Anaerobic digestion of bio-waste: A mini-review focusing on territorial and environmental aspects

Abstract

Scientific and industrial experiences, together with economical and policies changes of last 30 years, bring anaerobic digestion among the most environmental friendly and economically advantageous technologies for organic waste treatment and management in Europe. In this short review, the role of anaerobic digestion of organic wastes is discussed, considering the opportunity of a territorial friendly approach, without barriers, where different organic wastes are co-treated. This objective can be achieved through two proposed strategies: one is the anaerobic digestion applied as a service for the agricultural and farming sector; the other as a service for citizen (biowaste, diapers and wastewater treatment integration). The union of these two strategies is an environmental- and territorial-friendly process that aims to produce renewable energy and fertiliser material, with a low greenhouse gas emission and nutrients recovery. The advantage of forthcoming application of anaerobic digestion of organic wastes, even for added value bioproducts production and new energy carriers, are finally discussed. Among several advantages of anaerobic digestion, the role of the environmental controller was evaluated, considering the ability of minimising the impacts exploiting the biochemical equilibrium and sensitivity as a quality assurance for digestate.

Keywords

Review anaerobic digestion biowaste integrated approach energy recovery biomethane biohydrogen nutrient recovery

Introduction

The anaerobic digestion (AD) of solid biowaste has had a strong scientific background since early 1980s, in fact first articles in the literature dealing with AD of solid biowaste were published by a lot of authors, such as Cooney and Wise (1975), Diaz and Trezek (1977), Stenstrom et al. (1982), Fannin et al. (1984), Ahring and Westermann (1985a,b), Shmidell et al. (1986), Traverso andCecchi (1988), Mata Alvarez et al. (1990) and Cecchi et al. (1986, 1988a,b, 1989, 1990a,b,c,d). The first International Symposium on Anaerobic Digestion of Solid Wastes (ISAD-SW), held in Venice in 1992 (Cecchi et al., 1993), was the first successful attempt of AD specialists aggregation (more than 40 countries represented and more than 200 delegates) and an important discussion opportunity about solid waste treatment. During these years, since the mentioned Symposium, there was an increasing concern about municipal solid waste (MSW) disposal, which was changing its waste-to-resource status, approaching the new concept of separate waste collection. The conference output highlighted the importance of using a source-sorted organic fraction of MSW (SS-OFMSW) or separately collected-organic fraction of MSW (SC-OFMSW), which improves the ADs yields and removal efficiencies (Battistoni et al., 1993; Mata-Alvarez et al., 1993; Owens and Chynoweth, 1993; Wellinger et al., 1993), the opportunity of composting the AD dewatered effluent for a good quality soil amendment production (Engeli et al., 1993; Vallini et al., 1993; Vermeulen et al., 1993) and the biological treatability of AD liquid effluent inside a waste water treatment plant (WWTP). This was the starting point of the concept that composting and AD are not competing technologies but synergic ones, and WWTPs should be part of a territorial approach for reclaiming material, energy and nutrients, mainly phosphorous (Battistoni et al., 1997, 1998a, 2000, 2001, 2005). During the second ISAD-SW, held in Barcelona in 1999, other important aspects of the AD process were deeply discussed, such as the positive energy balance of AD solid waste treatment (Edelmann et al., 2000), the reduction of fossil fuels utilisation, the reduction of CO₂ emissions (Baldasano and Soriano, 2000) and the degradation of organic micropollutants and organochloride compounds. Global climate change and life cycle assessment (LCA) started to be the focus of new research issues, together with the fate of micropollutants, inorganic and organic ones. The organising scientific committee summarised these outputs in a position article, together with a worldwide overview of AD (Verstraete et al, 2000) emphasising that there must be an improved communication between various waste management sectors and compost users, in order to guarantee a future of organic recycling. Hence, proper technology and land planning can upgrade the end product of digestion as a form of sequestered carbon: lower carbon footprint. The ISAD-SW conference took place again in 2002 in Munich and in 2005 in Copenhagen. In this last meeting, the AD of solid waste and energy crop (SW&EC) fusion was proposed and discussed during the International Water Association – Anaerobic Digestion (IWA-AD) group meeting, addressing a strong increase of co-digestion of various substrates, that allow the balance nutrient requirement, together with the request of a high-quality end product. The concept of a territorial approach for the AD process was born, which means the sharing of services in a wide area where there are rural and urban settlement. ISAD-SW&EC was successful in its two editions in 2008 and 2011, held in Hammamet and Vienna, respectively, where the widespread use of AD technology in the agricultural sector was confirmed, pointing out the new issues related to the AD effluent final use (Nitrate Directive and quality control procedure) and nutrient removal/recovery (reclamation) technologies.

Today, the importance of the AD process is irrefutable. In fact, it counts over 13,800 biogas plants in Europe (in 2012) and more than 7400 MW_el of installed capacity (European Biogas Association, Biogas report 2012) providing the following advantages: AD is suitable for stabilising various organic substrates; a number of commercial processes are available (mesophilic/thermophilic, dry/wet, continuous stirred tank reactor (CSTR)/plug flow (PF), etc.); renewable energy can, in part, replace fossil fuels (i.e. biomethane); in AD effluent nutrients can be recovered (especially phosphorus); digestate is suitable as fertiliser/amendment.

About this last concept, there are several advantages of using digestate as amendment even from co-digestion of activated sludge and food waste, such as the nutrient and organic carbon content, the low pathogens content (after a short post-composting treatment or using thermophilic temperature), and the biological stability (low respiration activity).

In Europe, thanks to a strong energetic policy support, Germany is the first country to produce 5067 kilo tonne of petroleum equivalent (ktpe) of biogas; this is done mainly by decentralised agricultural plants, municipal waste methanisation plants and centralised co-digestion plants. The United Kingdom and Italy, with 1764 and 1095 ktpe of biogas produced in 2012, are ranked second and third, respectively, but they still showed a higher percentage of biogas produced by landfill, mainly in the UK. Compared with the UK, the Italian agricultural sector has changed during last 15 years thanks to national incentives on renewable energy production, and as a result of the European Nitrate Directive (91/676/EEC), increased the number of decentralised agricultural plant, municipal waste methanisation plant and centralised co-digestion plant. Probably Italy, and the Veneto Region in particular, was the first European country where full-scale biowaste treatment plants were developed and the concept of co-digestion of sludge and biowaste implemented (Bolzonella et al., 2006; Cecchi et al., 1994; Pavan et al., 2000). Spain then incorporated the concept, and outranked Italy in the application thanks to European structural funds (Mata-Alvarez et al., 2000).

