The plankton ocean digester (POD) provides ecologically sound energy, fresh water, and food production with the carbon sequestering cycle illustrated below.

Fresh water, a February 2009 insight - Anaerobic digestion produces the molecules used for forward osmosis, carbon dioxide and ammonia. In reverse osmosis, pressure forces pure water through a membrane excluding salt molecules. The pressure required is proportional to the salt concentration and is known as the osmotic pressure. In forward osmosis, one concentrates molecules that can be removed without significant energy on the “pure water” side of the membrane. When the removable molecule concentration exceeds the salt concentration, the osmotic pressure works to draw pure water through the membrane. Forward osmosis researchers are using carbon dioxide and ammonia for the removable molecules. Carbon dioxide will dissolve in sufficient concentrations for seawater forward osmosis at about 50 atmospheres pressure. Not including the cost of dissolving the initial carbon dioxide supply, the energy cost of recycling and recompressing carbon dioxide in a seawater forward osmosis process is about 0.4 kWh/m³ of fresh water produced. The bacteria in high-pressure anaerobic digestion may produce the dissolved carbon dioxide at the necessary concentration and pressure and avoid the 0.4 kWh/m³energy cost. Also, a high-pressure anaerobic digestion biomethane production facility would require relatively little additional structure to incorporate forward osmosis seawater desalting.

Simultaneously produce: 400 million MWh (California’s demand for a year), and 100 million tonnes of CO2 removed from air (Germany’s vehicle emissions for a year), and sustainably recycle nutrients, and increase species diversity, and either 200 million tonnes of fish (feeds Africa for a year) or 2 million hectares less ocean dead zone. Calculations based on current experience with algae growth and anaerobic digestion suggest 10,000,000 hectares of ocean surface (0.03% or about the size of Cuba) would produce World Eco Challenge volumes each year. This 2008 video was prepared prior to the forward osmosis insight.

Bacterial decomposition is a natural link in the carbon cycle. In water lacking oxygen, naturally occurring and ubiquitous bacteria convert organic matter into water, methane, carbon dioxide, and plant nutrients. Humans have employed “anaerobic digestion” for centuries. It occurs naturally in swamps, landfills, at wastewater treatment plants, and anywhere organic matter accumulates without oxygen. The digestion occurs at typical water body temperatures ranging from 3 to 40 ºC on microalgae, kelp, zooplankton, fish waste, and fish.

In Figure above, carbon dioxide from the atmosphere is captured by plankton, algae, or kelp. The plankton eventually die and are digested into methane, carbon dioxide, and the fertilizer needed to grow more algae. The methane is renewable natural gas, a common source of energy, which produces "neutral" carbon dioxide when burned. The carbon dioxide is easily separated from the methane at ocean temperatures and pressures and can be sequestered in the deep ocean.

[ Research Needs][Economics][Sustainability][Paper][About us]

MarkCapron@PODenergy.org