university of wisconsin system solid waste research … · solid waste research program . 2...
TRANSCRIPT
Increased Recovery of Solid Waste by Improving Scalability for Anaerobic Co-Digestion of Organic Solid Wastes
June 2014
P.I. Benjamin T.W. Bocher, Ph.D. Co-P.I. Philip J. Parker, Ph.D., P.E.
Contributing Authors: Dylan Friss Trevor Rundhaug Marc Stern
Civil & Environmental Engineering
University of Wisconsin-Platteville
UNIVERSITY OF WISCONSIN SYSTEM SOLID WASTE RESEARCH PROGRAM
2
Abstract
Anaerobic digestion of currently landfilled solid wastes was demonstrated as a
technologically viable and environmental advantageous alternative to landfilling three
currently landfilled solid wastes (cafeteria waste, organic fraction of municipal solid waste, and
screenings from a cheese factory).
Anaerobic co-digestion of currently landfilled organic solid wastes removed between 75
and 80% of total solid material. Use of existing municipal digesters could make anaerobic co-
digestion a low-capitol alternative to landfilling. Utilization of excess capacity, as is common,
would directly translate into diversion from landfills, meaning anaerobic digestion has the
potential to divert 80% to 100% of readily biodegradable substrates if biosolids are
landfilled or disposed of via alternative methods, respectively.
Addition of certain substrates may lead to beneficial synergistic effects—degrading
more solids, for example, when two substrates are added compared to the sum of anaerobically
degrading each waste separately. Pilot-scale testing is recommended for wastewater treatment
facilities located near consistent feedstocks of readily biodegradable organic solid wastes that are
currently landfilled.
Anaerobic digestion of organic solid wastes throughout Wisconsin represents low
hanging fruit in the ongoing discussion of how to divert more solid waste from landfills.
Legislative action may be the needed impetus for implementation of this viable treatment
technology.
4
1.0 INTRODUCTION
In 2011, the United States generated about 250 million tons of municipal solid waste
(MSW) and recycled 87 million tons of this material, a 34.7 percent recycling rate. (Municipal
Solid Waste, 2012). For the more than 90 million tons (56 percent) of landfilled MSW that is
organic, a viable alternative to landfilling might be anaerobic digestion. Use of anaerobic
digestion has the potential to decrease total solids in landfills as well as mitigate negative impacts
on the environment.
Supplying the required volume of landfill space is becoming increasingly challenging due
to the difficulties of siting a landfill; this ever-increasing land required by landfills stresses the
development of homes, businesses, and crops (Mercury, 1998). Thus, all diversion is valuable.
Landfilling poses potentially serious problems for human and environmental health.
Although landfill liners are designed to remain impermeable “forever,” eventually, all liners will
fail; when this happens, there are various potential pathways of environmental contamination
(e.g., groundwater) (Schiopu and Gavrilescu, 2010). If landfill gas is not captured, methane
(CH4), a greenhouse gas that is more than twenty times as harmful as CO2, is released untreated
into the atmosphere. While technologies to capture landfill biogas and treat the leachate have
improved, the limitations of mass transfer in landfills cap the extent to which organics can be
converted into these resources. Thus, there is a need for solutions that not only reduce the
negative impacts of landfills on the environment and the health of the general public, but also
better utilize the fraction of solid waste that can be transformed into a valuable resource.
1.1 Anaerobic Digestion Pathway
Anaerobic digestion is one means to convert a greater fraction of this currently landfilled
organic solid waste into valuable products like CH4. Anaerobic digestion is an interdependent
process consisting of four general biological reactions, each carried out by a different guild of
microorganisms: hydrolysis, acidogenesis, acetogenesis, and methanogensis (Figure 1) (Batstone
et al., 2002). Hydrolysis is the enzymatic breakdown of complex polymers into monomers like
glucose and amino acids. As such, this step is crucial to successful digestion of solid wastes. For
example, MSW is broken down from large organic polymers such as proteins, fats and
carbohydrates into amino acids, fatty acids, and simple sugars. After hydrolysis breaks down the
large polymers, acidogenic bacteria further convert the organic matter into volatile fatty acids
(VFA) via a fermentative reaction wherein some carbonic acids, alcohols, ammonia, and reduced
sulfur are also produced (Batstone et al., 2002). The subsequent conversion of VFA into acetate,
5
hydrogen gas (H2), and carbon dioxide (CO2) from the metabolized biomass products of
acidogenesis is termed acetogenesis. A specialized trophic group of Archaea then generates CH4
from either acetate (CH3CH2CHOO-), or H2 and CO2 (Batstone et al., 2002).
Figure 1. Anaerobic Degradation pathway: four general reactions (hydrolysis, acidogenesis, acetogenesis, and methanogenesis) and corresponding microbes (Figure Courtesy of Prince Mathai).
1.2 Statement of Objectives
The aim of this research was to study a novel method of reducing the total mass of solid
waste that is currently disposed of in landfills. This could be accomplished by diverting readily
biodegradable organics into anaerobic digesters where this waste could have additional positive
effects on the environment through the generation of renewable energy in the form of CH4 gas.
Many anaerobic digesters are operated under capacity—sometimes significantly so (Gray et al.,
2008); thus, solid wastes could only be added to the extent that they only utilize currently unused
digester volume (i.e., no reduction in current treatment of municipal biosolids would occur).
This project was designed to allow for the utilization of existing excess capacity in
municipal anaerobic digesters for co-digestion (degradation of two or more substrates in the same
anaerobic digester at the same time) of municipal wastewater with additional industrial wastes or
6
MSW. Hence, the ultimate goal (though beyond the scope of this initial study) of this research is
greater implementation of anaerobic co-digesters treating municipal wastewater and organic solid
wastes, thereby diverting more solid wastes from landfills and increasing CH4 production at
existing, under-loaded wastewater treatment plants.
To achieve this goal, lab-scale reactors were operated to collect data that could be
provided to operators to help them better predict full-scale performance of anaerobic co-digesters.
Lab testing provided information that aimed at allowing digester operators to more accurately
assess the viability of co-digestion of solid wastes in municipal wastewater digesters as a disposal
alternative and the effect of solid wastes on CH4 generation. Lab tests proved the feasibility of co-
digestion of solid wastes such as cheese screenings, cafeteria waste, and organic fraction of MSW
(OFMSW).
