The use of the Kaldnes suspended carrier process in treatment of wastewaters from the forest industry

•

Pergamon

Waf. Sci. Tech. Vol. 35, No. 2-3, pp. 123-130, 1997. Copyright © 1997 IAWQ. Published by Elsevier Science Ltd Printed in Great Britain. 0273-1223/97 $17·00 + 0·00

PH: S0273-1223(96)00923-7

THE USE OF THE KALDNES SUSPENDED CARRIER PROCESS IN TREATMENT OF WASTEWATERS FROM THE FOREST INDUSTRY Eva Dalentoft and Peter Thulin Purac AB, Box 1146, S-221 05 Lund, Sweden

ABSTRACT One pilot plant study and two full scale studies have been carried out seeking for the optimal use of the Kaldnes suspended carrier process in treatment of wastewaters from the forest industry. The wastewater used in all three cases came from secondary fiber mills. The studies show that the Kaldnes process as a highly loaded stage (typically 15-25 kg COD/m 3·d) in series with an activated sludge stage forms an efficient, stable and competitive combination process both regarding investment and operating costs. This is especially true when treating wastewaters with a composition that makes them unsuited for treatment in an activated sludge process. The flexibility and compactness of the Kaldnes suspended carrier process also makes it an ideal choice for upgrading of existing treatment plants. © 1997 IAWQ. Published by Elsevier Science Ltd.

KEYWORDS Aerobic biological treatment; biofilm; high load; moving bed biofilm process; secondary fiber mill effluent; suspended carrier INTRODUCTION The Kaldnes suspended carrier process is a biofilm process that takes advantage of both the activated sludge process and conventional fixed film systems, such as trickling filters and submerged aerated filters, without the disadvantages often found with these processes (Rusten et af., 1994). The core of the process is the biofilm carrier element which is made of polyethylene and has a size of 7 x 10 mm. The density is slightly less than that of water (0.92-0.96 g/cm3), which makes it easy for the elements to be kept in suspension. The elements are formed to give a large protected surface and to optimize bacterial growth. The filling of the carrier elements in the reactor may be varied allowing for flexibility in the specific biofilm area, but normally the reactors are filled up to 67 % of their volume with biofilm carrier elements corresponding to a total area of about 500 m 2/m 3 and an effective inner area of 333 m 2/m 3 water volume. Due to the shape of the carrier elements, only 12 % of the water is displaced. The biomedia is kept in suspension by air (aerobic processes) or mixers (anoxic processes). The biofilm carriers are kept in the reactor by installing a sieve (light opening about 7 mm) at the outlet of the reactor. To prevent clogging of the sieve either air can be sparged over the surface (aerated reactors) or for agitated (anoxic) reactors the agitation is arranged to move the carrier elements constantly over the surface of the 123

E. DALENTOFf and P. THULIN

124

sieve. For aeration coarse bubble aeration is used with good efficiency compared to that of fine bubble diffusors; the presence of the carrier elements improves the oxygen transfer considerably. There are several advantages of the Kaldnes suspended carrier process compared with the conventional activated sludge process. Operation at very high loads is possible; typically organic loads up to 25 kg Chemical Oxygen Demand (COD)/m 3.d are used, but up to 60 kg COD/m 3.d has been applied (Broch-Due et ai., 1994). Further, the process is well mixed, and since the main part of the active biomass grows on the surface of the biofilm elements, the process is insensitive to load variations and other disturbances (Hem et ai. 1994). No recycling of biomass to the reactors is needed. All the suspended biomass is caused by the excess sludge production. Therefore, chemical precipitation can be applied to the sludge separation stage without lowering the sludge age of the process. The process is also tolerant to fibers in the incoming wastewater. Another interesting feature of the Kaldnes process is that it may be operated at elevated temperatures (up to 50°C) compared to that applicable to a conventional activated sludge process (Strehler and Welander, 1994). The optimal selection of process technology for a wastewater treatment plant varies for different forest industries. Factors that must be considered are wastewater characteristics, investment cost, operating costs, sludge production, space available, effluent demands to be met, and whether or not there is an existing treatment plant that may be used. The Kaldnes suspended carrier process may be used for all kinds of applications; it can be used by itself in a single- or multi-stage reactor configuration or in combination with other process technologies. However, a relatively high investment cost puts limits to the use of the process. In this paper, the optimal use of the Kaldnes suspended carrier process for treatment of wastewaters from the pulp and paper industry is discussed. Pilot plant experiments have been carried out comparing treatment efficiencies in a two-stage Kaldnes process with those in a combination process consisting of a Kaldnes stage in series with an activated sludge stage. Results from two full-scale plants, both using the Kaldnes process in combination with the activated sludge process, are also presented. In all cases wastewater from secondary fiber plants has been used. These wastewaters are difficult to treat satisfactorily in an activated sludge plant since the composition causes sludge bulking and an alternative way to treat these wastewaters is needed. METHODS Pilot plant experiments Pilot plant experiments have been carried out comparing two process combinations. The first one consisted of two Kaldnes stages in series (K-K), and the other process was a Kaldnes stage in series with an activated sludge stage (K-AS). Both systems were followed by a sedimentation step. Flow sheets of the two process alternatives are shown in Figures 1 a and 1 b. The volumes of the Kaldnes-stages were 1 m 3 and the volume of the activated sludge stage was 8 m 3. The ratio between the Kaldnes volume and the activated sludge volume in the K-AS stage was chosen with regard to obtaining reasonable sludge loads in the activated sludge stage. The wastewater used came from a secondary fiber mill, Alier S.A in Spain. At this mill all kinds of wastepapers are used as raw material and the products include packaging for industrial purposes and testliner. The concentrations of the wastewater that were treated in the pilot plant are shown in Table 1. Sufficient nitrogen and phosphorus were added to the wastewater at all times and the oxygen concentration was kept ~tween 3 and 4 mg/l in the Kaldnes stages and >2 mg/l in the activated sludge stage. The average pH of the mfluent wastewater was 6.8 and no pH regulation was needed. The average wastewater temperature during the trials was 28°C.

