Design of On-Site Treatment Systems in Caliche Soils

Abstract

Caliche soils, weathered limestone soils of high calcium carbonate content and and low organic content, are common in regions with limestone sedimentary geology and arid to semi-arid climate. Currently, there are no standards other than hydraulic soil properties for the design and construction of on-site wastewater treatment systems in caliche soils. Experimental studies were performed to investigate biological transformation rates for organic carbon to inorganic carbon and organic nitrogen to nitrate, using four different caliche soils and the range of recommended hydraulic loading rates. The experimental results indicate that oxygen demanding materials readily decay over short distances in caliche soils. Efficiency of removal for TOC was >97%. However in three of the four caliche soils, nitrate concentrations were significantly higher than the background and the control cases. The high calcium carbonate content of the caliche soils leads to solutions well buffered with respect to pH and alkalinity. Nitrification rates are very rapid in these soils which are buffered at the optimum pH range for Nitrosomonas growth. The low organic carbon content of caliche soils results in low denitrification rates. Nitrate in the effluent is the limiting pollutant in the determination of appropriate loading rates, drain field area, and land requirements. Phosphate and metal aqueous concentrations are acceptably low and are limited by solubility control in the open system equilibrium model.