Manuscript

Food Waste Digestion at Inland BioEnergy in Chino, CA

Processing of pre-consumer and industrial food waste and biogas

yields

Author: Christian Tasser*, P.E.,

Carollo Engineers, Inc.

*ctasser@carollo.com

Khalil Kairouz, P.E. PhD, Carollo Engineers

ABSTRACT

The Inland BioEnergy (IBE) food waste digester project located in Chino, CA has been in full

operations since 2012. IBE is one of the largest dedicated food waste digester projects in the State

of California. The anaerobic digesters (AD) are currently processing up to 300 tons per day and

the plants permitted capacity is 600 tons per day. The purpose of this paper is to introduce

California's first dedicated food waste digestion project along with lessons learned in regards to

pre-treatment issues and biogas yields of various types of substrates. The digester feed stocks are

from industrial food processors and pre-consumer food waste from grocery markets pre-processed

at a material recovery facility. Bench-scale biogas yield tests are compared with actual full-scale

biogas yields and used to determine sustainable feed rates to ensure long-term operational stability

of the digesters.

KEYWORDS

Keywords: High strength organic waste, bioslurry, pre-treatment, screening, food waste,

substrates, toxicity, organic loading rates, ratio COD/VSS,

Introduction

The Inland BioEnergy (IBE) food waste digester project located in Chino, CA has been in full

operations since 2012. IBE is one of the largest dedicated food waste digester projects in the

State of California. The anaerobic digesters (AD) are currently processing up to 300 tons per

day, while the plants permitted capacity is 600 tons per day. The digesters are treating residues

from industrial food processing and organics from groceries, which are pre-processed to a

bioslurry. The AD facility consists of two completely mixed digesters, with a diameter of 55

feet, a sidewater depth of 60 feet and a volume of 1.2 Million gallons each. The generated

biogas is used in internal combustion engines with a combined maximum capacity of 3 MWe.

The purpose of this paper is to introduce California's first dedicated food waste digestion project

along with lessons learned in regards to pre-treatment issues and biogas yields of various types

of substrates. The digester feed stocks are from industrial food processors and pre-consumer

food waste from grocery markets which was pre-processed at a material recovery facility

(MRF).

In the interest of an economical and streamlined operation of the digesters it is important to pre-

select, pre-screen and pre-process the organic substrates to a level, which reduces onsite

mechanical processing requirements. The type and the size of the organic solids will further

determine the mixing requirements and the total biogas yield as well as how fast volatile solids

can be digested in the reactor. The following substrates were tested at the site to determine the

treatability and expected biogas yields:

- BioSlurry

- Juice and soda beverages concentrate (syrup)

- Juice wastewater

- Bakery wash water

- Food processing waste sludge

- Sludge from creamery

- Mixture of Ketchup and salad dressings

- Fish waste (tuna)

- Ice cream

- Biodiesel glycerin

The experience at Inland BioEnergy which spans over a period of over 5 years, is based on a

diligent evaluation and preselection of food waste substrates. The liquids are coming from

industrial food and beverage processing facilities as off-spec products or in form of wastewater

sludge. The bioslurry is generated from pre-consumer food waste such as fruit and vegetables

pre-processed at the Material Recovery Facility in Fontana. The substrate selection and feed flow

rates are based on COD, VSS, and other analytical parameters. As reported by Appleton, Rauch,

et al. in WERF 2017 conventional digester operating parameters (e.g., volatile solids loading rate

(VSLR)) alone may not be sufficient to characterize HSW co-digestion operation. For example a

municipal digester, which receives Greek yoghurt acid whey, achieves reliable co-digestion

performance at VSLRs above the recommended limits.

In addition to wet chemical analysis, actual biogas yields were determined through the bench-

scale tests of various substrates using seed sludge from the digesters. Even certain beverages with non-sugar based sweeteners, such as aspartame would still show a

COD, but not have much of a benefit to the biogas yield in digesters. For example a beverage

company operating a digester at its bottling plant noticed a reduction of biogas yield with the

introduction of diet drink syrups.

