Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2001-10-30
2004-01-20
Barry, Chester T. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S739000, C210S805000, C210S800000, C210S631000
Reexamination Certificate
active
06679993
ABSTRACT:
The present invention relates to the removal of contaminants, particularly suspended and organic materials, from waste water, and is of particular but by no means exclusive application in the purification of sewage.
Existing processes for the treatment of waste water, for the removal of suspended material and other contaminants, include mechanical, biological and physico-chemical processes, or combinations of these. Mechanical processes include filtration techniques, or the removal of solid suspensions through settling and differences in specific gravities. Biological techniques employ microorganisms to remove contaminants, particularly dissolved organic substances, from the liquid waste by incorporation into a biomass which is more easily separated from the liquid stream than the original contaminants. A significant portion of this biomass is generally converted to inorganic materials either before or after separation from the liquid stream. Physico-chemical techniques exploit the reactivity of certain minerals or other chemicals with organic or inorganic contaminants and may be used, for example, to supplement a biological purification technique, such as by chemical addition for improved precipitation and/or flocculation (e.g. the addition of lime for pH adjustment).
One existing physico-chemical technique is described in EP 177543, which discloses a process for the removal of suspended solids, biogenic nutrients and dissolved metal compounds from water contaminated with organic and/or inorganic substances, by dosing a completely mixed type activated sludge reactor with an agent of grain size less than 200 &mgr;m and other characteristics specified in that document, and which contains a minimum of 50 mass percent of rock granules containing at least 25 mass percent finely ground natural zeolite containing preferably clinoptilolite and/or mordenite. In a preferred form of the technique disclosed in this European Patent, sewage is passed through a primary settling tank, followed by a mixing tank and then an absorption zone, and an aeration basin, and finally through a secondary settling tank. In the settling tank, primary effluent and activated sludge are separated and a fraction of the settled sludge is recirculated into the mixing tank.
Zeolite of specified grain size is fed into the mixing tank so that the water leaving the secondary settling tank has a considerably decreased level of suspended material. Pre-treated sewage leaving the secondary settling tank is led through one of several zeolite beds that are filled with suitably prepared material of proper grain size and of high clinoptilolite and/or mordenite content.
These beds are flushed with purified water to remove sludge floccules from the upper layers; the flush-water is then fed back to the primary settling tank.
However, existing processes (employing various suspended growth media, including ground zeolite) have a number of disadvantages including that the growth media are separated from waste sludge prior to disposal, may usually be of synthetic manufacture, and do not have optimal surface area and pore volume characteristics. The process of EP 177543 does not separate the growth media from the waste sludge prior to disposal, it proceeds only up to a complete biological oxidation of contaminating carbon compounds.
Processes of this kind depend on the settleability of suspended material in the waste water. A particle of matter in suspension in a liquid will tend to settle under quiescent conditions if the specific gravity of the particle is greater than that of water. Passage of the particle through the liquid will be resisted by frictional forces, and hence the settling rate will be appreciable only if there is a reasonable difference in specific gravity between the particle and the liquid.
Activated sludge is a flocculant suspension consisting predominantly of bacteria. Because bacterial density is very close to that of water individual bacteria will not settle, and separation of activated sludge from water is dependant on the formation of aggregates containing many bacteria (i.e. flocs).
The way in which flocs settle depends on both their nature/quality and their concentration in the liquid. Many settleability parameters have been proposed in an effort to measure sludge quality as a specific entity unrelated to concentration, but with limited success.
In the course of such physico-chemical processes, the settleability of the sludge may be used to determine the sludge quality and hence optimize treatment of the sludge. Numerous parameters are used for assessment of the settleability of activated sludges, but unfortunately most of these fail to define sludge quality unambiguously, and even when supplemented by additional information regarding the test conditions the results are often not very helpful.
Part of the problem is that available parameters are often applied for purposes other than those for which they were intended, while in some cases standardisation of procedures leads to improved consistency in measurement at the expense of applicability to operating situations.
As with quality parameters in general, different parameters are required for different aspects of settleability. The permissible rise rate in clarifiers, for example, relates directly to the settling rate of the sludge while satisfactory decant of intermittently aerated plants depends on the distance the sludge has settled before decant commenced (and not necessarily on whether it did this at a uniform rate or not).
The most commonly used parameters for assessment of activated sludge settleability are:
the sludge volume index (SVI)
the stirred sludge volume index (SSVI)
the mass concentration of suspended solids (MLSS)
parameters known as V
0
and n (or k), which are used to determine the “steady rate” settling velocity of the sludge at various concentrations.
Both SVI and SSVI suffer from difficulties in that they are affected by sludge concentration to an extent which is not completely predictable (and hence they do not uniquely identify the quality of the sludge).
V
0
and n do seem to reflect the quality of a given sample of sludge, but the test procedure is laborious and may not result in unambiguous values, particularly if not carried out over a suitable range of sludge concentrations. There is also little information available on the changes in V
0
and n response to changes in plant conditions. As such, there are some doubts about application of the results to operating situations.
A number of researchers have attempted correlations between V
0
and n and either SVI or SSVI, and—while general correlations do seem possible—true correlation should probably not be expected because the parameters do not really measure the same thing (reasonable correlation being a reflection of influence by similar factors rather than a true relationship).
The shortcomings with SVI were addressed by Stobbe (1964), who recognised that the SVI is essentially independent of concentration at low sludge concentrations and developed the Diluted Sludge Volume Index (DSVI). However, the DSVI may, in practice, be dependant on the concentration of the sludge under examination, and so may not produce a unique value.
It is also recognized that different types of settling occur, and four distinct settling ‘zones’ have been designated on the basis of floc behaviour. These are known as: 1) the free settling zone, 2) the hindered settling zone, 3) the compression zone, and 4) the transition zone. Not all settleability parameters are appropriate in all zones.
Thus, care must be taken in employing settleability parameters that can correctly represent the characteristics of the particular settling zone being dealt with. In addition, the applicable regime of any particular settleability parameter depends on a number of factors. For example, when using SVI it is necessary to determine the MLSS of the sample (which is usually not possible on site); at low to moderate MLSS, the SVI increases with MLSS, but with a proportionality that is difficult to predict for any given MLSS
Charuckyj Leonid
Cooksey Peter
Barry Chester T.
Larson & Taylor PLC
Zeolite Australia Limited
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