2.0 METHODS AND MATERIALS
2.1 Collection and Characterization of Food Wastes
In this project, the intent of co-digestion was to reduce the mass of landfilled, solid waste
and increase CH4 production in existing municipal digesters. Three currently landfilled solid
wastes—OFMSW, cafeteria waste, and cheese screenings waste—were collected over a two-
month period (Table 1). The OFMSW was comprised of food scraps (e.g., fruit, vegetable, dairy,
and grain waste) from multiple households. This waste was blended to obtain a well-mixed
consistency. Cafeteria waste was obtained from a university cafeteria; the current practice is to
grind this waste in a food processor before disposal in a dumpster. Three samples of this substrate
were collected on three different days. Before analysis, these three samples were mixed together
to provide a homogeneous substrate. Cheese screenings, a finely screened homogenous mixture
of small pieces of cheese removed from process water, were collected on two different occasions.
Samples of OFMSW, cafeteria waste, and cheese screenings waste were stored in a freezer at
–5°C until further use. The “source” samples of the three organic solid wastes were analyzed for
total chemical oxygen demand (TCOD), total solids (TS), and volatile solids (VS) in triplicate
following Standard Methods: 5220 D, 2540 B, 2540 E, respectively (Table 2). Each sample was
diluted to produce a “slurry” that flowed and could then be used to create a liquid “feed,” which
was actually fed to the CMFRs (Table 3).
7
Table 1. Collected Substrates Type of Substrate Weight* (lbs.)
Cafeteria 50 Cheese Screenings 80
OFMSW 25 *Weights are approximate values within 10%.
Table 2. TCOD, TS, VS Data for Solid Waste Sources Used to Make Slurries for CMFRs
Cafeteria a Cheese b OFMSW c
TCOD (g/L) 38 150 120
TS (g/L) 284 537 159
VS (g/L) 273 506 155 Cafeteria, Cheese, and OFMSW were diluted in tap water
a Solid waste from university cafeteria. b Solid waste from cheese screenings. c Solid waste from organic fraction of MSW.
Table 3. TCOD, TS, VS Data for Solid Waste Slurries Used to Make Feed for the CMFRs Cafeteria Cheese OFMSW
TCOD (g/L) 6.7 29 50 TS (g/L) 25 136 96 VS (g/L) 24 123 92
2.2 CMFR Set-Up and Operation
Continuously mixed flow reactors (CMFR) were constructed using acrylic cylinders (4.5
in. I.D., 0.25 in. thick walls, and 12 in. tall) with acrylic disks (5.125 in. diameter, 0.25 in. thick)
used for the tops and bottoms of the reactors (3.1 L total volume) (Figure 2) (Ridout Plastics, San
Diego, CA). Methylene chloride was used to attach the bottom to the cylinder. The top was
connected by cutting a 0.25 in. inset groove and using two-part epoxy to bond the top to the
cylinders. Two 0.5 in. diameter holes were cut in the top where an acrylic gas port and feeding
tube were fixed using methylene chloride. A 3 in. piece of Norprene® tubing was attached to the
feeding tube and clamped off while not in use. Also, a 3 in. piece of Norprene® tubing was
connected to the gas port on one end and, on the other end, to a gas valve that was connected to a
gas bag.
8
Figure 2. Schematic of 2-L CMFRs that were fed solid waste and synthetic municipal wastewater. All measurements have units of inches, including fractions.
Lab-scale experiments were carried out in CMFRs with a 1.5-L working volume and a
15-day hydraulic retention time (HRT). CMFRs were maintained at 35±1 °C and continuously
mixed at 150 rpm in a shaker incubator (New Brunswick, I2500 series, Edison, NJ). Each day,
100 mL of effluent was removed from each CMFR vessel and replaced by 100 mL of “feed”
substrate to supply a feeding rate of 1g TCOD/(LCMFR*d); a nutrient solution (described by
Schauer-Gimenez et al., 2010 except that Na2MoO4.2H2O was used instead of NaMoO4.2H2O)
was added daily with the feed. The control CMFRs (CMFRs #1-3) were fed only a synthetic
municipal wastewater (SMW). As has been done previously, Diamond Adult dog food (990 g/L
TS and 920 g/L VS) was used by blending 10.8 g dog food with tap water for a total volume of
333 mL (32 ± 1 g/L total solids, TS, and 30 ± 0.5 g/L volatile solids, VS) to more accurately
mimic municipal wastewater (Bocher, 2012); 100 mL of this mixture was fed to the three control
CMFRs. The remaining nine CMFRs were co-digesters, as they were fed a combination of SMW
and one of the following organic solid wastes in an 80/20 ratio (based on TCOD): OFMSW
9
(CMFRs #4-6), cafeteria waste (CMFR #7-9), and cheese screenings (CMFRs #10-12). Hereafter,
the combination of solid waste and municipal wastewater is termed substrate. The lab-scale co-
digestion of each of these three solid wastes in triplicate was compared with the triplicate control
digesters.
Since seed cultures with diverse microbial communities have been shown to improve the
efficiency of anaerobic digestion (Wittebolle, 2009), anaerobic seed was collected from three
unique full-scale anaerobic digesters: a municipal wastewater digester (Platteville, WI), an
agricultural digester (Waterloo, WI), and a brewery wastewater digester (La Crosse, WI). All
digesters were seeded with a mixture of biomass from each of these three full-scale digesters as
described below (Table 4).
Table 4. Initial CMFR Seeding
CMFRa Municipal Digester
(mL)
Agricultural Digester
(mL)
Brewery Digester
(mL)
Deionized Water (DI)
(mL)
Volatile Solids (VS) (g/L)
Total Chemical Oxygen Demand (TCOD)
(g/L) 1-3 500 500 150 350 29.4 33.6 4-6 500 500 300 200 37.4 43.9 7-9 500 500 300 200 39.4 43.9 10-12 500 500 500 0 47.6 57.5
aSMW (#1-3), OFMSW (#4-6), Cafeteria waste (#7-9), Cheese screenings (#10-12)
The following analytical methods were used:
• The pH was measured using an electrode probe (Accumet, Fisher Scientific, Singapore,
Indonesia).
• Biogas production was monitored daily using the water displacement method.
• Once at steady-state operation, the CMFRs were tested for TS concentration, VS
concentration, and CH4 generation using Standard Methods 2540 B, 2540 E, and 2720 C,
respectively (APHA, 1998).
