BERGEN COUNTY UTILITIES AUTHORITY DIGESTS CHEESE WHEY – GETS GAS
AN ENERGY-COMMUNITY WIN-WIN
Ralph Eschborn1 (Corresponding Author*), Mohammad Abu-Orf1, Metin Duran2, Keith
Weisman3, Michael Williams4
1
AECOM; 2Villanova University; 3Alaimo Group; 4Bergen County Utilities Authority (BCUA)
*1700 Market Street, Suite 1700, Philadelphia, PA 19103
ABSTRACT
Bergen County Utilities Authority (BCUA) operates a 413 ML/d (109 MGD) wastewater
treatment facility in Little Ferry, New Jersey. Digester gas produced in the anaerobic digesters is
fed to a Combined Heat and Power (CHP) facility. Increasing digester gas production has the
immediate benefit to BCUA of reducing purchased natural gas used to operate the Internal
Combustion (IC) engines.
Two major producers of cheese products are located just 5 km (3 miles) away. BCUA engaged a
study team to perform bench scale testing to experimentally determine the potential for increased
gas production from whey co-digestion as well as potential deleterious effects on digester
operation. Bench testing consisted of Biochemical Methane Potential (BMP) and nutrient
deficiency testing.
Based on the encouraging bench testing findings, BCUA commenced a full-scale demonstration
of CW co-digestion in October, 2010. Primary objectives were to confirm gas production
potential and assess the impact on solids generation, which represents an off-setting cost to the
savings from reduced natural gas purchases. Preliminary results, while qualitative, were quite
favorable, and extended full scale testing has continued through June 2011. No significant
adverse effects have been noted during extended testing. Drawing firm conclusions with respect
to additional gas produced and net solids generated has proved challenging for reasons discussed
below. The most demonstrable benefits are a “win-win” for the community. The cheese
producers are no longer faced with intolerable costs for disposing of the whey. One has
abandoned plans to relocate, and the other is now expanding production.
KEYWORDS
Co-digestion, Anaerobic Digestion, Cheese Whey, Digester Gas, Combined Heat and Power,
Thermal Dispersion Gas Metering, Biochemical Methane Potential
INTRODUCTION
Bergen County Utilities Authority (BCUA) operates a 413 ML/d (109 MGD) wastewater
treatment facility in Little Ferry, New Jersey, in a densely populated area. The facility has a
service area population in excess of one-half million people. Solids processing consists of
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feeding, a blend of thickened primary and secondary solids at rates ranging from 1325 to 1893
m3/d (350,000 to 500,000 gallons per day) to 5 anaerobic digesters, with a combined capacity of
18,395 m3 (4.86 million gallons). Detention times range from 10 to 14 days, with an average of
12. Digester gas is utilized to provide heating and generate electricity. Since 2008, digester gas
produced in the anaerobic digesters is fed to a Combined Heat and Power (CHP) facility
consisting of two, dual fuel (digester and natural gas) Internal Combustion (IC) engines.
Increasing digester gas production has the immediate benefit to BCUA of reducing purchased
natural gas used to operate the IC engines. One means of increasing digester gas production is
the co-digestion of readily biodegradable substrates, such as concentrated waste streams from
food processing operations.
Fortuitously, a major producer of cheese products, Biazzo Dairy, is located just 3 miles (5 km)
away, and is seeking a lower cost disposition of their by-product cheese whey. The locations of
the Little Ferry facility and the Dairy are shown in Figure 1.
Biazzo
Dairy
BCUA Little Ferry
Wastewater Treatment
Facility
Figure 1 Locations of the Little Ferry WWTF and Biazzo Dairy (~ 3 driving miles apart)
While the thought of generating immediate savings through reduced natural gas purchases was
tempting, BCUA was concerned that their digesters were already heavily loaded.
Bench Study Goals and Approach
Recognizing that an assessment of the digestibility of cheese whey prior to a full scale trial
would improve the likelihood of success and minimize risks, BCUA engaged a Study Team
comprised of the Alaimo Group, AECOM and Villanova University to perform bench scale
testing to experimentally determine the potential for increased gas production as well as potential
deleterious effects on digester operation. Secondary objectives included identifying the preferred
form of the cheese whey byproduct stream to co-digest and assessing any other cost impacts –
specifically the potential for increased solids generation.
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The Alaimo Group (Alaimo) is the General Consulting Engineer to the BCUA. Alaimo
engaged the services of AECOM to draw on their specialized expertise in digester process
engineering. AECOM, in turn, contracted with Villanova University and Dr. Metin Duran to
perform the bench testing.
