sewage treatment plants

a sewage treatment plant is a packaged sewage-works that converts raw sewage into water (effluent) and solid waste (sludge). these ‘packaged’ plants come in all shapes and sizes but treat the waste using the same principles - combining oxygen and food (organic waste) to speed up a natural biological breakdown. 

all treatment plants need routine maintenance to keep them working at their optimum levels but treat them right and they are an environmentally friendly way of solving an off-mains drainage problem when a pumping station is not possible.

how it works?

primary treatment

sewage treatment plants create an artificial environment to speed up the natural process of breaking down the pollutants in sewage. the treated effluent can then be safely discharged into a local watercourse or soakaway system.

there are three main process types:

• rotating biological contactor (rbc); 
• aerated filter; 
• submerged aerated media (sam).

solid matter (sludge) separates from liquid waste and settles at the bottom of the primary settlement tanks, while lighter material forms a layer of scum on the top of the liquid. sludge and scum need to be removed regularly by a vacuum tanker and will then be disposed of at a large municipal wastewater treatment works.

if it's not removed, the sludge will build up and eventually carry over into the secondary treatment stage, where it could interfere with the biological process, block the biological filter or pollute the watercourse or soakaway. putting this right can be really expensive, so don't wait until things go wrong before you give us a call.

secondary treatment

the remaining liquid is broken down by live, naturally occurring micro-organisms (biomass) until the quality is high enough for it to be safely discharged to a watercourse or soakaway. the biomass is fed with oxygen to speed up the digestion process.

final settlement

this final stage allows the contents of the humus tank to settle out. where discharge consent is more stringent, tertiary treatment is likely. this adds significantly to overall process costs (i.e. nitrification units, sand filters and reed beds). not forgetting the need for more frequent tanker de-sludging.

final disposal

if a watercourse is available and you've been given permission by the environment agency, this is the simplest way to dispose of the treated effluent. at the moment the most common consent issued is:

• biological oxygen demand (bod) - 20mg/l
• total suspended solids (ss) - 30mg/l 
• ammonia - 20mg/l

maintaining the system

regular tanker visits are essential to prevent sludge and scum building up in the tank. equally as important is having the system maintained by a properly trained engineer. serious engineers are british water accredited.

day to day you’ll need to keep the system and electrics well looked after – we recommend you get into the habit of inspecting the plant every day to check everything’s working properly. its crucial the electricity supply is never turned off, as this will stop the treatment plant from working.

desludge and service requirements

your sewage treatment plant should be desludged and serviced atleast one a year. however, each system is different and you should always arrange a desludge and service inline with the manufactures recommendations. this will ensure your sewage treatment plant is in proper working order and that your not breaking the law.

when it all goes wrong

neglect your plant at your peril. not only will the consequences be unpleasant, if you pollute the surrounding area you could face prosecution and some serious fines.

if the biomass is harmed, it will usually recover with time and patience. but until it does, you’ll have to live with a pretty foul smell. prevention is better than cure.