Biogas yield or toxicity testing services are offered by commercial laboratories and universities.

The time for coordination and costs involved for these tests to be conducted off-site would be

some of the disadvantages compared to on-site bench-scale tests. Also, the interaction between

the various cultures of acid and methane formers in the existing digester can be better predicted

by using seed sludge with microorganisms from the current operation to better predict the

compatibility of the new feed stock with the on-going operations.

On the other hand highly soluble COD from juice bottling plants will take time to adapt as they

shift the population to other types of methane formers. Adaptations can include a shift from

Methanosarcina barkeri (metabolizing short carbon molecules methanol, acetate) to

Methanosaeta concilii.

Long-chain fatty acids (LCFA) can inhibit methane production by methanogenic archaea. The

effect of oleate and palmitate on pure cultures of Methanosaeta concilii and Methanosarcina

mazei was assessed by Silva et al. in 2016 by comparing methane production rates from acetate

before and after LCFA addition. In particular, the robustness of M. concilii might contribute to

the observed prevalence of Methanosaeta species in anaerobic bioreactors used to treat LCFA-

rich wastewater.

For the bench test and for full-scale operation the ratio of new substrate to seed sludge has to be

carefully chosen in order to avoid foaming, inhibition through organic overloading or specific

compounds such as ammonia, protein/sulfur compounds, etc.

This simple and effective bench-scale set-up was also used to evaluate the level of pre-screening

for the food waste slurry. Bioslurry was generated with a hammermill and bioseparator from

DODA with 10 mmm screens at the MRF, which was processed further onsite at IBE using a 5

mm DODA screen or a 2 mm Vincent screw press. The fiber reject was processed through the

bench set-up to determine the loss in biogas potential when removing these organics. Results

showed that the loss in biogas generation when removing these solids was negligible, because of

the recalcitrant lignocellulose materials since they are very slowly hydrolyzed without other

technical means such as enhanced mechanical-physical or chemical hydrolysis (Cambi or

Thermal hydrolysis processes).

The additional screening for removal of contaminants is benefiting the overall operation by

protecting the downstream pumps from various types of debris (glass, metal, plastics etc.)

coming from the transfer tanker trucks or from previous glitches in the processing at the MRF.

In the interest of an economical and streamlined operation of the digesters it is therefore

important to pre-select, pre-screen and pre-processes the organic substrates to a level, which

reduces onsite mechanical pre-processing requirements. The type and the size of the solids will

further determine the mixing requirements and the total biogas yield as well as how fast the

volatile solids are digested in the reactor. The remaining solids will have an impact on

dewaterability of the digestate in screw presses, centrifuges or other type of equipment. The

quantity and type of polymers used will also be influenced by the positive or negative charges of

the solids, the age of the sludge, the mixing efficiency, biogas removal or entrapment in the

sludge, time for polymer to unwind and age and contact time between negatively charged sludge

and cationic polymer prior to the dewatering step.

As the food to mass ratio in the digester needs to be maintained a partial return of thickened

sludge to the digester should be considered (similar to reseeding of fresh substrate in dry batch

fermenters). This is especially the case in dedicated industrial digesters (merchant facilities)

where a high COD loading, but low VSS loading can cause a wash-out of microorganisms. The

solids concentration is a simple but not the most adequate means to assess if there is sufficient

biomass available to digest the organic feed materials. The analysis of the VSS/TS ratio would

be better indication, which can also be performed onsite.

More sophisticated means are microscopic observations in universities or labs to quantify the

types and quantity of methane formers.

A good starting point for the dosing of fats, oil and greases to municipal co-digestion has been to

stay below 10% by volume or VSS loading. Other factors which can impact the co-digestion are

acidity of substrates causing corrosion of pumps. Or physical contamination causing build-up of

grit, glass, plastics and metals. These contaminants and solids will change the viscosity and the

mixing energy requirements. Another operational consideration would be, if the additional

biogas generation can be handled by existing blowers, sulfur scrubbers and methane destruction

equipment such as flares or cogeneration plant.

