How do you convert raw sewage into clean water?
- 14 April, 2020
Raw sewage from the typical household produces effluent at a strength estimated by British Water to be 300 BOD. This is similar to 300 parts per million! BOD is the term used for Biological Oxygen Demand.
For many generations Septic Tanks were the standard way of treating sewage. Septic tanks work on the anaerobic principle, that is, the bacteria can live without oxygen. The bacteria live on the organic material in the effluent and produce unpleasant gases in the process. They are not called septic for nothing!
As sewage comes into the tank at 300 BOD treated effluent is released at the outlet pipe at the other end at 200 BOD. So we can see how inefficient the septic tank is. It retains, at best, only one third of what goes into it! The greyish dirty water then is sent to a soakaway where the anaerobic bacteria in the soil do their best to further digest the organic material. Bacteria don’t live for long, they live, eat, breed and die. When they die they leave a black sludge.This sludge fills the pores of the soil so even in free draining sandy soils the ground under and around the soakaway gets saturated and the water in the soil then backs up into the septic tank, then into the drainage pipes around the house. A blocked soakaway may cause the need for a septic tank repair.
Eventually the soakaway is replaced if there is room in the garden.
Modern Sewage Treatment Plants work on the principle of aerobic digestion. That is, they need air or oxygen to operate. Sewage Treatment Plants come in different forms, from the filter bed (which introduces air by spraying water over stone media in the big filter beds operated by the big Water Companies such as Anglia water.) to Klargester’s Biodiscs (where discs dip slowly into and then out of the effluent giving the bacteria what they need. That is, food, water and air!). The smaller packaged plants blow air up through the effluent, much like in an aquarium.
Now we have sewage final effluent as clean as 20 BOD or better. Sometimes down to 10 BOD. Compare that with roof water, say 5 BOD or tap water which of course is zero BOD. This water can safely discharged to the river system as fish can cope with it easily enough. This water is still carrying pathogens so is still not to be treated without care.
So this is how the two forms of treatment of sewage produce two very different standards of final effluent. Only the 20 BOD water can be discharged into rivers, the Septic Tank water has to be kept in the garden!
Learn more about the sewage treatment rules in your area on our General Binding Rules page.
Summary
It is clear to see just how important a well functioning and maintained septic tank is, for many reasons. It saves your house backing up with sewage, it helps the environment by keeping harmful wastewater out of the waterways, and it helps your fellow neighbours by keeping biohazard risks down! Untreated raw sewage at 300 BOD, according to British Water estimates, is an extreme biohazard for both animals and humans alike. An essential key to the advancement of human civilisation, is figuring out what do we do with our poop? And how can we make that as seamless as possible to what else is around us.
If your septic tank is overflowing, has a chemical imbalance, you have a blocked soakaway, or any other septic tank problems, it reduces its effectiveness. This means dirtier water is discharging from the septic tank, or backing up into your house. If you have any septic tank problems, contact us at Allerton. We can get it good as new, and ensure you’re following local Environmental Agency Guidelines.
FAQ
Q: What is a ConSept?
A: A Septic Tank Conversion, a cesspool, a well, or some other underground tank to a sewage treatment plant.
Q: When will my unit need emptying?
A: ConSept in a GRP septic tank would be once every 2 years, but in a brick tank it’s better to empty every year. Discuss with the Service Engineer; they’re in a better place to make judgement.
Q: If my septic tank goes into a soakaway, do I need permission to install a ConSept?
A: NO. But you do if you decide to outfall it into a ditch.
Q: What are the challenges in sewage treatment?
A: Converting horrible sewage into relatively clean water and using nature to do it seems like a miracle at times. To do it is not so easy though. That is the constant challenge.
Q: Is planning permission required for a sewage treatment plant?
A: Yes. For links and more information, check out our page on General Binding Rules.
Q: What is the best domestic sewage treatment plant?
A: They are all good. They have to satisfy rigorous testing with human sewage for a long period of time. Personally I prefer the WPL Diamond.
Q: What is the biggest problem with conventional sewage treatment?
A: If there is excessive use of chemicals, then the bacteria are killed. We need the aerobic bacteria to break down not just the organic matter, but the chemicals as well. We use bleach and other agents for cleaning the house and chemicals on our hair and bodies. We use chemicals to sterilise our hands. We use all sorts of powerful agents in the kitchen sink. All this has to be neutralise by our friendly bacteria. If too much chemical product is used, then they can’t do their job properly. Oil and grease suffocate the aerobic bacteria, allowing the anaerobic bacteria to make the whole lot septic again. Putting spent oil into a bottle is better than down the sink. So, everything is permitted but in moderation!
Q: How does a small sewage treatment plant work?
A: All sewage systems work by adding oxygen to the sewage which passes through. These days it is mainly done by passing fine air bubbles through the water, allowing the bacteria air as well as food and water. They convert the sewage to harmless gases in the air.
Q: Does a sewage treatment plant need a soakaway?
A: Not necessarily. It can quite easily go to a ditch or river or other running water. Soakaways are fine though. Effluent from a septic tank has 200 ppm. Water from a sewage treatment plant has less than 20ppm. Rainwater is quite clean and about 5 ppm. Tap water is zero of course
A: The Environment Agency and British Water reckon on a 4 bedroomed house being 6 PE (population equivelent). This generally needs a plant about 2 metres in diameter by just less than 3 metres deep. Much bigger than a septic tank.
Q: How much does it cost to install a sewage treatment plant UK?
A: To simply install a sewage treatment plant it would cost between £3300 and £4000 in normal conditions. Rock makes it more expensive but running sand can double the cost. Try digging a hole on the beach where the tide has just gone out!
Q: How long do sewage treatment plants last?
A: Allerton have units that we still service that’re 25 years old and going strong. Klargester are even older! They are not made for a limited life as white furniture is. They do need to be maintained though. Fortunately there are no moving parts in the WPL units. The Diamond Sewage Treatment Plant relies on a blower, and when we service the unit we make sure the blower is good for another year at least. We are constantly refurbishing and replacing them so as to keep them working constantly. I cannot think of anything else that lasts as long.
Q: What is a BioDisc sewage treatment?
A: Read our blog post Engineers at Work for more information about BioDiscs
Q:What is package sewage treatment plant?
A: This is a piece of kit which converts raw sewage to clean water. Not clean enough to drink of course as it can still carry E. coli and other pathogens. It is high in nitrates and phosphate. It should not be sprayed onto plants but can be used as underground irrigation. Roots have the ability to take what they want and to leave the pathogens in the ground.
Q: How do you make a sewage treatment plant?
A: People over the centuries have tried very hard to make an effective sewage treatment Plant. You need chemists and engineers working together, to get the right balance of air, water, volumes and flows.
Q: What is the purpose of a sewage treatment plant?
A: A sewage treatment plant coverts foul water into clean water, not perfect but good enough to go into rivers and streams.