Kaldnes suspended carrier process

125

KALDNES 2

KALDNES 1

A)

EXCESS SLUDGE KALDNES

ACTIVATED SLUDGE

B) RECYCLED SLUDGE

.".,

1

EXCESS SLUDGE ACTIVATED SLUDGE

IN

----r+t-------+-I------..I-------I-.-.

OUT

C) RECYCLED SLUDGE EXCESS SLUDGE

Figure 1. Flow sheets of (A) the K-K process, (B) the K-AS process, and (C) the AS process.

Table 1. Average wastewater data Total Chemical Oxygen Demand (TCOD), mg/l

Pilot plant 1 350

Apura 1 600

K-AS full scale plant 500

·Tota"i"i3l'o·i"o·gi"caTOxygen·····......···......·······55"6"········..········ ·········.. ·····960..·· ....··..·..··..·······..··..........2'ifO'......···············.. Demand (TBOD ), mg/l

s ·Tc6i57TB·ob;···..·..····..················..··..··..·. .·..·..·..2:·5·····......···.. ···· ·············....1·:·7"·..··....······ · ....····........··......(8······ · . ··········· 'Total··Sus·pe·nde·'(soiTds..···..·..·· ··················50"6"·······.. ········· ·..·.. ·····.. ·:(·1·5·0....·········· ······..·..··..·..····<.. 1'00····· ···..·.. ···· (TSS), mg/l

·pH···..····..································..········· ........·..··..·..·0·

Temp., C

··········

·..·····

··..·..······..·

·······..··..······6:·8···················· ···················7·· ······..····· ···········..··· · 7:·5..·· 28

25-30

30

··

· .

Upgrading of an existing treatment plant with the Kaldnes process At Apura in Germany, tissue paper is produced using a variety of wastepapers as raw material. Due to increased production the load to the existing biological treatment plant, consisting of a trickling filter and an activated sludge plant in series, was expected to increase and an extension was needed. The pretreatment of the wastewater consists of an equalization basin and a drum screen that removes large particles. The water is cooled in heat exchangers and taken to a primary clarifier for removal of fibers. Then the pH is adjusted in a neutralization stage and the wastewater is ready for the biological treatment. The data of the wastewater to be biologically treated is presented in Table 1.

