What does FOG do in its journey to the sea?
Down the household pipes
The drain pipe from the house to the street has a diameter of only 100mm, the same diameter as a margarine tub.
The pipe from the kitchen sink to the drain is only 40mm in diameter.
What happens when FOG is poured down the sink into the household drain pipes?
Fat, oil and grease do not just flow down to the sewer in the street. Hot fat cools and sets solid on the pipe wall within seconds, and cooled oil spreads itself over the first few metres of pipe. Some of the oil will be pushed along by the force of the water into the street sewer, but most won't go far beyond the plug hole.
Why won't FOG flow down into the sewers?
Fat melts when it is hot, and goes solid when it cools back again. Oil flows easily when it is hot, but oozes slowly when it cools back again.
Fat and oil don't dissolve in water. People pour hot water down the sink to remove FOG, but that water is cooled down by the pipes after a few metres, so it won't have any real effect. It just moves the gunge along.
So how does FOG ever get to the sewers?
The pipe walls are so tacky that food scraps and sanitary products stick to them. Fat and oil coats any tree roots in the pipe leading to the street,
forming a sticky web. The sticky areas catch small food scraps and any small objects that are put down the toilet.
Either more food scraps come and scrape some of this away, or rotting bits fall off and are flushed away.
Why aren't blockages happening all the time?
The gunge builds up inside the pipe, with all sorts of objects stuck in it. It gets scraped away by solid objects or it rots away. If the scraping or rotting away is enough then the drain or sewer will still work okay. If the buildup is faster than the scraping or rotting away then the drain or sewer blocks up totally. Usually, it's okay.
Through the sewer under the street
The FOG poured down the sink by the all the households in the street clumps together into large lumps of gunge in the sewer pipe under the street.
These clumps are occasionally big enough to cause a blockage. Sewage then backs up and flows out through the lowest point in the system above the blockage. This can be a sealed manhole, a gully trap or a toilet inside a house.
Captured at the treatment plant
Finally, the lumps of FOG reach the sewage treatment plants, where they are removed. They interfere with the bacterial treatment of the sewage and can't be allowed out into the river or sea.
The NZ North Shore City local authority estimates that more than 4 tonnes per day ends up at their wastewater Treatment Plant.
The lumps of FOG are removed and sent to the landfill at the expense of the Local Authority, who charge you for doing it.
Or off to the sea
Despite everything, traces of FOG remain in the treated water that goes into the sea through the outfall.
See our Keep a Healthy Sewer Near You! (88k pdf) flyer for more information.
Next
For more information, go to the Life-cycle of FOG page.
For information on how to prevent FOG, go to the What to do right page.
FOG isn't all that goes down your the drain. There's also Sewer Litter.