However, the overall vision of the AD process implementation still has obstacles that should be overcome. In particular, the opportunity of a territorial approach, without barriers, where food wastes, the organic fraction of MSW, agricultural residues, waste from food processing plants and livestock effluents, and other organic waste, such as diapers, are co-treated, is not completely considered. That is, all the advantages linked with the application of the AD as an environmental and territorial friendly process, aimed to produce renewable energy and fertiliser material, with a low greenhouse gas (GHG) emission and nutrients recovery, are still to be fully exploited.

This gap is mainly owing to controversial interpretation of legislation, both at national and European levels, probably caused by an exceeding of precautionary principle application (for example the debate on organics and inorganics micropollutants limits). In this context, the cultural limit, that is an insufficient knowledge of the AD process, probably could play a role in the final decision. In fact, the AD process could fail by inhibitory and toxic episodes owing to its great sensitivity to these toxic compounds, especially at a thermophilic working temperature (Ahring and Westermann, 1985b; Chen et al., 2008). It seems possible to speculate (the literature is not so exhaustive) that, if the process is stable and efficient, land utilisation of digestates would become feasible. That is, the AD process becomes controller of the environmental impact: If some substrates are toxic for the process, it fails, and the effluent could not be used as a fertiliser. This controller activity could not be carried out in the majority of the composting systems, because the reactors, where the process takes place, are completely different. Composting takes place in piles with a low mixing degree, causing no homogeneous zones inside. These piles are improperly called ‘reactors’, and in fact usually a proper process control is very difficult (i.e. the loss of water during the oxidation process often causes a material dehydration (total solid contents over 30–40 gTS kg⁻¹) with a consequent possible shutdown of biological activity). An attempt of overpassing this problem is the adoption of a long retention time (more than 2 months) to obtain a more or less homogeneous stabilised material, and using closed and more mixed reactors. Different is the AD process, which takes place in a completely mixed reactor that offers guaranties about homogenisation, even of toxic compound concentration inside the reactor. Hence different legislation has to be used for the final disposal of compost and digestate; in one process (composting) the control of the toxic compound level is obvious and necessary, while in the other (AD) is part of the process itself. Although these aspects, aerobic composting of digestates is required as a polishing/quality upgrading before land application.

Previous reviews on AD are mainly related to specific technical aspects (Angelidaki et al., 2003; Cecchi et al., 1988b; Mata-Alvarez et al., 2011). In this short review/position article, the objective is to give an applicative approach of two strategies, based on results obtained by several studies and pilot/full-scale applications. In agreement with these assumptions/assertions, in this article two approaches were discussed aiming to evaluate the territorial application of the anaerobic co-digestion process: One approach is addressed to the primary production sector and the other approach to that of urban services focusing, in this case, to the technological issues that are the base of a proper process management and success. The concept of the AD process will be developed as the ‘environmental controller’ emphasising the limits of literature and reporting our laboratory and in-the-field experiences. Finally, the paths toward which the AD process will move in the near future will be briefly indicated.

Two strategies to reach the environmental territorial sustainability

Anaerobic co-digestion of agricultural residues, energy crops, food industry residues and livestock effluent is an attractive and widespread technology and is the most important example among the two proposed territorial approaches. Looking at the exponential diffusion of the AD process at agricultural level, the AD technology should be considered a crucial integration of different actors’ needs, for example solving the necessity of stabilising organic matter produced from farming activities and to share land for fertiliser spreading achieving nitrate directive limits. The centralised form of AD, designed as a consortium of different users, can be an instrument for a territorial strategy. The second strategy regards the AD of biowaste and sewage sludge as a result of waste and water treatment cycle integration. In that way the wastewater treatment plant becomes a territorial service for citizens, providing a new concept of treatment plant.

These two strategies can even be linked together in an overall view of a sustainable and environmental friendly approach and a service for society in terms of energy and material recovery.

The AD as a service for the agricultural and farming sector

Waste-to-bioenergy and waste-to-resource challenges were the driving force of biogas plant diffusion in Europe, attracting lots of interests and involving mainly the agricultural and farming sectors. In 1985, Danish Government developed and implemented a demonstration programme to show the potential of large-scale manure-based biogas plants. As reported by some authors (Angelidaki and Ellegaard, 2003; Raven and Gregersen, 2007), Denmark is known for its centralised biogas plant concept, where a community of farmers cooperate in an organisation to supply and digest the manure in a centrally located biogas plant. Advantages of this approach were evident in that fact that it produced renewable energy, enabling the recycling of organic waste and reducing chemical fertiliser use, with a consequent reduction of the GHG emission.

If the common substrates are livestock effluents, such as pig or cattle slurry and manure, that are available all year, then a co-digestion approach is usually carried out using maize silage or similar cultivation. This is the typical rural approach, which could allow a food competition of crops harvesting for energy production purposes (Poggi-Varaldo et al., 2014), as happened in South America with bioethanol production from crops (the so called ‘tortilla war’). Furthermore, especially in Mediterranean areas, there are lots of available biomasses and food-processing industry wastes suitable for bioenergy production through AD. The seasonal availability of these biomasses, which can cover the whole year, assures a continuous energy production, and opens the concept of a territorial service of AD. In fact, the food industry processes raw material from agriculture almost all the year, for example tomatoes (August to October) to produce tomatoes sauces, or fruits (August to December) to produce juices, giving back the organic residuals to the biogas plant.

The feasibility of a centrally located AD facility, in the Mediterranean region, was successfully verified by Fountoulakis et al. (2008), co-treating slaughterhouse, olive mill and winery waste, which resulted in an increase of methane production at a thermophilic temperature ranging from 15% to 35% and observing the synergistic effect of mixing different substrates. Moreover the Mediterranean regions substrates availability was evaluated by Petruccioli and Santori (2013); these substrates and many others available can be co-treated in a centralised plant. Ward et al. (2008) reviewed the ways of AD optimisation of agricultural resources; among all, co-digestion studies have recognised ways of improving biogas yield and reducing hydraulic retention time (HRT). Mata-Alvarez et al. (2011) reviewed the co-digestion of solid waste, and observed that it is important to choose the best blend ratios in order to favour positive interactions (positive synergisms and nutrient and moisture balance), to avoid inhibition (ammonia, lipid degradation products) and to optimise methane production. In that way AD, and especially co-digestion, became a territorial service for treatment of various substrates, for energy recovery and for the production of soil amendment for agriculture, upgradable in a consortium feature.

