Here it is folks, by popular demand. The Septic Tank FAQ for
England and Wales Version 1.2.

Unless otherwise stated the measurements and constructional standards
contained in this document are drawn from the British Standards Code of
Practice for the Design of Small Sewage Treatment Works and Cesspools
BS 6297 : 1983 and Regulation H2 of the Building Regulations 1985 and
its accompanying Approved Document of 1st April 1990.

It is an offence under the Building Act 1990 to construct a new septic
tank or cess pit which does not conform with the Building Regulations
however the regulation in question does not require full compliance
with the BS. The Approved Document recommends the BS however it is a
defence to a charge of breaching the regulation if you can prove that
the installation you build is as good or better than BS 6297.

You should note that if your own installation pre-dates the British
Standard and/or the Building Regulations it is unlikely to be large
enough due to the increase in water consumption over the last 20 years
and in the case of a septic tank may be incapable of producing a final
effluent of a sufficiently high quality. In particular many
older installations discharge direct to a ditch or other watercourse
without the consent of the Environment Agency, this is now a
criminal offence under the Water Resources Act 1991. (see below Q.21)

The information in this document does not cover installations designed
to treat trade effluent.

Please note also that this FAQ does not cover issues arising out of the
Town and Country Planning Acts or Health and Safety legislation. However,
in the event of sufficient requests and/or bribes I might be persuaded to
produce an appendix to the FAQ.

1. What is a septic tank?

2. What do they look like?

3. Is size important?

4. So how big should it be?

5. What goes into the tank?

6. Is one tank enough?

7. So what else do you need?

8. I'm not digging all those bloody holes - is there another kind of
secondary treatment?

9. Now can I discharge the effluent to the environment?

10. What problems can I expect with my septic tank?

11. So what are the er.... end products?

12. How often should I de-sludge my septic tank?

13. Okay I got to ask - what's in the sludge?

14. If it isn't a septic tank - what is it?

15. What is a Buchan Trap

16. Ah ha - so you don't know?

17. Why have I got a septic tank or cess pit?

18. Can I get mains drainage?

19. So if I've got to have my own system where should I put it?

20. And how should I build it?

21. Anything else?

22. When was the first septic tank installed in GB?

1. What is a septic tank?

A septic tank is a self- contained installation for the treatment of
sewage. It takes the form of a settlement tank in which sewage is
retained for a sufficient time for organic matter to undergo anaerobic
decomposition. The sewage is liquefied, a thick oily scum forms on top
of the tank contents and sludge falls to the bottom of the tank. The
relatively clear liquid in between is then (ideally) treated further
or (often) discharged direct to the environment.

2. What do they look like?

The septic tank proper consists of either two tanks in series or a
single, larger tank divided into two by baffle boards with vertical
or horizontal slits. This to ensure that treatment is not prejudiced by
surges in flow, typically caused by multiple discharges from different
appliances at the same time.

The tanks are normally square or rectangular and older ones are almost
invariably made of brick. It is now possible to buy prefabricated units
and some of these are circular. The floor of the tank usually slopes
into a central channel to allow for desludging.

The tank should be not less than 1200 mm deep below TWL (top water level)
and input to the tank should be by a pipe feeding at a depth of not less
than 450mm below TWL. The pipe should be laid as flat as possible and in
a direct line to the centre of the tank to reduce turbulence. The output
pipe should be 250 mm below the inlet pipe.

3. Is size important?

Yes, it is essential that the sewage is held in the tank long enough for
decomposition to take place, if the tank is too small the resulting
discharge will be even more polluting than normal and may even include
what are charmingly known as "gross solids". These may block pipe-work,
clog the bio-filter (see below) or block the soakaways (see below) or
may even be discharged direct to the environment leading to pollution,
land affected by "sewage sickness", risk of rat infestation and disease.

4. So how big should it be?

BS : 6297 provides a formula.

C= (180P + 2,000)

Where C is the size of the tank in litres and P is the population to be
served. This assumes desludging on an annual basis. Children count as
adults.

5. What goes into the tank?

All foul drainage not only from lavatories but also baths, showers,
sinks, dish-washers and washing machines.

Particular care must be taken with washing machines, in many instances
they are connected directly to the rainwater gulley that takes water
from the roof down pipe. This is because it is often easier to bash a
hole in the kitchen wall to the outside and run a pipe to it than it
is to plumb into the foul drainage. In many rural and village settings
the discharge of detergent can have disastrous effects on local
watercourses to which surface water is often discharged direct.

On the other hand rainwater from roofs and hard standings such as
yards should be excluded from the tank if at all possible.

6. Is one tank enough?

No, the liquid that leaves a septic tank is still grossly polluting
and will require secondary and possibly tertiary treatment.

Some older septic tanks discharge their effluent direct to a ditch
or watercourse. Even if that ditch or watercourse is normally dry this
constitutes an offence under the Water Resources Act 1991 (see Q.21 below)

7. So how many do you need?

You need secondary treatment and the simplest form of that is by soakaway.
Either (rarely) a pit filled with brick bats or clinker which allows
the effluent to soak into the ground or (more commonly) a system of porous
or perforated land drains laid fairly shallowly.