Higher concentrations of proteins with sulfur containing molecules will result in higher hydrogen

sulfide concentrations in biogas. Also, a lower pH due to more soluble organics and higher

concentration of acetates (for example vinegars) will shift the dissolved sulfides to the biogas

side as hydrogen sulfide (H2S) or increase ammonia toxicity.

In the long-term the increase in methane concentrations due to co-substrates and the overall

higher digester gas volumes opens up new opportunities to utilize the biogas for other purposes

such as CNG truck fueling or injection to the grid.

Figure 1 - Two Digesters with 1.2 Million gallons at Inland Bioenergy in Chino, CA retrofitted

with LANDIA Gas Mixing System

Methodology

Biogas yield and toxicity testing

In order to determine the biogas yields a bench-scale set-up was used. A 2-L Erlenmeyer bottle

was filled with seed sludge (2,000 mL) from the full-scale reactor and 30 ml of the various

substrates were heated on a stirred plate to 98 Deg F for a duration of 3 weeks. Biogas was

collected in a 1-Liter graded measuring cylinder and the volume of biogas was measured based

on the water displacement. About the standardized procedures for the Biochemical Methane

Potential (BMP) developed by McCarty at Stanford Owen et. al. (1979) reported in detail. BOD

assay indicates how much organic pollution can be degraded in an aerobic process, whereas the

BMP is the correlative measure in the anaerobic process (Speece, 1996). Fat containing

substrates typically cause higher methane content in the biogas (Table 1) and hence generate

lower amounts of biomass (excess sludge) due to low cell synthesis.

Theoretical methane yield (YCH4, m3 STP/kg substrate converted) can be calculated from the

elemental composition of a substrate:

CcHhOxNnSs

YCH4=22.4*(c/2+h/8+x/4−3n/8−s/4) / (12c+h+16x+14n+16s)

Table 1 shows the substrate, a common elemental formula, and the theoretical methane yield for

each.

Table 1: Theoretical methane yield (m3 STP/kg substrate converted) for several

biomass sources

Substrate Elemental formula Theoretical methane yield

(m3 STP/kg)

Carbohydrates (CH2O)n 0.37

Proteins C106H168O34N28S 0.51

Fat C8H15O 1.0

Plant biomass C5H9O2.5NS0.025 0.48

Carbohydrates class of organics provides more energy to these organisms than proteins and fat

where the biomass yield is 10 times higher (y = 0.35 g cells/g COD consumed, vs. 0.038 for fats

(Speece)).

For the bench-testing the total biogas was measured and reported including the CO2 production

to simulate the actual rate of full-scale digester gas production. This would also be indicative of

potential foaming issues or rapid bed expansion through biogas formation and/or protein

lowering liquid surface tension). The substrates, which were tested and found to be suitable for

treatment in the full-scale digester were analyzed onsite for COD, TSS, VSS, VDS, TN, TP and

other constituents using HACH test kits and Spectrometers.

Daily plant monitoring data for the digesters were used to compare the expected biogas yield

with full-scale test results.

RESULTS

The time for the adaptation of the microorganisms in digesters to the new substrates was

predicted with the bench-scale testing The acclimation timewas used as a basis for the

operations plan by implementing a gradual increase of the new substrate digester feed ratesover a

period of 5 to 10 days. The bench-scale testing approach is therefore not only helpful in

determining the biogas yield, scum layers formation, stratification, but also the time required to

develop the methane from co-substrate. The time to generate methane from various substrates

can be only one day for juices, over two days for bioslurry to over 5 days for organic waste high

in organic fibers.

The results of the analytical tests for COD and TSS for various substrates are shown in TABLE

2. The results show the variability of the materials based on seasonal changes and pre-processing

conditions at the various industrial food processing plants.

