Purification of Wastes from a German Yeast Plant DIETER LONDONG, Manager Civil Engineering Department and Lippeverband Essen, Western Germany

INTRODUCTION

The yeast plant, the waste purification of which is subject of my report, is situated on the eastern border of the Rhenisch-Westphalian industrial district within the area of the "Lippeverband". This is a public-legal association of all towns, villages and larger industrial works in the western catchment area of the River, a tributary to the River. The duty of this association is water-management especially the purification of waste waters. By joining all parties causing wastes and benefiting from water-engineering projects, the association is in a position to distribute the cost of such projects to a great number of municipalities, villages and industrial undertakings, re­ sulting in essential cost savings to the individual party in many cases. In the region south of the "Lippeverband," i.e. in the catchment area of the River, the "Emschergenossenschaft" carries out similar duties in personal union with "Lippever­ band."

YEAST PRODUCTION AND YEAST WASTES FROM THE PLANT The yeast plant processes beet-sugar molasses to compressed yeast and alcohol. The alcohol is extracted only from the first production stage, where mother yeast is produced. In the subsequent aerobic process free of alcohol, the mother yeast continues to grow with a high rate of propagation. The final product is dewatered, sold as so-called "compressed yeast" and is used mainly for baking. For the cultivation of yeast, molasses are used as nutrient substrate. The nu­ trient salts missing for the production of protein by the cells, are added in the form of ammonia and phosphoric acid (P2O5). The total water consumption including cooling water amounts to 850,000 cu m/yr. 350,000 cu m of wastes are produced annually. This gives, with five working days per week, 1350 cu m/working day. Reports on the water consumption of compressed yeast works in Europe vary be­ tween 40 and 100 cu m/ton of molasses processed and between ten and 30 cu m of wastewater produced per ton of molasses. The wastes from the yeast plant are composed of: 1) Burnt slop from the burners for alcohol distilling. (This slop is very highly loaded with alcoholic sub­ stances, weak acid is discharged rather evenly at temperatures of 190 F); 2) Wort from the alcohol-poor production of yeast, in other words: the consumed molasses widely

-770- •771 free from sugar. (The wort also has a high oxygen depletion but is discharged inter­ mittently.); 3) The washer waste from the two-stage yeast separation discharged about in the same rhythm; and 4) The filter waters from the partial dewatering of the yeast and the cleaning and purification waters, however, not very important with regard to quantity or pollution. The cooling waters are separated from the operating wastewaters and are also discharged separately. Quantity and composition of the wastewaters vary very much in the course of a day. Table I shows an analysis of combined wastes from the yeast plant for 48 hr. Several of such measurements averaged the following mean pollution loads:

BOD5 12,000 kg/working day

total nitrogen 1,000 kg/working day

total phosphorus 120 kg/working day

For the BODs/ton of molasses processed, papers in the German competent journals report between 160 kg (1) and 188 kg up to 238 kg (2). These figures were confirmed also by our measurements. The following expositions are based on a mean value of 170 kg BODs/ton of molasses. The mixed yeast wastes contain mainly proteins and their products of decom­ position, furthermore carbohydrates, organic acids and mineral salts, in a genuine solution or in a much suspended condition. These wastewaters are especially characterized by: 1) high contents of organic substances, especially of low organic acids; 2) the tendency for anaerobic acid fermentation; 3) the formation of hydrogen sulphides by reduction of the sulphates and/or decomposition of the proteins; and 4) a more or less pronounced brown color. In former years, the wastewater from the compressed yeast plant was passed directly into a small receiving stream near the plant. In this stream and in all other receiving waters up to the Lippe River 24 km away, considerable defects, damages and odor nuisances could be observed. When all efforts for a treatment of yeast wastes by the plant itself had remained without success, because of technical or economical difficulties, Lippeverband sug­ gested after the war to treat the yeast wastes together with the domestic wastes from the 80,000 inhabitants of the town of , situated along the Lippe River, in a common sewage treatment plant to be run by Lippeverband. The high percentage of domestic wastes and the iron contents of the wastewater from the town of Hamm, caused by two wire drawing mills, gave rise to expected good results. In order to provide connection for the wastes from the yeast works, to this common purification plant in Hamm, however, a 21 km long pipeline had to first be built.

