Capturing the lost phosphorus

Abstract

Minable phosphorus (P) reserves are being depleted and will need to be replaced by recovering P that currently is lost from the agricultural system, causing water-quality problems. The largest two flows of lost P are in agricultural runoff and erosion (∼46% of mined P globally) and animal wastes (∼40%). These flows are quite distinct. Runoff has a very high volumetric flow rate, but a low P concentration; animal wastes have low flow rates, but a high P concentration together with a high concentration of organic material. Recovering the lost P in animal wastes is technically and economically more tractable, and it is the focus for this review of promising P-capture technologies. P capture requires that organic P be transformed into inorganic P (phosphate). For high-strength animal wastes, P release can be accomplished in tandem with anaerobic treatment that converts the energy value in the organic matter to CH(4), H(2), or electricity. Once present as phosphate, the P can be captured in a reusable form by four approaches. Most well developed is precipitation as magnesium or calcium solids. Less developed, but promising are adsorption to iron-based adsorbents, ion exchange to phosphate-selective solids, and uptake by photosynthetic microorganisms or P-selective proteins.

Chemistry: Reuse water pollutants

“…Also discarded are forms of nitrogen and phosphorus that would be valuable for making fertilizers. Precipitated by adding calcium, iron or aluminum salts, 90% of the phosphorus ends up buried in landfill because the precipitates cannot be taken up by plants and are often contaminated with toxic metals 8,9 . Likewise, more than 80% of the nitrogen is lost through conversion to nitrogen gas by microbes.…”

“…One option is recovering both as struvite, a slow-release fertilizer that is precipitated by adding magnesium and lime. This is commercially viable at the high phosphate and ammonium concentrations (hundreds of milligrams per litre) found in sludge or livestock wastewater, but it is ineffective for domestic wastewater 8 .…”

“…Two emerging technologies -ion exchange and electrodialysis -capture and concentrate phosphorus and nitrogen enough to be recovered from effluent as struvite 8 . In the first, phosphate ions are swapped with anions (such as carbonate) or ammonium ions swapped with cations (such as sodium ions) and adsorbed by materials such as ironbased hydroxides, zeolites and polymers.…”

“…For example, membranes currently cost hundreds of dollars per square metre. And electrodialytic extraction (at a recovery rate of 90%) of phosphorus and nitrogen consumes roughly 0.23 kWh m -3 and 0.14 kWh m -3 , respectively -around two-thirds of the energy consumed in the activated-sludge process 8 . Use of MXCs may partly offset that energy input by generating electricity, but microorganisms and biomolecules aggravate membrane fouling 10 .…”

“…For example, AnMBRs can remove up to 98% of the organic matter (around 18 kilograms per cubic metre) from petrochemical effluent, producing 100 times more methane than is achievable with domestic wastewater. Livestock wastewater is rich in organic molecules and phosphorus, making it an important potential source of energy and fertilizer 8 .…”