PSA B2 SOTA

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.

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 takes 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 been 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.