Biogas production from anaerobic digestion of food waste and relevant air quality implications

ABSTRACT

Biopower can diversify energy supply and improve energy resiliency. Increases in biopower production from sustainable biomass can provide many economic and environmental benefits. For example, increasing biogas production through anaerobic digestion of food waste would increase the use of renewable fuels throughout California and add to its renewables portfolio. Although a biopower project will produce renewable energy, the process of producing bioenergy should harmonize with the goal of protecting public health. Meeting air emission requirements is paramount to the successful implementation of any biopower project. A case study was conducted by collecting field data from a wastewater treatment plant that employs anaerobic codigestion of fats, oils, and grease (FOG), food waste, and wastewater sludge, and also uses an internal combustion (IC) engine to generate biopower using the biogas. This research project generated scientific information on (a) quality and quantity of biogas from anaerobic codigestion of food waste and municipal wastewater sludge, (b) levels of contaminants in raw biogas that may affect beneficial uses of the biogas, (c) removal of the contaminants by the biogas conditioning systems, (d) emissions of NO_x, SO₂, CO, CO₂, and methane, and (e) types and levels of air toxics present in the exhausts of the IC engine fueled by the biogas. The information is valuable to those who consider similar operations (i.e., co-digestion of food waste with municipal wastewater sludge and power generation using the produced biogas) and to support rulemaking decisions with regards to air quality issues for such applications.

Implications: Full-scale operation of anaerobic codigestion of food waste with municipal sludge is viable, but it is still new. There is a lack of readily available scientific information on the quality of raw biogas, as well as on potential emissions from power generation using this biogas. This research developed scientific information with regard to quality and quantity of biogas from anaerobic co-digestion of food waste and municipal wastewater sludge, as well as impacts on air quality from biopower generation using this biogas. The need and performance of conditioning/pretreatment systems for biopower generation were also assessed.

Introduction

Bioenergy is energy produced from biomass. It can be in the form of electricity (biopower), renewable gas (biomethane), or liquid transportation fuels (biofuels). Biopower can diversify energy supply and improve energy resiliency. Increases in biopower production from sustainable biomass can provide many economic and environmental benefits, including creation of green jobs, promotion of local economic stability, and reduction of water and air pollution including greenhouse gas (GHG) emissions (O’Neill and Nuffer, 2011; California Energy Commission [CEC], 2012).

Energy recovery from food wastes

Total municipal solid waste (MSW) generation in the United States in 2012 was 251 million tons. Food waste is the second largest component of MSW, and it represents 15.5% of total MSW generated. Due to difficulties in recovery/reuse of food waste, only 4.8% of the generated food waste was diverted from landfills and incinerators for recovery, mainly by composting (EPA, 2014a). Food waste can be readily digested under anaerobic conditions for capture of energy content, and the residuals may be beneficially reused as fertilizer or soil amendment. Food waste has three times the methane (CH₄) production potential of biosolids (376 vs. 120 m³ gas/ton) (EPA, 2014b). Yields from anaerobic digestion can be as high as 3,200 standard cubic feet (scf), 90.6 m³, methane per ton of raw food waste. Assuming an electricity cost of US$0.10/kWh and an efficiency of 35% for converting biogas energy to electricity, energy in food waste would be about $33/ton (Kraemer, 2012).

Anaerobic digestion of food wastes in wastewater treatment plants

Biogas is the main desirable product from anaerobic digestion of organic carbon. Quality and quantity of biogas will be affected by many parameters, including pH, temperature, feed composition, loading rate, mixing condition, reactor design, and residence time. Although almost any organic material can be processed with anaerobic digestion, the level of digestibility is the key factor in its successful application, if biogas production is the goal. The more digestible the feed is, the higher is the gas yield potential. The biogas or methane yield is measured by the amount of biogas or methane that can be produced per unit mass of volatile solids (VS) contained in the feedstock after a given amount of time under a given temperature (Banks et al., 2011; Zhang et al., 2007).

Many wastewater treatment plants (WWTPs) use anaerobic digestion to reduce the volume of biosolids before disposal and/or reuse. The amount of biogas produced by existing WWTPs in California alone could fuel 125 MW of power generation capacity. Co-digestion is the treatment of a mixture of at least two different substrates with an aim of improving efficiency of anaerobic digestion. A recent study by California Energy Commission (CEC) estimated that, using existing infrastructure, co-digesting fats, oil, and grease (FOG), food processing waste, and dairy waste at the existing WWTPs could increase the biogas yield potential to 450 MW of capacity, representing 2,500 gigawatt hours (GWh) per year (Kulkarni, 2009). Food waste can either be digested at facilities specifically designed for the organic portion of MSW, or co-digested at WWTPs. WWTPs are ideal for accepting food waste diverted from landfills because the facilities are often located in urban areas and have a short haul distance (i.e., less carbon footprint), have experiences in operating anaerobic digesters, and have existing infrastructure in place to capture biogas. In addition, large treatment facilities could use the produced electricity and heat onsite (EPA, 2014b). It should be noted that preprocessing of food waste is often required because WWTP digesters can be damaged by highly fibrous material, metal, and plastic. Co-digesion should also not exceed the design capacity of WWTP digesters with regard to flow, solid loading, and biogas handling (Kraemer, 2012).

Many WWTPs are now in different stages of anaerobic digestion of food waste (Institute for Local Self-Reliance [ILSR], 2012; Kraemer, 2012; Moriarty, 2013; EPA 2014c). East Bay Municipal Utility District (EBMUD) in Oakland, CA, was the first large-scale WWTP in the nation to convert postconsumer food scraps to energy through anaerobic codigestion. Table 1 compares anaerobic digestion of food waste and municipal wastewater sludge under mesophilic conditions and mean cell residence time (MCRT) of 15 days. The data were extracted from a project report by EBMUD (2008). As shown, the food waste tested had a higher VS content than municipal wastewater sludge (86.3 vs. 77%). Food waste is more digestible, as indicated by the larger VS destruction (73.8 vs. 38–57%) after 15 days of anaerobic digestion, and has a larger methane formation potential, 6–8.5 versus 5 ft³/lb (0.37–0.53 vs. 0.33 m³/kg) total solid (TS) applied. The methane concentrations in the biogas produced from these two types of feed are essentially the same, 64 versus 63%.

Table 1. Comparison of anaerobic digestion of food waste and municipal wastewater sludge (source: EBMUD, 2008).

Parameter	Units	Food waste	Wastewater sludge
VS in feed (% of TS)	%	86.3	77
VS destruction	%	73.8	38–57
CH4 content	%	64	63
Methane production	ft^3/lb TS applied	6-8.5	5

Note. 1 ft³/lb = 0.0623 m³/kg.

