Septic Tank Treatments, Tablets and Enzymes: Do Additives Actually Work?

For a normal household septic tank, no. The bacteria, enzyme and yeast treatments sold at Bunnings, supermarkets and online are not needed, because your tank already holds far more of the right bacteria than any dose can add. NSW’s own owner guide says a well maintained system receiving the right amount of wastewater should not need additives, and the US Environmental Protection Agency goes further and does not recommend them at all.

That doesn’t mean every product is a scam or that every one will hurt your tank. Some do reduce scum and sludge inside the tank. The trouble is where that material goes next, and that no additive can remove the sand, grit and other solids that only a pump-out takes out.

This guide goes deeper than the short additives section in our septic safe products guide.

What’s on the shelf, and what it promises

Septic treatments come as powders, liquids, sachets and flushable tablets, and nearly all fall into a few types.

If you searched ‘septic tank tablets’, note that AWTS chlorinator tablets are a different product: they disinfect treated water just before irrigation. They are not a septic treatment, and don’t belong in a conventional tank. Our guide to AWTS chlorine tablets covers them.

What NSW’s own guidance says

NSW’s Easy Septic Guide, the owner booklet available from the Office of Local Government, answers the question under the heading ‘Magic enzymes … do additives work?’. Its answer is short. Additives such as enzymes and cleansers are only suitable for problems that are minor and temporary, and it gives two examples: antibiotics in the system, or occasional water overuse. A well maintained system that receives the correct amount of wastewater should not need them. And the guide adds that no additive will help a system that needs pumping out or has a failing trench.

The current Onsite Wastewater Management Guidelines (April 2025), which replace the 1998 ‘Silver Book’ many older approvals were issued under, don’t mention additives at all. What they do recommend is measuring sludge and scum every year and desludging on that evidence.

So the NSW position is not ‘never’. It is ‘you shouldn’t need it, and it won’t fix a full tank or a failing trench’.

What independent testing found

Most of the independent research we could find is American.

The US EPA. Its Septic Tank Additives Fact Sheet (updated 2024) says additives are not recommended for household wastewater because septic systems already contain bacteria, enzymes, yeasts, fungi and other microorganisms. Its reasons: some products may interfere with the breakdown of waste, contribute to clogging and contaminate groundwater; they’re unnecessary; and many have only limited third-party research behind the claims. On biological products specifically, the EPA accepts that some can reduce scum and sludge in the tank, but says the long-term effect on the soil’s ability to treat wastewater is unknown, and some studies suggest the broken-down material changes the quality of the effluent leaving the tank.

Washington State University Extension. Its Clark County fact sheet points out that the bacteria or enzymes in a dose are small compared with what’s already in the tank, so they add little, if anything. If something toxic has killed off the bacteria in your tank, the new bacteria you pour in will likely die too. It also cites a study of 48 septic tanks that found no difference in sludge levels between tanks that used bacterial additives and tanks that didn’t.

University of Kentucky. A 2012 extension publication concludes that additives have little positive effect on septic performance and in some cases may be harmful, and advises pumping every three to five years instead.

The Ontario Onsite Wastewater Association, an industry body, is more open. It accepts a bacterial product may help a tank knocked around by medications or heavy cleaning products, but says added bacteria compete with the adapted ones in a healthy tank, enzymes have little scientific support, and independent studies show additives don’t stop sludge building up.

Put together, the picture is consistent. Additives may do something in the tank, but they don’t replace pumping, and none of these sources found evidence that they make a system last longer.

Why no product can replace a pump-out

The sales pitch for many treatments is that bacteria will ‘eat’ the sludge so you pump out less often. The problem is what sludge is made of. Part of it is material no bacteria or enzyme can break down: sand, grit, dirt and bits of plastic, as WSU and the Ontario association both point out. That fraction builds up whatever you add, and only a vacuum truck removes it.

The NSW guidelines set the trigger for a pump-out on measurement, not on a calendar or a product. A tank must be desludged no later than when sludge and scum together take up a third of the tank’s depth, or when either comes within 150 mm of the bottom of the inlet or outlet. For an appropriately sized tank with full occupancy, the guidelines say every three to five years is common. How to work out your own interval is in how often to pump a septic tank in NSW.

