PFs if there is
sufficient change
in elevations or by mechanical means (windmills, aeratores, etc.). After piloting gives the final DO requreimentsrequirements, the site selection
process will drive this desiciaiondecision.
· Regulations
require a discharge DO concentration of
7.0 mg/L
· A turbulent
zone will aerate the water prior to discharge.
There are four possible configuarionsconfigurations
of the removal system. Some characteristics will be common to all, such as the
wetlands liner, a plastic barrier overlain with a geotextile cloth to protect
it from the R/PFwetland
media, a distribution and bar screen structure at the entry to all
for laminar flow. Typically
there will two depths in each R/PF to accommodate different biological processes. Shallow ( 18-24
inches ) for primarily aerobic organisms and deep ( 24-48 inches) for
primarily anaerobic organisms.
:
The wetlandR/PFs will be sized to
remove all the available nitrate and ammonia without the addition of any additional
nutrients. Maximum nitrogen
removal will occur 3-5 years after construction to allow for the development and natural
decomposition of the
rhizosphere. While this will result in not removeing all nitrogen from the onset very begining of the systems
use, it will reduce costs in not requireing replacing spent
additives. Using 1 ½ 1- 31/2 stone as
the media, initial sizing of the three
systems would be:
|
Location: |
|
|
|
Great Pond |
2.5 |
1.5 |
|
Green Pond |
1.25 |
1.25 |
|
Bourne Pond |
1 |
1.2 |
* R/PF 1 - Planted Rock Filter
** R/PF 2 - Unplanted Rock filter
Combining
the Planted and Unplanted R/PFs provides the optimum configuration with varying
depths to accommodate aerobic and anaerobic processes. Therefore the total area
required
is:
·
4.5 Acres for Great pond
·
2.5 Acres for Green Pond
·
2.2 Acres for Bourne pond
Here
the R/PFwetlands will be the same size, with the addition of limestone for
ph and alkalinity control. Initial sizing is :
|
Location |
Tons of limestone |
Cubic feet of
limestone |
|
Great Pond |
201 |
3660 |
|
Green Pond |
40 |
732 |
|
Bourne Pond |
45 |
815 |
Under
this scenario, the limiting factor is assumed to be available carbon. The wetlandR/PFs would be constructed
with both inert stone and anthracite coal (low sulfur) to provide the required carbon. While coal is practicallyally insolumbleinsoluble, microbial action on
the material surface will release available carbon over time, thus powering
denitrification.
|
Location: |
Tons of Coal, |
Tons of Coal, |
|
Great Pond |
168 |
100 |
|
Green Pond |
34 |
34 |
|
Bourne Pond |
37 |
45 |
Here
both limestone and coal will be added to the constructed wetlandR/PFs to ensure that all
necxessary environmental conditions are met from day 1.
Piloting
of these four scenarios will give more accurate sizing and costs.
All locations for the site of the system are located within the tributary boundaries. Therefore, the design of a subsurface planted rock filter suggests that the best place to locate the system is directly in the tributary leading to the ponds with a bar-screen and head-works located at the culverts under Route 28. The subsurface flow through the rock filter would be engineered to achieve the necessary detention for denitrification and to insure an un-interrupted flow from the tributaries to the ponds. The moving water would be 3 -6 inches below the surface.
The benefits of a
sub-surface flow, planted or unplanted rock filter permits a great
deal of flexibility in site hydrology. It can be
constructed in-stream and not significantly impede the flow to the ponds. We have developed data from the Solar Aquatic
R/PF in Weston that demonstrates that the water flow rate is
reduced
over
time by
40% as the plant rhizosphere develops and thereafter a steady-state is achieved
as the root decomposition and mineralization equals new root production. Detailed
engineering will illuminate the issues and flooding can be
accommodated by surface flooding when storm volumes exceed the capacity for subsurface flow.
The value of
upland property not located within the 100 foot buffer zone from wetlands may too
expensive to consider a taking by eminent domain for the purposes of mitigating
eutrophication in the ponds.
While this is a political decision, it may be less onerous
to use land already within the existing tributaries and their wetlands
to construct such a facility.
Careful consideration to engineering and construction sequencing to allow for the diversion of tributary flows while the P/RF systems were being constructed so as not to disrupt the tributary hydrology.
Additional testing of the
existing water quality (lower pH from humic, fulvic or sulfuric acids will enhance some
of the reaction kinetics) and flow conditions is required. While there has been extensive evaluation of
the wastershed,
not all the parameters required for treatment (i.e. pH) have been
analyzed. Once the background data is
fully deveoloped, a series of pilots
can be designed to deteminedetermine the efficacy of the our proposed systems.
