Liquid purification or separation – Processes – Chemical treatment
Reexamination Certificate
2000-07-18
2002-04-09
Hoey, Betsey Morrison (Department: 1724)
Liquid purification or separation
Processes
Chemical treatment
C210S743000, C210S754000
Reexamination Certificate
active
06368511
ABSTRACT:
BACKGROUND OF THE INVENTION
Industrial and municipal wastewater treatment produce large amounts of sewage sludges, generally as a mixture of “primary sludge” (PS) and “secondary sludge” (surplus activated sludge SAS). The sludges, to reduce/utilize the organic sludge fraction, are in part subjected to a digestion and even after their thickening are present as very high-water-content raw sludges having low dry matter contents of, for example, only 4 to 5% by weight. In Germany this involves annually approximately 80 million tons of raw sludges, equivalent to approximately 3.6 million tons of dry matter per year.
The invention is directed at a considerable improvement in dewatering of industrial and municipal sewage sludges and relates especially to a novel process for sewage sludge conditioning as an important process step prior to actual sewage sludge dewatering.
Each sewage sludge conditioning treatment, such as the conventional organic conditioning with polyelectrolytes or else the conventional inorganic conditioning with lime, is intended to ensure or improve the dewaterability of the sewage sludges (which are still rich in organics even after digestion), since without a specific conditioning they would be difficult to dewater. The purpose of the invention is a novel sewage sludge conditioning which leads to a considerable improvement in the subsequent sewage sludge dewatering.
Every current sludge treatment has the purpose of utilization or elimination and generally proceeds via the processing substeps of flocculation/thickening, sludge reduction (for example digestion, hydrolysis) and/or—directly—flocculation/thickening, conditioning, dewatering, drying and incineration. Sewage sludge incineration plants generally include the residual drying which is still required of the mechanically dewatered sewage sludge (for example fluidized-bed furnaces and multiple hearth furnaces). The final purpose of sludge treatment is, according to the German Technical Regulations on Waste, generally sewage sludge mineralization by incineration.
Critical factors in the overall costs of sewage sludge mineralization by incineration are, in addition to the costs of the other upstream stages, the incineration costs. If it were to be possible to reduce considerably the water load on the sewage sludge, these could be correspondingly reduced both with respect to capital costs (for example smaller plant size) and also with respect to operating costs (for example reduced consumption of auxiliary fuel).
The purpose of much current work on conditioning with respect to dewatering and on dewatering itself is a sludge which is dewatered as highly as possible (see, for example, [1]). A main starting point for improvements of mechanical dewatering is still the upstream conditioning of the sludges high in organics. The conditioning must be precisely matched to the sludge present in the individual case itself and at the same time to the dewatering technique employed (for example decanters, fire filter presses, chamber filter presses, membrane filter presses).
Conventional conditioning is preferably carried out using organic conditioning aids, for example polyelectrolytes (PE), but not rarely also using inorganic conditioning aids such as slaked lime and/or other substances promoting dewatering, for example pulverized coal or fly ash from coal combustion.
Especially in the case of inorganic conditioning, the amount of sludge dry matter originally present (SDMo) is noticeably increased by the amount of conditioning aids added (CA) to the higher amount of dry matter then present SDM+=SDMo+CA. The CA load of the original sludge dry matter yCA (as the ratio CA/SDMo) is set, for example, to values of 0.25 kg of CA/kg of SDMo, frequently even substantially higher. This increase in the amount of original sludge dry matter by 25% or more is a reason (but not necessarily a valid one) for organic conditioning using PE frequently having been preferred recently to inorganic conditioning.
In addition to the described organic or inorganic conditioning, even earlier, other possible methods for sludge conditioning have been investigated. One of the investigated possible methods relates to oxidative partial degradation of organic sludge components, in particular the “slimy microbial sludge fractions”, which obviously greatly impede sludge dewatering. Such oxidative treatment of sewage sludges based on the Fenton reagent (hydrogen peroxide H
2
O
2
and divalent iron ions Fe
++
as catalyst, added, for example in the form of FeCl
2
) or based on ozone are the subject of extensive work which also considers ancillary questions such as AOX degradation or odour reduction (as a specific partial degradation).
A substantial part of the prior art on H
2
O
2
conditioning is described in summary in [2]. Despite many studies (cf., for example, [3], [4], [5]), although using H
2
O
2
conditioning the dewatering results achieved on various dewatering machines were somewhat improved, the improvement was not substantial, that is to say the dewatering results, in our opinion, were still unsatisfactory.
The dewatering result is usually described by the dry matter content achieved DM+(% by weight), i.e. the ratio DM+=water/(water+SDM+), which also includes the conditioning aid CA added in variable amounts in the inorganic conditioning (SDM+=SDMo+CA). What is termed the water loading yH
2
O of the original sludge dry matter, that is to say the ratio yH
2
O=water/SDMo appears far more suitable for engineering evaluation of dewatering.
Conditioning not only has a beneficial effect on the extent of the degree of dewatering achieved, but also on the dewatering rate (e.g. filtration times, pressing times) and thus the output of a dewatering machine used.
The effect of an oxidative conditioning on sewage sludge dewatering in, for example, belt filter presses and chamber filter presses which is beneficial in trend, especially of acidic oxidative conditioning using the Fenton reagent, is therefore known in principle. However, contrary reports are found, for example relating to the type and amount of the iron catalyst or of corresponding transition metals. From the multiplicity of these studies on the acidic oxidative conditioning of sewage sludges using the Fenton reagent, the procedure in principle and unit operations which participate are substantially known from earlier work:
Acidification proceeds as far as starting pHs of from 3 to 4.
Preferably, H
2
O
2
/Fe
++
molar ratios≦5:1 are employed.
Most frequently, the cheaply available Fe(II)SO
4
.7 H
2
O (green vitriol) is used, which, however, in the event of a subsequent neutralization with lime, would have the disadvantage of considerable gypsum formation.
The H
2
O
2
requirement is sludge-dependent; the minimum H
2
O
2
requirement is 1.0 to 1.5% by weight of H
2
O
2
(based on SDMo).
The oxidation reaction proceeds somewhat rapidly, as a function of temperature (minimum time required 10 min).
Elevated temperatures (e.g. 60 to 90° C.) which are, however, associated with the disadvantage of considerable residual filtrate pollution for example with COD, BOD, NH
4
+
, accelerate the oxidation.
In some work, specific post-conditioning was claimed explicitly.
However, the references previously made to post-conditioning are concerned only with:
a) an additional organic PE post-conditioning, with reference frequently being made to the high pressure sensitivity of H
2
O
2
-preconditioned and PE-post-conditioned sludges, and/or
b) a “neutralization” as increasing the pH with, for example, NaOH, Mg(OH)
2
or Ca(OH)
2
back to the vicinity of the neutralization point (avoidance of corrosion) and furthermore to a final pH of a maximum of 8.5, the latter with the aim of rebinding heavy metals which have dissolved in the interim in the previously acidic environment and/or with the aim of decreasing the COD/BOD residual pollution of the later filtrate.
The object underl
Lemke Joachim
Melin Thomas
Vosteen Bernhard
Weissenberg Heinz-Günther
Bayer Aktiengesellschaft
Eyl Diderico van
Gil Joseph C.
Henderson Richard E. L.
Hoey Betsey Morrison
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