Waste and Energy Implications
Biosolids
Summary of Recommendations
Anaerobic digestion is recommended as a replacement for
Gainesville Regional Utilities’ (GRU’s) current
wastewater treatment practice of aerobic digestion. The
anaerobic digestion process would generate methane that
could be used to either generate electricity or to
supplement a process that would create a high quality
fertilizer that would be free of the health hazards of
the current Class B biosolids that are spread at
Whistling Pines Ranch. Modifying GRU’s wastewater
treatment process to use anaerobic digestion of sewage
sludge would have the benefits of (a) providing an
alternative source of energy through production of
methane, (b) reducing emissions of various greenhouse
gases, (c) destroying harmful microorganisms in the
sewage sludge, and (d) providing a valuable soil
amendment or fertilizer and thereby reducing the need
for commercial fertilizers that are produced from
natural gas. Anaerobic digestion is recommended as a
replacement for the current practice of aerobic
digestion. If the County decides to stop the practices
of landspreading Class B biosolids, the ECSC would
recommend using the methane from anaerobic digestion to
facilitate the production of Class AA fertilizer from
biosolids.
Incineration (thermal oxidation) of sewage sludge
biosolids, an alternative treatment process, has the
following adverse effects: (a) the energy needed to dry
the sewage sludge, which is about 95% water, would
exceed the energy produced from burning it, (b) carbon
from the sludge would be released into the atmosphere
as carbon dioxide, a greenhouse gas, and (c) the energy
value and agricultural value of the biosolids would be
lost.
Recommendations in this report can help to achieve
various goals of the Alachua County Board of County
Commissioners by promoting the reuse of products
derived from wastewater, providing the foundations for
new businesses that sell or use Class A or Class AA
biosolids fertilizer, enhancing Alachua County’s
efforts to ensure clean soil (by reducing the risk of
adverse impacts from landspreading potentially
hazardous Class B biosolids), and fostering
sustainability through development of industries that
use locally produced fertilizers as replacements for
commercial fertilizers that are made with natural gas,
most of which now comes from foreign countries.
Definitions and Process Description
Biosolids: The term biosolids was coined by the Water
Environment Foundation, formerly known as the
Federation of Sewage Works Associations, to lessen
public objection to spreading sewage sludge on the land
[1]. Now the term is used to differentiate between raw,
untreated sewage sludge and treated sludge (biosolids)
that might be suitable as a soil amendment or
fertilizer.
Anaerobic Digestion: Anaerobic digestion is a process,
conducted in the absence of oxygen, in which bacteria
decompose organic material into methane and carbon
dioxide [2]. In wastewater treatment, the process is
conducted in closed, heated vessels that are free of
oxygen and nitrates, and these conditions force the
bacteria to decompose sulfates . The resulting product,
called the digestate, has less odor and fewer pathogens
[3]. When the process of digestion is conducted at
approximately 122–150 °F (50–60 °C), called the
thermophilic temperature range, pathogens are killed
and the biosolids can attain a Class A or even Class AA
rating [3, 4].
Aerobic Digestion: Another process for treating
wastewater is aerobic digestion, which is conducted at
ambient temperatures and requires air to be pumped into
the water using aerators. Bacteria used in the aerobic
digestion process produce carbon dioxide and the
resulting solid product is typically Class B biosolids
that contain more pathogens than Class A or Class AA
biosolids. The aerobic digestion process is typically
quicker than an anaerobic process.
Methane extracted and captured from the anaerobic
digestion process can be used to generate electricity
or heat. A rough estimate of the methane produced from
wastewater is 18,250 kg per 1,000 persons per year [5].
Another estimate is that the wastewater from one person
can produce enough methane to generate 2.2 watts of
electricity [6]. For the population served by GRU
wastewater treatment facilities, this would create
about 0.5 megawatts (enough to run 5,000 100 W bulbs,
(but I think that you have similar info for 1 MW, do
not need both)*** if somebody gives me a specific
example of what .5 megawatts can power, I can include
it here. It is also in the list of key terms* ***) . If
combined heat and power were used, a total of about 1
megawatt could be produced.
The GRU Biosolids Management Plan, in Exhibit 5-44,
proposed an anaerobic digestion model, but the
temperature used in the analysis was not optimal and
might not have fully represented the potential of an
anaerobic system [4, 7]. Within the thermophilic
temperature range (approximately 122–150 °F), one study
found that methane output is maximized at approximately
127 °F. At this temperature methane output is
approximately three times what it would be at 135.5 °F
and double what it would be at the 95 °F (35 °C)
second-stage temperature recommended by the GRU
Biosolids Management Plan. At the optimal temperature,
methane output is also higher than it would be at the
primary digestion temperature (107 °F, 42 °C)
recommended to GRU in section 5.4.3 [4, 7].