Biogas from anaerobic digestion

Using biogas in power generation (when compared to fossil fuels) avoids additional GHG emissions (Razbani et al., 2011). With regard to beneficial uses of biogas generated from anaerobic digestion, internal combustion (IC) engines are the most common ones used in WWTPs (O’Neill, 2012). A flaring system and a boiler are often needed to manage excess biogas during outage or maintenance of the cogeneration system. Small WWTPs may need to supplement their biogas with natural gas to fulfill the minimum fuel requirement (CH2M HILL, 2014).

Main components of biogas that are generated from anaerobic digestion are methane and carbon dioxide (CO₂). However, biogas also contains other trace gases, moisture, particulate matters (PMs), and contaminants such as volatile organic compounds (VOCs), sulfur compounds, siloxanes, and ammonia. The composition of biogas can be different in different plants and even different in a specific plant due to the differences in feed composition and operating conditions of its anaerobic digesters. The presence of several trace compounds in raw biogas produced from anaerobic digestion may have adverse effects on beneficial uses. Removal of these trace compounds is often done through pretreatment (or conditioning). The most significant components targeted in biogas conditioning/pretreatment are hydrogen sulfide (H₂S), siloxanes, moisture, PM, ammonia, and CO₂. Hydrogen sulfide is a toxic product formed from sulfates and organic sulfur compounds in the feedstock under anaerobic conditions. During combustion, H₂S will react with oxygen to form SO₂, then sulfurous acid (H₂SO₃) and sulfuric acid (H₂SO₄). These acids are corrosive to downstream equipment such as IC engines (Razbani et al., 2011). Stringent H₂S limits are usually imposed by regulatory agencies. Gas treatments for H₂S removal include adsorption, chemical scrubbing, and biological scrubbing using biotrickling filters (Huertas et al., 2011). Iron sponge adsorption is one of the most commonly used H₂S removal systems, and alternative iron oxide adsorbents such as SulfaTreat, Sulfur-Rite, and Sulfa-Bind are also being used (CH2M HILL, 2014).

Siloxanes are often used in cosmetics, detergents, and building materials and are frequently found in household waste and wastewater. If siloxanes are present in the feedstock to the anaerobic digesters, the low-molecular-weight siloxanes will volatilize into biogas. When this biogas is subsequently combusted in an IC engine, turbine, or boiler, siloxanes will be converted into silicon dioxide and deposited internally in the machine, exhaust manifolds, and turbochargers, increasing wear and tear. Although food waste slurry should contain few or no siloxanes, they are often contained in biogas from codigestion with wastewater sludge. Activated carbon adsorption is currently the best available technology for removing siloxanes from biogas. Upstream removal of H₂S and moisture are important for optimal performance of the granulated activated carbon (GAC) absorbers. Silica gels are an alternative to GAC and they are gaining acceptance as an option for siloxanes removal for their faster removal rates (CH2M HILL, 2014).

Nitrogen in the food waste typically enters the digesters as organic nitrogen and a significant fraction of it is hydrolyzed in the process, leaving the digesters as ammonia in the digestate. The level of ammonia concentration in the digestate of municipal WWTPs can be as high as 1,300 mg/L. Consequently, biogas also contains ammonia at a concentration in equilibrium with that in the digestate. Ammonia in the ambient air poses health risks and it can be a precursor to airborne particles. Ammonia in the biogas can react with water to form ammonium hydroxide (NH₄OH), which will corrode certain metals, such as aluminum and copper, making bearings more susceptible to corrosion from ammonia (Razbani et al., 2011). It should be noted that odorous compounds are often generated under anaerobic conditions. Proper odor control in areas of food waste processing and biogas generation and utilization may be needed.

There are many federal, state, and local regulations governing biogas production and uses in various aspects (air, water/wastewater and waste). As an example, the “Permit Guidance for Anaerobic Digesters and Co-digesters” provides the basic permitting framework and requirements for anaerobic digestion projects in California (California EPA, 2011). Although a biopower project will produce renewable energy, the process of producing bioenergy should harmonize with the goal of protecting public health and the environment. It is therefore important to understand the emission profile of biogas from its creation to end use to ensure that the affected communities are not further harmed. In addition, for example, many air districts in California are designated as nonattainment in regard to air quality standards of ozone and PMs. Emissions of nitrogen oxides (NO_x), precursors for ozone generation, and other compounds from stationary engines that utilize biogas are of concern. The NO_x emission limits can be as low as 9 parts per million by volume (ppmv) in some air pollution control districts (Drake, 2011; U.S. Department of Agriculture [USDA]/EPA/Department of Energy [DOE], 2015; Warner, 2009). Consequently, meeting air emission requirements is critical to the successful implementation of any biopower project.

Objectives of this project

Although anaerobic codigestion of food waste is a viable process, full-scale operation of this process is still new. There is a lack of readily available scientific information on the quality of raw biogas, as well as on potential emissions from power generation using this biogas. The information is needed for determining conditioning requirements of raw biogas for beneficial uses, for selecting power-generating equipment, and for air quality permitting.

The overall objectives of this research were (1) to develop scientific information with regard to quality and quantity of biogas from anaerobic codigestion of food waste and municipal wastewater sludge, (2) to assess the need and performance of conditioning/pretreatment systems for biopower generation, and (3) to develop scientific information with regard to impacts on air quality from biopower generation using this biogas.

Project approaches and methods

Treatment process trains

All field data of this research project were collected from the WWTP of Central Marin Sanitation Agency (CMSA), San Rafael, California. The CMSA processes and disposes of approximately 10 million gallons per day (MGD), 0.44 m³/sec, of wastewater and has treated in excess of 116 MGD (5.08 m³/sec) during peak rainfall periods. The incoming raw wastewater goes through bar screens, aerated grit chambers, primary clarifiers, biological treatment units (i.e., biotowers + fine-bubble aeration tanks for activated sludge process), secondary clarifiers, chlorine contact tanks, and then dechlorination before discharge into central San Francisco Bay through a 2-mile (3.2-km) outfall. The primary sludge (PS) and the thickened waste activated sludge (TWAS), which is thickened by rotary drum thickeners (new), are fed to two mesophilic anaerobic digesters. The biogas produced in the digesters is used to generate electricity by an IC engine for treatment plant and facility uses and to heat plant process water. When digester biogas is not available, the IC engine switches to natural gas (CMSA, 2013).

