Anaerobic purification of waste water containing sulphate and organic material

Abstract: Waste water containing sulphate and organic material is treated in a two-step anaerobic biological plant in two fluidized-bed reactors. In the first reactor (1) (acidification stage) at least 80 % of the sulphates are reduced to hydrogen sulphide which is then removed for at least 70 % in a gas stripper (13) by a purge gas (12). The effluent from the stripper (13) is then digested in the second (methanic) reactor (22).

Description: This invention relates to a process for the purification of waste water containing sulphate and organic material by anaerobic waste water treatment. During anaerobic waste water treatment sulphate, if present, will be reduced predominantly to hydrogen sulphide. In most cases this is disadvantageous as it results in a decrease or even complete inhibition of the methanogenic capacity of the sludge and the production of a malodorous effluent. It is difficult to inhibit this sulphate reduction. Therefore the application of anaerobic purification techniques to highly contaminated waste waters is often limited by the high concentrations of SO₄²⁻. Examples are the vinasses of fermentation processes such as alcohol, lactic acid, citric acid, penicillin. In such cases, SO₄²⁻-concentrations of up to 4 g/l may occur (= 1330 mg S/l). However, the application of anaerobic purification in this field is highly attractive, owing to the very high COD involved. Typical COD-loads are 10-60 ton/day, leading to expensive purification reactors. The alternative route used in the majority of cases to handle these highly contaminated wastes is further concentration to a slurry that can be sold as a fodder additive. It appeared, however, that in many cases the fodder market is not sufficiently large to serve as an outlet for this slurry. Another route is to use such slurry for irrigation/fertilization of agricultural land, but here again, there are limits to the amount that can be applied per hectare (2 471 acres). Owing to these problems, and because of the CH₄-production, anaerobic purification has become increasingly important for these highly contaminated waste waters. However, the high level of the SO₄²⁻ concentration appeared to be a major obstacle. A general possibility to avoid the problems arising from the presence of SO₄²⁻ in waste water is to bring about complete reduction of the SO₄²⁻ to H₂S and to remove this H₂S continuously from the water to alleviate its toxic effect on methanogenesis. French patent application FR-A-2484990 discloses the continuous purification of sulphate-containing waste water by treatment with anaerobic methanogenic bacteria and the removal of sulphides formed from the sulphates using a stripping gas. The elimination of the sulphides is carried out in an external gas stripper. The sulphate containing waste water is purified in an anaerobic waste water reaction space wherein at the same time the acidification and methanization takes place. To keep the acidification and methanisation working, the sulphide concentration is kept below 0.1 g/l in the reactor. Therefore the ratio of feed to this reactor must be 0.5 to 0.02 times the liquid feed to the gas stripper. British Patent Application GB-A-2143810 describes a process wherein primary sulphur oxide-containing wastes, for example containing gypsum (CaSO₄), are biologically converted. The process of this British Patent Application is carried out in an anaerobic reactor with suspended growth biomass. Preferably a continuous upflow expanded bed reactor or a fixed film reactor is used. Although it is suggested that the conversion rate in this process will be from about 8 to about 32 kg/m³ of reactor space per day in the examples the average conversion rate is 1 kg SO₄²⁻/m³ of space per day at most. The hydraulic liquid velocity in the reactor is about 1.2-2.4 m/h. However applying this hydraulic liquid velocity in combination with the applied retention time of one day results in very wide reactor designs, which are economically unattractive for industrial application. The pH in the process of British Application GB-A-2143810 remains in the range of 7.0 to 8.0, regardless of the pH of the influent. The stripping of hydrogen sulphide in those pH ranges is unsatisfactory; the higher the pH, the worse the efficiency of the gas-stripper. As a result of extensive research and experimentation a new process has now been found which is surprisingly especially suitable for the purification of sulphate containing waste water. The SO₄²⁻-reduction capacity in the