• The TCOD was measured using Standard Method 5220 D. Pre-made COD vials (Hach,
2125915, Loveland, CO) were also utilized.
• The SCOD test was measured with the same procedure as the TCOD test, except that
prior to pipetting into the digestion vials, the effluent was filtered through 0.45 µm filter
paper (Gast® 0322-V4B-G18DX, Benton, Michigan).
• While at steady-state, samples of biogas from each CMFR were collected and analyzed
using Gas Chromatography (GC) (Hewlett Packard 5890 series II, DE, Wilmington, DE)
10
to measure the percent CH4 being produced in the CMFRs. Helium was used for the
carrier gas at a rate of 4.5 mL/min. The injector temp was 100°C, oven temp was 40°C,
and the detector temp was 250°C. To make a standard curve for the GC samples,
standard vials of 5, 28, 60, 84, 99.9 % CH4 were prepared and analyzed for CH4 content
on the GC.
2.3 Anaerobic Digestion Potential (ADP) Assays
The anaerobic digestion potential (ADP) assays are a novel lab-scale assay designed to
more accurately provide operational data of full-scale anaerobic co-digestion of MSW and
organic solid waste than the commonly used biochemical methane potential (BMP) test. Current
BMP assays are limited because they represent the maximum volume of CH4 that could be
produced per unit mass rather than the actual volume of CH4that will likely be produced by the
digester. Although still a batch process, the ADP assay improves upon the BMP assays by
allowing digester operators to measure other parameters affecting the digester performance, such
as the TS and TCOD removals, along with the specific rate of CH4 production.
2.3.1 Anaerobic Digestion Potential (ADP) Setup
The ADP assays were performed in triplicate for each of the four digesters using 160 mL
serum bottles with the characteristics outlined in Table 5. Initially, three different TCOD loading
rates (F/M of 1.4, 2.0, and 2.8) for each substrate were tested in order to find an optimal and non-
rate limiting loading rate. The total volume consisted of (1) either MSW, OFMSW, cafeteria
waste, or cheese screenings as the substrate (measured as TCOD), (2) biomass from the
aforementioned seed (measured as VS), and (3) nutrient solution as described by Schauer-
Gimenez et al., 2010 except that Na2MoO4.2H2O was used instead of NaMoO4.2H2O. Because
the TCOD, VS, and total volume in each of the ADP assays were the same in all vessels, the
volume of biogas produced could be compared among the different substrates at each loading
rate. In addition to the different substrates, triplicate blanks with only the biomass (measured as
VS) from each CMFR and nutrient solution were set up in order to measure the volume of CH4
produced from endogenous decay. Before sealing with rubber butyl stoppers (Geo-Microbial
Technologies, Ochelata, OK), each ADP was sparged with a 30%/70% (v/v, CO2/N2) gas for two
minutes.
11
Table 5. ADP Setup Characteristics
F/M TCODfed (Substrate)
VSseed (Biomass)
Total Volume
[g/L] [g/L] [mL] 1.4 7 5 50 2.0 10 5 50 2.8 14 5 50
2.3.2 Anaerobic Digestion Potential (ADP) Operation
The ADP assays were continuously mixed at 150 rpm using a shaker table (Cole Parmer
Orbital Shaker, Model OS 200,Vernon Hills, IL) held at 35±1 °C in a temperature controlled
room (Environette Controlled Environmental Room, Lab-Line Instruments Inc., Melrose Place,
IL). The biogas production was measured with a glass syringe with wetted barrel (Cadence Inc.
Perfektum®, Matched Numbered Glass Syringe, and Cranston, RI). Biogas volume in the ADP
assays was measured approximately every five hours; the duration between measurements
gradually increased as the biogas production slowed. Once the biogas production reached quasi-
steady state, the ADP tests were no longer monitored. High loading rates of TCOD produced over
150 ml of biogas in each ADP assay (exceeding the total headspace volume). Therefore, biogas
was released to prevent cracking of the bottles and to safely measure biogas production. Due to
this release of biogas, the ADP rate (mL CH4/(g VS-d)) was estimated indirectly, using TCOD
removal (Equation 1). The blanks produced less than 50 mL of biogas. The CH4 content, pH, as
well as TS, VS, TCOD, and SCOD concentrations were then measured in the ADP assays using
the same methods as described for the CMFRs. The conductivity of the effluent was measured
using an electrode probe (Orion, Boston, MA).
Equation 1. CH4 Content Based on COD Removal 𝑔 𝑇𝐶𝑂𝐷 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 ∗ !"# !" !"!
! !"#$ !"#$%"&
𝐹𝑖𝑛𝑎𝑙 𝑉𝑜𝑙𝑚𝑒 𝐵𝑖𝑜𝑔𝑎𝑠 (𝑚𝐿)∗ 100% = 𝑀𝑒𝑡ℎ𝑎𝑛𝑒 𝐶𝑜𝑛𝑡𝑒𝑛𝑡 (%)
3.0 RESULTS AND DISCUSSION
3.1 CMFR Operational Analysis
Of the initial twelve CMFRs, six (#1, 2, 3, 4, 9, and 11) were at steady state by day 38.
CMFRs 5, 6, 7, 8, 10, and 12 were excluded from further analysis because defective seals allowed
12
oxygen inside the vessels. A summary of the reactor numbers and their ability to generate useful
results is provided in Table 6.
Table 6: Substrate Type and Inclusion in Analysis
CMFRs Substrate Type Reactors Included in Analysis
Reactors not Included in Analysis
1-3 SMW a 1,2,3 4-6 OFMSW 4 5,6 7-9 Cafeteria Substrate 9 7,8
10-12 Cheese Screenings 11 10,12 a SMW = synthetic municipal wastewater
Oxygen is toxic to methanogens; thus, the presence of oxygen was evident in a 268 %
increase, 435 % increase, and 601 % increase in SCOD concentration for OFMSW CMFRs (i.e.,
a lack of COD removal from inhibition of methanogenesis caused by O2), cafeteria CMFRs, and
cheese screenings CMFRs, respectively (Figure 3). CMFRs 8 (cafeteria waste) and 12 (cheese
screenings) also exhibited toxicity in a sharp decrease in pH (Figure 4). It is likely that these
changes in SCOD concentration and pH were accompanied by VFA accumulation, which was
caused by the inhibition of methanogensis resulting from oxygen toxicity.