The Central Marin Commercial Food-to-Energy Program is a public–private partnership between CMSA and Marin Sanitary Service (MSS). The amount of food waste that could be collected, provided all the 500 food waste generators in the area participate into the program, is estimated to be up to 15 tons per day. The preconsumer commercial food waste is collected and then transferred to the MSS Transfer Station for processing, by uses of hoppers, belts, and magnet, and then transported to CMSA for further treatment (Dow and Garbarino, 2013). On most days, the facility receives more than 15,000 gallons/day (56.7 m³/day) of FOG, mostly from restaurants in the MSS service area, and as of July 2016, the facility recieves approximately 7.5 tons/day of food waste up to 6 days per week.

The FOG and food waste are typically received in the mornings. They are then mixed, ground, and recirculated in a storage tank for a couple of hours. The slurry is then screened, by using a drum screen paddle mixer, to remove materials that are not readily digestible before being fed into the digesters (typically in late afternoons). The food waste slurry is fed to each digester on alternate days to be co-digested with PS and TWAS.

To upgrade for co-digestion of FOG and food waste, CMSA overhauled its 1985 anaerobic digestion system by installing new covers, mixers, biogas purification equipment, and support systems (Creer, 2012). The gas mixing system was replaced with a pump mixing system. The floating cover of each digester was replaced with a two-layer plastic membrane roof top with air in between to regulate the pressure inside the digesters. The covers were replaced to increase available digester volume and to avoid potential difficulties that could arise from using floating covers (Kennedy/Jenks Consultants, 2008). The two digesters at the project site are currently running in a mesophilic, single-stage, and continuous mode (Dow and Garbarino, 2013).

The biogas from the two anaerobic digesters goes through an H₂S removal system and a siloxanes removal system before being fed to the IC engine for power generation. Figure 1 is the process flow diagram of biogas generation, conditioning, and utilization.

Figure 1. Process flow diagram of biogas generation, conditioning, and utilization.

The raw biogas generated from anaerobic digestion is first fed to a recently installed H₂S-removal system (Mi SWACO, Chesterfield, MO). The system consists of two vessels (10 ft [3.05 m] diameter and 353 ft³ [10 m³] packing) filled with a synthetic blend of iron oxide media (SulfaTreat 410 CHP; 4 × 16 mesh) operated in series and in a down-flow mode. The design gas velocity is 3.35 ft/min (1.02 m/min) and the design effluent H₂S concentration is 15 ppmv with design influent H₂S concentration of 400 ppmv (maximum).

The effluent from the H₂S removal system goes through a condenser to reduce moisture content before being fed to two 2,500-lb (1,135-kg) activated carbon adsorbers (Model SAG 48V, Applied Filter Technologies, Snohomish, WA) for siloxanes removal. The two vessels (48 inch [1.2 m] diameter × 72 inch [1.8 m] straight side, 45º bottom cone) operate in parallel and in an up-flow mode, and the design effluent concentration is <100 ppbv of total siloxanes with influent concentrations of 2 to 6 ppmv. CMSA currently budgets for one SulfaTreat media and two siloxanes media bed disposals per year (CMSA, 2014).

The effluent from the siloxanes removal system is fed to the IC engine (Waukesha P48GLD, GE Power & Water, Waukesha, WI) for cogeneration. The historic average of run time on biogas is around 8 hr/day. With co-digestion of food waste and FOG, the runtime on biogas has increased to over 22 hr or longer (Dow, personal communication, 2016).

Experimental approaches

The following experimental approaches were used in this study:

1. To assess production rate and composition of raw biogas from anaerobic codigestion of food waste and municipal wastewater sludge.

2. To evaluate removals of reduced sulfur compounds and siloxanes from raw biogas by the gas conditioning systems and to evaluate the energy content of the biogas as well as the performance and robustness of the conditioning systems.

3. To determine characteristics of emissions from the IC engine and the reliabilities and efficiencies of the system.

Sampling plan and analytical methods

It should be noted that all the biogas and IC engine emission samples collected in this study are for the purpose of research only, and are not intended for use for regulatory compliance. Although samples were collected with care, the sampling approach might not meet all the requirements that are needed for compliance data. Historical data of this facility prior to codigestion were also included in the data analysis when appropriate, so that comparisons could be made.

Biogas production and characteristics

TS and VS of various components of the feed (i.e., PS, TWAS, FOG, and food waste) were sampled and analyzed on a daily basis. Selected samples were also analyzed for chemical oxygen demand (COD). The loading rates of PS, TWAS, FOG, and food waste were recorded and used to calculate the mass loading rates to the digesters. Operational temperature and pH of the digesters were also recorded. TS and VS of the digestate were analyzed on a daily basis to facilitate the determination of VS destruction. Concentrations of ammonia and volatile acids (VA) and alkalinity of the digestate were also determined daily. Analytical methods used were Standard Method 2540G (TS and VS), SM 5220C,D (COD), SM 4500-H⁺ B (pH), EPA 170.1 (temperature), 4500-NH₃ H (ammonia), SM 2320 (alkalinity), and SM 5560 (VA).

Biogas conditioning

A portable biogas analyzer, Gas Data GFM416 (Gas Data Limited, Whitley, Coventry, UK) was acquired for this project. The measurement ranges of the analyzer are CH₄ (0 to 100%), CO₂ (0 to 100%), O₂ (0 to 25%), and H₂S (0 to 5,000 ppmv). Effluent samples of the biogas from the digesters, after the H₂S removal system, and after the siloxanes adsorbers were taken and analyzed for CH₄, CO₂, O₂, and H₂S twice daily by this portable biogas analyzer. The analyzer was routinely calibrated according to manufacturer’s specifications. Selected samples were also analyzed for siloxanes (gas chromatography/flame ionization detection [GC/FID]) and VOCs (TO-15), as well as for EPA Method 3C, “Determination of Carbon Dioxide, Methane, Nitrogen, and Oxygen From Stationary Sources,” with testing by certified laboratories.

Emissions from the IC engine

A portable emission analyzer, ECOM J2KN Pro Industrial OCNX-IR (ECOM America, Ltd., Gainesville, GA), was acquired for this project. The unit is equipped with sensors for CH₄, CO₂, CO, NO₂, NO, SO₂, and O₂. The emissions were surveyed by the portable emission analyzer twice daily (once when the engine was fueled by biogas and the once when the engine was fueled by natural gas). The analyzer was routinely calibrated according to manufacturer’s specifications. The range, accuracy, and resolution of the portable analyzer can be found in ECOM (2016).