OPERATION OF THE PIPELINE TO THE SEWAGE TREATMENT PLANT IN HAMM The pipeline was laid of cast-iron pipes of 250 mm diam for an operating pressure of six atmospheres during the years 1950/51. Emptying drains were pro­ vided at several deep points of the pipeline and automatic air relief valves in the neighborhood of the high points. It was soon found that the formation of digester gas in the pipeline and their safe removal offered an extremely difficult technical problem. Gas formation was much favored by the high temperature of wastes (83 F on the average) and the TABLE I

ANALYSIS OF COMBINED WASTE FROM A YEAST PLANT

Time Settle- COD COD Total Total (Tuesday able (settl.) BOD. (raw) BOD5 Nitrogen Phos. to Flow, Solids, (KMn04) (settl.) (KMn04) (raw) (N) (p2°5> Thursday) 1/sec ml/1 pH mg/1 mg/1 mg/1 mg/1 mg/1 mg/1 8- 10.00 19.3 0.3 5.4 5680 6190 5980 10- 12.00 20.0 0.2 5.5 5860 9100 11300 492 47.5 12- 14.00 21.3 0.2 5.2 5560 8820 10600 109.0 14- 16.00 16.9 0.9 5.1 7550 14000 24900 684 16- 18.00 19.6 0.6 5.1 9210 11600 8300 69.5 18- 20.00 20.1 0.7 5.25 5920 7700 12800 687 20- 22.00 20.6 0.6 5.25 5610 3680 5980 22- 24.00 18.2 0.7 5.4 4530 6560 5100 476 38.0 0- 2.00 18.8 1.6 5.3 6220 8000 9360 2- 4.00 21.8 8.0 5.3 11500 8850 10700 772 94.5 ? 4- 6.00 19.7 0.9 5.3 6940 5450 6790 6- 8.00 20.4 0.2 5.3 6160 4600 8050 567 61.5 8 10.00 19.6 0.9 5.25 5680 22500 6880 10 12.00 18.1 1.8 5.1 6110 9750 17500 456 37.0 14.00 8.8 3.5 5.1 8430 13700 12 8100 14 16.00 3.5 0.8 5.1 7480 11100 18100 587 47.5 18.00 16.3 13.0 5.25 9230 11500 16 14800 22300 18 20.00 18.5 10.0 5.4 11600 14200 953 130.0 22.00 21.4 0.3 5.55 5150 7550 20 5090 22' 24.00 20.3 0.2 5.6 4960 8400 12800 479 62.0 0 2.00 21.0 6.5 5.4 6740 10400 10200 14400 850 142.0 2- 4.00 20.6 8.5 5.2 12100 14500 4 6.00 18.5 16.0 5.2 7950 1150 10800 26900 886 150.0 6 8.00 21.7 4.0 5.15 5720 1800 Average: 18.54 0.34 7160 10350 8570 15400 657 82.5 - 773 flowtime of about one day. The gas is very noxious and forms explosive mixtures with air. During operation of the pipeline it was soon found to be a serious disadvantage that the gases rising from the shafts in a way not to be foreseen or controlled, could be ignited by passing motor cars and that the toxic gases could also endanger anyone passing by. Besides, the separated fermentation products occasionally blocked the air relief valves with the result that the pressure in the pipeline increased then, in certain sections, beyond the nominal pressure in a dangerous way. Therefore, the automatic relief valves were soon replaced by valves operated by hand. Figure 1 shows the daily quantities of gas and its composition, as an average from several measurements. Quantity and composition of the gas change with the time of flow. The possibilities to reduce the high gas formation of 120 to 150 cu m/day were studied in comprehensive laboratory tests. Much success was reached with alterations of the pH-value and with inhibitors. The cost of these processes, however, were very high. The most economic solution was found by cooling of the burnt slop. Today, about 30 to 70 cu m of gas per day are formed at an average temperature of the wastes of 75 F. Since that time, the air relief valves need only to be operated once a day. During weekends, the pipeline is regularly scavenged with fresh water. Although the reduction in gas production resulted in a decrease in head loss through the pipeline, the measured pressure line (Figure 1) is still far above the theoretical curve for gas-free wastes. From our experience, we would recommend the following principles for the building of similar pipelines: 1. The flowtime should not exceed, if possible, eight to ten hr, and the velocity should always be greater than 0.7 m/sec. 2. The pipeline should be laid out straight with as few high and low points as possible. All high points must be equipped with air relief valves. They should be