Villanova investigated the feasibility of co-digestion of primary and secondary solids and cheese
whey using Biochemical Methane Potential (BMP) testing.
In addition, a nutrient deficiency study was conducted on anaerobic digester biomass operated by
BCUA to assess whether there were any possible micro- and macronutrient supplementation
needs for the anaerobic biomass in the BCUA digesters for optimal metabolism.
Two cheese whey streams were sampled for study: a hot, dilute, pre-reverse osmosis (RO)
material generated in cheese manufacture; and this same stream after it is cooled to ambient and
concentrated by RO.
These two different locations represent alternate sources of byproduct cheese whey from the
dairy with a trade-off in attributes: the higher temperature of the hot CW is an advantage in that
it reduces the pre-heating burden on the digesters; however, it is more dilute. The RO step
concentrates the biodegradable material by removing over one-half of the water. Since the
BCUA digesters have short detention times, adding the more dilute hot CW stream would have a
greater impact on shortening the detention time and potentially reducing volatile solids
destruction. Reduced volatile solids destruction would not only reduce gas production, it would
increase digested solids and solids disposal cost.
Whey and Biosolids Sampling
In preparation for bench scale testing, Villanova researchers visited the BCUA and Biazzo Dairy
facilities and collected the following samples:
 Digester solids (for use as “seed” for the BMP testing)
 Thickened primary/secondary solids (digester feed)
 Cheese Whey before centrifugation (Hot CW)
 Cheese Whey after Reverse Osmosis treatment (Cold CW)
Samples were preserved on ice and transported to Villanova and refrigerated on the same day as
they were collected. (Note: “Hot” as the designation for the pre-centrifuge CW refers to the
temperature at the time of sampling. All samples were chilled for transport after being taken.)
Characteristics of these streams as well as the BCUA solids to be co-digested are shown in Table
1. Hot and Cold CW samples (“before” and “after” the RO process) were analyzed in terms of
Total Chemical Oxygen Demand (CODT), Total Solids (TS), Volatile Solids VS, pH, alkalinity,
ammonia and total phosphate. It is important to note that “hot” CW sample is CW prior to
centrifuging, which removes particulate matter, especially fats and colloidal material, from the
CW. Thus, more accurately, these samples are “pre-centrifuge” and “after RO”.
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Table 1 Characteristics of CW samples, digester contents, and feed
Sample
Digester (Seed
sludge)
Feed sludge
CW before
centrifuge
(hot)
CW after RO
(cold)
CODT
(mg/L)
22600
TS
(mg/L)
20700
VS
(mg/L)
14200
pH
Alkalinity
Ammonia
(mg/L CaCO3)
(mg/L)
7.05
4600
NA
Total P
(mg/L)
NA
45500
72500
36200
65200
27800
56100
NA
5.75
NA
1400
NA
20
NA
2360
195000
177700
153900 5.87
2600
24
THTM
NA: Not analyzed, THTM: To high to measure and dilution causes loss of sensitivity
The data show that both CW samples have highly oxidizable (potentially biodegradable) organic
matter as measured by CODT regardless of the RO treatment. CW before centrifuge (“hot” CW)
contains 72,500 mg/L of total COD and the RO process concentrates these organics 2.7 fold.
METHODOLOGY
Bench Testing Methods and Apparatus
Co-digestion feasibility was determined according to the standard Biochemical Methane
Potential (BMP) test using 165 mL capacity serum bottles (Owen et al., 1979). Each serum
bottle contained seed (biomass from BCUA digester), feed sludge, several initial concentrations
of “hot” and “cold” CW, and tap water for normalizing the total liquid volume. The bench-scale
testing used165-ml capacity serum bottles each with 60 mL operating liquid volume. Each
parameter was tested in triplicate to ensure data quality.
Four different initial concentrations were tested for each CW sample. These concentrations,
along with the corresponding CW to feed sludge ratios, are reported in Tables 2 and 3 for “cold”
and “hot” CW samples, respectively. These ratios were selected to cover the ranges that would
be experienced if up to roughly 200% of the Biazzo Dairy cheese whey were co-digested with
current BCUA primary and secondary solids production. For example, the expected average
“hot” CW flow of 189 m3/d (50,000 GPD) that could be co-digested by the BCUA would
represent roughly 12% of digester feed volume. Lower feed volume ratios were used for cold
CW, to adjust for the higher concentration of biodegradable material.
Table 2 Ratios of cheese whey (CW) after RO (“cold”) and feed sludge tested
Cheese whey
(mg/L COD eq.)
