Definition

Bioconversion is defined as the conversion of organic waste(s) into a methane energy source by a fermentation process involving living organisms.  This process is generically known as anaerobic digestion (AD).  AD is a naturally-occurring process commonly utilized as a pollution control means in municipal sewage treatment and livestock waste handling.  McElvaney Associates Corporation applies its BioConverter technology (an extension of traditional AD) to these and other organic waste streams which might not otherwise be treated or "treatable".  Some examples of these other types of waste are:  pre- and post-consumer food waste, green waste (cut grass, shrub and tree trimmings, etc.), waste paper (magazines and junk mail, mixed residential, etc.), FOG (Fats, Oils, and Grease), and "high-strength" wastewaters.

Feedstock Preparation

Input wastes are combined with additional liquid in a mixing device to form a slurry.  The resulting feedstock is generally limited to approximately 10% TS, due to processing equipment considerations.  Once the "batch" is complete, it is loaded into the BioConverter(s) with a pump, passing through a trash removal device for contaminant removal.

BioConversion

The bioconversion (gasification) of the feedstock is performed in the BioConverter(s).  This process is affected by several distinct groups of bacteria working in concert.  The first group, hydrolytic bacteria, break down organic compounds to fermentation products, such as organic acids, alcohols, and carbon dioxide (CO₂).  The second group, transitional bacteria (acetogenic, homoacetogenic), convert the products of the first group to acetate, hydrogen, and CO₂.  These are the products which are actually converted to methane (CH₄) and CO₂ by the third group, methanogenic bacteria.  Each group relies on the next to consume it's products, which prohibits inhibition that occurs when excess concentrations of these compounds are allowed to develop.

Output Processing

• Gas

  BioGas as produced contains approximately 65% CH₄, 34% CO₂, 0.5% hydrogen sulfide (H₂S).  The gas is "scrubbed" to remove the H₂S.  With equipment modification, the clean BioGas may be utilized in engine generators, cooking and/or refrigeration appliances, gas burners, etc.  Clean BioGas may also be further processed into vehicle fuel (CNG, replacing gasoline or diesel) by removing the CO₂ with a membrane separation system.

   

• Solids

  Since BioConversion is never 100% efficient, non-converted and non-convertible solids (VS and NVS) remain in the effluent from the system, requiring further processing.  Effluent is pumped over a vibrating screen to remove "coarse" solids.  These solids are placed in a dryer for pasteurization and moisture adjustment.  The result is BioSoil, an organic soil amendment which is packaged in bags for sale.

   

• Liquid

  The screened effluent is further processed with an ultrafilter.  This device separates bacterial solids from liquid and dissolved solids.  A portion of the concentrated bacterial solids is packaged in bottles for sale.  The "filtrate" is stored and shipped in bulk to commercial users.

Operating Parameter(s) Calculation

The first step is to define the feedstock.  Analyses are conducted to determine TS, VS, Nitrogen, Sulfur, Phosphorous, Iron, Cobalt, Nickel, Molybdenum, and Selenium.  Once TS is known, the batch size and system volume can be established.  Batch TS% is generally kept around 10%, due to processing equipment limitations.

• Here's an example using 1000 kg of Food Waste:

   

  Batch Size/System Volume

  Total Weight of Material	=	1000	kg
  Food Waste TS%	X	10	%
  TS in Food Waste	=	100	kg
  TS% of Batch	/	8	%
  Batch Size	=	1250	kg or liter
  HRT of System	X	30	days
  Required System Volume	=	37500	liter

  ^

• The Organic Loading Rate is determined using TS, VS%, and System Volume, as follows:

   

  Organic Loading Rate

  TS in Food Waste	=	100	kg
  VS% of TS	X	90	%
  VS in Food Waste	=	90	kg
  System Volume	X	37.5	m3
  Organic Loading Rate	=	2.4	kg VS/m3-day

   

• Methane Yield can be calculated after performing a batch fermentation in the lab to determine feedstock digestibility and produced gas quality (density):

   

  Methane Yield

  VS in Food Waste	=	90	kg
  Digestion Eff.(digestibility)	X	80	%
  VS Converted	=	72	kg
  Gas Density(@ 65% CH4)	X	1.14	kg/m3
  Total Gas	=	63.2	m3
  Methane(CH4)%	X	65	%
  Total Methane	=	41.1	m3 CH4
  VS in Food Waste	/	90	kg
  Methane Yield	=	.46	m3 CH4/kg VS added

   

• Methane Production Rate is determined using Total Methane and System Volume:

   

  Methane Production Rate

  Total Methane	=	41.1	m3 CH4
  System Volume	/	37.5	m3
  Methane Production Rate	=	1.1	m3/m3-day

  ^

Nutrient Requirements

In addition to the energy (in the form of carbon [C]) in the VS, the bacteria require certain nutrients to be able to bioconvert that energy.  These nutrients are successively limiting.  That is, enough of one nutrient must be available in excess for a subsequent nutrient to provide any improvement in process efficiency.

