“Nothing grows out of diamonds. Out of dung, flowers do grow,” used to sing Fabrizio De André in 1967. Asking the poet’s for forgiveness, fifty years on we can go a step further and state that out of sludge, fuel can grow. We are talking about biomethane obtained from wastewater, i.e. the so-called sewage sludge, black water’s waste products.

This is Biometanow’s objective, the project of circular economy and recovery of resources developed by Gruppo CAP, the public company managing the integrated water service in 134 municipalities of the Milan metropolitan area and an additional 64 in the Monza, Brianza and Pavia provinces, totalling 6,483 kilometres of water network, 782 wells and 294 water treatment plants. Overall, every year, the structure provides about 200 million cubic metres of water to over 2 million people. 

Before going into the details of such experimentation, it is good to take a step back to better understand the #waterevolution of Gruppo CAP, an innovative framework including several projects and development activities of the processes within a circular economy. In particular, with regard to water treatment, the Milan-based company is committed to developing a wide and complex recovery policy of nutrients that, through synergies with the separate waste collection cycle, will enable the recovery of resources and materials from sludge, organic matter and in general from the water cycle, preparing itself to transform the main water treatment plants into as many biorefineries which could produce not only biomethane, but also fertilizers, electricity, biopolymers and nutrients, besides, of course, treated water. Indeed, re-using wastewater can contribute to meeting several water needs both in environmental terms and in some production sectors, agriculture or horticulture, to irrigate orchards and pastures and for water crops. But also for industrial uses, such as concrete production and dust control.

Despite some red tape slowdown, Gruppo CAP is already prepared to experiment the possibility to reuse waters deriving from the treatment of wastewater in the Assago plant, thanks to which it could, for instance, heat or cool Assago’s Mediolanum Forum (450,000 cubic metres of volume, a built area of over 40,000 square metres, a central arena with a capacity to accommodate up to 12,700 viewers). Or it could provide mechanical sweepers cleaning up city streets (8,400 litres a week), irrigate parks or gardens or supply car washes with over 13,000 cubic metres of water a year.

But the widest and most advanced front is the sewage sludge: the waste par excellence containing many nutrients, with enormous potential for energy, biogas and biofuel production. Gruppo CAP, in its four treatment plants (Robecco sul Naviglio, Peschiera Borromeo, Bresso-Niguarda and Sesto San Giovanni) already produces electricity through cogeneration of biogas obtained from anaerobic digestion processes. In two other structures (Pero and Truccazzano) it is carrying out the necessary measures to activate production. While in the Bresso-Niguarda plant it also recovers heat from treated water to heat offices and other work premises linked to the structure.


Microalgae and Treatment Plant

It is through minor projects, the small pieces of a mosaic, that a bigger project is achieved leading to sustainability and efficiency. This is clearly proven by the experiment of introducing microalgae into the treatment process in the Bresso-Niguarda treatment plant managed by Gruppo CAP.

Objective: To improve the plant’s performance both from an environmental and energy point of view in line with the circular economy. Thanks to a €300,000 loan by Fondazione Cariplo and the collaboration of Milano-Bicocca University and the Polytechnic of Milan, a pilot plant for the cultivation of microalgae was started, it will remain operational for the next two year to test the system.

Indeed, microalgae can improve the quality of sewage naturally reducing the content of micropollutants. Moreover, they can multiply just thanks to the nutrients (nitrogen and phosphorous) contained in wastewater thus reducing their concentration. In addition, their metabolism requires CO2 that can be obtained from exhaust emissions produced by the very plant. And finally, algae in excess can be used in the anaerobic biodigester, thus increasing the production of biogas and biomethane. 


Setting aside the out-dated logic of waste and focussing on the valorisation of waste characteristics, high-quality sludge treatment produces fertilizers both for agriculture and flower and plant farming. In the two plants of Cassano d’Adda and Settala, Gruppo CAP has already completed the experimentation activity achieving industrial scale of the process in Peschiera Borromeo started in August 2017 with a yearly production estimated at around 12,000 tonnes of fertilizers per year. Since January 2017, the production of fertilizers started in Rozzano too: about 2,000 tonnes of mixed composted soil improver a year. While in the Sesto San Giovanni plant fermentation of treatment sludge for the production of volatile fatty acids is underway. Initially, they will be used to replace chemical reagents used in the treatment process, but on which – at a later stage – new trials will be carried out aimed at the production of bioplastics, in collaboration with the Smart Plant project, funded by the European Community within the Horizon 2020 financial plan. 

But the most interesting initiative is undoubtedly that of the Bresso-Niguarda treatment plant, where the first filling station with methane obtained from sewage was created. Thus, the treatment plant of Milan’s metropolitan area has turned into a biorefinery and filling station. This project was made possible by the collaboration of actors with both the technical and scientific expertise: Gruppo FCA, the CNR-LIA Institute and the engineering company Ecospray. 

The Bresso-Niguarda treatment plant collects house, industrial sewage and rain from the municipalities of Paderno Dugnano, Cormano, Cusano Milanino and Cinisello Balsamo, an area with 300,000 inhabitants. The normal treatment process causes – inside digesters – the formation of biogas containing about 65% methane, while the second main component is carbon dioxide. Since natural fossil gas contains normally between 85 to 98% methane, biogas can achieve similar quality only after undergoing upgrading. This process basically consists of three steps: biogas cleaning to remove impurities, followed by upgrading to remove CO2. And finally post-treatments to remove from off gas methane so that it is not released into the environment. It is in this transformation process that zeolite filtration technology developed by Ecospray comes into play. Once processed in this way, methane reaches nearly 99% purity, and after compression it is ready to be used in cars.

At full capacity, the Bresso plant alone will be able to produce over 340,000 kg of biomethane, enough to power 416 vehicles each driving 20,000 km. Last November, at EcoMondo, one of the leading novelties was Fiat Panda Natural Power #BiometaNow, powered with biomethane produced by the Bresso-Niguarda plant. Tests carried out on the first 10,000 km run by the vehicle showed a 97% emission reduction compared to the same petrol-powered model.

Unfortunately, the recurring problem in this field, as in any application of the circular economy, is the fact that laws lag behind and do not offer clear and determined support. A symptom of the lack of a wide-ranging vision. The positive impact that the nationwide adoption of this process in treatment plants would have on the environment is very clear. Gruppo CAP’s long term objective is to create about 50 biorefineries in Lombardy able to transform sewage into fuel thus supplying biomethane to a network of about 1,000 filling stations scattered all over Italy. 

A virtuous idea combining economics and ethics, as highlighted by Alessandro Russo, Chairman and CEO of Gruppo CAP. “Every year, we process about 70,000 tonnes of sewage sludge whose disposal costs currently amount to €100 per tonne. This is a rather hefty charge that is added to your bill. This is why we decided to invest in circular economy projects aiming to create value from what is currently waste.”



#waterevolution, www.gruppocap.it/il-gruppo/waterevolution/il-progetto




Top image: Faucet: Monica Stromann/the Noun Project