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Technologies / Phosphorus recycling

ENVIRONMENTAL TECHNOLOGIES BY AGROVIT:

The technology of phosphorus recycling from the sewage sludge ash with manufacturing slow action controlled release phosphate fertilizers

Role of phosphorus in the sustainable development. Phosphorus, as a major prerequisite for plant growth and crop production, is one of the principal elements for modern civilization. In the physiology of plants, phosphate participates in such vital processes of plant growth and development as changing enzyme properties, chemical energy transfer and others. After being assimilated by plants, phosphorus enters the food chain of humans and animals. Phosphorus is also a major player in some types of industrial production, for example - in detergent industry.

Unlike some other natural resources, phosphorus is irreplaceable because of its biochemical role. Comparing the role of phosphate in the plant growth with that of external energy (such as thermal and light sources) one can say that both are equally important, but phosphate has no alternatives, unlike, for example, solar energy that can be replaced by artificial light sources, if necessary.

The world production of P2O5 has been around 38 Mio t per year over the last decade; between 80% and 85% of the production has been used as a fertilizer. The ultimate source of phosphorus for the agricultural and industrial use is mineral ore. Natural depletion of phosphate ore resources is a real danger of the nearest future.

Phosphorus chain. Another problem is the low efficiency of the phosphorus utilization by its end-users - plants. Typical polycrystalline phosphate fertilizer is assimilated at a rate of only 30-40%. With the given dosage of 300-400 kg phosphate fertilizer per hectar per annum, no less than 200 kg is washed out from each hectar into the ground water catchment area and creates further uncontrollable flow of phosphoric compounds into lakes and seas. In the absence of efficient purification and recycling systems for agricultural run-offs, extensive use of phosphorus in fertilizers results in severe pollution. For example P-rich washout from the fields to the fresh water reservoirs (e.g. Baltic sea) causes eutrophication - an irreversible degradation of aquatic ecosystems where high nutrient concentrations stimulate blooms of phytoplankton and bentos, breaking the fragile balance of marine ecosystems. Altogether, about 50% of P pollution comes from agricultural sources.

Another half of the phosphorus inflow enters the ecosystems through urban sewage facilities with human faeces and detergent washouts. In the water purification plants phosphorus sediments in the form of sludge. Before, sludge was deposited in the fields thus partly compensating for the permanents washout of phosphorus. However further disposal of sludge for agricultural use is forbidden by recent EU directives. Another directive prevents landfilling of sludge in special deposits as a waste with more than 5% contents of dry carbohydrate and potential source of organic toxins. These measures leave only one way out- sludge incineration. It eliminates all organic substances but produces ash - rich in phosphorus but highly volatile material containing heavy metals in amounts preventing further utilization. Ash needs smaller but advanced storage systems. About 300 000 tons of such ash is generated in European sludge incinerators every year, and these amounts rapidly grow.

Thus the phosphorus chain that begins at the phosphorite mines outside Europe bifurcates at the end-user locations and further ends either in the European water basins (agricultural input) or in the landfills (urban and industrial input). Both branches have serious environmental implications.

These problems are nowadays addressed in the form of standalone measures (perfection of incineration and storage procedures, decrease in the fertilizer use through precision agriculture and similar technologies). No concise large scale system involving both branches of phosphorus pollution is currently in place.

Phosphorus cycle? A prospective technology of phosphorus utilization cannot avoid coupling of the phosphorus sinks to its sources. The world sewage sludge generation is around 25-30 Mio t per year; up to 5 Mio t of P2O5 can be recovered from this source.

Attempts are made to process sewage sludge into various fertilizing agents. In a typical case such technologies address only one "sludge" part of the phosphorus chain. Thus the recycling effort is decoupled from the agricultural effort to decrease use of fertilizing agents.

In this proposal we suggest a complex technology that combines sewage sludge processing with the production of new fertilizing agents with high assimilation levels and low washout. Such technology could pave the road to really sustainable phosphorus use and could simultaneously reduce the Europe's dependence on phosphorite imports solving major environmental problems.

The input for our technology is the ash resulting from sewage sludge incineration. The concrete research objectives may be tentatively broken into two main stages. The first stage deals with the inevitable necessity of ridding the ash from hazardous compounds (heavy metals, excess iron etc). We should note that the economic feasibility of such routines depends also on the possibility of collection of the metallic admixtures as a useful byproduct.

The resulting material rich in phosphorus and free from pollutants will represent the input for the next stage - manufacturing new materials comprising slow release complex kinetics fertilizers. Such vitreo-ceramic materials with controlled physical-chemical properties represent a new generation of fertilizing agents which furnish plants and soil microflora with nutrients "on demand", depending on soil temperature, humidity and agrobiochemical activity of the plant roots. Existing types of such fertilizers demonstrate almost complete (over 90%) assimilation and, as a consequence, very low runoff.

In the end, our research results will lay the foundation for development of an economically feasible and environmentally friendly technology of sustainable management of phosphorus turnover, which will have important local and global implications.