The response time for a digester between an increase in feed rates to result in an increase in

biogas production can be just a matter of hours once the microorganisms in the digester have

been acclimated.

Table 2: Analytical Test Results for Various Food Waste Substrates

Substrates COD (mg/L) TSS

Bakery wash water 17,000-50,000 < 1 %

Juice wastewater 34,000-50,000 < 1 %

Food processing waste sludge 80,000-120,000 5 - 10%

BioSlurry 120,000-150,000 10 - 15 %

Sludge from creamery 160,000-180,000 10 - 12 %

Syrups 200,000 - 900,000 <1 %

Ketchup, Mayo, Salsa 200,000 - 300,000 8 - 12 %

Fish waste (tuna) 300,000 - 350,000 10 - 15 %

Ice cream 400,000 - 500,000 <1 %

Biodiesel Glycerin 900,000 - 950,000 3 %

The FIGURE 2 is showing the biogas yield test results for the bioslurry. The tests were

performed over a period of 3 weeks. The bioslurry was screened to 2 mm in a full-scale screw

press. The biogas yield for the 31 mL of substrate was 2800 mL collected over the test duration

of 21 days. In subsequent tests the fibers from fruits and vegetables with particle size larger than

2 mm removed in the screw press were also tested for biogas yield. It was demonstrated, that

the lignocelluloses in the food fiber as well as the seeds were very difficult to hydrolyze to

become amenable for anaerobic digestion. Therefore this material was removed from the

digester feed material and sent to composting off-site. The following check list was developed

to define the quality requirements for bioslurry:

Checklist of Pre-treatment and Quality Requirements for Food Waste Slurry:

• Removal of contamination such as glass, rocks, plastics, metals, etc.

• Particle size: slurry screened to <2 mm

• Pumpable slurry with total solids below 15%

• pH range 5 to 8 SU

• Continuous testing of substrate to include the following parameters: TS, VSS,

VDS, sCOD, BOD, TKN, TP, sulfide, ammonia

• Perform a biogas yield analysis

Figure 2 - The cumulative biogas production over a period of 21 days

Comparison between Biogas Yield Predictions and Operational Data

0

500

1000

1500

2000

2500

3000

3500

4000

4500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Cu

mu

lati

ve G

as (

ml)

DaysBakery Soda Beverage Orange JuiceTuna waste Creamery sludge Powder Starch (10%)

The biogas production trends over a period of one month and the solids concentrations, biogas

generation, variations in VA/PA ratios, VFA are shown in the following graphics (Figure 3

through Figure 7). Two digester were operated with one having higher solids concentrations with

high food waste slurry reactor D2. The consistent feed material in terms of COD, VSS and TSS

showed the best performance in biogas generation. Minimal fluctuations in VA/PA ratio

(alkalinity buffering acid) and VFA based on the adapted microbiology, ideal food to mass ratio

and the consistent feed managed in coordination with the MRF.

Figure 3- Digester #1 with lower solids concentration showing the actual biogas versus estimated

biogas production

Figure 4- Digester #2 with higher solids concentration showing the actual biogas versus

estimated biogas production

Figure 5- The high solids digester D2 with food waste slurry and digester D1 with liquid

substrates (lower TSS)

Figure 6- The higher solids digester D2 shows a lower VA/PA ratio is more stable in operation

due to a more consistent feed stock than the liquids digester D1

Figure 7- The digester with more liquids food waste has a lower pH and shows more variations

over time with changes of input materials.

DISCUSSION

After completing the biogas yield tests for various substrates it was confirmed, that substrates

such as glycerin from biodiesel processing and juice and soda beverage concentrates had the

highest biogas yields. Glycerin was found to contain residues of methanol from the biodiesel

processing and was therefore not accepted for the full-scale digester due to safety

considerations. Juice concentrates delivered in 5 gallon plastic lined boxes (BIB) would have

required a mechanical de-packaging machine to be implemented to reduce the processing time

and associated costs.

