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Björs, Marie, 2023. Separation and acidification of digested animal manure : properties of the future organic fertilizers. Second cycle, A2E. Uppsala: SLU, Dept. of Soil and Environment



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
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

Main title:Separation and acidification of digested animal manure
Subtitle:properties of the future organic fertilizers
Authors:Björs, Marie
Supervisor:Aronsson, Helena
Examiner:Dahlin, Sigrun
Series:Examensarbeten / Institutionen för mark och miljö, SLU
Volume/Sequential designation:2023:06
Year of Publication:2023
Level and depth descriptor:Second cycle, A2E
Student's programme affiliation:NY011 Agricutural programme - Soil/Plant, 300.0hp
Supervising department:(NL, NJ) > Dept. of Soil and Environment
Keywords:Digestate, Eutrophication, Farmers’ survey, Plasma activation, Screw press
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Deposited On:01 Jun 2023 07:07
Metadata Last Modified:02 Jun 2023 01:00

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