Figure 3. Soluble chemical oxygen demand of CMFRs 5, 6, 8, and 12 as a function of time.
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
28 30 32 34 36 38 40 42 Soluble Ch
emical Oxygen De
man
d, SCO
D (m
g/L)
Time (Days)
Reactor 5
Reactor 6
Reactor 8
Reactor 12
13
Figure 4. pH of CMFRs 8 and 12 as function of time.
3.2 Steady State CMFR Characteristics
The steady state metadata of the CMFRs are reported in
Table 7. The CH4 content was measured for three consecutive days (39 to 42) at steady
state. CH4 production statistically increased in OFMSW and Cafeteria waste as compared to
the control.
Table 7). For CMFRs 1-3 (Synthetic Wastewater), 4 (OFMSW), 9 (Cafeteria), and 11
(Cheese Screenings), respectively, the initial TCOD concentrations (33.6, 43.9, 43.9, and 57.5
g/L TCOD, respectively) (Table 4), decreased to steady state concentrations of 9.6, 9.4, 8.6, and
9.0 g/L TCOD during days 40 to 42.
Table 7; Figure 5). From this data, the TCOD removal was calculated for each substrate.
Table 7). A statistical increase in TCOD removal over the control was calculated in the
cafeteria waste and cheese screenings waste.
Table 7). Steady state SCOD concentrations in the effluent were statistically similar for
all substrates .
6.4 6.5 6.6 6.7 6.8 6.9 7
7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8
0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45
pH
Time (Days)
Reactor 8
Reactor 12
14
Table 7). The VS concentrations in CMFRs 1-3 (SMW), CMFR 4 (OFMSW), CMFR 9
(cafeteria), and CMFR 12 (cheese screenings) all decreased until steady state conditions.
Table 7). The trend to steady state depicted the data asymptotically approaching a steady
state condition near 9.0 g/L (Figure 6). In addition, Figure 6 exhibits how the variability in the
initial VS concentration (Table 4) for each replicate CMFR had little effect on the steady state VS
concentration.
Table 7). The TS results exhibited a similar trend as the VS results, but with higher
steady state concentrations.
Table 7; Figure 7).
Table 7. Steady State Characteristics of CMFRs (a) CH4 Production (b) Solids Concentrations (c) COD results. All values are presented as average ± standard error. (a)
Substrate CH4 Content (%) CH4 Production (mL CH4/g VS‐d) pH SMW 51 ± 1 159 ± 6 7.28 ± 0.02
OFMSW 50 ± 5 198 ± 24 7.32 ± 0.02 Cafeteria 53 ± 2 181 ± 8 7.27 ± 0.04
Cheese Screenings 48 ± 1 163 ± 9 7.37 ± 0.07
(b)
Substrate TS (g/L) TS Removal (%) VS (g/L) VS Removal (%)
SMW 13.4 ± 0.3 79.8 ± 0.5 8.9 ± 0.2 86.6 ± 0.3 OFMSW 13.4 ± 0.8 77 ± 1 8.7 ± 0.9 85 ± 2 Cafeteria 13.7 ± 0.1 76.1 ± 0.2 9.4 ± 0.5 83.6 ± 0.9
Cheese Screenings 14.0 ± 0.5 76.0 ± 0.8 9.4 ± 0.2 84.0 ± 0.4
(c) Substrate TCOD (g/L) TCOD Removal (%) SCOD (mg/L)
SMW 9.6 ± 0.2 37 ± 1 490 ± 10 OFMSW 9.4 ± 0.6 37 ± 4 450 ± 30 Cafeteria 8.6 ± 0.2 43 ± 1 480 ± 20
Cheese Screenings 9.0 ± 0.2 40 ± 1 470 ± 30
15
Figure 5. Total chemical oxygen demand (TCOD) as a function of time.
Figure 6. Volatile solids (VS) as a function of time. Note: Because the loading rate was based on TCOD, there was as slight difference in the VS loading rates among the four substrate types. OFMSW is seen below the cafeteria substrate.
8
10
12
14
16
18
20
21 26 31 36 41
Total Che
mical Oxygen De
man
d, COD (g/L)
Time (Days)
OFMSW
Cafeteria
Cheese Screenings
SMW
Loading Rate
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40
Volatile Solids, VS (g/L)
Time (Days)
SMW Loading Rate
SMW (g/L)
OFMSW Loading Rate
OFMSW (g/L)
Cafeteria Loading Rate
Cafeteria (g/L)
Cheese Screenings Loading Rate
16
Figure 7. Total solids as a function of time. Note: Because the loading rate was based on TCOD, there was as slight difference in the TS loading rates among the four substrate types. OFMSW is seen below the cafeteria substrate.
3.3 CMFR Discussion
TS removals ranging from 76 to 80% during continuous operation indicate that anaerobic
co-digestion of readily biodegradable substrates with domestic wastewater could divert up to
80% more biodegradable solid waste from landfills if biosolids are landfilled and 100%
more biodegradable solid waste from landfills if biosolids are disposed of via alternative
methods.
Table 7). Given that co-digestion would use excess capacity in existing anaerobic
digesters, there would be more degradation, more CH4 generation, and less landfilled solid
wastes.
While the data show that the greatest TS removal was in the SMW CMFRs, all of the
solid wastes studied resulted in at least 76% TS removal, and, thus, could be co-digested in the
many municipal digesters have unused capacity, thereby diverting these wastes from landfills. For
example, the East Bay Municipal Utility District in San Francisco demonstrated that addition of
0
20
40
60
80
100
0 10 20 30 40
Total Solids, TS (g/L)
Time (Days)
SMW Loading Rate
SMW (g/L)
OFMSW Loading Rate
OFMSW (g/L)
Cafeteria Loading Rate
Cafeteria (g/L)
Cheese Screenings Loading Rate
Cheese Screening (g/L)
17
OFMSW actually increased the methane yield by 300% (Gary et al., 2008). Furthermore, this
increase was present at 15- and 10-day retention times, which are at or below typical minimum
operating retention times. Therefore, these substrates represent a vast, untapped potential for
diversion of significant quantities of currently landfilled solid wastes AND for production of
valuable, renewable energy.