Tests on IC engine emissions were also conducted by a Bay Area AQMD-certified source tester (Total Air Analysis, Carson, CA) on two separate days during the study period. For each test run, samples were taken and analyzed for air toxics (i.e., formaldehyde, polycyclic aromatic hydrocarbons [PAHs], polychlorinated dibenzodioxins/furans [PCDD/F] and VOCs) by a certified laboratory, Quantum Analytical Services (Carson, CA). These IC engine emissions tests were conducted when the engine was fueled by the biogas. The analytical methods used were CARB 428 (PCDD/F), CARB 429 (PAHs), TO-14 (VOCs), and EPA 323 (formaldehyde).

Results and discussion

Characteristics of feed and digestate of anaerobic digestion

Characteristics of feed to the anaerobic digesters

The two digesters started to receive FOG in November 2013 and food waste in February 2014. Due to considerable daily fluctuations in quantities and quality of FOG and food waste during the 12-week sampling period (08/08/14–11/07/14), determination of characteristics of feed and digestate on a daily basis would not be meaningful. Instead, analyses here were conducted by using the average values to delineate the trends. The typical hydraulic residence time of anaerobic digestion was 36 days.

Table 2 provides some statistics on characteristics and flow rates of the feed streams to the anaerobic digesters. (The raw data of this research project can be found in Kuo [2015].) As shown, the average TS values for PS, TWAS, FOG, and food waste were 4.4 ± 0.6%, 4.5 ± 0.7%, 3.1 ± 2.1%, and 20.6 ± 3.3%, respectively. The data indicate that the TS concentrations for PS, TWAS, and food waste are relatively consistent, while those for FOG vary considerably. The average percentages of VS in the TS were 84 ± 2%, 83 ± 1%, 91 ± 4%, and 90 ± 3% for PS, TWAS, FOG, and food waste, respectively. As expected, the organic contents of FOG and food waste (91% and 90%) are higher than those of PS and TWAS (84% and 83%). Samples were also taken from the combined feed stream of FOG and food waste; the TS, percentage of VS, and COD were 5.5 ± 5.7%, 88.6 ± 7.2%, and 39,900 ± 28,600 mg/L, respectively. The average TS and VS values are comparable to the corresponding flow-rate weighted average values of the FOG and food waste.

Table 2. Characteristics and flow rates of feed streams to the anaerobic digesters.

	PS			TWAS			FOG			Food Waste
	TS (%)	VS (%)	Rate (gal/day)	TS (%)	VS (%)	Rate (gal/day)	TS (%)	VS (%)	Rate (gal/day)	TS (%)	VS (%)	Rate (ton/day)
Count	59	59	65	59	59	65	60	60	58	70	70	71
Maximum	7.8	88	42,072	5.3	92	26,685	11.0	98	20,500	29.3	94	8.6
Minimum	3.5	80	17,630	3.8	80	8,930	0.4	77	4,000	12.9	72	1.7
Median	4.3	84	28,069	4.6	83	16,016	2.4	92	10,500	20.4	91	3.6
Average	4.4	84	28,588	4.6	83	16,098	3.1	91	11,543	20.6	90	4.1
Std. dev.	0.6	2	4,769	0.7	11	2,967	2.1	4	4,316	3.3	3	1.9

Note. 1 gallon/day = 3.785 L/day; 1 ton/day = 2,000 lb/day = 908 kg/day.

By using the average values of TS, VS, and loading rate of each stream (i.e., PS, TWAS, FOG, and food waste), the corresponding values of the total feed stream to the anaerobic digesters were calculated and are shown in Table 3. The average total flow rate to the digesters was 57,300 gallons/day (217 m³/day), which contained 4.5% TS, and the VS percentage was 85.1%. The average mass loadings of TS and VS were 21,500 and 18,300 lb/day (9,760 and 8,310 kg/day), respectively. The contributions of PS, TWAS, FOG, and food waste to the TS and/or VS in the total feed were 49, 28, 15, and 9%, respectively. FOG and food waste combined represents approximately 25% of TS or VS fed to the digesters. In other words, the addition of FOG and food waste increased the VS loading to the digester by one-third (25%/75%), when compared to that without the addition. This implies the biogas formation would increase at least 33% with codigestion since the VS of FOG and food waste are more readily digestible than that in the municipal wastewater sludge.

Table 3. Characteristics and loading rates of total feed to the anaerobic digesters.

	PS	TWAS	FOG	Food waste	Total daily feed
Loading rate (gallons/day)	28,588	16,098	11,543	1,077	57,305
TS (%)	4.4	4.6	3.1	20.6	4.5
VS (%)	83.7	82.7	91.5	90.4	85.1
TS (lb/day)	10,543	6,155	2,943	1,854	21,495
VS (lb/day)	8,828	5,089	2,692	1,676	18,285
Percent in TS of total feed	49.0	27.8	14.7	9.2	100.0
Percent in VS of total feed	48.3	27.8	14.7	9.2	100.0

Note. 1 gallon/day = 3.785 L/day; 1 lb/day = 0.454 kg/day.

Characteristics of the digestate

Table 4 provides some statistics on characteristics of the digestate. Assuming the flow rates of the feed and the digester effluent are the same, the calculated VS destruction is 11,460 lb (5,180 kg) VS/day and the VS destruction efficiency is 64.9% (lb VS destructed/lb VS applied). The destruction efficiency is in line with the data in the literature, as shown in Table 1 (EBMUD, 2008). The average values of pH, temperature, alkalinity, and NH₄⁺-N were 7.2 ± 0.0, 99.7 ± 0.3ºF (or 37.6 ± 0.2ºC), 4,853 ± 179 mg/L, and 1,137 ± 83 mg/L, respectively. As shown, all these operational parameters are in narrow ranges, and this implies the anaerobic digesters were being operated under stable conditions. The ammonium concentrations were around 1,150 mg/L, and that did not seem to be inhibitory to biological activities, as evidenced by increases in biogas production.

Table 4. Characteristics of the digestate.

	TS (%)	VS (%)	pH	Temperature (°F)	VA (mg/L)	Alkalinity (mg/L)	NH4^+-N (mg/L)
Count	59	59	59	60	60	60	58
Maximum	2.2	70	7.3	100.7	162	5,300	1,322
Minimum	1.9	63	7.2	99.3	90	4,600	1,008
Median	2.0	68	7.2	99.7	122	4,850	1,119
Average	2.0	67	7.2	99.7	124	4,853	1,137
Std. dev.	0.1	2	0.0	0.3	18	179	83

Note. °C = (°F – 32) × (5/9).