process of the present invention is far greater than hitherto thought to be possible (up to 30 kg SO₄²⁻/m³ space reactor per day). The present invention relates to a process for the purification of waste water containing sulphate and organic material by anaerobic biological waste water treatment in a two-step process, an acidification stage followed by a methane fermentation stage, characterized in that at least 80 percent of the sulphate is converted into hydrogen sulphide in the acidification stage which takes place in a fluidized bed reactor, and the hydrogen sulphide concentration in the acidification reactor is kept below 200 mg S/l, and at least 70 percent of the resulting hydrogen sulphide is removed from the waste water. The optimization of the sulphate conversion in the acidification stage of a two stage anaerobic digestion and the removal of hydrogen sulphide from the acidified waste water will prevent the inhibition of the methane fermentation by hydrogen sulphide, which will benefit the methane fermentation. Moreover as the aqueous effluent leaving the acidification stage contains minimal amounts of sulphates and sulphides, the biogas formed in the methane fermentation stage will, as a result of this process, contain only minimal amounts of hydrogen sulphide. The hydrogen sulphide recovered from the acidification may be converted into a product of greater value such as sulphur, NaHS-solution, SO₂ and SO₃ and/or H₂SO₄ (for example H₂SO₄ with a concentration of 20%). In such processes, if internal gas stripping takes place, sometimes the efficiency in keeping the acidifying biomass inside the reactor, because of the gas present in the system, has to be controlled. When a gas stripper is situated outside the acidification reactor, the conditions in the process have to be chosen so as to achieve a high liquid recirculation over the gas stripper in case of high sulphate conversions. For example when the sulphate concentration in the influent amounts to 1000 mg S/l the recirculation ratio has to be chosen to remain below a desired sulphide concentration in the anaerobic reaction space. Therefore, in the preferred embodiment of the invention, a fluidized bed reactor is used for simultaneous acidification and sulphate reduction. Advantageously superficial liquid velocities of 5-30 m/h are applied in this reactor. In the fluidized bed reactor carrier particles preferably having an average dimension smaller than 3 mm, more preferably smaller than 1 mm, the density of the carrier is preferably at least 1200 kg/m³, more preferably 1500-4000 kg/m³ and the mean concentration of the carrier under process conditions in the reactor is preferably smaller than 400 kg/m³ reactor volume. For example, river gravel, silver sand, eiffel-lava are suitable carriers. In a preferred embodiment of the invention, the hydrogen sulphide is removed using a purge gas. In another preferred embodiment of the invention the hydrogen sulphide is removed by creating a zone of subatmospheric pressure above the reaction liquid so that the H₂S can be removed from this zone. The reaction liquid is the liquid in the acidification process and/or the liquid in the optional gas stripping device, situated externally to the acidification reactor. In a gas-lift fluidized bed reactor, the purge gas may be used to obtain the gas lift in the reaction space for internal gas stripping. In still another preferred embodiment of the invention a fluidized bed reactor with an external gas stripper is used. The external gas stripper gives the possibility of obtaining more independent process parameters. When using a purge gas in the stripper, gas strippers known per se or which are conventionally designed can be used. When subatmospheric pressure is created above the liquid in the gas stripper, all the dissolved gases will escape from the liquid for example H₂S, CO₂, CH₄, etc. An advantage of this method is that the H₂S is concentrated in the gas leaving the gas stripper, while the methane formed in the methanization stage is substantially free of H₂S. These gases from the external gas stripper are removed optionally together with the gas formed in the acidification stage. In all these embodiments, it is important to know the maximum sulphide level that may be maintained in the reactor without inhibition of the acidification. As is shown by R.L. Starkey in "Sulphate-reducing bacteria-physiology and practical significance", University of Maryland (1960) the sulphate reducers can tolerate sulphide concentrations up to 2000 mg S/l, corresponding to the reduction of 6 g