:
· Existing water quality and flow conditions
· Types and
rates of reactions
To
understand what is meant by the Rremovoal pathways for asy, nNitrification and
denitrification can be represented by the following generalized reactions
(Crites 2000):
Conversion of ammonia
to nitrate (as typified by Nitrosomonas)
NH4+
+ 1.5O2 => NO2- + 2H+ + H2O
Conversion
of nitrite to nitrate by Nitrobacter
NO2-
+ 0.5O2 => NO3-
Overall
conversion of ammonia to nitrate
NH4+
+ 2O2 => NO3- + 2H+ + H2O
Ammonia is also used
by some organisms for the synthesis of biomass, according to the following
reaction (Crites 2000)
4CO2
+ HCO3- + NH4+ + H2O
=> C5H7O2N + 502
The general
reaction for heterotrophic denitrification is:
Autotrophic
denitrification can be represented by the following reactions:
2NO3-
+ S + H2O + CaCO3 = CaSO4 + N2
(Cornell(?) course website)
0.422H2S
+ 0.422HS- + NO3 + 0.347CO2 + 0.865HCO3
+ 0.0865NH4+ σ
0.844SO42- + 0.5N2 + 0.0865C5H7O2N
+ 492H+ (Nahar)
55S
+ 20CO2 + 50NO3 + 38H2O + 4NH4+
=> 4C5H7O2N + 25N2 + 55SO42-
+ 64H+ (Lampe, Zhang)
Limestone dissolves in
water when in contact with acidity, as shown in the following general reaction:
H+
+ CaCO3 => Ca2+ + HCO3-
Based
on the stoichiometric equation:,
7.1 mg/L of SO42- produced for every 1 mg/L NO3--N reduced,.
(Flere, Zhang) eEach
mole of NO3 denitrified produces 0.844 mole SO4 if sulfide is used as the
electron donor. (Nahar)
The process of sulfate
reduction can be represented by the following reactions:
Organic
matter + SO42- =(bacteria)=> S2-
+ H2O + CO2
S2-
+ 2H+ => H2S
During
the year, there will be consiciderable seasonal variationsno in the systems performance. To begin with, the temperature to the water
will have a direct impact on the removal rates. While microbial reactions occur year-round, for each 10°C rise in temperature, there is a doubling of the
microbial reaction rate. This means
that in peak summer, the plants may well find themselves with insufficient
available nitrogen. On the opposite
side, the entire system will have to be sized to handle all influent nitrogen
microbially in winter at the reducedeuce reaction rates.
Planting
and
not harvesting the bioreactorssub-surface flow
rock filters
exposes them to the possibilities of creating unwanted animal habitat. This will be somewhat controlled by the harvesting cyccles, but will have to be
addressed should large numbers of a single speicies decide to relocate to the
new wetlandR/PFs. As plants will transpire a conciderableconsiderable amount of water,
there will the in unavoidable effect of ampliftying the impact of any drought, such as we are now expeiencinexperiencingg here in New England.
Anaerobic
degraedation pathways also generate small amounts of
hydrogen sulfide (H2S) that will be released to the atmosphere. Here the planting of the wetlandR/PFs becomes a major
advantage. While there will still be
generation of H2S, ( the rotten egg odor,) the plants will slow
down its
release to the atmosphere. at large.
The vast majority of this
will be chemically oxidized in the air to hydrogen sulfate, which is relatively odoerless. As concentrations
will be low, natural dilution with moving air will mitigate this issue. Once disbpursed to the air above
the vegetation, there will be little or no detectable noticble odoer. at all.
As
the R/PFsmarshes mature, there is the possiibility of the leading edges clogging with excess
biomass. While care will be taken to size the systems
to allow for this, unforseenunforeseen incidents can cause a
bloom
which may clog the marshR/PF. For this reason, the initial 6 of each wetlandR/PF will be constructed
of a larger stone
(3 - 6) and will be removeable.
Each
R/PFmarsh, while relativrelatively ly self- controlled, will still require some maintainecemaintenance.. This is dividedsplit into threewo segmentsemtns: Routine, vegetative and sStructural .and Vegetation.
Routine: The entry of the P/RFs will be protected from
unwanted trash by bar screens that will need periodic trash removal. Sediments carried down from the watersheds
will be collected and periodically removed from collection
basins.
Structural: The subsurface flow rock filtermarsh media will require minimal attention. In the event they are constructed with
depletable constituents, ( e.g. coal, limestone, ) these will need to be replaced
every 10 to 20 years. Should there be
apparent clogging at the inlets, this material may also have to be removed and
replaced at approximately the same intervals.
Vegetative: All the planted area will require harvesting
at least twice a year and always before the planted species go to seed. Final disposal of these harvests will be
determined after piloting has selected the exact species to be useds: Options
include drying and use as biofuel, use as mulch for nurseries, or composting to
create new soil. Annually
cores willl
be extracted to monitor root penetration and spread. This cost should be offset by the values of the harvested vegetation.