Biogas production and conditioning

Production and characteristics of biogas generated

Table 5 provides some statistics on characteristics of the biogas from the digesters. The concentrations of CH₄, CO₂, O₂, and H₂S in the raw biogas were 62.6 ± 0.7%, 36.1 ± 0.7%, 0.0 ± 0.0%, and 127 ± 75 ppmv, respectively. With the average methane concentration of 62.6%, the heating value of the raw biogas is slightly greater than 600 BTU/ft³ (22,340 kJ/m³).

Table 5. Daily biogas production rate and composition.

		Raw biogas			Post H2S adsorbers			Post siloxanes adsorbers
Date	Biogas flow rate (ft^3/day)	CH4 (%)	CO2 (%)	H2S (ppm)	CH4 (%)	CO2 (%)	H2S (ppm)	CH4 (%)	CO2 (%)	H2S (ppm)
Count	52	52	52	52	52	52	52	45	45	45
Maximum	330,079	64.3	37.2	323	64.8	37.5	43	64.8	37.0	0
Minimum	112,098	61.3	33.2	0	61.3	34.4	0	61.9	34.9	0
Median	199,827	62.7	36.4	133	62.8	36.3	0	63.6	36.1	0
Average	212,811	62.6	36.1	127	62.8	36.3	7	63.6	36.0	0
Std. dev.	46,997	0.7	0.7	75	0.7	0.6	13	0.5	0.4	0

Note. 1 ft³/day = 0.0283 m³/day.

The daily biogas extraction rate, not necessarily the biogas generation rate, depended mainly on two operating factors. The first was to keep the level of the dome and the pressure in the dome within the recommended ranges of the manufacturer; the second was to make sure that there was enough gas to run the IC engine during the peak power window, 12 to 6 pm. As shown in Table 5, the daily biogas flow rate was 212,800 ft³/day (6,030 m³/day). The corresponding biogas generation rate is 10.2 ft³ biogas/lb (0.64 m³ biogas/kg) TS applied or 6.4 ft³ CH₄/lb (0.40 m³ CH₄/kg) TS applied, which is within the range of 6 to 8.5 ft³ CH₄/lb (0.37 to 0.53 m³ CH₄/kg) TS applied for food waste in literature, while the corresponding value for the municipal wastewater sludge is 5 ft³ CH₄/lb (0.31 m³ CH₄/kg) TS applied (EBMUD, 2008). On the basis of VS destruction, the biogas generation rate was 18.5 ft³ biogas/lb (1.15 m³ biogas/kg) VS destroyed, or 11.6 ft³ CH₄/lb (0.72 m³ CH₄/kg) VS destroyed. The average H₂S concentration in the raw biogas was 127 ± 75 ppmv.

During the similar period in 2011 (08/18/11 to 11/07/11) when the anaerobic digesters only received PS and TWAS, the average biogas generation rate was 131,800 ± 26,800 ft³ (3,730 ± 760 m³)/day. With co-digestion of FOG, food waste, PS, and TWAS, the daily biogas yield has increased from 131,800 to 212,800 ft³ (3,730 to 6,030 m³)/day, a 61% increase. On a mass loading basis, the biogas yield has increased from 8.75 to 12.1 ft³/lb (0.55 to 0.75 m³/kg) VS entering the digesters, a 38% increase. The fact that the increase in biogas production (61%) is larger than that for the VS loading rate (38%) supports the argument that volatile solids in FOG and food waste are more readily biodegradable than those in the municipal wastewater sludge.

Also shown in Table 5, the average CH₄ concentrations in the raw biogas, effluent from the H₂S adsorbers, and effluent from the siloxanes adsorbers were 62.6 ± 0.7%, 62.8 ± 0.7%, and 63.6 ± 0.5%, respectively. These values are essentially the same, which implies that these two biogas conditioning systems have no, or insignificant, effects on methane concentrations of the biogas. On the other hand, the H₂S concentrations dropped from 127 ± 75 to 7 ± 13 ppmv by the H₂S removal system. The system appeared to meet the design specification. Hydrogen sulfide was not detected in the effluent of the siloxanes adsorbers, plausibly due to the additional removal of H₂S by GAC in the adsorbers.

Grab samples of raw biogas, biogas in the effluent of the H₂S adsorbers, and biogas in the effluent of the siloxanes adsorbers were analyzed for EPA Method 3C, and the data served as a quality assurance and quality control (QA/QC) check for the measurements of the portable biogas analyzer. The results are summarized in Table 6 and the values between the portable biogas analyzer and the certified lab are comparable.

Table 6. Results of EPA Method 3C testing (concentrations in % by volume).

Compound	Raw biogas	Post H2S adsorbers	Post siloxanes adsorbers
H2	<1.3	<1.3	<1.3
O2	<0.1	<0.1	<0.1
N2	0.7	0.6	0.4
CO	<0.1	<0.1	<0.1
CO2	34.9	34.7	34.4
CH4	64.3	64.6	65.1

Performance of the hydrogen sulfide removal system

Grab samples were also analyzed for reduced sulfur compounds. Out of 22 reduced sulfur compounds, H₂S and n-propyl mercaptan were the only two above the detection limit of 0.065 ppmv (Table 7). The concentration of total reduced sulfur compounds as H₂S in the raw biogas was 164 ppmv, and it was reduced to 14.6 ppmv by two H₂S adsorbers. It was further reduced to 10.4 ppmv by the siloxanes adsorbers. This serves as a QA/QC check for the H₂S measurements by the portable biogas analyzer and the results are comparable.

Table 7. Removal of the sulfur compounds in biogas by the conditioning systems (concentrations in ppmv)

Compound	Raw biogas	Post H2S adsorbers	Post siloxanes adsorbers
Hydrogen sulfide	163	14.0	10.0
n-Propyl mercaptan	0.824	0.604	0.367
Total reduced sulfur as H2S	164	14.6	10.4

Performance of the siloxanes removal system

Samples of raw biogas, biogas in the effluent of the H₂S adsorbers, and biogas in the effluent of the siloxanes adsorbers were grabbed on 10/08/14 and 10/29/14 and analyzed by a certified lab for siloxanes: hexamethyldisiloxane (L2), hexamethylcyclotrisiloxane (D3), octamethyltrisiloxane (L3), octamethylcyclotetrasiloxane (D4), decamethyltetrasiloxane (L4), decamethylcyclopentasiloxane (D5), and dodecamethylpentasiloxane (L5).

