Hygienisering av gödsel med urea och ammoniak
Ottoson, Jakob R.; Nordin, Annika; Vinnerås, Björn
Cow manure may contain substantial amounts of pathogenic microorganisms such as Salmonella, Campylobacter, enterohaemorragic E. coll (EHEC), parasites and viruses. Insufficiently disinfected manure has been shown to be a significant factor for the occurrence of pathogens in animal herds. Further, transmission of disease from manure to man has been verified, for example the outbreak of EHEC in Halland this summer (2005). Treatment of the manure before application to land is suggested, to decrease the risk of disease transmission to man or other animals, since microorganisms may survive for extended periods of time in soil. Storage, composting, anaerobic digestion and chemical disinfection are common practises. In outbreak situations, the cormnon practise is lime treatment of the manure. Lime raises pH to around 12 for a period of time, leading to a rapid pathogen die-off The problem is that the handling is complicated with salt precipitation making the incorporation difficult. Further, lime treatment leads to nitrogen losses with a lower agronomic value of the manure. In this project the possibilities of pathogen disinfection with urea or ammonia was evaluated. Urea is the globally most used nitrogen fertiliser. When applied to soil it is converted to ammonia and carbon dioxide by the enzyme urease that is naturally excreted by soil and intestinal bacteria. The same thing will happen if urea is added to manure. With a higher pH, more of the ammonia will be present in the uncharged form, which is disinfecting. At a lower pH, the ammonia will be present in the plant assimilable ion form. lf the ammonia is added as ammonia (aq), which is a stronger base than urea, pH will be higher. Thus, less ammonia needs to be added for the same disinfecting effect providing an economically interesting option. The addition of urea or ammonia raises the agronomic value of manure, however, it is important that the manure well is covered to avoid nitrogen losses. Earlier die-off studies in human faeces have reported a rapid die-off of especially E. coli, but also Salmonella after addition of 6% (w/w) urea. In this study we have added high numbers of Salmonella, enterococci and a bacteriophage to liquid manure (TS 12%) that was collected at Kungsängens gård in Uppsala. Enterococci are, after coliforms, the most common group of faecal indicator microorgnanisms. They are used as a complement to the coliforms since they are more tolerant to many types of environmental stress. Bacteriophages are viruses that infect bacteria, but not eucaryotic cells, and are therefore easy and harmless to handle. They have earlier been used as a model for viral die-off in several reported studies. The microorganisms were added in numbers between 106 and 108 colony- or plackforming units (CFU/PFU) per gram manure. After the organisms had the chance to adapt to the material over night, 2% urea or 0.5% ammonia was added to the material. There was also a control study with no addition of ammonia. The experiments were carried out in triplicates at 4 and 14 °C. Salmonella were severely affected by the treatment and could not be detected after 2 days at 14 °C and 6 days at 4 °C after the addition of ammonia (~pH 9.7). Also urea treatment (~pH 9.2) gave a significant die-off of Salmonella at 14 as well as 4 °C. The time for l log reduction (90%), decimal reduction, was reduced between 76% and 97% compared to the controls (Table, at the end of the summary). Enterococci was not affected in the same way as Salmonella even if the die-off was significantly quicker in ammonia as well as urea treated manure than in the control, in the latter case, however, only at 14 °C (see Table, at the end of the summary). Further, it took some time for the ammonia treatment to be effective. Urea addition seemed to be growth promoting, at least initially. This phenomenon has earlier been documented in studies of enterococci in urine. The phages were not affected at all by ammonia treatment but the reduction took place log- linearly (exponentially) for other reasons, probably by the presence of DNAse (enzyme breaking down DNA) in the material. The die-off was significantly quicker at 14 than 4 °C. Another Salmonella experiment was carried out, where an outbreak situation at a farm was simulated. Ammonia or urea was added to liquid manure in jars that were kept over the weekend. During the following week, 1% salmonella infected manure (107 CFU/ g) was added on top of the disinfected manure. This is what would happen at a farm if disinfection was performed during the time of an asymptomatic outbreak Infected manure was added during five consecutive days without any incorporation into the disinfected manure. After the following weekend, the contents in the jars were mixed and the material sampled. The Salmonella densities were 1 000 times lower in the ammonia treated, and 100 times lower in the urea treated, manure than in the control. After another two days, the Salmonella level was below detection limit in the ammonia treated manure, after four in the urea treated, while it was still possible to detect Salmonella in the control after 35 days when the experiment was ended. Ammonia, added as a water-soluble or in the form of urea, proved to effectively disinfect Salmonella in cow manure (liquid manure with a dry substance, DS, of 12 %). If a six log reduction (99,999%) is desired, less than a week treatment with 0.5% ammonia is needed in 14 as well as 4 °C. Treatment with urea for 10 days at 14 °C or 24 days at 4 °C will also lead to the same hygiene effect. This time can be shortened by raising the pH (and thereby the amount of free ammonia) in the material by the addition of a stronger base such as lime, ashes, sodium- or potassium hydroxide. The time for five log reduction without additives was 170 days at 4 and 42 days at 14 °C. The long-term ammonia disinfection effect in covered compartments leads to further treatment during transports in tanks. Otherwise, reinfection of hygienised material has caused problems during transports. Further, disinfection of manure is a safety barrier in case of an asymptomatic outbreak at a farm. Ruminants are an important reservoir for EHEC and only few strains induces infection in animals that still can shed a lot of bacteria of clinical importance for humans. The use of enterococci as an indicator of Salmonella occurrence after ammonia treatment would lead to an overestimation of the risk due to their tolerance and different behaviour in the material. We suggest that other indicators are used, for example E. coli or faecal coliforms, as indicators of Salmonella occurrence in ammonia treated manure. To be able to account for the uncertainty of the analysis the time to achieve five log reduction was simulated and results presented in the table below. The recommended treatment is based on the upper 95% value and simplified to; at least two weeks treatment with 2% urea at temperatures above 10 °C, one month at lower temperatures. For ammonia one week after the addition of 0.5% is sufficient while storage should include at least one summer season.
Gödsel; hygien; ammoniakhygienisering; hygienisering; salmonella
Rapport. Miljö, teknik och lantbruk
Publisher: Institutionen för biometri och teknik, Sveriges lantbruksuniversitet
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