The AD as a citizen and territorial service

The integration of the AD of biowaste and wastewater treatment, that is the co-digestion of sewage sludge and biowaste inside a wastewater treatment plant, is the way proposed by Cecchi et al. (1994) and implemented at full scale in 1999 as a service for the Treviso City (North Italy) (Pavan et al., 2000). In that approach, the biological nutrient removal (BNR) process efficiency was improved by adding the rapidly biodegradable matter (rbCOD) coming from biowaste fermentation (this makes for easier biological removal of nitrogen and phosphorus), and exploited the electric energy from biogas together with the phosphorous recovery from digestate by a crystallisation process (Battistoni et al., 1997, 1998a, 2000, 2001, 2005). Some considerations can be drawn on the results coming from the Treviso implementation of integrated cycles: The biowaste fermentation was carried out in the range 1–6 days and a liquid phase rich in rbCOD (up to nearly 50 g l⁻¹) was produced and added in the BNR plants inlet promoting biological nitrogen and phosphorous removal (yields up to 18.4 gCOD PE⁻¹ d⁻¹). The nutrients removal efficiency was similar to those of acetic acid but it is a negative-cost substrate since it was obtained from organic wastes. The authors (Battistoni et al., 1998b; Bolzonella et al., 2005b; Cecchi et al., 1999; Pavan et al., 2000) evaluated the global effect in a full-scale wastewater treatment plant with 20 t d⁻¹ of biowaste by anaerobic co-digestion of sewage sludge and OFMSW. This allows a significant increase in biogas production to be obtained, even if a higher oxygen demand and a larger sludge production was observed in the activated sludge process (5–10%).

On the other hand, digestate was usually sent to a composting plant to produce a high quality soil amendment. With this approach, biowaste management became a resource, minimising the size of the composting plant and allowing a better waste separation. The case study of Treviso City (about 100,000 inhabitants) can be extended to a larger area, i.e. the province of Treviso.

Treviso province (about 1 million inhabitants) produces more than 100,000 t y⁻¹ of biowaste (60%) and green waste (40%): considering the availability of one composting plant treating about 35,000 t y⁻¹, and one AD plant treating 3000 t y⁻¹ (of Treviso city), about 64,000 t y⁻¹ of biowaste + green waste have to be treated in plants located outside the province. This means high transport costs, especially in those cases where plants are located outside of the region.

Considering the situation of wastewater treatments, Treviso province has 90 WWTPs, 15 of those with more than 10,000 person equivalent (PE) of capacity and with an annual sludge production of 30,000 t y⁻¹ (20% of total solids content), with a provision of 45,000 t y⁻¹ when the sewage network is fully implemented.

Assuming the available volume of five existing AD plants to be fully exploited (total volume of about 12,000 m³ located inside wastewater treatment plants), the whole province could use AD for organic waste and sludge treatment (organic loading applied of about 4 kgTVS m⁻³ d⁻¹), with a final composting of digestate and sludge, working in a synergic way to produce renewable energy, avoiding cost of disposal and transport and lowering CO₂ emission. The electric energy and heat-recovery prediction is illustrated in Figure 1, supposing one million inhabitants and adopting an integrated approach.

The advantage of AD exploiting as a territorial and friendly approach is noticeable, in fact it is possible to recover up to 110 MWh d⁻¹ of electric energy and 390 t d⁻¹ of high quality compost, produced by the post aeration treatment of digestate and green waste.

Feedstock quality and availability: A facilities problem

From the process point of view almost all is known, studied and verified. In fact, different technologies were developed, such as: wet/dry and semi-dry digestion; continuous and batch reactors; single-phase and multi-phased; mesophilic and thermophilic working temperature.

When implementing the AD in a biowaste treatment process, most of the problems are practical issues (plant problems); hence simple, reliable and permanent solutions must be adopted in order to preserve the biological process and make management easier.

The importance of adopting an efficient separate collection system to obtain high-quality organic waste is as fundamental as for the valuable matter recovery (Hartmann et al., 2004). Door-to-door collection systems give the best quality characteristics in terms of inert material content and meets the quality requested for the AD process. Obviously, the organic waste obtained by mechanical selection must be forbidden. In fact, it can cause serious problems to plant facilities and can negatively affect the quality of the end product and, moreover, usually requires high-energy consumption for a proper selection (Cavinato et al., 2013). Nevertheless, a mild mechanical separation of the OFMSW is suggested, even if the collection is a door-to-door system with a high quality of biodegradable matter. Another approach could be the under-sink disposer, which assures the disposal of selected organic material (Battistoni et al., 2007; Bolzonella et al., 2003). Several pre-treatment technologies are available, but three are the technologies widely used to mechanically sort OFMSW prior to AD/co-digestion treatments: wet pulper, extrusion press and wet sorting system. All these technologies have the same objective of improving biogas conversion through the size reduction, which allows a better micro-organism contact, and inert material removal that can cause, in the long-term, the facilities consumption and/or accumulation inside the reactor. The functioning principles are reported here, putting together similar technologies.

Extrusion press: The waste is pushed inside the extrusion chamber under high pressure. In this condition the organic fraction is in part liquefied, passing through the extruder holes and the dry rejected fraction is discharged. Other pressure-based technologies are the use of a hammer mill, coupled with a fixed screen or screw press.

Hydropulper: After a first shredding step, biowaste is suspended in process water and disrupted for 0.5–1 h. After removal of the light fraction from the top and the heavy contaminated fraction, such as glass, plastics and stones from the conical bottom of the hydropulper, the biowaste suspension is pumped into the anaerobic digester.

Wet-refine system (Treviso): After a first soft shredding step, the biowaste is sent to a mixer/separator where the dry matter content is lowered to 7%–8% using sludge coming from a WWTP, and the floating (upper part, light material such as plastic) and inert (bottom part, heavy material as shell and bone) materials are withdrawn and finally disposed.

These technologies are aimed to produce a material with a low inert contaminant content, avoiding damage to piping and pumping systems (saving maintenance costs), inert material accumulation inside the reactor (causing a reduction of working volume) and consequently a high organic material suitable for biogas conversion.

In Giuliano et al. (2011), three biowaste treatment plants in Europe were analysed in terms of biowaste pre-treatment efficiency before the AD process. Mass balance, chemical physical parameters, waste classification and particle size distribution analysis were carried out on inlet substrates, rejected material and sorted waste. Among the results obtained, the particle size distribution after sorting steps and fed to the digester, gives interesting output. Fractions sizes were divided in three main categories: Coarse (more than 1 mm), middle (from 0.25 to 1 mm) and fine (less than 0.25 mm) fractions. Observing Figure 2, it is possible to recognise the effects of pre-treatment systems in terms of size reduction: coarse and middle fractions increase moving from wet pulper to wet selection technology, suggesting a preservation of the material proprieties of wet selection, due to a lower size reduction.

