Influence of Algae on Soil Saturated Hydraulic Conductivity: An <i>in situ</i> Treatment Option for Reducing Infiltration Beneath Unlined Algae Cultivation Ponds

Commercial production of algal biofuels is currently limited by high capital costs, including the cost of installation and maintenance of plastic pond liners, which mitigate seepage of cultivation fluids and control the release of salts and nutrients into the subsurface beneath outdoor algae cultivation ponds. However, studies of animal waste settling lagoons show that underlying soils ranging from sands to clay loams can exhibit reduced hydraulic conductivity within days to weeks after construction, reducing the need for plastic liners. The mechanisms of the hydraulic conductivity reductions, or “soil sealing”, are physical rearrangement of soil particles, buildup of fines, and the accumulation of microorganisms and their metabolic products within pore spaces.

metabolic products within pore spaces. In this study, laboratory-scale soil column experiments investigated a new application for old technology by using fluids that are low-cost and readily available in algae biofuel production to reduce the saturated hydraulic conductivity (K_s) of soils of varying textures by promoting physical and microbial pore clogging mechanisms. Three fluid treatments were supplied to a fine sand soil 1) a nutrient solution used in commercial algae cultivation (NS), 2) the nutrient solution with glycerol added (NSG) and 3) algae (Scenedesmus dimorphus) growing in the nutrient solution (NSA). Relatively small reductions of K_s by NS (44-63%) indicate that the culture broth alone was insufficient to promote soil clogging. Larger reductions (77-94%) were seen by addition of the carbon substrate glycerol to the nutrient solution, indicative of enhanced bacterial growth. However, the K_s values produced by the NSG varied widely over time. The magnitudes of K_s reductions by NSA were also large (84-95%) and remained stable.

The large, stable K_s reductions provided by algae, seen in the fine sand soil, were similar to those for a loamy sand (96-99%) and a loam soil (98%). The approximately two-order-of-magnitude K_s reductions imply that, like organic fines in animal waste holding ponds, algae can also significantly contribute to reductions in K_s of soils. This new technology appears to be successful in reducing the K_s of native soils and suggests the technology could be applied as an alternative to plastic liners.