Page 80 - North American Clean Energy January February 2015

biopower

Left. Anaerobic digester and feedstock pile at Rosendale
Dairy in Pickett, Wisconsin

Above. Dried residual from the KB BioEnergy anaerobic
digestion and composting facility in Akron, Ohio

Anaerobic Digestion Residuals Recovery

By Douglas Renk

he potential for renewable gas production and clean energy often dominate the spotlight in the anaerobic digestion

technology world. Anaerobic digestion (AD) is a biological process that’s been rapidly gaining traction across all sectors that

create organic waste. In the absence of oxygen, micro-organisms break down organic matter during anaerobic digestion,
releasing a combination of methane and carbon dioxide—also known as biogas.

From wastewater treatment plants and composting facilities to campuses, grocery stores, he management of phosphorus levels post-digestion can save farmers time and cut
and food processors, various establishments can beneit from AD. Biogas from anaerobic hauling costs, which could afect biomass crop production for the better. Phosphorus lev-

digestion provides an alternative to non-renewable and polluting energy sources, while els in manure limit the allowable amount that can be applied in a season, and many soils
utilizing waste as a resource. In fact, the Innovation Center for US Dairy estimates that are already at their maximum phosphorus levels. Manure may be applied to help meet
the United States currently has the potential to generate 11.7 million megawatt-hours of the nitrogen needs of a crop growing in soils with concentrations up to 50 mg/kg, so long
electrical power from anaerobically digesting manure and food waste alone.
as the phosphorus applied doesn’t exceed the amount taken up by the crops over a rota-

he beneits of AD, however, often overshadow the beneits of the residues and nutri- tion cycle.
ent management from the digestion process. Utilizing AD residues and properly manag- If a ield’s phosphorus level exceeds 100 mg/kg, then no additions in manure or ferti-
ing related nutrients, such as phosphorus, can make-or-break potential projects. he lizer are permitted. In eforts to allow more nitrogen requirements to be met by diges-
value of the AD nutrients can take a project considered uneconomical, based solely on tate, nutrient planners encourage phosphorus removal from digestate slurries in these

biogas or energy production value, and make it worthwhile.
areas, rather than supplementing nitrogen from other sources.
Of course, controlled application is the key factor in proper nutrient management.
Obtaining those nutrients
And, according to the Wisconsin DATCP Nutrient Management and Water Quality Sec-

he nutrient management beneits from anaerobic digestion can and should be con- tion, solid nutrient capture is valuable in reducing hauling costs. For example, if a farmer
sidered to help keep residuals on the positive side of the balance sheet. For example, pays a contractor 1¢ to 2¢/gallon to haul and spread manure, the material could be sepa-
agricultural waste management is typically not considered a proit center. Monetizing rated into two streams, whereby more concentrated material could be taken to further
the beneits in terms of removal and clean-up costs, or its value as a fertilizer, is vital but ields. he lower concentration could, then, be applied by drag-hose nearer to the farm.

challenging.
A full spectrum of valuable nutrients and minerals can be captured from AD residues, Waterway management
including phosphorus. When compared to the aerobic organic treatment processes, AD Although phosphorus and other nutrients are valuable to farmers, excess nutrients car-
retains nutrients throughout its process, transforming complex, organically bound phos- ried in wastewater pose a serious threat to waterways. Decades of management practice

phorus into a more biologically available form.
and regulatory eforts have been established to combat the growth of oxygen-depleting
algae, as a result of phosphorus runof. Unfortunately, prevention and recovery of nutri-
A limited resource
ents from waterways is diicult and expensive.
Phosphorus is an essential element for all cellular growth, and demand for it is growing. When watershed protection is mandated by regulatory compliance, nutrient recovery

Only 28 years of phosphate deposits remain in the US (assuming a two percent market technology is deployed for concern of ines or operational shutdown. But, phosphorus
growth), unless more recovery is developed. Until scarcity and increased production cleanup from a body of water can cost from $5/pound to $100/pound—the present nu-
costs drive phosphorus prices higher, incentives such as cost savings and reduced hauling trient market value is about $0.53/pound for phosphorus pentoxide (P2O5). It’s impor-
are currently being sought for implementing phosphorus recovery.
tant to note that reduction of nutrients in discharge isn’t a linear function. he cost of

80 JANUARY/FEBRUARY 2015 nacleanenergy.com