Production of biomass for use in the treatment of Fischer-Tropsch reaction water

Abstract

A process for producing a biomass for use in the treatment of Fischer-Tropsch (FT) reaction water includes introducing a nutrient component comprising Carbon (C), Nitrogen (N) and Phosphorus (P), and water into an aerobic reaction zone containing a sewage sludge, and maintaining, in the aerobic reaction zone and under aerobic conditions, a F/M ratio of 0.25-2 kg COD/kg MLSS, where F/M=Food to Microorganism Ratio; COD=Chemical Oxygen Demand, expressed as mg oxygen/B of liquid in the aerobic reaction zone; and MLSS=Mixed Liquor Suspended Solids, expressed as mg solids in the aerobic reaction zone/B of liquid in the aerobic reaction zone. The F/M ratio is maintained for a period of time, to produce a biomass suitable for use in the treatment of FT reaction water.

Claims

1. A process for producing in a reactor a biomass for use in the treatment of Fischer-Tropsch (FT) reaction water, the process comprising: introducing a nutrient component and water into an aerobic reaction zone containing a seed sludge, wherein the nutrient component comprises Nitrogen (N), Phosphorus (P) and a synthetic feed that comprises a Carbon (C) source consisting of acetic acid; maintaining, in the aerobic reaction zone and under aerobic conditions, a F/M ratio of from 0.4 to 1.0 kg COD/kg MLSS.Math.day for a period of time sufficient to produce an acclimatized biomass for subsequent use in the treatment of FT reaction water; and after the period of time, discontinuing the introduction of the synthetic feed to the aerobic reaction zone and, after the period of time, introducing a feed stream comprising an aqueous mixture of carbon compounds to a zone that is upstream of and in fluid communication with the aerobic reaction zone, where F/M=Food to Microorganism Ratio; COD=Chemical Oxygen Demand, expressed as mg oxygen/l of liquid in the aerobic reaction zone; and MLSS=Mixed Liquor Suspended Solids, expressed as mg solids in the aerobic reaction zone/l of liquid in the aerobic reaction zone.

2. The process according to claim 1, wherein the seed sludge is an aerobic domestic activated sludge.

3. The process according to claim 2, wherein the F/M ratio that is maintained in the aerobic reaction zone during the period of time is about 0.8 kg COD/kg MLSS.Math.day.

4. The process according to claim 1, wherein C and N introduced into the aerobic reaction zone are present in a C:N mass ratio of from 20:1 to 60:1, wherein C is expressed as COD.

5. The process according to claim 1, wherein N and P introduced into the aerobic reaction zone are present in a N:P mass ratio of from 2:1 to 5:1.

6. The process according to claim 1, wherein MLSS is about 800 mg/l or above.

7. The process according to claim 6, wherein MLSS is about 1500 mg/l or above.

8. The process according to claim 1, wherein pH in the aerobic reaction zone is maintained in the range of from 6.5 to 7.5 during the period of time.

9. The process according to claim 8, wherein a minimum alkalinity concentration of 75 mg/l as CaCO.sub.3 is maintained in the aerobic reaction zone during the period of time.

10. The process according to claim 1, wherein a dissolved oxygen concentration of 1.5 to 3.0 mg/l is maintained in the aerobic reaction zone during the period of time.

11. The process according to claim 10, wherein the dissolved oxygen concentration in the aerobic reaction zone is maintained at 2.5 to 3.0 mg/l during the period of time.

12. The process according to claim 1, wherein the aerobic reaction zone is maintained at a temperature in the range of 32? C. to 42? C. during the period of time.

13. The process according to claim 12, wherein the aerobic reaction zone is maintained at a temperature of about 37? C. during the period of time.

14. The process according to claim 1, wherein the biomass has a cell residence time (CRT) in the aerobic reaction zone of from 18 to 45 days during the period of time.

15. The process according to claim 14, wherein the biomass has a cell residence time of about 35 days during the period of time.

