Exploitation of Digestate in a Fully Integrated Biowaste Treatment Facility: A Case Study

The increase of biowaste generation has reached critical levels in many countries. The European legislation introduced the biowaste treatment and the organic recycling as central theme of its political agenda with the aim to promote the sustainable exploitation of this peculiar waste. The most utilized technologies applied to the biowaste treatment are based on the biological processes targeting to produce biogas or, more recently, biomethane to be used as fuel. The production of biomethane allows to produce a substitute of the fossil methane with a yield of about 0.07g CH4 /g biowaste ; the remaining fractions are waste coming from the pretreatment/refining steps, solid digestate or stabilized compost, and leachate. The sustainable treatment of these fractions is a mandatory issue to treat the biowaste in a reliable and sustainable integrated process since their amount is more than 85% and the impact of their treatment on environment and economy of the overall treatment process can be quite relevant. This chapter focused on the so-called smart facility that integrates processes based on thermochemical processes with the biological one targeting to increase the overall sustainability, the flexibility regarding the input biowaste composition, and the independency by the external factors affecting the waste trading.


Introduction
Biowaste can be defined as a mixture of vegetal and animal biomasses that are residues of human activities. It contains large carbon-based macromolecules that can be used as a renewable material for energy production, carbon sequestration, and soil conditioner and fertilizer. Nevertheless, biowaste management requires a specific attention in order to increase the overall sustainability of the treatment and to define guidelines to increase its effective recovery. In Europe, more than 118 millions of tons of biodegradable waste are produced each year; only 25% of this amount is collected and sent to the given recovery option [1]. The recovery treatments are mainly based on biological processes: the most used is the composting (low capital and operating cost, low value of products) followed by the integrated anaerobic/aerobic digestion (medium-high capital cost, low operating cost,

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: a. Acceptance, weight, and discharge of biowaste from the lorries b. Preliminary mechanical treatment and sorting c. Mixing between substrates having different moisture content and structure (e.g., lignin-based biowaste is added to food biowaste to increase permeability, moderate moisture content, and modify the C/H ratio) d. Pulping (only in case of wet digestion processes) e. Anaerobic digestion into one (unique bioreactor) or two stages (two in series reactors for hydrolysis and acetogenesis/methanation steps) f. Leachate recirculation and storage before treatment or delivery to the water treatment plant g. Aerobic stabilization of digestate (composting) h. Mechanical refining and foreign matter removal 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.

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.

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

Cogeneration
The biogas generated by the anaerobic bioreactors resulted to be 5,040,000 Nm 3 per year, corresponding to a production yield of 140 Nm 3 /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.

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 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 3 and sent to the external facility to be treated and disposed. The leachate corresponds to about 30-40% of the initial biowaste.

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 Nm 3 of air each Mg of digestate; that means that, in this specific case, 29,000 Nm 3 /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.

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][17][18]. 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: a. Treat the digestate in a more efficient way.
b. Exploit the not biodegradable waste by avoiding disposing it outside the facility boundary.
c. Treat the wastewater by using the heat produced by the integrated processes.

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 • The energy and heat production from the waste by minimizing the amount of waste to be disposed outside the facility boundary and its economic cost • The treatment of leachate by using an effective evaporation method carried out by using the accumulated sensible heat and the recovered heat from the waste conversion 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.

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, w digestate is the weight of the digestate and x H 2 O,digestate is the mass fraction of the moisture in the digestate.
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.

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 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.  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.

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.

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

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 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.

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.   Table 3.