Another example that affirms the findings of these lab-scale CMFRs is that anaerobic
digestion of MSW has been in use at full-scale in Toronto, ON, Canada for nearly a decade
(Barclay, 2012). The research performed at UW Platteville and described in this report points in
the same direction as the early research that allowed implementation in Toronto (Allen Kani,
2001). Therefore, pilot scale tests and an economic feasibility study would be appropriate at
this time if various waste producers, haulers, and municipalities are interested in pursuing this
work.
CMFRs treating cheese screening substrate and cafeteria substrate demonstrated a
statistically greater TCOD removal (40 and 43%, respectively) compared to the control (36%).
Table 7). This may indicate the possibility of synergistic effects from the addition of
the cheese screenings substrate and cafeteria substrate to the SMW. The percent of CH4 in the biogas was similar in CMFRs treating all four substrates, but,
compared to the control, CH4 production (mL CH4/gVS-d) was 20% greater in the CMFRs
treating cafeteria substrates and 40% greater in the CMFR treating OFMSW.
Table 7). The CH4 production results of cheese substrates show similar data as the
synthetic wastewater.
Table 7). The increases in CH4 production may be indicative of a synergistic
relationship between cafeteria or OFMSW and the SMW in the CMFRs (Table 6).
3.4 ADP Results
ADP assays were conducted using F/M ratios of 1.4, 2.0, and 2.8 g TCOD /g VS,
resulting in three different sets of data for each substrate (Table 8). As expected, blank ADP
assays resulted in negligible ADP values, solids removal and TCOD removal (Table 8),
demonstrating minimal activity in the microbial community from endogenous decay. The
18
heterogeneous nature (i.e., some large particulates were present) of the solid waste substrates was
one source of error in the ADP assay set up.
Table 8. ADP Results for (a) SMW, (b) OFMSW, (c) Cafeteria, and (d) Cheese Screenings (Values are Presented as the Average of Three Physical Replicates ± One Standard Error) (a)
Substrate SMW F/M (g TCOD/g VS) 1.4 2 2.8
ADP (mL CH4/gVS d) 26 ± 1 24 ± 3 NA
pH 7.09 ± 0.02 7.1 ± 0.01 5.15 ± 0.03 Conductivity (mS/cm) 121.2 ± 0.3 133.1 ± 1.7 113.7 ± 0.003
TCOD (g/L) 9.4 ± 0.09 11.6 ± 0.5 NA TCOD
Removal (%) 29 ± 1% 14 ± 3% NA
SCOD (mg/L) 500 ± 30 1200 ± 100 20000 ± 0 TS (g/L) 18 ± 0.4 1 ± 1 37 ± 1.3
TS Removal (%) 34 ± 1% 51 ± 3% 15 ± 3% VS (g/L) 9.6 ± 0.4 10.2 ± 0.5 27.9 ± 1.2
VS Removal (%) 54 ± 2% 63 ± 2% 25 ± 3% (b)
Substrate OFMSW F/M g TCOD/g VS 1.4 2.0 2.8 ADP (mL CH4/gVS d) 96 ± 20 97 ± 3 94 ± 6 pH 7.01 ± 0.03 6.96 ± 0.01 7.04 ± 0.01
Conductivity (mS/cm) 103.4 ± 0.2 107.1 ± 0.2 112.3 ± 1.4 TCOD (g/L) 7.6 ± 0.8 7.0 ± 0.3 8.0 ± 0.4 TCOD
Removal (%) 46 ± 6% 59 ± 1% 62 ± 2%
SCOD (mg/L) 900 ± 500 400 ± 100 337 ± 7 TS (g/L) 13 ± 0.8 13 ± 1.0 15 ± 0.3
TS Removal (%) 8 ± 6% 24 ± 6% 23 ± 1% VS (g/L) 7.4 ± 0.2 7.4 ± 0.1 8.2 ± 0.1
VS Removal (%) 31 ± 2% 44 ± 1% 50 ± 0.4% (c)
Substrate Cafeteria F/M g TCOD/g VS 1.4 2.0 2.8 ADP (mL CH4/gVS d) 63 ± 4 116 ± 11 105 ± 6 pH 6.83 ± 0.003 6.83 ± 0.01 6.83 ± 0.01
Conductivity (mS/cm) 92.5 ± 0.9 91.2 ± 0.6 88.4 ± 0.6
19
TCOD (g/L) 8.4 ± 0.4 8.0 ± 0.6 7.8 ± 0.3 TCOD
Removal (%) 41 ± 3% 53 ± 3% 63 ± 1%
SCOD (mg/L) 280 ± 20 270 ± 20 300 ± 20 TS (g/L) 12 ± 0.5 13 ± 0.3 13 ± 1.5
TS Removal (%) 9 ± 4% 13 ± 2% 27 ± 9% VS (g/L) 7.5 ± 0.3 7.7 ± 0.4 8.5 ± 1.1
VS Removal (%) 20 ± 3% 32 ± 3% 38 ± 8% (d)
Substrate Cheese Screenings Blank F/M g TCOD/g VS 1.4 2.0 2.8 0 ADP (mL CH4/gVS d) 87 ± 3 88 ± 4 105 ± 8 7 ± 1 pH 7.17 ± 0.01 7.34 ± 0.05 7.49 ± 0.03 7.22 ± 0.01
Conductivity (mS/cm) 136.2 ± 0.5 152.1 ± 2.3 169.4 ± 1.7 100.4 ± 0.3 TCOD (g/L) 7.5 ± 0.2 7.9 ± 0.4 7.8 ± 0.4 5.9 ± 0.2 TCOD
Removal (%) 50 ± 1% 56 ± 2% 65 ± 2% 18 ± 3% SCOD (mg/L) 260 ± 10 268 ± 4 339 ± 6 646 ± 10
TS (g/L) 16 ± 0.2 14 ± 0.9 14 ± 0.6 12 ± 0.7 TS Removal (%) NA 20 ± 5% 37 ± 3% NA
VS (g/L) 8.5 ± 0.2 8.0 ± 0.4 7.9 ± 0.4 6.5 ± 0.1 VS Removal (%) 21 ± 1% 39 ± 3% 52 ± 2% -30 ± 2%
3.4.1 SMW ADP Results
For SMW, the greatest TS removal (63%) was at an F/M ratio of 2.0, approximately 15%
and 150% more than the TS removal at F/M ratios of 1.4 and 2.8, respectively. The 1.4 F/M ratio
resulted in the highest TCOD removal (29±1%) and ADP as well as the lowest SCOD
concentration of the three different loading rates (Table 8). The F/M ratios of 1.4 and 2.0 yielded
statistically similar ADP values. The lower TCOD removal in the F/M of 2.0 (14±3%) compared
to the F/M ratio of 1.4 (29±1%) (Table 8) may have been evidence of the F/M ratio for the SMW
approaching an inhibitory level for the microbial consortia. In fact, inhibition of CH4 production
was evident when the F/M ratio was 2.8, as seen in the following: pH of 5.15±0.14 (indicative of
VFA accumulation), a negligible ADP value, extremely high SCOD and TCOD concentrations,
and VS removals at least 50% lower than those at lower F/M ratios (Table 8). Further evidence of
inhibited methanogenesis was seen in the dark brown color in the ADP assays at an F/M of 2.8,
compared to the usual black indicative of anaerobic conditions in healthy ADP assays (Figure 8).