· Plants checking for
root penetration (esp. if bamboo), preventing spreading of invasive plants by
chopping plants before seed, etc.
· Recharging substrate
· Additives
· Inlet/outlet
· Unclogging
· Liner
·
· Alternative to salt marsh hay for
gardens
· Animal ffodder and bedding,
biomass potential
·
· Fiber (packing
material, paper production)
· Construction
material (strawbale housing, insulation, thatched roofing)
· Fuel (palletized or shredded)
· Interior
decorating (dried plants)
· Substrate for
co-composting (manures, sludge, fish offal)
A detailed analysis of markets should be
undertaken in the following phases of work to determine to what extent
the harvesting costs could be offset by either sale of the material or allowing
product end-users the right to harvest at optimum times in the growing season.
In 1996, a similar R/PF (to upgrade lagoon
wastewater treatment system) was constructed in Walnut Cove, North Carolina for
a cost of $60,000 per acre (Wolverton). This could have been reduced to $30,000 per
acre had there not been a requirement for an impervious Bentonite
Geocomposite liner to prevent possible groundwater contamination. The proposed R/PF for Falmouth may or may not need a
liner.
If we use this high cost from Walnut Grove, adjust for the much
lower strength of the tributary waters, inflation, higher Massachusetts construction cost and combine it with the
graph from Paragraph 4.1, Option 1 the following construction costs (excluding land
acquisition, engineering and permitting) are projected:
|
Location |
R/PF 1 planted acreage (Cost) |
R/PF 2 unplanted acreage (Cost) |
|
Great Pond |
2.5 ($150,000) |
1.5 ($90,000) |
|
Green Pond |
1.25 ($75,000) |
1.25 ($75,000) |
|
Bourne Pond |
1 ($60,000) |
1.2 ($72,000) |
Additional costs will be incurred for the following tasks:
· Preliminary planning
· Engineering
· Pilot construction and
evaluation
· Final planning
· Final engineering
· Land acquisition
· Permitting
When Time Required
Periodicity Cost per hour Total annual
Weekly 2 hours - bar screen 52 $25 $2,600
Quarterly 8 hours - harvesting 4 800
Annual 16 hours - general 1 400
Total $3,080
It is not within the scope of this paper to quantify the benefits of reducing nitrogen in the ponds. What can be said is that the R/PF is the lowest-cost denitrification method for large volumes of tributary waters. Other denitrifying techniques, such as break-point chlorination, do not lend themselves to large flows and are politically infeasible in our chlorine-averse society.
Permitting any
construction in or adjacent to a river or a wetland will require a
significant level of effort. In
addition to preparing detailed hydrological and system engineering plans and
specifications, the engineer must address all Wetlands Act 310 CMR 10 concerns, such as
wildlife habitat and river protection, in a narrative that must be included in a Notice of Intent filing with the
Falmouth Conservation Commission. The
engineer would submit completed design plans and the Notice of Intent to the
Conservation Commission, MADEP, and the Natural Heritage Endangered Species Program.
Because the
project is located in an estimated habitat indicated on the Estimated Habitat
Map of State-Listed Rare Wetland Wildlife, the Natural Heritage and Endangered
Species Program must review the project. The
Natural Heritage and Endangered Species Program shall determine whether the
project is in fact part of a rare species habitat.
The construction of a well-landscaped R/PF in or adjacent to the
existing tributaries will alter the natural views that are
now seen from the bridges on Route 28. However,
the present un-managed areas are not aesthetic amenities that might attract one to the area. The landscaped, multi-functional R/PF may improve the
images rather than detract from them.
· use
waterforms to get sound of flowing water, planting fragrant flowers around edge
of wetland
· summary =
landscape for aesthetics
· Advantage =
no mosquito problems, no odors for subsurface
· Disadvantage
= odor for subsurface possibly remedy(?) = planting with thick mat of grass,
covering with compost
The passage of fish will be accommodated by installing species-appropriate channels in the R/PFs that can be opened and closed to accommodate migrating fish during the times of the year that said migration occurs. They could be engineered to be open at all times, but this would allow that volume of water flowing in the fish passages to by-pass the R/PF and thus reduce the denitrifying effect. A detailed engineering and economic assessment would determine if that would have a significant impact on the denitrifying benefit of the overall effort.
