Wast(ing) water

Twenty to forty percent of the water consumption in sewered cities is used for flushing toilets. This is often potable water brought to the cities at high cost. While the daily water use in industrialised countries ranges from 150 to 250 liters per capita, the volume of our excreta (urine and faeces) amounts to a mere 1.5 litres per capita per day, constituting less than one percent of the wastewater volume. But note what this one percent excreta contributes: the majority of the pathogens in wastewater, 90 percent of the nitrogen, 80 percent of the phosphorus and about 50 percent of the organic matter. These are in fact the major substances of concern regarding health problems and water pollution from sewage. Do we really need to dilute them in large quantities of water and spread pathogens to both waterways and groundwater?

Wastewater as a resource
Nitrogen and phosphorus are valuable plant nutrients, and phosphorus is a limited resource that the world will be running out of in some decades. A century ago these resources were recycled. In cities, the night soil was collected, sometimes mixed with peat and/or lime, and used as fertiliser. With the invention of the water toilet about 150 years ago, and development and installation of subterranean gravity sewer systems, these resources began being discharged to water, causing pollution. When Paris introduced flush toilets and sewers in the late nineteenth century the question arose, “What would happen with the vegetable production in the city outskirts?”

Do we need a 150-year-old technology?
We are still building and improving our sewer systems often at very high cost. But is it necessary to continue using a system developed 150 years ago? Would we have chosen the same water consuming system today if we had no sewers and were planning a new system? The paradox is that we are doing so, despite the interesting alternatives which exist and are emerging.  

Large parts of the world face water scarcity
and over 40 percent of the world population are in the situation where they have no sewers or no sanitary systems at all. Should they all be provided with standard flush toilets and large collecting sewers? Particularly in dry climates the water need alone will make conventional sewer systems very expensive and often not feasible.

Volvo engineers show the way
When Volvo was designing its new conference and recreation centre on a beautiful fjord north of Gothenburg they had to design a near zero-emission wastewater treatment system in order to obtain a building permit. Conventional systems were simply not good enough. The Volvo engineers then designed one of the first multi-apartment source separating systems in the world in the early 1990s. They used low flush toilets for collection of excreta, and combined this with a biogas reactor that received the excreta and grinded organic household waste. From the nutrient rich output from the biogas reactor they produced struvite – a mineral that contains both phosphorus and nitrogen, and returned this as fertiliser to neighbouring farms.

The Volvo engineers had understood that the majority of the pollutants were in our excreta. But they also realised that waste was a potential resource for agriculture. By collecting waste using a minimum of water they could both meet their discharge requirements and turn the pollutants into valuable resources – fertiliser and biogas!

To achieve this, the Volvo engineers were using a source separating system. Over the last two decades source separating technology has been developed mainly in smaller systems. This technology, however, is now ready for upscaling.

Will source separation work in a large city such as New York? Imagine a skyscraper where the water for toilet flushing has to be lifted hundreds of meters. If you could lift one litre instead of 10, large amounts of energy will be saved. By using modern vacuum toilets the flush volume can be reduced to about one litre. Vacuum toilets are already standard on all modern cruise liners, some having more than 1500 toilets. The collecting vacuum systems are becoming very energy efficient, making substantial water and energy savings possible. The development of vacuum systems in cruise ships have therefore paved the way for using such systems in any terrestrial building complex.

More than 90 percent water saving
More water could be saved if greywater is treated and recycled. Greywater is water from showers, sinks, kitchen and washing. Greywater is very low in nutrients and often meets drinking water requirements for nitrogen without further treatment. Commercial systems suitable for installation in the basement of large building complexes have been developed by German companies and are used, for example, in some new buildings in Arabian countries. In Oslo, the capital of Norway, greywater from 33 apartments at Klosterenga is treated to bathing water quality in a beautifully landscaped compact natural system in the courtyard of the building. The treatment area is partly utilised as a playground for children. The high quality effluent in the Norwegian system is made possible using a light∞weight aggregate especially developed for treatment wetlands. The excellent effluent quality allows direct reuse of the water for irrigation, groundwater recharge or for some in∞house applications. Water from such systems often is of better quality than many raw water sources currently used for drinking water production. In order to upgrade to drinking water, compact membrane systems designed for this purpose are already on the market. If treated greywater is reused for in-house applications more than 90 percent reduction in water consumption is possible. In arid areas this option is particularly interesting.

In addition, international research shows that source separating systems give an equal or higher reduction of pathogens than a traditional sewer systems, and a significant reduction in risk of exposure to pathogens. A major reason for this is that the resources are collected using a minimum of water and not mixed into the water cycle in the first place. This could therefore make an important contribution to the improvement of health particularly in the developing world, where conventional treatment systems are expensive, difficult to maintain, and simply not viable.

Leapfrog conventional systems
Source separating systems provide a decentralised treatment option. The systems can be implemented house by house or block by block in urban or peri-urban areas. The locally treated greywater, if not reused, can be discharged to the nearest storm drain, stream or river, thus reducing the need for secondary sewers that are the largest expense of a sewer system. In cities where secondary sewers are not yet established, such as Kuching in Sarawak Malaysia with a population of 0.5 million, substantial cost savings would be possible using a decentralised source separating system as compared to building new sewers in an existing city. Additionally, a source separating system would provide potential for large water savings as well as production of bioenergy and fertiliser for local agriculture. This is in fact a much more efficient and sustainable option, particularly as compared to centralised conventional options based on secondary treatment, where the nutrients would be lost to the sea. There are many cities like Kuching with no established sewer system. For such cities a decentralised approach would be worth considering. They have the option to leapfrog 150-year-old water consuming conventional technology and move right into a decentralised water saving system with potential for water and nutrient recycling and bioenergy production, provided they get access to this technology and are able to adapt it to their specific economic, environmental and cultural contexts. The challenge is now to find the most effective mechanisms to make such a shift in approach possible, and thus contribute to sustainable development both locally and globally.

Further information: www.umb.no