This was confirmed by a fine fractions trend, which has an opposite behaviour, showing the higher fine production associated to wet pulper, which was surely the more disrupting technique adopted. Hence, the wet pulper option lead to an output stream, which was richer in fine, at least 25% more, than other technologies; but the fine fraction is usually rich in inert, which are difficult to separate from the waste pulp obtained. Fine fractions, with a high inert concentration, were one of the causes of management problems in full-scale applications, owing to pipes clogging, digester volume reduction, pump abrasion, etc. Even if a higher organic material fragmentation probably lead to higher kinetics of biological conversion in digesters, owing to the higher surface/volume ratio of substrate this advantage could be not enough to balance the amount of other negative effects coming from the heavy presence of inert fine fractions. It seems to be much more profitable to not reduce too much the organics in size before digestion, demanding the degradation to the biological step of AD.

AD as an environmental controller

To make a territorial friendly approach possible, first of all the European and Members States legislation must be reconsidered. In fact, the Waste Framework Directive (2008/98/EC) identifies bio-waste as:

… biodegradable garden and park waste, food and kitchen waste from households, restaurants, caterers and retail premises, and comparable waste from food processing plants.

This definition does not include other organic materials, reducing the possibility of exploiting all the advantage of an integrated and territorial approach. Moreover, Article 22 of Waste Directive says that:

…the Commission shall carry out an assessment on the management of bio-waste that shall examine the opportunity of setting minimum requirements for bio-waste management and quality criteria for compost and digestate from bio-waste, in order to guarantee a high level of protection for human health and the environment.

Indicating the necessity of pointing out quality criteria, the legislation wrongly considers compost and digestate as the same thing. In fact, as reported in the premise, the AD process takes place in a CSTR, giving more guaranties and effectiveness from several points of view if compared with a composting reactor. However, composting can surely be part of a reclamation general strategy when post-composting of digestate mixed with bulking agent is used for quality soil amendment production (Di Stefano et al., 2008; Vallini et al., 1993).

Moreover, the Member States policy differs a lot considering each local situation. In fact, only some states allow the co-treatment of biowaste and agricultural residues with the use of digestate as fertiliser, or allow the direct grid injection of biomethane.

The huge potentiality of AD must overcome these weaknesses and must include the scientific opinion for new legislation based on environmental sustainability.

The AD process is environmentally friendly because of its low carbon impact, positive energy balance and its intrinsic property of being adaptable to most of organic substrates. About this last option, AD could help legislation to define the limits of a quality soil amendment or fertiliser (Kupper et al., 2014). In fact, this biological treatment is sensitive to a toxic amount of heavy metal, organic micro-pollutant, etc., coming with the feeding, acting itself as a controller for the effluent quality. Speculating on this concept and considering the sensitivity of AD micro-organisms to some micro-contaminants, it could be assumed that if the process fails at a specific concentration, and this concentration is under the regulation limit, the biochemistry imbalance becomes the ‘warning bell’ of a low quality effluent. Even if the complex biochemical system of AD biology suggests that with time acclimation can occur, in the literature several research studies on AD inhibition are reported (ammonia, pH, volatile fatty acids (VFAs), salinity, etc.) as reviewed by Chen et al. (2008), but few data about heavy metals or the organic compounds toxic range are indicated, and often are really contrasting, based on specific case studies. The wide range of concentrations is mainly owing to different chemical–physical forms assumed by heavy metals (precipitated as sulphide, hydroxides or carbonates, adsorbed to solid fraction, biomass or inert fraction, or forming metal complex during AD), and the effective inhibition carried out by the soluble heavy metal form. Moreover, the solids content provides a protection from this inhibition, and for this reason it should be easier to compare data expressed as milligrams of metal per gram of solid or volatile solid rather than milligrams per litre as usually reported (Chen et al., 2008). In order to compare some toxic values reported in the literature, Table 1 indicates the limits of heavy metals at the end-of-waste (EoW) (JRC Scientific and policy reports, 2014) proposal, expressed in milligrams of metal per litre assuming the AD process with 25 and 35 gTS l⁻¹, and compared with toxicity limits (strong inhibition or 50% of biogas production reduction, Inhibition Concentration IC₅₀).

Table 1.

Comparison of end-of-waste heavy metal limits and Inhibition Concentration 50.

	EoW, 2014	EoW, 2014	EoW, 2014	Parkin and Owen, 1986	Lo et al., 2012	Fermoso et al., 2009	Altas, 2009	Lin and Shei, 2008	Yue et al., 2007
	mg kg⁻¹ d.m.	mg l⁻¹ @ 25 gTS l⁻¹	mg l⁻¹ @ 35 gTS l⁻¹	Strong inhibition	IC₅₀	IC₅₀	IC₅₀	IC₅₀	IC₅₀
Cu	200.0	5	7	0.5 (sol) 50–70				6.5	6.4
Zn	600.0	15	21	1.0 (sol)	0.482		7.5	4.5
Pb	120.0	3	4.2
Ni	50.0	1.25	1.5	30	7.239	118	35
Cr tot	100.0	2.5	3.5	Cr(VI) 3.0 (sol) 200–250 Cr(III) 2.0 (sol), 180–420	0.124		27	60	18
Cd	1.5	0.0375	0.0525				36		4.4

EoW: end-of-waste.

The values reported are in milligrams of metal per litre, so the TS content is not clear. But for some components, for example zinc, the toxicity values are below the limits. Comparison is difficult, but that could be an effective way to address the objective of EoW criteria on digestate (which is aimed to create specific criteria in order to specify when certain waste ceases to be waste and obtains a status of a product) adopting the biology of AD as a controller itself. Moreover, pathogen depletion is achievable, especially at a thermophilic working temperature.

In this context, it seems clear that effluent quality is a hot issue from an environmental impact point of view, and there is still an open discussion about the necessity of having a positive list of input material in order to consider digestate usable as a fertiliser or to set output limits. Recently the EoW criteria (JRC Scientific and policy reports, 2014) set some limits, both on input material (avoiding sewage sludge) and on digestate final quality, based on a European survey of about 25 AD samples of biowaste, manure + biowaste, manure + energy crops, suggesting concentration limits for heavy metals. Table 2 reports the limit values of the EoW proposal, Italian low fertilisers (D.lgs 75/2010) and on agricultural sludge disposal (D.lgs 99/1992), and compares them with experimental data considering the co-digestion of biowaste alone, biowaste and sludge, and winery waste mixed with sludge (Cavinato et al., 2014; Da Ros et al., 2014). It is possible to observe how the heavy metal content comply with all the regulation limits, even with sludge addiction. Values that are above the limit for copper and zinc are linked with the wine harvesting treatment process and sewage sludge, and can give some suggestion on the amount of winery waste treatable. It is evident that studies are required to determine mechanisms and rates of heavy metals release after applications of digestates and composts into soils.

