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Significance of phosphorus in anaerobic digestion residuals recovery


Gas production and energy potential typically dominate the spotlight in the anaerobic digestion (AD) technology world, while benefits of residues and nutrient management from AD are overshadowed. However, utilizing AD residues and considering the value of properly managing nutrients, such as phosphorus, may make-or-break projects considered uneconomical based on energy production value alone.

Nutrient management benefits from digestion should be considered to help keep residuals on the positive side of the balance sheet. For example, agricultural waste management is typically not considered a profit center. Monetizing the benefits in terms of removal and cleanup cost or value as fertilizer is vital, but can be challenging. A full spectrum of valuable nutrients and minerals can be captured from AD residues, such as phosphorus (P).

Phosphorus is an essential element for all cellular growth and demand for it is growing in the U.S.; only 28 years of phosphate deposits remain in the U.S. assuming a two percent market growth, unless more recovery is developed. Still, until scarcity and increased production costs drive phosphorus prices higher, incentives such as cost savings and reduced hauling are sought for implementing phosphorus recovery.

When compared to the aerobic organic treatment processes, anaerobic digestion retains nutrients throughout the process, transforming complex, organically bound phosphorus into a more biologically- available form.

The management of phosphorus levels post-digestion can save farmers time and cut hauling costs. Phosphorus levels in manure limit the allowable amount that can be applied in a season, and many soils are already at their maximum phosphorus levels. Manure may be applied to help meet the nitrogen needs of a crop growing in soils with concentrations up to 50 mg/kg of P; however, the P applied is not to exceed the amount taken up by the crops over a rotation cycle. If a field’s P levels exceed 100 mg/kg, then no P additions in manure or fertilizer are permitted. Nutrient planners encourage phosphorus removal from digestate slurries in these areas in efforts to allow more nitrogen requirements to be met by digestate, rather than supplementing nitrogen from other sources.

Controlled application is the key factor in proper nutrient management. Solid nutrient capture is valuable in reducing hauling costs, according to Sara Walling, section chief with Wisconsin DATCP Nutrient Management and Water Quality. For example, if a farmer pays a contractor one to two cents per gallon to haul and spread manure, the material could be separated into two streams where more concentrated material can be taken to further fields, while the lower concentration can be applied by drag-hose nearer the farm.

Phosphorus recovery processes

  • Fluidized bed crystallization-struvite
  • Dissolved Air Flotation/decanter
  • Phase separation/clarifier
  • Mechanical separation/centrifuge
  • Membrane separation
  • Polymer/belt press/dryer
  • Biological uptake-managed wetland or aquatic growth

Although phosphorus and other nutrients are valuable to farmers, excess nutrients carried in wastewater pose a serious threat to waterways. Decades of management practice and regulatory efforts have been established to combat the growth of oxygen-depleting algae as a result of phosphorus runoff. Unfortunately, both prevention and recovery of nutrients from waterways is difficult and expensive. When watershed protection is mandated by regulatory compliance, nutrient recovery technology is deployed for concern of fines or operational shutdown. For example, phosphorus cleanup from a body of water can cost from $5/pound to $100/pound; the present nutrient market value is about $0.53/pound for phosphorus pentoxide (P2O5). It is important to note that reduction of nutrients in discharge is not a linear function. The cost of reducing the first 90 percent of phosphorus in a waste stream may cost $5/pound, however, it may cost $50/pound to reach 90 to 99 percent reduction.

One method of meeting nutrient reduction requirements is through Nutrient Credit Trading (NCT) or Water Quality Trading, which focus greatly on phosphorus. NCT’s are earned by capture of nutrients from non- point sources by virtue of base conservation activities, such as strip farming, no-till farming, and contour farming. Capture techniques such as these can be as effective in nutrient planning as recovery of P from AD liquid and manures. With proper documentation, phosphorus removed from AD effluent can qualify for NCTs.

Some nutrient recovery techniques manipulate pH and require the addition of minerals to optimize recovery from the solution. Phosphorus recovery is aided by anaerobic digestion as the process converts and homogenizes inputs into more available form.

The management of organic material through anaerobic digestion has many offerings outside the gas and energy production. There are a multitude of uses for residues from anaerobic digestion: animal bedding, compost, fertilizer, fiber, and biochar to name a few; this material recovery component distinguishes renewable biogas from solar, wind and hydro energy. High product standards for residuals management enhance overall digester value and benefit sustainable agriculture by replacing depleted resources.

Douglas Renk is a biological commissioning engineer with BIOFerm Energy Systems