• AIR RELIEF VALVES

/MEASURED-LINE CHi] IGASCOMPOSITON

GASPRODUCTION mJ/d

30

TREATMENT PLANT HAMM

Figure 1 - Head losses and gas production in waste yeast pipeline. -774-

located only at sites where the environs cannot be endangered or molested by gas emission. Automatic air relief valves do not always operate in a reliable way and can also be used only where there is no danger of poisoning or explosion. 3. The yeast water should be cooled down to at least 75 F.

COMBINED WASTEWATER PURIFICATION IN THE SEWAGE TREATMENT PLANT IN HAMM The municipal sewers of the town of Hamm supply, on the average, 650 1/sec to the treatment plant of Lippeverband. The wastewaters are characterized by the following values:

Average settleable solids about five ml/1 Average BOD^ about 150 mg/1

Average KMn04 about 210 mg/1 Average iron about 20 mg/1 Average heavy metals (Cu, Ni, Zn) about two mg/1 Average pH 6.7 Average temperature 59 F

Figure 2 shows the flowsheet and Figure 3 a photo of the treatment plant. The aeration tank II is equipped with coarse bubble aeration and the aeration tank I with mechanical aerators. The two-stage aeration has proved to be of advantage, after the two aeration tanks had been operated at first simultaneously with mixed wastes. As only one final sedimentation tank is available for both stages and as each stage has thus not its own sludge circulation, the two-stage process is not 100 per cent. The theoretical detention time of the yeast waters in the aeration tank I is 45 hr, the BOD5 loading about five kg/cu m/d and the sludge loading about 0.67 kg BODs/kg MLSS. The average removal of BOD5 in this stage is 65 per cent and the power consumption about 0.3 kWh/kg BOD5. In the aeration tank II, the average BOD5 loading from the mixed wastes amounts to 2.6 kg/cu m and the average sludge loading to 0.65 kg BODs/kg MLSS. In the yearly average, 80 per cent of the common BOD5 is removed. Much more than in other high-loaded sewage treatment plants, the degree of BOD removal depends on water temperature. Figure 4 shows the annual development curves of water temperature and degree of BOD-removal as monthly averages in the course of three years. After interrupting the yeast wastes load at the weekend, the treatment plant responds to the heavy inflow during Monday by increased BOD-effluent-values at the beginning of the week. Putting the high number of average daily values of the percentage BOD-removal into an order according to sludge loading and influent BOD5, we obtain the curves of Figure 5, the scatter of which, however, is not shown. Only water temperatures be­ tween 59 and 64 F were taken into consideration. The curves confirm the known trend that the degree of BOD-removal increases with smaller sludge loadings and higher influent concentrations. So far, no operating difficulties by sludge bulking or foam formation were ex­ perienced. The sludge volume index varies between 35 and 1 50 ml/g, with 68 ml/g on the average. 775-

PRIMARY SEDIMENTATION FINAL SEDIMENTATION V " 600 cum AERATION II V = 9000cum t " -1 5 mm V=4680cum f 38 h LIPPE GRIT CHAMBER f 2 h RIVER SCREEN M Q = 18cum/s

TOWN SEWAGE AVFLOW 650l/s 15 MGD D YEAST WASTES AVFLOW 15 l/s 0.35 MGD 5 l/s U 560 l/s RAW SLUDGE "ca y^ -Jfc GAS AIQOcum/d "SEE!? ^ESLUDG^\_D|C3E5TER •&•=• cN V= 4000 cu m

DIGESTED SLUDGE TO DEPOS

Figure 2 - Flowsheet of the Hamm sewage treatment plant.