Cheese whey/ Feed Sludge
(COD CW/ COD feed as %)
195
488
975
1950
4.3
10.71
21.4
43
Cheese whey/ Feed sludge
(% by volume)
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1
2.5
5
10
Table 3 Ratios of cheese whey (CW) before centrifuge (“hot”) and feed sludge tested
Cheese whey
Cheese whey/ Feed Sludge
Cheese whey/ Feed sludge
(mg/L COD eq.)
(COD CW/ COD feed as %)
(% by volume)
145
363
726
1452
3.2
8
16
32
2
5
10
20
Total biogas production from serum bottle reactors was
monitored for the 30-day incubation period and the gas phase
was analyzed for its CH4 and CO2 content. Total gas was
measured via a volume displacement device – see Figure 1.
Biogas produced was analyzed for its CH4 and CO2 using a
Hewlett Packard™, Model 6890 gas chromatograph equipped
with a thermal conductivity detector (TCD).
Recognizing that food processing byproducts are frequently
deficient in nutrients needed to support the anaerobic biomass,
nutrient deficiency testing was also conducted using a slightly
modified form of anaerobic toxicity assay (ATA) testing (Owen
et al., 1979). Various concentrations of iron, nickel, and cobalt
as well as a “cocktail” of micro- and macro-nutrients were
tested. The micro- and macro-nutrients (Fe, Ni, Co, Cu, Se,
Mo) were added with a synthetic substrate and compared to
performance with no additives.
Figure 2 Liquid displacement device
for total gas measurements
Full Scale Whey Handling and Testing
As discussed in detail below, the bench testing showed rapid increases in gas production with
proportional increases for the amount of CW fed indicating that there were no initial inhibitory
effects and the CW is highly co-digestible. The nutrient deficiency (ATA) testing showed no
deficiencies – the treatment facility’s biomass contained a sufficient nutrient reserve to
accommodate the needs of anaerobic whey co-digestion. The testing also showed that the gas
production potential of the pre-centrifuge whey was considerably higher than the post-centrifuge
whey. Based on the encouraging bench testing findings, the study team recommended a fullscale co-digestion trial based on the addition of hot, pre-centrifuge whey. In the course of
scoping the logistics of a full-scale trial, it was determined that the Biazzo dairy’s storage system
was not set up to conveniently store and load pre-centrifuge whey, so BCUA commenced a fullscale demonstration of post-centrifuge, hot (pre-RO) cheese whey co-digestion in October, 2010.
Biazzo was able to make a dedicated tank truck available for transporting CW from the dairy to
the BCUA. A full load represents approximately 21 m3 (5,500 gallons) of whey. Transporting
all of Biazzo’s hot, pre-RO cheese whey production to BCUA would involve transporting 9
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loads per day. BCUA has two 198 m3 (5,240 gallon) stainless steel grease storage tanks which
are idle. These tanks are suitable for running a full-scale trial and for routine operation for an
interim period. The tanks are equipped with small pumps 0.3 – 1.0 L/s (5-15 gpm) to feed the
digesters. See Figure 3.
Figure 3A Unloading whey to storage tanks
Figure 3B Whey transfer pump
While the bench testing provided strong encouragement to proceed with a full scale trial, BCUA
elected to start conservatively with one tank truck per day feeding as single digester and using
the other 4 digesters as controls. This approach provided confidence in what is added, and
minimized overall exposure. After several weeks of operation with no observed undesirable
impacts, this level was increased to two tank trucks per day. Averaging in weekend and other
interruptions to delivery, the average monthly rate of sustained whey co-digestion has been ~ 25
m3/d (6500 gallons per day). At this level, feeding a single digester, the volumetric ratio of whey
to facility primary/secondary solids is ~ 8%. On a COD (substrate) basis, the feed ratio is ~
11%.
Second Source
After full scale trials commenced using Biazzo Dairy whey, a second cheese manufacturing
dairy, Montena-Taranto was identified also within 5 driving km (3 miles) of the facility. Figure
4 shows the location of Montena-Taranto.
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Figure 4 Location of Montena Taranto Dairy
The Montena Taranto (M-T) whey characteristics are similar to the Biazzo whey, as shown in
Table 4. The M-T whey VS content and COD were comparable. The M-T whey pH was
slightly lower, but this is balanced by a higher Alkalinity. Beginning in April 2011, BCUA
converted to co-digestion of M-T whey for continued full scale trials.
