Process for treating waste water

Waste waters are treated with hydroxyl producers in the presence of titanium-containing silicates as catalyst.

INTRODUCTION AND BACKGROUND OF THE INVENTION
 The present invention relates to a process for treating waste water.
 A wide variety of processes and combinations of processes are used for
 treating waste water such as, for example, chemico-physical methods such
 as precipitation and flocculation, or adsorptive processes, for example
 using active carbon, or the removal of harmful substances by
 biodegradation or oxidation of harmful substances by different methods.
 The oxidative treatment of waste water with oxygen-containing gases, using
 titanium, silicon or zirconium-containing mixed catalysts at high
 temperatures and under pressure is known (EP 0 257 983 A).
 In order to be able to perform waste water treatment under ambient
 conditions, compounds which form hydroxyl radicals, for example hydrogen
 peroxide, are used in many cases for the oxidation of harmful substances.
 Frequently, however, hydrogen peroxide alone is not reactive enough. It
 has to be activated in an appropriate manner.
 UV light may be used, for example, for activation purposes. UV light causes
 amplified production of hydroxyl radicals from hydrogen peroxide. This
 hydroxyl radical production is also catalyzed by dissolved iron salts. The
 combination of hydrogen peroxide and dissolved iron(II) salts is called
 Fenton's reagent (O. Specht, I. Wurdack, D. Wabner "Mehrstufige
 Pilotanlage zur oxidativen Abwasserbehandlung nach dem Fenton-Verfahren
 (H.sub.2 O.sub.2 /Fe-Katalysator), Chemie Ingenieur Technik 9/1995).
 However, the use of Fenton's reagent has serious disadvantages (I. Wurdack,
 C. Hofl, G. Sigl, O. Specht, D. Wabner "Oxidativer Abbau von AOX und CSB
 in realen Abwassern: Vergleich verschiedener "Advanced Oxidation
 Processes", 3rd GVC conference "Verfahrenstechnik der Abwasser- und
 Schlammbehandlung", VDI-Verlag 1996): The reaction proceeds only at a very
 acid pH values between 2 and 3. Thus, the waste water has to be initially
 acidified, and then neutralized again after oxidation with Fenton's
 reagent, before it can be passed into a sewage treatment plant or
 receiving waters. This results in considerable salting out of the waste
 water and the production of considerable amounts of a sparingly soluble
 iron hydroxide precipitate which has to be removed. In addition, there is
 a high, non-specific, consumption of H.sub.2 O.sub.2 during this reaction.
 It is accordingly an object of the invention was to get round the
 disadvantages of Fenton's reagent mentioned above. Salting out of the
 waste water should be avoided and the non-specific consumption of H.sub.2
 O.sub.2 should be restricted as far as possible.
 SUMMARY OF THE INVENTION
 The above and other objects of the invention can be achieved by a process
 for treating waste water which is characterized in that the waste water is
 treated with hydroxyl radical producers, such as for example hydrogen
 peroxide, in the presence of titanium-containing silicates as catalyst.
 In one embodiment of the invention, metal silicates are used as catalyst.
 Furthermore, a titanium-containing metal silicate selected from the group
 consisting of titanium silicalite TS1, titanium silicalite TS2,
 titanium-containing ZSM5, titanium-containing DAY, titanium-containing
 beta-zeolite and mixtures thereof may be used as catalyst.
 Crystalline metal silicates with regular micropores or mesopores are
 sometimes very effective catalysts for the synthesis of various products.
 In particular, microporous products with the general composition
 represented by the formula (SiO.sub.2)1-x(TiO.sub.2)x, in which titanium
 atoms replace some of the silicon atoms in the crystal lattice, have
 gained industrial importance as oxidation catalysts. Here, titanium
 silicalite-1 (U.S. Pat. No. 4,410,501 incorporated herein by reference)
 and titanium silicalite-2 (J.S. Reddy, S. Sivasanker, P. Ratnasamy, J.
