Wood Ash Based Treatment of Anaerobic Digestate: State-of-the-Art and Possibilities

Abstract

The problem of current agricultural practices is not limited to land management but also to the unsustainable consumption of essential nutrients for plants, such as phosphorus. This article focuses on the valorization of wood ash and anaerobic digestate for the preparation of a slow-release fertilizer. The underlying chemistry of the blend of these two materials is elucidated by analyzing the applications of the mixture. First, the feasibility of employing low doses (≤1 g total solids (TS) ash/g TS digestate) of wood ash is explained as a way to improve the composition of the feedstock of anaerobic digestion and enhance biogas production. Secondly, a detailed description concerning high doses of wood ash and their uses in the downstream processing of the anaerobic digestate to further enhance its stability is offered. Among all the physico-chemical phenomena involved, sorption processes are meticulously depicted, since they are responsible for nutrient recovery, dewatering, and self-hardening in preparing a granular fertilizer. Simple activation procedures (e.g., carbonization, carbonation, calcination, acidification, wash, milling, and sieving) are proposed to promote immobilization of the nutrients. Due to the limited information on the combined processing of wood ash and the anaerobic digestate, transformations of similar residues are additionally considered. Considering all the possible synergies in the anaerobic digestion and the downstream stages, a dose of ash of 5 g TS ash/g TS digestate is proposed for future experiments.

1. Introduction

The growth of the human population and the change in their diet (e.g., more consumption of animal products) imply devoting more land to food production at an alarming rate [1]. The problem of our current agricultural practices is not limited to land management but also to the unsustainable consumption of essential nutrients for plants, such as phosphorus [2,3]. The fluctuation in prices helps preserve the long-term availability of these mineral resources [4]. In the present situation, the development of profitable strategies for utilizing the waste materials is the only way to achieve sustainable development of the society [5]. Once a material is regarded as waste, its utilization is constrained by regulations [6]. In Europe, all waste-derived products must comply with Directive 2018/851, but there are shortcuts to achieve end-of-waste (EoW) status. Generally, when a residue is produced in large amounts and has a composition suitable for a particular application, the European parliament applies EoW regulations that do not need to be transposed by the governments of each member state [7].

The production of digestate was around 180 million tons per year in the EU28, before the UK withdrawal in 2020 [8]. In the UK, 7.5 million tons of anaerobic digestate and 2.7 million tons of compost were produced in 2018 [9]. A common policy is being developed to improve the management of all nitrogenous materials employed as soil amendments [10,11]. In 2009 [12], the EU27 produced 256 million tons of municipal solid waste (MSW). Much more animal manure is applied to land as a soil amendment than anaerobic digestate because of the high CAPEX and OPEX associated with implementing anaerobic digestion (AD) in the farms [13]. The biological treatment of organic residues via AD is a promising technology for energy recovery in the form of biogas and the production of fertilizers [14]. The AD was first applied to deal with the sewage sludge (SS) produced during the primary and secondary treatment of wastewater, but its use to deal with agro-waste and the MSW with around 70% of organic material has been subsequently encouraged [15,16]. The AD technology fits with the current trends of using the biorefinery to sustainably satisfy the needs of society, which used to be covered by the petrochemical industry [17].

Vassilev et al. [18] estimated that approximately 476 million tons of biomass ash could be generated worldwide annually if the average ash content is 6.8% and the burned biomass is assumed to be 7 billion tons. This amount resembles the 780 million tonnes of coal ash generated every year [18]. According to Pitman [19], the wood ash (WA) production could be estimated by considering that 1% of the wood incinerated is left as ash. According to the UK Forestry Commission [20], 2.68 million tons of wood fuel were used in 2016. Even considering the combustion of other types of biomass, the amount of biomass ash currently produced in the UK is lower than that coming from the coal power plants. In 2016, the amount of coal ash produced in the UK was around 6 million tons [21]. However, this scenario is changing, and by 2025 the amount of biomass ash is expected to be greater than that of coal ash [22]. In 2007, Sweden produced 0.3 million tons of WA, and most of this material was disposed of in landfills [23]. The preparation of a blend of organic manure and ash to enhance the circular economy has been addressed in the literature from several points of view:

• To enhance the AD process [24].
• To improve the properties of the soil [25].
• To promote crop growth [26].
• To control the release of the nutrients [27].
• To achieve better dewatering of the digestate [28].
• To promote the self-hardening and granulation of the digestate [29].

The anaerobic digestates, animal manures, and slurries are appreciated by the farmers as organic soil amendments. Improving the nutrient profile of the digestate might be a way of increasing its value and decreasing the cost of transportation. Moreover, there are more urgent challenges that need to be addressed, such as the pollution associated with the use of these materials [10]. It is necessary to optimize the conditions of the WA-based treatment of the anaerobic digestate to improve the stability of the soil organic amendment, improve the nutrient use efficiency, and reduce the contamination of the environment. The objectives of this review are (a) to discuss the underlying chemistry of the blends of anaerobic digestates and WA, and (b) to design a process that only requires affordable and widely available resources to prepare the novel fertilizer.

2. Upstream Processing

2.1. State-of-the-Art of AD

The AD process is also known as bio gasification due to the intervention of microorganisms to convert the components of the organic matter to methane and carbon dioxide. The most accepted model, on how the reactions occur in the anaerobic digester in the absence of oxygen, describes 4 different stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Figure 1). The limiting step is hydrolysis, while the methanogenesis is quick, preventing the accumulation of volatile fatty acids (VFA) in the system. In this way, the concentration of VFA in the anaerobic digest ester could be used as an indicator of the correct stationary operation of the bioreactor [17].

2.2. Use of WA as an Additive for Enhancing the AD

The addition of WA to the feedstock of AD can be understood as a way of promoting the stabilization of the anaerobic digestate because the greater the biogas production during the AD, the lower the emission of methane and carbon dioxide occurrence during storage and land application of the soil organic amendments [30]. Concerning the use of WA as an additive for AD, to the authors’ knowledge, 6 experimental studies have been reported so far on this topic (

Table 1. Summary of the experimental studies on the use of WA in the AD.

Reference	Type of Ash and Incinerator Type of Substrate Type of Anaerobic Digester Ash Dose	Biogas Production Composition of Anaerobic Digestate
[33]	Bauhina monandrina plant ash Pig waste and cassava peel waste (25% w/v) and distilled water (75% w/v) 45-day biochemical methane potential (BMP) experiment conducted in 2.8 L digesters Ash seeding rate not specified	Ash-amended digester provided more than twice the biogas production during the 45-day incubation (2345 mL) The pH of the digester sown with WA was 4.40, but the methanogen archaea were able to adapt to the acidic conditions of the digester media
[34]	Not specified type of WA Brewery spent grains with rumen liquor 1 L conical flasks with 100 g spent grains of 14 days of hydraulic retention time (HRT) Ash to grains ratios 0:1, 1:5, 1:4, and 1:3. Several dilution ratios of the feedstock with water were tested (1:4, 1:6, 1:8, and 1:10 w/v)	Greatest release of biogas obtained with the lower feedstock to water ratio (1:4 w/v). Enhancement of biogas production: WA > poultry litter (PL) > urea WA led to an initial increase in the carbon content, due to the accumulation of the VFA. Once the methanogenesis reached a stationary state, the carbon-to-nitrogen mass ratio (C/N) decreased, and an organic manure with better fertilizing properties than the initial feedstock of AD was obtained
[35] Cited by the 3 articles below employing WA in AD	MSW fly ash from industrial incinerator 850–1050 °C. The flue gases were treated with semi-dry scrubber using Ca(OH)2 as a flushing agent, powdered carbon, and filter bag Synthetic MSW substrate and anaerobic sludge seeding 35 °C leaching/percolate (100 mL/day) bed reactor. A schematic diagram can be found in the Supplementary Material (Figure S1) 0, 10 and 20 g MSW fly ash/L MSW (equivalent to 0.2 and 0.4 g/g vs. or 0.17 and 0.33 g/g TS, respectively) Lo et al. [36] tested 100 g MSW bottom ash/L MSW, 2 g MSW bottom ash/g vs. or 1.67 g MSW bottom ash/g TS	Biogas production rate in ash-added digesters was higher than in control experiment (i.e., unamended BMP) but total yield followed the trend: 20 g/L > 0 g/L/ > 10 g/L > 100 g/L [36] Drop in the pH due enhancement of the hydrolytic and acidogenic processes. After the 84 days of operation, a decrease of volatile solids (VS) and accumulation of the VFA due to a decrease of methanogenic activity
[37]	Wood bottom ash (WBA) 2 mm sieved and 105 °C dried 2 mm sieved cattle slurry (5 weeks AD) 1 L; 37 °C; 50 rpm; 20 day retention time. 0 and 0.5 g ash/g total solids (TS), which is equivalent to 0.96 g WA/g vs. or 9.65 g WBA/L digested cattle slurry	Biogas stopped after the addition of ash, and higher production rates were found when the doe of ash decreased down to 0.25 g/g TS (0.48 g/g vs. or 4.83 g/L digested cattle slurry). The biogas increased the content of CH4 The ammoniacal nitrogen (NH4+-N) was not affected by the ash amendment but increased. With excessive ash dose, C/N increased due to the accumulation of VFA.
[38]	Wood fly ash from combined heat and power plant using wood residues as a fuel of a fluidized bed incinerator. Thickened wastewater treatment plant (WWTP) sludge 37 °C for 4 weeks. Schematic of the setup in the Supplementary Material (Figure S2) 0, 0.4, 0.7 and 1 g ash/g vs. or 0, 0.28, 0.49, and 0.70 g ash/g TS or 0, 0.008, 0.015, 0.022 g ash/L, respectively	After 3 to 5 days, the production of biogas is comparable to the control experiment because the accumulation of VFA decreases the pH The elevated pH at the beginning of the AD was responsible for the longer lag phase when adding the WA to the digester
[39]	Boiler ash granular (0.85–4.75 mm) and powdered (<0.075 mm): Conventional fixed grate biomass combustion system fueled by pine, spruce, and fir bark and some dry wood shavings Fermented WWTP sludge BMP at mesophilic conditions (38 °C). Schematic can be found in the Supplementary Material (Figure S3) WA doses ranging from 0.16 to 3.7 g/g vs. fermented WWTP sludge, which is equivalent to 0.14 and 3.2 g ash/g TS or 5.72 and 133.94 g ash/L, respectively	WA increased the lag phase of AD and decreased the total biogas produced WA accelerated the biodegradation of propionic and butyric acids. Even at a rate of 2.2 g ash/g VSsubstrate the WA poorly contributed to the total alkalinity; thus, the authors recommend using a buffer (e.g., 20 mmol/L Na2CO3 & KHCO3 even when adding the ash). The authors found that the granular WA was able to remove NH4+-N from the liquid phase due to adsorption via cation exchange

