FIGURE 4-2 Dissolved solids control in a simplified open-pond cultivation system.
NOTE: Make-up water addition (Fin) and water purge (Fout) are used to control the critical concentration of total dissolved solids in the pond water (xcontrol). If the pond is well-mixed, the concentration of total dissolved solids in the purge (xout) is equal to the concentration in the pond (xcontrol).
In addition to evaporative losses, water can seep into and out of open ponds, particularly if they are clay-lined or if liner failure occurs. Water percolation is strongly influenced by the composition and texture of the underlying soils (for example, clay versus sand). Seepage rates are typically on the order of 5 to 6 millimeters per day (Weissman et al., 1989; Boyd and Gross, 2000), which is low compared to rates of evaporative water loss in many regions of the United States. In contrast to open ponds, closed photobioreactors are not affected by surface evaporation and seepage, and the lowest reported values for estimated water use are associated with closed systems. However, the water requirements of a photobioreactor system depend on its actual configuration and operating conditions.
The reclamation and recycling of water are key determinants of the total water requirements of both open-pond and closed photobioreactor systems. Whether and how much of the harvest water can be reclaimed and reused depend on the efficiency of separation processes, the quality of the return water, and the sensitivity of the algal culture itself to changes and impurities in the return water, including any waste products produced by the resident algae (Murphy and Allen, 2011).
The water requirement for processing of algal biomass to biofuel is small relative to evaporative losses during cultivation in open-pond systems. Water use for processing algal biomass to fatty-acid methyl ester (FAME) was estimated to be 1 liter per liter of biodiesel produced. Water loss during the drying of algae to prepare the biomass for processing to fuel is unavoidable, and some water also is unavoidably lost during the extraction of oil from algae and esterification of algal oil. However, Pate et al. (2011) stressed that evaporative water loss under operating conditions involving the inland use of water with a high salt content will result in salinity increases unless fresh water is used to make up for the loss or steps are taken either to mitigate or adapt to salt build-up. The use of inland saline water in algal biofuel production also could have other potential environmental effects (see Chapter 5).