R&D on Wastewater Treatment Technology with Rotating Biological Contactors

Transcription

1 1999D R&D on Wastewater Treatment Technology with Rotating Biological Contactors 1. Introduction and Experiment Overview Wastewater treatment facilities at refineries should be selected and installed so that the pollutants in the wastewater can be effectively removed at the points close to the sources of such pollutants. From the results of preliminary research on a model refinery, it was found that alkyl phenol was one of the major contributors among COD (chemical oxygen demand) source substances contained in the refinery wastewater, and its source was not the effluent from cracking facilities but desalter. We then tried to develop a biological treatment method which had good efficiency for the desalter effluent. Even though it shows high oil content and drastic fluctuations in its properties, the objective was to develop, as a biological treatment method, rotating biological contactors (RBC) wastewater treatment system, which is considered suitable for such conditions. In 1996, an RBC bench plant composing of different types of disks was installed, and basic data on the treatment of desalter effluent were collected. In 1997, an RBC bench system equipped with pretreatment and post-treatment facilities was designed and installed, and studies were done on optimum arrangements of RBC units and on suitable combinations of pretreatment and post-treatment. In 1998, the number of RBC units was increased, and coagulating sedimentation facilities and sludge dehydrators were added so as to increase the performance of the bench system; a comprehensive system good for commercial plant was thus developed. 1.1 Design and construction of comprehensive RBC bench system For optimization of the arrangement of RBC units, in 1997 two RBC units of which tank was partitioned into four chambers were used, and the relationship between COD concentration and COD removal rate per unit surface area was analyzed. With this method, however, no meaningful results were obtained, so in 1998 the number of RBC units was increased so that various combinations of arrangements could be tested. A coagulating sedimentation unit as post-treatment and a sludge dehydrator were also added. The RBC bench system was thereby advanced to the extent that it could be evaluated as a comprehensive system development. A flow scheme of the comprehensive bench system is shown in Figure After being cooled in a cooler and passing through two equalization tanks, desalter effluent is distributed to each RBC unit by means of a weighing distribution tank. RBC treatment water goes to the sedimentation tank either directly or after undergoing coagulation reaction in a coagulation reaction tank, where sludge is allowed to precipitate and supernatant is drained off. Sludge precipitated in the sedimentation tank passes through a scum storage tank and is dehydrated by dehydrator. 1

2 1.2 Optimization of RBC units arrangements In designing a wastewater treatment facility that uses RBC units, optimization of their arrangement, the core of the treatment system, becomes a vital issue and for this purpose it is convenient when the relationship between COD concentration and COD removal rate per unit surface area can be clarified. Accordingly, in 1997, two rotating disks were used to partition a water tank interior into four layers, and an analysis of COD and COD removal rate was attempted, but meaningful results could not be obtained by this method, as mentioned previously. In the current fiscal year, therefore, experiments were conducted in which various practical arrangements for wastewater treatment were compared. Figure Flow scheme of comprehensive RBC bench system 1.3 RBC in combination with induced air flotator (IAF) In the case of biological treatment of oily wastewater, IAF is a general process of pretreatment. In the previous fiscal year, IAF was installed as RBC pretreatment, but the results were not good, so in the current fiscal year, tests were conducted on cases of IAF as RBC post-treatment. From results of inorganic coagulant tests done before, it was found that polyaluminum chloride (PAC) is more effective. In beaker tests of desalter effluent, the PAC concentration increased up to 100 ppm and COD declined, but with the increase in PAC concentration, scum also increased and ph declined. From above results, in tests with a comprehensive bench system, the PAC concentration was determined at 50 ppm and 2 ppm of anion polymer coagulant was added. The flow rate of desalter effluent for treatment was 5 T/hr. 1.4 Coagulation se dimentation of RBC treated water RBC treated water contains large quantities of suspended solids (SS) which do not settle well. Accordingly, tests were done on coagulation treatment of RBC treated water and the effectiveness of this approach was confirmed. As for treatment conditions, 5 to 10 ppm of PAC was added, together with 1 ppm of anion polymer coagulant. 2

3 1.5 Sludge dehydration and reuse of dehydrated sludge (cake) Sludge exhausted from RBC units was dehydrated by means of a belt-press type dehydrator and rates of dehydration were investigated. The cake was also analyzed, and the possibilities for its reuse were explored. 2. Results and discussion 2.1 Operability of comprehensive RBC bench system Operability and performance of each unit in the comprehensive RBC bench system was found to be almost satisfactory as expected. In contrast to the present RBC, however, two newly installed RBC required more time for the formation of microorganism layers. As for ph fluctuation of the effluent, ph adjustments in the No. 2 adjustment tank and the coagulation tank are required to be done with manual operation. Automation of these adjustments will have to be desired for commercial plant. 2.2 Optimization of RBC unit arrangements A comparison was investigated for the following two cases. In case A, two RBC units were aligned in parallel in the early stage and influent was made to flow through them. In the late stage there was only one RBC unit, where the two flows of the early stage connected. In case B, influent flowed through one RBC unit in the early stage, and then flow was divided into two RBC units arranged in parallel in the late stage. Desalter effluent treated with stripping was used for this experiment. As shown in Figure 2.2-1, it was indicated that the removal rate was slightly higher in case A at the normal operation, but case B exhibited a slightly superior tendency in attenuating of fluctuations. (A) Early stage parallel arrangement (B) Late stage parallel arrangement 3

