Abstract
Ammonia recovery from synthetic and real anaerobic digestates was accomplished using hydrophobic flat sheet membranes operated with H2SO4 solutions to convert ammonia into ammonium sulphate. The influence of the membrane material, flow rate (0.007, 0.015, 0.030 and 0.045 m3 h−1) and pH (7.6, 8.9, 10 and 11) of the digestate on ammonia recovery was investigated. The process was carried out with a flat sheet configuration at a temperature of 35 °C and with a 1 M, or 0.005 M, H2SO4 solution on the other side of the membrane. Polytetrafluoroethylene membranes with a nominal pore radius of 0.22 µm provided ammonia recoveries from synthetic and real digestates of 84.6% ± 1.0% and 71.6% ± 0.3%, respectively, for a membrane area of 8.6 × 10−4 m2 and a reservoir volume of 0.5 L, in 3.5 h with a 1 M H2SO4 solution and a recirculation flow on the feed side of the membrane of 0.030 m3 h−1. NH3 recovery followed first order kinetics and was faster at higher pHs of the H2SO4 solution and recirculation flow rate on the membrane feed side. Fouling resulted in changes in membrane surface morphology and pore size, which were confirmed by Atomic Force Microscopy and Air Displacement Porometry.
Highlights
A change in the perception of the uses of anaerobic digestion has occurred over past decades
Conventional nitrogen removal in domestic wastewater treatment plants (WWTPs) relies on nitrification, which is the conversion of ammoniacal conversion of NO3− to N2 gas nitrogen (NH3-N) to NO3−, [14,15,16]
PTFE membranes exhibited a superior performance to PVDF membranes in terms of NH3 recovery at a pH of 10 (Figure 2)
Summary
A change in the perception of the uses of anaerobic digestion has occurred over past decades. Anaerobic digestion was initially considered as a cost-competitive technology for organic matter stabilization in wastewaters and solid waste, a sustainable platform for renewable electricity and heat generation via biogas production, and more recently, it has been regarded as the potential core of a multiproduct biorefinery. Conventional nitrogen removal in domestic WWTPs relies on nitrification, which is the conversion of ammoniacal conversion of NO3− to N2 gas nitrogen (NH3-N) to NO3−, [14,15,16]. In this context, 1.5 and denitrification, which is the kWh of electricity are typically required to remove 1 kg of N in denitrification/nitrification or anammox processes [17]. There is an urgent need to develop and implement cost-competitive and sustainable NH3 recovery technologies in high strength wastewater such as digestates [20,21,22]
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