Pilot-scale magnetic recovery of vivianite from digested sewage sludge.

Phosphorus (P) is an essential resource for food production and chemical industry. Phosphorus use has to become more sustainable and should include phosphorus recycling from secondary sources. About 20% of P ends up in sewage sludge, making this a substantial secondary P source. There is currently a technological gap to recover P from sludge locally at wastewater treatment plants (WWTP) that remove P by dosing iron. Vivianite (Fe3(PO4)2•8(H2O)) is the main iron phosphate mineral that forms during anaerobic digestion of sewage sludge, provided that enough iron is present. Vivianite is paramagnetic and can be recovered using a magnetic separator. In this study, we have scaled up vivianite separation from lab-scale to bench- and pilot-scale. Bench-scale tests showed good separation of vivianite from digested sewage sludge and that a pulsation force is crucial for obtaining a concentrate with a high P grade. A pilot-scale magnetic separator (capacity 1.0 m3/h) was used to recover vivianite from digested sewage sludge at a WWTP. Recirculating and reprocessing sludge allows over 80% vivianite recovery within three passes. A concentrated P-product was produced with a vivianite content of up to 800 mg/g and a P content of 98 mg/g. P recovery is limited by the amount of P bound in vivianite and can be increased by increased iron dosing. With sufficient iron dosing, the vivianite content can be increased, and subsequently more P can be recovered. This would allow compliance with existing German legislation, which requires a P recovery larger than 50%.

Competition of electrogens with methanogens for hydrogen in bioanodes.

Bioelectrochemical systems (BES) can provide an energy efficient way to recover nutrients from wastewaters. However, the electron donors available in wastewater are often not sufficient to recover the total amount of nutrients. This work investigates hydrogen (H2) as an additional substrate for bioanodes. This hydrogen can be produced in the fermentation of complex organic waste or could be recycled from the cathode. Understanding how to influence the competition of electroactive microorganisms (EAM) with methanogens for H2 gas from different sources is key to successful application of H2 as additional electron donor in bioelectrochemical nutrient recovery. Ethanol (EtOH) was used as model compound for complex wastewaters since it is fermented into both acetate and H2. EtOH was efficiently converted into electricity (e-) by a syntrophic biofilm. Total recovered charge from 1 mM EtOH was 20% higher than for the same amount of acetate. This means that H2 from EtOH fermentation was converted by EAM into electricity. Low EtOH concentrations (1  mM) led to higher conversion efficiencies into electricity than higher concentrations (5 and 10  mM). Thermodynamic calculations show this correlates with a higher energy gain for electrogens compared to methanogens at low H2 concentrations. Cumulatively adding 1 mM EtOH without medium exchange (14 times in 14 days) resulted in stable conversion of H2 to e- (67%-77% e-) rather than methane. With H2 gas as electron donor, 68 ±â€¯2% H2 was converted into e- with no carbon source added, and still 53 ±â€¯5% to e- when 50  mM bicarbonate was provided. These results show that under the provided conditions, electrogens can outcompete methanogens for H2 as additional electron donor in MECs for nutrient recovery.

(Bio)electrochemical ammonia recovery: progress and perspectives.

In recent years, (bio)electrochemical systems (B)ES have emerged as an energy efficient alternative for the recovery of TAN (total ammonia nitrogen, including ammonia and ammonium) from wastewater. In these systems, TAN is removed or concentrated from the wastewater under the influence of an electrical current and transported to the cathode. Subsequently, it can be removed or recovered through stripping, chemisorption, or forward osmosis. A crucial parameter that determines the energy required to recover TAN is the load ratio: the ratio between TAN loading and applied current. For electrochemical TAN recovery, an energy input is required, while in bioelectrochemical recovery, electric energy can be recovered together with TAN. Bioelectrochemical recovery relies on the microbial oxidation of COD for the production of electrons, which drives TAN transport. Here, the state-of-the-art of (bio)electrochemical TAN recovery is described, the performance of (B)ES for TAN recovery is analyzed, the potential of different wastewaters for BES-based TAN recovery is evaluated, the microorganisms found on bioanodes that treat wastewater high in TAN are reported, and the toxic effect of the typical conditions in such systems (e.g., high pH, TAN, and salt concentrations) are described. For future application, toxicity effects for electrochemically active bacteria need better understanding, and the technologies need to be demonstrated on larger scale.

Clinoptilolite-based mixed matrix membranes for the selective recovery of potassium and ammonium.

A clinoptilolite-based mixed matrix membrane (MMM) was developed and studied for the selective recovery of ammonium and potassium. Adsorption of sodium (Na(+)), potassium (K(+)) and ammonium (NH4(+)) was investigated with single salt and equimolar salt solution under static and dynamic conditions. Furthermore, the adsorption capacity of clinoptilolite was investigated when embedded in the MMM and in clay form. Two conditioning methods were compared: HCl and NaCl. Conditioned clinoptilolite with NaCl gave higher static adsorption capacities than with HCl which alters the chemical structure of clinoptilolite. The adsorption of Na(+) was not detected in the static adsorption experiments and results showed that Na(+) adsorbed during the conditioning process it was exchanged by K(+) and NH4(+).The clinoptilolite embedded in MMM reduced the porosity of the MMM so the highest adsorption capacity was reached when the amount of polymer was the lowest: 30 wt% polymer and 70 wt% clinoptilolite. The application of MMM in a dead-end filtration cell (dynamic adsorption) resulted in higher adsorption capacities compared to static conditions and comparable results between synthetic solutions and diluted urine samples. This indicates that MMM is a suitable method for the recovery of K(+) and NH4(+) directly from a diluted urine matrix. The desorption (recovery) of K(+) and NH4(+) from MMM was higher using water at 60 °C than using an acidic treatment.

Ammonium recovery and energy production from urine by a microbial fuel cell.

Nitrogen recovery through NH(3) stripping is energy intensive and requires large amounts of chemicals. Therefore, a microbial fuel cell was developed to simultaneously produce energy and recover ammonium. The applied microbial fuel cell used a gas diffusion cathode. The ammonium transport to the cathode occurred due to migration of ammonium and diffusion of ammonia. In the cathode chamber ionic ammonium was converted to volatile ammonia due to the high pH. Ammonia was recovered from the liquid-gas boundary via volatilization and subsequent absorption into an acid solution. An ammonium recovery rate of 3.29 g(N) d(-1) m(-2) (vs. membrane surface area) was achieved at a current density of 0.50 A m(-2) (vs. membrane surface area). The energy balance showed a surplus of energy 3.46 kJ g(N)(-1), which means more energy was produced than needed for the ammonium recovery. Hence, ammonium recovery and simultaneous energy production from urine was proven possible by this novel approach.

Effects of ammonium concentration and charge exchange on ammonium recovery from high strength wastewater using a microbial fuel cell.

Ammonium recovery using a two chamber microbial fuel cell (MFC) was investigated at high ammonium concentration. Increasing the ammonium concentration (from 0.07 to 4 g ammonium-nitrogen/L) by addition of ammonium chloride did not affect the performance of the MFC. The obtained current densities by DC-voltammetry were higher than 6A/m(2) for both operated MFCs. Also continuous operation at lower external resistance (250 Ω) showed an increased current density (0.9A/m(2)). Effective ammonium recovery can be achieved by migrational ion flux through the cation exchange membrane to the cathode chamber, driven by the electron production from degradation of organic substrate. The charge transport was proportional to the concentration of ions. Nonetheless, a concentration gradient will influence the charge transport. Furthermore, a charge exchange process can influence the charge transport and therefore the recovery of specific ions.