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State of the art and innovative aspects of the project

Provide a description of the state of the art of the technique or method addressed. Elaborate on the technical description of the processes or methods and/or proposed innovation(s), new elements, improvements. Describe the previous research and experience carried out in preparation for the project implementation, including feasibility studies. Please take into account that the innovative nature of the proposed actions can be evaluated from different perspectives: a) relative to the technologies applied by the project (technological innovation), b) relative to the way technologies are implemented (innovation in processes or methods) and, c) concerning the business and economic models developed by the project (economic and business innovation). These different dimensions of the innovatory nature have to be compared with the state of the art at global (world) level. N.B. Geographical technology or practice transfer alone (without a genuine development of innovative character) can not be considered as innovative. Equally, projects which involve pure research and development or merely preparatory activities (studies, surveys, etc) can not be considered innovative per se.

description of the state of the art of the technique or method addressed

The promotion of waste minimisation and recycling are important components of modern waste management strategies: to this end anaerobic digestion has been reported as the most cost-effective method.

Anaerobic co-digestion of the organic fraction of municipal solid waste (OFMSW) and sewage sludge is a sustainable and an appropriate treatment alternative due to bioenergy and nutrient recovery while combining the treatment of two largest municipal waste streams.

By co-digestion, volume of the organic wastes is reduced and stabilized, a residue that can be used for soil conditioning is produced, and energy in the form of methane is recovered.

Sludge occurring at municipal wastewater treatment plants (WWTP) is considered as one of the most appropriate co-substrates for co-digestion with the OFMSW. With the large amount of sewage sludge produced in WWTPs and the large number of existing anaerobic digesters to stabilize it, the anaerobic co-digestion of OFMSW with sewage sludge is especially attractive.

Co-fermentation of biowastes in WWTP may affect plant operational performance in terms of degree of degradation, gas production, drainability, and backload. In addition, dilution of potential toxic compounds, improved balance of nutrients, and synergistic effect of microorganisms are the other benefits of co-digestion including hygienic stabilization under appropriate operational conditions.

Summarizing:

• anaerobic co-digestion of sewage sludge and OF-MSW is a well-known process;
• it is considered a preferable choice;
• apparently there are not major drawbacks.

As seen in the description of the environmental problem targeted, this methods is not opposed by EU legal framework.

So why isn't COWS-like projects on every WWTP and MSW manager?

Core technology providers focus their efforts primarily on new facilities where their whole process can be purchased without the need of extensive works and modifications as it happens on existing plants.

One key point is pre-treatment, which is crucial for throughput performance and quality of residual digestate and organic waste.

Pre-treatment is essential for:

• separating biodegradable, useful ingredients of MSW-OF from contaminants or contrary elements;
• breaking down OF waste to particles small enough to maximize mixing with sludge;
• reduce the volume of residual fraction.

In the majority of cases currently available, pre-treatment technologies and devices have been:

• pushed by providers of digester process technologies;
• derived from other industrial field, mainly pulp and plastics industry.

It should also be noticed that - while in a new plant a patented process can be used, there is little space for patenting integration of co-digestion in existing WWTP using "public-dominion" technology.

In conclusion there is a technology gap around integration of pre-treatment subsystems into existing WWTP AD processes.

Past experimentation conducted by COWS proponents

Starting from 2005 SEARIS and the water supply utility GAIA (producer of sewage sludge) set up an experimental line for AD codigestion of wastewater sludge and OF-MSW. Results from that experiment are the proof-of-concept of COWS innovative prototype and demonstration line. A paper reporting the experimentation was published (Bolzonella D, Battistoni P, Susini C, Cecchi F, (2006) "Anaerobic co-digestion of waste activated sludge and the OF-MSW: The expereinces of Viareggio and Treviso plants (Italy)" Water Science & Technology 53(8) 203-211). Caterina Susini, Ph.D. is the candidate Project Manager of COWS project.

Viareggio WWTP treats the wastewater coming from the Viareggio Municiaplity and neighborough, covering a capacity of some 75.000 people equivalent. The sludge line adopts a pre-thickening (2 dynamic pre-thickeners with a working volume of 350 m3 each) and the anaerobic digestion of the sludge, carried out in two digesters of 3000 and 1500 m3, respectively. After digestion, sludge are dewatered achieving some 20% TS in the dewatered stream.

The experimental co-digestion line was implemented by using a pilot scale sorting line able to treat 3 - 5 tons per day of organic waste coming from source sorting collection.

