IL31328A - Method of separating inorganic impurities from sewage and of making a soil improving composition from the impurities separated - Google Patents

Description

METHOD OF SEPARATING INORGA IC IMPURITIES PROM SEWAGE AND OF MAKING A SOIL IMPROVING COMPOSITION FROM THE IMPURITIES SEPARATED ru an ΡΙΠ*ΤΠΒ ypip ai»e JOHAN RAGNA1'. LILJEGRE , Ulasgatan if, 7?4. 00 ORSA.

'.' METHOD OF SEPARATING INORGANIC IMPURITIES FROM SEWAGE AND _ OF__ MAKI__N_G_A__-_S_O_IL I_M__P_R__O__V_I_N_G___j_C_O_M_P__O__S_ITI_O_N__F__R_O_M__T_H_E__-__I_M_P_U_R_ITIES SEPARATED.

This invention relates to a method of separating inorganic impurities from sewage and of making a soil improving composition from these impurities. By this method the inorganic salts are separated with the aid of precipitating agents from the sewage which as a rule has been caused to flow through a biologically operating plant for removing organic impurities therefrom, and are obtained in the form of a sludge.

In connection with the purification of sewage it has not been possible hitherto to provide any economically useful method of separating inorganic impurities, so-called nutritive salts. Attempts have been made at removing int.al. •the sodium ammonium phosphates by precipitation, but these attempts were practically altogether unsuccessful, primarily perhaps because of the extremely high costs of the precipitating agents. Another serious problem is that also after dewatering . and drying the precipitated sludge contained so much bacteria and similar organisms that it was not suitable as soil improving composition without costly sterilizing procedures. One would thus have to carry the high costs of procuring the precipitating agents and also the further costs of making the precipitated sludge salable, which contains- valuable soil improving substances. a method of separating, the inorganic impurities from sewage and besides of preparing from the separated sludge a sterile soil improving composition. This- method comprises the steps of adding to the sewage , on the basis of the measured values of the quantity of sewage, the pH-value and hardness thereof and the content of the impurities therein as well as with due consideration of the desired degree of purification, a precipitating agent in the form of lime or the like ,' causing the sewage thus treated to flow into one or more sedimentation and/or flotation pools for the separation of the precipitates before the water is discharged into the receiver, preferably mechanically heavily dewatering the precipitated sludge containing lime and inorganic substances while returning the waste water to said pools, mixing the dewatered sludge with sludge preferably dewatered in a similar manner and deriving from a biologically operating purification plant for sewage, and adding an amount of unslaked lime to said sludge mixture,' which is so adapted to the moisture content of the dewatered sludge mixture that during the reaction of the unslaked lime with the remaining water in the sludge mixture under formation of slaked lime, said sludge mixture is desiccated and heated to a temperature over 100°C for producing a sterile soil improving composition from the solids of the sludge -mixture and the slaked lime .

The method suggested according to the present inven tion will be more fully described in the following with reference to the accompanying drawing which diagrammatically illustrates a sewage purification plant adapted to operate in accordance with the method. plant for topological purification, in which- the organic constituents of the sewage-have been separated. The in-, fluent sewage, termed "highly purified", contains as impuri-ties substantially inorganic matter. At the inlet of the plant there are arranged means 2 and 3 for measuring the quantity of the influent sewage and the degree of pollution thereof. Measuring of the degree of pollution can be effected in various ways for example optically, chemically by measuring the oxygen consumption, or electrically. Also disposed adjacent the supply conduit is a device 4 for supplying precipitating agents, preferably lime, and with the aid of this device there is added to the sewage an amount of lime which is automatically adjusted to the influent volume of sewage and its degree of pollution and corresponds to the desired degree of purification. The water admixed with precipitating agent is then caused to flow into one or more sedimentation and/or flotation pools 5 where a vigorous flocculation arises by reaction between the inorganic substances and the precipitating agent. The flocks gradually sink and settle in the form of sludge at 'the bottom of the pool or rise to the surface thereof.