). Furthermore, Alavi-Borazjani et al. [31,32] provided an insight into the feasibility of the utilization of biomass ashes in AD and biogas upgrading. According to Alavi-Borazjani et al. (2020), biomass ashes are cheap sources of alkali suitable for controlling the excessive acidification of anaerobic digesters. Biogas production is also enhanced due to the supplementation with the macro and micronutrient present in biomass ashes, which are required for the anaerobic microbes, and only a reference of WA was mentioned in relation to the biogas upgrading. Raw biogas of approximately 60% methane does not have enough quality for its introduction into the natural gas grid. Most frequently used purification technologies (water/amine scrubbing, pressure swing adsorption, cryogenic distillation, and membrane) require an excessive amount of energy and chemicals. The main targeted compounds to be removed from the biogas are CO₂ and H₂S. Given the complexity of the WA, there is a knowledge gap on the absorption capacity of bulk WA, and gathering more information to fully appreciate the valorization opportunities of this material is required [31,32].

2.3. Stability and Maturity of the WA Amended Anaerobic Digestate

In some UK and European regulations, the terms stability and maturity are used indistinctly to describe the properties of a soil organic amendment [8,40,41]. Stability is related to reactions affecting the fate of carbon, which is the most abundant element in organic materials, and the maturity focuses on all the other elements (Figure 2). Some are nutrients necessary for plant growth (e.g., N, P, K, etc.), while others are phytotoxic compounds (e.g., Cd, Hg, Pb, etc.) that limit seed germination and root development. It is important to mention that the excess of any type of nutrients has a detrimental effect on the soil biota. Because nitrogen is the most abundant of these nutrients, its mineralized form, NH₄⁺-N, could measure maturity [42,43,44].

The aim of determining the stability is to determine the fate of the carbon present in the labile and stable organic matter, since the inert organic matter will not suffer any degradation [45]. As displayed in Figure 2, the carbon could be (a) assimilated by the microorganisms for their growth, (b) lost via respiration, also known as carbon mineralization, and to a lesser extent, (c) lost due to leaching of the low organic molecular weight compounds. The carbon use efficiency (CUE) only accounts for the C used for the microbial growth, hence this parameter could be used to directly measure the stability of an organic amendment for more efficient nutrient management. However, the most common way of determining the stability is by measuring the losses via respiration [8,41,46,47,48], although the test is not carried out in similar conditions to the land application but in AD conditions as per the BMP protocol.

Figure 2. Evaluation of the quality of an organic amendment in terms of stability and maturity [42,49,50,51,52,53].

Figure 2. Evaluation of the quality of an organic amendment in terms of stability and maturity [42,49,50,51,52,53].

In a BMP test of an organic amendment (Figure 3), the release of biogas due to microbial activity depended on the composition expressed as the C/N and the relative amount of this material with respect to the microbial biomass (i.e., substrate-to-inoculum ratio, S/I), expressed as organic loading rate (OLR) in a continuous reactor. Together with the HRT, the OLR (Equation (1)) is one of the most important parameters in the operation of an anaerobic reactor, and both parameters are used for the design of the anaerobic digester [54,55]. The scale of the OLR for the operation of a continuous reactor, equivalent to the S/I in a batch reactor. Figure 3 included the OLR in the X axis based on the Figures 4 and 5 of Rincón et al. [56], who reported similar trends of biogas release and vs. removal. Reproduced with the permission of Biochemical Engineering Journal (Elsevier). In their original manuscript, the OLR was expressed on chemical oxygen demand (COD) basis, however, it is recommended to use the vs. instead [57].

$$\mathrm{OLR}\left(\mathrm{kg}\frac{\mathrm{VS}}{{\mathrm{m}}^3\mathrm{day}}\right)=\frac{\mathrm{Input}\left(\frac{\mathrm{kg}\mathrm{substrate}}{\mathrm{day}}\right)\times \mathrm{Labile}\mathrm{matter}\left(\frac{\mathrm{kg}\mathrm{VS}}{\mathrm{kg}\mathrm{substrate}}\right)}{\mathrm{Net}\mathrm{digester}\mathrm{volume}\left({\mathrm{m}}^3\right)}$$

(1)

The highest CUE, and therefore the highest stability of the organic amendment, were obtained at the lowest C/N (Figure 3) because the microbes have all the nutrients that they need to build their cell structures and do not need to get rid of excess carbon. On the other hand, when a low amount of substrate is available (OLR < 6), the microbes are in starvation mode, and they assimilate the carbon for their growth more efficiently, compared to higher OLR conditions under which more carbon is lost in microbial respiration due to excessive microbial activity (Figure 2). Since less carbon ends up in the microbial biomass, which is measured as part of the VS, the CUE decreases. Despite the different physiology of the terrestrial microorganisms from the consortium of the AD, the trend of CUE measured in an aerobic environment is useful to explain the fate of the carbon during batch and continuous AD operations (Figure 3). There are similar processes involved in the nutrient turnover in the AD and once the soil organic amendment is applied to land. The maturity profile is offered in the Supplementary Material (Figure S4).

Theorem 1.

Microbes get rid of any nutrient in excess available in the medium (e.g., anaerobic digester, soil solution, etc.), hence the composition of the organic amendment should be as similar as possible to the composition of the microbes.

If carbon is in excess (C/N ≥ 35), the microbes cast aside this element via respiration or extracellular polymeric substances (EPS) segregation [59]. The composition of the AD feedstock has been traditionally measured using the C/N, and this parameter has been employed for other types of fermentations, such as composting and land application of the organic soil amendment [60]. Möller & Müller [61,62] emphasized that a more representative ratio to express the composition of the organic soil amendment would be organic carbon (C_org) to organic nitrogen. Some researchers reported carbon as the main component of the wood ashes [62,63]. According to Forbes et al. [64], the continuum of combustion products such as char, ash, and charcoal are referred to as black carbon. This feature makes the properties of the wood ashes more suitable to be employed as a land amendment, to restore the soil as a carbon sink, maintain an adequate microbial activity in the soil, appropriate management of the nutrients to boost crop growth, and prevent pollution via leaching and gaseous emissions [65]:

Proof of Theorem 1.

