**2. Configuration of the base case anaerobic digestion facility**

### **2.1 Description of the unit processes**

The standard configuration of anaerobic digestion facility consists of the following sections:


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


The data and the information utilized in this work are based on a full-scale facility that utilizes a dry-batch technology to perform the anaerobic digestion, and it is integrated with the composting plant to obtain the mineralization of the digestate. The facility is located in the industrial area of Naples, Italy [5], and treat biowaste from household separate collection and restaurants.

The block diagram of the integrated processes is reported in **Figure 1**.

With reference to the unit processes labels reported in **Figure 1**, a short description is reported in the following paragraphs.

### *2.1.1 Pretreatment and sorting*

The organic fraction of municipal solid waste contains a certain amount of foreign matter constituted by inorganic and organic nonbiodegradable materials such as glass, ceramic, metals, plastic bags, plastic closures, wires, etc. The size distribution of this fraction ranges from few millimeters up to several centimeters, allowing the removal of large objects by means of manual sorting and sieving in a trommel. Generally, the minimum size of the holes installed to remove the foreign matter is 5 cm. A photo of the waste removed by using the mechanical sorting after the bag opener (light-intensity shredding) is reported in **Figure 2**.

### *2.1.2 Anaerobic digestion*

The anaerobic digestion is then carried out by using eight batch reactors operated by recurring to an operation mode by including the following phases: emptying (a), filling and mixing (b), and reaction (c). The reactors are sequentially operated in order to have a semicontinuous operation. Steps (a) and (b) require a couple of days to be carried out, that is why each bioreactor starts the reaction phase with a delay of two days; the reaction phase has a duration of 28 days. Each reactor is filled with about 200 t of fresh biowaste after the removal of a part of the digestate formed by the preceding cycle. The digestate remaining in the batch reactor (about 50%) is mixed with the fresh one, acting as an inoculum for the microbial growing kinetics.

The process is a dry-batchwise since the solids fraction in the reacting mass is larger than 30%. The level of moisture that ensures the microbial activity inside the reactors is maintained by feeding the leachate collected from the bottom of each reactor at the top of it. It is important to highlight that the reactors are not stirred and that heat transfer and water percolation are limiting factors for the process:

### **Figure 1.**

*Block diagram and unit processes included in the reference case.*

without the recirculation of preheated leachate and the mixing with activated digestate, the process shall not occur in an appreciable way.

## *2.1.3 Cogeneration*

The biogas generated by the anaerobic bioreactors resulted to be 5,040,000 Nm<sup>3</sup> per year, corresponding to a production yield of 140 Nm<sup>3</sup> /t. This biogas is conditioned in order to remove hydrogen sulfide, ammonia, and moisture that results to be 5% in the final gas. The composition of biogas is variable, but the mean values are 60%v of methane and 40%v of carbon dioxide. The combustion of biogas is carried out in two 500 kWe Jenbacher engines that produce the electricity introduced in the public grid. The corresponding produced heat is recovered and used to enhance the composting process rate and drying the final compost.

### *2.1.4 Leachate storage tank*

The leachate is produced during the anaerobic digestion thanks to the percolation of interstitial water from the substrate; a part of leachate is heated and

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

**Figure 2.** *Waste removed by the biowaste in the pretreatment stage.*

recirculated inside the anaerobic bioreactors in order to keep the substrate humidified. The rest of the leachate is stored in a tank having a volume of 1000 m<sup>3</sup> and sent to the external facility to be treated and disposed. The leachate corresponds to about 30–40% of the initial biowaste.

### *2.1.5 Composting*

The aerobic stabilization of the unconverted volatile solids occurs in order to mineralize the substrate for a period of 90 days. The composting process requires air not only for chemical oxidation of volatile solids but also for the heat removal and odor dilution in the indoor environment. The aerobic treatment requires 16,000 Nm3 of air each Mg of digestate; that means that, in this specific case, 29,000 Nm3 /h need to be continuously extracted from the warehouse and sent to the air treatment modules in order to be cleaned up. The air treatment system receives this stream containing odorigenous molecules including organics, acids, and ammonia; it is composed of a scrubbing unit followed by two biofilters; this system is designed in order to remove odor molecules from the conveyed air stream before the diffusion in the outdoor environment. Electricity consumption for air recirculation, biofilter replacement, and wastewater treatment are expenses for this stage of the overall process.

The aerobic stabilization is followed by the maturation and refining phases (F + G). The refining process aims to remove the foreign materials and obtain a homogenized size distribution. The moisture level in the compost is lowered at 5% by using the heat recovered by section C. This phase is required in order to produce a compost that can be sold on the market of fertilizers.

### **2.2 The critical issues of the present configuration**

The mass balance of the plant in the present configuration is reported in **Table 1**. Data are in agreement with those obtained by other anaerobic facilities assessed in the scientific and technical literature [16–18].


### **Table 1.**

*Mass balance of the reference facility.*

The amount of produced waste, whose amount is depending on the separate collection performance, and that of leachate both represent a negative feature as well as for environmental and economic reasons: the delivery and disposal of leachate at external facilities requires about 50 €/Mg, while the tipping fee of the waste is more than 150 €/Mg. The impact of transportation should also be included in the environmental impact assessment and in the cost evaluation since the distance between treatment plant for waste and leachate can be quite large.

Moreover, the present configuration is economically sustainable only if electricity and/or biomethane is sustained by incentives. The value of green certificates for biogas is variable, but for 1 MW biogas facilities, an indicative value of 104 €/MWh can be used [19].

Despite the incentives for the obtained products (electricity/methane), and considering that the compost has a very low value, the cost of waste disposal and that of composting (aeration) result in the increase of biowaste tipping fee that, in Italy, leads the cost of the management of separate collected waste over 190 €/t [20]. Other countries in Europe have lower tipping fees for biowaste treatment due to less restrictions about digestate use (e.g., it is not mandatory to mineralize the digestate prior to the soil scattering) and a lower cost of waste disposal due to an efficient network of waste treatment facilities. This latter cost is anyway continuously increased in the last years due to the difficulty to process inside the Europe borders the plastic waste [12].

Based on these considerations and on the European guidelines about the proximity criterion, an improvement of the industrial layout of the facility can be proposed in order to reduce the impact and the cost of the whole system. This improvement is based on the integration inside the facility boundary of the processes that allow to:

