October 19, 2011 | General

Bringing Simplicity To Digestate Treatment (Italy)

BioCycle October 2011, Vol. 52, No. 10, p. 58
Four different full-scale plants were monitored, two operating on swine manure and two on digestate from biogas plants fed with manure and silage.
Alessandro Chiumenti

TO comply with the European Commission’s Nitrates Directive, Veneto Agricoltura coordinated, and the Veneto Region in Italy have financed, the RiduCaReflui Research Project, with the main objective of evaluating the most promising processes for treatment of digestate and farming effluents.
Anaerobic digestion represents a great opportunity, not only for the revenues related to the production of energy but also from an environmental point of view. Nitrogen (N) content of digestate is not affected by the process in terms of quantity, however where surplus N is a problem (as in the Po Valley of Italy), dedicated solutions must be implemented.
Main technologies monitored in the research studies are represented by nitrification-denitrification, liquid/solid separation, membrane filtration (ultra- filtration and reverse osmosis), digestate drying, vacuum concentration and unconventional composting process of liquid digestate.
The general approach is represented by comprehensive testing of the plants – evaluating performance, quality of input and output streams of each process, mass flows, nutrients flows and energy consumption, and finally, monitoring of the emissions, where applicable. Possible improvements in the set-up of each process were also evaluated.
Composting of liquid manure and/or digestate with the CLF MODIL Digester consists of processing liquid inputs on a bed of bulking/absorbing dry material (straw, wood chips, sawdust, corn stalks, etc.) to yield a solid product. The process takes place in a composting plant, represented by a track up to 120 m long (130 yd), 20 m wide (22 yd), and about 1.6 m deep (2 yd). The plant has concrete walls with the function of supporting and guiding a bridge, which distributes the liquid effluent on the bed of absorbing material and turns and aerates the feedstocks. The liquid input of the process is collected in a channel parallel to the track and loaded by means of equipment mounted on the bridge: different pumping/spreading solutions are adopted according to the content of solids in digestate.
Turning operations are performed by means of screws, mounted on the bridge itself, acting also as aeration equipment by forcing air inside the feedstock, by means of a compressor. The aerobic conditions introduced by aeration of the biomass determine exothermic reactions that cause self-heating of the feedstock, up to 60°C, with relevant evaporation of water. Each treatment cycle lasts for a period of four to five months.

Experiment And Conclusions
The main objective of the monitoring was to assess the environmental sustainability of the process, mainly in terms of gaseous emissions. The specific emission rate from the emitting surface was determined by means of a photo-acoustic Bruel & Kjaer 1302 multigas monitor and implementing the static chamber method. Four different full scale plants were subject to monitoring, two operating on swine manure and two on digestate from biogas plants fed with manure and silage. For each plant different cycles were monitored.
The main benefits derived from the system are: Relevant mass reduction, with a remaining solid representing about 15 percent of the input; Transformation of the input into a solid output; and Relevant nitrogen reduction (60-70%) with consequent reduction of land for agricultural utilization and eventual reduction of costs related to land concessions.
This biodigestion process proves to be very effective for the reduction of N, exceeding in many cases 70 percent both from digestate and manure, with a very limited percentage of N emitted as NH3 and N2O (about 2% of input N) and without significant odor emissions. The biological processes that determine these results are not completely clear at this date. The discovery of 107 UFC/g of bacteria involved in the nitrogen cycle, the consideration that the environment in the feedstock is not favorable to the classic nitrification-denitrification process, and the fact that input feedstock is characterized by high ammonia concentration, seem to support the hypothesis of deammonification processes, with transformation of organic N and ammonia into molecular N. It has been recently accepted that in addition to the main biological processes, represented by N fixation, mineralization, nitrification and denitrification, that different processes, like deammonification, also are involved in the nitrogen cycle, illustrating its complexity.
In terms of final product quality, the humification index (HI) on different samples – using the Sequi method as “not humified fraction:humified” fraction – is 0.26 HI, well below 0.5 HI, which is stable. Nitrogen content is about 5 gN/kg (> 95% is organic N, even if the input has a high percentage of ammonia), C/N < 20. The final product is solid with a moisture content of about 70 to 80 percent (with higher levels corresponding to higher load of the liquid input).
For plants installed after anaerobic digestion, the option of drying the entire production of the solid output could be evaluated, considering that the thermal energy produced by the biogas CHP unit is, in this case, sufficient for the scope. The cost of the treatment plant could be better recovered by installation of photovoltaic panels on the roof of the structure. Energy consumption of the system is limited to 2-3 kWh/m3 of input manure/digestate; quantifying the cost of biomass amendment is more difficult, as it depends on the specific material being used and on several other factors such the cost and availability on the market.
Results of monitoring the different plants demonstrates the environmental sustainability of the process, both in terms of emissions of greenhouse and acidifying gases and in terms of odors, underlining that air treatment is unnecessary. The system does not represent the only possible solution for digestate management, but proves to be efficient, flexible and simple.

Alessandro Chiumenti, PhD, is a researcher in Agricultural Mechanization at the Udine University. Dr. Chiumenti has published several articles, written books and contributed to publications covering several aspects of solid and liquid waste management.

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