Whereas this design is a subsurface horizontal flow
planted
and/or unplanted rock filter; the surface is always dry and has a
significant weight-bearing capacity. Therefore, the area that this
design would occupy should be used for other community purposes. The facility would be amenable to a
multi-functional area that could accommodate some or all of the following paved or unpaved activities:
·
Public access to the ponds from Route 28 for
fishing or walking
·
Nature walks with benches and bird viewing stands
·
Boat storage and ramps
· Aquaculture
facilities
· Canoe or kayak
rentals
·
Basketball, Bocce or shuffleboard courts
Some of these activities could be revenue sources if the
town offered concessions for controlled commercial activities.
Beale, C.V., and S.P. Long.
1997. Seasonal Dynamics of Nutrient Accumulation and Partitioning in the
Perennial C4-Grasses Miscanthus x giganteus and Spartina cynosuroides. Biomass and Bioenergy: 12: 6: 419-428.
Hosoi, Y., Y.
Kido, M. Miki, and M. Sumida. 1998.
Field Examination on Reed Growth, Harvest, and Regeneration for Nutrient
Removal. Water Science and Technology,
38:1: 351-359.
Kleinhenz, Volker and David J. Midmore. 2001. Aspects of Bamboo Agronomy. Advances in Agronomy, 74: 99 153.
Larcher, Walter, 1995. Physiological Plant Ecology, 3rd edition. Springer-Verlag, New York, p. 168.
MA Massachusetts
Ooffice of Ccoastal
Zzone
Mmanagement. The
Coastal Wetlands Ecosystem Protection Project Water Chemistry. http://www.state.ma.us/czm/wacHEM.HTM
Alken-Murray
Corp., Alken Enz-Odor 6 Product Bulletin, Sulfur
Oxidizing Denitrifier, http://www.alken-murray.com/EZ6p:
., 2001
Bezbaruah, Zhang,
Stansbury Lab-Scale Subsurface Flow
Constructed Wetlands for Nitrogen Removal from Municipal Wastewater, Water
Environment Federation, WEFTEC 2001 Conference Proceedings
Caves and Karst, http://www.geosc.psu.edu/People/Faculty/FacultyPages/Kubicki/cavesandkarst.html
Cornell, Something to Grow On, http://www.cals.cornell.edu/dept/flori/growon/macronut.html
Crites,
Tchobanoglous, Small and
Decentralized Wastewater Management Systems, WCB/McGraw-Hill, 1998
Data
gathered from 4.5 years of operation of a Solar Aquatics R/PF marsh system in Weston,
Massachusetts under a MADEP Groundwater Discharge Permit by Bruce Strong, plant
operator
Egan, T.J. Burroughs and T. Attaway. 1995 Packed Bed Filter Proceedings of 4th Biennial Symposium on Stormwater Quality. Southwest Florida Water Management District. Brookville, FL pp.
264-274 Reported in Article 97, Technical Note # 76 from Watershed Protection
Techniques 2(2): 372-374
Flere, J.M., T.C.
Zhang, Remediation of
Nitrate-Contaminated Surface Water Using Sulfur and Limestone Autotrophic
Denitrification Processes, Proceedings for the 12th Annual
Conference on Hazardous Waste Research, May 1997, Great Plains/Rocky Mountain
Hazardous Substance Research Center
Lampe, D.G., T.C.
Zhang, Evaluation of Sulfur-Based
Autotrophic Denitrification, Proceedings of the HSRC/WERC Joint Conference
on the Environmental, May 1996, Great Plains/Rocky Mountain Hazardous Substance
Research Center
Nahar, S., P.
Fox, R. Wass, Sulfur Driven Autotrophic
Denitrification in Constructed Wetlands, Water Environment Foundation,
WEFTEC 2000 Conference Proceedings
North Pacific
Trading, Specialty Products Division Garden Science, Gypsum, http://www.gypsumsales.com/gardenscience,
North Pacific Group, Inc., © 2000, 2001
Pennsylvania DEP,
The Science of Acid Mine Drainage and
Passive Treatment, Department of Environmental Protection, Bureau of
Abandoned Mine Reclamation, 2001
Sulfur
denitrification/Sulfur denitrifier info webpage,
http://www.env.t.u-tokyo.ac.jp/~kiyo/sulfur-E.html
USEPA, Coal Mine
Drainage (CMD) Treatment Methods, from A
Citizens Handbook to Address Contaminated Coal Mine Drainage, USEPA
Publication, September 1997
Virginia Tech,
Soil Microbiology BIOL/CSES 4684 course info pages:
Thiobacillus, http://www.bsi.vt.edu/biol_4684/Microbes/Thiobacillus.html
Sulfur Oxidation,
http://www.bsi.vt.edu/chagedor/biol_4684/Cycles/Soxiddat.html
Sulfur (Sulfate)
Reduction, http://www.bsi.vt.edu/chagedor/biol_4684/Cycles/Sreduct.html
Wolverton,
BC, Growing Clean Water, WES, Inc.,
Picayune, MS, 2001