MANURE DIGESTERS

The technology of anaerobic manure digestion has the potential to accomplish the following:

1. Greatly reduce odour levels during manure processing, creating a relatively odour-free end product (closed vessel processing confines odourous compounds which are converted to other compounds). Digested manure, which could be subsequently applied to land, would not have more odour associated with it than would composted manure.
    
2. Reduce pathogen levels in the final products. Anaerobic digestion greatly reduces pathogen levels. Additional pre- or post-digester technologies can ensure pathogen-free end products.
    
3. Conserve nutrients - more than 90% of nutrients entering anaerobic digesters are conserved through the digestion process. By conserving nitrogen during digestion, the N:P ratio of the treated manure is more favourable for plant growth. Reducing the demand for additional mineral nitrogen helps decrease the use of natural gas for production of new mineral nitrogen, as well as reduce greenhouse gas emissions associated with nitrogen fertilizer production.
    
4. Reduce greenhouse gas [ghg] emissions -  Since anaerobic digestion operates in a closed system, substantial reductions in greenhouse gas emissions (methane, nitrous oxide) are achieved.   Ammonia losses, while not of direct ghg concern, are also reduced. Ammonia is now on the list of toxic substances under the Canadian Environmental Protection Act (CEPA).
    
5. Kill Weed Seeds and Reduce Herbicide Use - It has been observed that weed seeds lose their viability while passing through an anaerobic digester. In conventional manure management systems, weed seeds maintain most of their viability as they pass through animal guts and as well as through the storage and handling steps, so that the seeds are repeatedly cycled back to the fields. After a few years with A.D. processing, weed pressures are substantially reduced, thus reducing herbicide requirements.
    
6. Co-generation and Energy Independence - Anaerobic digesters produce biogas (approx. 65% methane, 35% CO₂) which can be captured for supplying energy (heat, electricity) for the operation, thereby achieving substantial cost recovery. With the increasing privatization of power generation utilities, the "net cost" of power delivered to the farm is steadily increasing, and energy independence for the farmer is becoming a more attractive option. Often the delivered cost of electrical power can be at least double the original generating costs.  The decision to co-generate electricity should initially be driven by the savings for meeting the farm's power requirements, rather than based on potential sales of excess power back to the grid, if allowed.
   
   However, in Ontario, several significant developments have changed the economics for considering A.D. Systems for farmers:
    
   1. The Feed-In Tariff (FIT) Program pays for electricity generated from farm digesters at 19¢/kwh (≤ 100 kW), 18.5¢/kwh (>100 kW to ≤ 250 kW), & 16¢/kwh (>250 kW to ≤ 500 kW) when put into the grid, with a multiplier of 1.35 for for peak-time generation. Being able to store biogas for peak-demand generation offers farm-based A.D. systems an important advantage over other less predictable sources of renewable energy, such as wind or solar, both in terms of income for the farmer as well as increased efficiency/stability for the grid (decreased line losses, distributed generation).
       
   2. OMAFRA introduced the Ontario Biogas Systems Financial Assistance Program which provides financial assistance with some of the capital costs associated with the design, construction and startup of farm A.D. systems (program now closed).
       
   3. Nutrient Management Act Regulation 267/03 (OMAFRA) permits the importation of up to 25% food-grade organic wastes from off-farm sources (e.g. fats, oils, greases) under specified conditions, which can significantly boost the biogas output efficiency and substantially improve the profitability of the digester for the farmer.
      
      In some other jurisdictions, Net metering laws are now starting to be implemented, which may improve the economics for co-generation on smaller farms. In this scenario, the farmer can only off-set his on-farm electricity costs by exporting surplus electricity to the grid.
       
7. More predictable as sources of plant nutrients - The final products of anaerobic digestion are quite homogenous and are more predictable as sources of plant nutrients since they are in a more mineral form (50% of carbon (labile) is converted to biogas - methane & CO₂). If there is treated manure in excess of land base requirements, this homogenous product lends itself well to further processing for off-farm "value-added" products (adding supplemental nutrients can make this a valuable "organic fertilizer"). If the end product is dried and pelletized, it can be stored, transported and applied with existing fertilizer application equipment. Recycling of livestock nutrients back to cereal production sources should be an essential part of any long-term agricultural sustainability plan.

Because of the large initial capital costs associated with the design, testing and construction of an on-farm digester plant, very careful planning is essential. The most efficient design is likely to be unique and quite dependent upon the operation. The digester option is likely to be more attractive to larger operations, or to well-established existing operations wanting to expand and modernize their manure management systems. One of the greatest challenges in Canada for the A.D. industry is the lack of experienced design-build companies with a proven track record.

When considering manure digesters as an option, it may enhance the chances of success if local municipalities or other agricultural industries (vegetable or fruit processing, slaughter houses) or commercial industries (distillers, bio-fuel production) are considered as potential partners.... any industry generating non-toxic organic wastes as by-products of their operations. This appears to be the trend of several European digesters documented in the Digester Compendium. The addition of off-farm fatty wastes, animal rendering wastes or vegetable/cooking oils can act as an accelerant for methane production, increasing outputs by up to 4-fold. 

One of the key benefits to the farmer for generating and exporting renewable energy is that it provides new type of revenue stream, one that is relatively independent from the cyclic nature of crop and livestock commodities, and thus can provide some degree of stabilization of farm income, especially when commodity prices are depressed. Livestock will continue to generate manure regardless of what farm commodity prices are!

Since farm-based energy systems can operate 7/24, they make an important contribution to "distributed baseload capacity" for the electrical grid, complimenting less-reliable wind and solar power sources, as noted above.  Electrical transmission losses are typically in the 5-7% range, and having the generating capacity distributed across the countryside increases overall grid stability, while reducing transmission losses.

The buyers of green power may not always be governments, but also private industry looking for emissions credits.  A positive consequence of such a policy would be that farmers would coincidently solve quite many of the current environmental issues associated with livestock manure management through economic avenues, rather than as a result of prescriptive regulations (e.g. odours, pathogens, greenhouse gas emissions). As noted above in Sect. 7, treated manures lend themselves very well to value-added processing which is essential for developing a new business activity for recycling livestock nutrients.

See related articles:

1. Making manure processing a "Standard Practice"
2. Changing the Paradigm for Manure Management
3. Sustainable Nutrient Management in Agriculture and Closing the Loop on Large-Scale Nutrient Flows
4. Commentary on Nutrient Recovery Systems
5. A New Paradigm for Agriculture: "FOOD + BIO-ENERGY PRODUCTION" [pdf]

Bruce T. Bowman,
  WebMaster