Anaerobic Digestion

Authors:

This system is also called: AD, Digestion,

Principles of Operation

Anaerobic digestion is the conversion of organic matter to biogas which is flammable if the methane content of the biogas is high enough. Bacteria and Archea that perform the individual stages of anaerobic digestion are naturally present in the feces and organic matter that enter the digester or can be introduced from an already functioning digester. The conditions within the digester can determine which organisms proliferate.

The subprocesses of anaerobic digestion are degredation, hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Of these, hydrolysis is often the rate limiting step followed by methanogenesis. Typically all subprocesses are carried out concurrently in a single vessel, however large scale facilities are increasingly splitting up the processes in order to provide conditions optimal for each sub-process. For example it is advantageous to split up the acid forming phases (called acid-phase digestion) from the methanogenesis phase because it allows for an overall reduction in reactor volume needed although it increases complexity (See Modes of Operation).

Modes of Operation

Three different temperature ranges are ideal for anaerobic digestion. Each temperature range selects for a different set of organisms that carry out the process. These temperatures represent local optimums, however biogas can be successfully produced outside these ranges and the rates of organic matter degredation and biogas production generally increase with increasing temperature.

Psycrophilic digestion 15°C (59°F)
Mesophilic Digestion 30-38°C (85-100°F)
Thermophilic digestion 50 to 57°C (122-135°F)

Psyochrophilic and mesophilic digestion are commonly used at the building scale level, while mesophilic and thermophilic is typically operated at large scale facilities.

Operating Requirements and Conditions

Condition	Optimum			Operating Range	Unit	Notes
	Psycrophilic	Mesophilic	Thermophilic
Temperature	15	30-38	50-57	15-57	°C	NA
pH	7			-	-	-

Maintenance and Repair

Maintenance Schedule

Times are based on Mesophilic Digestion

Frequency	Action	Who Performs?	Time to Complete
Residence Time of 28 days	Remove Liquid	Operator	N/A
Residence Time of 28 days	Remove Solids	Operator	N/A

Failures and Solutions

Problem	Symptom (s)	Fundamental Cause	Solution
low gas production	low pH	Neutral volatile fatty acids or lactic acid penetrate the cell walls of methanogens and inactivate the organisms	Reduce inputs, especially carbohydrates, until symptoms relieve, or dilute with water, or add buffers like CaCO3
low gas production	high ammonia	Ammonia can penetrate the cell walls of methanogens and inactivate the organisms	Reduce inputs until symptoms relieve, dilute with water, or add buffers like CaCO3
non-flammable gas	high moisture in gas	Moisture is evaporated during burning and can reduce the temperature below the level that a flame can be sustained.	Add a cooling trap where moisture can condense in the gas line
non-flammable gas	high CO2 in gas	Biogas must be above ?% methane for combustion. Inadvertent introduction of oxygen can increase CO2 production.	Add a cooling trap where moisture can condense in the gas line

Methanogens are typically the most sensitive organisms in the process but are the most crucial because they perform the final step of converting acids to methane. Methanogens can be sensitive to high amounts of acid or high amounts ammonia. A low pH indicates high amounts of acid which can occur by putting too much organic matter in the digester. This is especially the case with carbohydrates (e.g. from food waste) which break down quickly to acids causing acid build up. A high pH could The quickest indicator of conditions that are hostile to methanogens is pH, which should be near 7.

Non-flammable gas could be due to too much moisture in the gas or not enough methane in the gas. This is common during digester startup.

Instances of the System

Household Scale Anaerobic Digesters

Make	Model	Start of Production	End of Production	Number Produced
Home Biogas	2.0	October 2018	In Production	-