Abstract
The demand for phosphorous (P) fertilizer is projected to increase by 100% by the end of the century, and the use of nitrogen (N) fertilizer is increasing exponentially as well. Current single-use nutrient management practices have led to environmental impacts of mining, extensive greenhouse gas emissions and surface water eutrophication. Recovery and reuse of nutrients would reduce solubilizing plant available N and P in the environment. Thus, the embedded energy in fertilizer production can be retained while minimizing the conversion of stable forms of N and P. Wastewater was identified as a potential source to recover nutrients, as wastewater contains a large load of nutrients. The purpose of this work is to demonstrate simultaneous N and P recovery from settled sewage and Net-Zero Water (NZW) sewage in a continuous-flow reactor by electrochemical pH shifting technique, termed electrohydromodulation (EHM). In achieving this objective, first currently available nutrient recovery technologies are reviewed, and electrochemical processes were identified as a promising process to recover nutrients from untreated sewage. Then EHM reactors were constructed and their performance was evaluated in batch mode followed by simultaneous N and P recovery in a continuous-flow apparatus. The EHM nutrient recovery process was optimized based on the operating energy and recovery efficiency, and an economic analysis is carried out comparing the process with commercial fertilizer production. Performance of the EHM was first tested in batch mode using municipal sewage and mineral-spiked settled sewage to simulate NZW sewage. Results indicated that pH can be shifted over 11 with a reasonable energy demand using CMI-7000 multi-valent cation exchange membrane (MCEM), and a voltage below 7 V, to recover over 90% PO4. The high pH achieved in batch experiments indicated that gas-liquid stripping for N recovery can be conveniently coupled with P recovery, as both processes favor high pH. Simultaneous N and P recovery was successfully demonstrated in a continuous-flow apparatus by coupling the EHM reactor with gas-liquid stripping and absorption for N recovery. Recovery of 89% and 97% of average total N & P from municipal primary effluent was achieved in the continuous-flow bench reactor, at a total (EHM, microfiltration, stripping and absorption) energy demand of 1.21 kWh/m3. Phosphorus was recovered as amorphous calcium phosphate, and nitrogen as an ammonium sulfate solution. Non-toxic, inexpensive graphite electrodes were intact from fouling over the course of 2 years of experimentation, indicating the robustness in long term operation of the process. The pH modulation in the EHM process was modeled in Matlab Simulink with an accuracy of ±0.4 pH units and the current efficiency of the water electrolysis reaction at 4.07 V was estimated as 71.6%. Economic analysis of the process indicated that nutrient recovery is economical compared to commercial fertilizer production when environmental benefits are considered. Based on recovery efficiency and process economics the EHM process developed in this work can be potentially incorporated in NZW treatment plants and conventional wastewater treatment plants. The major operating cost of the process was calculated as 5.5% of the cost of wastewater treatment in Miami, Florida. Given the low operating cost and recovery efficiency, potential applications may include (a) fertilizer recovery from central wastewater treatment plant; (b) recovery from future net-zero water (nearly closed-loop direct potable water reuse plants) at any scale; (c) pretreatment at wastewater treatment plants to allow compliance with stringent indirect potable reuse nutrient discharge standards to protect groundwater and surface water; and (d) as a septic tank add-on to address the emerging issues with septic tanks due to sea level rise, to control nutrient pollution in the environment.