The amount of drain required is calculated by reference to the
percolation test laid out in BS : 6297

(1) Dig three holes 300 mm square. Each hole should be 250 mm below
where it is proposed that the bottom of the pipes should be and should
be dug along the proposed route of the pipes.
(2) Fill each hole with water and leave overnight
(3) Fill each hole with water not more than 350 mm deep and time how
long, in seconds, it takes for each hole to empty
(4) Divide the time in seconds by the depth in mm to get V
(5) Add the three figures and divide by three.

Special rules apply if the result is less than 24 or if the holes
have to be laid very deeply.

If V > than 140 BS : 6297 regards the land as unsuitable for soakaways
and an alternative must be found.

If V is between 100 and 140 BS : 6297 says that underdrains will
have to be laid.

However if V > 100 the EA regards the land as unsuitable for soakaways.
(See Q.21 below)

The area of drainage required is calculated as

Area = Population to be served x V x 0.25

If soakaways are used where the ground is not suitable ponding of the
sewage will occur and there is a risk of run-off to watercourses,
blockage etc.

In addition, in certain areas the water table is so high that the EA
will always object to soakaways for fear of damaging the aquifer
(see Q21. below)

8. I'm not digging all those bloody holes - is there another kind of
secondary treatment?

Yes, the other type of secondary treatment is a bio-filter. This is
a smaller version of the large circular tanks that can be seen at
sewage treatment works which have sweeper arms that dribble effluent
over beds of clinker.

In conjunction with a septic tank this job is done by a square tank
filled with clinker or other inert material which is coated with a
living biomass which digests the organic material allowing clean(ish)
water to drain from the bottom. Because of the depths involved it may then
require a pump to bring it up to a level where it can be discharged.
It is essential that the effluent is spread evenly over the surface of
the clinker and there are a number of mechanisms to ensure it happens;
the simplest and least effective is a series of perforated pipes which
require regular inspection and maintenance to prevent blockage.
If the effluent is not spread properly it will "pond" in one place and
the bacteria will be unable to digest the amount of material
they are presented with. Uneven distribution can be seen by the presence
of green algae on the areas which are not being properly dosed.

Best modern practice is to have a third simple settlement tank to
allow undigested matter, including dead biomass, to settle out prior
to discharge, a practice sometimes called "polishing".

9. Now can I discharge the effluent to the environment?

See Q.21 below

10. What problems can I expect with my septic tank?

Too much rainwater and ground water will prejudice treatment.

Excessive use of detergents and disinfectants. Too much detergent and
the contents of the septic tank may emulsify instead of forming the
three layers described above and all later stages of treatment will
tend to block. Too many disinfectants or antiseptics will kill off
the bacteria necessary for the treatment process, all septic tanks are
living systems and therefore all vary so it is impossible to give specific
guidance. Normal domestic use should cause no problems although any home
industry which involves excessive use of these products should be avoided.

Some water softeners produce brine as a by-product and this should not
pass into the septic tank due to the antiseptic effect of the salt.

(See Q.4 ante for questions of size)

11. So what are the er.... end products?

Sludge (which remains in the septic tank) and sewage effluent consisting
of water, nitrates, nitrites and, unless your system is working at
optimum efficiency, tiny residual particles of sewage.

12. How often should I de-sludge my septic tank?

The constructional standards and dimensions in BS 6297 assume that
desludging will take place annually. So long as both inlet and outlet
pipes are free, bi-annual emptying is probably sufficient unless the
tank is undersized for the population served.

13. Okay I got to ask - what's in the sludge?

Anything too heavy to remain in suspension and anything which will
not decompose.

14. If it isn't a septic tank - what is it?

Chances are you've got a cess pit (or cesspool or dead well). Put simply
a large tank which holds domestic sewage until it can be removed by a
contractor.

Older ones are usually brick and circular and too small, newer ones are
usually prefabricated out of some form of plastic and conform to BS 6297,
Building Regulation H and its accompanying Approved Document which
stipulate a minimum size of 18,000 cubic meters and gives a suggestion
that a minimum of 45 days storage be provided.

The average family of 3 produces about 7 cubic meters of sewage in
about three weeks and this is the capacity of the average tanker used
for the emptying, consequently the larger the tank the more trips it
will take to empty it.

15. What is a Buchan Trap?

Buchan traps have nothing to do with septic tanks or cesspits.

16. Ah ha - so you don't know?

Oh all right.

Early Victorian sewerage engineers were worried at the possibility of
"miasma" (disease-bearing smell) escaping from the sewers into peoples'
houses by way of newly installed and new-fangled toilets. The Buchan
trap was a method of preventing this in the same way that U- and
S-bends prevent smells getting into modern homes.

The Buchan trap was badly designed and subject to blockage.

17. Why have I got a septic tank or cess pit?

Septic tanks and cesspits or pools are self-contained installations for
dealing with foul drainage in the absence of mains drainage.

18. Can I get mains drainage?

You have the right to connect to and discharge domestic sewage into
any nearby sewer and the sewerage undertaker has very limited rights to
stop you. If there is no nearby sewer you can requisition one, if you're
prepared to pay for it in a complicated formula which takes into account
the income from sewerage charges.