According to the British Standards Institution’s Publicly Available Specification (BSI PAS 110:2014) for anaerobic digestate [46], the upper limit for a stable organic amendment in the 28-day test is 450 mL biogas/g VS, which is in agreement with Figure 3. It should be noted that this threshold value was established in the conditions of no inhibition of the test, which could be related to an inappropriate S/I (i.e., OLR > 10 g VS/L/d) and/or low nutrient content (i.e., C/N > 35). Profiles of biogas production indicating inhibitory conditions of the BMP test are displayed in the Supplementary Materials (Figures S5–S7). It is important to highlight that nitrogen plays a role in enhancing biogas production but supplementing the organic amendment with other nutrients is advised [66], which is the reason for seeding the AD feedstock with ashes [67]. Equation (2) represents how the heterotrophic microbial biomass (C₅H₇NO₂) is chemically built from glucose (C₆H₁₂O₆), as a source of carbon, and NH₃, as a source of nitrogen:

$$3{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6{}_{\left(\mathrm{s}\right)}+8{\mathrm{O}}_2{}_{\left(\mathrm{g}\right)}+2{\mathrm{NH}}_3{}_{\left(\mathrm{g}\right)}\to 2{\mathrm{C}}_5{\mathrm{H}}_7{\mathrm{NO}}_2{}_{\left(\mathrm{s}\right)}+8{\mathrm{CO}}_2{}_{\left(\mathrm{g}\right)}+14{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$$

(2)

The maximum theoretical CUE is approximately 56% (2 × 5/3/6 × 100) in aerobic fermentation [68], in agreement with Figure 3. The reason for measuring the stability as a release of biogas could be the fact that there is no accurate quantification of the microbial biomass available in the biodigester, and this is often measured as suspended vs. [69]. The polymerase chain reaction to quantify only the active cells in the anaerobic digester is in the development stage and requires equipment advances [70]. The CUE measured in soil science is a more straightforward technique that relies on the extraction of the microbial carbon with a solution of 0.5 M K₂SO₄ after fumigation with chloroform [71].□

3. Downstream Processing

As in the previous section about upstream processing, this section of downstream processing explains the possible synergistic effects found by adding the WA to the anaerobic digestate. The WA can be added to the anaerobic digestate after the biodigester to ease the management of the organic material by developing a process complying with green chemistry principles [72], such as minimum input of energy and resources, due to all the synergies involved. The role of the WA as sorbent is essential to produce a controlled-release fertilizer derived from the anaerobic digestate. It should be noted that there are other materials (e.g., ion exchange resins) that could also be employed for the same purpose [73]. The sorption (adsorption and/or absorption; [62]) will take place from the moment in which the wood ash touches the anaerobic digestate; thereby this phenomenon is implicitly involved in each step of the process of Figure 4.

The dose of WA depends on the intended properties of the fertilizer to be manufactured. The following are some of the considerations that were taken into account to establish the blending ratio of 5 g TS WA/g TS anaerobic digestate (Figure 4):

• Enhancement of AD could be achieved by preparing the feedstock with coal fly ash (CFA) using a dose as high as 2% (w/w) [76].
• Precipitation of struvite and adsorption of phosphate achieved by preparing a suspension with up to 3% (v/w) ash in swine wastewater [77].
• Application to land of both raw materials following a blending of up to 5% (w/w) of ash [78].
• Mitigation of CH₄, CO₂, NH₃, N₂O associated with the storage of cattle slurry with 4.6% TS content by adding charcoal or biochar at a rate of 4.5% (w/v) [79] and around 11% (w/w) [80].
• Adjustment of the pH to 5.5 of untreated pig slurry and co-digested pig slurry by adding 2% and 3.5% (w/w) of powdered aluminium sulfate (Al₂(SO₄)₃), respectively [81,82]
• Supplementation of the anaerobic digestate by means of a WA dose up to 9.99% (w/w) or 3.09 g TS ash/g TS digestate to improve the nutrient ratio (C:N:P), the availability of phosphorus, and the microbial activity in the soil [60].
• Agreement with the regulations regarding the maximum content of heavy metals present in the anaerobic digestate [46,83]. The share of WA should not be greater than 15.51% (w/w) relative to the anaerobic digestate or 1.47 g TS WA/g TS AD. These results were obtained by considering the maximum content of heavy metals in the WA values established in the UK Quality Protocol of PL ash [84]. The content of Zn was found to be the limiting factor. The assumption of these calculations (Tables S1 and S2) are described in the Supplementary Material.
• Prevention of a large volume of dewatered digestate obtained via filtration by using as much CFA as the dry matter of the digestate (i.e., 1 g TS CFA/g TS digestate) to assist the dewatering process [28].
• Moure Abelenda et al. [85,86,87,88] tested alkaline and acid conditions to minimize the volatilization of NH₃, and carbon and PO₄³⁻ solubilization. They obtained better results (i.e., lower availability of nitrogen, carbon, and phosphorus) under acid conditions (4.39 g TS WA/g TS digestate) than under alkaline conditions (5.51 g TS WA/g TS digestate).
• Alkaline stabilization of sewage sludge via liming with a dose of CaO as high as 40% (w/w) or 8 g TS CaO/g TS sewage sludge to decrease the pathogens (Méndez et al., 2002). A dose of 3.82 g TS CaO/g TS digestate or 224.5 g CaO/L digestate (5.88% TS) was required for reaching a pH 12 and removing 51.2% of the NH₄⁺-N due to NH₃ volatilization [74]. Limoli et al. [74] reported that a low dose of 45 g/L increased the TS content of the manure digestate by 42.7%. When the organic material had higher dry matter (25.4% TS content), a dose of 50 g CaO/kg SS represented an increase in the TS content of approximately 30% and just 2 units of pH. This liming effect reduced the availability of heavy metals in the SS [89].
• Reducing phosphate availability by adding 5.6 kg of CFA to each kilogram of dairy slurry [90] could present a dose of greater than 110 g TS CFA/g TS slurry if the moisture content of the organic manure is 95%.
• Preparation of granules prepared with 100% (w/w) biomass ash showed the best mechanical properties. Decreasing the content to 80% bio ash and 20% dewatered SS (45% moisture) significantly affected the compressive strength of the pellets [29]. The lowest dose of bio ash and Ca(OH)₂ that Pesonen et al. [29] tested corresponded to a 5.19 g TS bio ash + Ca(OH)₂/g TS hygienized SS.

3.1. Pasteurization and Sterilization

According to the animal by-product regulations for biogas plants in the UK [91], unless the AD is done in the conditions of

Table 2. Conditions to produce a sanitized digestate when using animal by-products (e.g., manure and slurry) as feedstock in AD [91].

Minimum Temperature	Minimum Time	Maximum
57 °C	5 h	50 mm

, pasteurization (70 °C for 1 h) of the digestate is required before using the digestate for any application [46]. A considerable reduction in the number of pathogens can be achieved during fermentation, especially if it is done in thermophilic conditions [17]. Also, the best control of the digester was obtained in the one-stage non-mixing thermophilic reactor [92].

Greater doses of WA than that used to improve the performance of the AD can be employed with pasteurization and sterilization purposes. While sterilization implies the destruction of all life forms in the anaerobic digestate, the pasteurized material might contain beneficial or harmless microorganisms [46]. The aim is to keep the pH of the organic manure above 12 for at least 2 h [93,94]. This is often achieved using a rate of application of WA to organic material of 0.1 g Ca(OH)₂ per g TS [93,95]. It should be noted that calcium represents approximately 18% of the total weight of the WA [96], thus this element is one of the main components of this material [97]. This alkaline treatment, similar to the use of lime, reduces the storage time of organic manures by 3 months while still preventing the contamination of the crops by pathogens [98,99]. The shares of the calcium as oxide, hydroxide, and carbonate in the wood ashes are mainly determined by the temperature of incineration and storage conditions. Under 500 °C, carbonates and bicarbonates predominate, while oxides require temperatures around 1000 °C [96]. During storage, the reaction with the moisture and the CO₂ in the atmosphere leads to the formation of hydroxides and carbonates, although converting to CaO is possible via calcination of the ashes at temperatures over 500 °C [100,101].

3.2. Nitrogen Recovery Technologies

In this subsection, only the processes of NH₃ stripping, manufacturing of (NH₄)₂CO₃, and struvite (MgNH₄PO₄·6 H₂O) precipitation and sorption of the nitrogen in the soil organic amendments are described for the exploitation of this element as fertilizer because these technologies are the most convenient for the implementation with wood ashes [102]:

3.2.1. NH₃ Stripping Processes from the WA Anaerobic Digestate

Limoli et al. [74] tested the addition of the CaO to the anaerobic digestate to increase the pH and promote the volatilization of NH₃. Less than 1% of the NH₄⁺ & NH₃ is volatilized as part of the biogas released during the AD [61]. Limoli et al. [74] described a stepwise mechanism involving the ammonium dissociation (NH₄⁺ → NH₃ + H⁺) and the mass transfer in the water-air interface. Since the WA primarily consists of calcium and other alkaline elements, this material can be used to increase the pH of the anaerobic digestate and promote the volatilization of NH₃. Limoli et al. [74] did not consider how much of this nitrogen ends up in a trap containing a sulfuric acid (hereinafter H₂SO₄ trap). The resulting 40–60% ammonium sulfate ((NH₄)₂SO₄) solution used can be as commercial-grade fertilizer (Equation (3)), although this will depend on the organic contamination of the liquid fertilizer [103]. If the gas-liquid contact system employs a high volume of air to strip the NH₃ from the anaerobic digestate, the cost of the process increases since the absorption in the H₂SO₄ solutions is hindered. It should be noted that the recovery of the NH₃ from the gaseous stream has complications, and often the H₂SO₄ traps require calibration to determine the efficiency of this technology [104].

