**1. Introduction**

The biogas production from agriculture waste in anaerobic digestion is a profitable direction for their energetic use [1, 2]. The byproduct of anaerobic digestion is called a digestate, which consists of both liquid and solid fractions. They can be separated using screw separators, presses, or decanter centrifuges. Liquid fraction can be used to fertilize farmlands because it

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contains substantial amount of the elements necessary for plant growth. Liquid fraction can also be further treated and recirculated to the fermenter [3]. Solid fraction can also be used for agriculture or as a solid fuel, e.g., in the combustion process [4].

**2. The treatment of raw digestate**

parameters of the fermenter were used [11]:

• The total batch fed to a digester—1.2 m<sup>3</sup>

**2.1. Mechanical dewatering of digestate**

the photograph of the screw separator.

of the process.

pH Water content Loss of

**Table 1.** Properties of raw digestate.

ignition

**Parameter**

• The total load of organic compounds—2.3 kg VS/m<sup>3</sup>

• Residence time in the pre-fermentation tank—3 days • Residence time in the fermentation chamber—20 days • Residence time in the post-fermentation tank—20 days

**Tables 1** and **2** show the properties of the raw digestate.

network meter. In separation process test 0.035 m3

• The set temperature during the fermentation process—35 to 40°C

• Feedstock moisture—90%

A digestate from a pilot biogas plant located at the Experimental Station in Bałdy, Poland (N54° 36′ 1.8073″, E20° 36′8.5295″) was used in this research. The following technological

Small-Scale Energy Use of Agricultural Biogas Plant Wastes by Gasification

http://dx.doi.org/10.5772/intechopen.71700

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Raw digestate from biogas plant has been pre-dewatered in screw separator with slot filter with a filter gap of 0.5 mm. Raw digestate contained 94.55% of water. Energy consumption of the mechanical dehydration process was measured using the Schneider ION7650 electrical

As a result of the separation process, 30 kg of liquid fraction and 5 kg of solid fraction was obtained. Energy consumption during the experiment was 0.02 kWh. The tests were carried out without the pump forcing the digestate pressure in the separator. Digestate was fed to the separator only under its hydraulic pressure. Absence of pressure force does not interfere with separation operation and performance results may be lower than expected. The usage of a forced pump can increase productivity but can also increase the energy consumption

O5 K2

N N-NH4 P2

pH % % D.M. % D.M. mg/kg D.M. % D.M. % D.M. % D.M. % D.M. % D.M. 8.24 94.55 68.97 7.16 1830.0 2.38 6.61 1.28 3.48 1.28

of raw digestate was used. **Figure 1** shows

O MgO CaO Na2

O

### **1.1. Digestate treatment in Europe**

Nowadays, the digestate in the regulations is considered as waste in Europe. It does not have the status of a biofuel or an alternative fuel and is most commonly used as a soil improver. Such use often requires additional tanks to store the digestate mass to allow it to be used during fertilization periods. This can generate significant capital expenditure on the construction of liquid fraction storage tanks [5]. Another problem that causes the necessity of processing digestate is too small cultivable area where it can be used directly. Agricultural use of digestate is limited by the maximum allowable nitrogen dose of 170 kg N ha−1 y−1 [6]. For this reason, European countries have begun to use the separation of digestate to solid and liquid fraction. These fractions differ in physicochemical properties. As a result, a much smaller area for storage of digestate is required, while liquid fraction is again used to dilute the substrates to the fermentation process to the required 12% of dry matter. Liquid fraction can also be used as fertilizer by acquiring mineral compounds. It is characterized by lower phosphorus content and a higher content of nitrogen and potassium. Solid fraction is used in areas with low phosphorus content, liquid fraction on the other hand—in phosphorus saturated areas [7].

For fertilizer purposes, the liquid fraction is used either directly or in the production of mineral fertilizers through further purification. Further purification can be achieved through ultrafiltration in order to remove solid particles, followed by the reduction of high concentrations of nitrogen through stripping or crystallization of struvite [8]. These methods are used extensively in Germany and the Netherlands, and are mainly derived from a pig farm where ammonia load is very high.

For separation, screw separators with slot filter are most commonly used, which have previously been used successfully to separate liquid manure. Those devices feature low energy consumption because of low pressure of pumped digestate and low rotational speed of the screw shaft. Dry matter content obtained is about 30% of the solid fraction, while the liquid fraction remains about 4% of the dry matter. More advanced equipment, such as decanter centrifuges or belt presses, allow more efficient operation, but are rarely used in small biogas plants because of high investment costs.

Often solid fraction of digestate is processed in composting process, which reduces its volume, moisture, and improves fertilizer and storage properties [9]. Separation devices are often directly integrated with composting reactor, e.g., container with moving floor and aeration system or drum reactor.

Another method often used is drying in belt or drum dryers using the heat from CHP units. Dried digestate is used to produce pellets for energy purposes or for use as bedding for animal farm [10].
