source of waste would be reduced or eliminated. Anaerobic digestion is another method of waste disposal and can generate electricity as a coproduct (Chapter 2). For the disposal of waste biomass, blow down of solids from production and recycling ponds, and saltwater, companies are considering landfilling waste, underground injection, and diverting processed water to sewage systems.
Solid waste from algal biofuel manufacturing processes is most likely to be generated as sludge from an anaerobic digester from which the volatile organic acids have been converted to methane and CO2; the methane is useful as a fuel supplement for the process. Anaerobic digestion in many cases is followed by aerobic digestion to convert dissolved solids to sludge, concentrated by settling in the large aerobic settlers. Such systems have been operated commercially for decades and most likely will be incorporated into algal biofuel production + concentrations of digested processes. Golueke et al. (1957) reported average NH4+ sludge in the range of 1600 to 1850 milligrams per liter for anaerobic digestion of algae, which is comparable to some of the high values reported for piggery waste (Sukias and Tanner, 2005; Sukias and Craggs, 2011). Another source of solid waste is the spent synthetic plastic liner from open ponds or closed bioreactors that will need to be disposed periodically.
According to Jim Sears (J. Sears, A2BE Carbon Capture, personal communication on September 22, 2011), who chaired the “Committee on Technical Standards” for the Algal Biomass Organization, “there are as many proposed processes for producing algal biofuels as there are companies.” Thus, whether generation of waste products would be a concern cannot be known until operations at commercial scale are in place and compositions can be ascertained. Maximizing recycling would reduce the need for waste product disposal.
5.9.2 Opportunities for Mitigation
Recycling of nutrients is the obvious mitigation for waste generation. Algenol, an algal biofuel company, plans to recycle seawater waste for cultivation (less than 6 liters of seawater waste per liter of ethanol is produced if photobioreactors last greater than 6 years).
If digested sludge is produced, municipal waste treatment plants usually spread the nutrient-rich waste on designated land, with the benefit of conditioning and nourishing the soil. Algal fuel process sludge from wastewater treatment is not expected to be significantly different. The National Pollutant Discharge Elimination System permitting process governs discharge of sludge; most states have a permitting process under this federal program. The composition of the sludge is monitored to ensure compliance with the permit.
5.9.3 Sustainability Indicators
Many sustainability indicators relevant to waste are described elsewhere, for example, quantifying recycling of nutrients and salinity of ground water. If saline wastewater is injected to groundwater, then sustainability indicators also could include annual volume injected per volume of reservoir per year.
5.9.4 Information and Data Gaps
Information is needed about the types and rate of waste generation for most algal biofuel production processes. When and if processes move toward commercialization, state and local regulations will govern the acceptable disposal of waste, which will necessarily be well characterized by then.