Because it has significant global warming potential,
the large amount of methane produced from wastewater
can be problematic if it is not captured and is instead
released into the atmosphere. Both the carbon dioxide
and the uncaptured methane produced during digestion
can contribute to global warming, but each unit of
methane produces approximately 25 times the global
warming potential over a 100-year period as the same
amount of carbon dioxide [8, 9] (see Table ??). These
gases trap relatively more solar radiation than other
gases do, and the result is a relative warming of the
atmosphere [8]. The Intergovernmental Panel on Climate
Change has determined that these warming effects
threaten to increase average temperatures, change
precipitation patterns, raise sea levels, and increase
the frequency and intensity of extreme weather events
[10].
When used as fertilizer, biosolids must be evaluated to
determine their levels of metals; organic compounds
such as polychlorinated biphenyls (PCBs), dioxin,
pharmaceuticals, and cleaners; and biological pathogens
[11]. Because of Florida’s low level of industry, there
have traditionally been low levels of metals in the
wastewater stream. In the Clean Water Act of 1993, the
U.S. Environmental Protection Agency (EPA) defined the
standards for the allowable levels of these
contaminants [12]. The EPA also developed additional
reporting guidelines [13].
Management of Biosolids in Alachua County
In Alachua County, biosolids are produced from sewage
sludge that is treated by GRU at its Main Street and
Kanapaha Water Reclamation Facilities (commonly called
wastewater treatment facilities). The wastewater is
collected from the City of Gainesville and other
portions of Alachua County that have GRU sewer service.
GRU also provides biosolids treatment and handling for
the University of Florida and the smaller communities
of Hawthorne, High Springs, and Waldo [4]. The city of
Alachua has its own wastewater treatment facility with
final disposal of biosolids on land owned by the city
[14].
Once some of the wastewater has been separated from the
solid sludge, it has a water content of either 5.3% or
16.0%, for thickened and dewatered sludge respectively
[4]. Because of the difference in water content, the
quantity of sludge is given in dry tons per day, which
for the year 2006 was 13.9 tons per day for undigested
sludge or 9.85 tons per day once it has been digested
[15]. The decrease in mass can be attributed to the
conversion of biosolids to carbon dioxide, methane and
other gases by the digestion process. The current GRU
treatment process is aerobic in which air is blown
through the sludge and the digestion process decomposes
some organic material into carbon dioxide and other
gases that are released into the atmosphere.
After the Class B biosolids are processed by GRU at the
wastewater treatment facilities, they are transported
by truck to Whistling Pines Ranch, a site near Archer,
and spread on the farmland for growing hay and other
crops. When applied to the soil, biosolids can add
nutrients, such as nitrogen and phosphorus, that help
plants grow. Adding nitrogen and phosphorus to the soil
in this way can become problematic if the amounts added
exceed the agronomic amount that can be absorbed by the
crops being grown [4].
For the past 25 years, GRU has been landspreading Class
B biosolids on 1175 acres of farmland at Whistling
Pines Ranch [4]. The present contract expires in 2009
[4] after which it may be renewed if GRU and the
current owner agree. Thus, there is little danger of
GRU being left without a place to dispose of the
biosolids if GRU were mandated to convert to a new
wastewater treatment technology.
GRU has proposed to purchase the Whistling Pines Ranch
property so that landspreading can continue
indefinitely [16]. Because the land is in
unincorporated Alachua County, GRU submitted an
application to Alachua County for a Special Exception
to allow this continued use. Frequently, nearby
residents object vigorously to the practice of
landspreading Class B biosolids. County residents and
the Archer City Commission oppose continued
landspreading of biosolids on Whistling Pines Ranch
[17].
The safety of landspreading sewage sludge, which might
be toxic because of heavy metals, radio-nuclides,
pathogens, and untreated organic chemicals, is
questionable [18, 19, 20], but because there is
relatively little industry, Florida has traditionally
had low levels of metals in its wastewater [21].
Another class of contaminants in biosolids are
endocrine disruptors, sometimes called pharma
pollution. One example of the effects of endocrine
disruptors was seen recently in Lake Apopka. These
chemicals threatened the local alligator population by
causing serious reproductive problems [22, 23].