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

HTC mass and energy balance
With reference to the section of the plant reported in Figure 3 and destined to the hydrochar production and thickening of leachate, the mass balance is reported in Table 4. The flow ID are referred to in Figure 3.
The yield of hydrochar referred to the initial biowaste results to be 7%. The amount is quite similar to the compost yield, obtained by using the aerobic stabilization instead of the HTC process. The main differences are the following: a. The HTC process requires three cycles by day, each as long as 6 hours, and two closed batch reactors to reach the given yield.
b. The HTC process does not emit odors and fugitive/diffuse emissions.
c. The HTC process requires an area quite lower than that necessary for composting.
d. The HTC can be conveniently coupled to a leachate treatment plant based on the evaporation/thickening standard processes available in the industrial market by using the heat content of liquid/vapor at the reactor outlet.
The energy balance of the HTC section is proposed in Table 5.
The heat necessary for the heating of the slurry from the input temperature (exit of filter) up to 220°C is 866 MJ/Mg (at 42.6 bar); once the reaction temperature is reached, the carbonization begins by absorbing heat from the environment until exothermic reactions begin. The heating time has been fixed in 0.4 h (1440 sec), so an installed heating power of about 10 MW is necessary to provide the heat in the specified time interval.
The thermal energy necessary to provide for the evaporation of water has been obtained by subtracting that requested to bring the water into vapor phase at 100°C and 1 bar to the thermal energy of the water medium present in the reactor. The amount of heat to carry out this process is 1366 MJ/Mg that corresponds to a thermal power of 9.7 MW, by assuming an evaporation time equal to that necessary for slurry's heating. By using the energy content of water after the reactions are completed, it is possible to obtain the evaporation for about 78% of water The most important feature is that the leachate produced by the facility is reduced at 44% of that produced by anaerobic digestion sector, without increase of thermal heat, but that used for HTC reaction.
Further evaporation is technically possible and can be also economically feasible if heat demand is fulfilled by the third section of the plant: the gasification with energy recovery.
The overall feedstock energy balance is reported in Table 6.
The energy balance shows in brief that: • The biogas contains 42% of the initial feedstock energy content • The removed waste contains 37% of the initial feedstock energy content • The hydrochar contains 20% of the initial feedstock energy content The energy content of waste corresponds to about 3800 MW of chemical energy that can be used to produce energy by means of a gasification process, described in the following paragraph.

Gasification mass and energy balance
With reference to the section of the plant reported in Figure 3 and destined to the conversion of waste into energy and heat, the data in Table 7 are the basis for calculation. Data refer to the the typical waste resulting from the sorting of biowaste treated in the reference facility, just before being fed to digestors. These data are the starting point for calculation of calorific value, stoichiometric oxygen demand, bottom ash production rate, and other process parameters. Enthalpy to be provided, MJ 13,920 Negligible 13,920 Table 5.

Evaluation of enthalpy flows for the unit processes E and F.
The gasification process is carried out in a plant that is composed of two sections: a. Thermal conversion under partial oxidation conditions b. Energy recovery (with two options: electricity and heat or only heat) The conversion section is composed of the gasifier itself and the thermal cracking reactor where the tar undergoes cracking and upgrading. The gasifier is fed with the waste once shredded and compacted in order to increase the bulk density at a suitable value to guarantee a constant mechanical feeding. The waste is then converted into syngas by recurring to a thermal cracking at high temperature, sustained by plasma torches, providing a thermal load of 125 kW.
The gasifier temperature as obtained by the energy balance results to be 1350°C in the homogeneous phase. The air-to-waste ratio has been fixed at 2.13 and corresponds to an equivalence ratio of 0.25.
The main input and output parameters are reported in Table 8.  Table 6. Energy balance.

Conclusions
The integration between the anaerobic digestion plant of biowaste with thermochemical processes such as hydrothermal carbonization coupled with the evaporation/thickening process and gasification of nonbiodegradable waste has been proposed and described.
The mass and energy balances have been reported in order to evaluate the feasibility of the proposed integrated "smart" facility.
The advantages obtained by using the smart facility design can be briefly summarized as follows: • The waste production destined to disposal decreased by 95%.
• The leachate to be disposed decreased by more than 66%.
• The electricity produced is 5370 MWh/year.
• The hydrochar can be sold in the market or used as fuel in the gasifier. By an economic point of view, the capital cost of the smart facility increases by a factor of 3. Anyway, the operating costs strongly decreased by considering the savings connected to the above listed advantages.
The strong reduction of transportation of waste and leachate by trucks, the minimization of air pollution and odor emission, the possibility to cover the electricity cost by self-producing itself without grid losses, the reduction of waste volume of more 95%, and the production of clean biofuel (biomethane) and a highadded value char (hydrochar), all these positive features allow to consider the smart facility and even a green facility.