$$2{\mathrm{NH}}_{3\left(\mathrm{g}\right)}+{\mathrm{H}}_2{\mathrm{SO}}_{4\left(\mathrm{aq}\right)}\to 2{\mathrm{NH}}_{4\left(\mathrm{aq}\right)}^{+}+{\mathrm{SO}}_{4\left(\mathrm{aq}\right)}^{2-}\to {\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_{4\left(\mathrm{s}\right)}$$

(3)

The reason for the costly amount of alkali required to basify the anaerobic digestate is the fact that there are 3 buffer equilibria (Equations (4)–(6)) responsible for the pH in the anaerobic digestate [61]:

$${\mathrm{NH}}_4^{+}{}_{\left(\mathrm{aq}\right)}\leftrightarrow {\mathrm{H}}^{+}{}_{\left(\mathrm{aq}\right)}+{\mathrm{NH}}_3{}_{\left(\mathrm{aq}\right)}$$

(4)

$${\mathrm{CO}}_2{}_{\left(\mathrm{aq}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\leftrightarrow {\mathrm{H}}_2{\mathrm{CO}}_3{}_{\left(\mathrm{aq}\right)}\leftrightarrow {\mathrm{H}}^{+}{}_{\left(\mathrm{aq}\right)}+{\mathrm{HCO}}_3^{-}{}_{\left(\mathrm{aq}\right)}\leftrightarrow 2{\mathrm{H}}^{+}{}_{\left(\mathrm{aq}\right)}+{\mathrm{CO}}_3^{2-}{}_{\left(\mathrm{s}\right)}$$

(5)

$${\mathrm{CH}}_3{\mathrm{COOH}}_{\left(\mathrm{aq}\right)}\leftrightarrow 2{\mathrm{H}}^{+}{}_{\left(\mathrm{aq}\right)}+{\mathrm{CH}}_3{\mathrm{COO}}^{-}{}_{\left(\mathrm{aq}\right)}$$

(6)

Since oxides, hydroxides, and carbonates are the most common forms of metals in wood ashes [96], the reactions of the WA in an aqueous solution, involving the alkali and alkaline metals, should be (Equations (7) and (8)):

$${\mathrm{K}}_2{\mathrm{O}}_{\left(\mathrm{s}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to 2{\mathrm{K}}_{\left(\mathrm{aq}\right)}^{+}+{\mathrm{O}}_{\left(\mathrm{aq}\right)}^{2-}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to 2{\mathrm{K}}_{\left(\mathrm{aq}\right)}^{+}+2{\mathrm{OH}}_{\left(\mathrm{aq}\right)}^{-}$$

(7)

$${\mathrm{CaO}}_{\left(\mathrm{s}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to {\mathrm{Ca}}_{\left(\mathrm{aq}\right)}^{2+}+{\mathrm{O}}_{\left(\mathrm{aq}\right)}^{2-}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to {\mathrm{Ca}}_{\left(\mathrm{aq}\right)}^{2+}+2{\mathrm{OH}}_{\left(\mathrm{aq}\right)}^{-}$$

(8)

After the dissociation of the alkaline oxide, the oxide anion (O²⁻) rapidly reacts with water [105,106], leading to the formation of a hydroxide anion (OH⁻). Also, basic cations (e.g., Ca²⁺ and K⁺) increase the pH because the electric charge of the liquid digestate needs to be neutral, thus the concentration of H⁺ is lower [61].

$${\mathrm{K}}_2{\mathrm{CO}}_3{}_{\left(\mathrm{s}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to 2{\mathrm{K}}_{\left(\mathrm{aq}\right)}^{+}+{\mathrm{CO}}_{3\left(\mathrm{aq}\right)}^{2-}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to 2{\mathrm{K}}_{\left(\mathrm{aq}\right)}^{+}+{\mathrm{CO}}_{2\left(\mathrm{g}\right)}+2{\mathrm{OH}}_{\left(\mathrm{aq}\right)}^{-}$$

(9)

$${\mathrm{CaCO}}_3{}_{\left(\mathrm{s}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to {\mathrm{Ca}}_{\left(\mathrm{aq}\right)}^{2+}+{\mathrm{CO}}_{3\left(\mathrm{aq}\right)}^{2-}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to {\mathrm{Ca}}_{\left(\mathrm{aq}\right)}^{2+}+{\mathrm{CO}}_{2\left(\mathrm{g}\right)}+2{\mathrm{OH}}_{\left(\mathrm{aq}\right)}^{-}$$

(10)

3.2.2. Manufacturing of (NH₄)₂CO₃

It is worth mentioning that Equations (4) and (5) are connected by the precipitation of (NH₄)₂CO₃, which naturally increases the pH of the anaerobic digestate. This is, in fact, another route to recover the NH₄⁺ contained in the anaerobic digestate. The NH₄HCO₃ is a marketable fertilizer that contains 18% of nitrogen and minimizes the losses of nitrogen during the processing of the anaerobic digestate. Unlike in the stripping of NH₃ by increasing the pH of the anaerobic digestate and subsequent reaction with the H₂SO₄, raising the pH of the digestate by adding WA is not useful because the high pH prevents the release of the CO₂. It should be noted, however, that the WA could be, though as a source of CO₃²⁻-C to increase the share of inorganic carbon in the anaerobic digestate. In the process designed by Drapanauskaite et al. [107], the simulated liquid fraction of the anaerobic digestate contained 20 times more HCO₃⁻ than NH₄⁺. The CO₃²⁻-C content is usually not reported in AD studies [55] but the total alkalinity [49] (expressed as a concentration of CaCO₃), although this parameter includes the buffer effect of the VFA (e.g., acetic acid represented in Equation (6)) and the NH₄⁺-N as well [108]. Astals et al. [49] reported that the mass ratio CO₃²⁻-C/NH₄⁺-N was lower than 10 in the anaerobic digestate of pig manure and glycerol. In the case of studies focused on using the anaerobic digestate as an organic soil amendment, most studies only report the amount of C_org, and they consider the amount of CO₃²⁻-C negligible (Theorem 1) for the calculation of the C/N [109,110,111]. Otherwise, they directly calculate the C_org/N without recognizing any role for the CO₃²⁻-C [42,112]. Considering that the biogas is approximately 40% v/v and the alkaline pH of the anaerobic digestate [61], a considerable amount of CO₂ needs to remain dissociated in the form of CO₃²⁻-C. The quantification of CO₃²⁻-C is interesting from the point of view of controlling the removal of NH₄⁺-N, because to achieve the formation of NH_3(aq) in a stripping process, a significant amount of alkaline reagent needs to be spent on shifting the equilibria described in Equations (4)–(6) [113]. However, experimental studies developing process for the recovery of the NH₄⁺-N via stripping do not clearly stablish the content of CO₃²⁻-C in the anaerobic digestate [74,114,115].

In the novel process simulation performed by Drapanauskaite et al. [107], the anaerobic digestate was subjected to distillation at 3.3 bar with the condenser operating at 49 °C to produce a liquid stream rich in HCO₃⁻ and NH₄⁺. The solid NH₄HCO₃ was obtained in a crystallizer at 12 °C and was finally recovered via drying [107]. According to Drapanauskaite et al. [107], approximately 100% of the NH₄⁺-N was used to manufacture the NH₄HCO₃. Drapanauskaite et al. [107] compared the open-loop process of (NH₄)₂SO₃, which implies using H₂SO₄ as an external chemical, with the NH₄HCO₃ process where both reagents (HCO₃⁻ and NH₄⁺) are initially present in the anaerobic digestate. In addition to the lower capital cost of the manufacturing of NH₄HCO₃, the operating cost was also lower due to the lower consumption of utilities.

3.2.3. Struvite Isolation Using WA as a Source of Magnesium

Another strategy to remove the excess of NH₄⁺-N from the anaerobic digestate using the WA is via isolation of struvite [116]. However, the production of this slow-release fertilizer is expensive due to the cost of the source of magnesium and the alkaline agent needed to reach the target pH for the crystallization of this material [77,100,101]. Huang et al. [77] reported that 0.31 US Dollar/kg PO₄³⁻-P can be saved if straw ash is employed instead of NaOH. Both Sakthivel et al. [101] and Huang et al. [77] recovered more than 96% of the phosphate in the ureolysis urine and swine wastewater, respectively, using different doses of WA. While Sakthivel et al. [101] employed 2.7 mol Mg/mol P; Huang et al. [77] used 1.2 mol Mg/mol P. This might be related to the fact that only 50% of the magnesium in the WA of Sakthivel et al. [101] was water-soluble (WS), thus only this amount was available for the formation of struvite. It would be possible to increase the availability of the magnesium via calcination of the WA at temperatures higher than 600 °C [101]. Another option would be to perform aeration when adding the WA to the anaerobic digestate to remove the CO₃²⁻-C [77]. Drosg et al. [116] explained that the magnesium should be added in excess, according to the molar ratio 1.3:1:0.9 for Mg:N:P. This also agrees with the dose of magnesium proposed by Miles & Ellis [117] (1.25:1:1), and both nutrient ratios recommended for the precipitation of struvite are slightly different from the stoichiometry of the chemical reaction (Equation (11)). It should be noted that Equation (11) implies the release of H⁺ [61,118], but other authors prefer to represent the drop of the pH as consumption of OH⁻ [116].