Table 4 Comparison of Biazzo and Montena Taranto wheys
Sample
CODT
TS
VS
pH
Alkalinity
Ammonia
(mg/L) (mg/L) (mg/L)
(mg/L CaCO3)
(mg/L)
Digester (Seed
22600
20700
14200 7.05
4600
NA
sludge)
NA
M-T after
64100
63000
52400 5.4
4700
centrifuge (hot)
CW* before
72500
65200
56100 5.75
1400
20
centrifuge (hot)
CW* after RO
195000 177700 153900 5.87
2600
24
(cold)
*Biazzo Dairy
Total P
(mg/L)
NA
NA
2360
THTM
RESULTS
Bench Testing – BMP Findings
The cumulative CH4 generations from serum bottles that received feed sludge and Cheese Whey
(CW) at various initial concentrations are presented in Figure 5 and Figure 6 for “cold” and “hot”
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CW, respectively. Each data point in these figures is the average cumulative methane production
in triplicate serum bottles.
Figure 5 Cumulative CH4 production from co-digestion of “cold”
CW (after RO) at various concentrations and feed sludge
Figure 6 Cumulative CH4 production from co-digestion of feed sludge
and “hot” CW (before centrifuge) at various concentrations
As expected, cumulative CH4 production increases with the increasing initial concentrations of
CW added to the serum bottles. This is true for both CW before centrifuge and after RO
treatment. The fact that the active reactors supplemented with high doses of CW produced
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nearly proportionately more CH4 than the low dose reactors clearly indicates that CW had no
inhibitory effect on digestion of primary and waste activated sludge at the levels tested, as well
as indicating the ready anaerobic digestibility of CW.
Comparing the gas production for hot CW fed at a 10% volumetric feed ratio (representing
approximately the feed ratio for co-digestion of 100% of Biazzo cheese whey) indicates that gas
production would be boosted approximately 40% over current gas production resulting from
digestion of primary and secondary solids alone.
It is important in co-digestion feasibility studies to assess the optimal loading rate of the codigestion candidate that could be applied without any detrimental effect on the overall
metabolism of digester biomass. The COD recovery at different loading rates is useful in that
regard. Percent COD recovery is based on COD equivalent of CH4 and it is the ratio of CH4
generated (in COD equivalent) to the COD added. The COD equivalent of CH4 is 395 mL of
CH4 per gram COD at 35 0C (incubation temperature), which is derived from the stoichiometry
of CH4 oxidation. The estimate does not account for the fraction of substrate used for cell
synthesis, which is typically less than 10% of the substrate digested. The COD recovery is based
on net CH4 generated by digestion of CW. Net CH4 from digestion of CW is the difference
between cumulative CH4 from active reactors and that from the seed control. Two control
samples used in the study were seed and feed control. Seed control accounts for the CH4 that
resulted from digestion of residual volatile solids in the digester sample (seed) collected from the
BCUA plant. Feed control, on the other hand, was used to determine the amount of CH4 from
digestion of sewage sludge. Figure 7 presents the percent COD recovered as a function of CW
loading, expressed as initial CW to feed sludge ratio in the feed.
Figure 7 Relationship between % COD recovery and CW to feed sludge
ratio expressed as % by volume
Feeding Cold CW (after RO) at full feed rate would be approximately 4% by volume; and
feeding Hot CW (pre-centrifuge) at full rate would be approximately 12% by volume. At these
values, the %COD recovered (as a percentage of theoretical) was approximately 75% and 130%
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respectively. Figure 7 indicates that CW before the centrifuge treatment (“Hot” in Figure 3) has
a higher COD recovery, and thus, had higher specific CH4 production, at all loadings compared
to CW after the RO (“Cold” CW in Figure 3). The COD recovery in excess of 100% for the
addition of Hot CW suggests that co-digestion of with Hot CW has a synergistic effect leading to
greater mass of sludge digestion. However, the solids and CODT concentrations measured at the
end of the incubation period were more or less the same in the control and “Hot” CW added
bottles indicating that there was no greater degree of feed sludge digestion. The most likely
explanation for greater than 100% COD recovery from “Hot” CW is the fact that fats, which
would be removed during centrifuging, have a higher energy content, on a mass per mass basis,
than carbohydrates and proteins, upon which the theoretical estimate is based.
Therefore, Hot CW, having greater fat content than the Cold CW samples, shows a greater than
100% CH4 recovery compared with theoretical levels when it is anaerobically digested. Based
on this higher gas yield and greater COD content, and the absence of any observable inhibitory
effect (as discussed in Section 2.4, below), running full-scale trials with the Hot CW stream as a
co-digestion feedstock would be the preferred choice.