Figure 3 - Photograph of the Hamm treatment plant. 776-

>F 77 2 5TEMP°C

BOD5- 90' REM. 7.

Figure 4 - Waste temperature and BOD removal at the Hamm sewage treatment plant.

B0D5-REM7. B0D5-REM .7. 100 100

80 804 INFLUENT-BOD5 AVERAGE VALUE 60 60

AO-f £.0

20 WASTE WATER TEMP 15-18 °C 20 59 65 0 I I I 1 r r 0 0 0.2 0.4 06 0.8 1.0 1.2 U LOADING INTENSITY k9 B0D5 kg MLSSd

Figure 5 - Relationship between BOD loading, removal, and influent concentration. •777-

An improvement of BOD-removal could be attained if operations could be made really two-stage by establishing an intermediate sedimentation tank. Besides, a certain equalization of loads during the weekends could be of advantage. The average power consumption for both stages of aeration comes to 0.85 kWh per kg of BOD5-removal. With the former parallel operation of both sections of the tank, it was possible to test separately the power consumption of the mechanical aerators in tank I and of the bubble aeration in tank II. As Figure 6 shows, the mechanical aerators proved to be much better (3). The production of waste sludge amounts, on the average, to one kg per kg of BOD5-removal. Waste sludge and the sludge from the primary sedimentation tank are concentrated in a thickener to 220 cu m/day on the average with 6.5 per cent solids and are then pumped in a heated digester with a load of 2.25 kg of organic matter/cu m. About 460 1 of gas/kg of organic matter are produced. The German yeast producing industry is exposed at present to a strong compe­ tition from foreign countries with the result that the high cost of wastewater puri­ fication, due to an unfavorable location, threaten the economic existence of many yeast works. Despite the advantages given by Lippeverband, yeast production in the yeast plant will also not be economical in the long run. It is planned to displace the production works in the near future to a site more favorable with a view to wastes purification.

TESTS FOR THE TREATMENT OF YEAST WASTES WITH A VIEW TO THE FORTHCOMING DISLOCATION OF THE YEAST PLANT In the catchment area of the Emscher River with its 2.8 million inhabitants and a great number of large industrial works of any kind, all wastewater will be treated in future in a single biological purification plant with an average capacity of about 20 cu m/sec. The principal wastewater receiver is the Emscher River itself which, with low water flow, carries up to 90 per cent wastes. The mixing of different types of wastewater on its 80 km long course, promotes an optimum equalization of

kWh/kg BOD5-REM kWh/kg BOD5-REM 2.0 2.0

200 300 400 INFLUENT - BOD5 PPM

Figure 6 - Power consumption per kg BOD removed. 778 water quantities, concentrations and nutrients. This facilitates also essentially the treatment of organically much loaded wastewaters of one-sided composition. For these reasons, it was obvious to try in a testing plant (Figure 7) how the wastes from the yeast plant could be purified, in case of the movement of the works to the area of the Emscher River, in the large-scale treatment plant already mentioned. First, the 0.1 per cent yeast wastes were added to the Emscher water, causing an in­ crease in BOD5 by seven per cent. In another test plant run parallel, original Emscher water was treated and, in other purification plants, also other difficult wastes. When doing so, the data agreed for the large-scale treatment plant were maintained, e.g. aeration times of two h, sludge contents between 1.2 and 1.5 kg MLSS/cu m and a loading of 1.1 kg BOD5/CU m/day. With these data and a uniform addition of fresh yeast wastes from a lightly aerated storage tank, a sludge load of 0.88 kg BODs/kg MLSS and a BOD-removal of 80 per cent were obtained. A comparison with the parallel treatment plant, however, showed that only about 40 per cent of the addi­ tional increase in BOD due to the yeast wastes had been eliminated. By increasing the sludge contents and reducing the sludge load to 0.48 kg BODs/kg MLSS, the rate of elimination of the increase of yeast wastes could be increased to about 50 per cent. In a further series of tests, it was possible to remove the increase in BOD nearly com­ pletely. In doing so, the flow distance of the Emscher River was simulated for 1 2 hr and the wastes of two large chemo-pharmaceutical works were added to the feed at the same time. In the case of common treatment in the purification plant at the mouth of the Emscher River, it is probable that the levy for wastes treatment to the yeast plant could be essentially reduced. This shows clearly the advantages of a common treat­ ment of different wastes in a large-scale purification plant.

