Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
1998-09-02
2001-01-09
Barry, Chester T. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S605000, C210S609000, C210S620000, C210S630000, C210S767000, C210S175000, C210S919000, C210S920000
Reexamination Certificate
active
06171499
ABSTRACT:
The present invention relates to a method for the treatment and the energetic upgrading of urban and industrial sludge issued from the sewage treatment plants.
The method subject of the claim
1
is characterised by a <<novel>> combination of known and separately tested equipment, allowing to <<liberate>> the energy contained in the sewage sludge and to obtain an energetic positive balance, equal to ≅5.7 tons equivalent petrol/year/1000 equivalent inhabitants.
The method subject of the claim
1
resolves completely the problem of the sewage sludge elimination without pollution transfer, protecting the soil, the ground water and the environment against the bacterial and olfactive pollution and against that of the heavy metals; it is characterised that:
1) Its investment cost is competitive and amounts to about 10% of that, for the same equivalent inhabitants number, of a household refuse incineration plant fitted with heat recovery and regulation conformed exhaust gas cleaning.
2) Its automatism is high-efficient with all the securities and allows its automatic running without any human presence, on the understanding that it doesn't use any under pressure steam.
3) Its maintenance is simple and non-expensive; the maintenance staff could come either from the on site sewage treatment plant or from an external maintenance company.
4) Its total cost for the complete sludge treatment per ton is quite competitive in comparison with the cost of a regulation conformed sludge elimination.
It follows an economic incitement to make new investments related to the process subject of the invention, which are therefore profitable and creating a great number of fixed employment by the equipment suppliers.
Today, the problem caused by the sewage sludge makes the local authorities and the State anxious, because of its big and increasing volume, and the risks of its toxic, bacterial and olfactive pollution toward the soil, the ground water and the environment. It is therefore urgent to dispose of an optimised method of treatment and valorisation of this sludge which is reliable, durable, non-polluting, cheap and applicable to all cases; that is what the <<process subject of the invention>> proposes, for all the sewage treatment plants with a capacity of 30,000 and more equivalent inhabitants.
The current sewage sludge destination is, after its mechanical dehydration, either the agricultural land spreading or the refuse disposal area, and sometimes its joint incineration with the household refuse, including or not a preliminary thermal drying. This latter destination is expensive and non-optimised regarding to the energetic upgrading of the sludge.
The sewage sludge land disposal is going to disappear due to the dumping prohibition in the European Countries announced for 2002.
The sewage sludge agricultural land spreading, currently more and more widespread, is concerned by the high sludge content of P
2
O
5
and the relatively low plants need of phosphoric fertiliser which makes the phosphorus as a restricting component and considers the sludge as a <<late>> fertiliser releasing slowly the fertilising elements.
There is a risk of diseases conveying to the man and the animal by the sewage sludge land spreading or land disposal; for example in Switzerland the disease risk is limited because the spread sludge on the agricultural land must be always <<sterile>> and not contain more than 100 germs (enterobacteria) per gram and any contagious worm-egg. Such an obligation doesn't exist in France.
Even after hygienisation, the use of the sludge as a phosphoric late fertiliser must be carried out with <<parsimony>> because of its brought heavy metals, to avoid at a very long-term (see the following table) a soil saturation with heavy metals; in fact the time limit in years to reach the soil saturation with the heavy metals brought by the sludge land spreading of 2.5 t DS/ha/a, in accordance with the Swiss regulation for soil protection, amounts as indicated in the <<EAWAG-News Nr.28 of September 1989>>, for two heavy metals sludge contents, as follows:
Medium
Low
content
Time limit
content
Time limit
Metal
mg/kg DS
in years
mg/kg DS
in years
Zinc
1500
150
100
240
Copper
800
80
250
240
Cadmium
5
180
1
900
Mercury
5
160
1
800
The proceeding of soil <<filling>> till the regulation limit values are reached due to the progressive generalising of the sewage sludge agricultural land spreading, is not in accordance with a good management of the humanity natural patrimony; one has to be careful because the decontamination of polluted soil would be a tedious and much expensive task for the future generations. Besides the prohibition of the sewage sludge land disposal at short-time (2002), is in this consideration very reassuring.
Concerning the organic toxic elements also contained in the sewage sludge that are: LAS (linear alkylbenz{acute over (e)}ne sulfonate), NP (nonylph{acute over (e)}nol), PAH (polycyclic aromatic hydrocarbon), Sn-OC (pewter organic combinations), PCB (polychlorobiph{acute over (e)}nyles), HCB (h{acute over (e)}xachlorobenzene) et LI (lindane), it fails a better risk evaluation of the sewage sludge agricultural use according to the <<EAWAG-News Nr. 28 of September 1989>>.
The method subject of the present invention, allows to stop these inconveniences; it includes the <<energetic upgrading>> of the sludge preliminary dried till 92% DS (dry solid), as a <<booster fuel>> easily storable and to feed it automatically into the household refuse incinerating furnaces, fitted with heat recovery and exhaust gas cleaning installation in order to separate the heavy metals, which will be treated according to the regulation and to the protection of the soil and the ground water.
The dried sludge quantity amounts on average, in France, to <<4.5%>> of the household refuse for the same number of equivalent inhabitants. Its use as a booster fuel will allow to fill the <<hollows>> of the boiler-furnaces thermal load due to the variability of the household refuse humidity and composition, hence of its LHV (low heat value), and to warrant a <<constant>> steam production and equal in so far as possible, to the nominal capacity of the incinerators, thanks to the dried sludge appropriate storage and automatic feeding equipment which is a part of the process subject of the invention, and will be installed on the site of the nearest household refuse incineration plant.
REFERENCES:
patent: 3552333 (1971-01-01), Salamon
patent: 3702596 (1972-11-01), Winther
patent: 3954069 (1976-05-01), Loken
patent: 4753181 (1988-06-01), Sosnowski
patent: 5186111 (1993-02-01), Baria
patent: 5360546 (1994-11-01), Tomita
patent: 5410973 (1995-05-01), Kunstler
patent: 5630366 (1997-05-01), Lesoille
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