Conversion Glossary

Alternative daily coverBoard-approved materials other than soil used as a temporary overlay on an exposed landfill face. Generally, these materials must be processed so that they do not allow gaps in the face surface, which would provide breeding grounds for insects
and vermin.
Public Resources Code section 41781.3
stipulates this practice is recycling, not disposal, and authorizes the Board to adopt regulations, such as
Title 27
California Code of Regulations, section 20690
. Approved materials include processed green materials, sludge, ash and kiln residue, compost, construction and demolition debris, and special foams and fabrics. Also see

Daily/Intermediate Cover and Alternative
Daily/Intermediate Cover Guidelines
A dedicated unit process for controlling the anaerobic decomposition of organic material. Typically consists of one or more enclosed, temperature controlled tanks with material handling equipment designed to prevent
the introduction of oxygen from the atmosphere.
Biochemical oxygen demandBiochemical oxygen demand is the amount of oxygen required for complete (aerobic) biological decomposition of a material. The standard laboratory method (BOD5) tests the amount of dissolved oxygen consumed in a closed aqueous system over a
five-day period. It is a fairly direct, but time-consuming measure of biodegradability of liquid streams.
Biomixer A rotating drum often with a trommel screen used for size reduction and pretreatment of the organic fraction in mixed municipal solid waste (MSW) for sorting. Can be aerated to encourage biological breakdown. Can be operated at retention times
from several hours to several days.
Bioreactor landfillA landfill operated as a bioreactor using leachate recycling (or other management schemes) to increase the rate of organic decomposition and biogas production. Not to be confused with
anaerobic digester
CombustionA rapid conversion of chemical energy into thermal energy. The reaction is exothermic.
matter is oxidized with sufficient air (or oxygen) for reactions to go to completion. The carbon and hydrogen are oxidized to carbon dioxide and water, respectively.
CompostRefers to stabilized and screened
material ready for horticultural or agricultural use. If anaerobically digested material is used as compost, it must be biologically stabilized, typically through aeration and maturation.
DigestionEither in the presence of oxygen (aerobic) or in an oxygen-depleted atmosphere (anaerobic), digestion is the process in which microbes digest biogenic carbonaceous materials and emit any number of energetic, inert gases and liquids.
Extended Producer Responsibility (EPR)EPR as it relates to solid waste management refers to responsibility placed on waste producers for disposal or mitigation of their own waste. "Producer pays" or "take-back" laws are examples of implementing EPR.
FermentationConversion of substrates, such as sugar, by microorganisms, such as yeast, bacteria, and fungi, in the absence of oxygen, into products, usually ethanol with carbon dioxide by-product and some increase in cell mass.
Fischer-TropschFischer-Tropsch synthesis is a process for producing mainly straight-chain paraffinic hydrocarbons from a
synthesis gas having the correct mixture of CO and H2. Catalysts are usually employed. Typical operating conditions for FT synthesis are temperatures of 390-660 °F and pressures of 15-40 atmospheres depending
on the desired products. The product range includes the light hydrocarbons methane (CH4) and ethane (C2), LPG (C3-C4), gasoline (C5-C12), diesel (C13-C22), and waxes (>C23). The distribution of the products depends on the catalyst and
the process conditions (temperature, pressure, and residence time). The synthesis gas should have low tar and particulate matter content to avoid progressive contamination of the catalysts. Biomass derived synthesis gas for FT liquid production
is pre-commercial. However, it may be more easily commercialized than coal since it has smaller quantities of contaminants to remove in the synthesis gas cleaning process.
GasificationProduction of energetic gases from solid or liquid
organic feedstocks usually by partial oxidation. Primary energetic gases produced are hydrogen, carbon monoxide, and methane, along with an inorganic ash residue.
Gray wasteThe material left over after separation of recyclables and putrescible material from the mixed waste stream. Composed mostly of inorganic material, gray waste usually contains a significant amount of organic material. Depending on its composition,
gray waste can be treated biologically or burned prior to final disposal.
Hydraulic retention time
The average length of time liquids and soluble compounds remain in a reactor. Increasing the HRT allows more contact time between substrate and bacteria but requires slower feeding and/or larger reactor volume.
HydrogasificationGasification using hydrogen gas to react with the carbon in organic materials to produce a methane rich gas effluent, and provide heat for the process. Any pyrolytic products present are usually converted into methane.
is often used as a precursor process that can enhance the hydrogen reaction kinetics, despite the presence of water in the feed. Since oxygen is not intentionally introduced, carbon oxides are reduced and methane increased as the hydrogen
pressure is increased. Toxic hydrocarbons, like furans and dioxins, are chemically reduced by hydrogasification to less hazardous chemical compounds.
HydrolysisA chemical or biological process in which water is added to other molecules (the conditions are wide ranging and many molecules can be hydrolyzed). Hydrolysis is a pretreatment or preliminary step in
fermentation processes that ultimately yield biogas or ethanol. For cellulose and hemicellulose, a variety of hydrolysis methods can be used to break down the long chain polymer into simple glucose molecules.
Efficiencies of hydrolysis vary among methods and feedstocks.
IncinerationA generic term in the industry that connotes any process that combusts waste. Literally, "to make into ash."
