**3. Configuration of the integrated anaerobic digestion facility**

### **3.1 Description of the unit processes of the alternative configuration**

The configuration of anaerobic digestion facility presented in **Figure 1** has been integrated with unit processes allowing:

• A sustainable production of a stable product (hydrochar) useful for agriculture purposes in place of compost

*Exploitation of Digestate in a Fully Integrated Biowaste Treatment Facility: A Case Study DOI: http://dx.doi.org/10.5772/intechopen.92223*


The unit processes able to reach the abovementioned targets have been reported in the integrated block diagram in **Figure 3** and described in detail in the following paragraphs.

## *3.1.1 Pulping and filtering*

The digestate contains about 70% of moisture, as resulted from the mean values of the proximate and ultimate analyses reported in **Table 2** [21].

In this stage, it is conditioned in order to be suitable to undergo a thermal process aimed to hydrochar production, instead of the aerobic biological process aimed to compost production.

The hydrochar production process (F) occurs with very good efficiency if the organic fraction of digestate is in a pulping state, well mixed with water, and with a given water/dry matter ratio; this means that the mixing and pulping preprocessing has to be realized.

The pulping process allows to deal with another negative feature of digestate composition: the presence of foreign matter (inorganic or not reactive organics). In fact, a critical characteristic of digestate is the content of foreign matter such as glass, stones, and plastics that cannot be removed by the sorting made in the pretreatment stage where only the coarser fraction of foreign matter having a mean size larger than 5 mm is removed during the presorting; the remaining amount is not negligible and can be responsible of a dramatic decrease of the value of the final product if not removed. The pulping process between the digestate and the added water allows to separate the foreign matter by filtering the mixture following standard techniques applied in well-known wet anaerobic processes pretreatment. In this specific case, the digestate is mixed with the leachate produced by the anaerobic digestion in such a way to prepare a homogeneous pulp by respecting a given value of the parameter R (Eq. 1). In Eq. 1, *wdigestate* is the weight of the digestate and *xH*2*O*,*digestate* is the mass fraction of the moisture in the digestate.

$$\mathcal{R} = \left(\boldsymbol{w}\_{\text{digestate}} \bullet \boldsymbol{\varkappa}\_{\text{H}\_2\text{O},\text{digestate}} + \boldsymbol{w}\_{\text{H}\_2\text{O},\text{added}}\right) / \left[\boldsymbol{w}\_{\text{digestate}} \bullet \left(\mathbf{1} - \boldsymbol{\varkappa}\_{\text{H}\_2\text{O},\text{digestate}}\right)\right] \tag{1}$$

The pulping process can be favored by the preheating of leachate at about 70–80°C and by an intense shredding of the pulp itself. In this way, the foreign matter can be removed with very high efficacy (**Figure 4**, right) by floating (low-density fraction), by sedimentation (high-density fraction), and by sieving. The pulping creates the best conditions for this filtering/cleaning process and for the following reacting process reported in step F.

### *3.1.2 Hydrothermal carbonization*

The hydrothermal carbonization (HTC) is a thermochemical process occurring in the presence of subcritical, liquid water: the target of this "wet or hydrous pyrolysis" is making products with higher carbon content, that are biologically inert, and with physical characteristics that make them suitable for agricultural or

**Figure 3.**

*Block diagram and unit processes of the proposed integrated facility.*


**Table 2.**

*Proximate and ultimate analyses of digestate (dry basis).*

industrial purposes. The product obtained from the hydrous pyrolysis is called hydrochar to distinguish it from the biochar that is obtained by dry pyrolysis [22, 23]. During the hydrothermal process, the volatile solids contained in the digestate are surrounded by water which is kept at liquid state by allowing the pressure to rise until the endogenous value reached at the reactor temperature. The production of gases is very limited (1–5%), while dissolution of elements in the water can have a certain extent such as 5–20% [24]. The carbonization requires a reaction time of hours (1–12 h), depending on the reaction's temperature utilized, and occurs in a closed reactor by allowing the elimination of diffuse release of odors. *Exploitation of Digestate in a Fully Integrated Biowaste Treatment Facility: A Case Study DOI: http://dx.doi.org/10.5772/intechopen.92223*

**Figure 4.** *Photos of digestate (left) and pulp (right).*

Moreover, the high temperature destroys pathogens and active organic molecules. The resulted hydrochar may contribute to climate change mitigation and soil amelioration [24]. The HTC process is basically a decomposition process where chemical reactions such as hydrolysis, dehydration, decarboxylation, aromatization, and condensation occur [25]. The hydrolysis's activation energy is lower than the other reactions; this lowers the decomposition temperature of the main constituents of biomasses: cellulose and lignin, for example, decompose between 180 and 220°C.

The reactor where the HTC is carried out is a batch reactor, filled with the pulp obtained by mixing, stirring, and filtering digestate and leachate and heated up to the reaction temperature in a given heating time. A schematic process flow diagram of the HTC section is reported in **Figure 5**.

The pressure inside the reactor is determined by the temperature setup for the reaction; the temperature vs. pressure correlation can be obtained by the Antoine equation.

In the specific case of this simulation, the HTC operating conditions are reported in **Table 3**.

### *3.1.3 Evaporator*

This stage realizes the evaporation of water under the form of steam by using the pressure difference between the reactor and the evaporator and the sensible heat of the liquid water. The evaporation allows to realize the separation of the pure water under the form of vapor from a concentrated flow, having a higher boiling temperature. The concentrated flow is rich in nitrogen and carbon; it is sterile and could be used as fertilizer, in case the regulations allow this application. The worst case is to consider it as leachate, as in the case of **Figure 3**. The steam can be condensed or lost in the atmosphere, depending on specific cases.

### *3.1.4 Dewatering and drying*

The wet hydrochar is sent to dewatering, which is quite efficient due to its hydrophobia [26], and finally dried.

### *3.1.5 Gasification*

The waste produced by the anaerobic digestion facility consists mainly of plastic bags and dishes, foils, and a limited amount of metals. It is basically a combustible

### **Figure 5.**

*Process flow diagram of HTC section.*


### **Table 3.**

*HTC reactor and process parameters.*

material having a moisture content until 20% and it is very dirty. Its fate is the landfilling or energy recovery in large incineration plants.

The integration of the AD plant with a small-scale gasifier allows to reduce the disposal costs and the production of electricity and heat necessary for the other processes.

The gasification unit basically consists of a downdraft reactor equipped with a fixed bed as support for the primary reactions of the combustible material with water and air, three plasma torches aligned with the bed surface, a secondary zone where homogeneous reactions take place, and a secondary stream of air that is added to favor the reactions' completion. The gasification reactions occurring on the plasticbased waste are responsible of a large production of hydrocarbons and aromatics (PAHs), partly transformed into tar, that need to be minimized in the syngas than the primary and secondary methods [13, 15, 27, 28]. In this specific case, the minimization of tar content is guaranteed by a mechanism of thermal cracking coupled with the saturation of produced radicals promoted in a secondary cracking reactor, located at syngas exit. The thermal cracking is realized by obligating the syngas by passing throughout a plasma plume composed of oxygen and hydroxyl radicals.

### *3.1.6 Energy production*

The syngas is then sent to a cogeneration system to produce electricity and heat. Data related to the production of energy are reported in the following paragraphs.

A possible alternative is to convert the syngas into heat and use it to feed the leachate treatment process and increase the evaporation yield.