$${\mathrm{NH}}_4^{+}{}_{\left(\mathrm{aq}\right)}+{\mathrm{Mg}}^{2+}{}_{\left(\mathrm{aq}\right)}+{\mathrm{H}}_{\mathrm{n}}{\mathrm{PO}}_4^{\mathrm{n}-3}{}_{\left(\mathrm{aq}\right)}+6{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to {\mathrm{MgNH}}_4{\mathrm{PO}}_4·6{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{s}\right)}+{\mathrm{nH}}^{+}{}_{\left(\mathrm{aq}\right)}$$

(11)

According to Drosg et al. [116], materials such as anaerobic digestate contain more NH₄⁺ & NH_3, hence the addition of orthophosphoric acid (H₃PO₄) is necessary to reach the target nutrient ratio. Escudero et al. [119] studied how the removal of NH₄⁺-N from anaerobically treated effluents was affected by the source of orthophosphate PO₄³⁻-P and Mg²⁺ and the nutrient ratio. They found an NH₄⁺ removal of 95% in 30 s when using the best sources of magnesium (e.g., MgCl₂·6H₂O and MgSO₄·7H₂O) and a molar ration Mg:N:P of 1:1:1 [119]. Sakthivel et al. [101] reported that the solid precipitate was not pure struvite due to the high content of CaCO₃ in the WA. They found a phosphorus content of 3% in the precipitate that is lower than the struvite (13%) or the diammonium phosphate (46%). Although the phosphorus content in the precipitate was greater than that in the initial WA, the estimated value of the precipitate was lower than the WA because 60% of the potassium initially present in the WA remained undissolved in the phosphate-depleted ureolysed urine [101]. Huang et al. [77] reported the competition reaction between struvite (Figure 4) and K-struvite (MgKPO₄·6H₂O). These authors reported a greater share of K-struvite at pH 10, which was higher than the optimum pH range (7.5 to 9) for the precipitation of struvite [118]. According to Huang et al. [77], at pH 9.5, and with a dose of plant ash of 6 mol K/mol NH₄⁺-N, the amounts of struvite and K-struvite are the same. In the structure of the struvite (Figure 5), the Mg²⁺ can coordinate with the PO₄³⁻ and the NH₄⁺ despite being surrounded by six water molecules arranged according to an octahedral geometry.

Theorem 2.

The optimum pH for the precipitation of struvite is the pH of zero point charge (pH_zpc), at which the surface charge of the source of magnesium (e.g., WA) is neutralized, thus the magnesium is able to sorb WS NH₄⁺ and WS PO₄³⁻ simultaneously.

The pH_zpc could be identified by plotting Q (Equation (12)) versus the pH, a second-order fitting calculated (generic form: Q = a(pH)² + b(pH) + c), and the minimum value of the quadratic regression determined (Figure 6). Unlike the measurement of the zeta potential, the pH_zpc is a simpler technique that does not require specialized equipment. Nevertheless, both parameters can be used interchangeably since the zeta potential is the electrical voltage at the interface that separates mobile fluid from the fluid that remains attached to the particle surface [121].

$$\mathrm{Q}=\frac{1}{\mathrm{W}}\left({\mathrm{C}}_{\mathrm{a}}-\left[{\mathrm{H}}^{+}\right]+\left[{\mathrm{OH}}^{-}\right]\right)$$

(12)

• Q (mol/L/g dry adsorbent), surface charge.
• W (g/L), dry mass of WBA-based adsorbent in the aqueous system (i.e., analyte).
• C_a (mol/L), concentration of the acid titrant in the aqueous system.
• [H⁺] & [OH⁻], concentration of H⁺ and OH⁻ resulting from the direct measurement of the pH in the aqueous system (pH = −log([H⁺]); [H⁺]·[OH⁻] = 10⁻¹⁴).

Figure 6. Determination of the pH_zpc of the WBA representing the surface charge of the WBA adsorbent, Q, as a function of the titrate aqueous solution. Elaborated from the description provided by Leechart et al. [63] and Shah et al. [122].

Figure 6. Determination of the pH_zpc of the WBA representing the surface charge of the WBA adsorbent, Q, as a function of the titrate aqueous solution. Elaborated from the description provided by Leechart et al. [63] and Shah et al. [122].

Proof of Theorem 2.

While the systems had a lower pH than the pH_zpc, the surface of the WA-based adsorbents became positively charged (Equation (13)), favoring the adsorption of anionic species.

$${\mathrm{Surface}}_{\left(\mathrm{n}-1\right)}^{+}\stackrel{+{\mathrm{OH}}^{-}}{\leftarrow }\underset{+{\mathrm{H}}^{+}}{\to }\underset{{\mathrm{pH}}_{\mathrm{zpc}}}{\underbrace{{\mathrm{Surface}}_{\mathrm{n}}^0\left(\mathrm{s}\right)}}\stackrel{+{\mathrm{OH}}^{-}}{\leftarrow }\underset{+{\mathrm{H}}^{+}}{\to }{\mathrm{Surface}}_{\left(\mathrm{n}+1\right)}^{-}$$

(13)

source: [63].

Furthermore, since the pH of minimum solubility of CaO and MgO are 11.0 and 12.4, respectively, the surface of CaO and MgO could support the adsorption of the dye when the pH of the system was greater than pH_zpc (Equations (14) and (15)).

$$5{\mathrm{Ca}}^{2+}{}_{\left(\mathrm{aq}\right)}+4{\mathrm{OH}}^{-}{}_{\left(\mathrm{aq}\right)}+3{\mathrm{HPO}}_4^{2-}{}_{\left(\mathrm{aq}\right)}\to {\mathrm{Ca}}_5\left(\mathrm{OH}\right){\left({\mathrm{PO}}_4\right)}_3{}_{\left(\mathrm{s}\right)}+3{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$$

(14)

$$5{\mathrm{Mg}}^{2+}{}_{\left(\mathrm{aq}\right)}+4{\mathrm{OH}}^{-}{}_{\left(\mathrm{aq}\right)}+3{\mathrm{HPO}}_4^{2-}{}_{\left(\mathrm{aq}\right)}\to {\mathrm{Mg}}_5\left(\mathrm{OH}\right){\left({\mathrm{PO}}_4\right)}_3{}_{\left(\mathrm{s}\right)}+3{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$$

(15)

□

The sorption and precipitation processes are investigated as a way of reducing the cost of the purification of the water and preventing the eutrophication of underground water. Yagi & Fukushi [123] studied the mechanisms of adsorption and precipitation of calcium phosphate onto monohydrocalcite (CaCO₃·H₂O). According to Yagi & Fukushi [123], the sorption behavior was determined mainly by concentrating phosphate in the wastewater. The CaCO₃·H₂O is known to be more reactive than the conventional CaCO₃ [123]. The clean effluent obtained by Yagi & Fukushi [123] could be coupled with air or CO₂ bubbling to remove any trace of Ca²⁺ ion via precipitation of CaCO₃ [124]. Similarly, Brennan et al. [80] tested whether the addition of lime to the cattle slurry minimized the emission of NH₃, N₂O, CH_4, and CO₂ and the leaching of WS PO₄³⁻.

3.3. Acidification of the Blend of WA and Anaerobic Digestate to Improve the Nutrient Management

The commercial acids, such as H₂SO₄, HCl, HNO_3, and H₃PO₄, or easily fermentable compounds which ultimately lead to the formation of organic acids due to the microbial metabolism, such as the acetic acid (C₆H₁₂O₆ → 3 CH₃COOH), represent the most obvious choices to reduce the pH of the organic manures [125,126,127]. However, better stabilizations have been reported with the use of salts such as Al₂(SO₄)₃ [82], FeCl₂ [80] and FeCl₃ [128], and CaCl₂ [129,130]. The CaCl₂ does not necessarily need to be produced with quicklime (CaO; Equation (16)) but with slaked lime (consisting mainly in Ca(OH)₂), limestone (CaCO₃), or a mixture of these compounds that might be present in the WA. Similarly, the Al₂(SO₄)₃ (Equation (17)) or the Fe₂(SO₄)₃ could be produced from CFA [131]. It is noteworthy to mention that the Al₂(SO₄)₃, FeCl₂, and FeCl₃ (Equation (18)) are among the best acidifying additives for the stabilization of animal slurry anaerobic digestate [115,131,132]. These salts are widely employed in wastewater treatment for coagulation and precipitation of compounds [82,116]. This use is in line with the properties of WA as sorbent [133,134,135,136]. The activation of raw materials (e.g., ores and ashes) for the manufacturing of the salts might be why they are able to decrease the pH of the organic manures.