 Mol. Catal. 71 (1992) 373) may be mentioned in particular. Thus, for
 example, the reaction of olefins with hydrogen peroxide to give epoxides
 (EP 100 119), the reaction of aromatic compounds with hydrogen peroxide to
 give hydroxyaromatic compounds (DE 31 35 559), the reaction of aliphatic
 hydrocarbons with hydrogen peroxide to give alcohols and ketones (EP 376
 453) and the reaction of cyclohexanone with hydrogen peroxide and ammonia
 to give cyclohexanone oxime (EP 208 311) are known.
 According to the invention, microporous or mesoporous, titanium-containing
 metal silicates, in particular titanium silicalite-1, may be used as a
 catalyst in the presence of hydroxyl radical producers, e.g. hydrogen
 peroxide, for the oxidation of constituents of waste waters under mild
 reaction conditions, e.g. at neutral pH and room temperature.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention will now be described in greater detail.
 Waste waters which can be treated using the process in accordance with the
 invention are those which contain oxidizable organic or also inorganic
 compounds. The process is especially advantageous when the constituents of
 the waste water are not readily biodegradable. Thus, waste waters from the
 chemical and pharmaceuticals industries, waste waters from the metal
 processing and petroleum industries, ground waters which are contaminated
 with organic substances and waste waters from vent gas scrubbers can also
 be treated in addition to municipal sewage. The harmful substances which
 can be treated are primarily a variety of substituted aliphatic and
 aromatic organic compounds such as, for example, sulfur-containing organic
 substances with mercapto or sulfo functions, carboxylic acids and amides,
 amines, aliphatic hydrocarbons, mononuclear and polynuclear aromatic
 compounds, chlorinated aliphatic and aromatic compounds, aliphatic
 homocyclic and heterocyclic compounds.
 Using the process according to the invention, partial oxidation of the
 harmful substances in waste waters may be achieved. The biodegradability
 of harmful substances may be increased by partial oxidation.
 Since in many cases, in particular in the case of non-biodegradable waste
 waters, the harmful substances which are present are not fully known and
 also cannot be identified, additive parameters such as chemical oxygen
 demand (COD, determined according to DIN ISO 6060 or DIN 38409/41) or
 total organic carbon (TOC, according to EN 1484) are used to characterize
 waste waters. The process according to the invention therefore has as an
 objective in the first place a certain reduction in the COD or TOC present
 and also an improvement in biodegradability, as measured in standard tests
 according to DIN EN 29 888 (Zahn-Wellens process). As a result of partial
 oxidation of the harmful substances present, inhibition of nitrification
 may also be decreased or increased, this being determined by a standard
 Degussa method, SOP UT-001.
 The concentrations of the constituents of waste waters may be in the range
 from a few mg COD/1 to 100 g COD/1, mainly between 0.1 and 20 g COD/1.
 The titanium-containing silicates used as catalysts maybe used in powdered
 form in concentrations between 0.05 and 100 g/l, preferably between 0.5
 and 10 g/l, and/or as moulded items such as e.g. granules, extrudates or
 beads. Titanium silicalite-1 is particularly suitable as catalyst. The
 catalyst powder may contain between 1 and 5% (w/w) of titanium dioxide,
 preferably between 2 and 3%.
 The reaction may be done under comparatively mild reaction conditions. The
 pH value is between 2 and 9, preferably between 6.0 and 8.0.
 The process may proceed at a temperature between freezing point and boiling
 point, preferably in the temperature range between 15 and 40.degree. C.
 Catalysis may be performed in a suspension reactor with or without complete
 separation of the catalyst. The catalyst may be allowed to settle out
 after completion of the reaction, when small catalyst losses have to be
 accepted in the run-off for the treated waste water.
 In another embodiment, the suspension reactor may be combined with
 crossflow filtration, wherein the suspension is allowed to flow past a
 porous area/membrane and a pressure difference is applied between the face
 being flowed over and the filtrate side so that some of the solution flows
 through the area/membrane at right angles to the direction of flow (patent
 application DE 196 17 729.4).
 In another embodiment of the invention, titanium-containing silicates are
 used as moulded items in a fixed bed reactor, through which the waste
 water to be treated continuously flows in the presence of H.sub.2 O.sub.2.