SO₄²⁻/l. Their growth is most rapid at reactions close to neutrality but the limiting conditions are about pH 5.5-9.0. During the experimentation, it was found that in this pH region the required reactor volume will decrease with an increasing pH, while on the other hand, the purge gas flow which is necessary to achieve the allowable sulphide concentration will decrease with lower pH values. It was found during experimentation that the pH in the acidification process is advantageously 5-8, while the temperature in the acidification process is 25-45° C. Preferably the pH is maintained at 6-7 in the acidification process. The sulphate reduction and the stripping of hydrogen sulphide produces an increase of the pH. For example an increase of the pH of 1-2 units is observed when waste water having a sulphate concentration of 4000 mg/l is used. Therefore the pH has to be controlled, for example by adding acid to waste water. Another possibility is the use of a purge gas containing carbon dioxide, preferably biogas containing carbon dioxide, which will reduce or even prevent this acid consumption. Preferably at least 80% of the hydrogen sulphide is stripped by the purge gas. The purge gas then contains suitably at least 0.5% (v/v) of hydrogen sulphide, preferably about 2-5% (v/v) of hydrogen sulphide. The acidification efficiency is calculated as: Preferably, such conditions are determined so that the acidification efficiency of the effluent will have a value of at least 40%. Typical acidification percentages are 45-70% depending on the kind of waste water to be purified. For example with space loadings of the acidification of 50 kg COD/m³.day and an overall efficiency of about 60%, the conversion capacities will be 30 kg COD/m³.day. The sulphate reduction capacity in the fluidized bed acidification stage will increase rapidly in the first few days after the introduction of the necessary microorganisms. For example, after inoculation with suitable sludge originating from a waste water treatment process, the sulphate reduction increased to almost 100% within two weeks in a fluidized-bed process, when waste water with a sulphate level of 1500 mg/l was treated. The effluent of the acidification stage of the present process is treated in a methanization stage. In an embodiment of the invention two acidification stages are connected in series, in order to obtain very high conversions of the sulphate. It will be appreciated by those skilled in the art that any suitable methane fermentation reactor can be used in connection with the present process, such as a UASB reactor, a fluidized-bed reactor, an anaerobic filter or a down-flow stationary fixed film process. The invention will be illustrated by the following examples in conjunction with the accompanying Figure Figure 1 shows schematically an embodiment of the process of the present invention in which an external gas stripper is used. Example 1 The flow sheet of the experimental system is shown in Figure 1. The experiments were performed in a fluidized bed reactor (1) with a total volume of 1000 ml. The reactor volume without the settler (2) was 820 ml. Sand with a diameter of 0.25-0.4 mm was used as carrier material. The reactor was inoculated with 50 ml sludge from a pilot-plant acidifying fluidized bed reactor. The internal diameter of the reactor was 4 cm, the height 48 cm. The superficial liquid velocity was about 15 m/h. This was achieved by a peristaltic tubing pump (3). The hydraulic retention time in the reactor was adjusted to about 4 h. The methane fluidized bed reactor (22) was identical to reactor (1). The reactors were placed in a thermostatic controlled case. The temperatures in the reactors were adjusted to a temperature of 35 ± 1 °C (acidification stage) and 32 ± 1 °C (methanization stage). The hydrogen sulphide was stripped with a purge gas (12) consisting of technical N₂-gas in a gas stripper (13) with a diameter of 4.5 cm and a volume of 0.47 1. The nitrogen was distributed by means of a tube of sintered glass. The ratio purge gas flow / influent waste water flow was adjusted about 25 ± 5, in the gas stripper itself, the ratio purge gas flow / recirculation flow was about 0.3. The waste water (4) consisted of a diluted yeast waste water with a COD of 10-14 g/l. The sulphate additions were 0.4 ml 36N H₂SO₄ /l influent and 1.24 g K₂SO₄ /l influent, corresponding to 14.4 meq acid/l and resulting in a sulphate concentration of 1.5 g/l 5 ppm Fe⁺⁺ and 4 ppm phosphate (PO₄³⁻) were also added. The different analyses were performed in grab samples. VFA, COD and sulphate