Patent
1999-07-29
2000-12-26
Wyse, Thomas G.
610624, 610630, 610631, 610903, C02F 330
Patent
active
06163932&
DESCRIPTION:
BRIEF SUMMARY
Wastewater is treated in treatment plants by mechanical (primary), biological (secondary) and sometimes also (tertiary) chemical treatment. When using mechanical treatment, suspended solids are separated by some form of sieving, followed by separation, usually sedimentation. The removal of organic matter as well as nutrients (nitrogen and phosphorus) and bacteria, however, is not very high.
For further removal, biological treatment is needed, which means that micro-organisms transform organic matter into bio-mass, or sludge, provided that correct physical-chemical conditions are at hand. To enable this process, access to oxygen, or aerobic conditions, is required, which is obtained by some form of aeration system. Furthermore, nutrients like nitrogen and phosphorus must be present, which is the case in municipal wastewater in abundance. When using biological treatment, more than 90% organic removal can be achieved, as well as high removal of bacteria. Nutrient removal, however, will be limited to some 30% nitrogen and phosphorus removal by assimilation into the bio-mass, which is removed as excess sludge for separate treatment and recovery, or disposed of.
For further reduction of finely dispersed impurities, remaining after the previous treatment, and particularly for the removal of phosphorus, chemical treatment can be used. Chemical treatment involves the addition of a coagulant, which is able to coagulate and flocculate remaining impurities into chemical flocs, also precipitating phosphorus, resulting in more than 90% removal of phosphorus. When this tertiary chemical treatment is introduced as post-precipitation after the primary and biological treatment, the removal of organic matter, phosphorus and bacteria can be considerably increased. The coagulant can also be fed into the primary settling stage, pre-precipitation, or in the biological stage, simultaneous precipitation, which in many Swedish treatment plants has resulted in the possibility of using the existing tertiary stage for other purposes, or as an extra safety.
The obtained removal of nitrogen is thus by assimilation, or some 30%. If the sludge is stabilised for possible reuse in agriculture, some of the assimilated nitrogen is released, and re-circulated with the reject water after dewatering, which will result in only 15-20% nitrogen removal.
Below, the conventional methods for wastewater treatment will be presented, in FIGS 1-5, with emphasis on biological nitrogen removal.
In water treatment as well, nitrogen can give problems. The increased use of fertilisers, not rarely overdosed, has resulted in higher contents of nitrates in rivers and lakes, as well as in the groundwater, all being used as a portable water source. Nitrate can be reduced to nitrites, which is toxic to infants.
In order to achieve further removal of nitrogen, ion exchange might be used, though quite costly. Instead biological treatment methods have been developed to first biologically degrade nitrogen compounds to nitrate, nitrification, and then remove nitrogen by dissimilative reduction to gaseous nitrogen by biological denitrification. For potable water treatment nitrogen is often present as nitrate, and biological denitrification can be used.
All treatment of (waste) water includes the separation of suspended matter, either in the untreated (waste) water, or in the biological or chemical treatment stage. Biological treatment involves the transformation of organic matter by micro-organisms into biological flocs, which are separated from the water. Chemical treatment means the transformation of colloidal or fine suspended solids, as well as dissolved matter like phosphorus, into chemical flocs, which are separated from the water.
Normally, the transformation processes are discussed, being very complex. The separation of flocs, however, is more often giving problems, not least at peak flows.
With the presented invention the transformation process with nitrification and denitrification can be more effective, as well as the separation properties of the flocs, for an
REFERENCES:
patent: 3394814 (1968-07-01), Albertson
patent: 5356537 (1994-10-01), Thurmond et al.
Scanvironment AB
Wyse Thomas G.
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