$${\mathrm{CaO}}_{\left(\mathrm{s}\right)}+2{\mathrm{HCl}}_{\left(\mathrm{aq}\right)}\to {\mathrm{CaCl}}_2{}_{\left(\mathrm{aq}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$$

(16)

$$2\mathrm{Al}{\left(\mathrm{OH}\right)}_3{}_{\left(\mathrm{s}\right)}+3{\mathrm{H}}_2{\mathrm{SO}}_4{}_{\left(\mathrm{aq}\right)}\to {\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3{}_{\left(\mathrm{s}\right)}+6{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$$

(17)

$${\mathrm{Fe}}_3{\mathrm{O}}_4{}_{\left(\mathrm{s}\right)}+8{\mathrm{HCl}}_{\left(\mathrm{aq}\right)}\to {\mathrm{FeCl}}_2{}_{\left(\mathrm{s}\right)}+2{\mathrm{FeCl}}_3{}_{\left(\mathrm{s}\right)}+4{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$$

(18)

The CaCl₂ is one of the key components of the specially formulated products to stabilize the organic manures [137], and this compound is also widely used as an extracting agent for soil analysis. A solution of 0.01 M of CaCl₂ provides the same ionic strength [138] as the average salt concentration in many soil solutions (i.e., the medium in which surface and aqueous solution reactions occur in the soil). The rationale of solubilizing the elements to minimize the gaseous emissions agrees with the higher stability of the organic manures with higher moisture content [132]. On the other hand, when manures are diluted with water, most of the H⁺ ions tend to remain attracted to the solid particles and are not released to the aqueous fraction. The simple dilution of manure with water reduces the emissions [132] but does not acidify the medium. Small amounts of CaCl₂ provide Ca²⁺ ions that replace some of the H⁺ ions adsorbed in the solid particles, thus decreasing the pH of the medium. The stabilization of the organic slurry via the addition of CaCl₂ is explained by the solubilization of nutrients in the moisture of the organic slurry, ion exchange capacity of Ca²⁺, H⁺ release from the surface of the solid particles, and pH drop. The acid effect of these salts once dissolved in the aqueous phase of the organic slurries can be explained by a number of mechanisms. For example, the addition of FeCl₃ acidifies the aqueous solution by removing the sulfide and the phosphate, according to Equations (19)–(21) [61].

$${\mathrm{FeCl}}_3{}_{\left(\mathrm{s}\right)}\to {\mathrm{Fe}}^{3+}{}_{\left(\mathrm{aq}\right)}+3{\mathrm{Cl}}^{-}{}_{\left(\mathrm{aq}\right)}$$

(19)

$${\mathrm{H}}_2{\mathrm{S}}_{\left(\mathrm{aq}\right)}+2{\mathrm{Fe}}^{3+}{}_{\left(\mathrm{aq}\right)}\to {\mathrm{S}}_{\left(\mathrm{s}\right)}+2{\mathrm{Fe}}^{2+}{}_{\left(\mathrm{aq}\right)}+2{\mathrm{H}}^{+}{}_{\left(\mathrm{aq}\right)}$$

(20)

$${\mathrm{H}}_{\mathrm{n}}{\mathrm{PO}}_4^{\mathrm{n}-3}{}_{\left(\mathrm{aq}\right)}+{\mathrm{Fe}}^{3+}{}_{\left(\mathrm{aq}\right)}\to {\mathrm{FePO}}_4{}_{\left(\mathrm{s}\right)}+{\mathrm{nH}}^{+}{}_{\left(\mathrm{aq}\right)}$$

(21)

3.3.1. Activation of the WA as Sorbent to Improve the Properties of the Blend with the Anaerobic Digestate as Slow-Released Fertilizer

Commercial acids are also employed to activate the ash and to prepare a cheap and sustainable sorbent. For example, Mor et al. [139], who used the calcination at 500 °C and HCl to activate the rice husk ash, reported an increase in the adsorption of the WS PO₄³⁻ with a maximum removal (91.7%) of WS PO₄³⁻ at pH 2 and the lowest removal at pH 10. According to Mor et al. [139], because at high pH, there are more OH⁻ interacting with the surface, thus decreasing the WS PO₄³⁻ adsorption. It is necessary to bear in mind that the removal of PO₄³⁻ [140] and the NH₄⁺ [119] from the WS phase via struvite crystallization is not promoted at acidic pH. The dose of acid needs to be as much as it is required to reach the pH_zpc without a further decrease (Figure 5). Leechart et al. [63] found that the pH_zpc for the WBA employed was 10.8. When a pretreatment was applied to the WBA with distilled water (WBA-H₂O) or sulfuric acid (WBA-H₂SO₄), the pH_zpc was 10.9 in both cases [63].

Unlike acidification, carbonation uses CO₂ rather than commercial acids to decrease the pH of the adsorbent. In addition to the reagents, the suitability of each activation treatment depends on the origin of the adsorbent and the primary mechanism for capturing the nutrients (

Table 3. Implications of the activation treatments that affect the suitability for a particular application (e.g., preparation of a slow-release fertilizer, liming agent, recovery of nutrients from wastewater, etc.). Elaborated by considering the information of [62,63,100,122,140].

Activation Procedure	Carbon Content	pH	Reactivity
Carbonization	Increased, due to loss of volatile compounds containing H, O, S, and N	Slightly increased, due to the accumulation of ash with alkali and alkaline elements	Decreased. Mainly charred materials and recalcitrant compounds
Carbonation	Increase, in the form of inorganic carbon	Decrease, due to the neutralization of the alkaline element with carbonic acid	Decreased because carbonates are less soluble than oxides.
Calcination	Decrease, since all volatile compounds have already been lost in the carbonization	Increase, due to the release of CO2	Increase, due to the formation of oxides
Acidification	Decrease, due to the dissociation of the CO3 -C and subsequent CO2 emissions	Decrease, due to the dissociation of the commercial acids	Slightly increase 1
Wash	Slightly decrease, due to the removal of impurities	Slightly decreased due to the solubilization of alkalis	Slightly decrease, due to extraction of oxides
Milling and sieving	Slightly increase. Might enhance carbonation reactions during storage	Slightly increase. Might enhance the reaction for acidic and alkaline salts	Increase, due to greater availability of the elements

¹ The oxides of the alkaline elements are more soluble and reactive than the carbonates. It is considered that the acidification of the alkaline oxides (e.g., CaO) with commercial acids, rather than with CO₂, surpasses the preparation of adsorbents (e.g., CaCO₃) and lead to the formation of coagulants and flocculants (e.g., CaCl₂; Equation (16)).

). Janiszewska et al. [141] employed the carbonation at 800 °C by putting it in touch with the biochar, which had been prepared at 400 °C in an N₂ atmosphere, with a constant flow of CO₂ (150 mL/min) for 180 min. According to Janiszewska et al. [141], the CO₂ activation employed aimed to increase the porosity of the carbonized wastes. The mechanism relies on the volatilization of a significant part of the carbon content to enable the formation of a microporous structure in the biochar [141]. Leechart et al. [63] used the Brunauer, Emmett, and Teller (BET) isotherm to determine the sorptive surface area. It should be noted that the BET isotherm considers the possibility that the monolayer in the Langmuir adsorption isotherm could act as support for further adsorption. The nitrogen adsorption-desorption isotherm, employed for the BET study of these adsorbents, was characterized by clear hysteresis loops due to the capillary condensation occurring in the mesopore range [63]. The H₂O treatment removed some tars and led to a higher development of porosity and surface area than the treatment with 0.1 N H₂SO₄. The WBA-H₂O still had a lower surface area than AC. The average pore diameter of WBA, WBA-H₂O, and WBA-H₂SO₄ was of mesoporous size (i.e., 2–50 nm) while the activated carbon had micropores (<2 nm).

Ma et al. [142] described a more complicated activation procedure for producing an adsorbent based on wheat straw. They reported how the pH and the level of swelling (i.e., hydration) affect the adsorption of WS NH₄⁺ and WS PO₄³⁻ [142]. According to the results of Ma et al. [142], the optimum pH for the adsorption of these ions is between 4 and 8. The explanation that Ma et al. [142] provided for the trend of the WS NH₄⁺ is that at acidic pH, the H⁺ ions will compete with the WS NH₄⁺ for the anionic groups in the wheat straw derived adsorbent. On the other hand, at alkaline pH, more WS NH₄⁺ will be in the form of WS NH₃ that will subsequently escape from the aqueous solution. According to Ma et al. [142], the adsorption of WS PO₄³⁻ showed less dependence on the pH. Ma et al. [142] described the impact of the pH and the swelling capacity (i.e., hydration) of the performance of the wheat straw-derived adsorbent. The high swelling of the wheat straw-derived adsorbent obtained at pH 3 and above was associated with the high diffusion of water molecules and ions into the pores and through the internal skeleton of the adsorbent. The highest swelling was obtained at a pH 7 because acidic and alkaline solutions have H⁺ and OH⁻ ions, respectively, that reduce the difference in osmotic pressure between inside and outside the porous structure of the wheat straw derived adsorbent [142]. As described by Awad et al. [143], the swelling phenomenon is often associated with an increase in the adsorption of the organic materials in the WS phase. This can also be seen in the structure of the struvite (Figure 5), in which the nutrients NH₄⁺ and PO₄³⁻ remain attached to the magnesium hexaaqua (Mg²⁺(H₂O)₆).