Figure 7 - Pilot tanks at Emschermundung treatment plant. —————— •" '•"-

•779

FURTHER INVESTIGATIONS ABOUT THE AEROBIC TREATMENT OF YEAST WASTES A similar conception to that above described with regard to the discharge of wastes and their treatment in the area of the Emscher River, exists also for the Seseke brook, a tributary to the Lippe River with an average water flow of 3.2 cu m/ sec. The Seseke brook serves as sewer for manifold industrial wastes and domestic wastes from 200,000 inhabitants. The Seseke water is biologically treated in a purifi­ cation plant for 45 minutes by the activated sludge process. In this case, it had to be examined whether it was technically and economically acceptable to treat also the wastes from a ten km away yeast factory with a daily BOD5 loading between 6000 and 8000 kg. The results of investigations offer a useful supplementation to the knowledge on the above described aerobic treatment of yeast wastes. During the tests for a common treatment with the Seseke wastes, one testing plant was fed only with Seseke wastes and a second was additionally loaded with a corresponding percentage of yeast wastes (Figure 8). Then, the water quantities increased by 0.5 per cent and the BOD5 by about 40 per cent. The BOD5-concentration of the yeast wastes varies also for these yeast works within wide limits, with an average to 5000 mg/1. The proportion of BOD5 to the contents of nitrogen and phosphorus compounds is, on the average, 100 : 9 : 0.5. According to Kilgore and Sawyer (4), the supply of nutrients to the micro-organism should be satisfactory for a biological removal even without addition of other wastes or nutrient salts with regard to the nitrogen, and should be just sufficient with regard to the phosphorus. Table II shows the results of the test series described in the column S3, compared with those of Emscher water (El and E2) and the operating results of the treatment

Figure 8 - Pilot plant at Sesekemundung treatment plant. 780-

TABLE II

SUMMARY OF COMBINED PURIFICATION OF YEAST WASTES

Designation H 1 E 1 E2 S 3 Test time 1966- 6/7/67- 6/7/67- 7/15/68 1968 6/20/67 7/7/67 8/14/68 Detention time hours 3 2 2 0.75

BOD5 min mg/1 90 85 60 28 max 800 106 140 82 influent av 280 94 84 53 COD min 197 183 104 influent max mg/1 255 269 219 (KMn04) av 300 226 217 146 Per cent of 0 yeast wastes Per cent 65 in BOD5 50 ~7 ~7 ~40 pH ~6.7 ~7.3 ~7.3 ~7 water min 43 61 66 63 tempera­ max degrees F 77 66 70 73 ture av 59 70 73 66 BOD5 kg BOD/cu m/ loading av day 2.3 1.13 1.0 1.7 sludge min 1.20 1.70 3.4 contents max kg MLSS/cu m 1.43 2.55 7.8 av 5 1.29 2.10 5.0 sludge kg BOD/kg load av MLSS/day 0.45 0.88 0.48 0.35 min 35 70 73 30 SVI max ml/g 150 92 111 111 av 68 81 98 65 DO av mg/1 1.0 ~4 ~4 3.4 Per cent of return sludge Per cent 85 30 30 94 BOD5 min 57 55 65 64 max Per cent 98 91 89 87 removal av 80 80 81 76 COD re­ min 50 46 28 moval max Per cent 67 69 66 (KMn04) av 60 56 59 49 removal of increase in BOD5 due to Per cent 70 40 50 80 yeast sludge pro­ kg MLSS/kg duction av BOD removed 1.0 not measured Power consumption kWh/kg BOD for BOD5 removed removed 0.85 not measured 1.24 • 781 -

TABLE II (Continued)