20
Figure 8. Color difference between healthy (right) and acidic ADPs.
3.4.2 OFMSW ADP Results
ADPs with F/M ratios of 2.0 and 2.8 treating OFMSW yielded statistically similar TS
removals (24 – 23%) and TCOD removals (approximately 60%), all of which were greater than
those at an F/M ratio of 1.4 (Table 8). However, all three F/M ratios had statistically similar ADP
values (90 - 100 mL CH4/g VS-d).
3.4.3 Cafeteria Waste ADP Results
In ADP assays treating cafeteria waste, TS removals increased with increasing F/M ratios
(Table 8). The ADP values were statistically similar in the 2.0 and 2.8 F/M ratios (116 and 105
mL CH4/g VS-d, respectively), while the 1.4 F/M ratio had a slower rate (63 mL CH4/g VS-d).
The ADP values may suggest that the F/M ratios of 2.0 and 2.8 were non-rate limiting and the 1.4
F/M ratio was substrate-limited. At all three different F/M ratios of the cafeteria substrate, the pH
and conductivity were statistically similar (Table 8), suggesting that the microbial communities
for all loading rates were uninhibited. The pH of the cafeteria substrate was slightly lower than
that of all other substrates treated. This may be a function of feed pH, with a greater composition
of acidic foods (e.g., citrus fruits). TCOD, VS, and TS removals increased as F/M ratio increased
(Table 8). The trend did not fit with the effluent SCOD concentration, which exhibited no
statistical differences (Table 8).
3.4.4 Cheese Screenings ADP Results
Greater F/M ratios in the cheese screenings substrate correlated with an increase in ADP
value, conductivity, TCOD removal, VS removal, and, most importantly, TS removal (Table 8).
21
However, the F/M ratios of 1.4 and 2.0 were more similar throughout with the 2.8 F/M ratio
exhibiting significant statistical increase in efficiency of the aforementioned characteristics. For
instance, the ADP values at the two lower F/M ratios resulted in nearly identical averages (87 and
88 ml CH4/g VS-d), while the 2.8 F/M ratio had a 20% increase in ADP value (105 mL CH4/g
VS-d) (Table 8). The TS removal was 85% higher at an F/M ratio of 2.8 than the TS removal at
an F/M ratio of 2.0.
3.5 ADP Discussion
As with the CMFR testing, ADP assays verified that OFMSW, cafeteria, and cheese
screenings would all make suitable co-digestates. The difference between operational data at an
F/M ratio of 2.8 and at F/M ratios of 1.4 and 2.0 in the cheese screenings substrate suggest there
may have been some limit in the two lower F/M ratios (e.g., filamentous bacteria outcompeting
microbes essential to anaerobic degradation). Therefore, batch operation or similar lab-scale work
ought to operate at an F/M ratio between 2.0 and 2.8. Both cafeteria and cheese screening TS
removals increased with F/M ratio. Overall, the currently landfilled substrates all had greater
TCOD removal and ADP values as well as lower VS concentrations than the standard SMW
ADPs. In fact, the SMW ADPs resulted in the lowest TCOD removal and ADP values of all
wastes tested. However, the VS removal was the highest in the SMW ADPs, at 63±2% at an F/M
ratio of 2.0 (Table 8).
Since the OFMSW ADPs resulted in the highest TCOD removal with 50±0.4% for the F/M
of 2.8 (Table 8), they may be the optimal substrate if greater CH4 production is the ultimate goal.
Because it reached the greatest rate of CH4 production, is readily available at UW Platteville,
and is already ground into small pieces suitable for direct transfer to anaerobic digesters,
cafeteria substrate may be the best co-digestate of the three tested in this research. Since
most full-scale digesters operate at a retention time greater than the 15 days used in this study, the
cheese substrate (a complex substrate with a greater fat content than the other substrates tested in
this research) may yield greater degradation rates for actual full-scale digesters that are
operated at longer retention times.
A recommendation for future work is to develop a quantitative correlation between batch
(ADPs) and continuous (CMFRs) operation. Until such a relationship is established, the optimal
means of implementing co-digestion at full scale remains to first conduct lab-scale and then pilot-
scale tests. Other research has prescribed a more detailed protocol for the scale-up from lab
studies to pilot testing to full scale (Sell, 2011). If 50 or 100 substrates were examined at both
22
continuous and batch operation, a quantitative and predictive model may result that would yield
predictive results simply from lab-scale testing (Bocher, in press). As more and more
municipalities move to triple stream (recyclables, organics, and other solid waste) solid waste
management, this data may become available and would certainly prove valuable to successive
solid waste producers and anaerobic digester owners interested in partnering to divert solid waste
from landfills, thereby increasing CH4 generation.
3.6 Synthesis Discussion
In addition to the previous examples of full-scale facilities successfully treating solid
organic wastes (Gary et al., 2008; Barclay, 2012), a foundational study assessed the overall
environmental performance (i.e., life cycle assessment) of various disposal options for organic
solid wastes (Mar and VanDuinen, 2011). This study found that, compared to landfilling these
wastes, fewer greenhouse gas emissions resulted when wastewater treatment that included
anaerobic co-digestion similar to that conducted in this study was implemented to dispose of the
organic solid wastes. If all CH4 was captured and emissions control technologies were in place at
the landfill, this benefit would be reduced. More broadly speaking, wastewater treatment that
included anaerobic co-digestion of readily degradable organic solid wastes outperformed not only
two different scenarios for landfilling organic solid wastes, but also other disposal options
(including various wastewater treatment plant configurations, a waste to energy incineration
facility, and one composting alternative), as evidenced in lowered greenhouse gas emissions as
well as less total energy consumption. The evaluation of these 12 alternatives ought to provide
greater reassurance to move forward with the recommendations of the research described in this
report.