Mass Burn IncinerationCombustion of solid wastes without sorting or significant pre-treatment. The whole solid waste “mass” if used as feedstock. Heat and power may or may not be recovered.
Materials recovery facility (MRF)A facility where mixed MSW is sorted in order to recover material for reuse or recycling. In California, the “post MRF fraction” is typically landfilled.
Mechanical-biological treatment A waste processing system that combines a sorting facility for materials recovery (the mechanical portion) with biological treatment, either aerobic or anaerobic, for stabilizing the organic fraction before landfilling.
Mechanically separated organic fraction of
municipal solid waste (OFMSW)
Organic material separated from the mixed waste stream by mechanical means (i.e., trommels, screens, shredders, magnets, density dependent mechanisms). Isolating the OFMSW from mixed waste is less effective using mechanical
separation as compared with source separation.
Municipal solid waste
MSW includes all of the solid wastes that are generated from residential (homes and apartments) sources, commercial and business establishments, institutional facilities, construction and demolition activities, municipal services, and treatment
plant sites. Hazardous wastes are generally not considered MSW. Some regions or countries consider only residential solid waste as MSW.
Organic fraction of municipal solid waste
The biogenic fraction of MSW. OFMSW can be removed from the waste stream at the source (source separation), or downstream by mechanical separation, picking lines a combination of the two. The wood and paper fraction is more
recalcitrant to biological degradation and is therefore not desired for biochemical conversion feedstocks.
Organic Material containing
carbon and hydrogen.
Organic material in MSW includes the biomass components of the waste stream as well as hydrocarbons usually derived from fossil sources (e.g., most plastics, polymers, the majority of waste tire components, and petroleum
PretreatmentIn reference to MSW, pretreatment can refer to any process used to treat the raw MSW stream before disposal. This includes separation, drying, comminuting,
, biological treatment, heating and others.
PyrolysisA thermal degradation of carbonaceous material in an oxygen-free reactor. Pyrolytic oils, fuel gas, chars, and ash are produced in quantities that are highly dependent on temperature, residence time, and the amount of heat applied.
Rankine cycleThe Rankine vapor power cycle is the most widely used thermal cycle for electrical power generation throughout the world. It is commonly called a "steam cycle" when the working fluid is water. It consists of a boiler where heat is added to
liquid phase pressurized working fluid (water) to create a high temperature and pressurized vapor (steam if the working fluid is water). The high-pressure steam is expanded through a turbine, which turns a generator creating electrical
power. The low-pressure steam coming out of the turbine is condensed to liquid by cooling after which the pressure of the relatively low temperature liquid is raised by a boiler feed pump or pumps to repeat the cycle. Rankine cycle efficiencies
depend on plant size, fuel, and design and typically vary from about 10 percent for very small (500 MWe) supercritical units. Typical solid-fueled biomass and waste
fired power plants (~10-100 MWe) have net efficiencies of about 17-25 percent.
Solids retention time (SRT)The average length of time solid material remains in a reactor. SRT and HRT are equal for complete mix and plug flow reactors. Some two-stage reactor concepts and Upflow Anaerobic Sludge Blanket (UASB) reactors decouple
HRT from the SRT allowing the solids to have longer contact time with microbes while maintaining smaller reactor volume and higher throughput.
Source-separated OFMSW (SS-OFMSW) Organic solid waste separated at the source (i.e., not mixed in with the other solid wastes). Often comes from municipal curbside recycling programs in which yard waste and sometimes kitchen scraps are collected separately
from the rest of the
stream. The precise composition of SS-OFMSW can change significantly depending on the collection scheme used.
Steam reformingAn endothermic chemical conversion process used to make hydrogen and carbon monoxide from superheat steam and hydrocarbon gases, such as methane using a nickel based catalyst.
Synthesis gasA mixture of carbon dioxide, carbon monoxide and hydrogen gas formed via
for the express purpose of synthesizing products.
Total solids (TS)The amount of solid material (or dry matter) remaining after removing moisture from a sample. Usually expressed as a percentage of the as-received or wet weight. Moisture content plus TS (both expressed as percentage of wet weight) equals
100 percent.
Ultimate methane potential A standard laboratory technique used to measure the anaerobic biodegradability and associated methane yield from a given substrate. The test is run until no further gas production is detected and can last up to 100 days. The results can be
influenced by the substrate concentration and particle size, the inoculum source, the food to microorganism ratio, and the presence or build-up of inhibitory compounds among others. (Also known as ultimate biomethane potential, BMP, and
Volatile solids (VS) Volatile solids (VS) amount is used as an indicator or proxy for the biodegradability of a material, though recalcitrant biomass (i.e., lignin) which is part of the VS does not degrade anaerobically. VS is the amount of combustible material
in a sample. It is determined by an analytical method called “loss on ignition” which is the amount of matter that is volatilized and burned from a sample exposed to air at 550 ºC for 2 hours. The organic (carbon containing
matter) is lost and the remaining matter is the mineral or ash component of the original sample. VS is usually reported as percent of
total solids
(TS), where TS are the sum of the VS and ash components.

Additionally, a glossary of Biomass Terms is maintained by the National Renewable Energy Laboratory (NREL).

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For more information contact: Organic Materials,