No. of samples S= single samples ~600 D 15 S 31 S 15 D D=24-h- samples plant Hamm (HI). With a sludge load of 0.35 kg BODs/kg MLSS which will apply also to the large-scale treatment plant, total BODs-removal came to 75 to 80 per cent. Also about 80 per cent of the additional yeast load was eliminated. Air consumption, of the coarse bubble aeration was by 30 per cent above that for the normal treatment plant. Furthermore, it was studied to which extent and at what cost the yeast wastes can be treated aerobically for themselves. Some curves of BOD removal for yeast wastes from batch tests are plotted in Figure 9. It will be seen that BOD removal is the faster, the most the activated sludge is adapted. Significant relationships between removal effect and sludge contents, however, could not be found in this case. For one of the tests shown in the middle of Figure 9, we found a considerable deviation from the general characteristics of BOD removal. In the course of this test, BOD5 increased temporarily, while COD decreased steadily as with the other test series. It may be that intermediate products develop sometimes during the decompo­ sition of proteins which have the effect of inhibiting decomposition or, at least, require a longer time for exposure. Besides, it should be of principal importance for the aerobic decomposition of yeast wastes that they represent the remainder of a nutrient solution already much consumed by aerobic microorganisms. As generally known, easily biodegradable nutrients are mostly utilized during the decomposition of the substrate. The products of elimination to be removed slower and more difficult and possibly having also anti- biotical effects remain in the solution. It is likely that these matters are not eliminated during a further short-term aerobic biological treatment, to the same extent as com­ parable matters of other wastes (5). During the continuous BOD removal tests, the times of treatment were varied between 18.2 and 5 1 h and the sludge loads between 0.21 to 0.94 kg BODs/kg MLSS. The results are summarized in Tabie III. BOD5 removal always amounted to about or more than 80 per cent, with BOD5 in the effluent, however, of nearly 1000 mg/1. The wastewater had still a brown color and also a light characteristic odor of yeast wastes. Two test series could be carried out near the works without equalization of quantities and concentration. At the time of the tests, however, the plant was operating fully for six days a week. For the other test series (S5 to S7), a lightly aerated intermediate storage tank had to be used additionally which permitted a yeast waste water feed also over the weekend. You will find in Table III, in column H2, also the operating results from the first treatment stage in the purification plant Hamm. The sludge volume index varied during these tests between 40 and 400 ml/g. The very light, finely flaked activated sludge always contained numerous flagellates and amoebae, but also single types of vorticelly in test S6 and even numerous opercularia. Sphaerotilus and sulphur bacteria were not found. For some time, there was a slight tendency, during all tests, to bulking and occasional foaming. This, however, was sup­ pressed by adding activated sludge from a purification plant. The reason for the change of the sludge could not be cleared. As the phosphorus contents were very low, it may be that a temporary deficiency of phosphorus was the cause. It is also not out of the question that disinfectants occasionally used during yeast production or, perhaps, inhibiting intermediate products of protein decomposition, contributed to this 782-

BOD5 PPM SUSP SOLIDS kg/m' WASTE SLUDGE PRO YEAST WASTE MIXED LIQUOR AER TIME 24h ORIGINAL AER TIME 24 h kgSS/kgBODs-REM I 7 700 A 100 985 15.1 169 0.57 11 7 700 4 000 1100 6.0 8 8 094 III 7 700 4 540 1440 3.4 4 3 0 29 100 100 7. 1 80 80 60 TF •60 JIKx- 40 40 b*« ^~NON ADAPTED 20 ACTIVATED SLUDGE 20 •} II (TREATM. PLANT SESEKEM.) 1 1 10 TIME 21 24 h

BODs PPM SUSP SOLIDS kq/mJ WASTE SLUDGE PRO YEAST WASTE MIXED LIQUOR AER. TIME 24 h ORIGINAL AER TIME 24 h kgSS/kgBODs-REM. A 7725 3250 368 - - - B 7330 2220 450 - - - C 7387 2994 1250 - - - 1007. 80 60 40 20 (TREATM. PLANT HAMM) -t—t 1 1 1 1 1- 0 10 12 14 16 18 20 22 24h

