Sewage, also called blackwater, is a complex mixture of contaminants containing pathogens, toxic chemicals, heavy metals, debris, nutrients, nitrates and phosphates. It is wastewater from both domestic and industrial sources, from faeces and urine, to bleach and cleaning products, paint, solvent and oils – everything that is flushed down a toilet or poured down a drain.
Most homes in the UK are connected to the mains sewers, whereby sewage travels through a system of pipes to be collected by an industrial treatment plant. It is then treated with chemical and mechanical processes to remove contaminants and separate sludge from liquid. The liquid is discharged into the nearest large body of water or reused, and the sludge is incinerated or partially used in agriculture as a controlled fertiliser.
In the UK there are strict government regulations and standards related to sewage collection and treatment, but still 300 million gallons of raw or partially treated sewage is discharged directly into the sea, destroying aquatic life and posing significant risks to human health. Worldwide sewage dumping is an even bigger problem. This environmental threat, combined with the energy intensive processes of conventional sewage plants, has facilitated the development of sustainable sewage systems.
Ecological Sewage Systems
Ecological or biological sewage systems are based on natural ecosystems that use ecological processes for water purification and nutrient recycling. Their exact design is dependent on location, climate and population, but all involve wastewater being passed through a managed or constructed environment where a diversity of plant and animal organisms transform the waste in the water.
Aquatic plants such as water hyacinths, reeds, rushes, lilies, and duckweeds, break down toxic chemicals, nitrates and phosphates through their root microbes, and bioaccumulate heavy metals in their stems and leaves. These plants are an integral part of an ecologically-engineered whole system that provides an oxygenated environment in which bacteria, fungi, snails, fish and aquatic invertebrates can thrive and also play their part in the water purification process. There are two main types of ecological sewage systems: constructed reed beds, and solar aquatic systems.
Constructed Reed Bed Systems
Also known as artificial wetland systems, Constructed reed bed systems are the reconstruction of freshwater wetland ecosystems to treat wastewater. They are often used in combination with other conventional treatment facilities, including septic tanks. There are three basic types of wetland construction:
- Horizontal Flow – wastewater is continually fed through an inlet / outlet gradient system
- Vertical Flow – wastewater is applied in batch and allowed to drain each time
- Pond Systems – a series of shallow ponds linked by a constructed wetland container
Most reed bed models are land-intensive, but they are highly energy-efficient (requiring no energy for treatment processes) inexpensive to build, low maintenance, productive, have minimal sludge generation, are aesthetically appealing, and create valuable habitat for wildlife.
Solar aquatic systems model the processes of wetland ecosystems in a controlled and intensified environment. Anaerobically treated wastewater is passed through a series of tanks in a greenhouse, using sequenced ecologies of specified aquatic plants and animals to break down sewage. The resulting water is pure enough to be used for non-drinking purposes and can be safely discharged into the environment. Dr. John Todd of Ocean Arks International pioneered this technology in a patented design called ‘Living Machines’. He has now developed this into the design of restorers, assemblies of engineered ecologies that can be floated as a raft to treat sewage in constructed canals or lagoons.
Biogas plants are being developed in the UK and Europe in combination with conventional sewage plants to capture gas from sludge and introduce it into the National Gas Pipeline. They are already used widely in developing countries as complete sewage systems. In a biogas plant, sewage is piped into a sealed container called a digester, where it ferments producing a mixture of methane and carbon dioxide along with slurry. The anaerobic conversion process destroys pathogens and renders the slurry harmless and odourless, so it can be used directly on the land as a fertiliser. The methane is piped directly out from the top of the digester.
Sewage: A Valuable Resource
Ecologically designed sewage systems are a good example of sustainable design. They are energy-efficient, inexpensive, effective and environmentally friendly, and can be applied at any scale, from a single home to a large city. Yet this technology fails to realise the full potential of sewage as a resource. Although sewage contains contaminants, it also holds nutrients that can be used to improve soil fertility, along with the ability to produce natural gas. Technologies such as biogas plants that maximise sewage as an energy and nutrient source need to be developed on a global scale. It’s time that sewage was viewed as a valuable resource, and not just a problem to be treated.