B2-Environmental problem targeted
Contents
ENVIRONMENTAL PROBLEM TARGETED (max. 10.000 characters)discuss
B2.1
Source-separated collection of the organic fraction of municipal solid waste works well in Viareggio and Camaiore (Lucca, IT): from November to April, Sea Risorse Spa (SEARIS) collects 20 tons daily.
Collected door to door and from large producers, the organic fraction is then sent to the nearest composting plant capable of handling such large amounts.
Every day, from November to April, a truck with two hook-lift containers transports 20 tons of OF-MSW from Viareggio to Montespertoli (Florence, IT) covering 100 Km on the way there and back.
Together, Viareggio and Camaiore reach almost 100,000 resident inhabitants, while their intense tourist activity greatly contributes to the amount of produced and collected waste. As a result, from May to October, the number of daily trips to Montespertoli increase significantly, reaching peaks of up to nine trips per day.
On a yearly basis, about 700 trucks cover the distance to Montespertoli in order to deliver the output of the efficient waste collection. Each trip produces approximately 540 Kg. of CO2, for a total of more than 350 tons in a year. Delivery of OF-MSW from Viareggio to Montespertoli alone amounts to 27 TOE.
B2.2
For the composting of 15,000 tons of OF-MSW, Montespertoli plant uses 1,050,000 kwh with equivalent emissions of 560.7 tons of CO2 and a consumption of 241 TOE.
In itself, this could be considered a "local" problem in that it is due to the lack of a nearby composting plant (although, were such a plant available, the energy cost and environmental impact of the treatment would not be eliminated).
Presently (pre-COWS, business-as-usual) at Viareggio (the COWS project site), the existing wastewater treatment plant sends out for landfilling or composting 5,200 tons of sludge per year, requiring 260 250 Km trips yearly with emissions equivalent to 330 tons of CO2 and over 50 TOE.
This is not a local situation since it concerns the majority of wastewater sludge treatment plants in Europe.
There are some 36.000 wastewater treatment facilities in Europe today which adopt the anaerobic stabilisation of sewage or waste activated sludge: very often these plants are oversized, due to the low sludge mass loadings originated from wastewater treatment and the low biogas yields of sludge (Bolzonella D., Pavan P., Battistoni P., Cecchi F. (2005). Mesophilic anaerobic digestion of waste activated sludge: influence of the solid retention time in the wastewater treatment process. Process Biochemistry 40(3-4), 1453-1460).
At the same time, the total amount of municipal solid waste is continuously rising. Consequently, there are millions of tons of solid waste being produced every year which have to be safely disposed without any negative impact to the environment. As an effect of EC waste regulations pushing towards source-separation strategies for MSW management, about 400.000 tons/year of OFMSW coming from source sorted or separate collection approaches are produced.
This scenario suggests that making a solution out of this two problems is desirable. Anaerobic co-digestion of sewage sludge with OF-MSW seems to be an attractive alternative to current disposal strategies (Dereli RK, Ersahin ME, Gomec CY, Ozturk I, Ozdemir O. (2010) Co-digestion of the organic fraction of municipal solid waste with primary sludge at a municipal wastewater treatment plant in Turkey. Waste Management & Research 28(5):404-410).
On the other hand, the availability of energy in sufficient and sustainable amounts is a pillar or EU 2020 strategy. Anaerobic digestion as a pre-treatment prior to landfill disposal or composting offers several advantages, such as minimization of masses and volume, inactivation of biological and biochemical processes in order to avoid landfill-gas and odor emissions, reduction of landfill settlements and energy production in the form of methane.
Therefore, anaerobic digestion of bio-degradable solid wastes can be considered an alternative option to improve the environment condition caused by organic solid waste while at the same time taking advantage of it as an environmentally-friendly source of energy.
Anaerobic codigestion is hence considered one of the most promising ways to give a proper disposal to OF-MSW coming from source sorted or separate collection systems, the yields of anaerobic digestion (AD) process for the treatment of 100 kg of OF-MSW being reported to be 35 kg compostable fraction with 22 and 44 KWh electricity and heat energy, respectively (J. Mata-Alvarez. Biomethanization of the Organic Fraction of Municipal Solid Wastes. IWA Publishing, 2002. ISBN 1900222140).
These figures show that co-digestion of sewage sludge and OF-MSW, would potentially lead to recovery of some 112.000 electrical MW and 180.000 thermal MW in Europe.
Moreover, AD has less GHG release potential than aerobic composting. Finally, bringing OF-MSW to existing AD sludge treatement facilities may also reduce further GHG emissions thanks to shorter storage before delivery, when highly biodegradable content may be released. Daily deliver to a co-digestion plant avoids anaerobiotic pockets that could release CH4.
B2.3
A number of EU legal instruments addresses the issue of treatment of bio-waste.
The Green paper on the management of bio-waste in the European Union COM(2008) 811 final states that biological treatment (including composting and anaerobic digestion) may be classified as recycling when compost (or digestate) is used on land or for the production of growing media. If no such use is envisaged it should be classified as pre-treatment before landfilling or incineration. In addition, anaerobic digestion (producing biogas for energy purposes) should be seen as energy recovery. General waste management requirements such as environmental and human health protection during waste treatment and priority for waste recycling are laid down in the revised Waste Framework Directive which also contains specific bio-waste related elements (new recycling targets for household waste, which can include bio-waste) and a mechanism allowing the setting of quality criteria for compost. Landfilling of bio-waste is addressed in the landfill Directive which requires the diversion of biodegradable municipal waste from landfills.