With reference to experimental data, both substrates (sewage sludge, SS, and SS-OFMSW) fed to the digester exhibited good characteristics in relation to the codigestion process requirements: the SS present a volatile solid content of 69 %TS while the OFMSW shown high TS content (some 30%), and high TVS content (75 %TS); well balanced nutrient content was observed too.

An additional improvement was given by the sorting treatment which allowed to obtain a stream with a content of 89% of putrescible matter.

Good characteristics of substrates determined the good performances of the process: SGP improved from 0.21 to 0.26 m3/kgTVS treating 3 ton/day of SS-OFMSW, which means an additional OLR of 0.19 kgTVS/m3d, on a total OLR of 1.21 kgTVS/m3d.

Monitoring of the process covered three months of experiment: it was observed a progressive evolution of stability parameters, as alkalinity, which required much more than 1 HRT to achieve a stable situation. Collected data allowed to hypothesize similar positive behaviour of the process also using higher loading rate.

The experiment remarked the importance of pre-treatment.

Subsequent technology screening have been done, driving to the conclusion that a full scale prototype pre-treatment sub-unit can be implemented to demonstrate how to upgrade sludge treatment plant to co-digestion in a viable, sustainable way.

Some patented processes that have been successfully proven their reliable performance in full-scale plants processes are BIOCEL (batch system), DRANCO, Valorga, KOMPOGAS (one-stage dry system), Waasa, BTA (one- stage wet system), Schwarting-Uhde (two-stage wet system), Linde-BRV (two stage dry system).

In order to control the odor emission; the system is housed in a closed building that is kept at a slight under-pressure. The temperature is controlled at 35–40ºC by spraying leachate, which is pre-heated by a heat exchanger, from nozzles on top of the digesters. Typical retention time in this process is reported to be 15 – 21 days