As shown in Table 8, only D3, D4, and D5 were detected in the raw biogas (the detection limit is 13.0 ppbv). The siloxanes adsorbers reduced the D3 and D5 concentrations to below or close to the detection limit, and the removal of D4 was also 80% or better. However, it should be noted that the concentrations of D3 in the effluent of the H₂S adsorbers were higher than those in the raw biogas on both days (295 vs. <13.0 and 270 vs. 204 ppbv). In addition, the concentrations of L2 in the effluent of the siloxanes adsorbers were higher than those in the effluent of the H₂S adsorbers in both days (84 vs. <13.0 and 63.4 vs. 13.0 ppbv). The causes for the increases were not identified. One plausible reason is that these samples were not taken in a synchronized manner. The total effluent concentrations of siloxanes were 99.1 ppbv on 10/08/14 and 117.9 ppbv on 10/29/14, which are either at or slightly above the design specification of 100 ppbv.

Table 8. Removal of siloxanes in biogas by the conditioning systems (concentrations in ppbv).

	10/8/2014			10/29/2014
	Raw biogas	Post H2S adsorbers	Post siloxanes adsorbers	Raw biogas	Post H2S adsorbers	Post siloxanes adsorbers
Hexamethyldisiloxane (L2)	<13.0	<13.0	84	<13.0	<13.0	63.4
Hexamethylcyclotrisiloxane (D3)	<13.0	295	<13.0	204	270	<13.0
Octamethyltrisiloxane (L3)	<13.0	<13.0	<13.0	<13.0	<13.0	<13.0
Octamethylcyclotetrasiloxane (D4)	161	225	15.1	202	202	41.6
Decamethyltetrasiloxane (L4)	<13.0	<13.0	<13.0	<13.0	<13.0	<13.0
Decamethylcyclopentasiloxane (D5)	1160	570	<13.0	523	334	13.9
Dodecamethylpentasiloxane (L5)	<13.0	<13.0	<13.0	<13.0	<13.0	<13.0

These samples were also analyzed for VOCs using EPA Method TO-15. Out of a long list of compounds analyzed for, only one alkene (propene), four alkanes (cyclohexane, 2,2,4-trimethylpentane, hexane, and heptane), two ketones (2-butanone and 4-methyl-2-pentanone), four aromatics (benzene, toluene, ethylbenzene, and xylenes) and tetrahydrofuran were detected. Table 9 tabulates the concentrations of those compounds detected in the raw biogas, effluent of the H₂S adsorbers, and effluent of the siloxanes adsorbers.

Table 9. Removal of VOCs in biogas by the conditioning systems (concentrations in ppbv).

	10/8/2014			10/29/2014
Compound	Raw biogas	Post H2S adsorber	Post siloxanes adsorber	Raw biogas	Post H2S adsorber	Post siloxanes adsorber
Propene	1,730	1,700	1,590	1,150	1,180	1,140
Cyclohexane	<6.5	<6.5	9.9	<6.5	<6.5	<6.5
2,2,4-Trimethylpentane	<6.5	7.2	84.4	<6.5	<6.5	49.9
Hexane	29.8	29.7	110	17.1	16.4	52.4
Heptane	130	106	238	53.2	54.1	140
2-Butanone	31.2	35.8	39.2	<13.0	<13.0	<13.0
4-Methyl-2-pentanone	<6.5	<6.5	13.7	<6.5	<6.5	10.6
Benzene	20.5	21.4	72.1	17.4	17.9	35.3
Toluene	735	690	1,860	1,430	1,430	2,130
Ethylbenzene	19.4	17.0	55.7	18.4	20.9	41.8
m- and p-Xylenes	18.4	16.0	20.8	14.7	15.7	22.7
o-Xylene	6.7	<6.5	<6.5	<6.5	<6.5	<6.5
Tetrahydrofuran	88.5	<6.5	10.3	<6.5	<6.5	<6.5

IC engine emissions

Historic source test results on IC engine emissions

Since the biogas generated at the site is not sufficient for around-the-clock power generation, the engine is fueled by natural gas when biogas is not available. The IC engine has been source-tested by Bay Area Air Quality Management District (BAAQMD) annually to check for compliance. For each test, the IC engine was fueled by biogas as well as by natural gas. The results from these official source tests provide a valuable opportunity to compare the emissions from the same engine on the same day by using biogas versus natural gas.

Table 10 provides some statistics of the annual source test results from 2008 to 2014 and the IC engine has been in compliance. As shown, the average emissions from natural gas (NG)-fueled and biogas-fueled operations are: NO_x at 15% O₂ (38 ± 19 vs. 37 ± 20 ppm); CH₄ (692 ± 83 vs. 1,065 ± 224 ppm); nonmethane organic carbon (NMOC) as C₁ (45 ± 33 vs. 18 ± 6 ppm); total organic carbon (TOC) as C₁ (735 ± 65 vs. 1,075 ± 227 ppm); CO₂ (7.5 ± 0.5 vs. 12.8 ± 1.6%); CO at 15% O₂ (122 ± 9 vs. 142 ± 14 ppm); O₂ (7.9 ± 0.4 vs. 6.9 ± 0.7%); and SO₂ (4 ± 3 vs. 7 ± 5 ppm).

The NO_x concentrations in the exhausts using NG and biogas are essentially the same (38 vs. 37 ppmv). With regard to emissions of organic compounds, NG-fueled operations emit less CH₄ (692 vs. 1,065 ppmv), but higher NMOC (45 vs. 18 ppmv). The average CO₂ emission from using biogas is higher (12.8 vs. 7.5%), probably due to the higher CO₂ concentrations in the biogas. The average CO concentration from biogas-fueled combustion was higher (142 vs. 122 ppmv). The average SO₂ concentration from biogas-fueled combustion was also higher (7 vs. 4 ppmv), probably due to the presence of reduced sulfur compounds in the biogas.

Daily monitoring of IC engine emissions

Table 11 provides some statistics of the daily IC engine emission data. As shown, the average emissions from NG-fueled and biogas-fueled operations are: NO₂ (30 ± 3 vs. 23 ± 4 ppm); NO (17 ± 4 vs. 10 ± 3 ppm); CH₄ (1,200 ± 170 vs. 1,680 ± 200 ppm); CO₂ (7.2 ± 0.1 vs. 7.7 ± 0.1%); CO (122 ± 9 vs. 142 ± 14 ppm); O₂ (8.0 ± 0.1 vs. 7.2 ± 0.1%); and SO₂ (0 ± 0 vs. 0 ± 0 ppm). The data from daily monitoring are comparable to those of the source tests.

Table 10. Statistics of IC engine emission data using natural gas versus biogas (official source test results from 2008–2014).