When the excess oxygen in the influent sewage from the biological purification is maintained at a suitable relatively low level, nitrogen and ammonium, when hydrate of lime is used as precipitating agent, will escape in the form of small bubbles, from the water in the sedimentation and/or flotation pool 5· Bubbling results in that buoyancy will be imparted to the more light-weight flocculations so that they will float to the water surface from where they can be removed in the conventional manner. No supply of carbonic acid or the like is thus necessary to accelerate, The remaining fully purified water is then discharged into the receiver. 'The collected sludge, is preferably transferred first to a sludge container and thence to a ■mechanical dewatering plant 6 which separates most of the water from the sludge. The waste water, is returned to the separating pool for further treatment. The sludge separated - the sludge of nutritive salts - is to be a .constituent part of the desired soil improving composition and is therefore mixed in this substantially dry form with a biological sludge obtained from a conventional plant for biological purification of sewage and similarly dried. It should be observed that the mechanical water separating device can be utilized for instance in smaller plants alternatively for sludge of nutritive salts and biological sludge, but that the waste water from the sludge of nutritive salts has to be thoroughly separated from the biologically operating part of the plant in order' not to destroy the bacterial flora therein.

However, drying of the sludge of nutritive salts and biological sludge can also take place at a time in one and the same centrifuge or like apparatus, and in some cases this has even proved to be preferable, for the biological sludge has a considerably lower specific gravity than the sludge of nutritive salts, but by effecting mixing of the sludges already before drying it is possible to provide a considerable improvement of the dewatering degree. It should be mentioned that also the addition of precipitatin agent facilitates an efficient mechanical dewatering as the precipitating agent- increases the specific gravity of the sludge. At mechanical drying the increase of dry - sludges: which have been centrifuged together is naturally allowed to return to the inorganic purification part of the plant .

The well dried sludge types are mixed in a mechanical mixing device 7 and unslaked lime is added. At the addition of unslaked lime there arises a strong generation of heat when the lime reacts with the water of the moist sludge and this results in the sludge mixture being dried to powder form and in the sludge powder being steriliz since, the temperature will amount during slaking to more than 100°C. To attain this the sludge mixture must have a relatively low water content. At a high water content such a high temperature would not be reached because in such a case too much heat would be consumed for drying the sludge mixture.

Experiments have shown that the water flowing from the sedimentation pool into the receiver .- provided that the biological purification plant ahead of the device for separating nutritive salts functions in a satisfactory manner - contains so small amounts of impurities 'that it can be ranked equal with such domestic water as is used in many places, but is considered less suitable as drinking water.

The soil improving composition obtained in the manner described contains in addition to the organic substances deriving from the biological sludge, lime which is of vital importance to agriculture, and also in various forms minerals and other substances which must indispensably be added to the soil and which had hitherto to be bought in the form of fertilizers.

While the invention h s been de understand that the invention is -.not limited to this . very '. embodiment, but can. be' modified in several ways within the scope of the appended claims.

Claims (3)

WHAT IS CLAIMED fe;L A I M S.

1. A method of separating inorganic impurities from sewage and of making a soil improving composition from -these ■ impurities , in which the inorganic salts are separated with the aid of precipitating agents from the sewage which as a rule has been passed through a biologically operating plant for removing organic impurities therefrom, and are obtained in the form of a sludge, ■ comprising the steps of adding to the sewage, on the basis of the measured values of the quantity of sewage, the pH-value and hardness thereof, and the content of the impurities therein as well as with due consideration of the desired degree of purification, a precipitating agent in the form of lime, hydrate of lime or slaked lime, causing the sewage thus treated to flow into one or more sedimentation and/or flotation pools for the separation of the precipitates before the water is discharged into the receiver, preferably mechanically heavily dewatering the precipitation sludge containing lime and inorganic substances while returning the waste .water to said pools, mixing the dewatered sludge with sludge preferably dewatered in a similar manner and deriving from a. biologically operating purification plant for sewage, and adding an amount of unslaked lime to said sludge mixture, which is so adapted to the moisture content of the de-watered sludge mixture that during the reaction of the unslaked lime with the remaining water in the sludge mixture under formation of slaked lime, said sludge mixture is desiccated and heated to a temperature ove 100°C for producing a sterile soil improving composition from the solids of the sludge mixture and the slaked .lime. ■

2. A method as claimed in claim 1, in which dewatering of the two. sludge types is effected with the aid of a centrifuge, a separator or like apparatus.

3. - A method of separating inorganic impurities from sewage and of making a soil improving composition from these impurities , substantially as hereinbefore described with reference to the accompanying drawing.