Water that has been used for almost any purpose usually turns to waste water after its use. In most 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, and natural capacities are limited, waste water needs to be recycled. This article covers the way that water is purified, to complete the water cycle.

Technology for Waste Water Treatment

Waste water occurs in very many different places, with different degrees of contamination and in different amounts. It arises both continuously and in batches. So the best treatment philosophy is to clarify water close to where it arises. Specialised technologies can be applied for the removal of each particular contaminant.

These can often be found in industry. Some factories consume huge amounts of fresh water for their production processes. They are often allowed to take it from creeks, as long as they treat it after use and return it into their natural flow. Legislation provides lists naming critical parameters that have to be met for any industrial branch. The operators have to monitor some of them regularly and submit their results to local authorities.

Unlike industrial waste water, household 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 produces waste water of different amounts, at different times and with different pollutants in it. Therefore, sewage has to run down pipes and sewage systems to be collected and treated in centralised plants.

The following describes the various stages of treatment at such a plant.

Collection and Screening of Waste Water

The first step in treatment is fairly simple, technology-wise. It is just collecting sewage, wherever it comes from. Collection tanks are not only good for having some place where waste water can run to, they also are buffers that never run empty, once there is no water arriving; and they have enough capacity to keep huge amounts of waste water, for example, the amount that would be produced if all New Yorkers flushed their toilets at the same moment. So, collecting waste water is good for treatment plants not to run dry or the other way round to be flushed away.

The next step is the first one that actually affects the waste water and its composition, and can justifiably be called the first step in the treatment process. Waste water is "screened" to remove big pieces of rubbish, wood, tyres, or whatever from the water. As leaving a tyre, for example, for the micro-organisms to break down, would take a very long time, the waste water passes a set of comb-like 'screens' with increasing density of teeth, before passing on to the subsequent tanks. Tyres will probably be held back at the first screen, french fries boxes at a medium -toothed screen and cigarette filters at the finest.

Nitrification and Denitrification

The sewage may now enter the biological treatment section. Usually there are two major steps, nitrification ("nitri") and denitrification ("deni"), sometimes accompanied by additional steps such as 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 bonded 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 waste water contents. Oxidation numbers of nitrogen can vary over a wide range, which is documented by the following list:

• -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.

The oxidation that takes place during nitri usually results in oxidation number -3 to +5 and +3, the latter if "incomplete".

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

Both nitri and deni come along with "burning" carbon. 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 tanks, consuming a considerable amount of electrical power.

Conversely, the deni eliminates substances that contain bonded oxygen. Therefore, it is not necessary to inject oxygen here. Bacteria are able to use the oxygen from nitrates and
nitrites for burning carbon, thus producing nitrogen N₂), carbon dioxide (CO₂) and clean water. This type of reaction is called "anoxic", because 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 must now be kept back their tanks and not run out with the treated water. For this reason bacteria, they are immobilized, meaning they 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 tank, having a nitri section at one end and a deni section at the other end.

Well, how is this sludge, hosting our bacteria, kept in its tank? The answer is: by sedimentation and decanting. Sedimentation is the scientific expression for letting things sink to the bottom. Sinking happens because of the density of what sinks is greater than that of the surrounding medium, here: water. Sedimentation is driven by gravity or can be accelerated by centrifugation way, this entails, of course, a higher demand for electrical power.

As you can imagine, sedimentation is a slow process that is only possible if there is no turbulence. Therefore, water has to pass very slowly through these parts of a treatment plant. Very often, tanks are conical in shape, like 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 funnel set-up additionally provides a self-filtering effect. Particles that are carried with the water, flowing in upward direction, have to pass the already sinking particles. It has been verified that particles are prevented from rising by the movement of the particles already on their way down.

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

Links to related pages of the project

This entry is part of the Project

"Biological Waste Water Treatment".

• An Overview of Waste Water
• Waste Water Composition
• Biological Background for Waste Water
  Treatment
• A Waste Water Treatment Plant