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How a Septic Tank Works

How a Septic Tank Works

 

Septic tanks handle sewage in many rural areas around the world. Invented in Napoleonic times [in France], they are the ideal biological digester; but to most people they are a ‘grey’ area. They cost nothing to run, work without steam, and are very simple to understand and look after.

 Schematic Septic Tank

Many people have heard of septic tanks, Few people know how they work. Very few people understand them [and this includes process engineers, municipal inspectors and architects].

 

However, they are probably the most misunderstood unit in the home, and are usually far too small for the task they are designed to do.

 

In South Africa, and some other countries, you can buy an empty septic tank from a plumbing supplier, or an agricultural general dealer. These are usually ‘rotomoulded’ plastic [HDPP] and supplied in 900 lt – to - 3,600lt. capacity.

 

The majority of builders know little about their workings and they channel all the waste water from the house into them, and dig a pit for their brick rubble and waste materials, which becomes the ‘soakaway’. This is wrong ! Septic Tanks are designed for the load they are to process. Human waste takes 18 – 21 days to break down [done by bacteria called a ‘biomass’].

 

Only the sewage [from the loos]  and the kitchen waste water drainage enters the tank. The kitchen drains via a grease trap or ‘gulley’ to protect the tank from complex refined fats that occur in modern foodstuffs [chips, snacks and bread spreads &c.]. It does not matter how many loos, bathrooms and bedrooms there are in the house, The calculation of the size of tank is based upon residents [+ any staff] using the system. Basically you need a 7,000lt tank for a family of 5 people, plus a house girl once a day and a gardener at week ends. This allows for a couple to stay over once a fortnight. [maybe it’s your son’s mate from school].

No ‘grey water must enter the tank [except the kitchen drain – protected by a grease gulley/trap. Faecal material takes 18-21 days to degrade anaerobically: Scientific Fact.

 

You should allow 1,000lt capacity per resident plus 20% for visitors. We get cries for help from middle aged couples who’ve converted the family home into a 10 person B & B [them and 8 guests], and they wonder why the septic system gives trouble ‘It was alright in Dad’s day’, so they pour all manner of chlorine, caustic soda and chemical ‘drain cleaners’ from the supermarket to remove the smell and blockages.

A typical septic tank is a pair of 9” [250mm] brick boxes separated by a common dividing wall. We show the baffled [‘T’ piece] design, but it may have a perforated dividing wall to allow the water to pass from Ist Chamber to the Second. It has ‘T pieces’ [which act as baffles to control the movement of scum]. Without these the soakaway would fill with solids and stop working [become fatted up]. They are ‘T pieces’ so that the covers can be removed and they can be rodded vertically to remove wads of toilet paper or feminine textiles. With a bend this would not be possible, because there is limited access.


Both chambers are designed to be full of water, at the same level as the inflow [from the house] and outflow sewer [to the soakaway]. The Primary has a ‘crust’ of faecal solids, but the Secondary should only have black water with bubbles [looking like the sky on a winter’s night]. There may be more chambers, depending upon the size of tank, and the loading to be imposed upon it’s roof. [eg; vehicles].  

 

The tank is usually 1.5m deep – OC - [occupied volume]. With a head space of +-300mm,  making it 1.8m deep overall. Allow 100mm for concrete base and reinforced roof, and you arrive at approx 2.00m. [any deeper, and the sides of the hole would fall in, unless a back-hoe is used.] So we establish that the average tank is 1.5m deep. ‘A rule of thumb’ is to allow 1,000 lt capacity per resident [it makes no odds how many bathrooms or loos – it’s people that count]. Plus allow an extra 1,000lt for overnighting friends & the kids ‘sleep-overs’ &c.

 

A convenient width is 1.5m as precast lintels, laid side by side and screeded with concrete  can be used to form the roof – giving a mean width of 1.2m, so a 7,500lt tank is 1.5m deep x 1.2m wide and 4.2m long [1.5 x 1.2 x 4.2 = 7.560 lts.]; adequate for a family of 6 [Mum Dad three kids and Granny + a house girl and a gardener once a week]. Such a tank should run happily ever-after. [every 20 -25 yrs it needs to be de-silted, as 0.9% of sewage is undegradable, and sand blows into the drains !].

 

How does it work: When you flush the toilet, you release between 7 and 9 lts of water from the cistern into the bowl, which washes your deposit round the bend and down the sewer. It enters the Primary Chamber, via the ‘T piece’ and the force of gravity ensures that the deposit enters beneath the crust. Being lighter than water, it floats up under the crust, and begins to be broken down [degraded] by the microbes living and thriving there. [they’re cunning little devils & know where their ‘food’ is]. This then displaces an equivalent volume into the Secondary chamber, and out of the Secondary [maybe via a Tertiary – in large tanks] – also via a ‘T piece’ [to retain any scum] & into the soakaway, where it ‘rests’ before seeping out through the ‘medium’ [stones] into the surrounding soil, where natural soil microbes are waiting to finally ‘clean it up [it’s called ‘polishing’] prior to the clean water seeping down to the water table  [likely to be fed by aquifers] and the story starts again with the borehole

 

The system struggles quietly until the builder has claimed his retention, and then the trouble starts: usually expensive trouble ! We speak from experience; we see it time and again.

 

Beware the honey sucker. It mucks up the garden, and just draws off excess water, plus the essential microbes which make the tank work. The fat lining the soakaway remains; and next month it needs doing again !  And ‘the men’ always arrive when you have guests !. Pong !!!!

 

Yes, it’s as simple as that, but invariably the tank is too small for the load, or the soakaway soil has a high clay or rock content.  So it gives trouble – you can’t blame it !.                       

 

Any queries: bob@biosystemssa.co.za
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Bob Hadley

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