4 Figure Comparative tests of different arrangements 2.3 RBC in combinations with induced air flotator (IAF) As shown in Figures and 2.3-2, in comparison to the case of independent RBC, when IAF was carried out as RBC pretreatment, treatment effect was unexpectedly lowered. It is thought that the coagulant had an adverse effect on microorganism reaction. On the other hand, when IAF was carried out as RBC post-treatment, an improvement effect was noted over the case of independent RBC, as shown in Figures and This is supposed to be due to the removal by IAF of microorganisms and colloids separated from the disks in RBC. When desalter effluent was treated by a combinated system of RBC and IAF, the COD removal efficiency came up to 72.3% and the phenol removal efficiency to 83.4%. Figure Effect of IAF (RBC pretreatment, COD) 4

5 Figure Effect of IAF (RBC pretreatment, phenol) Figure Effect of IAF (RBC post-treatment, COD) 5

6 Figure Effect of IAF (RBC post-treatment, phenol) 2.4 Coagulation sedimentation of RBC treated water Figure shows the changes in COD from feed water to coagulation sedimentation treated water when RBC treated water has undergone coagulation sedimentation, where feed water means desalter effluent followed with stripping treatment. The COD of RBC treated water was found to be lowered about 2 to 10 ppm by coagulation sedimentation. What is more, the SS in RBC treated water showed fluctuating between 5 and 50 ppm, but in water treated by coagulation sedimentation under optimum conditions, they were not more than 1 ppm. Figure Effect of coagulation sedimentation on RBC treated water 6

7 2.5 Sludge dehydration and reuse of cake When RBC treated water underwent coagulation sedimentation, the water content in the sludge was 99.6% in average, and after this sludge had been dehydrated by a belt- press dehydrator, that in the cake was 78.1% in average. In order to investigate reuse of this cake in compost, its contents were analyzed for metal components, etc. The results are given in Table The level of metal components was adequately low, and the levels of phosphoric acid and potassium were also low. The weight percentages of carbon, hydrogen and nitrogen in dry-base cake were 37 to 40%, 7% and 2%, respectively. These results suggest that the cake could be usable as compost if its components were arranged adequately. Table Analysis of metals, etc., in dehydrated sludge Measurement 1 Measurement 2 Standard value (ppm) (ppm) Phosphoric acid less than 2% Potassium Copper less than 600 ppm Zinc less than 1800 ppm Mercury less than less than less than 2 ppm Cadmium less than 0.5 less than 0.5 less than 5 ppm Arsenic less than 0.01 less than 0.01 less than 50 ppm Lead less than 0.5 less than 0.5 Chrome less than 0.5 less than Conclusion 3.1 Design and construction of a comprehensive RBC bench system To advance the RBC bench system installed last year, it was designed and constructed to be possible to investigate optimum arrangements of RBC and its combinations with coagulant sedimentation treatment, and to be equipped with a sludge treatment facility. The comprehensive system exhibited good operability and performance as expected. 3.2 Optimization of RBC unit arrangements Comparative tests were performed on various combinations of arrangements in parallel and in series, and a set of information was obtained for optimum arrangements when emphasis is put on COD removal rate and on attenuating of fluctuations. 3.3 RBC in combinations with induced air flotator (IAF) In the previous fiscal year, IAF was investigated as RBC pretreatment, but it was found that better results could be obtained by IAF as RBC post-treatment, which means that effective information for the design of a comprehensive system were obtained. 7

8 3.4 Coagulation sedimentation of RBC treated water It was suggested that when coagulation sedimentation was performed as RBC post-treatment, SS and COD could be eliminated more effectively than in the case of dependent RBC. 3.5 Sludge dehydration and reuse of cake When effluent from the desalter system was treated by RBC units, the sludge byproduct was dehydrated and analyzed, and it was found that the sludge can be dehydrated satisfactorily, and the cake could be usable as compost. 4. Summary As a result of preliminary research, a biological processing technology was developed for direct treatment of desalter effluent, which we found to be the main source of pollutants in refinery plant wastewater. At the core of the processing technology developed is the RBC. Various combinations with pretreatment and post-treatment were investigated, and a comprehensive system good for commercial plant was developed. It thus concluded to construct a wastewater treatment system of refinery, as shown in figure 4.1-1, which is thought to be the most effective within the current system. With the system developed, the target was reached for COD removal rate, which is 70% or more. The system can withstand fluctuations in COD of about 40% in feed water. As for oil content, there are no problems if approximately 100 ppm of oil is contained in the feed water. With respect to denitration tests, on the other hand, although favorable results were obtained in small-scale test, scale-up and maintenance of processing stability were not successful. There must be further developments for when denitration is necessary. Effective usage of sludge byproduct also remains an issue. In addition, more data should be collected on such things as RBC unit arrangements, rotation speed and effects of aeration. Yet although these issues remain, we insist that this technological development has already reached the level of commercial plant, and in the future we hope to draw wide attention to it and work to see it spread. Atmospheric distillation Vacuum distillation Catalytic cracking Stripper Desalter Biological treatment Oil separator Filter Discharge Catalytic reforming Hydrodesulfurization Figure Stripper Optimization of a process wastewater treatment system at refinery Copyright 1999 Petroleum Energy Center all rights reserved. 8