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Animal wastes as a potential source of recoverable phosphorus

By Phil Hobbs, Phil Haygarth and Dave Chadwick.
Institute of Grassland and Environmental Research, North Wyke, Okehampton, Devon, EX20 2SB.

Over recent years, housing for livestock has increased substantially. In many cases, such intensification has lead to larger volumes of stored waste, which are more anaerobic in nature. Pain et al (1998) estimated that approximately 90 million tonnes of farm manure was produced in the UK in 1993. Farm manure was previously considered part of the natural means of fertilising the land. These larger wastes volumes are chiefly from cattle, pig and poultry and are considered a problem. First in terms, of when and how often to landspread and second, if wastes cannot be spread on the land, how they can be stored or disposed of effectively without risk of transfer to watercourses. In particular phosphorus (P) contributes to watercourse pollution in various chemical forms and oxidative states by causing eutrophication and associated algae blooms.

Using average P concentrations of the different manure types (MAFF, 1994) and the volume of manure generated we estimate that about 100,000 tonnes of phosphorus is present in manure, chiefly from cattle, poultry and pigs. Manure is collected and stored in different forms. Poultry manure (4.3 mt ) is collected as a paste like material with no added straw or absorbent bedding. Whereas cow and pig manure has straw added to about 50% of the 75 and 11 mt respectively. The other 50% is as slurry which is a mixture of urine and faeces combined with some rain water or washings. Cattle waste contains 45 kt of P but at lower concentrations than that found in pig or poultry manure.

Each of these animals has different ways of digesting and excreting waste. The cow has a rumen to digest plant material and phytase to breakdown inositol hexaphosphate. Phytase is lacking in some animals, such as the pig and chicken, which are monogastric.

The absorption of phosphorus by animals is controlled by a range of co-dependent factors that include dietary levels, vitamin D and others, as well as the health and age of the animal. Generally, about 70% of phosphorus ingested is excreted (Church 1979). Faeces generally contain more phosphorus than urine and this can be further categorised into inorganic, acid soluble, lipid and residual phosphorus (Barnett 1994).

Wastes that are stored anaerobically change in composition. Studies by Gerritse and Zugec (1977) using 32PO4 ions on the dry matter of pig slurry demonstrated that phosphorus was cycled homogeneously to all forms within 10 to 20 weeks. The ten slurries varied in dry matter composition from 5 to 10% and the total P content was 20g kg-1. The composition was approximately 15% organic P and 85% inorganic P. The latter was mainly present in the form of crystals or precipitates of calcium phosphate and was found in the solid fraction of the waste. Only 5% of the inorganic P was soluble.

Gerritse (1982) also produced a model for phosphorus cycling in slurry using labelled phosphate (32PO43-). It can be seen that microorganisms play a central role in cycling P forms in farm waste and manipulating this P cycle could be a possible source of recovering P effectively. Gerritse (unpublished) also found that this model was true for other farm wastes although not specifying which ones.

The value of P as phosphate in wastes is estimated as £77 million in the UK (based on 25p per kg) (Chadwick, unpublished). However, the economics of phosphorus recovery from farm wastes is difficult to evaluate. Certainly, there are higher concentrations of P in farm wastes (and especially in poultry manure) than human sewage. However economic recovery is dependent upon not only the amount present but also the means of extraction and the form of P species produced from the process. Barring interdependence of each stage mentioned above, then if microbial action were effective in modifying and/or concentrating P into an easily extractable and appropriate product this would be the preferred least-cost method.

Reference List

Barnett, G.M. (1994) Phosphorus forms in animal manure. Bioresource Technology 49, 139-147.

Church, D.C. (1979) Digestive physiology and nutrition of ruminants. In Nutrition, Vol 2 ed D C Church . O and B Books, Inc., Corvallis, OR, pp62-71.

Gerritse, R.G., Dewilligen, P. And Raats, P.C. (1982) Transport and Fixation of Phosphate in Acid, Homogeneous Soils .3. Experimental Case-Study Of Acid, Sandy Soil Columns Heavily Treated With Pig Slurry. Agriculture And Environment 7, 175-185.

Gerritse, R.G., Zugec I . (1977) The phosphorus cycle in pig slurry measured from 32PO4 distrubution rates. J Agriculture Science 88, 101-109.

MAFF (1994) Fertiliser Recommendations for Agricultural and Horticultural Crops. RB209 Sixth Edition. HMSO. Pp112

Pain, B.F., van der Weerden, T.J., Chambers B.J, Phillips, V.R. and Jarvis, S.C. (1998) A new inventory of ammonia emission from UK agriculture. Atmospheric Environment 32, 309-313.