16. Biomass produced by the process of claim 1.

Description

DESCRIPTION OF THE DRAWINGS

(1) In the drawings,

(2) FIG. 1 shows a simplified block diagram of a plant for carrying out a process according to the invention in the treatment of industrial waste water in the form of FT reaction water;

(3) FIG. 2 shows, for the Example, a photograph of floc structures of a biomass produced in accordance with the invention, with the floc structures being round and dense, with filamentous backbones internal to the floc with nitrifier colonies;

(4) FIG. 3 shows, for the Example, a photograph of abundant dense nitrifier colonies;

(5) FIG. 4 shows, for the Example, a photograph of an unhealthy number of Rotifers breaking up the flow structure;

(6) FIG. 5 shows, for the Example, photograph of flocs being of small and irregular size, with low filamentous abundance;

(7) FIG. 6 shows, for the Example, a photograph of flocs displaying excessive filamentous growth,

DETAILED DESCRIPTION OF THE INVENTION

(8) Referring to FIG. 1, reference numeral 10 generally indicates a plant for carrying out the process of the invention, in particular a process for treating industrial waste water in the form of FT reaction water.

(9) The plant 10 includes a reactor, generally indicated by reference numeral 11. The reactor 11 comprises, sequentially, an anoxic zone 14, a primary aerobic reaction zone 16, and a secondary aerobic reaction zone 18. It is possible for the reactor to consist of more reaction zones, depending on the desired design. The plant 10 further includes a solid-liquid separation zone, which in a preferred embodiment is a clarifier 22. Other types of solid-liquid separators such as filters may instead, or additionally, be used. A transfer line 20 leads from the secondary aerobic zone 18 to the clarifier 22. An effluent withdrawal line 24 leads from the clarifier 22. A sludge recycle line 26 leads from the bottom of the clarifier 22 to the anoxic zone 14 of the reactor 11.

(10) A recycle line 28 leads from the aerobic zones 16 and 18 to the anoxic zone 14. This allows for recycling of nitrate rich sludge to the anoxic zone during the stages of cultivation for nitrification/denitrification.

(11) A water stream 32 leads into the primary aerobic zone 16, as does a nutrient component stream 34. A synthetic feed line 36 is also provided for adding synthetic feed to the primary aerobic reaction zone 16, and this line feeds into the nutrient component stream 34.

(12) The plant 10 is commissioned, and the biomass is cultivated and adapted for use in treating FT reaction water, as follows:

(13) Seeding and Initial Growth Phase

(14) On day 1 of the seeding and initial growth phase seed sludge from an aerobic domestic sewage treatment plant is added to the primary aerobic reaction zone 16.

(15) The primary aerobic reaction zone 16 is then filled with a predetermined amount of water, ensuring that aerating devices (not shown) in the zone 16 are covered. The aim is to attain a MLSS concentration, after dilution, of ca. 1500 mg/l, (where ca means about or approximately).

(16) The primary aerobic reaction zone 16 is then fed with water, via stream 32, and synthetic feed and nutrients via stream 34. The synthetic feed is fed into the nutrient stream 34 via a separate synthetic feed stream 36. This synthetic feed stream 36 is only operational during the cultivation phase and will be isolated once real feed, derived from the FT process, is added to the plant 10 for treatment. The synthetic feed is used to ensure attainment of the unique species selection and ecological combinations necessary to produce suitable biomass to treat the FT reaction water. The cultivated sludge is preferably maintained using the synthetic feed until FT reaction water is produced for treatment.

(17) In stream 34, nutrients are added to the synthetic feed in such a manner as to ensure a C:N mass ratio of from 20:1 to 60:1 and a N:P mass ratio of from 2:1 to 5:1. The flow rate of water into the primary aerobic reaction zone is determined by the desired F/M ratio and an MLSS concentration of at least 1500 mg/l.

(18) The C:N mass ratio is preferably fixed, at 60:1. The nutrient component feed stream is added at such a rate that an F/M ratio of about 0.8 kg COD/kg MLSS is constantly maintained in the primary aerobic reaction zone 16. The MLSS concentration is a result of the F/M ratio of 0.8 kg COD/kg MLSS and the dilution effect during the filling of the primary aerobic reaction zone with water.