The downside of the Hot CW feedstock is its greater volume, which will reduce detention time
and adversely affect volatile solids destruction, but based on the kinetic constants derived from
the BMP testing (see Section 2.5), this effect is expected to be modest, i.e., on the order of 1-2%.
Volatile solids destruction was measured for the serum bottle tests. Results are shown in Table
5.
Table 5 Serum bottle Volatile Solids (VS) in/out and % VS removed (VSr)
FEED
Feed VS in
Whey VS in (mg) % VS r
(mg)
No Whey
167
Hot CW 5% bv
167
Hot CW 10% bv
167
VS out*
35%
100
17 (+10%)
36%
101
34 (+20%)
36%
102
* Normalized to 100%
While the precision of this testing does not support drawing quantitative conclusions, the
increase in VS out with increasing Hot CW loading suggests that solids production will increase
modestly as a result of co-digestion with BCUA solids. Any increase in digester effluent solids
as a result of reduced detention time would add to the VS increase due to CW addition. The
combined effect of these factors suggests that digested solids production will increase by 2-4%.
This represents a cost increase that has to be weighed against the value of increased gas
production, and confirming the increase in digested solids became a key co-objective of a full
scale trial, along with documenting a gas production increase.
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Nutrient Deficiency Testing Findings
As with many food processing byproducts, cheese whey is rich in COD but may lack the
nutrients necessary for biomass development. Successful co-digestion will rely on an adequate
supply of nutrients being provided by the primary and secondary solids. To account for potential
nutrient deficiencies, nutrient deficiency assays were conducted. These assays involve running
BMP tests to observe whether added micro- and macro-nutrients have beneficial effect on biogas
production. If no additional biogas production results with the addition of micro- or macronutrients, a satisfactory level provided by the primary and secondary solids is indicated. The
effects of nutrient supplementation on BCUA anaerobic digester biomass were evaluated by
slightly modified form of anaerobic toxicity assay (ATA), originally developed by Owen et al.,
(1979). Various concentrations of iron, nickel, and cobalt as well as a “cocktail” of micro- and
macro-nutrients were tested.
Each serum bottle contained seed (biomass from BCUA digester), feed sludge, 224 mg acetate as
the primary substrate for methanogens, nutrients and tap water for normalizing the total liquid
volume. Similar to the BMP bottles, the total liquid volume was 60 mL and each parameter was
tested in triplicate.
Figure 8 shows the results for various nutrient “cocktails” added as digester supplements.
Cumulative CH4 (mL)
250
200
150
Control
Acetate Ctrl.
100
Micronutrient
Macronutrient
50
Vanderbilt
0
0
10
20
30
40
Time (day)
Figure 8 Effects of macro-nutrient and micro-nutrient cocktails, and a combination of macronutrient and trace metal cocktails (Vanderbilt Media) on BCUA digester biomass
Figure 4 shows that methane production was virtually unchanged relative to the control that had
the same level of acetate (readily biodegradable substrate) added but without the supplements.
This is a strong indication that co-digestion of cheese whey with BCUA solids will not be
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hindered by nutrient deficiencies. A number of similar scouting tests were run with the same
favorable finding.
Kinetic Modeling
While it may be misleading to use batch data gathered from batch Biochemical Methane
Potential (BMP) tests to predict performance of a intermittently fed Continous Stirred Tank
Reactor (CSTR), which is the typical configuration for full-scale anaerobic digestors, a model
based on first order degradation kinetics may provide some qualilative insight on methane yield
as a function of time for full-scale co-digestion of cheese whey (CW). Based on the previous
findings which suggested that Hot CW was the preferred feedstock for co-digestion, modeling
was carried out only for Hot CW. The model developed is an exponential growth model that
predicts CH4 yield as a function of time and may roughly represent hydraulic retention time
(HRT) of full-scale digesters. It is expressed as follows.
Y = Ymax(1-e-kt)
Equation (1)
where;
Y: Cumulative methane yield (cubic feed of methane at 35 oC per cubic foot of active digester
volume)
Ymax: Maximum methane yield (cf CH4/cf digester volume)
r: First order reaction rate constant (1/time)
t: Time
To develop a model, for each CW loading, cumulative CH4 production was normalized per unit
active reactor volume. Then, the exponential growth model expressed by Equation 1 was applied
to the normalized cumulative CH4 production data using Curve Expert curve fitting software to
determine the constants Ymax and k for each CW loading. It is important to note that Y in the
model includes CH4 from degradation of feed sludge as well as of CW. These constants are
presented in Table 6. In each case, data fit well to the model as indicated by r2 values 0.98 or
above.
