Discursos que circulan sobre salud mental, en términos de lo que se dice, se

Effluent Biomass crop Harvest /process Conversion plant Heat/power generation Digester Sort Recycle Landfill RDF Paper/plastic Organic material

Municipal solid waste

Digested sludge

Figure 31

Urban sewage treatment and MSW can both yield energy, while nutrients from sewage can be recycled to grow energy crops.87

CH4

CH4

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(OFMSW) and organics-rich industrial wastewater can also be used. The resulting biogas can be distributed via local gas grids, used as a transport fuel or converted to electricity by gas engines or micro-CHP plants. Landfill gas capture

Landfill gas can be used directly on site, either by burning to produce heat or as fuel for gas engines to generate power. The gas must be scrubbed to remove impurities before it can be used in an engine or injected into a gas pipeline.

Besides providing a source of renewable energy, capturing landfill gas also reduces greenhouse gases. Methane is a much more potent greenhouse gas than CO₂, so burning it has a net positive effect on emissions.

Landfill gas used to be seen as a problem, but recent advances in our knowledge of landfill behaviour and the decomposition of MSW have created interest in maximis- ing the amount of gas that can be extracted from landfills. Many countries are now essentially operating landfills as bioreactors to produce more gas and to stabilise the result- ing waste more quickly95_.

In 2008 the city of Vaasa in Finland hosted a visionary housing exhibition on the theme of sustainable energy for buildings. Vaasa has an old landfill, and from this came the idea of using landfill gas to supply electricity and heat to the houses and apartments in the exhibition. Finnish engine manufacturer Wärtsilä built a 20 kW_e power plant

based on SOFC fuel cell stacks from Topsoe Fuel Cell (Fig- ure 32). With a heat output of 14–17 kW the unit can sup- ply up to ten households.

The fuel cell power plant in Vaasa has run for more than 2,000 hours on landfill gas, with very positive results. The unit was dismantled, inspected and returned to service in autumn 2010. It then ran for a further 5,000 hours before the next inspection in the first quarter of 201196_.

Biomass-fuelled small-scale CHP

Small-scale CHP units based on Stirling engines (Figure 33) convert solid biomass to electricity and heat very effi- ciently. They can be used in large urban buildings wherever a supply of biomass, such as wood chips, is available within easy reach. Read more about Stirling engines later in this chapter.

Algal biomass

An innovative way to produce biomass in future cities could be to convert flat rooftops into facilities for growing blue-green algae (cyanobacteria). At the right temperature and with the right nutrients algae can grow far more rap- idly than other forms of biomass, so they could be continu- ally cropped to produce biofuels or to run small CHP plants. Roofs could be designed to harvest water, collect solar energy and grow algal biomass at the same time97_.

Figure 33

35 kWe SD4-E bio-fuelled Stirling engine manufactured by Stirling.dk. Picture: Stirling.dk

Figure 32

Wärtsilä 20 kW SOFC fuel cell power plant using gas from an old landfill to provide sustainable electricity and heat in the Finnish city of Vaasa. Picture: Wärtsilä.

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Urban agriculture

According to the UN Food and Agriculture Organization (FAO), “urban agriculture” describes the production of crops and livestock within cities and towns and their sur- rounding areas. Urban agriculture can involve anything Biodiesel can also be produced from microalgae grown in

sewage treatment plants, taking up nutrients from the wastewater and so helping the water purification process98_.

Figure 34

Vertical farming, design by Blake Kurasek. Illustration by Blake Kurasek.

Biogas Digester - Methane and carbon dioxide by-products. Methane & CO2 tanks - Methane used for heating biogas digester, apart- ment cooking and heating. Black water gravity filtration system through perimeter hydro- ponics Grey water collection

& sand filtration system

- Used to irrigate soil crops and flush toilets. Black water collection - From building sewage. Blackwater liquid and solid separation tanks - Solid waste dried in kiln for fertilizer. - Liquid separate sent to hydroponic filtration loop.

Black water gravity fed filtration system through perimeter hydroponics - Nutrients removed by plants as source of fertilizer. - By-product of fresh water used in gray water collection or safely returned to Lake Michigan.

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from small vegetable gardens in back yards to farming of community land by associations or neighbourhood groups. It is commonly practised on fallow public and private spaces, wetlands and underdeveloped areas; rarely is it found on land specifically designated for agriculture. In many countries urban agriculture is informal and some- times even illegal. Competition for land is a frequent source of conflict. Other contentious issues include the environmental impact of urban agriculture and food safety concerns, particularly relating to livestock production. While data are scarce, urban agriculture is an important reality in many developing countries. Up to 70% of urban households participate in agricultural activities, according to the first systematic quantification of urban agriculture conducted by the FAO. The study is based on data from 15 developing and transition countries for which comparable statistics are available99_.

Urban agriculture seems particularly important in low- income countries. But even in more developed economies, a significant proportion of urban households are involved in farming.

With few exceptions, poor urban dwellers are more likely to participate in crop and livestock production than richer

households. In many countries more than half of all urban households in the poorest expenditure quintile rely in part on their own agricultural activities to satisfy their food needs. Urban agriculture has been made smarter through the development of new concepts like “vertical farming”, a greenhouse-inspired concept that scales up a niche tech- nology (Figure 34, Figure 35)100_{. Another example is Aero-}

Farms, a company based in Ithaca, NY, USA that has devel- oped an indoor, soil-less system of urban agriculture. By growing leafy greens in a cloth medium under LED lights, the technique eliminates the need for sunlight and pesti- cides, and dramatically reduces water consumption (see also De Groenten uit Amsterdam in chapter 5.

These concepts make farming smart and distributed, in the same way that energy is becoming smart and distributed. That paves the way for urban farming as a key concept in smart cities. Urban farming can rely on the availability of organic waste, heat and electricity produced within the city. The distances over which the food will need to be trans- ported are small, and the food will be fresher and healthier since it will not need to spend days or weeks in transit.