were determined on supernatant samples prepared by centrifugation at 15000 RPM for 10 minutes. The COD mixed samples were collected from the overflow through the tube system. During the experiment the pH was adjusted (6) only by the addition (11) of sulphuric acid to the influent. This additional supply of sulphuric acid above the addition of 14.4 meq acid/l to the waste water, was necessary to repress the pH increase which is attributed primarily to the sulphate reduction process (see equation 1) in combination with the fact that the waste water was buffered weakly. SO₄²⁻ + H⁺ + 4H₂ → HS⁻ + 4H₂O (1) Hydrogen sulphide is a weak acid and will act as a buffer. For bisulphide (see equation 2) the maximum buffer index is obtained at pH = pKa = 7. H⁺ + HS⁻ ⇄ H₂S pKa = 7 (2) For those reasons the reduction of 1.5 g SO₄²⁻/l will increase the alkalinity by about 15 meq/l. After the inoculation the sulphate reduction increased in about ten days to a level of nearly 100%. About 82-92% of the H₂S produced was stripped in the gas stripper. The space loading of the acidification stage was about 66 kg COD/m³.day based on the reactor volume of 1 l. The average of the flow rate was 0.25 l/h and the influent concentration 11 g COD/l. The overall efficiency was about 67% and the conversion capacity was 44 kg COD/m³.day. The sludge conversion capacity for volatile fatty acids came to 4 kg COD/kg VSS.day. When the combined sulphate reduction and acidification process reached a steady state the end product was mainly acetic acid. The methane fermentation which occurred partly in the acidification stage and the removal of hydrogen sulphide by the purge gas both result in a COD removal in the combined sulphate reduction and acidification process. The biogas production (5) was substantial with a COD removal of 900-1400 mg COD/l. The sulphate conversion capacity was about 8.1 kg SO₄²⁻/m³.day. Example 2 The procedure of example 1 was repeated but the sulphate concentration of the influent was increased to a value of 4.3 g SO₄²⁻/l. Moreover 1000 mg/l of propionic acid was added to the influent. Other conditions were maintained as in Example 1 except for changes mentioned later on. After a period, the process ran steady while 4.0 g SO₄²⁻/l was reduced. The sulphate reduction capacity was 21 kg SO₄²⁻/m³.day (based on the volume of reactor and a flow rate of 0.25 l/h), corresponding with the conversion of 13.9 kg COD/m³.day. The sludge conversion capacity was about 2 kg SO₄²⁻/kg VSS.day. The space loading of the acidification stage was 84 kg COD/m³.day based on the reactor volume of 1 l and an hydraulic retention time of about 4 h. The average of the flow rate was 0.25 l/h and the influent concentration 14 g COD/l. When the overall efficiency was about 67% the conversion capacities were 56 kg COD/m³.day. The sludge acidification capacity for volatile fatty acids came to 6 kg COD/kg VSS.day. When the combined sulphate reduction and acidification process had reached a steady state, the end product was mainly acetic acid. About 1.5-2 g H₂S-COD/l waste water was removed by stripping. Together with the production of 1.4 g biomass COD/l waste water, this resulted in a COD reduction (centrifuged effluent samples) of about 3 g COD/l. At least 85% of the produced H₂S was stripped by the purge gas. The H₂S content of the purge gas (14) after leaving the gas stripper was 2-5%. The sulphate reduction and the stripping of hydrogen sulphide produced an increase of the pH with 1-2 units. The control of the pH to repress this increase to a value of 6.7 required about 5 meq acid/g COD. The effluent of the acidification stage was combined with the recirculation (24) of the methane fluidized bed reactor (22) (see figure 1) and both were pumped (21) into the methane reactor. The effluent of the methane reactor was removed via outlet (25). The methane reactor started with about 600 ml sludge from a full-scale methanization fluid bed process, supplied with 200 g sand of 0.2-0.4 mm and 50 g of 0.8-1.25 mm. The gas production (23) increased in ten days until the conversion capacity was about 14 kg COD/m³.day. The VFA were converted in that period with an efficiency of 90%, and the overall (acidification + methanization) COD-removal efficiency was 60%. The overall reactor loading was about 41 kg COD/m³.day and the conversion was 24 kg COD/m³.day. Example 3 Industrial soybean waste water enriched with 3700 mg/l sulphate was used. Experimental conditions were maintained as in Example 1 except for the methane reactor where, in this example, a UASB-reactor (UASB = upflow anaerobic