Table 6 Kinetic constants, under various CW load, for exponential growth model
Cheese whey loading
Ymax
(cf CH4/cf active digester volume)
(% volume CW/total feed volume)
0 (feed sludge only)
2.87
2
2.96
5
3.13
10
3.27
20
3.46
Figure 9 shows an example best fit for CW loading of 5% of total feed volume.
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k
(1/day)
.3401
.3489
.3483
.4089
.4239
Methane yield (cf/cf reactor vol.)
S = 0.17818128
r = 0.98962264
3.6
6
3.0
5
2.4
4
1.8
3
1.2
2
0.6
1
0.0
0
0.0
5.5
11.0
16.5
22.0
27.5
33.0
Time (days)
Figure 9 Model results for 5% “hot” cheese whey in feed (vol./vol.)
This kinetic model may prove useful to compare predicted versus actual performance in full
scale trials.
Full Scale Trials
When full scale trials were commenced in October 2010, the first observation was a noticeable
rise in digester floating cover height with the addition on whey. This phenomenon has been
routinely observed throughout extended full scale testing.
Figure 10 shows cover rises observed during feeding of the first truckload of whey received on
each day for a representative16 day period in May, 2011. A load of whey is unloaded over a 5hour period. During that period the whey-receiving digester also received its share of
primary/secondary solids. In all but one instance, the cover rise exceeded the level increase that
would have occurred due to liquid addition, alone. The average cover rise during this period was
165% of the rise due to liquid addition alone, clearly showing that rapid gas evolution was taking
place during whey pumping.
Gas composition is monitored by BCUA for the combined 5 digester gas stream. The BCUA
staff has noticed a slight dip in methane content during periods of whey addition – methane
content is approximately 1% lower. This would translate to a 5 percentage-point drop in
methane content for the whey-receiving digester. This drop is attributed to the low pH of the
whey, which would shift the dissolved/gaseous CO2 equilibrium during whey addition. Even
with discounting for the increase in CO2, the majority of the gas evolution during whey addition
is methane.
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Cover Rise (Inches)
(First Load ‐‐ 5 hour feed)
Maximum Rise due to liquid
addition
20
18
16
14
12
10
8
6
4
2
0
May 3‐18
Figure 10 Whey receiving digester floating cover rises during whey addition
Where’s the Gas?
Each BCUA digester is equipped with a thermal dispersion gas flow meter. Integrated gas flow
is recorded for each digester. Table 7 shows the measured gas production for Digester 1, the
whey receiving Digester, compared to the average gas production for the other four digesters for
April and May 2011. The April – May period was selected as a period of steady whey codigestion.
Table 7 April-May 2011 Digester Data – Volatile Solids (VS) fed and Gas Production
VS – lbs/d & Δ%
April
May
2-Mo. Average
Gas – KCFD & Δ%
VS fed
Gas
VS fed
Gas
VS fed
Gas
Digester 1
155
24,268
157
23,897
156
23,526 +2%
+3%
+1%
+5%
+1%
+4%
Digester 2 – 5 avg
23,101
150
24,145
150
23,623
150
While a modest increase in gas production was noted, these results are not in line with expected
gas production. Digester 1 loading is shown in Table 8.
Table 8 Digester 1 Whey and Primary + Secondary solids feed, Apr-May 2011
Digester 1 Feed
April
May
2-Mo. Average
Volume & COD
Volume – gpd
Volume
COD
Volume
COD
Volume
COD
COD – lbs/d & Δ%
Digester 1
8,541
4,566
6,273
3,354
7,407
3,960
Whey
+10.3%
+14.5%
+7.2
+10.2%
+8.7%
+12.3%
Digester 1
82,842
31,436
86,966
33,001
84,904
32,219
Primary + Secondary
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Based on a COD loading of 12% over the primary + secondary solids feed to Digester 1 during
the April – May period, the high biodegradability for the whey would suggest an increase in gas
production more like +20%, five times higher than the measured gas increase of 4%. These
results beg the questions: What is going on? Where is the gas?
Volatile Solids Destruction
Volatile Solids (VS) destruction was calculated for the Digester 1 versus Digesters 2-5. VS
destruction data for Apr-May 2011 is presented in Table 9.