BIOCEL. The system is based on a batch-wise dry anaerobic digestion. The total solids concentration of organic solid wastes as feeding substrate is maintained at 30–40% dry matter (w/w). The process is accomplished in several rectangular concrete digesters at mesophilic temperature. The floors of the digesters are perforated and equipped with a chamber below for leachate collection. Prior to feeding, fresh biowaste substrate and inocula (digestate from previous feeding) are mixed then loaded to the digester by shovels. After the loading is finished, the digesters are closed with air tight doors. In order to control the odor emission; the system is housed in a closed building that is kept at a slight under-pressure. The temperature is controlled at 35–40ºC by spraying leachate, which is pre-heated by a heat exchanger, from nozzles on top of the digesters. Typical retention time in this process is reported to be 15 – 21 days (ten Brummeler, 2000). A full-scale BIOCEL plant is reported to have successfully treated vegetable, garden and fruit wastes with the capacity of 35,000 tons/year. Approximately 310 kg of high-quality compost, 455 kg of water, 100 kg of sand, 90 kg of biogas with an average methane content of 58% and 45 kg of inert waste are produced from each ton of waste processed (CADDET, 2000). [edit] DRANCO DRANCO. The DRANCO (dry anaerobic composting) process employs a one-stage anaerobic digestion system, which is followed by a short aerobic maturation phase. Although mostly operated under thermophilic temperature (reportedly to be 50-55 °C), mesophilic operation (35-40 °C) can also be applied for specific waste streams (de Baere, 2008). The DRANCO process is typically a vertical plug-flow reactor. The digester is fed from the top of the reactor and the digested slurry is removed from the bottom at the same time. Usually one part of the digested slurry is used as inoculum and mixed with six to eight part of fresh substrate. A small amount of steam is introduced to the mixture in order to maintain the temperature. The pre-heated mixture is then pumped to the top of the reactor through feeding tubes. There are no mixing devices needed in the reactor other than the natural downward movement of the waste caused by fresh feeding and digestate withdrawal (Vandevivere et al., 2002; Edelmann and Engeli, 2005; de Baere, 2008). The rest of the digested slurry is dewatered and the solid residue from the process is then stabilized and sanitized aerobically during a period of approximately two weeks. The DRANCO process is considered to be effective for treatment of solid wastes with 20-50 % TS. The typical retention time is 15 to 30 days, and the biogas yield ranges between 100 and 200 m3/ton of input waste (Nichols, 2004). [edit] VALORGA Valorga. The Valorga system is a one-stage dry anaerobic digestion process which uses a vertical cylindrical reactor which can be operated at both, mesophilic and thermophilic temperature. In order to obtain a horizontal plug-flow process, the digester is equipped with a vertical median partition wall on approximately 2/3 of their diameter. The biowaste substrate is fed through a port placed on one side of the partition wall and the digestate withdrawal port is placed on the other side. The vertical mixing is performed by internally recirculated high-pressure biogas injection every 15 minutes. The pre-treatments prior to feeding include: dry ballistic separation to remove the heavy fraction and other contaminants, crushing of biowaste to obtain particle size < 80 mm, adjustment of solids content to 25 -32 % by mixing with process water, and pre- heating by steam injection (Fruteau de Laclos et al., 1997; Karagiannidis and Perkoulidis, 2009). The retention time of this system is typically 18 – 25 days at mesophilic temperatures with a biogas yield of 80 to 160 m3·ton-1 of feedstock, depending on the type of solid waste (Nichols, 2004). One technical drawback of the system design is that gas injection ports are easily clogged when treating relative wet (< 20 % TS) feed stock (Vandevivere et al., 2002). Edelmann and Engeli (2005) reported that the operation of a thermophilic Valorga digester in Switzerland was stopped for a relatively long time because of large quantities of sediments (sand, gravel etc.) in the base of the digester, hampering the function of the mixing equipment and reducing the active volume of the digester significantly. [edit] KOMPOGAS KOMPOGAS. The KOMPOGAS system is a one-stage dry anaerobic digestion process. The fermentation process takes place in a horizontal plug-flow reactor at thermophilic temperature (typically 55-60 °C). The reactor is equipped by slowly rotating and intermittently acting impellers to ensure mixing and help the re-suspension of heavier materials. Prior to feeding, the solid waste is mechanical pre-treated in order to remove the impurities and reduce the size of the substrate (KOMPOGAS, 2007). A total solids content adjustment by addition of process water is done to have a TS concentration to around 23 to 28 %. If the TS values are lower than this range, heavy particles such as sand and glass tend to sink and accumulate inside the reactor while higher values can cause excessive resistance to the flow (Chavez-Vazquez and Bagley, 2002). The retention time of the system ranged from 15 – 20 days. Due to mechanical constraints, the volume of the KOMPOGAS reactor is limited. If the solid waste generation is relatively high, the capacity of the plant can be facilitated by installing several reactors in parallel, each with a capacity of either 15,000 or 25,000 tons/year (Nichols, 2004). The KOMPOGAS system is reported to run very stable, however, it has to be stressed that it is important to feed an appropriate mixture of wastes. A KOMPOGAS plant which was run exclusively with protein-rich food wastes first experienced an inhibition due to high ammonia concentrations (Edelmann and Engeli, 2005). Nishio and Nakashimada (2007) reported that three types of waste (i.e., garbage and rejects from hotels, yard waste, and old paper) were mixed at various ratios to control the C/N ratio before feeding to the KOMPOGAS plant. The plant ran at stable operation for at least two years and generated biogas at a rate of about 820 m3/ton of VS. [edit] WAASA Waasa. The Waasa process is a wet, one-stage anaerobic digestion system and is operated at both, mesophilic and termophilic temperatures. This completely mixed process is maintained in a vertical reactor which is subdivided internally to create a pre- digestion chamber by which the possibility of short-circuiting should be prevented. A relatively complex pre-treatment including mechanical sorting and waste washing has to be done prior to feeding. The sorting facility produces by-products such as relatively high-calorie RDF (Refuse-Derived Fuel) stream, ferrous/non-ferrous