(19) The water, nutrients and synthetic feed components are gradually added to fill the primary aerobic reaction zone 16 over a period of days, such that after the initial growth phase, and, in the case where more than one aerobic reaction zone is employed, upon equalisation across more than one aerobic reaction zone, an MLSS of about 1500 mg/l or above is achieved across the aerobic reaction zones.

(20) A typical nutrient component feed stream consists of diluted macro nutrients, micro nutrients and a carbon source as indicated in Table 1.

(21) TABLE-US-00001 TABLE 1 Nutrient dosing in a typical nutrient component feed that includes acetic acid as a carbon source. Element Macro/micro Element Constituent provided nutrients concentration (mg/l) CH.sub.3COOH C Macro 10 000-20 000 CH.sub.4N.sub.2O N Macro 500-1000 H.sub.3PO.sub.4 P Macro 100-200 K.sub.2HPO.sub.4 P and K Macro/micro 1-2 and 3-5 MgSO.sub.47H.sub.2O Mg and S Macro/micro 3-5 and 3-6 CaCl.sub.22H.sub.2O Ca Micro 1-4 FeSO.sub.42H.sub.2O Fe Micro 0.5-2.0 MnSO.sub.45H.sub.2O Mn Micro 0.2-0.8 ZnSO.sub.47H.sub.2O Zn Micro 0.2-0.8 CuSO.sub.45H.sub.2O Cu Micro 0.05-0.2 CoCl.sub.26H.sub.2O Co Micro 0.05-0.2 NiCl.sub.26H.sub.2O Ni Micro 0.05-0.2 Na.sub.2MoO.sub.42H.sub.2O Mo Micro 0.05-0.2 H.sub.3BO.sub.3 B Micro 0.01-0.1 KI I Micro 0.01-0.1

(22) The pH in the primary aerobic reaction zone is controlled between 6.8 and 7.5 by dosing (not shown) an alkali solution, such as NaOH or KOH. It is preferred that a minimum alkalinity concentration of 75 mg/l as CaCO.sub.3 is maintained to enhance floc formation.

(23) Dissolved oxygen (DO) concentration is maintained at 1.5 to 3.0 mg/l in the primary aerobic reaction zone to limit filamentous growth.

(24) During the seeding and initial growth phase the temperature in the reactor 11, in particular in the aerobic reaction zones 16 and 18, is maintained at ca. 37? C.

(25) Equalisation of the Sludge Over Both Aerobic Zones

(26) Once the primary aerobic reaction zone 16 is filled completely and the MLSS is >1500, the sludge therein is then distributed over both aerobic reaction zones i.e. the primary aerobic reaction zone 16 and the secondary aerobic reaction zone 18. After distribution, the resulting MLSS in each aerobic reaction zone should preferably be at least 1500 mg/l.

(27) The two aerobic reaction zones are then filled in parallel over a number of 10 days with water and nutrient component feed streams. The nutrient component and synthetic feed stream is used to maintain the F/M ratio at about 0.8 kg COD/kg MLSS and water is used to gradually increase the volumes of the sludge in the primary and secondary aerobic reaction zones. The F/M of 0.8 kg COD/kg MLSS and the dilution effect during the filling 15 of the zones influence the MLSS concentration in both aerobic reaction zones, which should be maintained at above 800 mg/l, preferably at 1500 mg/l or above.

(28) Equalisation of Sludge Over the Total Reactor Volume

(29) Once the sludge in aerobic reaction zones 16 and 18 has grown sufficiently to fill the aerobic reaction zones, the anoxic zone 14 and the clarifier 22 will also have been filled with water.

(30) The water flow in the water stream 32 is decreased such that the water levels in the reactor 11 and clarifier 22 are maintained at full, however, without any effluent being discharged to the clarifier 22 i.e. zero up-flow velocity in the clarifier 22. Thereafter a sludge recycle pump (not shown) is started and operated at maximum capacity so that sludge is discharged into the sludge recycle stream 28 and fed to the anoxic zone 14. Water, and nutrients including the synthetic feed, are then introduced into anoxic zone 14, via streams 38 and 40 respectively. At this stage the flow of the nutrient component stream 40 to the anoxic zone 14 is such that an F/M of 0.8 kg COD/kg MLSS is maintained throughout the total reactor volume while not exceeding an organic loading rate (OLR) of 1.2 kg COD/m.sup.3.Math.day. The sludge is continuously monitored to ensure that it is of good quality. Once homogenisation is achieved i.e. the sludge concentration in the aerobic reaction zones 16, 18 and recycle stream 28 is the same, the flowrate of the nutrient component feed stream 40 into the anoxic zone 14 is increased. The flow rate of the water in stream 38 into the anoxic zone 14 and stream 32 into the aerobic reaction zones 16, 18 is also increased to the extent that an effluent is discharged from the reactor ((or alternatively aerobic reaction zones 16 and 18) to the clarifier through effluent discharge stream 20.

