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on Wastewater Treatment Technology with Rotating Biological



              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 1.1-1.
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    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.

Desalter          FWT
                                                     RBC units   PAC     NaOH Polymer coagulant

        No. 1           No. 2Weighing distribution
      equalization     equalization                              Coagulation reaction tank
        tank            tank tank                                                  Sedimentation

               Oil storage tank

       Recovery oil                                                          Scum storage tank
                               acid                                                    Cake
           Figure 1.1-1   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.

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

                                                                     RBC IN
                                                                     (A) Early stage parallel case
                                                                     (B) Late stage parallel case
   COD (mg/l)




                      20          40            60             80              100              120

                  Figure 2.2-1   Comparative tests of different arrangements

2.3     RBC in combinations with induced air flotator (IAF)
As shown in Figures 2.3-1 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 2.3-3 and 2. 3-4. 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%.

                                                                                       TANK IN
                                                                                       RBC IN
                250                                       Without IAF    With IAF      No.4 RBC OUT
                                                                                       No.3 RBC OUT
                                                                                       No.2 RBC OUT




                      20   30          40     50         60         70       80        90        100

                  Figure 2.3-1   Effect of IAF (RBC pretreatment, COD)

                                                                                           TANK IN
       35                                                            With        Without   No.4 RBC OUT
                                                                     IAF          IAF
                                                                                           No.3 RBC OUT






                   20           30       40         50        60            70     80      90        100

             Figure 2.3-2            Effect of IAF (RBC pretreatment, phenol)

                                                                                                RBC IN
                   200                                                                          RBC OUT
                                                                                                IAF OUT




                            0        5         10        15            20         25       30         35

             Figure 2.3-3            Effect of IAF (RBC post-treatment, COD)

                                                                                         RBC OUT
                                                                                         IAF OUT



                  0            5          10         15             20     25          30           35

               Figure 2.3-4        Effect of IAF (RBC post-treatment, phenol)

2.4    Coagulation sedimentation of RBC treated water
Figure 2.4-1 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.








                         Feed water        RBC IN                    RBC OUT       Coagulation
                                                                                sedimentation OUT

               Figure 2.4-1        Effect of coagulation sedimentation on RBC treated water

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 2.5-1.
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 2.5-1 Analysis of metals, etc., in dehydrated sludge
                             Measurement 1           Measurement 2           Standard value
                                (ppm)                   (ppm)
     Phosphoric acid             400                     370                       less than 2%

     Potassium                    0.682                   2.725

     Copper                        1.4                     1.7                less than 600 ppm

     Zinc                         64.8                     69                less than 1800 ppm

     Mercury              less than 0.0005       less than 0.0005               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 0.5

3.      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.

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

       cracking                                                              Filter


           Figure 4.1-1     Optimization of a process wastewater treatment system at

                       Copyright 1999 Petroleum Energy Center all rights reserved.


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