Table 2.

Comparison of end-of-waste and Italian limit values with heavy metal content in co-digestion effluents.

	EoW 2014	Italian regulation on fertilisers	Italian regulation on use of sludge in agriculture	37 °C	55 °C	37 °C	55 °C	37 °C	55 °C
		D.lgs 75/2010	D.lgs 99/1992	Biowaste digestion	Biowaste digestion	Biowaste and WAS co-digestion	Biowaste and WAS co-digestion	Winery waste and WAS co-digestion	Winery waste and WAS co-digestion
Cu mg kg⁻¹ d.m.	200.0	230.0	1000.0	68.1	52.5	138.0	105.8	929.0	927.0
Zn mg kg⁻¹ d.m.	600.0	500.0	2500.0	155.0	129.0	452.0	352.0	1198.9	1120.7
Pb mg kg⁻¹ d.m.	120.0	140.0	750.0	17.3	7.8	0.2	0.1	114.6	99.4
Ni mg kg⁻¹ d.m.	50.0	100.0	300.0	42.1	27.0	17.4	23.5	25.8	24.2
Cr tot mg kg⁻¹ d.m.	100.0	0.5		85.9	51.5	34.8	29.4	48.4	42.7
Cd mg kg⁻¹ d.m.	1.5	1.5	20.0	0.2	0.3	0.1	0.1	1.6	1.4
Hg mg kg⁻¹ d.m.	1.0	1.5	10.0	0.2	0.1	0.1	0.1	0.4	0.3
As mg kg⁻¹ d.m.	10.0			0.3	0.2	0.2	0.1

EoW: end-of-waste; WAS: waste activated sludge.

The analysis on digestate suggested by EoW is aimed to develop a proper approach of digestate use considered among the objectives (enable disposal, reduce the dependence on land application, reduce the volume for lowering transport and disposal cost) to ensure more sustainable use of digestate products, to remove and recover substances, and produce a customised fertiliser increasing digestate value, creating new markets for digestate products.

In order to evaluate what is the best process or strategy that can be applied in a particular situation, LCA must be implemented. As mentioned by Poggi-Varaldo et al. (2014), processes and services must have the lowest impact on the environment as possible, and the LCA analyse is one of the most complete tools thanks to its holistic view and its systematic approach and standardisation.

Innovative prospective of biowaste treatment

The waste management is often focused on treatment in order to meet the environmental legislation, but there are many visions of sustainable use of biowaste included in the biorafinery concept (Fava et al., 2013; Poggi-Varaldo et al., 2014). There are research studies aimed to fully recover added value chemicals (such as bio-plastics, enzymes (Escamilla-Alvarado et al. 2013)) and energy (biohydrogen and biomethane, bioelectricity (Poggi-Varaldo et al. 2009; Villano et al. 2012). All these aspects are components of a biorefinery approach.

Biomethane

The advantage of biogas upgrading to methane (>90%) is the increased heating value and the consequent possibility of using it as automotive fuel or directly injected into the gas grid. There are several methods of biogas upgrading, such as physical absorption (pressurised water scrubbing, organic physical absorption), chemical absorption (ammine scrubbing) and pressure swing adsorption, membrane treatment (gas-permeation) (Andriani et al., 2014; Petersson and Wellinger, 2009). Application of membrane technology is not widespread, but seems to be the most adaptable to different plant configurations offering a single or multiple stage approach and a multiple compressor variation (Basu et al., 2009; Sholtz et al., 2013).

Biohydrogen and biohythane

An advanced way of exploiting AD is the two-phase approach aimed to produce hydrogen gas in the first-phase (dark fermentation) and biogas in the second-phase (methanisation). These two gasses can be used separately or mixed in order to obtain biohythane, a gas mixture composed by 10% H₂, 30% of CO₂ and 60% of CH₄, which enhances combustion (better thermal efficiency and power output compared with biogas combustion) and has reduced hydrocarbons emissions. The feasibility of biowaste treatment alone or co-digested with sludge has been studied (Cavinato et al., 2011a, 2011b, 2012; Chinellato et al., 2013; Giuliano et al., 2014), especially taking into account the sustainability of the process without external chemicals additions.

Bioplastics

VFAs are produced during the acidogenic fermentation of the AD metabolic pathway (Bolzonella et al., 2005a; Sans et al., 1995). Adopting specific operative conditions (for example low HRT, pH range control, micro-organism speciation), it is possible to produce VFA from a variety of organic wastes that can be used in several way, such as the biological production of biodegradable plastics (Lee et al., 2014; Valentino et al., 2014). For example polyhydroxyalkanoates (PHA) are biodegradable polymers that can be synthesised by micro-organisms. The PHA content in accumulating micro-organisms can be improved by optimising the operational conditions of the cultivation reactor by feeding a specific VFA or solving a critical factor for PHA accumulation (Mohan and Reddy, 2013). In that way it possible to achieve a PHA content of 40%–77% using different substrates, such as fermented food waste, fermented waste activated sludge and sugar cane molasses (Reddy and Mohan, 2012; Reis et al., 2011; Shen et al., 2014). Other authors have shown that it was possible to accumulate PHA in sequencing batch reactors (SBRs) (Valentino et al., 2013), and short-cut SBR-treating AD supernatants, simultaneously with nutrient removal (Fatone et al., 2014; Frison et al., 2014).

Conclusions

AD of bio-waste, as a territorial and environmental friendly process, can be achieved in the near future, by developing the following concepts.

- AD has to be considered as a territorial service both for agricultural and urban sectors: The centralised form of AD, designed as a consortium of different users, can be an instrument for a territorial strategy and, if located inside a wastewater treatment plant, can become a territorial service for citizens, providing a new concept of treatment plant.

- In order to preserve the AD biological process and make management easier, the pre-treatment of separate collected biowaste must adopt simple, reliable and permanent solutions. This has to be confirmed by system analysis (LCA and similar tools).

- AD has a low carbon impact, positive energy balance and, as a biological process, has an intrinsic property of being adaptable to most of organic substrates, obtaining in most of cases a material that responds to legislation limits of a quality soil amendment.