(31) Dissolved oxygen, pH and temperature are controlled as mentioned above in zones 16 and 18.

(32) Once the clarifier 22 has begun overflowing and the system has stabilised after the split of the water and nutrients, de-sludging will start at such a rate as to maintain a biomass CRT of 18-45 daystaking into account biomass losses via the clarifier. The C:N ratio in the nutrient component feed streams 34 and 40 will be managed to maintain a ratio of 20:1.

(33) Microscopic analyses to determine sludge quality is important. The main objective during this period of cultivation is to grow sufficient sludge of good quality. An acceptable sludge quality is:

(34) MLSS=3500 mg/l

(35) SVI (Sludge Volume Index)<150 ml/g

(36) Solid round flocs

(37) Low in filamentous content

(38) Protozoa presence

(39) Sludge is aerobic in nature

(40) Introduction of FT Reaction Water

(41) Once the sludge has been cultivated to the extent that it can treat FT reaction water, FT reaction water effluent from an FT process is gradually introduced into the reactor 10 via the nutrient stream 40. The FT reaction water is fed into the nutrient stream 40 via a separate FT reaction water stream 42.

(42) The FT reaction water enters the reactor 10, at the anoxic reaction zone 14. Thereafter it moves into the primary and secondary reaction zones 16 and 18. The product from the reactor overflows into the clarifier 22, wherein treated water is removed overhead via a treated water stream 24, and the biomass reports to the bottom of the clarifier 22. The biomass from the clarifier 22 is fed to the anoxic reaction zone 14, via stream 26.

(43) The flow rate of FT reaction water feed stream 42 is increased gradually over a 72 h period while at the same time decreasing the nutrient component in the nutrient stream 40 flow rate in such a manner as to maintain an organic loading rate (OLR) of ca 1.2 kg COD/m.sup.3.Math.d.

(44) The temperature of the zones 16, 18 is maintained at ca. 37? C.

(45) Minimum phosphate concentrations (10 mg/l as PO.sub.4.sup.3?) are maintained in the FT reaction water stream 42 to the reactor 11.

(46) The pH, temperature and DO are controlled as herein before described.

(47) During the treatment of the FT reaction water, sludge is harvested to maintain a CRT of 18 to 35 days, preferably 18 days.

EXAMPLE

(48) Aspects of the plant 10 were tested by means of laboratory scale experiments.

(49) The experiments were conducted using six 0.2 m.sup.3 pilot reactors, hereinafter referred to as Reactors 1 to 6. Each reactor comprised two zones, representative of the primary aerobic reaction zone 16 and the secondary aerobic reaction zone 18 in the plant 10.

(50) During the seeding and initial growth stage, seed sludge was added to the primary reaction zone and cultivated using acetic acid as a primary carbon source.

(51) The biomass concentration in the sludge was permitted to increase to the extent that on transfer of the sludge to the secondary aerobic reaction zone of the reactor the MLSS concentration was above 1500 mg/l throughout the system. After the distribution of the sludge between the primary and secondary aerobic reaction zones the primary zone was operated anoxically and the secondary aerobic reaction zone was operated aerobically with an internal recycle to an anoxic zone for denitrification purposes. This was done for the promotion and selection of nitrifying and denitrifying bacterial ecology in the system.

(52) The experiments were conducted in two modes. In the first mode each reactor was operated at a different F/M ratio within the range of from 0.2 to 2 kg COD/kg MLSS.Math.d. In this instance, the start-up MLSS was kept at 3500 mg/l across all the digesters. In the second mode of operation the start-up MLSS across Reactors 1 to 3 was varied from 800 to 3500 mg/l, while the F/M ratio was kept constant at 0.8 g COD/gMLSS.Math.d.