Table 9 April-May 2011 Digester Data – VS Destruction (VSr) & predicted Gas Production*
VSr – lbs/d & %VSr
April
May
2-Mo. Average
Gas – KCFD & Δ %
VSr
Gas
VSr
Gas
VSr
Gas
Digester 1
8,402
126
8,092
121
8,247
124
35.6%
+19%
33.2%
+16%
34.4%
+18%
Digester 2 – 5
7,041
6,943
6,992
106
104
105
average
30.5%
28.8%
29.7%
* Gas production based on 0.93 m3/kg, VS destroyed (15 cu. ft. biogas/lb., VS destroyed)
As shown in Table 9, the percent Volatile Solids destruction (%VSr) increases substantially
when whey is added – from a %VSr of 29.7% for the non-whey receiving control digesters
Digesters 2-5) to 34.4% for Digester 1, the whey receiving digester. Typical values for biogas
production range from 0.74 – 1.12 m3/kg, VSr (12-18 cu. ft./lb., VSr) (Metcalf & Eddy 3rd
Edition, 1991). Using a mean value of 0.93 m3/kg (15 cu. ft./lb.), VSr, gas production from
Digester 1 was estimated to be 18% greater than gas production from the non-whey receiving
control digesters, consistent with the expected gas production indicated by bench BMP testing.
So, qualitatively by cover rise, and quantitatively by bench testing and gas production calculated
by Volatile Solids destruction, co-digestion of whey is resulting in a substantial increase in gas
production; however, this has not been confirmed directly by gas flow measurements.
Solids Production
Mass balances were prepared around the digesters to estimate the incremental solids production
resulting from co-digestion of whey. Taking small differences between large numbers can lead
to large errors, but the use of mass balances provides a semi-quantitative estimate of additional
solids produced. Table 10 presents mass balance data for Digester 1 for the April – May 2011
period of steady whey co-digestion.
Eschborn, Ralph
Bergen County Utilities Authority Digests Cheese Whey – Gets Gas
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Table 10 Solids production Data for Digester 1 – Apr – May 2011
Total Solids (TS) –
April
May
2-Mo. Average
lbs./d & Δ%
Digester 1 measured
21,559
22,905
22,232
TS out
+4.9%
+4.3%
+4.6%
Digester 1 Calculated
755
555
655
Solids from Whey
+3.7%
+2.5%
3.1%
Non-Volatiles
Digester 1 Estimated
20,557
21,960
21,259
Solids without whey*
* Solids generation estimated based on VS destruction achieved in Digesters 2-5 for the same
Apr-May period, and Primary + Secondary solids fed to Digester 1
The indicated increase in solids production is 4-5%, with as Table 10 shows, about two-thirds of
the solids coming from non-volatiles in the whey. The net solids production increase, excluding
the non-volatiles in the whey, of under 2% shows that the whey is highly biodegradable under
full scale conditions and/or that the adverse impact on volatile solids destruction for the primary
+ secondary solids due increased hydraulic loading is quite modest. Detention times for Digester
1 during the April – May period were reduced by 9% due to the addition of the whey stream.
To determine the actual impact on biosolids production, the estimated 4-5% increase in solids
production needs to be corrected for Total Dissolved Solids (TDS). The diary whey is “salty”
and these dissolved solids show up in a Total Solids (TS) measurement, but would be largely
removed during dewatering prior to biosolids transport.
DISCUSSION
The discrepancy between the rapid digester cover rises during whey addition and the nearly 20%
increase in gas production indicated by volatile solids destruction on one hand, and the less than
5% increase in gas production indicated by direct measurement of gas flow prompted an inquiry
into gas metering.
Figure 11 shows a comparison of Digester 1gas flow, as measured, and gas flow, as calculated
using volatile solids destruction. Monthly total measured gas flow was compared to calculated
flow based on monthly volatile solids destruction data. Two periods were examined: a ninemonth period just prior to the start of whey co-digestion, and the Apr – May 2011 period of
steady whey co-digestion. The red diagonal in Figure 11 is the “identity line”, i.e., if there was
perfect agreement between measurements and calculations, all points would fall on this line.
What is immediately evident from the graphical portrayal of Figure 11, is that estimated gas
production based on volatile solids destruction, varied by roughly 300% during the period
preceding whey co-digestion, while measured gas production varied by only 25% between the
highest and lowest readings. All calculated values were below the identity line. Estimated gas
production during the April – May period of whey co-digestion showed the closest approach to
identity, but were still below measured values.
Eschborn, Ralph
Bergen County Utilities Authority Digests Cheese Whey – Gets Gas
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Since gas production is expected to vary no more than +/- 20% per unit of volatile solids
destroyed (Metcalf & Eddy, 3rd Edition, 1991) and estimating volatile solids destruction using %
volatiles in and out (the Van Kleeck method) is expected to have only modest error when taking
monthly averages, a strong inference is drawn that the gas flow meter is largely insensitive to gas
flow.