metal fractions, paper and plastic fraction. The washing process comprises a wet separation process that removes coarse inert materials and sand from the organic fraction. Process water is added to fresh substrate to the desired concentration of total solids (10-15% TS). The slurry is mixed with small amount of inocula, pre-heated with steam injection and pumped to the pre-chamber which is operated in a plug-flow mode with retention times of one or two days before digestion in the main reactor. The mixing in the digester is performed by mechanical impellers and injection of a portion of the biogas into the bottom of the digester tank (Williams et al., 2003). Nichols (2004) reported a full-scale Waasa process plant which was run at both temperatures parallelly. The thermophilic process required a retention time of 10 days compared to 20 days in the mesophilic process. A modified Waasa process (Vagron) treating the mechanically separated organic fraction of municipal solid waste in Groningen, the Netherlands was reported to reach a stable operation at an OLR of 7.7 kg VS ·m-3 ·d-1 (Luning et al., 2003). The biogas production was reported within the range of 100-150 m3/ton of feedstock with 20-30% internal biogas consumption for the pre-heating of the feeding substrate. The volume reduction reached approximately 60%, and the weight reduction was about 50- 60% (Williams et al., 2003). [edit] BTA BTA. The BTA process consists of two major steps: the hydro-mechanical pre- treatment and the anaerobic digestion processes. During the hydro-mechanical pre- treatment the solids are diluted in hydropulpers with recirculated process water in order to obtain a maximum solids content of 10%. The light impurities like plastics, foils, textiles, wood etc as well as heavy impurities like stone, batteries, metals etc are removed by means of a rake and a heavy fraction trap. This process results in a thick, pumpable suspension that is fed to the digester. The grit removal system can be optionally added in order to separate the remaining finest matter like sand, little stones and glass splinters. Although commonly applied as single-stage system, BTA also offers a multi-stages system depending on the size of the plant. Single-stage systems are mainly for relatively small, decentralized waste management units whereas multi- stages systems are mainly for plants with capacities of more than 50,000 tons/year. The temperature in BTA process is maintained in the mesophilic range, normally at 35 °C and the digester is considered as a completely mixed reactor. Mixing is performed by biogas injection. The digestion residue is dewatered by a decanter centrifuge and generally sent to aerobic post-treatment. The water demand of the process is met by recirculating the process water. Depending upon the waste composition and local requirements, excess process water is sent to the sewage system, or will be additionally treated on-site before it can be discharged. The generated biogas can be recovered for use in gas engines or co-heat and power (CHP) stations. Depending on the waste composition, the gas yield ranges between 80 and 120 m3/ton of biowaste (Kübler et al., 2000; Chavez-Vazquez and Bagley, 2002; Nichols, 2004; Haines, 2008). [edit] Schwarting-Uhde Schwarting-Uhde. The Schwarting-Uhde process adopts a two-stage wet anaerobic digestion process which is performed in a series of two vertical plug-flow reactors. The first reactor is operated at mesophilic temperature for hydrolysis and acidification processes while the second reactor is operated at thermophilic temperature for methanogenesis. The source-sorted biowaste is shredded to reduce the particle size and diluted to a TS concentration of around 12 %. The slurry is pre-heated to the intended temperature by heat exchangers and then pumped through a series of perforated plates placed within the reactor, which is employed to ensure the uniformity of upward movement and to maintain plug-flow conditions. Mechanical stirrers are not needed in for mixing purposes. An adequate mixing is obtained by raising and lowering the column of liquid in the tank, thus creating turbulence at the perforated plates via time-controlled impulse pumps. The retention time in both reactors is about 5 to 6 days making an overall retention time of 10 to 12 days. Biogas is collected at the top of the digesters, whereas settled heavy solids, which accumulate at the bottom of the reactors, are frequently removed via screw pumps. This process design offers an advantage in decreasing the potential formation of a thick floating scum layer which is commonly plaguing wet anaerobic digestion. However, due to the high risk of perforated plates clogging, the Schwarting-Uhde process is only suitable to treat relatively clean highly biodegradable biowastes (Lissens, et al., 2001; Vandevivere et al., 2002). A full-scale Schwarting–Uhde plant was reported to have stable operation at an OLR of up to 6 kg VS·m-3 ·d-1 (Thrösch and Niemann, 1999 in Trzcinski and Stuckey, 2009). A successful solids elimination of 55 – 60 % was reported to be achieved by a Schwarting-Uhde plant treating sludge from a wastewater treatment plant (EC, 1995). [edit] Linde-BRV Linde-BRV. The Linde-BRV process can be considered as two-stage dry anaerobic digestion. After pre-treatment to reduce the particle size and to remove impurities, the solids concentration of source-separated biowastes is adjusted to 34 %. The slurry is then pre-digested in an aerobic upstream stage where the organic materials are partially hydrolyzed (Vandevivere et al., 2002). After 2 days of retention time, the pre- digested slurry is pumped to a rectangular shaped concrete digester in horizontal plug- flow mode. The mixing is accomplished by several agitators of transverse paddles. The horizontal plug-flow movement is ensured by a walking floor installed on the bottom of the reactor which also functions to transport the sediments to the digester’s discharging end (Nichols, 2004; Zaher et al., 2007). The process is commonly kept at thermophilic temperature although modification to mesophilic is also possible. Some of the heating is done outside the digester with a short heat exchanger, but primarily heating occurs within the digester walls using a heat exchanger. In the termophilic process, the retention time is reported about 21-25 days with an OLR of 8 kg VS ·m-3 ·d-1 (Vandevivere, 2002; Zaher et al., 2007).

technical description of the processes or methods and/or proposed innovation(s), new elements, improvements

innovative nature of the proposed actions

a) relative to the technologies applied by the project (technological innovation)

b) relative to the way technologies are implemented (innovation in processes or methods)

c) concerning the business and economic models developed by the project (economic and business innovation)