TECHNICALLY SPEAKING
A powerful push for phosphorus capture and recovery
With global phosphorous stocks dwindling, recovery of this agricultural necessity from sewage is becoming increasingly viable, says Frank Rogalla of Aqualia
ithout phosphorus we cannot produce food – but at current consumption rates, reserves could be depleted in the next 50 to 100 years. Projections suggest that global phosphorus demand could grow at 2.3% annually just to feed the growing world population, an estimate that was made before the growth of biofuels. The very concept of biofuels as a viable “renewable” source of energy might not hold if one of the fundamental elements is growing more scarce. Within a few decades, a “peak phosphorus” crunch could seriously threaten agriculture as global reserves of high-quality phosphate rock decline. At present extraction rates (around 40 million t/a P2O5), reserves are unlikely to last much more than 100 years.
W Recovered phosphate pellets magnified
In large parts of the developing world, phosphate supply is insufficient, both in terms of total application and imbalance in the N to P to K ratio. If many of the areas being farmed today were to receive sufficient phosphate to prevent mining of soil reserves, this in itself would substantially increase world demand. On the other hand, in some regions of Europe and the USA, there is an oversupply of phosphate to agriculture due to the large combined input of phosphate in the form of fertilisers and organic manures.
Increased imbalances
Part of the answer lies in better husbandry of phosphorus reserves: an effort that may require the creation of an international body to monitor the use and recycling of phosphorus. As animal wastes offer a potentially large source of phosphates for recovery, stricter controls on intensive livestock farming may become the norm, to avoid localised manure surpluses and their disposal.
An assessment for Western Europe shows that the phosphate excreted by livestock in this region could be some 50% more than the amount currently applied as mineral phosphate fertiliser. If a limited quantity of up to 40% of the phosphate contained in manure, mainly from animals in stables, could be collected and spread onto agricultural land, 4-5M tonnes of mineral fertiliser phosphate could be replaced by a more efficient use of manure, mainly in Europe and the USA.
Phosphorus capture could be required at all significant sewage treatment works, and its recovery from wastewater will become economically attractive. The technology of phosphate recovery is straightforward, yielding both the value of the recovered phosphorus, and significant savings in both treatment costs and disposal of the residual sludge.
Realistically, logistical factors, such as the cost of transport and the scale of installations, would make recovery an economic option only in the case of large, geographically concentrated waste streams. In rural areas, agricultural sludge or manure spreading will probably always remain the best option for recycling nutrients. In the UK even a conservative estimate of the potential for phosphorus recovery and recycling (50% recovery applicable to 25% of sewage and to 15% of animal wastes) represents half of industrial phosphate demand. Currently, the excess of nutrients from sewage treatment works, increasingly also from animal wastes, was prevented from reaching surface water through a strategy of phosphorus removal, not recovery. This is where phosphorus is transferred to sludge, either in an organic form as in biological phosphorus removal, or as a chemical precipitate, in iron or aluminium salts. The majority of phosphorus removal in Europe uses chemical precipitation, often simultaneous with secondary biological treatment, yielding a mix of precipitate with organic sludge which is of limited agronomic value. Because higher concentrations of precipitation chemicals are required than actually combine with the available phosphorus, these methods lead to an increase of sludge production around 40%.
Recovery of phosphorus for recycling, rather than its transfer into sewage sludges, may offer
34 Water & Wastewater Treatment December 2010
economic and environmental rewards for the water industry. For the phosphate industry it holds out the promise of a significant source of sustainable raw material. These benefits must be compared with the investment and running costs of phosphorus recovery installations.
Routes to recovery
Phosphorus removal by traditional precipitation will generally preclude recovery for recycling, as the resulting iron or aluminium compounds are incompatible with technologies used in the phosphate industry. They either require excessive energy input, to separate the phosphates from the added precipitation chemicals, or interfere with the industrial process.
For either the solvent extraction or electro- thermal reduction to yield high purity phosphate compounds, the upper limit is considered 1-2% of iron and aluminium. Incinerated sewage sludge, generated through precipitation, easily contains ten times as much with up to 20% iron (as Fe2O3). But several technologies are now implemented at both demonstration and full scale to show possible routes for phosphorus recovery. These processes isolate the recovered phosphorus in the form either of a calcium phosphate or magnesium ammonium phosphate (struvite). As a preliminary step to generate a concentrated liquid phosphate stream for recovery, biological phosphate removal looks very promising. In a side steam, the nutrient rich sludge can yield a liquor containing phosphorus in excess of 100mg/l, which would be particularly appropriate for phosphorus recovery. Full-scale struvite recovery processes were first introduced more than ten years ago in Japan and the Netherlands: The DHV Crystalactor fluid bed process, a the full scale installation at the AVEBE potato processing plant in the Netherlands (150m3/h)
The Unitika (Osaka) process was applied at the Ube Industries Sakai plant for industrial wastewaters and commissioned in 1998 at the Shimane Prefecture sewage works, Japan (45,000m3/d)
The Geochem Research/Delft University Earth Sciences stirred precipitation process extracts struvite from 700,000t/a of calf manure at Putten, Netherlands in 1998
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