D10: Impact of Contaminants - Hydromod

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Integrated Water Resource Management for Important Deep European Lakes and their Catchment Areas EUROLAKES D10: Impact of Contaminants FP5_Contract No.: EVK1-CT1999-00004 Version: 4.0 Date: 25/07/01 File: D10-vers.4.0.doc Page 92 of 136 Mechanical treatment 1. Screening or straining: Typically a coarse screen and fine screens with a min. slots spacing down to 3mm is applied 2. Sedimentation for removing of sand and grit 3. Flotation for removing of oil and grease 4. Pre (or primary)- sedimentation prior to biological treatment for removing of undissolved settable organic and inorganic compounds. With these mechanical treatment processes a reduction in the total waste-load of up to 30 % can be achieved. Pre - sedimentation is not always applied since it is not always wished to fully reduce carbonaceous pollution load in order to keep up a the required ratio C : N : P for the following biological treatment process. Screenings are usually washed and deposed or used in other biological solid waste processes (like composting), the removed grit is washed and deposed or reused for different purposes. The settled sludge from the pre (or primary sedimentation) is referred to as primary sludge and usually submitted to further anaerobic treatment. Biological treatment Activated sludge process (processes with suspended micro -organisms growth) The principle of biological wastewater treatment is based on the ability of microorganisms to deteriorate certain organic substances contained in natural waters. This natural self purification capability of surface waters is technically applied and intensified in the activated sludge process which is mostly applied in plants designed to keep the effluent standards of western European countries. The activated sludge process essentially involves a phase in which the water to be purified is brought into contact with a bacterial floc in the presence of oxygen (aeration phase) followed by a phase of separation of the purified wastewater from this floc (clarification). In fact these processes amount to an intensification of the phenomena that occur in the natural environment. The difference lies in the greater concentration of micro organisms which result in a greater oxygen volume demand. Moreover, in order to maintain the bacterial mass in suspension, it must be artificially mixed. An activated sludge facility always includes a so called aeration tank, in which water to be purified comes into contact with the purifying bacterial mass, • a clarifier, in which the purified water is separated from the bacterial growth
Integrated Water Resource Management for Important Deep European Lakes and their Catchment Areas EUROLAKES D10: Impact of Contaminants FP5_Contract No.: EVK1-CT1999-00004 Version: 4.0 Date: 25/07/01 File: D10-vers.4.0.doc Page 93 of 136 • a re-circulation device for the return of the biological sludge from the clarifier to the aeration tank. This arrangement enables the aerated tank to support the quantity of or concentration of micro-organisms required to maintain the desired level of purification • a device for the extraction and disposal of excess sludge, or surplus sludge, or surplus bacterial growth, which is permanently synthesised from the substrate, • a device supplying oxygen to the bacterial mass in the aeration tank, These plants are usually fed continuously with wastewater, and the processes are applied in separate reactors. However it is also possible to apply these processes (aeration, clarification, excess sludge removal...etc) in the same reactor with discontinuous feeding of the wastewater to different reactors. This mode of operation requires different plant design and process layout and is referred to as SBR (sequencing batch reactor) process. The compounds contained in the wastewater are aerobically degraded by excellular enzymes, by metabolising, by aerobic fermentation processes, by transformation of the compounds to gases (like CO2 and N2 ),by building up additional bio mass or by adsorption to the activated sludge floc. So the compounds contained in the wastewater leave the plant either in the form of gas (CO2) in the form of bio mass (approx. ratio 50 : 50 in aerobic processes) or the not biodegradable compounds in the residual effluent. The capability of the plant is depending on the prevailing bacteria strains. Since different bacteria strains have different growth rates the “sludge age” (the retention time of the bacteria in the system) is the key point in the design of biological treatment plants. The more specialised the bacteria have to be, the lower is the growth rate and consequently the longer the provided retention time has to be. For plants designed only for the removal of carbonaceous compounds a sludge age of 3 days is sufficient, for plants designed for nitrogen removal the sludge age must be minimum 10-12 days depending on the temperature. According to the Council Directive 941/271/EEC of 21 May 1991 concerning urban wastewater treatment most treatment plants have to be designed for enhanced nutrient removal up to the year 2005, what means that the aerobic sludge age will have to be minimum 10-12 days. This means an increase in the sludge age of up to 300 % compared to existing plants only designed for reduction of carbonaceous compounds. As will be described below this will have a great influence on the reduction of endocrine substances since the high sludge age is a precondition for the presence of bacteria which allow the degradation of compounds which are present in the wastewater in only very low concentrations.
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D10: Impact of Contaminants - Hydromod

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