[nadine chatwin]

nadine chatwin asked the Naked Scientists:
   
Name two similarities and two differences between a farm septic tank system and a city sewage treatment plant.

What do you think?

[Mazurka]

A septic tank in its most basic terms is a holding tank which allows the anaerobic breakdown of waste products.  Typically, the discharge from a septic tank is below the surface in a "drain field" which is a large enough, dry area, with sufficiently high porosity / permeabillity to allow the partially reated effluent to percolate away.  They can be very effective if used right, but if someone pours something that kills the bugs that breakdown the waste into the system, it can become ineffective and stink...

A waste water treatment works will have a number of stages.  These are
pre treatment
Primary Treatment
Secondary Treatment
Teritary treatment

There are a number of different technologies for each stage and the equipment that a waste water treatment works needs depends on how many people would be connected to it, how clean the final discharge needs to be and what other waste (such as industrial effluent) needs to be treated.

Typically, at the start of the process there are the inlet screens to sieve out solid waste such as plastic etc, although if you look at the stuff screened out it appears to mainly be false teeth and tomato seeds.

Then it goes to the primary sedimentation tanks - where solids and liquids are seperated off;

Solids (sludge) can be dried/ composted, further treated, digested anaerobically to produce gas, spread on land as a soil improver. The liquid effluent is then (often) put into a trickle filter tank, (the big round tanks full of gravel with arms rotating around) which primarily breaks down the waste aerobically. 

Tertiary treatment (often called polishing) typically involves sand filter beds, (although charcoal is sometimes used) and other techniques to remove high levels of particular substances (such as nitrate and phosphorous) 

Finally, it is increasingly common for there to be ultra violet treatment of the now clean water to disinfect it prior to discharge directly back to the river

[Geezer]

A septic tank in its most basic terms is a holding tank which allows the anaerobic breakdown of waste products.  Typically, the discharge from a septic tank is below the surface in a "drain field" which is a large enough, dry area, with sufficiently high porosity / permeabillity to allow the partially reated effluent to percolate away.  They can be very effective if used right, but if someone pours something that kills the bugs that breakdown the waste into the system, it can become ineffective and stink...

If the bugs are doing their stuff properly, the solids precipitate in the tank, and only waste water enters the drainfield. If the solids are not removed periodically, they reach a level where they clog up the drainfield. That's when things get very expensive, and extremely unpleasant.