Farms and Biogas

Dairy farms and other confined animal feedlots, especially larger ones, are under increasing public and regulatory pressure to manage their animal manure to control environmental problems. A major concern is odour, which has been a prime force behind local ordinances to control feedlot expansion. There are also potential problems with storing and spreading the manure, along with the potential for spills. Anaerobic digesters have come into their own over the last several years for their potential to address some of the environmental impacts of manure management while providing farmers with economic benefits (Nelson and Lamb).

In a large scale system raw manure is processed using an oxygen free container (heated if in colder climates), allowing digestion that began in the cow’s stomach to continue and be enhanced. Products of anaerobic digestion of livestock manure include a combustible gas (i.e. biogas), liquid effluent, and digested solids.

The liquid effluent is a low-odour fertilizer with characteristics closer to commercial fertilizers that provide more flexibility to farmers in land application. This can often be substituted for the increasingly expensive commercial fertilizers. The phosphorus (P) rich digested solids are commonly used as bedding for cows, but also have value as soil supplements either on agricultural lands or for landscapers and greenhouses (Kramer).

The development of anaerobic digesters for livestock manure treatment and energy production has accelerated at a very face pace over the past few years. Factors influencing this market demand include: increased technical reliability of anaerobic digesters through the deployment of successful operating systems over the past decade; growing concern of farm owners about environmental quality; an increasing number of states and federal programs designed to cost share in the development of these systems; and the emergence of new state energy policies designed to expand growth in reliable renewable energy and green power markets (AgSTAR Program). 

A vast array of anaerobic digesters have been developed and placed in operation over the past fifty years, and a variety of schemes could be used to classify the digestion processes. When considering waste from a dairy, the most important classification is whether or not it can be used to convert dairy waste solids to gas while meeting the goals of anaerobic digestion. The goals of dairy waste anaerobic digestion are as follows: 

·          Reduce the mass of solids 

·          Reduce the odours associated with the waste products 

·          Produce clean effluent for recycle and irrigation 

·          Concentrate the nutrients in a solid product for storage or export 

·          Generate energy 

·          Reduce pathogens associated with the waste 

The processes that have been used for digesting dairy waste can be subdivided into high rate and low rate processes. Low rate processes consist of covered anaerobic lagoons, plug flow digesters, and mesophilic completely mixed digesters. High rate reactors include the thermophilic completely mixed digesters, anaerobic contact digesters, and hybrid contact/fixed film reactors. 

Western nations have until recently, been much less enthusiastic about biogas technology than most third world countries. The basic older style digester is labour intensive, and until systems became mechanised and automated there was little uptake in the West. These automated large scale farm biogas plants are on the scale of town sewage plants and capital costs can run into millions of dollars. 

Most of this section has been about large farm digesters, but medium biogas systems with volumes ranging from 50 m3 upward for cattle, pigs and/or poultry can still be built using the Chinese and Indian designs seen on page 7. Of course this size digester is not just the domain of farmers, and restaurant owners or collectors of food waste could construct a digester as their waste products a perfect feedstock. 

In the Build-A-Biogas-Plant  Member’s Area documents provide guidance on screening for project  opportunities, selecting a gas use option and conducting site-assessments to identify technically appropriate and cost-effective biogas recovery processes for your farm.

References

Kramer, 2008. Wisconsin Agricultural Biogas Casebook, http://www.build-a-biogas-plant.com Member’s Area (accessed 22 April 2010).

Nelson and Lamb, 2002. Final Report: Haubenschild Farms Anaerobic Digester, http://www.build-a-biogas-plant.com Member’s Area (accessed 22 April 2010).