What does flushable mean?

Most of us know (I hope) that we should only put the 3 'P's down the loo (pee, paper and poo). However, sewer blockages prove that we often ignore this advice [1]. So what does it mean when an item is labelled as flushable? I found a protocol for testing this [2] which is a bit long in the tooth (2008) but I doubt it has changed hugely since then.

This post is not really about energy at all, rather plastic and how to avoid it blocking up our sewers. I have experienced a sewer system clogged with wet wipes and believe me it was smelly. I did not actually have to get into the pipe to clear it; I just viewed it from the ground level but the sewage worker did get in, wearing rubber waders. I did not envy him. This particular blockage mainly consisted of wet wipes which are a common cause of blockages as they contain plastic and are not fully biodegradable. These did not come from us but from a neighbour; we have a shared drain.

I also discovered an alternative approach which is to apply something to ordinary toilet paper to make it 'wet'. We know that ordinary toilet paper disintegrates thoroughly when wet and does not harm to the sewer system. For example Wype is a gel that you can use to effectively turn toilet paper into a safe wet wipe. (by the way I do not get paid for advertising or endorsing anything and I have not actually tried the product myself either but it does seem like a very good idea. There is a question as to the cost of these relative to the wet wipes they replace.

Context

The law anticipates that domestic sewage contains waste from toilets, and water that has been used for cooking and washing only; in addition it is illegal to dispose of anything to the sewer that may cause harm to the sewer, or interfere with the free flow in the sewer. 

As I understand it, the test protocol can be summarised as:

1. Does the sample get past the toilet and through the drains 98% of the time.
2. In case it does not, does it disintegrate within 48 hours?
3. After disintegrating, can the remains do harm to treatment plants?
• Such as clogging up the screens used to prevent large particles from entering.
• The screens are periodically cleared by backwashing
• If the particles are very sticky or fibrous this could make backwashing ineffective.

Tests on the item being flushed

• Is it likely to be contaminated with faeces or other organic matter?
• If not, it does not need to go down the toilet anyway.
• Is it safe for the receiving environment (typically soil and/or water)
• For example it could be used as fertiliser on a farm
• Does it break up in the drain within 24 hours leaving material that does no harm to treatment plant or pumps.

Tests with the drainline rig - a simulation of the drains from a dwelling into the sewers: these assume the flush uses 6l of water and the drain (10m long) slopes at 1:80. Also there is a 30cm section of pipe with sharp grit attached (i.e.. glued) to the bottom to mimic sediment. This section starts 8m from the sewer entry point so near the end. The test is repeated 100 times.

• WC Flush Clearance test. Does it clear the toilet?
• If so: Drainline test: Does it clear the drainage system.
• > 98/100 pass - OK
• < 90/100 pass - fail
• between 90 and 98/100 - do a disintegration test as described below.

The product is considered to have sufficiently disintegrated if:

• 100% (by dry weight) passes through a 10mm sieve and
• 80% (by dry weight) passes through a 5mm sieve within 24 hours
• NB. anything bigger than 6mm is regarded as a pollutant. Most waste water treatment plant inlets and combined sewer outlets have a 6mm sieve.
• For a sewer disintegration test, the sample is subjected to 48 hours of gentle agitation, to simulate turbulence in a sewer on the way from the household drains to the treatment plant.

At this point the material will hopefully contain only small, harmless, organic particles. In the treatment plant these particles will be formed into sludge and could ultimately be used as fertiliser on farms which means mixing them up with soil. They should be unrecognisable within a short space of time and be safe for soil organisms.

This summary of the protocol is a bit confused in places partly because I am. The document mentions a decision tree but I did not see one. However, the purpose of each testing stage is clear enough and together they seem (to me) to cover most conceivable eventualities. It is encouraging that the tests are so comprehensive.

References

[1] Public urged to bin ‘harmful’ wet wipes as over a fifth admit to flushing down the toilet (Water UK) Feb 2023

[2] Test Protocol to Determine the Flushability of Disposable products (Water.org.uk) March 2008