ADVANTAGES OF AEROBIC DIGESTION
• Volatile solids reduction meets or exceeds that of anaerobic digestion.
• The stabilized sludge is free of offensive odor and an excellent fertilizer.
• Supernatant BOD (Biological Oxygen Demand) concentrations are lower than that of anaerobic digestion.
• Operation is relatively easy.
The major disadvantage of aerobic digestion is the cost of supplying oxygen to the micro-organisms. Power cost for supplying oxygen, however, may not be as significant at smaller power plants.
Other disadvantages are that the stabilized sludge sometimes has poor dewatering characteristics and process performance depends strongly on temperature. Aerobic digestion is more commonly used at smaller municipal wastewater treatment plant with wastewater flows less than 5 mgd (million gallon day)
The major difference between aerobic and anaerobic digestion is that micro-organisms are supplied oxygen in aerobic digestion. Aerobic digestion fundamentally follows the principles of the biological mechanism of endogenous respiration. When little food is available to the biomass population, micro-organisms consume their own protoplasm as an energy source, thereby reducing the volatile solids concentration of the sludge.
ADVANTAGES OF ANAEROBIC DIGESTION
The manure handling system of any farm is made up of many different components, each with a different function and purpose. An anaerobic digester, although only one component of the system, can greatly improve the environmental performance and efficiency of the overall system. The main effect of anaerobic digestion is conversion of organic matter to biogas. This conversion has many potentially beneficial environmental and management side effects.
The end products of aerobic digestion are carbon dioxide, water, and ammonia or nitrates. However, the entire cell cannot be biologically transformed to these end products. Approximately 25 percent of the cell is typically non-biodegradable.
By removing organic matter, the digester reduces the organic matter-loading and associated oxygen demand on downstream manure handling components. This may allow the downstream components to be smaller, operate more efficiently and function with less environmental impact. Anaerobic pretreatment may be a more economical method of converting an anaerobic lagoon to an aerobic lagoon, compared to mechanical aeration. Digester effluent is more stable than raw manure. It contains more stable organic material and less volatile odorants. Thus, storage and land application of digester effluent greatly reduces odor nuisance compared to raw manure.
Uses for digested solids
Manure solids are stabilized through anaerobic digestion. What was once reactive, partially digested material has been processed into stable microbial biomass and precipitated nutrients, although the majority of nutrients remain with the liquid. The potential to dry and transport digester solids is greatly improved over raw manure. The solids can be recycled and used for bedding or a soil amendment on the farm. The reduction in moisture content also increases the feasibility of selling the solids to farms that are greater distances away. In the right market conditions, composting the digested solids can result in a value-added product that can be sold to homeowners, gardeners or the landscape industry.
Plant nutrients are conserved and transformed during anaerobic digestion. Ammonium is created from manure proteins. This can be a benefit or a nuisance. If injected immediately into the soil, ammonium-rich effluent is highly available for plant growth. On the other hand, if digester effluent is stored under anaerobic conditions, ammonium will convert to ammonia gas and escape to the atmosphere. Since digesters are also a reducing environment, the potential exists for capture of ammonium and soluble phosphorus through precipitation as struvite.
Many metals are precipitated during anaerobic digestion. Sulfur is reduced to sulfide, which is generally a bad thing since it can escape as hydrogen sulfide gas. However, the digester environment can be manipulated so that sulfides are precipitated along with potentially harmful metals such as Ni and Zn.
Anaerobic digestion results in the reduced emission of greenhouse gases. This may seem ironic, since the methane contained in the resulting biogas is a powerful greenhouse gas. An anaerobic digester is a controlled environment that captures the methane. After capture, it is either flared or used to generate electricity and/or heat.
When flared, the carbon dioxide formed in the combustion has less heat trapping potential than the original methane, and it is essentially recycled atmospheric carbon. What is released to the atmosphere through combustion of methane was once plant material formed through photosynthesis from atmospheric carbon dioxide.
When used for energy generation, the biogas replaces power that might have otherwise been created through conversion of fossil fuel. Regardless, if the biogas is flared or used for energy generation, the farmer may be eligible for carbon credit payments.
Anaerobic Digestion on Farms
With all of the potential benefits, one might wonder why relatively few farms utilize these systems. One major reason is that anaerobic digesters are expensive to install and operate. The economic benefits have, in the past, been limited to a reduction in electricity purchased by the farm, which is not enough to offset the costs of the system.
As the interest in renewable energy sources increases, farms are increasingly able to apply and receive carbon credits. Some farms also accept off-farm waste, collecting tipping fees, to co-digest with manure. In many states, more favorable net-metering laws have also made the economics more favorable. Power generated by the digester is valued at retail costs rather than wholesale costs.
The decision to install a digester is often driven by additional considerations, such as nuisance issues. A digester greatly reduces the odor potential of the manure, which also greatly reduces neighbors' complaints and the potential for lawsuits.
At the current time, anaerobic digestion is slowly but surely increasing as a manure treatment method in the United States. In determing whether or not anaerobic digestion is environmentally and economically relevant, one must consider the consequences of creating a biologically derived energy carrier, i.e., "Biogas" (Methane) versus dehydrating the effluent and using a high efficiency combuster/turbine. Surprisingly, there is an adequate supply - the closed cycle combustion is vastly more efficient and friendly to the environment. The tradeoff is suppresion of any nutrient byproducts to fertilize the agricultural production.
Anaerobic biogas generation will only represent about a maximum of twenty percent (20%) of the available energy harvest that would otherwise be liberated by combustion. (Combustion is actually more environmentally conscious than biogas generation.)