- The sensitivity of AD micro-organisms to a toxic amount of heavy metal, organic micro-pollutant, etc., coming with the feeding, could be a feature to control the effluent quality.

- Forthcoming AD technologies will allow added value chemicals (bioproducts such as bioplatics, enzymes and solvents) and energy (biohydrogen, biomethane, bioelectricity) to be recovered.

Footnotes

Declaration of conflicting interests

The authors declare that there is no conflict of interest.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

References

Ahring

Westermann

(1985a) Methanogenesis from acetate: Physiology of a thermophilic, acetate-utilizing methanogenic bacterium. FEMS Microbiology Letters 28(1): 15–19.

Ahring

Westermann

(1985b) Sensitivity of thermophilic methanogenic bacteria to heavy metals. Current Microbiology 12: 273–276.

Andriani

Wresta

Atmaja

. (2014) A review on optimization production and upgrading biogas through CO₂ removal using various techniques. Applied Biochemistry and Biotechnology 172: 1909–1928.

Angelidaki

Ellegaard

(2003) Codigestion of manure and organic wastes in centralized biogas plants, status and future trends. Applied Biochemestry Biotechnology 109: 95–105.

Angelidaki

Ellegaard

Ahring

(2003) Application of the anaerobic digestion process. Advanced Biochemical Engineering and Biotechnology 82: 1–33.

Altas

(2009) Inhibitory effect of heavy metals on methane-producing anaerobic granular sludge. Journal of Hazardous Materials 162: 1551–1556.

Baldasano

Soriano

(2000) Emission of greenhouse gases from anaerobic digestion processes: comparison with other municipal solid waste treatments. Water Science and Technology 41: 275–282.

Basu

Asim

Angels

. (2009) Membrane-based technologies forbiogas separations. Chemical Society Reviews 39: 750–768.

Battistoni

Boccadoro

Fatone

. (2005) Auto-nucleation and crystal growth of struvite in a demonstrative fluidized bed reactor (FBR). Environmental Technology 26: 975–982.

10.

Battistoni

De Angelis

Pavan

. (2001) Phosphorus removal from a real anaerobic supernatant by struvite crystallization. Water Research 35: 2167–2178.

11.

Battistoni

Fatone

Passacantando

. (2007) Application of food waste disposers and alternate cycles process in small decentralized towns: A case study. Water Research 41: 893–903.

12.

Battistoni

Fava

Pavan

. (1997) Phosphate removal in anaerobic liquors by struvite crystallization without addition of chemicals. Preliminary results. Water Research 31: 2925–2929.

13.

Battistoni

Fava

Stanzini

. (1993) Feed and digester operative conditions as parameters affecting the rheology of digested municipal solid wastes. Water Science and Technology 27: 37–45.

14.

Battistoni

Pavan

Cecchi

. (1998a) Phosphorous removal in real anaerobic supernatants: modelling and performance of a fluidized bed reactor. Water Science and Technology 38: 275–283.

15.

Battistoni

Pavan

Cecchi

. (1998b) Integration of civil wastewater and municipal solid waste treatments. The effect on biological nutrient removal. In: Proceedings of European conference on new advances in biological nitrogen and phosphorus removal for municipal or industrial wastewaters, Narbonne, France, 12–14 October 1998.

16.

Battistoni

Pavan

Prisciandaro

. (2000) Struvite crystallization: a feasible and reliable way to fix phosphorus in anaerobic liquors. Water Research 34: 3033–3041.

17.

Bolzonella

Battistoni

Susini

. (2006) Anaerobic co-digestion of waste activated sludge and the OF-MSW: The experiences of Viareggio and Treviso plants (Italy). Water Science and Technology 53: 203–211.

18.

Bolzonella

Pavan

Battistoni

. (2003) The under sink garbage grinder: A friendly technology for the environment. Environmental Technology 24: 349–359.

19.

Bolzonella

Pavan

Battistoni

. (2005b). Anaerobic co-digestion of sludge with other organic wastes and phosphorous reclamation in BNR wastewater treatment plants. In: IWA specialized conference & exhibition ‘Nutrient management in wastewater treatment processes and recycle streams’, Krakow, Poland, 19–21 September 2005.

20.

Bolzonella

Pavan

Fatone

. (2005a) Anaerobic fermentation of organic municipal solid wastes for the production of soluble organic compounds. Industrial and Engineering Chemistry Research 44: 3412–3418.

21.

Cavinato

Bolzonella

Fatone

. (2011a) Optimization of two-phase thermophilic anaerobic digestion of biowaste for hydrogen and methane production through reject water recirculation. Bioresource Technology 102: 8605–8611.

22.

Cavinato

Bolzonella

Fatone

. (2011b) Two-phase thermophilic anaerobic digestion process for biohythane production treating biowaste. Preliminary results. Water Science and Technology 64: 715–721.

23.

Cavinato

Bolzonella

Pavan

. (2013) Mesophilic and thermophilic anaerobic co-digestion of waste activated sludge and source sorted biowaste in pilot- and full-scale reactors. Renewable Energy 55: 260–265.

24.

Cavinato

Da Ros

Pavan

. (2014) Treatment of waste activated sludge together with agro-waste by anaerobic digestion: focus on effluent quality. Water Science and Technology 69: 525–531.

25.

Cavinato

Giuliano

Bolzonella

. (2012) Bio-hythane production from food waste by dark fermentation coupled with anaerobic digestion process: A long-term pilot scale experience. International Journal of Hydrogen Energy 37: 11549–11555.

26.

Cecchi

Battistoni

Pavan

. (1994) Anaerobic digestion of OFMSW and BNR processes: a possible integration. Preliminary results. Water Science and Technology 30: 65–72.

27.

Cecchi

Battistoni

Traverso

. (1999) Civil wastewater and municipal solid waste integrated treatment. Design considerations by using ASM 2. In: Proceedings of II international symposium on anaerobic digestion of solid waste, Barcelona, 15–17 June 1999.

28.

Cecchi

Marcomini

Pavan

. (1990b). Enhancing performance. Anaerobic digestion of municipal solid waste. Biocycle 31: 42–43.

29.

Cecchi

Marcomini

Pavan

. (1990c) Mesophilic digestion of the refuse organic fraction sorted by plant. Performance and kinetic study. Waste Management & Research 8: 33–44.

30.

Cecchi

Mata-Alvarez

Pohland

(1993) Anaerobic digestion of solid waste. In: Proceedings of the international symposium on anaerobic digestion of solid waste, Venice, 14–17 April 1992.

