Karin Jørgensen, Lars S. Jensen1
Key words: Animal manure, solid separation, nutrients, composting
Abstract
Separation of animal slurry into a liquid and a bio-solid fraction is a new opportunity for farmers in Denmark to comply with strict environmental regulation. A comprehensive investigation has been carried out in order to investigate the chemical variability among bio-solids from different separation plant types. A great variability among the samples in several parameters like, ash, nitrogen and phosphorous content was found. Knowledge about this variability can contribute to future recommendations about alternative utilisation methods of the bio-solids, such as composting.
Introduction
Denmark has one of the highest animal production densities in Europe. Technologies for separation of animal slurry constitutes a new opportunity for farmers to transport excess nutrients in the form of bio- solids over longer distances to areas without nutrient excess, in order to comply with strict environmental regulation. By separation, the slurry is divided into a liquid fraction containing soluble components such as mineral nitrogen and potassium and a solid, fiber rich fraction (10-15%), containing the majority of the organic matter. The two most common separation systems are based on either chemical separation (CHEM) which has a high efficiency with respect to phosphorous removal from the liquid fraction or mechanical separation (MEC) more efficient in removal of total solids. Additionally, a number of biogas plants, digesting animal slurry anaerobically, have installed a decanting centrifuge, separating the manure mechanically after the biogas procedure (DEC/BIO). Currently (2007), about 50 animal manure separation units are operating on farms and biogas plants in Denmark, separating about 3 % of all the animal slurry (27 million tons annually) (Birkmose, 2007). There are many initiatives to develop alternative utilization opportunities for the bio-solids, aiming at either land application or bio-energy production. Composting of the bio-solids could be one alternative, by producing either a stabilized product of reduced volume and weight facilitating export to areas/countries poor in organic matter, or a high quality product to be sold as an alternative for peat based soil improvers or growth media. However, the processing and utilization of bio-solids will be greatly influenced by their quality, i.e. content of nutrients and degradability. The objective of this study was to determine the variability in quality parameters of separated animal slurry bio-solids in order to make a foundation for suitable future recommendations of the best utilization methods for the individual types of manure bio-solids.
Materials and methods
Sampling of about 50 different types of bio-solids from various animal slurry separation plants in Denmark was carried out in the spring 2007. The samples originated mainly from pig farms and biogas plants treating mixed (pig and cattle) slurry, and a few from dairy and fur animal farms. Most of the samples were collected immediately after separation, whereas the rest had been stored (2-14 months) on farm before sampling. All samples were analyzed for total solids (TS) and ash content. pH, electric conductivity was measured in a 1:5 w.w. water suspension, total nitrogen was measured by Kjeldahl distillation and ammonia nitrogen was measured at spectrophotomety in a 1:20 w.w. 1 M KCL suspension. Total phosphorus was determined by digestion with concentrated nitric acid followed by measurement with molybdenum colour reaction by spectrophotometry.
Results
A significant variation among the samples was observed. An overview of the preliminary results is presented in table 1. The results presented are mean values from all samples within the grouping of three separation plant types which substantiates in some cases a relatively large variation within the groups, because within group samples represents both fresh and stored samples and in some cases samples from different animal types. The highest ash content, Ec, N- total and P total is observed for DEC/BIO samples. The highest total solids content is generally found in MEC samples but the lowest P concentrations are also found here. The total nitrogen concentration is generally highest in DEC/BIO samples, whereas the ammonia concentration is almost the same for the three bio-solid types.
1 Department of Agricultural Sciences, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Copenhagen, Denmark.E-Mail [email protected].
Table 1: Quality parameters of 44 samples of bio-solids from separated animal slurry. All parameters are presented as mean values with standard deviation in parentheses.
TS % Ash % pH Ec (uS/cm) N total g/kg w.w. NH4-N g/kg w.w. P total g/kg w.w. DEC/BIO (n=10) 31.9 (6.5) 33.7 (8.9) 8.1 (1.2) 46.9 (24.3) 10.53 (2.28) 3.35 (1.83) 10.10 (5.64) CHEM (n=22) 24.2 (5.3) 18.9 (6.6) 8.2 (0.5) 35.8 (13.7) 8.57 (2.56) 3.80 (1.37) 5.55 (1.76) MEC (n=20) 34.3 (8.8) 8.8 (3.4) 7.8 (1.1) 25.8 (14.5) 6.64 (2.58) 3.22 (2.03) 2.55 (1.56) Discussion
The great variability among the three different types of bio-solid was expected due to the different nature of the samples. In this experiment we only have information about the separation type and animal type. All information about feeding strategy of the animals, slurry treatment and storage that has a possible influence on the bio-solid quality (Møller 2002), is not included. Especially pre- treatment of slurry in anaerobic digestion plants (bio-gas) or just longer storage periods of the slurry, where it undergoes a decomposition process can lead to a decrease in total suspended solids, and contributes to a transfer of nutrients, especially nitrogen and phosphorus, between different fractions and chemical forms in manure (Henze, et al 1996). This can contribute to the explanation of the increased ash and nutrient content in DEC/BIO samples. The CHEM separation procedure is a relatively new technology applied on animal slurry, where the solids are precipitated with a chemical polymer followed by a mechanical separation. This treatment facilitates separation of the smaller particles from the slurry which contains the main part of the phosphorus (Masse, et al 2005) – explaining the increased phosphorus content in CHEM samples compared to MEC samples. The proportion of ammonium nitrogen relative to the total nitrogen varies from 32 % in DEC/BIO samples to 44 and 48 % in CHEM and MEC samples, respectively. If this relative high ratio of inorganic vs. organic nitrogen is maintained during a composting process the compost will be very attractive from a farmers perspective, since he can expect a relatively high nitrogen efficiency for his crops if the compost is incorporated rapidly into the soil upon application in the spring.
Conclusions
A great variability among the 44 samples from three different separated animal slurry bio-solids was observed on various parameters. Future recommendations on the use of the bio-solids can become very dependent on the type of separation. Some will be suitable for composting while others may be more suitable either for use in biogas plants or for direct incineration with energy recovery. An ongoing investigation of the phosphorus distribution, in the raw bio-solids and in composted bio-solids can further contribute to the understanding of phosphorous availability and the possible use of animal slurry bio-solids.
Acknowledgments
We greatly acknowledge the farmers that have participated in this project, both by supplying us with samples and their willingness to answer questions about their separation plant. Further, thanks to The Danish Agricultural Advisory Service (DAAS), that has participated as a partner in this project by carried out a great deal of work in relation to information collection and sampling. The study was funded by grant 3304-VMP-05-052-01 from the Danish Ministry of Food, Agriculture and Fisheries.
References
Birkmose 2007: Status over anvendelse af gylleseparering I Danmark, 2007. http://www.lr.dk/planteavl/informationsserier/aktuelt/pl_aktuelt_07_108.htm. In Danish, (accessed 2007-12- 04).
Henze M, Harremoes P, La Cour Jansen J, Arvin E (1996). Waste Water Treatment: Biological and Chemical Processes, second ed. Springer, Berlin.
Masse L, Massé DI, Beaudette V, Muir M. (2005). Size distribution and composition of particles in raw and anaerobically digested swine manure. Transactions of the ASAE, 48 (5), pp. 1943-1949.