Rethinking buildings to make them net positive

The net zero challenge focuses on cutting energy and carbon, but buildings could have a positive impact on the environment, says Aecom’s Dave Cheshire

The basic concept of sustainability is that the needs of the present must be met without compromising the ability of future generations to provide for themselves. Population growth and dwindling resources, however, mean this definition is no longer valid. At the present rate of use, resources will run out.

We must design and construct structures that make a positive contribution to the planet. We can no longer think of buildings as consumers of resources. Groups are already looking at how buildings can make a net-positive contribution to the planet. Two ambitious initiatives stand out: the Living Building Challenge,1 which promotes buildings that generate more energy than they consume; and Cradle to Cradle,2 which prescribes design solutions for the elimination of waste.

Both initiatives advocate reimagining buildings as components that contribute to the planet, in terms of energy or regenerative architecture that renews nature. Whether it’s cleaning the air and releasing clean water, or adding to the biodiversity and providing sustenance to the biosphere, the ideas take inspiration from nature, where waste from one system becomes a nutrient for another.

CIBSE has produced a guide on how to respond to the sustainability agenda, for which I was a principal author, along with CIBSE’s Julie Godefroy. CIBSE Guide L: Sustainability3 includes sections on incorporating ecosystems into design. It redefines waste as a resource and takes a whole-system approach to design that considers the wider impacts on infrastructure.

Implementing net-positive buildings requires new principles. These include:

  • Systemic thinking, with buildings as part of ecosystems
  • Mapping resource flows and determining how waste from one system becomes a resource for another
  • A holistic approach to environmental and social topics that avoids the trap of considering each topic in isolation
  • Designing out complexity and using solutions that do not require layers of technology.

With this new perspective and these principles, let’s consider two examples of systems that are used in the majority of our buildings – the disposal of effluent and the provision of breathable air – and see what could be done differently.

Resource capture

The principles of toilets have not changed in more than 150 years, except that they have become more water-efficient. Looking at them afresh raises many questions. We collect, store, treat and distribute water to a level that it is so clean that it can be drunk and accessed in virtually every building in the UK – but we only drink 10% of the supply.

More than 40% of water (in a typical office) is mixed with faeces, urine, bleach and paper, and flushed down the drains, where it is often combined with freshly fallen rainwater. This warm, nutrient-rich effluent is transported to sewage works, where it is filtered and treated so it can be released back into the biosphere.

To make a positive contribution from our built environment, we need to redesign the system to avoid water use and capture nutrients. Composting toilets have been around for a long time, but they conjure up images of music festivals and isolated long drops in Canadian forests.

In a radical move, the Bullitt Center building4 in Seattle installed composting toilets in an office building as part of an objective to achieve the Living Building Challenge. They look like any other toilet, yet use virtually no water and result in nutrient-rich compost.

Sewage treatment plants are an efficient way of treating effluent and this makes sewage treatment in buildings hard to justify. However, the ability to avoid using water in toilets and to capture the nutrients from waste – and use them locally – may just tip the balance. It is possible to collect urine from (waterless) urinals and use urine separation in WCs, turning a waste product into a valuable resource. Urine is an excellent source of nitrogen, phosphorous, potassium and trace elements for plants5 and could even be used to make new materials and generate electricity.

Another key resource flow in our buildings is air. We pump our air full of particulates, nitrogen oxides, sulphur dioxide and many other pollutants that cause respiratory diseases, lung cancer, heart disease and more. We drag this polluted air into our buildings, force it through filters and around tight bends. We heat it up and cool it down, then push it through pinch-points and labyrinths of ductwork. The internal environment is loaded with volatile organic compounds that further pollute the air. This CO2-rich, warm air is recirculated via dusty ceiling voids before being expelled back into the environment. Fans use huge amounts of energy forcing the air around the building.

How could we do better? For starters, we could be more conscious about the air we breathe. The radical reduction in air pollution in our cities during the Covid-19 pandemic has given us a glimpse of how a future of sustainable transport could look. To clean up the city air we need to put an end to polluting fuels – from petrol and diesel-powered transport to diesel generators and wood-burning stoves.

Liberated from concerns about noise and air pollution, designers and buildings operators could consider other forms of ventilation, such as windows and mixed-mode ventilation. Where mechanical ventilation is necessary, we could design the building around the ductwork, creating generous voids for efficient, effective airflow – we did this for the University of Nottingham’s Centre for Sustainable Chemistry to create a net-zero carbon laboratory building. We would design out indoor air pollutants by selecting better materials and finishes. Finally, we could think of uses for the warm, moist, CO2-laden air we expel from buildings, such as the ICTA Rooftop Greenhouse Lab in Spain.6 This uses CO2 from the building it sits on to boost crop yields. In return, plants reduce the CO2 content of the air, which could be fed back into the building. 

Of course, using plants to clean the air is not new. Because of security concerns, little natural light enters New York’s Public Safety Answering Center,7 where workers take emergency calls 24 hours a day. Aecom installed a green wall that cleans the air. It uses phytoremediation to trap particulates, VOCs and CO2, releasing clean, oxygenated air back into the building. Perhaps, in future, plantrooms will be full of plants!

A new paradigm

Net-positive buildings can only be achieved by changing our perspective. Instead of individual structures, we should be thinking in terms of whole systems. The aim should be to optimise the overall efficiency of the whole system, instead of treating one part in isolation. We should be looking for opportunities to turn what is treated as waste in one system into a nutrient or resource for another.

We need to reach beyond our defined roles and understand the wider context and implications of design decisions. We have a lot to learn from nature, which can teach us how to restore some of the symbiotic relationships that we have with the biosphere.


1 Living Building Challenge 

2 William McDonough and Michael Braungart, Cradle to Cradle: remaking the way we make things 

3 CIBSE Guide L: Sustainability

4 Bullitt Center

5 Urine: the ultimate ‘organic’ fertiliser? Ecologist 

6 The water exchange between the greenhouse and the building of the RTG-Lab 

7 This living wall cleans the air inside New York’s new emergency center, Fast Company