Development, Implementation, and Improvements on an Effective Electrochemical Wastewater Treatment and Recycling Unit as a Sustainable Sanitation Solution for the Developing World

Abstract

In this thesis, I present my work on the development of a self-contained toilet wastewater treatment and recycling system, the “Caltech Solar Toilet”. The Caltech Solar Toilet technology is based on electrolysis of toilet wastewater with TiO2-coated semiconductor anodes and stainless steel cathodes. This is a potentially viable onsite sanitation solution in parts of the world that lack the needed infrastructure for centralized wastewater treatment. Prototypes of Caltech Solar Toilets were designed to fit in shipping containers in order to provide toilets and onsite wastewater treatment with clean water recycling. Units were designed to handle the waste of 25 users per day (or 130 L of toilet wastewater). The various prototypes were able to provide for the disinfection of pathogens, reduction of chemical oxygen demand (COD), [NH3], and color at an average energy consumption of 35 Wh L-1. The treated wastewater was recycled for use as toilet flushing water. The addition of a microbial fuel cell system for urine pre-treatment was investigated to lower the overall energy consumption of the Solar Toilets. The microbial fuel cell system used consisted of two stacks of 32 cells connected in parallel. An average power density of 23 mW m‑2 was produced at an effective current density of 65 mA m‑2 for more than 120 days. [NH3], total inorganic carbon, COD, and total organic carbon levels were monitored frequently to understand the chemical energy conversion to electricity as well as to determine the best electrical configuration of the stacks. Archaeal and bacterial population on selected anode felts and in the anolyte of both stacks were investigated as well. In addition to treating toilet wastewater, pilot-scale and bench-scale experiments demonstrated that electrolysis can remove phosphate by cathodic precipitation as hydroxyapatite at no additional energy cost. Phosphate removal could be predicted based on initial phosphate and calcium concentrations, and up to 80% total phosphate removal was achieved. While calcium was critical for phosphate removal, magnesium and bicarbonate had only minor impacts on phosphate removal rates at concentrations typical of toilet wastewater.