4.0 CONCLUSIONS
Anaerobic co-digestion of organic solid wastes removed between 75 and 80% of total
solid material. If the organic loading rate (OLR) of these currently landfilled organic wastes
makes up the difference between average operating OLR and the design OLR (i.e., if the co-
digestates utilize only the excess capacity in existing municipal anaerobic digesters), all of this
degradation directly translates into diversion from landfills. Thus, given sufficient excess
capacity, anaerobic digestion has the potential to divert 80% to 100% of readily
biodegradable substrates if biosolids are landfilled or disposed of via alternative methods,
respectively.
23
Anaerobic digestion of currently landfilled solid wastes was demonstrated as a
technologically viable and environmental advantageous alternative to landfilling these, and
other similar, solid wastes. Furthermore, existing excess capacity in many anaerobic digesters
treating domestic wastewater makes this a low-capital option that could be readily implemented
at numerous municipalities throughout Wisconsin (and other states with solid waste generators
and excess anaerobic digester capacity). Specifically, anaerobic co-digestion may be a viable
option for the diversion of OFMSW, cafeteria waste, and cheese screenings at the City of
Platteville’s municipal wastewater treatment facility. Other solid wastes may also be ideal
substrates at this plant.
Addition of some substrates may lead to beneficial synergistic effects—degrading
more solids, for example, when two substrates are added compared to the sum of the anaerobic
degradation of each waste separately. Synergism occurred in this study in the total chemical
oxygen demand (TCOD) removal. CMFRs treating cheese screening substrate and cafeteria
substrate demonstrated a statistical increase in TCOD removal (40 and 43%, respectively)
compared to the control (36%).
Table 7). Compared to the control, CH4 production (mL CH4/gVS-d) was 20% greater in
the CMFRs treating cafeteria substrates and 40% greater in the CMFR treating OFMSW.
Table 7). The increases in CH4 production may be indicative of a synergistic relationship
between cafeteria or OFMSW and the SMW in the CMFRs.
Table 7).
Anaerobic digestion of organic solid wastes throughout Wisconsin represent low
hanging fruit in the ongoing discussion of how to divert more waste from landfills.
Legislative action may be the needed impetus for implementation of this viable treatment
technology.
4.1 Correlation of Batch and Continuous Operation
The ADP assays (batch process) were not representative of continuous operation. In fact,
the TS removals present in the ADPs for solid waste substrates were, at most, just over one-third
of the TS removal measured in the CMFRs.
24
5.0 RECOMMENDATIONS
Based on the results of this research, pilot scale studies that include economic
feasibility (e.g., tipping and hauling fees as appropriate) are recommended at this time because
anaerobic co-digestion was a feasible disposal method for these currently landfilled wastes. All
substrates demonstrated similar TS removals, and the cafeteria substrate yielded the greatest
maximum ADP value (116 mL CH4/g VS-d) among all substrates (25 mL CH4/g VS-d for SMW,
95 mL CH4/g VS-d for OFMSW, and 105 for cheese screenings). The cafeteria and cheese
screenings were also the two greatest in terms of TCOD removal. Cafeteria waste is produced
about one mile from the Platteville wastewater treatment facility and is currently pulped before
disposal; therefore, cafeteria waste is likely the optimal substrate among those three examined
herein and co-digestion using this waste is recommended for pilot-scale operation. That said,
there might be reasons outside the technical scope of this research (e.g., greater quantity, more
consistent production, greater support from the waste producer) that make one of the other wastes
a more optimal substrate.
A next broader step in this study is to survey municipal digesters around Wisconsin to
determine the available capacity (i.e., the difference between typical OLR and design OLR).
Then, identify and characterize (i.e., determine the quantity and ADPs descriptors like CH4
generation rate, TS removal) the currently landfilled, readily degradable, organic solid wastes
produced and their various proximities from anaerobic digesters with excess capacity. Those
municipal anaerobic digesters with excess capacity that are located near sources of readily
biodegradable organic solid wastes currently being landfilled can then be prioritized as the
most suitable candidates to aid in diversion of solid wastes.
25
6.0 REFERENCES
Allen Kani Associates with Enviros RIS Ltd. (2001) WDO Study: Implications of Different Waste Feed Streams (Source-Separated Organics and Mixed Waste) On Collection Options and Anaerobic Digestion Processing Facility Design, Equipment and Costs. http://nerc.org/documents/toronto_report.pdf.
AECOM, & Beck, R. (2011). Food Waste Digester Phase 1 Feasibility Report (Dane Co., WI). Middleton, WI. <http://dnr.wi.gov/topic/Waste/documents/swip/20120508/DaneCoFinalFoodDigesterReport.pdf>.
APHA: Standard Methods for the Examination of Water and Wastewater (1998). 20th ed., American Public Health Association/American Water Works Association/Water Environment Federation, Washington D.C., USA. Barclay, Jody. 2012-01-23. City of Toronto Anaerobic Digester and Municipal. Solid Waste Dryer and
Homogenizer. Canadian Biomass Innovation Network. <http://cbin.gc.ca/projects/22>.
Batstone, D.J., Keller, J., Angelidaki, R.I., Kalyuzhnyi, S.V., Pavlostathis, S.G., Rozzi, A., Sanders, W.T.M., Siegrist, H. and Vavilin, V.A. (2002). Anaerobic Digestion Model No.1. STR No. 13, IWA Publishing, London, UK.
Bocher, B. T. W. (2012a). Relating Methanogen Community Structure and Function in Anaerobic Digesters. Marquette University.
Bocher, 2012: Bocher, Benjamin. (2012) Relating Methanogen Community Structure and Function in Anaerobic Digesters. Marquette University, Ph.D. Thesis.
Bocher, B. and Zitomer, D. (2012). Staged Anaerobic Digestion Alters Microbial Community Structure and Increases Methanogenic Activity WEFTEC, 2012: New Orleans, LA.
