An investigative field study of onsite residential lagoon systems was conducted to identify important demographic, hydraulic and hydrologic variables affecting components of lagoon water balances. The goal was development of a computer design model for residential lagoons more sensitive to the sources of water balance variance. Dimensional and operating measurements were made of thirty-three onsite residential lagoon systems plus two additional commercial systems in the Tulsa, Oklahoma area. Social and demographic data, as well as household water use habit data, were acquired by questionnaire from the thirty-six households served by the lagoon systems. Neighborhood demographic change analysis, based upon selected census tracts in Tulsa, was undertaken to identify predictable trends in household population and age characteristics. Predictive relationships developed from these analyses provided the basis for projecting long-term dwelling unit wastewater flow changes.

Lagoon water balance components included household wastewater influents, water surface incident precipitation, dike runoff precipitation, evaporation, and seepage. Computer design model relationships were developed from the most significant predictor variables. The design model is iterative, with one-month time steps, and attempts to optimize lagoon operating depth by accounting for water balance component changes based upon the results of the individual component predictive relationships. A twenty-five water balance is computed for the design life of the facility and options for stressing the adequacy of the lagoon design are provided.