sludge blanket) was used and as support material in the fluidized-bed acidification reactor, lava of 0.2-0.3 mm was used. After a period the process ran steadily while 2.8 g SO₄²⁻/l was reduced (75%). The sulphate reduction capacity was 20 kg SO₄²⁻/m³.day (based on the volume of the reactor) corresponding to the conversion of 12 kg COD/m³.day. The sludge conversion capacity was about 0.6 kg SO₄²⁻/kg VSS.day. The space loading of the acidification stage was 85 kg COD/m³.day based the reactor volume of 1.0 l and a hydraulic retention time of about 3 h. The average of the flow rate was 0.3 l/h and the influent concentration 11.4 g COD/l. When the overall efficiency was about 80% the conversion capacity was 68 kg COD/m³.day. The sludge conversion capacity for volatile fatty acids came to 2.2 kg COD/kg VSS.day. When the combined sulphate reduction and acidification process reached a steady state, the end product was mainly acetic acid. At least 70% of the produced H₂S has been stripped by the purge gas. The H₂S content of the purge gas (14) after leaving the gas stripper was 2-3%. In the combined acidification and sulphate reduction process, COD-removal efficiencies of about 34% were achieved. In this example the pH in the acidification stage was maintained at 6.5. At the end of the experiment the reactor and stripper contained 32.3 g VSS. The biomass concentration in the reactor was 31.5 g VSS/l while 2% of it was present as suspended solids. The support overgrowth amounts to 110 mg VSS/g support. The effluent of the acidification / sulphate reduction reactor was combined with the recirculation of the UASB-reactor. The hydraulic retention time in this reactor was about 9.0 h. The conversion efficiency of the volatile fatty acids was over 90%, the overall soluble COD removal efficiency was 85% and the overall total COD-removal efficiency was 80%. The reactor loading was about 15 kg COD/m³.day. The UASB-reactor was started with about 1.25 l granular sludge from an industrial scale UASB reactor, used for the purification of effluent from a sugar industry plant. The amount of the inoculated sludge was approximately 100 g dry suspended solids (about 80 g VSS).

Claims: Process for the purification of waste water containing sulphate and organic material by anaerobic biological waste water treatment in a two-step process, an acidification stage followed by a methane fermentation stage, characterized in that at least 80 percent of the sulphate is converted into hydrogen sulphide in the acidification stage which takes place in a fluidized bed reactor, and the hydrogen sulphide concentration in the acidification reactor is kept below 200 mg S/l, and at least 70 percent of the resulting hydrogen sulphide is removed from the waste water. Process according to claim 1, characterized in that the superficial liquid velocity in the fluidized bed reactor is 5-30 m/h and carrier particles having an average dimension smaller than 3 mm are used in the fluidized bed reactor, the density of the carrier being at least 1200 kg/m³. Process according to claim 2, characterized in that carrier particles having an average dimension smaller than 1 mm are used, the density of the carrier is 1500-4000 kg/m³ and the mean concentration of the carrier under process conditions in the reactor is less than 400 kg/m³ reactor volume. Process according to any one of claims 1-3, characterized in that the resulting hydrogen sulphide is removed using a purge gas. Process according to claim 4, characterized in that the purge gas contains carbon dioxide. Process according to claim 5, characterized in that biogas is used as purge gas. Process according to any one of claims 1-3, characterized in that the resulting hydrogen sulphide is removed by creating a zone of subatmospheric pressure above the reaction liquid and removing the hydrogen sulphide from the zone. Process according to any one of claims 1-7, characterized in that the resulting hydrogen sulphide is removed in a gas stripper situated outside the reactor used for the acidification process. Process according to any one of claims 1-8, characterized in that the hydraulic retention time in the acidification stage is 1-10 hours. Process according to any one of claims 1-9, characterized in that in the acidification process the pH is between 5 and 8 and the temperature is 25-45 °C. Process according to claim 10, characterized in that in the acidification process the pH is 6-7.

Inventors: MULDER ARNOLD, MULDER, ARNOLD
Applicants: BIOTHANE SYSTEMS INT BV
Application year: 1987
Main classification: C02F3
IPC codes: C02F3