3.3.2. Dewatering of the Blend of WA and Anaerobic Digestate

The studies of Ma et al. [142] and Mor et al. [139] provided contrary results regarding how the pH and the level of swelling/hydration affect the adsorption. The dehydration without acidification was tested by Zheng et al. [28]. They used a cationic surfactant (cetyltrimethylammonium bromide; CTAB) together with CFA to ease the filtration of the anaerobic digestate (Figure 7). The CTAB neutralized the negative charges on the surface of the flocs of the anaerobic digestates, reduced the electrostatic repulsion between the flocs, and promoted coagulation-flocculation. Moreover, the neutralization of the charges allowed the release of the extracellular polymeric EPS of the cells and fiber of the digestates and bound water molecules. The proteins and polysaccharides are components of the EPS that largely contributed to enhance the binding ability of water to the flocs of the digestate. Award et al. [143] divided the EPS into subgroups of main sources of carbon, nitrogen, and phosphorus. An example of EPS-phosphorus moieties would be nucleic acids [144]. After the release of the hydrated EPS from the flocs surface, the bound water that was coordinated with the EPS became free water [28]. On top of that, the CFA acted as a rigid lattice to reduce the filter cake compressibility and to provide drainage pathways for water to exit the filter cake [28].

The dewatering of the anaerobic digestate is required to reduce the cost of storage, transportation, and land application [9,10]. Although the AD plants are located in strategic sites, usually with several farms around and with plenty of nearby areas to apply the anaerobic digestate to land, the transportation of this material to remote locations is still required once the nutrient quotas are reached in the fields around the plants [107,144]. The most common approach for the solid and the liquid fractions obtained after the separation is their use as fertilizer. There is particular concern about the emission from the resulting solid fraction [145,146,147,148,149,150] due to the higher area for gas exchange compared to the liquid fraction (Figure 8). While the gaseous emitting area of the slurry is the horizontal cross-section of the storage tank, the shape of the pile of dried organic manure was described by Dinuccio et al. [146,147] as a truncated cone heap with a greater area to volume ratio.

Regueiro et al. [82] concluded that the Al₂(SO₄)₃ was able to reduce the gaseous emissions (NH₃, CO₂, CH₄, and N₂O) even after the solid-liquid separation (i.e., dewatering of the solid fraction) of raw and co-digested pig slurries. The Al₂(SO₄)₃ increased the TS in the liquid fraction but had the opposite effect on the solid fraction obtained after the solid-liquid separation. Regueiro et al. [82] attributed this fact to the role of the sulfate [150], and explained that the increase of the TS in the liquid fraction via the formation of low molecular weight carbohydrates derived from the acid hydrolysis of cellulose or hemicellulose. The acid pH provided by the Al₂(SO₄)₃ enhanced the retention of nutrients in the liquid fraction, minimizing the gaseous emissions from the solid fraction. It should be noted that the nutrients are more stable in the liquid fraction than in the solid fraction, due to the high moisture content (i.e., lower concentration) and lower surface area. Kavanagh et al. [132] explained that a slurry with 4% TS is more stable than the undiluted slurry with 7% TS. Thereby, the lower amount of TS in the solid fraction, the lesser the gaseous emissions. Regueiro et al. [82] found that the gaseous emissions were also minimized in the liquid fraction, due to its lower pH, compared to the liquid fraction obtained from the solid-liquid separation without acidification. It should be noted that the content of nitrogen, expressed in grams of nitrogen per gram of TS, decreased in the solid and the liquid fractions due to the increase of the TS, which resulted from the addition of Al₂(SO₄)₃ [82].

3.4. Commercial Processes for Manufacturing of a Granular Fertilizer based on Blends of Ash and Organic Manures

The features of the process of Limoli et al. [74] employing CaO and H₂SO₄ and the process of Zheng et al. [28] using CFA and CTAB were compared against processes implementing the granulation, as this step could improve the aesthetic properties of the blended fertilizer of WA and anaerobic digestate to be retailed (

Table 4. Identification of the synergies and possibilities in the process involving the mixture of anaerobic digestate or sewage sludge with WA or alike materials.

Process	Raw Materials	Blending Ratio	Advantages	Disadvantages
RecoPhos [153].	Sewage sludge and H3PO4.	Incineration to produce SS ash. Reaction of the SS ash with the H3PO4 using phosphorus molar ratio 1:12 of sewage sludge ash: H3PO4	Marketable granular product with similar properties to the triple superphosphate	The NH3 volatilized during the drying is not recovered. Emission of nitrogen oxides during incineration.
ADFerTech [154].	Anaerobic digestate, dolomite (CaMg(CO3)2), organic binders and coating.	Dolomite was added to the liquid fraction (>91% moisture) in a dose ranging from 10 to 200 g/L.	Improve the aesthetic properties of the liquid fraction of anaerobic digestate. Decrease the cost of transportation and storage.	Additives of the liquid fraction of the anaerobic digestate are suitable for land application.
Limoli et al. [74].	Anaerobic digestate, CaO, and H2SO4.	>95% moisture of anaerobic digestate. Dose of CaO to operate the stripping at pH 10.	Recovery of the NH3 volatilized. Enable the self-hardening and granulation of the NH3-depleted organic amendment.	Low fluency to be employed in a traditional stripping column (i.e., packed tower). High COD content of the filtrate.
Zhengh et al. [28].	Anaerobic digestate, CFA, and CTAB.	>93% moisture of anaerobic digestate Mass of CTAB up to half of the TS of the digestate Mass of CFA up to the TS of the digestate	Reduce the energy consumption of the filtration of anaerobic digestate. Possible to enhance the mechanical separation with adsorption.	Presence of CTAB and heavy metals of the CFA in the filtrate. High COD content in the filtrate.
Pesonen et al. [29]	Sewage sludge, wood-peat ash, and Ca(OH)2.	Up to 40% of SS (45% moisture) and up to 30% Ca(OH)2. The dose of ash can go up to 100%.	No need to include Ca(OH)2 to have high compressive strength. Low presence of heavy metals	Dewatering and sanitation The NH3 released is not captured
Jewiarz et al. [155].	Anaerobic digestate and woody biomass.	18–20% moisture of the anaerobic digestate. Up to 75% WA.	Save in energy for drying the anaerobic digestate by thermal drying in fueling the drum drier with the biomass to produce the ash.	The NH3 released is not captured. Not possible to include the biofertilizer together with the ash due to the high pH.

). There is a lack of holistic approach in the processes described below as none of them entirely complies with the following objectives:

• Use of the most cost-efficient way of achieving solid-liquid separation.
• Find the optimum carbon and nutrient profile of the main stream coming out of the process, intended to be used as an organic amendment.
• Self-hardening to provide this material with the best mechanical properties before and after the granulation.
• Minimize any waste streams with valuable nutrients or other pollutants that need to be removed before disposal.

The RecoPhos P 38 fertilizer is produced from SS following the same manufacturing procedure of the triple superphosphate (Ca(H₂PO₄)₂) from the phosphate rock [153]. This process aligns with the management strategy proposed by Chojnacka et al. [156] for organic wastes. Firstly, the valorization is carried out via complete dehydration and subsequent incineration to reduce the organic pollutants and to obtain ash with similar characteristics to the phosphate ores. Secondly, the solubility of the phosphorus contained in the ash is enhanced via reaction with H₃PO₄ (Equation (22)), prior to the preparation of the granular fertilizer. This technology could be improved since carbon and nitrogen are lost during the fluidized bed incineration. Furthermore, the isolation of the phosphorus contained in the ash only can be done via solubilization with phosphoric acid and subsequent drying. The RecoPhos might not be an energy-intensive process if, for instance, the drying of the SS prior to the incineration is done with the heat released in the combustion.

$${\mathrm{Ca}}_4{\mathrm{Mg}}_5{\left({\mathrm{PO}}_4\right)}_6{}_{\left(\mathrm{s}\right)}+12{\mathrm{H}}_3{\mathrm{PO}}_4{}_{\left(\mathrm{ac}\right)}\to 4\mathrm{Ca}{\left({\mathrm{H}}_2{\mathrm{PO}}_4\right)}_2{}_{\left(\mathrm{s}\right)}+5\mathrm{Mg}{\left({\mathrm{H}}_2{\mathrm{PO}}_4\right)}_2{}_{\left(\mathrm{s}\right)}$$

(22)

The ADFerTech process of Fivelman [154] is an energy-intensive process because, in addition to the preliminary solid-liquid separation, the subsequent processing of the liquid fraction involves drying (a) after the adsorption to enable the self-hardening and (b) after the agglomeration to have a 3% moisture content in the final granules. In this way, Fivelman [154] employed the adsorption technology to enhance the solid-liquid separation of the liquid fraction of the digestate. The granular fertilizer was produced with the TS remaining in the liquid fraction after adding organic polymer binders (e.g., carboxymethyl cellulose) to promote the aggregation of the nutrient-laden particles of the adsorbent. Fivelman [154] claimed that the amount of nutrients that remain in the liquid stream after the adsorption is low enough to be discharged directly to the local water bodies without further treatment. The main powder adsorbent that they used was dolomite in doses ranging from 10 to 200 g/L (mixed for 5 min at 20 °C), which was able to retain around 250 mg nitrogen and 300 mg phosphorus per gram of dolomite.

