You’ve Got Questions — We've Got Answers
WHAT ARE THE MAIN STEPS IN SEWAGE TREATMENT (OFF MAINS SEWAGE TREATMENT PLANTS)?
1) Primary settlement. A calming chamber is where raw sewage settles out. Usually only the liquor within the mid column can pass over to the aeration chamber or biozone via a dipper pipe. In a “complete” sewage treatment system all sewage is aerated and there is no primary settlement chamber.
2) Secondary treatment. Occurs in an aeration chamber or biozone (sewage treatment plants) where obligate aerobic bacteria breakdown complex nitrogenous products. Septic tanks are passive and so not aerated or have any mechanical treatment process. Obligate aerobic bacteria are not present and therefore little secondary treatment occurs within a septic tank. Septic tanks simply separate solids from liquids and discharge the untreated liquid phase into a drainage field in the soil. “Friendly” bacteria accumulate within the drainage field and provide degree of cleaning.
3) Secondary settlement. Another low energy chamber where suspended solids settle out of solution (they either float or sink)
4) Via a “dipper” or weir the cleanest proportion of liquor discharges out of the Sewage Treatment Plant to the environment, either passively by gravity, or via an electric pump.
5) Tertiary treatment or “polishing” occurs within the soil (as in a drainage field), plant roots such as reeds: Phragmites australis), watercourses or ditches etc. Tertiary treatment is also a function of bacteria and micro fungal action.
Reed beds are a fantastic way to tertiary treat or “polish” the final discharge from a sewage plant. Reed beds (also called Constructed Wetlands) can reduce nitrogen loading to near zero. This is very important if you live adjacent to a river, estuary, lake or the sea and are Off-main sewerage. The Environment Agency recognise the value of reed beds and often the installation of a tertiary reed bed will make the difference between being granted a Discharge Permit, or not.
WHAT WILL I SEE IN MY GARDEN ONCE A SEWAGE TREATMENT PLANT (STP) HAS BEEN INSTALLED?
Most likely you will only see a round or oblong green coloured lid about the size of a small kitchen table or surf board. There may also be 1 or 2 small metal or black plastic manhole lids (around 450mm in diameter) connecting the house to the STP. At any point where sewage pipes or foul drains change direction Building Regulations require the installation of manhole. This is to prevent blockages and provide an access point where drains can be rodded or cleaned.
A sewage treatment plant will also require the installation of a small kiosk, usually within 10m of the STP. The kiosk houses the aeration blower and electrical connections. They are generally 500mm cubed but bigger treatment plants require bigger kiosks.
DOES A SEWAGE TREATMENT PLANT (STP) MAKE ANY NOISE?
Noise depends on the type of Sewage Treatment Plant. STP’s that are aerated via a blower make a small hum. This usually cannot be heard standing 5-8 meters away.
There are makes of blower that are very quiet but of course they are more expensive!
WHAT IS THE ANNUAL RUNNING COST OF A SEWAGE TREATMENT PLANT (STP)?
Running costs will vary depending on the size of the STP, but for an average 3 bedroom house (with 6 persons resident) the cost will be approximately £50-80 per annum for electrics. The Sewage Treatment Plant will also require an annual service @ circa £85 plus parts (usually around £30), and an annual empty to remove the sludge @ £135-£185 depending on your local tanker companies' fees.
HOW MUCH DOES A SEWAGE TREATMENT PLANT (STP) COST?
Domestic sewage treatment plants for a single dwelling (family home with 6 x persons) costs between £8k to £12K. Cost is dependent on; size of STP (number of bedrooms), site access, site clearance (trees, scrub, bushes etc), soil type, distance from dwelling, condition and age of existing electrics, removal of spoil, connection to existing sewers, number of manholes, discharge option, the level or quality of the landscape finish required by the customer, whether the sewage treatment system, drains and discharge conform to the General Binding Rules (as required by the Environment Agency).
Building Control (BC) involvement (inspections by BC) in the installation of all Sewage Treatment Plants and drainage is required by law. Failure to engage with Building Control can result in a £6,000 fine for the installer.
CAN I CLEAN MY TOILETS AND SINKS WITH BLEACH OR OTHER TYPES OF STRONG DETERGENTS?
Normal weekly use with a cleaning product such as bleach, or similar, is okay but excessive use of these products in your toilets, drains, showers, baths and sinks will negatively affect the “friendly” bacteria that dwell in your Sewage Treatment Plant. A sewage treatment plant that has been dosed with large amounts of bleach will likely start to smell.
HOW LONG DOES IT TAKE TO INSTALL A SEWAGE TREATMENT PLANT STP)?
A domestic STP usually takes about a week to install, connect to the foul drains, connect and certify the electrics, Seed and Commission.
Large STP’s can take months.
DO I NEED A PERMIT FROM THE ENVIRONMENT AGENCY (EA)?
If your STP installation conforms to the EA’s General Binding Rules, and most domestic dwellings will conform if installed by ourselves, then NO.
Commercial systems are more likely to require a Permit. We will undertake all applications and liaise with the EA for you.
WHAT IS THE LIFESPAN OF MY SEWAGE TREATMENT PLANT?
The tank itself, if correctly installed should last 50 years. Aerators, blowers and other perishable components will require replacing from time to time.