48 23 Schoumans, O.F., 2007. Trends in de fosfaattoestand van landbouwgronden in Nederland in de periode 1998-2003. Alterra rapport 1537. Alterra Research Instituut voor de groene ruimte, Wageningen. http://library.wur.nl/ way/bestanden/clc/1855771.pdf. Schröder. J.J., F. de Buisonjé,, G. Kasper, N. Verdoes & K. Verloop, 2009. Mestscheiding: relaties tussen techniek, kosten, milieu en landbouwkundige waarde. rapport 287, Plant Research International B.V. Wageningen. http://edepot. wur.nl/50884. Phosphate from manure cannot fully replace mineral phosphate fertilisers. Crops with a short growing period and a high daily phosphate requirement will benefit from fertilisation with mineral phosphate fertilisers, particularly at a lower soil P status. Soils with ample or lower ratings benefit from row placement of water soluble mineral phosphate fertilisers. About 43% of the grassland has an ample or lower rating of the phosphate status of the soil, and 24% and 17.7% of the maize land and arable land respectively have an ample or lower rating (Schoumans, 200723). 24 25 Hjorth, M., K.V. Christensen, M.L. Christensen and S.G. Sommer, 2010. Solid–liquid separation of animal slurry in theory and practice. A review. Agron. Sustain. Dev. 30 (2010) 153–180. 4. Treatment of manure and other biosolids Animal manure contains essential plant nutrients. The composition of manure does not always match the needs of specific crops, farm types or regions (Schröder et al., 200924). In general, animal manure cannot substitute mineral fertilisers completely unless it is processed. Phosphate in animal manure is available in the long term, but in the short term availability does not necessarily match crop requirements (i.e. high-phosphate requiring crops such as lettuce or spinach). Manure treatment involves solid-liquid phase separation by sedimentation, centrifugation, drainage or pressurised filtration. The pre-treatment of manure includes the addition of chemicals to promote flocculation and coagulation and the formation of struvites (see section 6.3). Post treatment includes a number of separation techniques e.g. evaporation or membrane filtration of the liquid phase (Hjorth et al. 201025). Techniques using membranes (microfiltration, ultrafiltration, reverse osmosis, nanofiltration) are the most advanced but are still in the research and development phase. These techniques should eventually lead to the development of treatment methods that recover most of the nutrients (phosphate) present in manure and other biosolids, and lead to the production of solid or liquid fractions which can fully replace mineral fertilisers. Strangely enough, struvite is still not permitted as a fertiliser in the Netherlands, although it is widely used in agriculture in Japan, and studies in Sweden have shown that struvite is a better fertiliser than the best available commercial fertilisers. The few plants in the Netherlands that treat animal manure or process water must sell their product as a low-valued secondary phosphate ore to Thermphos. Manure processing techniques are also used to make products other than mineral fertilisers (Hjorth et al., 200926 ). Techniques for producing energy from manure and other biosolids include anaerobic digestion to produce methane, incineration, or other techniques such as pyrolysis, torrefaction and gasification to produce other forms of bioenergy. Energy production has substituted odour emission reduction as the main motive for processing manure or other biosolids. Adding olivine powder to digesters eradicates most of the odour problems. Separation of the liquid and solid phase is the most viable technique currently available. Other techniques are still in the research and development stage and the prospects for medium and long-term beneficiation are good. 26 Hjorth, M., A.M.Nielsen, T. Nyord, M.N. Hansen, P.Nissen, S.G. Sommer, 2009. Nutrient value, odour emission and energy production of manure as influenced by anaerobic digestion and separation. Agron. Sustain. Dev. 29: 329–338. Pagina 57

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