31.

Cecchi

Mata-Alvarez

Traverso

. (1990d) A new approach to the kinetic study of anaerobic degradation of the organic fraction of municipal solid waste. Biomass 23: 79–102.

32.

Cecchi

Pavan

Mata-Alvarez

. (1989) Anaerobic mesophilic digestion – Co-composting research in Italy. Bio-Cycle 30: 68–71.

33.

Cecchi

Traverso

Cescon

(1986) Anaerobic digestion of the organic fraction of municipal solid waste. Digester performance. The Science of Total Environment 56: 183–197.

34.

Cecchi

Traverso

Mata-Alverz

. (1988b) State of the art of R & D in the anaerobic digestion process of municipal solid waste in Europe. Biomass 16: 257–284.

35.

Cecchi

Traverso

Perin

. (1988a) Comparison of codigestion performance of two differently collected organic fractions of municipal solid waste with sewage sludge. Environmental Technology Letters 9: 391–400.

36.

Cecchi

Vallini

Mata-Alvarez

(1990a) Anaerobic digestion and composting in an integrated strategy for managing vegetable residues from agro-industries or sorted organic fraction of municipal solid waste. Water Science and Technology 22: 33–41.

37.

Chen

Cheng

Creamer

(2008) Inhibition of anaerobic digestion process: A review. Bioresource Technology 99: 4044–4064.

38.

Chinellato

Cavinato

Bolzonella

. (2013) Biohydrogen production from food waste in batch and semi-continuous conditions: Evaluation of a two-phase approach with digestate recirculation for pH control. International Journal of Hydrogen Energy 38: 4351–4360.

39.

Cooney

Wise

(1975) Thermophilic anaerobic digestion of solid waste for fuel gas production. Biotechnology and Bioengineering 12: 1119–1135.

40.

Da Ros

Cavinato

Cecchi

. (2014) Anaerobic co-digestion of winery waste and waste activated sludge: assessment of process feasibility. Water Science and Technology 69: 269–277.

41.

Di Stefano

Drennan

VerNooy

(2008) Laboratory-scale investigation of the curing process for anaerobic digestate. In: Proceedings of Vth international symposium on anaerobic digestion of solid wastes and energy crops, Hammamet, Tunisia, 25–28 May 2008.

42.

Diaz

Trezek

(1977) Biogas of a selected fraction of municipal solid waste. Compost Science 18(2): 8–13.

43.

EBA, 2012, EBA (European Biogas Association). Present status and future prospects of biogas/biomethane in Europe, http://www.european-biogas.eu

44.

EC, 1991, Council Directive 91/676/EC of 12 December 1991 concerning the protection of waters against pollution caused by nitrates from agricultural sources, OJ L 375, (accessed 31 December 1991).

45.

EC, 2008, Councul Directive 2008/98/EC of 19 November 2008 on Waste and repealing certain directives, OJ L312/3, (accessed 22 November 2008).

46.

Edelmann

Schleiss

Joss

(2000) Ecological, energetic and economic comparison of anaerobic digestion with different competing technologies to treat biogenic wastes. Water Science and Technology 41: 263–274.

47.

Engeli

Edelmann

Fuchs

. (1993) Survival of plant pathogens and weed seeds during anaerobic digestion. Water Science and Technology 27: 69–76.

48.

Escamilla-Alvarado

Poggi-Varaldo

(2013) Use of organic waste for the production of added-value holocellulases with Cellulomonas flavigena PR-22 and Trichoderma reesei MCG 80. Waste Management & Research 31(8): 849–858.

49.

Fannin

Conrad

Srivastava

. (1984) Anaerobic processes. Journal of the Water Pollution Control Federation 56: 586–593.

50.

Fatone

Frison

Katsou

. (2014). Integrating the selection of PHA storing biomass and nitrogen removal via nitrite for the treatment of the sludge reject water. In: Wastewater and Biosolids Treatment and Reuse: Bridging Modeling and Experimental Studies, ECI Symposium Series Otranto, Italy, 8–14 June, 2014. Available at: http://dc.engconfintl.org/wbtr_i/15.

51.

Fava

Totaro

Diels

. (2013) Biowaste biorefinery in Europe: opportunities and research & development needs. New Biotechnology. 32(1): 100–108.

52.

Fermoso

Bartacek

Jansen

. (2009) Metal supplementation to UASB bioreactors: From cell-metal intractions to full-scale application. Science of the Total Environment 407: 3652–3667.

53.

Fountoulakis

Drakopoulou

Terzakis

. (2008) Potential for methane production from typical Mediterranean agro-industrial by-products. Biomass and Bioenergy 32: 155–161.

54.

Frison

Katsou

Malamis

. (2014). Nutrient removal via nitrite from reject water and polyhydroxyalkanoate (PHA) storage during nitrifying conditions. Journal of Chemical Technology and Biotechnology. Epub ahead of print 6 October 2014. DOI: 10.1002/jctb.4487.

55.

Giuliano

Cavinato

Bolzonella

. (2011) SS-OFMSW sorting approaches oriented to anaerobic digestion: preliminary comparison for full scale implementation. In: Proceeding Latin American anaerobic digestion symposium, Latin American Anaerobic Digestion Symposium, Ouro Preto, Brasile, 23–27 October 2011. pp.554–562.

56.

Giuliano

Zanetti

Micolucci

. (2014) Thermophilic two-phase anaerobic digestion of SS-OFMSW for bio-hythane production: effect of recirculation sludge on process stability and microbiology over a long-term pilot scale experience. Water Science and Technology. 69(11): 2200–2209.

57.

Hartmann

Møller

Ahring

(2004) Efficiency of the anaerobic treatment of the organic fraction of municipal solid waste: Collection and pre-treatment. Waste Management & Research 22: 35–41.

58.

JRC Scientific and policy reports (2014) End-of-waste criteria for biodegradable waste subjected to biological treatment (compost & digestate): Technical proposals. Saveyn

Eder

(eds) Luxemburg: Luxemburg Publication Office of the European Union 2014.

59.

Kupper

Bürge

Bachmann

. (2014) Heavy metals in source-separated compost and digestates. Waste Management 34: 867–874.

60.

Lee

Chua

ASM

Yeoh

. (2014) A review of the production and applications of waste-derived volatile fatty acids. Chemical Engineering Journal 235: 83–99.

61.

Legislative Decree n.75 of 29th April 2010 “Revision of the Discipline on Fertilizers”, published OJ n121, (accessed 26 May 2010).

62.