Unfortunately, endocrine disruptors are not the only
chemicals that slip past the EPA’s regulation of
biosolids. In Florida, only 12 chemicals and a few
pathogens are tested in biosolids [24, 25]. The County
might want to explore the potential benefits of
advanced anaerobic digestion and additional processes
that might reduce the risks associated with the
current, partial treatment of biosolids.
Another consideration that affects continued
landspreading of biosolids is the expected change in
the regulatory environment. The Florida Department of
Environmental Protection (FDEP), which is responsible
for administering EPA regulations in the state, is in
the process of revising its rule 62-640 for
landspreading biosolids. A new subsection, 62640.100(1)(b), has been added in the 2007 draft that
states, “The Department [FDEP] intends to encourage the
highest levels of treatment, quality, and use for
biosolids” [26]. Although the GRU Biosolids Management
Plan states that promulgation of new, more strict
regulations would require treatment to Class A or Class
AA standards, at this point the draft-Florida (*** edit
by PW adds “draft Florida” here—is that right? ***)
rule does not prohibit landspreading Class B biosolids.
Producing Class A or Class AA Biosolids
Composting
/Flickr/Wikipedia/WWTFWikipedia.jpgThe solid digestate
produced in biological decomposition of sludge, either
by composting or anaerobic digestion is either Class A
or Class AA biosolids depending on the level of
pollutants remaining after treatment. Composting might
be the simplest, low-cost, low-tech process that will
produce these classes [4]. In composting sewage sludge,
another component of waste such as wood chips or yard
waste is used as a bulking agent to disperse the liquid
sludge and allow air to be circulated through it. With
adequate aeration, the solids in sludge decompose into
carbon dioxide and water vapor, with compost, a solid
humus-like material, remaining. If aeration is
inadequate, some methane is produced that is
undesirable since it is a greenhouse gas that the
composting facility is not likely to capture.
The two most common methods for composting are the
static pile and windrows [27]. For the static pile,
aeration is achieved by blowing air through pipes up
through the pile. Aeration of windrows is provided by
frequent mechanical turning of the pile. For each of
these methods, energy is required to accomplish the
aeration. However, there is a useful environmental
benefit of carbon sequestration, which is the
terminology for keeping carbon out of the atmosphere,
(*** for the main ECSC report, refer to the generic
introduction. For the sludge motion, somebody can
provide a definition ***) when the compost is used in
agriculture or plant growing [28]. The EPA estimates
that the net greenhouse gas sequestration is 0.05
metric tons of carbon equivalents per (standard) ton of
wet organic material composted [28].
Anaerobic Digestion
The preferred option for managing biosolids is
anaerobic digestion carried out in a closed, heated
vessel in the absence of oxygen. Thermophilic anaerobic
digestion is carried out at temperatures up to 130 °F
with retention times of two to three weeks, which
destroys pathogens (but not heavy metals). If
industries discharge to the wastewater, heavy metals
might be present in unacceptable concentrations unless
industrial pre-treatment of the wastewater is used to
remove them. Because of low levels of industry, Florida
in general, and Alachua County specifically, has low
levels of metals in both groundwater and wastewater
[21].
The decomposition of biosolids produces a gas of about
60% methane and 40% carbon dioxide, a solid digestate,
and process liquor [1, 7]. The methane might be used
for power production at 80% efficiency using combined
heat and power [2]. The waste heat can be used to heat
the anaerobic digestion vessels or for drying the
digestate for pelletized fertilizer. A net energy of
about 100 kilowatt hours per ton is realized, as well
as the digestate, which is Class AA biosolids, a
valuable soil amendment [6, 27].
In comparison to GRU’s practice of landspreading Class
B biosolids, Jacksonville Electric Authority (JEA) uses
anaerobic digestion and a supplemental drying process
to convert its wastewater to Class AA fertilizer. After
initial treatment, JEA’s biosolids are processed by
GreenTechnologies, Inc. and made into GreenEdge, an
organic fertilizer marketed in various outlets [29].
Green Technologies, Inc. is located in Alachua County.
Its president and CEO, Dr. Amir Varshovi, is a UF
graduate.
GRU has balked at not responded favorably to the
suggestions that it switch to anaerobic digestion,
citing capitol cost of $40,000,000. However, this cost
may be inflated by as much as 4X [*1, *2DO’K,Zaher].