The revised IPPC Directive (Directive 2008/1/EC), laying down the main principles for the permitting and control of bio-waste treatment installations, will cover all biological treatment of organic waste above a capacity of 50 tonnes/day. The incineration of bio-waste is regulated in the Waste Incineration Directive while the health rules for composting and biogas plants which treat animal by-products are laid down in the Animal By-products Regulation.
The RES Directive (Directive 2009/28/EC) also contains measures on how bio-wastes are to be counted towards renewable energy targets. EU legislation does not limit Member States' choices of bio-waste treatment options as long as they respect certain framework conditions, notably those set by the Waste Framework Directive. The choice of treatment options needs to be explained and justified in national or regional Waste Management Plans and Prevention Programmes.
Together with a definition of waste which, before the revision of the Waste Framework Directive, did not set clear boundaries for when a waste has been adequately treated and should be considered a product, this has led to a wide variety of policies and treatment methods in the EU, including different interpretations of Member States as to when treated bio-waste may cease to be waste and become a product that can move freely on the internal market or be exported from the EU. Standards on the use and quality of compost exist in most Member States, but differ substantially, partly due to differences in soil policies. While there is no comprehensive Community legislation, certain rules regulate specific aspects of bio-waste treatment, biogas production and compost use.
In this framework, COWS addresses clearly an environmental problem of European breath, with grounding on the most recent EU legislation and having EAP6 and Europe2020 as horizon. COWS demonstration of an innovative, proven solution that can be implemented widely in Europe, with low barriers to social acceptance and low lifecycle impact (because it goes astride existing facilities), adds EU value.
B2.4
Finally, COWS will help to clarify from bottom-up the regulation of co-digestion of waste with sewage sludge at sewage treatment plants. Collaboration between utilities under the Water Framework Directive and waste management utilities may lead to enhanced sectoral practices and possibly to specific guidelines for enabling the use of digestion capacity within the water industry (e.g. considering that most water utilities are energy-hungry industries).
STATE OF THE ART AND INNOVATIVE ASPECTS OF THE PROJECT (max. 10.000 characters)discuss
Note: for forests monitoring projects this box should not be filled in
SOTA 1 - 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 appropriate treatment alternative due to bioenergy and nutrient recovery while combining the treatment of two of the 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 amounts 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 positively 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 no major drawbacks.
As seen in the description of the environmental problem targeted, this methods is not opposed by EU legal framework.
So why aren't there COWS-like projects being entertained by 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 would be the case for 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 OF-MSW 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 fields, mainly pulp and plastics industry.
It should also be noted that, while in a new plant a patented process can be used, there is little space for patenting integrations 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 on 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 experiences 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 Municipality and neighboring areas, 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, sludges 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 presented a volatile solid content of 69 %TS while the OFMSW showed high TS content (some 30%), and high TVS content (75 %TS); well balanced nutrient content was observed too.
An additional improvement was provided 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: we observed a progressive evolution of stability parameters, such 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 has been done, leading to the conclusion that a full scale prototype pre-treatment sub-unit can be implemented to demonstrate how to upgrade of a sludge treatment plant to co-digestion in a viable, sustainable way.
The process at SEA PLANT
The facility in VIareggio is similar to most treatment plants: it uses conventional, non-patented, single-stage digestion system, in which all phases during the anaerobic degradation take place in the same reactor, even though the microbial requirements of the different phases are not the same. COWS addresses primarily this type of conventional wet, one-stage, batch-wise digestion systems.
COWS approach may be useful also in other patented processes that have successfully proven their reliable performance in full-scale plants processes (e.g. Waasa, BTA, Schwarting-Uhde), but such investigation falls out of the scope of the project.
Pre-treatment technologies
Pre-treatment is crucial for good process performances. Bio-wastes from separate collection as OF_MSW require extended pre-conditioning for wet co–digestion. Dry digestion offers the easier solution since the material doesn’t separate much as long as the dry matter content remains above 25% total solids. But it does not fit with sewage sludge co-digestion. Wet separation removes heavy contaminants (glass, stones) and floating materials (plastics, wood, rubber) after the addition of recycled water. The technology is well established and better suited for large-scale bio-waste digestion plants. It is generally accepted that wet separation processes achieve higher impurities removal compared to dry separation, however it is laborious and expensive. In the end pre-treatment process must always be waste specific, in accordance with the digestion process applied and adjusted to the product quality required. For COWS this means searching the best trade-off for conventional wet, one-stage, batch digestion system fed with OF-MSW from separated collection.
Hence a technology screening have been done after the experimentation reported, leading 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. As said most pre-treatment technologies are borrowed from other industrial sectors. Focus is on obtaining the best semi-fluid phase, the OF-MSW slurry to be mixed with sludge with the simplest, most stable and serviceable machinery. Technologies examined include:
- PROMECO extruder, derived from pulp industry use and with a good track record in waste treatment;
- ROS ROCA wet pre-treatment (hydro-mechanical);
- LINDE hydropupler;
- BTA Wet mechanical pre-treatment, pulper and grit removal;
- VM Press (extrusion press system);
Extrusion has been identified as the most promising technique, but this decision will reviewed after a new technology screening and invitation to express interest to these technology vendors.
The key factor is the availability of adjustement and calibration features that are not available in catalogue products and must be co-designed for COWS prototype.
The resulting COWS process will be a modified process in which incoming OF-MSW is pre-treated to obtain a slurry of controlled characteristics to be mixed with sewage sludge and fed in the existing digestion process to rise biogas production at a level convenient for CHP.
Applied technology innovation regards the pre-treatment unit to obtain a slurry of controlled quality.
COWS brings about an innovation in the way it upgrades existing facilities.
As a consequence also the economic model will change, because it changes the bottom line of waste processing.
