A Thermodynamic Analysis of a Primary Waste Stabilization Pond

Traditional design practice for waste stabilization ponds is based upon the premise that sufficient photosynthetic oxygen must be produced within the pond to satisfy oxygen requirements of the incoming waste flow. Thus, because algae production is proportional to pond surface area, surface organic loading rate is a principal design criterion (hydraulic detention time is the other) . That a possible adverse energy trade exists in the sequence of coupled reactions (aerobic waste degradation-photosynthesis) has been largely ignored. This work is focused on quantitatively articulating this energy trade, in terms of algae produced vis a vis waste degraded. This is done by: (1) defining the chemical reactions involved-both stoichiometrically and thermodynamically (the latter in terms of equilibrium conditions), (2) measuring terms in a daily mass balance model of an operating primary pond, and (3) evaluating the algae production potential for the pond studied, based upon available solar insolation. These results define respectively, (1) the calculated absolute lower limit of daily algae synthesis necessary for production of the stoichiometric oxygen to satisfy the daily influent BOD requirement, (2) a measured daily synthesis rate of algae to compare with the daily influent TOC (total organic carbon), under conditions of maximum sunshine in the annual cycle, and (3) the calculated absolute upper limit of daily algae synthesis, through the annual cycle, if all usable solar energy were utilized. Results, for a daily waste inflow of 415 kg Toe, showed: (1) 167 kg per day of algae TOC must be synthesized to provide the stoichiometric oxygen for 415 kg TOC waste, as glucose, (2) measured algae synthesis rate, in early July, was +12,600 kg TOC per day, and (3) algae production potential in early July was 44,000 kg algae TOC per day. The effluent flux was 110 kg TOC per day