Separation and acidification of digested animal manure : properties of the future organic fertilizers

Abstract: Agricultural areas with a high animal density contribute to eutrophication in waterbodies and seas worldwide due to accumulation of nutrients around animal farms. Animal manure is heavy and bulky, thus unpractical to transport long distances and new techniques have been developed to refine animal manure and make it easier to transport. In this paper mechanical separation of digested liquid animal manure was investigated, a method where the digestate is separated into a solid and a liquid phase. The solid fraction has a high phosphorus and carbon concentration as well as a high dry matter content, making it a good phosphorus fertilizer and easy to transport. The liquid fraction contains the main part of nitrogen and works well to spread on fields adjacent to the treatment facility. However, ammonium nitrogen in the liquid fraction risks being lost quickly to the atmosphere due to ammonia volatilisation. By lowering the pH of liquid fractions, the ammonia emission can be reduced. Two methods for lowering the ammonia emission were compared to see how lower pH might affect the mineralizing potential of nitrogen and carbon in the digested manure. To investigate their fertilizer value, an incubation experiment was conducted with different fractions of digestate from a biogas plant in Kalmar Sweden. On the biogas plant, pig, cow, and poultry manure as well as food and slaughterhouse waste were anaerobically digested and the digestate was separated with a screw press. To potentially lower the ammonia emission, a part of the liquid fraction was plasma activated, which lowered the pH from 8.2 to 4.4. All the samples from Kalmar were transported to Uppsala for the incubation experiment. In Uppsala another fraction of the liquid fraction was acidified with sulfuric acids to pH 5.5. The different fractions, raw unseparated digestate, solid fraction, non-acidified liquid fraction, acidified liquid fraction and plasma activated liquid fraction was incubated for 44 days to measure mineral nitrogen concentration, mineralization rate of nitrogen and carbon dioxide emission. The mineral nitrogen concentration was analyzed on several occasions during the experiment by AgriLab in Uppsala. By plotting the change of mineral concentration over time, the mineralization rate could be calculated. In addition to the incubation, additional cups were prepared the same way and placed in glass jars with falcon tubes with 50 ml 0.5 M NaOH. The sodium hydroxide in the falcon tubes captured the emitted CO2 which in turn could be estimated through titration with H2SO4. In this experiment, the nitrification was delayed when the pH was lowered while a netmineralization still occurred. The liquid fraction treated with sulfuric acid only had a delay for a few days and the mineralization rate was about the same as for the non-acidified liquid fraction. The plasma activated liquid fraction had a delay during the entire experiment (44 days) and the lowest net mineralization of all treatments. The plasma activated liquid fraction is probably still a good fertilizer due to the high initial nitrite/nitrate concentration compared to the other materials. A farmers survey was incorporated in the thesis to see if any fertilizers produced might interest Swedish framers. The survey consisted of 22 questions about current and future use of organic fertilizers as well as positive and negative properties of the different organic fertilizers on the market. Most of the farmers in the survey wanted a fast release of nitrogen and phosphorus and considered carbon important in organic fertilizers. According to the incubation experiments, only the liquid materials had a net mineralization during the first 44 days, which gives a positive delivery of nitrogen to crops in addition to the initial content of ammonium nitrogen. However, these materials can contribute to soil compaction due to a higher water content. Soil compaction was an important factor which might hinder farmers to use organic fertilizers according to the survey. The solid fraction on the other hand is at low risk for soil compaction problems and had a high carbon and Abstract phosphorus concentrations. However, the solid fraction caused net-immobilization of nitrogen which lowers the nitrogen fertilizing value of this fraction. Many farmers expresses that an organic fertilizer needs to be price worthy and must compete with easily available untreated manure that already is on the farm. The refined manure needs to have an added value like easily available nutrients, lower environmental impact, and lower contribution to soil compaction. For example, the solid fraction in this experiment might improve soil structure and increase phosphorus concentrations in phosphorus-poor soils. Additionally, the liquid fraction can be a good nitrogen fertilizer while lowering the phosphorus surplus in phosphorus-rich areas given that the solid fraction is transported away (i.e., better for the environment). Finally, this experiment showed that separated manure can be an attractive for farmers depending on the farms nutrient need. Keywords: Digestate, Eutrophication, Farmers’ survey, Plasma activation, Screw press