Wastewater Treatment From Oasis Environmental

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Common Septic Tank Queries Answered

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What is a Septic Tank?

A septic tank is a two, or three chamber system which retains sewage from a property for sufficient time to allow the solids to form into sludge at the bottom of the tank. Here, the sludge is naturally broken down. The remaining liquid, or effluent, in the tank then drains from the tank by means of an outlet pipe.

Septic tanks are usually installed where there is no mains drainage available. Most septic tanks are buried in the ground.


How does it work?

Any sewage treatment system is basically just a way of slowing up the sewage to allow sufficient time for enzymes to naturally break up the contamination. A septic tank is basically a collection point for sewage, which allows the sludge to settle out of it. The liquid effluent on top of the sludge has a relatively low level of contamination which can safely flow out of the tank into a soakaway and then into the ground in a drainage field. Naturally-occurring microbes, which live on the walls of the septic tank, produce enzymes, which help to break up the sludge into CO2 and H2O.


What is the difference between a septic tank and a Wastewater Treatment Plant (WWTP)?

Small treatment plants usually have access to a supply of electricity which drives a small pump to aerate the wastewater. Septic tanks have no power demand and no moving parts, so they are cheaper to buy, install and run, but the final effluent will not be as high quality. The effluent from a septic tank will always require further treatment in a soakaway.

Small wwtps are often referred to as Package Treatment Plants, or PTPs. This is because they arrive at your house on the back of a lorry, as one "package".


Why do you need to empty a septic tank?

It's unlikely that all the sludge at the bottom of the tank will disappear. It will build up over time, so needs to be taken out of the tank. A contractor will take the sludge, but put the watery effluent back in, which is good, because that effluent contains billions of the good microbes necessary for your tank to work. It's probably better not to refer to it as emptying your septic tank, but "desludging" your septic tank.


My tank has a scum on top. Is this OK?

Yes, as long as it is liquidy. This scum consists of Fats, Oils and Greases (FOG), from cooking and washing up etc. Over time the enzymes will break them down. If the scum gets too thick, it can dry out and get solid. This stops air getting to the effluent. The microbes in the effluent work better if they have access to oxygen from the air. If the scum is hard and dry, it's probably a sign that you should get in a contractor to empty the tank – sorry, desludge your tank.

How often should I desludge my tank?

Under normal circumstances, once a year is plenty. Certainly, if you desludge more often, you should be using bioaugmentation products.

We are installing a new tank. What do we need to do?

A really useful document is available from the Environment Agency. Pollution Prevention Guidelines PPG4 "Disposal of Sewage where no Mains Drainage is Available."  Here is a link to it


Make sure the person installing your tank has read this. Make sure they do a proper percolation test so that the soakaway is the right size.


What is Bioaugmentation?

The addition of bacteria. This augments the existing biological population, or biomass.


Why do we need to add bacteria?

Bacteria, like all living things, die. If the environment is perfect, the biomass will be self-sustaining; new bacteria replace those that die. But, a septic tank is rarely a perfect environment and some strains of bacteria find it harder to live there than others. The regular addition of a bioaugmentation product guarantees your tank has the right number of the right bacteria in it.


Is there anything I shouldn’t flush down the loo or pour down the sink?

Bleach or any "biocide". Paints, solvents or caustics. Try and limit the amount of kitchen fats and coffee grounds to a minimum. Your septic tank is a biological solution for treating biological waste. It cannot cope with physical blockages, so cigarette ends, sanitary products, balls of hair and any paper other than soft tissue will cause a physical blockage that will require physically unblocking. No amount of bacteria will move Junior's nappy


Can I just put a dead rabbit in my septic tank to get it started?

This is a classic example of using the wrong sort of bacteria. The bacteria which your tank needs, exist in nature in the soil, not an animal's gut. So, no. Ask us for a free start-up pack with the first year's supply that you buy.

My tank stinks! What’s wrong?

Probably, the biomass has died. You must add some bacteria to the septic tank immediately. Then try and work out what has killed the biomass. It's highly likely to be a cleaning product you use.


What are my legal obligations is I live in a property with a septic tank?

You need to maintain your septic tank in accordance with the manufacturer's instructions and you need to keep a record of all maintenance, this includes regular emptying.......sorry, desludging.

British Water is a trade association for the water industry. They have written some excellent Codes of Practice which cover the installation and maintenance of small wastewater treatment plants. They are free and available online at:


These may be useful, particularly if you don't have access to the original documentation that came when your septic tank was first installed.

You may need to Register your Septic Tank with the Environment Agency (EA). You may even need to obtain an Environmental Permit from the EA. Whether you need to or not depends on many things, including where you live and the amount of sewage discharging into your septic tank.

Basically, the best way to make sure you don't fall foul of the law is to check out the EA's website:



How much does it cost to register my Septic Tank with the EA?



How do I register?

You can do it by post, but it's very straightforward to register online on the EA website.


We have a reed bed. Will your products block this up?

No. They can only help.


What’s in OE42?

What you can see is bran. The bran carries 14 strains of bacteria, a bio-enhancer (which is a little food for the bacteria) and micro-nutrients which give them strength. All of these are much too tiny to see. There are 5 billion bacteria in each gram of OE42. So there are 125,000,000,000 bacteria in every 25 gram sachet.  The sachet is water soluble. It disappears in seconds.


How do we dose OE42?

Once a month, last thing at night, drop one sachet of OE42 into the loo, and then flush the loo.


Why last thing at night?

This means that the bacteria spend as much time as possible in the pipes leading to your septic tank forming a biomass on the inside of the pipes as well as in the tank itself.


My local farmer has offered to desludge my tank. Is this OK?

By law, desludging should be carried out by an operator registered for the carriage of such waste by the Environment Agency. It's unlikely that your friendly farmer is registered.


How do I find someone to desludge my tank?

Look up Waste Disposal in Yellow pages, or enter Waste Disposal in a search engine. If you're new to the area, ask your neighbours. If you have a septic tank, probably so do they. It's a great way to introduce yourself -much more original than asking for a cup of sugar!