Figure 11 Calculated versus measured Gas Production in KCF per month
Several potential causes for this apparent metering error have been identified:




Mis-calibration
Change in gas composition
Asymmetric or swirled flow due to upstream bends or fittings
Moisture in the digester gas impinging on the meter’s probe
Thermal Dispersion Mass flow meters are commonly used for measuring gas flow, as they have
the virtue of being insensitive to temperature and pressure changes; however, they require
calibration and are subject to errors due to changes in gas composition and asymmetric flow.
With respect to mis-calibration and asymmetric flow these sources of error are expected to
impart bias more than the substantial insensitivity observed and are thought to be only minor
contributors to the observed error. Gas composition can have a significant effect on readings,
but given the fact that the insensitivity was clearly found prior to the co-digestion of whey, the
normal variation in digester gas composition would not result in the insensitivity observed.
Ruling out the first three sources of error strongly points to moisture from the wet digester gas
compromising readings.
Figure 12A and 12 B show the flow meter installation on Digester 1 and the vendor
recommended installation for Thermal Dispersion meter probes.
Eschborn, Ralph
Bergen County Utilities Authority Digests Cheese Whey – Gets Gas
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Figure 12A – Digester 1 Flow Meter
Figure 12B – Flow Meter Mounting Positions
Based on vendor mounting guidance for meters in wet gas service, the BCUA meters are
mounted in a manner that may allow moisture to impinge on the meter probes and descend on to
the measuring elements, where the mass of the liquid will have a significant cooling effect,
creating the high and insensitive flow measurements observed.
CONCLUSIONS
Based on both bench and full scale trials, BCUA has found that cheese whey co-digestion
promises to deliver a substantial increase in gas production. While there is a lot of “noise” in the
full scale trial data, increased gas production is estimated to be between 4 and 18%. The lower
figure is based on direct gas flow measurement, but these gas flow meter readings are suspect,
based on data generated before whey co-digestion commenced.
Full scale trial addition levels are about one-half of the goal level which was bench tested with
satisfactory results, indicating that expansion of the co-digestion program to include all digesters
and higher digester loading rates could increase overall gas production by over 30%.
Whey co-digestion is accompanied by increased solids production. While it is estimated to be
less than 5%, this represents a significant potential cost off-setting the value of the gas generated.
Further trials are needed to refine gas and solids production estimates. Based on the workable
logistics demonstrated for whey receipt, storage and digester feeding, and the absence of any
observed undesirable side effects in digester operation, BCUA plans to extend full scale trials.
Eschborn, Ralph
Bergen County Utilities Authority Digests Cheese Whey – Gets Gas
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Perhaps the biggest payoff of the whey co-digestion program is the community “win-win” that
has resulted from converting high strength waste streams from two nearby diaries to a “green”
source of power. One dairy was investigating relocating out-of-state, to be closer to milk
production and reduce hauling costs for waste whey. With the development of a local outlet for
the whey, they have shelved relocation plans. The other dairy, whose economics were
compromised by the high cost of whey disposal, did not see sufficient justification to expand
their operations, but with the prospect of local disposition of their whey are now exploring
expansion.
ACKNOWLEDGEMENTS
The authors would like to express their appreciation to Robert Laux, Executive Director of the
Bergen County Utilities Authority (BCUA) and Eric Anderson, Director of the BCUA Water
Pollution Control Division and Chief Engineer for their support of the cheese whey co-digestion
program. Conducting full scale trials of cheese whey co-digestion would not have possible
without the full cooperation and concerted efforts of the BCUA Little Ferry wastewater
treatment facility staff. In particular, the authors would like to express their appreciation to
Walter Paul, Plant Manager, Edward Switay, Assistant Plant Manager, Michael Williams,
Manager of Permits and Regulatory Affairs, David Hudspeth, Chief Sewerage Plant Operator,
and Thom Chaiket, SCADA Engineer for their contributions to the Program.
REFERENCES
Owen, W.F., Stuckey, D.C., Healy J.B., Jr., Young L.Y., McCarty, P.L. 1979. Bioassay for
monitoring biochemical methane potential and anaerobe toxicity. Water Res. 13:485–492
Metcalf & Eddy, Third Edition, 1991. “Wastewater Engineering, Treatment, Disposal, Reuse”
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Bergen County Utilities Authority Digests Cheese Whey – Gets Gas
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