 The moulded items used may be, for example, 2 mm solid cylinders with 20%
 binder (e.g. silica sol).
 EXAMPLES
 1st Example
 500 ml of a model waste water with 2.1 g/l of phenol are initially placed
 in a heated and stirred glass beaker, heated to the start temperature (see
 below, 30.degree. C. or 80.degree. C.) and then 6.25 g of titanium
 silicalite and 12.75 g of 50% strength H.sub.2 O.sub.2 solution are added
 immediately after each other. The mixture without any catalyst is used as
 a blank sample. The pH of the model waste water is 7. The model waste
 water is not buffered. The residual COD is analyzed after different
 reaction times (&lt;=120 min).
 A clear reduction in the COD contained is achieved under very mild
 conditions with titanium silicalite, as compared to a blank sample with
 only H.sub.2 O.sub.2. A corresponding oxidative reaction with the aid of
 iron salts as catalyst (Fenton's reagent) would require a strongly acidic
 pH. An increase in temperature to 80.degree. C.causes a shortening of
 reaction time to less than one quarter.
 During the course of reaction there is a decrease in pH, which indicates
 the production of organic acids due to oxidation.
 The results for example 1 are shown graphically in FIG. 1.
 2nd Example
 500 ml of a model waste water with 2.4 g/l of ethanol are initially placed
 in a heated and stirred glass beaker, heated to 30.degree. C. and then
 6.25 g of titanium silicalite and 12.75 g of 50% strength H.sub.2 O.sub.2
 solution are added immediately after each other. The pH of the model waste
 water is 8.0. The model waste water is not buffered. The residual COD is
 analyzed after 120 min. For comparison, oxidation of the model harmful
 substance is performed in the same test system using Fenton's reagent at
 pH 2.5.
 With Fenton's reagent, a reduction in COD by 54%, as compared with the
 starting concentration, was observed after 120 min. With titanium
 silicalite, under substantially milder conditions, a degree of degradation
 of 40% is achieved at pH 8. Thus, salting out of the water, which is
 unavoidable when using Fenton's reagent, is avoided.
 The results for example 2 are shown graphically in FIG. 2.
 3rd Example
 A column with 100 ml of catalyst packing as moulded items (extruded solid
 cylinders of titanium silicalite, .O slashed.2 mm, length 3 mm, with 20%
 silicon dioxide as binder, calcined for 1 h at 550.degree. C.) is
 continuously supplied with a real waste water stream at 100 ml/h, adjusted
 to pH 6.0, to which had previously been added 5-10 ml of 30% strength
 H.sub.2 O.sub.2 solution. About 80 to 100 ml/h from the column run-off is
 returned to the head in order to maintain catalyst wetting. The amount
 supplied (100 ml/h) is removed from the column run-off.
 An increase of approximately a factor of 8 is observed in the COD values
 measured. This is possibly due to the fact that the constituents are
 oxidized or partly oxidized so that they are quantitatively accessed only
 during the subsequent COD measurement procedure. This is probably not the
 case in the original waste water. That is to say: the COD values measured
 in the original waste water are probably too low, which could explain the
 increase in COD values.
 The toxicity towards a nitrifying biomass (nitrification inhibition) is
 quantitatively eliminated by the oxidative treatment according to the
 invention (proven in the nitrification test according to SOP UT-001). At
 the same time, the fundamental biodegradability is improved in the
 Zahn-Wellens test procedure.
 Other treatment processes tested with this real waste water such as, for
 example, alkaline pressurized hydrolysis or oxidation with Fenton's
 reagent, show neither an increase in COD values measured nor a significant
 reduction in toxicity with regard to the nitrification properties of
 biomasses.
 The results for eliminating nitrification inhibition are shown graphically
 in FIGS. 3, 4 and 5.
 Further variations and modifications of the foregoing will be apparent to
 those skilled in the art and are intended to be encompassed by the claims
 appended hereto.
 German application 199 25 534.2 is relied on and incorporated herein by
 reference.