Hybrid membrane distillation reverse electrodialysis configuration for water and energy recovery from human urine: An opportunity for off-grid decentralised sanitation

Abstract

The integration of membrane distillation with reverse electrodialysis has been investigated as a sustainable sanitation solution to provide clean water and electrical power from urine and waste heat. Reverse electrodialysis was integrated to provide the partial remixing of the concentrate (urine) and diluate (permeate) produced from the membrane distillation of urine. Broadly comparable power densities to those of a model salt solution (sodium chloride) were determined during evaluation of the individual and combined contribution of the various monovalent and multivalent inorganic and organic salt constituents in urine. Power densities were improved through raising feed-side temperature and increasing concentration in the concentrate, without observation of limiting behaviour imposed by non-ideal salt and water transport. A further unique contribution of this application is the limited volume of salt concentrate available, which demanded brine recycling to maximise energy recovery analogous to a battery, operating in a ‘state of charge’. During recycle, around 47% of the Gibbs free energy was recoverable with up to 80% of the energy extractable before the concentration difference between the two solutions was halfway towards equilibrium which implies that energy recovery can be optimised with limited effect on permeate quality. This study has provided the first successful demonstration of an integrated MD-RED system for energy recovery from a limited resource, and evidences that the recovered power is sufficient to operate a range of low current fluid pumping technologies that could help deliver off-grid sanitation and clean water recovery at single household scale.

Membrane distillation of concentrated blackwater: Effect of temperature, solids concentration and membrane pore size

“…This resulted in a trend comparable to that for conductivity (Table 1, Figure 9b), in which ammonium rejection exceeded 95% removal for all fecal solids concentrations, equivalent to 200 mg/L in the permeate (Table 1). This is high relative to the permeate quality experienced in conventional sewage treatment due to the absence of flushwater which imposes dilution (Mercer et al, 2019). In recognition of this concentrative effect, the ISO30500 standard for discharge places a requirement to achieve a 70% reduction in nitrogen (International Organization for Standardization, 2018), which membrane distillation can safely achieve (Table 1) with a modest feed temperature.…”

Scale-up of reverse electrodialysis for energy generation from high concentration salinity gradients

“…A constant current was applied at each step until the voltage stabilised. The theoretical open circuit voltage which could be achieved using ideal membranes with a 100% permselectivity was calculated using the Nernst equation [ 24 ]: Where n is the number of cell pairs, R is the universal gas constant (J K −1 mol −1 ), T is the temperature (K), z is the valency of the ion, F is the Faraday constant (C mol −1 ), γ is the mean activity coefficient of the counter-ion and C is the concentration of the counter-ion, with the subscripts C and D referring to the concentrated and dilute feeds respectively. Maximum current was determined from the roots of the power/current curve produced.…”

Electrodialysis Applications in Wastewater Treatment for Environmental Protection and Resources Recovery: A Systematic Review on Progress and Perspectives

“…The integration between membrane distillation (MD) and RED was proposed to recover water and energy from urine in off-grid applications [ 434 ]. From real urine feed (12.65 mS/cm, 207 mg/L , 6.33 g/L COD), the MD produced a retentate with doubled conductivity (24.1 mS/cm) and a permeate at 0.21 mS/cm.…”