	Natural gas					Biogas
	Maximum	Minimum	Median	Average	Std. dev.	Maximum	Minimum	Median	Average	Std. dev.
Output (kW)	650	538	590	601	42	649	550	620	612	30
Flow rate (SCFM)	2,140	1,390	1,720	1,717	227	1,810	1,340	1,590	1,583	176
CO (ppm)	292	246	263	269	17	381	286	329	334	28
CO (ppm), converted to 15% O2	138	106	121	122	9	157	114	142	142	14
CO (gm/Hp-hr)	1.56	0.78	1.14	1.14	0.21	1.58	0.87	1.28	1.29	0.21
NO (ppm)	129	20	29	49	37	118	11	46	55	42
NO2 (ppm)	49	24	33	36	8	43	18	36	33	8
NOx (ppm), converted to 15% O2	77	24	28	38	19	66	13	38	37	20
NOx (g/Hp-hr)	0.93	0.37	0.45	0.55	0.20	1.02	0.21	0.51	0.54	0.30
Methane (ppm)	799	561	686	692	83	1,400	762	1,070	1,065	224
NMOC as C1 (ppm)	90	10	34	45	33	26	10	18	18	6
NMOC as C1 (g/Hp-hr)	0.21	0.02	0.08	0.10	0.07	0.07	0.02	0.04	0.04	0.02
TOC as C1 (ppm)	799	639	760	735	65	1,418	778	1,070	1,075	227
CO2 (%)	8.3	6.8	7.3	7.5	0.5	15.1	10.7	12.7	12.8	1.6
Oxygen (%)	8.5	7.2	8	7.9	0.4	8.2	6	6.8	6.9	0.7
SO2 (ppm)	11	2	2	4	3	16	2	6	7	5
SO2 (g/Hp-hr)	0.08	0.01	0.02	0.03	0.03	0.15	0.02	0.05	0.06	0.04
Estimated heat input (MMBTU/d)						191	191	191	191	0
Heating value of biogas (BTU/scf)						634	424	560	542	65

Note. 1 CFM = 0.0283 m³/min; 1 g/Hp-hr = = 1.34 g/kWh; 1 BTU/day = 1,055 J/day; 1 BTU/ft³ = 37.24 kJ/m³.

Table 11. Daily IC engine emission data.

	Natural gas							Biogas
	CH4 (%)	CO2 (%)	CO (ppm)	NO2 (ppm)	NO (ppm)	SO2 (ppm)	O2 (%)	CH4 (%)	CO2 (%)	CO (ppm)	NO2 (ppm)	NO (ppm)	SO2 (ppm)	O2 (%)
Count	38	38	38	38	38	38	38	54	54	54	54	54	54	54
Maximum	0.195	7.7	378	37	28	0	8.2	0.214	7.8	385	30	18	0	7.3
Minimum	0.092	7.1	224	21	10	0	7.2	0.127	7.6	160	17	4	0	7.0
Median	0.119	7.2	312.5	30	17	0	8.0	0.170	7.7	367	22.5	10.5	0	7.2
Average	0.120	7.2	312	30	17	0	8.0	0.168	7.7	360	23	10	0	7.2
Std. dev.	0.017	0.1	19	3	4	0	0.1	0.020	0.0	36	4	3	0	0.1

Toxics testing on the IC engine emissions

Tests on IC engine emissions, when the engine was fueled by biogas, were also conducted by a BAAQMD-certified source tester on 10/29/14 and 10/30/14.

Table 12 tabulates the emission test results from the source tester and the data are similar to those of the official source tests in the past 7 years (Table 10). The concentrations of CH₄, CO₂, CO, NO_x, and O₂ are also similar to those measured by the portable emission analyzer (Table 11). If the stringent 2016 regulations in the jurisdiction area of South Coast Air Quality Management District (SCAQMD) in southern California were enforced on this IC engine, the CO concentration (at 15% O₂) of 157 ppmv would be less than the 250 ppmv standard; however, the NO_x concentration (at 15% O₂) would be higher than the 11 ppmv standard (SCAQMD, 2012). It should be noted that the NO_x concentratons from this IC engine, using natural gas, are also higher than 11 ppmv, as indicated by the official source test results from 2008 to 2014 (see Table 10).

Table 12. IC engine emissions from tests conducted by the source tester.

	10/29/2014	10/30/2014
Output (kW)	613	615
Measured flow rate (SDCFM)	1,886	1,898
Calculated flow rate (SDCFM)	1,972	2,190
CO (ppm)	362	361
CO (ppm), converted to 15% O2	157	157
CO (g/Hp-hr)	1.63	1.81
NOx (ppm)	54.8	51.4
NOx (ppm), converted to 15% O2	23.8	22.3
NOx (g/Hp-hr)	0.33	0.35
Methane (ppm)	1,484	1,708
Methane (ppm), converted to 15% O2	643	741
TOC as C1 (ppm)	1,484	1,708
CO2 (%)	11.3	11.3
O2 (%)	7.23	7.24

Note. 1 CFM = 0.0283 m³/min; 1 g/Hp-hr = = 1.34 g/kWh.

With regard to air toxics, the formaldehyde concentrations were 31.7 ppbv (10/29/14) and 32.6 ppbv (10/30/14) in the IC engine exhausts. For the VOC analysis (EPA Method TO-14), two aromatics (benzene and toluene) and 10 halogenated organic compounds (1,1,1-trichloroethane [TCA], 1,2-dichloropropane, bromomethane, carbon tetrachloride, chloroethane, chloroform, cis-1,2-dichloroethylene [DCE], trichloroethylene [TCE], trichlorotrifluoroethane, and vinyl chloride) were detected (see Table 13). If the stringent 2016 SCAQMD regulations were enforced on this IC engine, the total VOC concentrations, which are at the level of a couple of parts per million by volume or less, would be less than the 30 ppmv standard (SCAQMD, 2012).

Table 13. VOCs in the IC engine exhausts from tests conducted by the source tester (concentrations in ppbv).

	10/29/2014	10/30/2014
Benzene	33	30
Toluene	12	11
1,1,1-Trichloroethane	0.1	0.1
1,2-Dichloropropane	1.4	0.9
Bromomethane	648	1,027
Carbon tetrachloride	0.1	0.1
Chloroethane	111	102
Chloroform	22	16
cis-1,2-Dichloroethene	17	23
Trichloroethene	0.2	0.4
Trichlorotrifluoroethane	0.5	<0.3
Vinyl chloride	167	604

From using CARB Method 428, octachlorodibenzodioxane (OCDD) was the only single PCDD/F species detected. In one of the two sampling events the masses of total heptachlorodibenzodioxine (HpCDD), tetrachlorodibenzofuran (TCDF), pentachlorodibenzofuran (PeCDF), and hexachlorodibenzofuran (HxCDF) were also above the detection limits. Table 14 tabulates the masses of the detected compounds in the collected samples and the corresponding concentrations (mg/dry standard cubic meter), calculated using the mass and the total sample volume (80.11 and 81.49 ft³ [2.27 and 2.31 m³] for 10/29/14 and 10/30/14, respectively). As shown, the concentrations are relatively low, at 1.95 × 10⁻⁸ mg/dscm or less. (In addition to concentrations, the discharge limits of these compounds from an IC engine typically depend on discharge flow rate, stack configuration, meterological conditions, and population density and distribution of affected communities. No specific discharge limits on these compounds were found from a literature search.)