Bocher, B. and Zitomer, D. Staged Anaerobic Digestion as a Means to Increase Specific Methanogenic Activity in International Water Association: 12th World Congress on Anaerobic Digestion. 2010: Guadalajara, Mexico.
Bocher, B. Maki, J., Johnson, M, and Zitomer, D. (in press). Relating Methanogen Community Structure and Anaerobic Digester Function.
Brown, D., Shi, J., & Li, Y. (2012). Comparison of solid-state to liquid anaerobic digestion of lignocellulosic feedstocks for biogas production. Bioresource Technology, 124, 379–86. doi:10.1016/j.biortech.2012.08.051
Fernandez, J., Pérez, M., and Romero, L.I., (2008) Effect of substrate concentration on dry mesophilic anaerobic digestion of organic fraction of municipal solid waste (OFMSW). Bioresource Technology, 99, 6075-80. doi:10.1016/j.biortech.2012.08.051
Gray, D., Suto, P., and Peck, C. (2008) Anaerobic Digestion of Food Waste. U.S. ENVIRONMENTAL PROTECTION AGENCY REGION 9. East Bay Municipal Utility District. <http://www.epa.gov/region9/organics/ad/EBMUDFinalReport.pdf>.
Komilis, D. P., Ham, R. K., & Park, J. K. (2004). Emission of volatile organic compounds during composting of municipal solid wastes. Water research, 38(7), 1707–14. doi:10.1016/j.watres.2003.12.039
Li, Y., Park, S. Y., & Zhu, J. (2011). Solid-state anaerobic digestion for methane production from organic waste. Renewable and Sustainable Energy Reviews, 15(1), 821–826. doi:10.1016/j.rser.2010.07.042
Mar, L. E., & VanDuinen, M. (2011). Final Report Life Cycle Assessment of Systems for the Management and Disposal of Food Waste. Prepared for InSinkerator,<http://www.insinkerator.com/en-us/Documents/Disposer/LCA-Executive-Summary.pdf >.
Mata-Alvarez, J., Macé, S., & Llabrés, P. (2000). Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Bioresource Technology, 74, 3–16. Mercury. (1998). Toxicological Profiles. <http://www.atsdr.cdc.gov/toxprofiles/tp46-c5.pdf> (April 25, 2014). Mercury Compounds. (2013, October 18). EPA. <http://www.epa.gov/ttnatw01/hlthef/mercury.html> (March 13, 2014). Municipal Solid Waste. (2014, February 28). EPA. <http://www.epa.gov/osw/nonhaz/municipal > (March 12, 2014).
26
Navaratnam, N. (2012). Anaerobic co-digestion for enhanced renewable energy and green house gas emission reduction, Ph.D. Thesis, Civil, Construction, and Environmental Engineering. Marquette University, Milwaukee, USARapport, J. (University of California-Davis), Zhang, R., Jenkins, B. M., & Williams, R. B. (2008). Current Anaerobic Digestion Technologies Used for Treatment of Municipal Organic Solid Waste. (pp. 1–90). Sacramento, CA.
Rapport, J. (University of C.-D., Zhang, R., Jenkins, B. M., & Williams, R. B. (2008). Current Anaerobic Digestion Technologies Used for Treatment of Municipal Organic Solid Waste. Environmental Protection (pp. 1–90). Sacramento, CA.
Salminen, E., Rintala, J., Lokshina, L. Y., & Vavilin, V. a. (2000). Anaerobic batch degradation of solid poultry slaughterhouse waste. Water Science and Technology, 41(3), 33–41. <http://www.ncbi.nlm.nih.gov/pubmed/11386301>
Schauer-Gimenez AE, Zitomer DH, Maki JS, Struble CA. (2010). Bioaugmentation for improved recovery of anaerobic digesters after toxicant exposure. Water Res. 44:3555–3564.
Schiopu, Ana-Maria and Gavrilescu, Maria. (2010) Options for the Treatment and Management of Municipal Landfill Leachate: Common and Specific Issues. CLEAN – Soil, Air, Water, 38(12), 1101 – 1110. doi: 10.1002/clen.200900184. http://dx.doi.org/10.1002/clen.200900184
Sell, S. T. (2011). A scale-up procedure for substrate co-digestion in anaerobic digesters through the use of substrate characterization , BMPs , ATAs , and sub pilot-scale digesters by. Iowa state University.
Shahriari, H., Warith, M., Hamoda, M., and Kennedy, K.J. (2012). Anaerobic digestion of organic fraction of municipal solid waste combining two pretreatment modalities, high temperature microwave and hydrogen peroxide. Waste Management, 32(1), 41-52. doi: 10.1016/j.wasman.2011.08.012. <http://www.ncbi.nlm.nih.gov/pubmed/21945550>.
Snyder, R., & Hedli, C. C. (1996). An overview of benzene metabolism. Environmental health perspectives, 104 Suppl 6(December), 1165–71. <http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1469747&tool=pmcentrez&rendertype=abstract>.
Trzcinski, A. P., & Stuckey, D. C. (2012). Denaturing Gradient Gel Electrophoresis Analysis of Archaeal and Bacterial Populations in a Submerged Anaerobic Membrane Bioreactor Treating Landfill Leachate at Low Temperatures. Environmental Engineering Science, 29(4), 219–226. doi:10.1089/ees.2011.0003 U.S. EPA Solid Waste and Emergency Response. (2009). Municipal Solid Waste Generation, Recycling,
and Disposal in the United States: Facts and Figures for 2009 (pp. 1–12). Washington, D.C. <http://www.epa.gov/wastes/nonhaz/municipal/pubs/msw2009-fs.pDF>.
Vinyl Chloride. (2013, October 18). EPA. <http://www.epa.gov/ttnatw01/hlthef/vinylchl.html> (March 13, 2014). WI DNR (2011). Landfill Annual Tonnage Capacity Report. <http://dnr.wi.gov/topic/Landfills/Fees.html>.
Yadvika, S., Sreekrishnan, T. R., Kohli, S., & Rana, V. (2004). Enhancement of biogas production from solid substrates using different techniques –– a review. Bioresource Technology, 95(1), 1–10.
Wittebolle L., Marzorati M., Clement L., Balloi A., Daffonchio D., Heylen K., et al. (2009). Initial community evenness favours functionality under selective stress. Nature 458 623–626.