From the various substrates tested syrup or juice waste delivered in bulk tanker loads were the

organic feed stocks with the lowest processing costs because they were free of contaminants.

The bioslurry had an ideal C/N ratio for anaerobic digestion and resulted in a high biogas yield.

However, the contaminants (fiber, plastics, metal, and glass) and the solid residuals from

digestate dewatering did also lead to higher overall operational costs due to labor, chemical

usage, and electricity costs.

Figure 8: Biogas yield test results for bioslurry

The digester feed COD concentration is a better measure of the latent energy content in solids.

Recommended operating ranges for traditional metrics, such as VSr, have been developed based

on experience with anaerobic digestion of conventional wastewater solids and may not apply to

HSW co-digestion operation.

The digester feed and the digested sludge COD concentration can be used to calculate the

anticipated co-digester methane production. With the digester gas methane fraction, the COD

concentration change across the digester is an independent check of digester gas flow meter

readings, which are often unreliable. With the COD concentration, methane production can be

checked directly, where 1 gram of COD stabilized through anaerobic digestion is equivalent to

0.35 L (5.61 cubic feet CH4 per pound COD) of methane at standard conditions of 0 degree

Celsius and 1 atm (Rittmann and McCarty, 2001). Third, the ratio of COD to volatile solids can

indicate the relative composition of the HSW and, accordingly, the anticipated digester gas

methane fraction. High-strength waste with a COD:VS ratio less than 1.60 indicates a higher

proportion of carbohydrate and a lower digester gas methane fraction. Conversely, HSW with a

COD:VS ratio greater than 1.60 indicates a higher proportion of lipid and a higher digester gas

methane fraction.

Digester feed COD and digested sludge COD provide the information necessary to benchmark

one co-digestion system's performance against conventional digesters or other HSW co-

digesters.

Nevertheless, care should be taken when inferring gas production from COD data alone. COD

may not be directly proportionate to the amount of anaerobically biological degradable organics.

Celluloses and other high molecular weight organics may register as COD but may not be

available to the microorganismsm in an anaerobic process and therefore lead to lower gas yields.

CONCLUSIONS

During the last years of full-scale digester operations the management used the bench-scale test

results for the selection of the ideal substrates. Subsequently the operators made daily

adjustments to the digester feed flow rates to maintain organic loading rates based on the actual

COD and VSS concentrations of the various food waste substrates.

For periodic quality control the operators need to keep sampling and testing the influent

substrates for changes in COD, TSS and VSS. Also, toxicity can be critical when dealing with

industrial processes which are subject to changes and can be impacted by clean-in-place

procedures including quaternary ammonia or other disinfection chemicals.

VSS loadding rates might not always be the most adequate measure for digester process control

when dealing with food waste containing high fats, oil and greases. The process control was

based on using COD and monitoring the food to microorganism ratio (utilization rate) was

another operational parameter, which is 0.5 to 1 g BOD/g biomass day (as a reference organic

loading rates can be 5 to 10 kg COD /m3 -d).

Monitoring of the substrate and a transparent communication with the operations staff, truck

drivers can be helpful in learning the sensitivity of the operations on both ends in conjunction

with the support from environmental and plant engineering staff.

The higher solids digester D2 shows a lower VA/PA ratio and is more stable in operation with a

more consistent feed stock than the digester receiving mostly liquids

The digester with more liquid food waste D1 has a lower pH and shows more variations over

time with changes of input materials.

REFERENCES

A. Ron Appleton, Jr. P.E., BCEE Tanja Rauch-Williams, Ph.D., P.E., 2017, "CO-DIGESTION

OF ORGANIC WASTE ADDRESSING OPERATIONAL

SIDE EFFECTS" , WERF,

Sérgio A. Silva, Andreia F. Salvador, Ana J. Cavaleiro, M. Alcina Pereira, 2016,

"Toxicity of long chain fatty acids towards acetate conversion by Methanosaeta concilii and

Methanosarcina mazei", Microbial Technology