• Nitrogen requirement is listed as 1 kg N/60 m³ CH₄ or 6 kg N/1000 kg COD.  It is six times greater for carbohydrate digestion than for proteins and fatty acids.  For 1000 kg of Food Waste (41.1 m³ CH₄), the Nitrogen requirement is:

   

  Required Nitrogen

  Total Methane	=	41.1	m3 CH4
  1 kg Nitrogen	/	60	m3 CH4
  Required Nitrogen	=	.69	kg N

   

• The amount of Sulfur required is related to Methane Production Rate.  It is essentially equal to the Nitrogen requirement.  For optimum bioconversion, the head gas should contain 0.5% H₂S, or approx. 23 mg/l dissolved Sulfide.
• The Phosphorous requirement is approx. 15% of the Nitrogen requirement:

   

  Required Phosphorous

  Required Nitrogen	=	.69	kg N
  Phosphorous % of N	X	15	%
  Required Phosphorous	=	.10	kg P

   

• The amounts of Iron, Cobalt, Nickel, Molybdenum, and Selenium required are 10 mg/l Fe, 5 mg/l Co, and 0.1mg/l of Ni, Mo, Se:

   

  Required Iron

  Batch Size	=	1250	kg or liter
  Iron	X	10	mg/l Fe
  Required Iron	=	.0125	kg Fe

   

  Required Cobalt

  Batch Size	=	1250	kg or liter
  Cobalt	X	5	mg/l Co
  Required Cobalt	=	6250	ppm Co

   

  Required Nickel, Molybdenum, Selenium

  Batch Size	=	1250	kg or liter
  Nickel, Molybdenum, Selenium	X	.1	mg/l Ni, Mo, Se
  Required Ni, Mo, Se	=	125	ppm Ni, Mo, Se

   

Common Terms

There are several terms which are used to describe any bioconversion process and it's parameters:

• Total Solids (TS)

  All organic matter contains some water.  The human body is approximately 70% water.  Total Solids (TS) is a measure of the actual solid content of a substance.  Only portions of the solid material are actually bioconverted.  TS is determined by weighing a sample, oven-drying it to remove all moisture, and then re-weighing the dried sample.  TS% is determined by dividing the "dry" weight by the "wet" weight.  The same human body is therefore 30% TS.

   

• Volatile Solids (VS)

  Volatile Solids (VS) is a measure of the solids (portion of TS) which are actually available for bioconversion.  VS is determined by "burning" the dried TS sample, which removes the "volatile" component.  What remains is non-volatile (see NVS below).  The sample is weighed again to determine this "ash" weight, which is subtracted from TS to determine VS.  VS% is found by dividing VS by TS.

   

• Non-Volatile Solids (NVS)

  Non-Volatile Solids (NVS) is what remains in a sample after removing the VS in a furnace.  NVS (mostly minerals in ash form) are not bioconvertible.  NVS% is determined by dividing NVS by TS.

   

• Hydraulic, Solids, Microorganism Retention Time(s) (HRT, SRT, MRT)

  Retention Time(s) refers to how long a given material is kept (retained) in the system.  The units are days.  Hydraulic Retention TIme (HRT) measures the length of time that liquid remains in the system.  HRT is determined by dividing system volume by feedstock volume.  Solids Retention Time (SRT) is the length of time that feedstock solids remain in the system.  An Upflow Solids Reactor (USR) retains the solids longer than the liquid (SRT>HRT).  Microorganism Retention Time (MRT) is the length of time that the anaerobic bacteria (microorganisms) remain in the system.  Longer MRT's, which can be achieved by using a growth matrix, promote increased system stability while simultaneously reducing nutrient requirements (see below).

   

• Organic Loading Rate (kg VS/m³-day)

  Organic Loading Rate is a measure of the organic material (VS), per Bioconverter volume, added to the system on a daily basis.  The units are kg VS/m³-day.  The value is determined during engineering.  For a given system size, higher organic loading rates generally result in lower bioconversion efficiency.  Any value greater than 3.3 kg VS/m³-day is considered high-rate bioconversion.

   

• Methane Yield (m³ CH₄ / kg VS added)

  Methane Yield is a measure of the quantity of methane produced from the VS which are added to the system.  The units are m³ CH₄ / kg VS added.  The value is dependent upon the type and digestibility of the feedstock and the retention time in the system.  It is also affected by the condition of the fermentation (raw gas quality).  1 kg VS 100% bioconverted into 100% methane would yield 1.4 m³.  More typically, 1 kg VS is 70% bioconverted into 65% methane, yielding 0.4 m³.

   

• Methane Production Rate (m³/m³-day)

  Methane Production Rate is a measure of the quantity of methane, per BioConverter volume, generated by the system on a daily basis.  The units are m³/m³-day.  A value of 1 m³/m³-day is reasonable.  Methane production rates are proportional to the sulfur required for bioconversion, because more H₂S is carried away during vigorous gassing.

   

• Volatile Acids Concentration

  Volatile acids are measured to determine the equivalent buffering capacity which may be needed for bioconversion to proceed. The relative concentration of volatile acids affects the overall pH. If the volatile acids concentration exceeds the ability of the bicarbonate alkalinity to maintain the pH above 6.5, then the fermentation turns acid and methane formation ceases.

   

• Bicarbonate Alkalinity (CaCO₃, mg/l)

  Bicarbonate Alkalinity is a parameter which provides an estimate of the buffering capacity of a fermentation. The units are mg/liter, expressed as CaCO₃. Bicarbonate alkalinity is usually derived from the solubilization of carbon dioxide, which results from the bioconversion of organic wastes.  During bioconversion, acids are formed as intermediary compounds. To the degree sufficient bicarbonate alkalinity is present, high loading rates of solids to the BioConverter can occur without the need to make pH adjustments.

   

• Chemical Oxygen Demand (COD, mg/l)

  Chemical Oxygen Demand (COD) is a parameter which provides an estimate of the quantity of organic material in a sample.  The units are mg/l.  The value returned is dependent upon the sample being tested.  Samples of feedstock may measure 100,000+ mg/l, while filtrate samples are generally around 2000 mg/l.  The test itself is an EPA-approved method which provides faster, more repeatable results than the more common Biological Oxygen Demand (BOD) test.