New Toilets Reduce Flushing: What Will Happen to Drains?

Traditionally toilets flushed 11 litres or 3 gallons of water every time they were flushed with the water all flowing along through a drainage system that was designed for large flows of water. Unless a drain was broken or roots entered the drain it was not common for a drain to block. That was of course until the advent of reduced flow dual flush toilets.

Now that toilet only flushes 4.5 litres of water on a full flush and 3 litres of water on a half flush into the drainage system. Coupled with reduced flows from every other tap this has produced a situation where drains designed for large flows of water have had the water flow reduced by 60% in most cases.

The increase in blocked drains due to the reduced water flows has been a big boost for plumbing companies with drains clogging up for no other reason than lack of water flow. Often there is a dual problem of incorrect fall on the drain which creates high spots in the pipe leading to the drain becoming clogged at that point.

In Singapore scientists at Nanyang Technological University (NTU) have invented a new toilet system that will reduce the amount of water needed for flushing by up to 90 per cent compared to current toilet systems. Dubbed the No-Mix Vacuum Toilet, it has two chambers that separate the liquid and solid wastes. Using vacuum suction technology, similar to those used in aircraft toilets it uses 0.2 litres of water for urine and 1.0 litres of water for solids.

The No-Mix Vacuum Toilet installed in a standard public toilet which is flushed 100 times a day, is expected to save 160000 litres of water a year. The NTU scientists are now looking to carry out trials by installing the toilet prototypes in two NTU public toilet facilities. This trial is important due to the issues that plumbers are already aware of with the drains blocking at 4.5/3 litre flushes so a 1.0/0.2 litre flush is expected to exacerbate this problem in existing buildings.

The No-Mix Vacuum Toilet with it’s two chambers that separate the liquid and solid wastes also will turn human waste into electricity and fertilisers. The No-Mix Vacuum Toilet will divert the liquid waste to a processing facility where components used for fertilisers such as nitrogen, phosphorus and potassium can be recovered. At the same time, the solid waste will be sent to a bioreactor where it will be digested to release bio-gas which contains methane. Methane is odourless and can be used to replace natural gas used in stoves for cooking. Methane can also be converted to electricity if used to fuel power plants or fuel cells. ‘Grey water’ (used water from the laundry, shower and kitchen sink) can be released back into the drainage systems without further need for complex waste water treatment, while leftover food wastes can be sent either to the bioreactors or turned into compost and mixed with soil, resulting in a complete recovery of resources.

The No-Mix Vacuum Toilet uses will be limited in traditional buildings but will be useful for eco resorts, new housing estates, hotels, resorts, and especially communities not linked to the main sewerage system or unable to be connected and so require their own sewerage facilities.

Associate Professor Wang Jing-Yuan, Director of the Residues and Resource Reclamation Centre (R3C) at NTU who is leading the research project, said that their ultimate aim is not only for the new toilet system to save water, but to have a complete recovery of resources so that none will be wasted in resource-scarce Singapore. “Having the human waste separated at source and processed on-site would lower costs needed in recovering resources, as treating mixed waste is energy intensive and not cost-effective,” Prof Wang said. “With our innovative toilet system, we can use simpler and cheaper methods of harvesting the useful chemicals and even produce fuel and energy from waste.”

The development of a No-Mix Vacuum Toilet has great potential for use in environmentally sensative areas where traditional sewerage systems cannot be installed.