The actual cost could be no more than GRU expects to
pay for Whistling Pines Ranch. To determine the cost
of constructing an anaerobic digestion facility, GRU
could issue a Request for Proposals for its
construction and then the City of Gainesville could
weigh the costs against the greenhouse gas impacts,
energy generation from methane, and other long term
costs and consequences.
In contrast, JEA replaced its incinerators with an
anaerobic digestion process and cited benefits such as
the absence of incineration ash that would otherwise go
to a landfill, reduction in air emissions, reduction in
natural gas use, and a reduction of withdrawals of 1
million gallons of water withdrawals per day [30].
Anaerobic digestion of waste to produce methane is an
increasingly important way to provide an alternative to
fossil fuel. It is being used in various European
cities to power city buses, on farms to supply power
needed to operate the farm (with energy left over to
sell), and in various locations to produce energy from
food waste [31]. The small community of Live Oak,
Florida looked for ways reduce electricity use in their
aerobic digestion treatment process and switched to
anaerobic digestion to save about $28,000 per year in
electricity costs [32]. Much of the savings was a
reduction in the need to operate the aeration pumps in
the old aerobic digestion process [32].
Generating energy from anaerobic digestion and using
the digestate as a Class AA fertilizer is a good
example of fully closing the recycling loop on a waste
stream. Advanced anaerobic digestion of GRU’s sewage
sludge would have the following positive effects
relative to the current aerobic digestion process.
Energy Analysis of Aerobic Digestion
/Flickr/RMCGreen/aerobicdigestion.png
1. Energy saved by avoiding aeration (approximately
2–3 megawatts) [4].
2. Energy content of methane produced by anaerobic
digestion using combined heat and power (approximately
1 megawatt) [6].
3. Savings in energy for thermophilic digestion
using ground (or wastewater) source heat pump for
heating of up to 50% [31].
4. (*** THIS STAGE MIGHT EXIST IN BOTH SCENARIOS
***) Reduction in carbon dioxide, methane and other
greenhouse gases released during aeration. The methane
reduction is important because it has about 25 times
the global warming potential as carbon dioxide [8] and
approximately 60% of the gas released during digestion
is methane and 40% is carbon dioxide [2, 7].
5. Reduction in ammonia and methane from
landspreading of biosolids [4].
6. Savings and reductions in greenhouse gas
emissions associated with the burning of fossil fuels
need to truck the Class B biosolids to the Archer site
and spread it. Based on an estimate from GRU that the
landspreading operation uses 244 gallons of diesel per
week, halting the landspreading process would save
about 141 tons of carbon per year (see Text Box ??).
7. Savings from the capital cost of the trucks
($436,000) and spreading equipment ($1,082,500) needed
to haul the sludge and the rental of another truck from
Whistling Pines Ranch ($30 per hour) [4].
8. Production of Class AA biosolids would eliminate
health concerns with pathogens and other toxic
substances in Class B biosolids.
9. /Flickr/kqedquest/AnaerobicDigesters.pngClass AA
biosolids would be useful for replenishing soil carbon,
for landscaping, and for use as a soil amendment. Class
AA biosolids could also be used to grow food for humans
and would reduce the need for artificial fertilizer,
which is produced from natural gas.
10. The economics of anaerobic digestion should
reflect the savings from not having to purchase the
Whistling Pines Ranch. The price was reported as $11.5
million by the Gainesville Sun [17] and counted as
approximately $14 million in the GRU Biosolids
Management Plan between tables 5-11 and 5-12 [4]. To
determine the cost of constructing an anaerobic
digestion facility, GRU could issue a Request for
Proposals for its construction and then the City of
Gainesville could weigh the costs against the
greenhouse gas impacts, energy generation from methane,
and other long term costs and consequences. (moved to
above)
Biosolids as Plant Nutrients
Florida’s sandy soils are low in nitrogen and humus
material, which generally necessitates the addition of
fertilizer in one form or another for growing healthy
crops. Environmental horticulture is now the number one
agricultural industry in Florida [33]. Cities,
counties, and highway departments require significant
amounts of soil amendment, compost, or mulch for
landscaping streets and other public areas and for
growing plants for their landscaping requirements
[personal communication, Meg Niederhoffer, Arborist,
City of Gainesville]. Biosolids are a rich source of
some of the needed nutrients that can reduce the
requirement for commercial fertilizers produced largely
from natural gas. The world food chain is now being
threatened by growing shortages of fertilizers produced
from natural gas [*3 Keith Bradshear and Andrew Martin,
NYTimes, April 30, 2008].