For most Hunter homes a routine septic tank pump-out is $350 to $500 for a tank up to 3,000 litres, and $400 to $650 for a 3,500 to 4,500 litre tank. Compare that with a year of monthly doses before you buy.

Yeast, sugar and the dead possum: the home remedies

Yeast. A common tip is to flush a packet of baker’s yeast down the toilet every so often. The Ontario association notes that yeast breaks down starches, but starches are only a small share of the waste in a septic tank. Clemson University’s home and garden extension service is blunter: there is never a need to add yeast, sugar or raw meat to a septic system.

Sugar and raw meat. Same answer. The idea is to ‘feed’ the bacteria, but Clemson’s fact sheet says all the microbes needed for decomposition are already present in human waste.

The dead possum. NSW’s Easy Septic Guide has a section on this one. On farms, a dead possum or sheep was sometimes thrown into a new tank to ‘kickstart’ it. The guide’s verdict is that the sewage flowing in from the day the house is connected supplies plenty of nutrients for the bacteria to start work. A carcass adds nothing.

Lime. One home treatment does appear in the official guide. For starting up a new or freshly pumped tank, the Easy Septic Guide suggests filling it with clean water and adding a cupful of lime down the toilet each day for seven days, saying it helps with odour and raises the pH, which encourages bacterial growth. It suggests the same if a tank turns smelly. The newer 2025 guidelines don’t repeat it. If you’re on an AWTS, check with your service agent before adding anything, since the aeration stage has its own balance to keep.

When an additive can do harm

This is the part the labels leave out. A septic tank works by holding waste still long enough for solids to sink as sludge and fats to float as scum, so that only the clearer middle layer flows out to the trench. The NSW guidelines note that a fully formed scum layer also keeps oxygen out, so anaerobic bacteria can digest the organic matter, and stops odours escaping.

A product that breaks up scum or sludge doesn’t make that material vanish. The Ontario association puts the question directly: if a degreaser breaks down solids, where do they go? If they stay suspended, they can flow out to the trench and speed up clogging. It says the same about enzymes that break up the scum layer and release fats and grease. The EPA’s concern about changed effluent quality is the same problem seen from the research side.

Solids reaching the trench are the expensive failure. The trench soil is meant to receive clarified effluent, not fine sludge and grease. Once the soil surface clogs, effluent can’t soak away, and you start seeing wet ground, smells or slow drains, the absorption trench failure signs we cover separately. Heavy clay, which absorbs slowly to begin with, has less margin to spare. Rejuvenating an existing trench runs $1,500 to $4,500. A new absorption trench is $8,000 to $14,000, and that work is carried out by licensed plumbing and drainage contractors we work with; their NSW licence number appears on every quote.

The harsher products are worse. WSU says strong acid and alkali line cleaners can sterilise a tank for days, letting raw sewage flow straight to the trench, and that hydrogen peroxide, once sold as a trench reconditioner, degrades soil structure. Solvent degreasers kill the tank’s bacteria and can contaminate groundwater. And odour products that work by killing bacteria, as the EPA and the University of Kentucky both point out, do the opposite of what a septic tank needs.

AWTS owners: ask before you dose anything

An aerated wastewater treatment system adds an aeration chamber, where a blower keeps aerobic bacteria working, after a primary chamber much like a septic tank. If the aeration stage is knocked back, poorly treated water heads on toward your irrigation area.

We haven’t found an NSW source that recommends additives for an AWTS. Check your owner’s manual for what the maker allows, and ask at your quarterly service. An authorised service agent tests sludge and checks how the system is running at each AWTS service, so a real problem with the biology gets picked up there, with a cause attached, rather than masked by a product.

What actually keeps the bacteria healthy

Let sewage do the seeding. Every flush adds bacteria. A new tank starts working as soon as the house is connected, and no government source we read recommends a bacterial starter.

Get the pump-out done properly. The 2025 NSW guidelines recommend leaving about 10 per cent of the sludge in the tank after a desludge to help the bacterial population regenerate, then refilling the tank with water to at least two-thirds full straight away so it can’t lift out of the ground. Ask whoever pumps your tank about both.

