Recycling and depolluting the phosphorus in livestock waste

A product of mined phosphate rock  phosphorus is an indispensable element for life. In agriculture, it is used in mineral form to boost the growth of crops and animals. However, if it is incorrectly assimilated or used in excess, large quantities are subsequently released to the environment through erosion and run-off. In areas with intensive crop farming and/or animal production, over 50% of the phosphorus in rivers is the result of such diffuse pollution. A further source is the phosphorus used in detergents that is released in household wastewater. The excessive quantities of this mineral cause eutrophication in rivers and the development of certain bacteria that impact on water quality, thus affecting fishing, fish farms, swimming areas and drinking water.

Recycling in addition to good agricultural practices

To limit these risks, the regulations on spreading of phosphorus-based products now encourage farmers to reduce quantities of fertiliser. But in intensive animal-production zones such as Brittany, the quantities of phosphorus (and nitrogen) produced via animal effluents and available as fertiliser exceed the amounts needed for crops. How can the manure be treated to recover the phosphorus so as to be transportable to other regions, thus limiting pollution levels?

At Cemagref, scientists are developing processes to recycle phosphorus, whether from wastewater-treatment plants or agricultural effluents, so that it can then be used as a direct substitute for phosphate-based mineral fertilisers. A team is paying particular attention to the various types of phosphorus contained in livestock effluents and how they are modified during treatment. Between 60 and 80% of the phosphorus is in the form of mineral particles that settle with the organic matter.

A four-step process

The team has designed a process to separate the phosphorus from the organic matter in the effluents. It comprises four steps, i.e.

1) dissolve the phosphorus using formic acid, thus making it recoverable in the liquid phase,

2) separate the solids from the solution containing the phosphorus,

3) precipitate the solution chemically by adding magnesia to crystallise the phosphorus, and

4) filter the solution to recover the phosphorus in mineral form for use as a fertiliser. The goal is to obtain large crystals which are easier to filter and dry. Process optimisation depends on improving the separation step, prior to precipitation. If the solids are simply decanted, only 50% of the phosphorus can be recycled, but the crystals are large enough to be filtered. Another technique involves adding a polymer and then straining the solution before precipitation, which results in 80% recovery of the phosphorus, but the crystals are small and difficult to filter. The researchers are continuing their efforts to optimise this essential crystallisation step in order to devise the best conditions to produce large crystals and improve recycling efficiency).

Take action prior to the depletion of natural phosphate supplies

On the basis of current knowledge, phosphorus recycling necessarily includes the acid-dissolution step, whose high cost constitutes the main economic obstacle to wider use of this recycling process. The work carried out at Cemagref on these economic aspects, in conjunction with the European university of Brittany, has revealed that recycled fertiliser is not competitive with imported, chemical fertilisers. However, that should change over the coming decades due to the increase in fertiliser prices caused by the progressive depletion of phosphate reserves and the resulting increase in extraction costs. Though estimations diverge on when phosphorus supplies will be depleted, currently exploitable reserves may be gone within the next century.