The Life-cycle of FOG
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?
A sewer man hole blocked with fat.
Fat, oil and grease do not flow freely to the sewer in the street. Hot fat cools and sets solid on the pipe wall within seconds, and cooled oil spreads itself over a longer length 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 freely when it is hot, and becomes a slow ooze when it cools back again.
Household drain pipes from the house are sunk well under the ground on their way to the street sewer. They never get hot from the sun. Even in a heat-wave, the temperature of the drain pipes is always well below the melting point of fat and the free-flow point of oil.
Fat and oil don't dissolve in water. Hot water poured down the sink melts fat, but it cools and sets solid again a bit further on. Hot water heats up oil so that it will flow freely, but it cools and sticks again a bit further on. After a few metres, the hot water is so much cooled by the pipes that it won't have any effect.
So how does FOG ever get to the sewers?
The pipe walls are so tacky that food scraps stick to them. Fat and oil also coat 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 away by solid objects or it rots away. If the building up is faster than the scraping or rotting away then the drain or sewer will become totally blocked. Usually, it's okay.
But, every time someone pours a dose of FOG down the sink they run the risk of it plugging the last little hole and causing a blockage that requires the skill of a plumber to clear it. Every time someone puts an object that they shouldn't down the toilet or gully trap they run the risk of it plugging the last little hole too.
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. This requires urgent clearance and cleanup action, usually by the Local Authority, and at its expense. It isn't their fault but no individual is to blame so they have to pay for it.
The number of different items that can end up in sewer blockages is amazing. Southwest Water in the UK have compiled such a list, worst first.
Captured at the treatment plant
Finally, the lumps of FOG reach the sewage treatment plants, where they interfere with the bacterial treatment of the sewage.
The NZ North Shore City local authority estimates that more than 4 tonnes per day ends up at the wastewater treatment plant.
The lumps of FOG are removed and sent to the landfill at the expense of the Local Authority.
Or off to the sea
Despite everything, traces of FOG remain in the treated water that goes into the sea through the outfall.
So that which is not captured by the Local Authority ends its journey as pollution in the sea.