There are many villages in which problems are experienced because
inferior or badly maintained septic tanks and cesspits leak sewage
effluent into local watercourses, in summer when water levels may be
low or non-existent the smell can be appalling and the danger of disease
very real.

Huge rancorous arguments tend to arise about whose fault the pollution
is and who should pay for remedial works and in all cases known to me
personally no solution has been found due to either the expense of
remedial works or the difficulty in getting everyone to agree to the
same solution.

A new section (S101A) has been added to the Water Industry Act 1991,
supposedly to deal with this situation. The new section provides for mains
drainage to be supplied to houses which where (a) built before 20th
June 1996 (b) have no mains drainage and (c) satisfy a complicated set
of criteria relating to the cost and to the environmental impact of the
unsatisfactory drainage. If these criteria are satisfied, and I doubt if
they will be very often, mains drainage has to be supplied at the cost
of the drainage charge payers of the company as a whole, rather than
just those who directly benefit from the new drainage.

19. So if I've got to have my own system where should I put it?

A number of considerations are common to cesspools and septic tanks :--

(a) not within 15 metres of the dwelling to be served

(b) not with 10 metres of a watercourse

(c) not on land prone to flooding or with a particularly high water
table

(d) within 30 metres of vehicle access for emptying and desludging
by tanker.

(e) down wind of the dwelling of possible

(f) down hill of the dwelling, pumping should be avoided if at
all possible.

20. And how should I build it?

BS 6297 contains detailed recommendations as to materials.

It is essential that rain and ground water be kept out. In some older
installations roof drainage and foul water are collected in the same tank.
This should be discontinued if possible. Excessive amounts of rain water
will prejudice the action of the septic tank and increase emptying costs
for a cesspit.

Both should be covered (a) to keep out the rain and (b) for reasons of
safety. If not covered it should be fenced.

Access will be required for emptying/desludging and that access must not
be smaller that 600 mm in any dimension. The access should permit
inspection of the inlet and in septic tanks the outlet too, it is
recommended that the access should be sufficiently large to permit rodding.
If it will be necessary at any stage of the cleaning process to enter the
pool or tank there should be two access points.

Ventilation will be required in both cases.

21. Anything else?

Yup - if your septic tank system discharges effluent (no matter how clean)
to a watercourse you are committing an offence under Section 85 of the
Water Resources Act 1991, rendering you liable on conviction in the
magistrates' court to up to 3 months in the slammer or a fine not exceeding
UKPounds 20,000 or on conviction at the Crown Court to up to six months
in jail and an unlimited fine.

The way round this is to apply to the Environment Agency for a
discharge consent, however the Agency may simply refuse consent (in
which case you are back where you started) although it is more likely
to grant consent subject to conditions regulating the "strength" of the
effluent. Breach of those conditions nullifies the consent and you are
once more liable to prosecution as stated above.

If the watercourse is a "main river" as defined by the Act you may also
have to apply for consent for the outfall structure from the EA as land
drainage authority. S.109 Water Resources Act 1991.

It is an offence under the Salmon and Freshwater Fisheries Act 1975 to
discharge anything poisonous or injurious to fish into waters which
contain them.

In addition the Environment Agency is a statutory consultee of
the local planning authority and will receive notice of all new
development. If that new development constitutes a pollution risk the
Agency is empowered to issue a "prohibition notice" under Section 86
of the Water Resources Act 1991 which makes it an offence to
discharge trade or sewage effluent on to land. Breach of such a notice
is also an offence as detailed above. These particular provisions are
among the worst drafted of a badly drafted Act, however I retain too
much residual loyalty to the EA to point out the loopholes.

22. When was the first septic tank installed in GB?

The first recorded septic tank in full use was installed in 1896 under the
supervision of Mr Donald Cameron, City Surveyor for Exeter and served the
entire town. The tank was 64 feet 10 inches long and 18 feet wide and varied
in depth from 7 feet to 7 feet 9 inches. A description of the tank may be read
in Mr Cameron's paper "A years experience of the Septic Tank System of
Sewage Disposal at Exeter" as presented in the 1897 Congress of the Royal
Sanitary Institute held at Leeds.

Acknowledgements:-

Mr Martin Edmonds - for suggesting an acknowledgement section
Mrs Madeleine Bell - Leeds City Environmental Health Department - for
information on the Buchan trap
Leeds City Library Business and Commercial Section - for access to BS 6297
and to the papers of the Royal Sanitary Institute
Environment Agency - for charging information and for their leaflet
"Disposal of sewage where no mains drainage is available" and especially
for the mistakes in it which I enjoyed immensely.
Mr Robert Totman and Mr Alan Drayson for questions.
Mr Keith Mathews for information supplied.

This FAQ is brought to you Catherine E. Carter Legal Training Services.

Please note that although a ridiculous amount of time and effort has
gone into its compilation I can accept no liability for loss or damage
incurred by anybody fool enough to spend money in reliance on it.

I don't have indemnity insurance - it isn't worth suing me, honest.