BOD5 PPM SUSP SOLIDS kq/mJ WASTE SLUDGE PRO YEAST WASTE MIXED LIQUOR AER TIME 24 h ORIGINAL AER. TIME 24 h kgSS/kg BOD5 -REM a 5283 2825 638 15.1 162 050 b 5283 3000 715 3.5 47 053

Figure 9 - Results of batch tests with yeast waste. TABLE III

SUMMARY OF SEPARATE PURIFICATION OF YEAST WASTES

Designation S 6 S 5 S7 H2 Test time 2/21/64- 6/23/6< 9/15/68- 8/15/68- 10/1/68- 11/10/66- 5/25/64 7/24/6^ 9/30/68 9/14/68 11/7/68 8/13/67 Detention hr 51 48 36.4 18.2 18.2 45 time

BOD5 min 1104 620 755 307 influent max mg/1 7450 3390 4820 5750 32800 av 4917 2375 4041 4177 4680 8979 COD min 1100 1910 influent max mg/1 10200 4285 (KMn04) av 4500 2860 7506 8194 7505 00 pH av 5.7 5.7 ~6 ~6 ~6 6 r water min 54 temperature max degrees F 68 av 50 66 63 70 59 61 BOD5 av kg BOD/cu m/ loading day 0.97 1.20 2.50 5.5 6.21 ~5.0 sludge min kg MLSS/cu m 3.24 2.40 5.1 contents max 10.30 4.67 12.1 av 4.49 3.23 5.5 8.3 7.1 7.5 sludge av kg BOD/kg load MLSS/day 0.21 0.42 0.38 0.68 0.84 0.67 min 61 212 ~ 40 SVI max ml/g 270 408 ~150 av 125 318 125 91 71 80 DO av mg/1 0.5 0.5 4.0 2.2 2.8 0.5 Per cent of return sludge av Per cent 300 300 300 150 150 100

BOD5 min 64 70 84 61 45 45 removal max Per cent 98 86 96 97 92 93 av 82 79 87 87 77 65 > en COD removal min (KMn04) max Per cent — -J av 58 59 56 sludge av kg MLSS/kg 0 3 production BOD removed 0.86 0.78 not measured 0.85 not 5' c measured a Power consumption kWh/kg BOD not measured 0.94 0.66 0.83 0.3 u for BOD5 removal removed No. of samples S=single samples 13D 7 D 10D 8D 20 D 79 S D=24-h- samples •785- phenomenon. Yeast wastes with their high contents of colloidally solved proteins have a strong tendency to foaming. For aerobic decomposition, these factors must be paid special attention. Figure 10 shows individual figures of decomposition obtained from the tes' series S5 and S7, as a function of influent BOD5. With otherwise nearly the same test conditions, the strong influence of the higher temperature in test S5 can be clearly seen. The waste sludge produced in a quantity of 0.9 kg SS/kg of BODs-removal thickened, after 20 hr of sedimentation, to about 50 per cent of the original quantity. From this rate, the specific values of 15 cu m waste sludge per ton of BOD5-removal and 0.08 cu m waste sludge per cu m yeast wastes can be derived. The sludge digested well in a heated digester within 20 days, with a loading of 1.5 kg of organic matter/ cu m/day with a gas production of 425 1 gas/kg of organic substance. After digestion, the sludge could be thickened once more by 50 per cent. The question whether we should prefer the common or the separate treatment of yeast wastes in the purification plant at the mouth of the Seseke brook, can be decided only after completion of the present studies on the anaerobic and combined anaerobic/aerobic digestion.

GENERAL CONCLUSIONS FOR AEROBIC TREATMENT Although the biochemical processes during the decomposition of yeast wastes may appear very complex to the engineer and are little known generally, some con­ clusions can yet be drawn from the above described tests and actual operations for the practice of the treatment of yeast wastes. According to our experience from the treatment plant Hamm and the testing plants, yeast wastes can be treated by the activated sludge process then mixed with other industrial or with domestic wastes, and also without mixing with other wastes. A condition, however, is a sufficient provision of the microorganisms with

B0D5-REM7. BOD 5- REM. 7.