(53) The pH across the reactors was maintained at 6.8 by dosing caustic soda (NaOH) as required. Temperature was maintained at about 37? C. and dissolved oxygen (DO) concentration was maintained at 2.5 mg/l. Nitrogen was dosed in the form of Urea at a C:N mass ratio of 20:1. Phosphorus was dosed in the form of Phosphoric acid at a N:P mass ratio of 2:1 to 5:1.

(54) The investigation was assessed by the key requirements of the cultivated sludge, such as the time to acclimatise the domestic activated sludge, acceptable sludge quality during and after the acclimatisation phase, optimal sludge growth and compliance to all set effluent parameters. The results of the experiments are set out in Tables 2 and 4 to 7 hereunder.

(55) The investigation was aimed at, inter alia, achieving a varied and balanced population of protozoa in the biomass. Protozoa types in the biomass of the seed sludge and the final cultivated sludge after the acclimatisation phase were investigated, and the results are provided in Table 2 hereunder.

(56) TABLE-US-00002 TABLE 2 Presence of Protozoa types in seed sludge and final cultivated sludge Protozoa Seed Sludge Final Cultivated Sludge Rotifers Present in the seed sludge Present in the final cultivated sludge. Excessive numbers of rotifers were found to breakup and destroy floc structure. Amoebae Rarely present in the seed Never present/observed in the sludge final cultivated biomass. Flagellates Present but in low numbers Dominant under excessive in the seed sludge organic loading during commissioning and results in dispersion of floc particles. Free- Present in seed sludge, but Found during initial stages of swimming was not excessive commissioning, but disappeared ciliates over time. Presence of ciliates in low numbers is indication of healthy sludge. Crawling Present in seed sludge, but Found during initial stages of ciliates was not excessive commissioning, but disappeared over time. Presence of ciliates in low numbers is indication of healthy biomass. Alternate with stalked ciliates as the dominant group of protozoa. Stalked Present in seed sludge, but Found during initial stages of ciliates was not excessive commissioning, but disappeared overtime.

(57) The sludge quality was determined by microscopic imaging of the sludge. The qualitative characteristics evaluated were the floc structure and size. The aim was to achieve medium to large, round compact; good settling floc with filamentous backbones internal to the flocs. The presence and abundance of healthy nitrifying colonies is also essential. The resulting sludge qualities are depicted in FIGS. 2-6.

(58) FIG. 2 shows floc structures of a biomass produced with the floc structures being round and dense, with filamentous backbones internal to the floc with nitrifier colonies, which indicates a biomass of good quality, with good settling properties. FIG. 3 shows a photograph of abundant dense nitrifier colonies which are preferred for optimised nitrification/desertification conditions in the process of the invention.

(59) On the other hand, FIG. 4 shows a photograph of an unhealthy number of Rotifers breaking up the flow structure, leading to poor, irregular floc structures resulting in poor sludge settling during clarification. Similarly FIG. 5 shows a photograph of flocs being of small and irregular size, with low filamentous abundance, and resulting in poor settling and unacceptable effluent qualities whereas FIG. 6 shows a photograph of flocs displaying excessive filamentous growth, leading to the unacceptable bulking daily classification.

(60) The assessed sludge quality was translated into quantitative values using the quantitative scoring system provided in Table 3 below.

(61) TABLE-US-00003 TABLE 3 The class score of activated sludge with use of microscopic analyses. Ranking Visual Score Excel- Robust flocks between 200-1000 ?m, some protozoa, 100 lent Healthy amount of Filaments. Abundant Nitrifier colonies. Good Medium flocks, protozoa, 1-10 free bacteria per 25 ?m.sup.2, 80 Healthy amount of Filaments. Healthy amount of Nitrifier colonies Average Small flocks, no protozoa, 20-30 free bacteria per 25 30 ?m.sup.2, several filaments. Low numbers of Nitrifier colonies Poor Pin flocks or no flocks, no protozoa, excessive free 10 bacteria, excessive filaments, Yeast and fungi. No Nitrifiers.

