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Waste Water Treatment

General Aspects

Waste water can be defined as water, which was contaminated by domestic, agricultural or industrial use. A distinction is drawn between waste water from private and commercial or industrial use.[1] From domestic use waste water usually results from washing processes or the sanitary use. Typical contaminations are detergents or feces.

Industrial waste water was used for raw material extraction, as solvent or reaction substrate or as cooling medium. Thereby cooling water is the principal part of industrial waste water. The contamination may vary strongly and the contaminants can be differed according to following groups:[1]

  • dissolved or undissolved substances,
  • readily degradable or persistent substances,
  • herbal nutrients, heavy metal compounds or salts.

 To protect bodies of water, rivers and lakes from contamination, waste water has to be treated to remove pollutants as far as possible. In general, waste water treatment can be subdivided into 3 general procedures. Mechanical waste water  treatment is used for a first rough separation of pollutants such as fabric residues, pieces of wood, plastic objects feces and sand. For that, rakes, filters and sedimentation basins are used. Biological waste water treatment implies a more advanced purification because dissolved pollutants are degraded as well. Microorganisms degrade the contaminants. Organic compounds are degraded through aerobic and non-aerobic processes. In this way, inorganic compounds and biomass are released as degradation products. It is important to note, that microorganisms have to be protected against acidic and basic solutions and diverse toxics. Hence, thorough upstream processing of the waste water is crucial for operation of biological waste water treatment. Chemical waste water treatment includes the use of chemical substances for procedures such as precipitations, neutralizations and flocculation’s. The objective of such procedures is the conversion of dissolved substances into insoluble compounds, which can be easily removed afterwards by mechanical treatment processes.[1]

The design of a waste water treatment plant is strictly oriented by this general procedures. Untreated waste water is pumped into a basin containing a rake for mechanical treatment first. Major pollutants are removed, collected and afterwards burned in a waste combustion plant. After this, the water is pumped into a downstream basin containing an oil- and sand filter. Oils, fats and sand are separated from the waste water and pumped into the digestion tank (see Figure 1)[1;2]

process_mechanical

Figure 1: Scheme of the section for mechanical waste water treatment.[1]

 

process_biological

Figure 2: Scheme of the section for biological waste water treatment.[1]

In the second section of a waste water treatment plant, waste water is biologically processed by pumping the water into an aerated basin containing a so called bacteria sludge. The air supply enables the degradation of carbon, nitrogen and phosphorus compounds by aerobic processes. At this stage, the water is almost purified. A downstream secondary sedimentation tank ensures the separation of the aggregated and sedimented bacteria. This sludge is either pumped into the digestion tank or back into the air supply basin. A scheme of the biological section of a waste water treatment plant is shown in Figure 2[1;2].

As mentioned above, chemical treatment represents the last step of waste water treatment. In multiple basins heavy metals, phosphates and nitrates are removed through precipitation. After this step, the water is pumped into adjacent waterbodies. The sewage sludge can be collected in the digestion tanks. In these tanks anaerobic fermentation occurs. The released methane can be used to generate electric energy and the solid residue can be utilized as fertilizer or, depending on its contamination, burned in a power plant.[1;2]

Contamination of Waste water with pharmaceuticals

Pharmaceuticals are one of the main products of European chemical industry with a share of 25 %. The pharmaceutical industry in Germany produces about 30 000 t of pharmaceuticals per year. Pharmaceuticals are usually very stable chemical compounds. Besides ensuring high activity, it is a crucial objective in pharmaceutical research to guarantee a long term stability of distinct products. In this way it is ensured that some drugs such as Diclofenac deploy their full

process_chemical

Figure 3: Scheme of the section for chemical waste water treatment.[1]

efficacy in the bowel by protruding gastric acid. On this account, most pharmaceuticals are discharged unchanged. Because waste water plants are not constructed for degradation of dissolved persistent chemical compounds, most pharmaceuticals are not removed during the treatment of municipal or industrial waste water and are released to natural water bodies. In recent years, it has been shown that this increasingly becomes a serious problem for the environment and mankind. An indicator for this is the growing accumulation of more than 180 of the 3000 accredited drugs in water bodies, that can already be observed today. Hence, the accumulation of popular drugs such as Diclofenac is subject to recent research with the objective of a decrease of the amount released to nature.[3]

70% of the ingested amount of Diclofenac remains unchanged and is egested by the human body. In Germany the annual consumption of Diclofenac reaches 90 t. The maximum concentration of Diclofenac in water bodies in Germany is 0.05 µg l according to legislation. In several German regions this value is exceeded. In recent studies it could be shown that trouts exposed to this Diclofenac concentration revealed multiple harms to animal skin, kidney or branchia.[3]

In future pharmaceutical degradation will become an important task in waste water treatment. In this context, application of Advanced Oxidation Processes in waste water treatment represent a new approach to handle this problem.

 



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