X X X * *• * V • • X 80- i • • 80 • . . -•—• • 60- • 60 • • 40- - 40

20- XTE ST S5 V VASTE WAT ER TEMP. 21 °C - 20 •TE ST S 7 WASTE WATER TEMP. 15 °C fi- 1 1 1 () 10 00 2000 3000 4000 5000 60 00 INFLUENT - BOD5 PPM

Figure 10 - Results of test series S5 and S7. 786 nutrients and a more or less high equalization of concentration. In one-stage treat­ ment plants, rates of elimination of about 80 per cent, as a function of several factors of influence, can be obtained, and these rates can be increased even to more than 90 per cent under favorable conditions. The combined treatment with other wastes has usually technical and economical advantages, because of the equalization of nutrients and concentration, especially when the percentage of yeast wastes is not dominating. In my opinion, however, treatment times should not be much below two hr, unless the sludge contents are extremely high. If there is no discharge of yeast waste to the plant during the weekend, worse removal results of decomposition must be reckoned with at the beginning of the week. When yeast wastes are treated separately, operating difficulties such as foaming or bulking can happen, especially when the proportion of nutrients is not satisfactory. The treatment period should not be taken less than 20 hr. For treatment times below 48 hr, an equalization tank cannot be designed and must then be lightly aerated in order to keep the wastes fresh. The relationship between purification effect and water temperature is a particu­ larly striking factor (Figure 11). It is more pronounced than in normal high rate activated sludge plants. There is a certain relation between sludge loading and BODs-removal. The relations are plotted in Figure 12 on the top. The values shown represent average results from all tests (without the batch tests). The scatter of the individual values increases clearly with higher sludge loadings. All percentages are below the curve for domestic wastes (6). In the lower part of Figure 12, no relation between purification and treatment- time can be found. A difference must be made, however, between the separate and the combined treatment of wastes. A significant relationship between production of waste sludge and sludge loading could not be determined. The values found varied between about 0.8 and one kg SS/kg

BOD5-REM 7. BOD5-REM.7. 100- 100

90' 90

80- 80

70 70 NTHLY AVE • 1966 60- o 1967 T 60 * 1968 50- 50 - r —i— —r- -I WASTEWATER 5 10 15 20 25 TEMP. °C P" —t— —i— 41 50 59 68 77 °F

Figure 11 - Influence of temperature on BOD removal at the Hamm sewage treatment plant. -787- ® O O COMBINED TREATMENT ADO* SEPARATE TREATMENT

BOD5-REM.7. BOD5-REM.7o 100 •100

90 • I - 90 5^. 80 •*1£JLi;1 .mW I "OE1 - 80 H1 S 2-0 *T S7<> T- 70- Laiw CURVE FOR 70 I &H2 DOMESTIC WASTES! 60- f- 60

50 50 i • • • 1 1 -'—'-1 0.5, ^ 1.0 0 k g nBOD5 1.5 BOD5-REM.7. kg MLSSd BOD5-REM.7, 100 •100

90- <>S5 S6 90 U.C1 t f*W2 80- TT"1 *--W3 - 80 *-S3 <>S7 70 - 70 AH2 SOH PARAMETER : - 60 TEST NO- 50 1 50 1 1 1 1 1 1 1—1 20 40 60 80 DETENTION TIME h Figure 12 - Removal effect of yeast wastes. of BOD5-removal. With 90 per cent BOD-removal, about 150 kg of waste sludge are thus produced per ton of molasses. Power consumption depends much on the degree of purification and the aeration system used. For 80 per cent aerobic removal values between 0.3 kWh and 1.5 kWh per kg BOD-removal were measured (see Tables II and III). For any plans of a combined treatment with other wastes, it must be taken into consideration that power consumption increases the more the wastes are diluted. The cost for the aerobic treatment of yeast wastes are considerable. On the basis of about one kWh per kg of BODs-removal for 90 per cent power consumption 788- comes to about 150 kWh per ton of molasses for 170 kg BOD5 per ton of molasses processed. This comes, in the Federal Republic of Germany, to about DM 22, - power costs per ton on the average. Adding the capital cost and the cost for attendance and maintenance, total cost amount to 25 to 40.- DM per ton of molasses processed or four to ten per cent of the production cost of the yeast.

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