Class B biosolids contain higher levels of pathogens
than Class A or AA. Farmers are warned that the value
of their land might be reduced if they apply Class B
biosolids and thereby need to disclose the potential
hazard to when the land is sold [34]. Furthermore,
Class B biosolids may not be used for food crops or in
areas that have public access.
The Class B biosolids that have been applied by GRU to
the same 1175 acres at Whistling Pines Ranch for the
past 25 years. There the concern is that the nutrients
nitrogen and phosphorus contained in the biosolids not
exceed the agronomic uptake of the crops being grown. A
preferable use of these nutrients would be to upgrade
treatment to anaerobic digestion that would produce
Class AA biosolids, which could have much more
widespread usage, as indicated above.
Following anaerobic digestion, biosolids may be dried
to produce a pelletized slow-release fertilizer such as
is done at JEA [personal communication, Dr. Amir
Varshovi, GreenTechnologies, LLC, Gainesville,
Florida.] and other similar facilities. This fertilizer
is marketed in Gainesville under the label GreenEdge
using a process developed by Dr. Amir Varshovi
[personal communication, Dr. Amir Varshovi,
GreenTechnologies, LLC, Gainesville, Florida.]. The
drying process takes additional energy. However, with
the design of an efficient heat exchanger for the dryer
and a more efficient anaerobic digester, no external
source of energy should be required [personal
communication, Bob Leetch, P.E., Manager Wastewater
Treatment and Reuse, JEA]. Use of ground source heat
pumps [*4gshp] and and the heat pipe drying technology
for drying rice invented by Khanh Dinh [*5 Kahn Dinh,
“Dehumidifier Heat Pipes for Rice Drying and Storage,”
presented at 6th International Heat Pipe Symposium—2000,
Chiang Mai, Thiland, Nov. 5, 2000] would further reduce
the energy required to dry the biosolids. Pelletizing
to produce a slow-release fertilizer is an additional
value-added step, but is not necessary in order for the
biosolids to be used in landscaping, as a soil
amendment, or in potting mixes.
Widespread use of anaerobic digestion and drying of
biosolids throughout Florida could displace much of the
biosolids being shipped into the State from the
northeast, which would save significantly in energy and
greenhouse gas emissions for shipping. Fertilizer
derived from biosolids could displace commercial
fertilizer derived from natural gas that is now in
short supply [*3].
Incineration (burning)
Incineration (burning) was presented by GRU consultants
(using the euphemism thermal oxidation instead of
incineration) as an option for disposal of sewage
sludge. The 2007 GRU Biosolids Management Plan reviewed
incineration and other waste disposal methods but
failed to consider the full impacts of greenhouse gases
[4]. The plan for landspreading of biosolids might also
be affected by proposed changes in Chapter 62-640 of
the Florida Administrative Code that require
registration of lands, restrictions on when and where
biosolids can be spread, prohibition of spray guns,
prohibition of some stockpiling, and requirements for
redundancy of equipment [26].
GRU’s Request for Proposals [15] for a planned biomassburning generator includes sewage sludge as a possible
fuel, and City Commissioners have expressed interest in
using this once the biomass burner is online. Burning
sewage sludge would be primarily a disposal mechanism
rather than an energy producing process (as opposed to
anaerobic digestion) because the energy needed to dry
the sewage sludge, which is about 95% water, would
exceed the energy produced from burning it. Burning the
carbon in sludge would also release 10–15 metric tons
of carbon equivalents per day into the atmosphere [4].
Thermal oxidation, although allowed by environmental
regulations is not advisable since (a) no energy is
produced, (b) the carbon is released into the
atmosphere as carbon dioxide, a greenhouse gas, and (c)
the energy value and agricultural value of the
biosolids would be lost.
Additional references;
*1. David O’Keefe, Full Circle Solutions, Inc.,
informal statement in presentation to WEIS, December
11, 2007.
*2. U. Zaher, D-Y. Cheong, B. Wu, and S. Chen, “Producing Energy and
Fertilizer from Organic Municipal Solid Waste,” Washington State
University, Ecology Publication No. 07-07-024, June 26, 2007.
*3. Keith Bradshear and Andrew Martin, NY Times, April
30, 2008.
*4. Ground Source Heat Pump reference.
*5 Kahn Dinh, “Dehumidifier Heat Pipes for Rice Drying
and Storage,” presented at 6th International Heat Pipe
Symposium—2000, Chiang Mai, Thiland, Nov. 5, 2000.