(62) The sludge quality was assessed at varied F/M ratios, provided in Table 4, and varied MLSS, provided in Table 7. The class scores at varied F/M ratio and MLSS are provided in Tables 5 and 8 respectively.

(63) Tables 5 and 8 illustrate the qualitative interpretation of sludge quality translated to a quantitative scoring system for sludge quality. It is clear from the results in Tables 5 and 8 that the F/M of 0.8 provides a desired growth rate and sludge quality.

(64) Table 6 illustrates sludge volume index (SVI) measurements qualities at Days 14 and Days 35 of operation, at the varied F/M ratio as provided in Table 4. It was desired to obtain an SVI of from of 50 to 300 ml/gram, but lower SVI values are preferred for solid liquids separation with clarifiers. Preferably the SVI should be lower than 150 ml/gram. It was found that at F/M ratio of below 1 kg COD/kg MLSS, the SVI value was significantly lower compared to 20 F/M ratios above 1 kg COD/kg MLSS. These results therefore indicate that the biomass produced in this lower F/M ratio range is of a good quality and possessed a desirable floc structure. In particular the results show that at an F/M ratio of 0.8 kg COD/kg MLSS the SVI value achieved is relatively lower, and thus most preferred.

(65) It was found that at day 35 and at an F/M ratio of 0.8 kg COD/kg MLSS the desired MLSS and good quality biomass were achieved. The systems were operated for three biological cell ages (CRT), namely 18, 25 and 35 days, to verify successful commissioning. Maintaining a CRT of 35 days was found to give best results. The final effluent was monitored to ensure compliance with the set effluent quality requirements.

(66) TABLE-US-00004 TABLE 4 Start-up operating parameters for activated sludge reactors with varying F/M ratio. Parameters Reactor 1 Reactor 2 Reactor 3 Reactor 4 F/M (kg COD/kg 0.2 0.4 0.8 1 MLSS) MLSS (mg/l) 3500 3500 3500 3500 DO (mg/l) 2.5 2.5 2.5 2.5 pH 6.8 6.8 6.8 6.8 Temperature (? C.) 37 37 37 37 Parameters Reactor 5 Reactor 6 F/M (kg COD/kg 1.5 2 MLSS) MLSS (mg/l) 3500 3500 DO (mg/l) 2.5 2.5 pH 6.8 6.8 Temperature (? C.) 37 37

(67) TABLE-US-00005 TABLE 5 The class score of activated sludge with use of microscopic analyses of the experimental reactors with varying F/M ratio, at differing biological cell ages. Reactor Reactor Reactor Reactor 1 2 Reactor 3 4 Reactor 5 6 Sludge 70 77 92 67 74 71 Ranking Day 14 Sludge 75 72 90 52 40 48 Ranking Day 35

(68) TABLE-US-00006 TABLE 6 Sludge Volume Index of the activated sludge with varying F/M ratio. Reactor Reactor Reactor Reactor Reactor 1 2 3 4 5 Reactor 6 SVI 101 109 80 150 143 147 (ml/gram) Day 14 SVI 99 94 83 178 206 198 (ml/gram) Day 35

(69) TABLE-US-00007 TABLE 7 Start-up operating parameters for the 0.2 m.sup.3 activated sludge reactors with varying MLSS concentration. Parameters Reactor 1 Reactor 2 Reactor 3 F/M (kg COD/kg 0.8 0.8 0.8 MLSS .Math. d) MLSS (mg/l) 800 1500 3500 DO (mg/l) 2.5 2.5 2.5 pH 6.8 6.8 6.8 Temperature (? C.) 37 37 37

(70) TABLE-US-00008 TABLE 8 The class score of activated sludge with use of microscopic analyses of the experimental reactors with varying MLSS concentration. Reactor 1 Reactor 2 Reactor 3 Sludge 76 92 83 Ranking Day 35

(71) The invention thus provides a means for cultivating a microbial biomass or sludge that can be used in the treatment of industrial waste water, particularly FT reaction water, to produce a treated purified effluent to the specification of beneficial use or discharge quality.