Page 15 - APN June 2017

Anaerobic digestion – keeping bugs in the system
A NA EROBIC digestion is a naturally occur- ring biological process involving micro-organ- isms breaking down or- ganic matter in environ- ments with little or no oxygen.
The process occurs nat- urally in swamps, water- logged soils (such as rice fields), deep water storage dams and in the digestive systems of animals.
A D produces biogas, which is a mixture of most- ly methane and carbon di- oxide, with trace amounts of other gases such as hy- drogen sulphide.
Purpose-built biogas plants use the natural ac- tivity of the micro-organ- isms to convert organic biomass materials, such as piggery manure, into biogas, which can be com- busted to generate elec- tricity and heat.
Biogas can also be treat- ed (or upgraded) to re- move non-methane com- pounds so it can be used as a direct replacement for natural gas.
Leper digester
The earliest recorded purpose-built anaerobic digester was constructed at a Bombay (India) leper colony in 1859.
AD reached England in 1895, where a septic tank was used to generate gas for street lighting in Ex- eter.
Soon after, in 1907, a patent was issued for the Imhoff tank, an early form of digester devel- oped in Germany.
Staged process
The AD process in- volves four distinct stages, as follows:
1. Bacterial hydrolysis of the organic feed materials to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria.
2. Conversion of sug- ars and amino acids into carbon dioxide, hydrogen, ammonia and organic ac- ids by acidogenic bacteria.
3. Conversion of the re- sulting organic acids into acetic acid, along with ad- ditional ammonia, hydro- gen and carbon dioxide by acetogenic bacteria.
4. Finally, conversion of the acetic acid to meth- ane and carbon dioxide by methanogens.
It is important to note that effective AD requires the progressive processing of the various substrates produced by distinct groups of living micro- organisms.
Disruption of any of the individual stages can ad- versely affect the overall AD process.
In addition to biogas, the AD process also produces a wet solid residue called digestate that is rich in nutrients such as nitrogen, phosphorus and potas- sium.
The digestate has con- siderable value as an or- ganic fertiliser and can be processed into commer- cial compost. Temperature talk
The AD process tem- perature affects the rate of digestion.
Many covered anaerobic ponds used in the pork industry are not heated and therefore the effluent temperature follows the ambient air temperature, resulting in seasonal fluc- tuation in biogas produc- tion. www.porknews.com.au
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Its a gas
More sophisticated (and considerably more expensive) in-ground and above-ground tank digesters are heated to maintain their operating temperatures in the mes- ophilic range (35-40C), resulting in minimal sea- sonal variation in biogas production.
It is also possible to op- erate digesters in the ther- mophilic range (55-60C), however the digestion pro- cess is subject to upset if not closely monitored. Sizing digesters
The organic loading rate and hydraulic retention time are the main oper- ating parameters used in sizing digesters in prac- tice.
Overloading the digester with feedstock may cause the AD process to become unbalanced, potentially affecting the pH and in-
by ALAN SKERMAN Leader, Pork CRC Bioenergy Support Program
hibiting methane produc- tion.
Additionally, the time taken to digest various substrate components varies and decreasing the HRT may result in wash- out of active bacteria and feedstock.
Conversely, longer HRTs require larger reactor ves- sels or covered ponds, substantially increasing the capital cost of the sys- tem.
AD inhibitors
Piggery effluent con- tains relatively high con- centrations of nitrogenous compounds such as pro- teins and amino acids that may be degraded to form high levels of ammonia during the AD process.
Although ammonia is an essential nutrient for growth of micro-organisms, high concentrations are poten- tially toxic to methanogens,
depending on other process conditions.
AD is also inhibited by the presence of heavy met- als and a range of cleaning
and veterinary chemical formulations commonly used in piggeries, such as chlorinated solvents, bio- cides, pesticides and antibi- otics, which may inadvert- ently enter the waste stream discharged from piggery sheds.
Available technologies
The main technologies currently used for on- farm AD at Australian piggeries include passive (unheated, unmixed), in- ground CAPs, mixed and heated in-ground hybrid digesters and above- ground mixed tank di- gesters.
Unheated, unmixed CAPs have been by far the most commonly adopted digester type, primarily due to their relative sim- plicity and lower capital cost.
Biologically speaking
In summary, it is im- portant to remember that all anaerobic digest- ers are living biological systems that rely on the balanced and sequential performance of a range of micro-organisms for their effective operation and consistent produc- tion of biogas.
While the various bac-
teria can survive ‘shocks’ resulting from such things as inconsistent piggery management practices or the use of different clean- ing products, careful man- agement is necessary to avoid major disruptions to the AD process and resulting on-farm biogas and energy production.
For further information on AD or services pro- vided through the Pork CRC Bioenergy Support Program, contact me on 07 4529 4247 or alan.sker man@daf.qld.gov.au
www.porkcrc.com.au
Consistent and reliable biogas production relies on the healthy functioning of distinct groups of living micro- organisms that break down the organic matter in piggery effluent.
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Australian Pork Newspaper, June 2017 – Page 15