Septic tanks, drains and dry toilets: what EM can and can't do

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“A capful of EM in the toilet once a month and your septic tank will run like clockwork.” You often come across advice like this, on forums and on some product labels. The reality is more nuanced. A well-designed tank needs no additive at all, and no product can replace emptying it.

EM can nevertheless be of some use in household drains and dry toilets, particularly against odours. This page sorts out what you can reasonably expect from them and what you shouldn't ask of them.

How a septic tank works

The septic tank, which in France is now called an “all-waters tank”, receives all the wastewater from the house. Inside the tank, without air, the contents separate into three layers:

• at the bottom, the sludge: the heavy matter that settles;
• in the middle, the water, partly clarified, which flows on to the drainage field or filter;
• on the surface, the scum layer: fats and floating matter.

The bacteria naturally present in wastewater break down part of the sludge and produce gases, which escape through a vent pipe above the roof. But some of the matter does not break down: sludge inevitably builds up. That is why emptying is compulsory once the sludge reaches 50% of the tank's working volume, on average every four years depending on the size of the tank and the number of occupants.

Did you know?

The wastewater of a single person contains billions of micro-organisms every day. The tank is never short of them: it seeds itself, from the first few days of use and after every emptying.

Biological additives: what the studies say

The market for septic tank “activators” is long-established and thriving. Yet independent trials of these products, whether based on bacteria, enzymes or yeasts, have shown no lasting benefit to the way a tank works. In a long series of trials on pit latrines, none of the twenty additives tested in the laboratory significantly speeded up the breakdown of sludge, and in the field, treated pits filled up no more slowly than those that received water alone. The main reasons are well known:

• the quantity of bacteria added is negligible compared with the amount that arrives naturally every day with the wastewater;
• wastewater is already rich in nutrients: there is no need to “feed” the microbial life;
• the microbial population recovers on its own within a few days after emptying or occasional use of a strong household product;
• some additives, by speeding up decomposition, can even stir sludge back into suspension, which then goes on to clog the drainage field.

To be fair: most of these studies looked at other commercial additives. However, an Australian trial in the early 2000s did test EM, in a sewage treatment plant and in several septic tanks: it found no clear benefit either on sludge volume or on the suspended solids discharged by the tanks. No independent trial shows that EM delays the need for emptying. Our page What science says about EM puts these results in the context of current knowledge.

Key points

A correctly sized, well-ventilated tank that is emptied on time needs no additive. No product, EM included, replaces emptying.

What EM can reasonably do

In household drains

Pouring the leftovers of your EM Clean dilutions down the sink, shower or toilet, as the manufacturer suggests, is harmless to a septic tank and may help limit odours in the traps. The quantities are tiny and highly diluted. An independent review of microbial cleaners in fact judged these products more useful for maintaining traps and drains than for everyday cleaning. Dilutions are explained in our page Natural cleaning with EM Clean.

What you shouldn't ask of them

• Unblocking a drain: a blockage should first be dealt with mechanically, using a plunger, a drain snake or by dismantling the trap. Biological products act far too slowly for this.
• Getting rid of a smell around the tank: a smell near the tank, the inspection chamber or in the garden is a sign that something is wrong. Check the ventilation, the lids and the sludge level, and call in your local wastewater service or a professional.
• Emptying less often: only measuring the sludge level determines when it's due.

If you have a package treatment plant or compact system, read its user manual. Some manufacturers rule out products that are too acidic or too alkaline. As concentrated EM is acidic, only pour in dilutions, never a neat container.

Emy's tip

Never pour cooking fat down the sink. Let it set in the pan, wipe it out with kitchen paper and put that in the bin: it's the single most effective thing you can do to protect your pipes and your tank.

Looking after your tank day to day

• Flush nothing but toilet paper down the toilet: wipes, sanitary products, cotton buds, medicines and chemicals (solvents, paints, oils) are all off limits.
• Use strong household products sparingly: bleach, drain unblockers, descalers. Occasional use is fine; heavy daily use is not.
• Spread your laundry over the week rather than doing load after load on the same day.
• Have the sludge level checked and keep your emptying receipts. In France, the installation is inspected periodically by your municipality's public off-mains sanitation service.

Dry toilets: sawdust, EM or fermentation?

The classic sawdust dry toilet

In a bucket dry toilet, each use is covered with a good handful of sawdust, wood shavings or dry shredded material. It is these carbon-rich materials that absorb moisture and limit odours. Properly managed, a dry toilet smells very little. If it smells strongly, it is almost always because there isn't enough dry material, the bucket is too full, or urine is pooling.

Urine is in fact the main source of odour: mixed with faeces, it quickly turns into ammonia. Urine-diverting toilets, which collect urine separately, smell noticeably less.

What EM can contribute

A light spray of EM in the bucket, on the sawdust, or some EM Clean deodoriser on the seat and around the bucket, can help limit odours, alongside a sufficient amount of dry material. For cleaning the bucket and bowl, a dilution of EM Clean multi-purpose cleaner works well.

You should, however, be aware of what research has shown on lactic fermentation toilets, where the aim is to ferment the waste in a sealed bucket, rather like bokashi:

• without a source of sugar, EM added on its own did not lower the pH or reduce odours;
• with molasses, fermentation did take place, but ordinary lactic acid starters did at least as well as EM, and the researchers also used plain sauerkraut juice as a starter;
• when urine was mixed with the waste, the environment instead became alkaline and the smell strong;
• adding charcoal, on the other hand, toned down the sour smell of fermentation.

These systems remain experimental and require real know-how. The principle of fermentation, and what makes it succeed, is explained in our page How EM works.

And then?

Whatever the system, the contents of a dry toilet must be composted separately, for a long time and under good conditions before any use. Neither sawdust, nor EM, nor fermentation makes it directly usable in the vegetable garden. If you already make bokashi in the kitchen, keep the two systems strictly separate: the bokashi bucket in our bokashi guide is for food waste only.

In summary

A septic tank works on its own, thanks to the bacteria brought in every day by wastewater: additives, biological or otherwise, have shown no lasting benefit, and nothing replaces emptying at 50% sludge. Pouring your leftover EM Clean dilutions down the drains is risk-free and may limit trap odours, but a smell near the tank signals a problem that needs checking. In dry toilets, dry material and urine separation do most of the work; EM is only a complement, and fermentation only works with a source of sugar.

Related products

EM Clean, multi-purpose EM cleaner | Classic - 2 L Bag-in-box

For the home, outdoors, animals... from 2 to 250 L

£13.83 €16.58

EM Clean, EM deodoriser | Mint

Naturally neutralises bad odours

£10.78 €12.92

Other topics in the guide

Vermicomposting30 sheets →Composting25 sheets →Bokashi24 sheets →