Table 14. Dioxins/furans in the IC engine exhausts from tests conducted by the source tester.

	10/29/2014		10/30/2014
	pg/sample	mg/dscm	pg/sample	mg/dscm
OCDD	31.2	1.38E-08	11.7	5.07E-09
Total HpCDD	7.77	3.42E-09	<9.55	<4.14E-09
Total TCDF	44.2	1.95E-08	<8.72	<3.78E-09
Total PeCDF	39.1	1.72E-08	<7.98	<3.46E-09
Total HxCDF	7.65	3.37E-09	<3.95	<1.71E-09

The exhausts were also collected and analyzed for PAHs using CARB Method 429. Table 15 tabulates the masses of the detected compounds in the collected samples and the corresponding concentrations. Ten PAH species were detected in the IC engine exhausts and naphthalene has the highest average concentration, at 6.1 × 10⁻³ mg/dscm (no specific discharge limits on these compounds were found from a literature search).

Table 15. PAHs in the IC engine exhausts from tests conducted by the source tester.

	10/29/2014		10/30/2014
	ng/sample	mg/dscm	ng/sample	mg/dscm
Naphthalene	13,300	5.86E-03	14,600	6.33E-03
2-Methylnaphthalene	8,240	3.63E-03	11,100	4.81E-03
Acenaphthylene	773	3.41E-04	815	3.53E-04
Acenaphthene	357	1.57E-04	458	1.98E-04
Fluorene	698	3.08E-04	352	1.53E-04
Phenanthrene	2,470	1.09E-03	1,750	7.58E-04
Anthracene	128	5.64E-05	<20	8.67E-06
Fluoranthene	246	1.08E-04	142	6.15E-05
Pyrene	193	8.51E-05	108	4.68E-05
Chrysene	85.1	3.75E-05	47.7	2.07E-05

Two biogas samples (one from each testing day) were also analyzed for higher heating values (HHV) using ASTM 1945-03. The gross heating value is 639 BTU/ft³ (23,800 kJ/m³) and the net heating value is 575 BTU/ft³ (21,400 kJ/m³) on average.

Conclusion

The findings from this study include the following:

1. With FOG and food waste making up approximately 25% of TS or VS loading to the anaerobic digesters, the digesters are being operated under stable conditions.

2. The biogas production rate is 18.5 ft³ biogas/lb (1.15 m³ biogas/kg) VS destroyed. With 33% percent more VS loading from FOG and food waste, the daily biogas production is 60% more.

3. H₂S is the dominant reduced sulfur compound in the raw biogas, while n-propyl mercaptan is another one detected. The on-site treatment system is capable of reducing the H₂S concentration below 15 ppmv.

4. With regards to siloxanes, only D3, D4, and D5 were detected in the raw biogas. The on-site activated carbon adsorbers reduced the D3 and D5 concentrations to below or close to the detection limit, and the removal of D4 was 80% or better.

5. In the raw biogas samples, only one alkene, four alkanes, two ketones, four aromatics, and tetrahydrofuran were detected using EPA Method TO-15 analysis. With regard to air toxics in the IC engine exhausts, formaldehyde concentration was 32 ppbv. Two aromatics and 10 halogenated organic compounds were detected using EPA Method TO-14.

6. The IC engine at the site used both NG and biogas on a daily basis. The NO_x concentrations (at15% O₂) in the exhausts using NG and biogas were essentially the same (38 vs. 37 ppmv). NG-fueled operations emitted less CH₄ (692 vs. 1,065 ppmv), but higher nonmethane organic carbon (NMOC) (45 vs. 18 ppmv). Both the average CO₂ and CO (at 15% O₂) concentrations from using biogas were higher than those using NG (12.8 vs. 7.5% for CO₂ and 142 vs. 122 ppmv for CO, respectively). The average SO₂ concentration from biogas-fueled combustion was also higher than that using NG (7 vs. 4 ppmv).

7. OCDD was the only single PCDD/F species detected in the IC engine exhausts, at an average concentration of 9.4 × 10⁻⁹ mg/dscm.

8. Ten PAH species were detected in the IC engine exhausts with naphthalene having the highest concentration, at 6.1 × 10⁻³ mg/dscm.

9. The IC engine fueled by biogas can meet stringent emission limits for CO at 15% O₂ (250 ppmv) and VOCs (30 ppmv). However, additional emission control may be needed to meet the low NO_x (at 15% O₂) limit of 11 ppmv.

Acknowledgment

The research team expresses its gratitude toward the commission contract managers, Marla Mueller and Simone Brant, and Guido Franco and Yu Hou of the California Energy Commission (CEC), as well as Robert Dole (recently retired) and many other staff members of the CMSA for providing great guidance and assisting the team in many ways. The research team also expresses gratitude toward the Technical Advisory Committee (TAC) members for providing valuable technical input and suggestions for this project. The TAC members are (in alphabetical order) Rizaldo Aldas (CEC), Chris Berch (Inland Empire Utility Agency), Ryann Bonner (Environ Strategy), Robert Cole (CMSA), Jason Dow (CMSA), Steven Fan (City of Los Angles), Jacques Franco (CalRecyle), Robert Gilles (San Joaquin Valley APCD), Ken Kumar (Energy Environmental Solutions), Tung Le (CARB), Angus MacPherson (CARB), Gary O’Neill (CEC), Lisa Van de Water (San Joaquin Valley APCD), and Robert Williams (UC Davis).

Funding

This project was funded by the California Energy Commission under the Public Interest Energy Research (PIER) Program (Agreement Number 500-11-030).

Additional information

Funding

This project was funded by the California Energy Commission under the Public Interest Energy Research (PIER) Program (Agreement Number 500-11-030).

Notes on contributors

Jeff Kuo

Jeff Kuo is a professor at the Department of Civil and Environmental Engineering, California State University, Fullerton (CSUF).

Jason Dow

Jason Dow is the General Manager of Central Marin Sanitation Agency (CMSA), San Rafael, CA.