Overview | Waste Water Composition | Waste Water Treatment
Biological Treatment of Waste Water | A Waste Water Treatment Plant

Water that has been used for almost any purpose usually turns to waste water after its use. In several cases it has to be treated so that it may be used again. The world is thirsty. People, industry, agriculture - everything depends on a sufficient supply of fresh water. As the demand for fresh water is considerable high and natural capacities are limited, waste water needs to be recycled. The aim of this article is to explain how water purification is achieved.

Technology for Waste Water Treatment

Waste water can be found in all sorts of quantities in various places. It's best to clarify water close to where it arises. Special technologies can be applied for the removal of special contaminants.
In industry, some factories need huge amounts of fresh water for running their production. They are often allowed to extract it from creeks, as long as they treat it after use and return it into the natural flow. Legislation provides critical parameters for industries to operate within. The operators have to monitor their output regularly and give their results to local authorities.
Apart from industrial waste water, sewage usually cannot be treated at the place of its origin. Taking a shower, washing a machine full of clothes or simply flushing the toilet are actions that produce waste water of different amounts, at different times, containing different pollutants. Therefore, sewage has to run down pipes and canals to be collected and treated in centralised plants. The sequence of typical treatment steps is as follows:

Collection and Combing of Waste Water

The first treatment step requires no specialist technology. It is the collection of sewage, wherever it comes from. Collection tanks are not only good for providing a place for waste water to run into. They are also buffers that do not run empty immediately once the flow of water stops, and they provide enough space to store huge amounts of waste water - for example, should the whole of a large city decide to flush their toilets at once. So, the collection of waste water is good for treatment plants as it means they will not to run dry or, alternatively, will not overrun.
The next step is rather more difficult. Waste water is combed to remove big pieces of rubbish, wood, tyres, or any large fragments of waste from the water. If a tyre was left in the water, for example, the micro-organisms would take a considerably long time to elimintate this. Therefore, on its way to subsequent basins waste water passes a set of combs with an increasing density of teeth. Tyres will probably be held back at the first comb, fast food boxes at a comb with medium-sized indentations and cigarette filters at the finest.

Nitrification and Denitrification

Sewage may now enter the biological treatment section. Usually there are two major steps, nitrification ('nitri') and denitrification 'deni'), sometimes accompanied by additional steps like phosphate elimination or secondary nitri/deni.

The nitrification process has something to do with the element nitrogen (N) as the name suggests. Nitrogen is an important nutrient for bacteria (see entry on biological background for waste water treatment for more details on that topic). The nitri takes bound nitrogen¹ of almost any kind to form nitrates (NO₃^-) and nitrites (NO₂^-).
Nitrification is an oxidation process. This does not only mean that oxygen is bonded to nitrogen where no oxygen was before. Furthermore it means that the oxidation number of nitrogen is increased. This is closely related to the dipole property of the water molecule, described in the entry on the composition of waste water. Oxidation numbers of nitrogen can vary widely, as shown below:

• -3: Ammonia/Ammonium (NH₃ / NH₄⁺), Urine;
• ±0: Nitrogen (N₂);
• +3: Nitrite (NO₂^-);
• +5: Nitrate (NO₃^-); and
• any oxidation number between -3 and +5 is possible.

Denitrification, in contrast, eliminates nitrates and nitrites forming molecular nitrogen (N₂). According to the list presented above, this means a reduction (the opposite reaction to oxidation, named so, due to the reduction of the oxidation number) from +5 or +3 down to zero.

Both nitri and deni occur when carbon is burned. Any carbon-based components of the waste water that are edible for those micro-organisms, are burnt, using oxygen. This means that oxygen consumption is pretty high in nitri, for both nitrogen and carbon are bonded to oxygen. Therefore air, which contains 20% oxygen, is artificially injected into nitrification basins, consuming a not-inconsiderable amount of electrical power.
In contrary, the deni eliminates substances that contain bonded oxygen. Therefore, it is often unnecessary to inject oxygen at this stage. Bacteria can use the oxygen from nitrates and nitrites for burning carbon - therefore producing nitrogen N₂), carbon dioxide (CO₂) and clean water at the same time. This is an 'anoxic' reaction as no molecular oxygen needs to be added.

Sedimentation and Decanting

After its biological treatment the waste water is almost clean and fresh again. However, the micro-organisms responsible for cleaning should now be kept in their individual basins and not run off with all the rest. As a result, bacteria are immobilised and are usually fixed to sand or sludge. This 'activated' sludge is pretty easy to handle. Sometimes it is even possible to run a plant with only one biological basin with a nitri section at one end and a deni section at the other.
So - how is this sludge, hosting our bacteria kept in its basin? The answer is provided by processes called sedimentation and decanting. Sedimentation occurs when solid elements sink to the bottom of water - it happens because the density of the sinking matter is higher than that of the surrounding medium - in this case, water. Sedimentation is driven by gravity or can be performed with centrifugation in an accelerated way (though obviously this would require more electrical power).
As you can imagine, sedimentation is a slow process that is only possible without any turbulence. Therefore, water has to 'run' very slowly through these parts of a treatment plant. Very often, tanks or basins are built in shape of a funnel. Water runs in at the bottom end, where the cross-section is narrow. When going upwards the cross-section increases and the flow rate slows down so that particles can sink.
The employment of a funnel-shape eases the process of filtration. Particles that are carried with the water, flowing in an upward direction, have to pass the already sinking particles. Although this seems to be nitpicking, it has been verified that particles would be carried farther, if there were no particles in their way, moving in the opposite direction.

Finally, when the clarified water reaches the top of those tanks, the treatment is complete. The now clean water is decanted, meaning it runs over the edge of the tank into a circular canal, before it can be discharged into natural circulation again.