Keep the killers out. Heavy bleach use, constant antibacterial cleaners, caustic drain cleaners, solvents and flushed medications harm a tank’s biology. Our products guide covers what to use instead.

Spread the water load. Several loads of washing in a row push waste through before it settles. Spreading them across the week avoids the ‘occasional water overuse’ the Easy Septic Guide mentions.

Measure, then pump. The NSW guidelines recommend checking sludge and scum depth every year, because you can’t judge it by lifting the lid and looking. That measurement, not a product, tells you when the tank needs emptying.

Consider an outlet filter. The guidelines recommend an outlet filter on septic tanks to catch suspended solids before they reach the trench. If your tank doesn’t have one, it’s worth asking about at the next pump-out.

If you’ve been using a treatment because the tank smells, is slow to drain or the trench looks wet, the product is covering a symptom. Call 0468 278 123 and we’ll tell you what we’d check first.

Frequently Asked Questions

Do septic tank treatments from Bunnings actually work?

For a normal household septic tank, there’s no good evidence they’re needed. NSW’s Easy Septic Guide says a well maintained system receiving the correct amount of wastewater should not need additives, and the US EPA does not recommend them because tanks already contain the bacteria, enzymes and yeasts they need. No treatment removes the sand, grit and other undigestible solids that only a pump-out clears.

Should I add bacteria to my septic tank after a pump-out?

No. Sewage flowing in from the house restarts the bacterial population, and the 2025 NSW Onsite Wastewater Management Guidelines recommend leaving about 10 per cent of the sludge in the tank to help it regenerate. The tank should also be refilled with water to at least two-thirds full straight after pumping.

Is it good to put yeast in a septic tank?

There’s no need to. Yeast breaks down starches, which make up only a small part of what’s in a septic tank, and Clemson University’s extension service says there is never a need to add yeast, sugar or raw meat to a septic system.

Can septic tank additives damage my absorption trench?

They can. Products that break up the scum layer or dissolve sludge may leave fine solids, fats and grease suspended, and those can flow out to the trench and clog the soil faster. Harsh line cleaners, solvents and hydrogen peroxide are worse, because they can kill the tank’s bacteria or damage the soil structure the trench depends on.

Will an additive mean I can pump out less often?

No. Part of the sludge in every tank is sand, grit and plastic that no bacteria or enzyme can break down, and the Ontario Onsite Wastewater Association reports that independent studies found additives don’t stop sludge building up. NSW guidelines say a tank must be desludged by the time sludge and scum fill a third of its depth, commonly every three to five years.

Is there any time a septic additive makes sense?

NSW’s Easy Septic Guide says additives are only suitable for minor, temporary problems, giving antibiotics in the system or occasional water overuse as examples. Even then, they won’t help a tank that needs pumping or a trench that is failing. If you’re on an AWTS, check your owner’s manual and ask your service agent before adding anything.

Sources and further reading

• NSW Office of Local Government, The Easy Septic Guide: the ‘do additives work’ section, the dead possum myth and the lime start-up suggestion.
• NSW Office of Local Government, Onsite Wastewater Management Guidelines (April 2025): the scum layer, outlet filters, desludging triggers, annual sludge measurement, and retaining sludge and refilling after a pump-out.
• US EPA, Septic Tank Additives Fact Sheet (updated 2024): why the EPA does not recommend additives, and the risks of each product type.
• US EPA, Frequent Questions on Septic Systems: additives not recommended, and their possible effect on drainfields.
• Washington State University Extension, Clark County, Septic Tank Additives: dose size, the 48-tank sludge study, undigestible solids, line cleaners and hydrogen peroxide.
• University of Kentucky Cooperative Extension, Impacts of Additives on Septic System Performance (HENV-505): odour products that kill bacteria, and additives having little positive effect.
• Ontario Onsite Wastewater Association, Septic Tank Additives: an industry view on bacteria, yeast and enzyme products, and suspended solids and grease reaching the leaching bed.
• Clemson University Home and Garden Information Center, Be Septic Safe: no need for yeast, sugar or raw meat.