Steenari and Lindqvist [157] explained that the self-hardening or solidification of a gram of WA, when adding between 0.3 and 0.5 g of water, depends on a number of ongoing reactions which can last from days to months. It should be noted the importance of the (moisture) adsorption capacity of the WA to promote self-hardening [158]. The hydration of CaO to produce Ca(OH)₂ is faster than the subsequent formation of CaCO₃ due to the mass transfer resistance, limiting the diffusion of CO₂ from the atmosphere. However, the CaCO₃ is more stable due to its lower solubility and alkalinity, providing the ash with better properties for cementation and for being used as a long-lasting liming agent. Pellets predominantly made with organic matter such as cattle manure rely less on these reactions to obtain appropriate durability (i.e., percentage of the mass of the pellet that remains after tumbling [159]). Due to the fact that organic matter can retain more water [157], Zafari et al. [159] obtained the highest durability (~97%) in the cattle manure pellets when these were prepared with the open-end die method at 50% (w/w) moisture content, 40 °C, and 6 MPa. According to Alemi et al. [160], both the density and porosity increased when the pellets of cattle manure and urea were prepared with higher moisture content (from 11 to 24% w/w) and a higher compressive force (from 2000 to 5000 N). Even after the preparation, Alemi et al. [160] reported an increase of up to 8% (w/w) in the moisture content due to the absorption of the water in the surrounding environment with a relative humidity of up to 80%.

Pesonen et al. [29] studied the production of fertilizer based on biomass ash which was obtained from the combustion of a mixture of 65% wood and 35% peat. As a nitrogen source, they used SS, which had been previously sanitized and dewatered (until a 45% moisture) in a resource-intensive process. Pesonen et al. [29] included Ca(OH)₂ in the fertilizer as well, with the main purpose of promoting self-hardening and increasing the compressible load that the granules can hold. Nevertheless, this material did not increase the compressible strength to prevent the deformation of the granules. They carried out the granulation in a rotary drum, after which the granules were left in a fume hood for 28 days at 21 °C. This allowed to reach 3% of moisture due to the loss of free water, since the compounds responsible for the self-hardening require higher temperatures to decompose. Pesonen et al. [29] concluded that the solidification was not enhanced because the main reason for this phenomenon was the reaction of CaO with the water supplying Ca(OH)_2, and, to a lesser extent, the subsequent formation of CaCO₃ due to the absorption of the CO₂ in the atmosphere, which is a much slower reaction.

According to Steenari and Lindqvist [157], ashes with a content of combustible matter greater than 10% are not suitable for self-hardening, which could explain why the organic waste was not a good binder for the curing process and the solidification of the ash was hindered in the study of Pesonen et al. [29]. The acidification could be useful to promote dehydration [74] and mechanical properties (e.g., durability and compressive strength) of the solid obtained after the self-hardening. Based on the study of Steenari and Lindqvist [157], from the point of view of the self-hardening of the blend, a greater amount of ash would be desirable since this would minimize the organic binder. This agrees with the results of Rao et al. [161], who investigated the preparation of organo-mineral fertilizers with the compost of pig waste solids (20% w/w), PL (26% w/w), spent mushroom compost (26% w/w), cocoa husks (18% w/w), and moistened shredded paper (10% w/w). The additives that they tested were feather meal (≤29% w/w), dried blood (≤29% w/w), (NH₄)₂SO₄ (≤27% w/w), nitro-chalk or 5Ca(NO₃)₂·NH₄NO₃·10H₂O (≤27% w/w), phosphate rock (≤15% w/w), Ca(H₂PO₄)₂ (≤21% w/w), and K₂SO₄ (≤21% w/w). Only when the share of compost in the mixture was greater than 50% (w/w), pelletization was not possible. On the other hand, Pampuro et al. [152] reported that none of the woody bulking agents they tested (woody biochar and wood chips) improved the compression resistance of the pellets made with composted pig solid fraction. However, they found that the pellets made with woody biochar and composted pig waste had higher durability than those made only with composted pig waste. According to Pampuro et al. [152], the greater durability of the woody biochar amended pellets was due to the low particle size of this material (<0.5 mm). The greater surface area of smaller particle promote sorption processes during the compaction for densification. Pampuro et al. [152] concluded that smaller particles enable the manufacturing of pellets of higher quality. Based on the results of Mudryk et al. [162], Jewiarz et al. [155] proposed the entire process for the production of this granular fertilizer. Although initial dewatering of the digestate was required, the greatest reduction of the moisture content, from 70% to 25%, was achieved in a direct contact drum dryer which employed the flue gases produced in the combustion of the biomass as a drying agent [155]. The process took advantage of the ash produced in the incineration but also it could employ the drying step to sanitize the digestate and to produce a pathogen-free material (i.e., biological stabilization). If the digestate contains a lot of fibrous material, the fragmentation is performed with a hammer mill after the drying. The process could be improved by blending the digestate with the biomass ash before the drying of the digestate. In this way, the absorption of the CO₂ and other components of the flue gases in contact with the liquid blend with high pH could be possible, given the high alkalinity of the wood ashes and that this material represented up to 75% of the final product. According to the description of the process provided by Jewiarz et al. [155], the blend should solidify for 3 h before the granulation. Steenari & Lindqvist [157] explained that curing the WA is necessary because the reactive oxides and soluble salts might negatively affect plants (e.g., pH shock and burning tissues) if the untreated WA is applied directly to land. Additionally, Jewiarz et al. [155] suggested inoculating the fungi Trichoderm to enhance nitrogen assimilation by the plants, although this biofertilizer cannot be in the same blend containing the ash because it would not survive at a high pH.

3.5. Inoculation of Biofertilizers in the Blended Fertilizer Prepared with Anaerobic Digestate and WA

After the removal of the pathogens by processing the anaerobic digestate with WA [163], it is possible to inoculate microbial species [164], which enhances the fertilizing effect of the blended fertilizer [155]. These microbes regarded as biofertilizers are divided into four categories [165]. The following are some examples of microbial species that can improve the fertilization in the rhizosphere:

• Vascular Arbuscular Mycorrhiza fungus:

  ○

  Glomus mosseae [165,166].

  ○

  Glomus intraradices [165].

  ○

  Glomus fasciculatum [166].

• N-fixer bacterias can be symbiotic and non-symbiotic:

  ○

  Azotobacter chroococcum [165,167].

  ○

  Azospirillum lipoferum [167].

  ○

  Azospirillum brasilense [166,167].

• Phosphate-solubilizing bacteria:

  ○

  Bacillus megatherium [165,167].

• Potassium-solubilizing bacteria:

  ○

  Bacillus mucilaginous [165].

Mahfouz & Shamf-Eldin [167] concluded that the growth of the plant Fennel (Foeniculum vulgare Mill.) was greater when using half the recommended dose of the NPK mineral fertilizer (i.e., synthetic material containing nitrogen, phosphorus, and potassium) and a mixture of Azotobacter chroococcum, Azospirillum lipoferum, and Bacillus megatherium, compared to the full NPK dose. The total carbohydrates in the dry plant material were impacted by the use of the mixture of biofertilizers. Similarly, the biofertilizers also increase the yield of essential oil in the plant [167]. According to Brar et al. [168], the best physical properties of the soil (e.g., water infiltration) and highest crop yield in maize-wheat rotation were obtained using a balanced application of mineral fertilizers and organic farmyard manure [168]. In addition to greater crop growth, better fertilization is also associated with less greenhouse gas emissions since the nutrients are employed more efficiently once the organic material is applied to the soil [169].

4. Conclusions

There are many possibilities for employing the WA as an anaerobic digestate additive. The treatments described in this manuscript ranged from enhancing anaerobic fermentation in order to reach the correct levels of stability and maturity to improving the properties of organic amendment as a controlled-release fertilizer, with additional processing steps after the biodigester. It remains to completely link the WA dose employed for the AD (up to 1 g TS ash/g TS digestate) with the purpose of the subsequent processing stage. Considering all the possible synergies that could surge from treating the anaerobic digestate with the WA, an approximate dose of 5 g TS WA/g TS anaerobic digestate is proposed for future experiments on the manufacturing of a granular fertilizer with the best mechanical properties (e.g., compressibility strength, durability, etc.).