Legislative Decree n.99 of 27th Jenuary 1992 “Implementation of Directive 86/278/EEC on the environment protection, in particular of the soil, when using sewage sludge in agriculture” published OJ n38, (accessed 15 January 1992).

63.

Lin

Shei

(2008) Heavy metal effects on fermentative hydrogen production using natural mixed microflora. International Journal of Hydrogen Energy 33: 587–593.

64.

Chiang

Tsao

. (2012) Effects of spiked metals on the MSW anaerobic digestion. Waste Management & Research 30: 32–48.

65.

Mata-Alvarez

Cecchi

Pavan

. (1990) The performances of digesters treating the organic fraction of municipal solid waste differently sorted. Biological Wastes 33: 181–199.

66.

Mata-Alvarez

Cecchi

Pavan

. (1993) Semi-dry thermophilic anaerobic digestion of fresh and pre-composted organic fraction of MSW. Digester performance. Water Science and Technology 27: 87–96.

67.

Mata-Alvarez

Dosta

Mace

. (2011) Codigestion of solid wastes: A review of its uses and perspectives including modeling. Critical Reviews in Biotechnology 31: 99–111.

68.

Mata-Alvarez

Mace

Llabres

(2000) Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Bioresource Technology 74: 3–16.

69.

Mohan

Reddy

(2013) Optimization of critical factors to enhance polyhydroxyalkanoates (PHA) synthesis by mixed culture using Taguchi design of experimental methodology. Bioresource Technology 128: 409–416.

70.

Owens

Chynoweth

(1993) Biochemical methane potential of municipal solid waste (MSW) components. Water Science and Technology 27: 1–14.

71.

Parkin

Owen

(1986) Fundamentals of anaerobic digestion of wastewater sludges. Journal of Environmental Engineering 112: 867–920.

72.

Pavan

Battistoni

Bolzonella

. (2000) Integration of wastewater and OFMSW treatment cycles: from the pilot scale experiment to the industrial realisation. The new full scale plant of Treviso (Italy). Water Science and Technology 41: 165–173.

73.

Petersson

Wellinger

(2009) Biogas upgrading technologies—developments and innovations. IEA Bioenergy. available at: http://www.en.esbjerg.aau.dk/digitalAssets/80/80449_iea-biogas-upgrading-report-2009.pdf (accessed October 2009).

74.

Petruccioli

Santori

(2013) An international network for developing research, technological transfer and dissemination on treatment and upgrading of Mediterranean agro-industrial wastes and effluents. Environmental Engineering and Management Journal 12: 113–116.

75.

Poggi Varaldo

Munoz Paez

Escamilla-Alvarado

. (2014) Biohydrogen, biomethane and bioelectricity as crucial components of biorefinery of organic wastes: A Review. Waste Management & Research. 32(5): 353–365.

76.

Poggi-Varaldo

Carmona-Martínez

Vázquez-Larios

. (2009) Effect of inoculum type on the performance of a microbial fuel cell fed with spent organic extracts from hydrogenogenic fermentation of organic solid wastes. Journal of New Materials for Electrochemical Systems 12: 49–54.

77.

Raven

RPJM

Gregersen

(2007) Biogas plants in Denmark: Successes and setbacks. Renewable and Sustainable Energy Reviews 11: 116–132.

78.

Reddy

Mohan

(2012) Influence of aerobic and anoxic microenvironments on polyhydroxyalkanoates (PHA) production from food waste and acidogenic effluents using aerobic consortia. Bioresource Technology 103: 313–321.

79.

Reis

Albuquerque

Villano

. (2011) Mixed culture processes for polyhydroxyalkanoate production from agro-industrial surplus/wastes as feedstocks. In: Fava

Agathos

Young

(Eds.), Comprehensive Biotechnology, 2nd Edn., Vol. 6, Amsterdam, The Netherlands: Elsevier, 2011, pp. 669–683.

80.

Sans

Mata-Alvarez

Cecchi

. (1995) Volatile fatty acids production by mesophilic fermentation of mechanically sorted urban organic wastes in a plug flow reactor. Bioresource Technology 51: 89–96.

81.

Schmidell

Craveiro

Peres

(1986) Anaerobic digestion of municipal solid wastes. Water Science and Technology 18: 163–175.

82.

Scholz

Melin

Wessling

(2013) Transforming biogas into biomethane using membrane technology. Renewable and Sustainable Energy Reviews 17: 199–212.

83.

Shen

. (2014) Production of poly(hydroxybutyrate-hydroxyvalerate) from waste organics by the two-stage process: Focus on the intermediate volatile fatty acids Bioresource Technology. 166: 194–200.

84.

Stenstrom

Bhunia

. (1982) Anaerobic digestion of municipal solid waste. Journal of Environmental Engineering 109: 1148–1158.

85.

Traverso

Cecchi

(1988) Anaerobic digestion of the shredded organic fraction of municipal solid waste. Biomass 16: 97–106.

86.

Valentino

Beccari

Fraraccio

. (2014) Feed frequency in a sequencing batch reactor strongly affects the production of polyhydroxyalkanoates (PHAs) from volatile fatty acids. New Biotechnology 31: 264–275.

87.

Valentino

Brusca

Beccari

. (2013) Start up of biological sequencing batch reactor (SBR) and short-term biomass acclimation for polyhydroxyalkanoates production. Journal of Chemical Technology and Biotechnology 88: 261–270.

88.

Vallini

Cecchi

Pavan

. (1993) Recovery and disposal of the organic fraction of MSW by means of combined anaerobic and aerobic biotreatments. Water Science and Technology 27: 121–132.

89.

Vermeulen

Huysmans

Crespo

. (1993) Processing of biowaste by anaerobic composting to plant growth substrates. Water Science and Technology 27: 109–120.

90.

Verstraete

Ven Lier

Pohland

. (2000) Developments at the second international symposium on anaerobic digestion of solid waste (Barcelona, 15–19 June 1999). Bioresource Technology 73: 287–289.

91.

Villano

Aulenta

Majone

(2012) Perspectives of biofuels production from renewable resources with bioelectrochemical systems. Asia-Pacific Journal of Chemical Engineering 7: S263–S274.

92.

Ward

Hobbs

Holliman

. (2008) Optimisation of the anaerobic digestion of agricultural resources. Bioresource Technology 99: 7928–7940.

93.

Wellinger

Wyder

Metzler

(1993) Kompogas, a new system for the anaerobic treatment of sources separated waste. Water Science and Technology 27: 153–158.

94.

Yue

Wang

(2007) Anaerobic digestion of cattail with rumen culture in the presence of heavy metals. Bioresource Technology 98: 781–786.