E. DALENTOFf and P. THULIN

126

To extend the existing biological treatment a Kaldnes stage was installed in parallel to the existing trickling filter to take the organic load off the subsequent activated sludge stage. The design figures for the Kaldnes stage were 23 kg total COD/m 3·d, 35°C and 3 mg 0ll. Installation of a full scale treatment plant A full scale treatment plant consisting of a Kaldnes stage in series with an activated sludge process is under commission. The raw material at this mill consists of 50% waste paper, 25% pulp and 25% cotton linters. The products are a large variety of customized fine papers. The wastewater generated in this mill is, because of its composition, difficult to treat in an activated sludge process as it will most likely cause sludge bulking. Therefore, a combination process with Kaldnes and activated sludge in series was selected. Before the biological treatment the wastewater is pretreated in a drum screen that takes away large particles from the water. It is then flocculated with alum and the sludge is removed in a primary clarifier. The data of the wastewater that enters the biological treatment plant are shown in Table 1. The effluent average consents to be met were 20 mg BODsll and 30 mg TSSIl. In addition, total nitrogen < 4 mgll and phosphorus < 0.25 mgll had to be met 90% of the time. The stringent qualifications for N and P called for a tertiary treatment, chemical precipitation and flotation, after the biological treatment. The design data for the Kaldnes stage was 9 kg total BOD s/m 3.d and 16 kg total COD/m 3·d based on the 90 percentile load. However, the filling of the Kaldnes reactor is only 50% and after adjusting for a filling of 67% the theoretical design load is 12 kg total BODslm3.d and 21 kg total COD/m 3·d. The design load for the activated sludge stage was 0.25 kg BODslkg VSS·d. The biological sludge is separated in a clarifier and the sludge is recirculated to the activated sludge stage. The overflow from the clarifier is further treated in a flotation unit to meet the consent for phosphorus. Analyses In all three cases analyses have been carried out to follow the performance of the treatment plant. The main parameters that have been analyzed are COD Cr total and soluble, BODs total and soluble, Total Suspended Solids (TSS), Volatile Suspended Solids (VSS), pH and temperature. For the two full scale plants 24 hrs composite samples were analyzed. For the pilot plant experiments grab samples have been analyzed using the closed tube method (Dr Lange) for the COD Cr ' Soluble COD Cr and soluble BODs were determined in the filtrate from a sample filtered through Whatman GFIA glass fiber filters. The TSS and the VSS were determined according to standard methods. The Sludge Volume Index (SVI) was measured according to normal procedures. RESULTS AND DISCUSSIONS Pilot plant experiments The first Kaldnes stage in the K-K process and the Kaldnes stage in the K-AS process were run in parallel. Throughout the experiment the load to these two stages was increased from about 10 to 40 kg total COD/m 3·d and 4 to 16 kg total BODs/m 3.d. The corresponding loads for soluble COD and BODS were 6 to 26 kglm 3.d and 3 to 10 kglm 3.d, respectively. For the K-K-stage the total load calculated for both stages was half of that in one stage. The load to the activated sludge stage varied to a great extent, but averaged 0.8 kg soluble COD/m 3·d. The sludge concentration aimed at in the activated sludge stage was 3 gil. Typically, at an organic load of 25 kg total COD/m 3·d to the first Kaldnes stages the reduction in these stages of soluble COD and BOD was 40-50%. With an organic load of 12 kg total COD/m 3·d based on both stages of the K-K process the total reduction of soluble COD was 70-80% and the reduction of total COD was up to 78%. In the K-AS process the reductions of both soluble and total COD were about 90% with a load of 25 kg total COD/m 3·d to the Kaldnes stage and up to 1.5 kg soluble COD/m 3·d to the activated sludge stage.

Kaldnes suspended carrier process

127

In Figure 2 the total COD-reductions in the two process combinations are shown. It can clearly be seen that the COD-reductions for the K-K process were lower than those for the K-AS process. One reason for this is that there is more TSS in the effluent from this process. The average concentration of TSS in the outlet of the K-K stage was 140 and from the K-AS stage it was 37 mg/l, but this only explains part of the lower COD-reduction. However, since there is no sludge recirculation in the K-K process chemical precipitation can be used in the final separation stage to improve the COD-reduction. As a separation stage a flotation unit might also be used. Generally, both systems showed a great tolerance to variations in the influent wastewater and a quick ability to regain treatment efficiencies after disturbances caused by mechanical breakdowns. No decrease in soluble COD-reduction can be seen when the organic load is increased up to 40 kg total COD/m 3·d. However, when the organic load increases, the TSS in the effluent also increases, e.g. at a load of 10 kg total COD/m 3·d to the K-K process the average TSS in the effluent was 130 mg/l and when the total COD load was increased to 20 kg TCOD/m3·d the average TSS in the effluent increased to 185 mg/I. These results agree with those found by Rusten et al. (1992). 100,0 , - - - - - - - - - - - - - - - - - - - - - - - - - , 90,0 ';ft

co :g

80,0 70,0

::::I

60,0

!

50,0

---+- K-K total tot %

"CI I

C

o(.)

~

_

K-AS total tot %

40,0 30,0 20,0 10,0 0,0 ++++++-H+++H-+++++++-H+++H-t-tH+H-t-tH+H-t-tH+H-t-tH+H-t-tH+H-t-tH+H-t-tH+H-H-t-' 05-sep-95

25-sep-95

15-okt-95

04-nov-95

Date Figure 2. Total COD-reductions (%) in the K-K and K-AS processes.

The pilot plant trials show that the K-AS process gives satisfactory treatment results for this wastewater. Better treatment results would most likely be reached in the K-K process if th~ load were decreased. Upgrading of an existing treatment plant with the Kaldnes process The results from Apura in Germany show 55-60% COD-reductions in the Kaldnes stage at total COD-loads of 15-17 kg/m 3.d. The temperature has been 25-30°C. The total performance of the biological treatment plant has increased remarkably after installing the Kaldnes process. In Figure 3 the sludge volume index in the activated sludge plant is shown with and without the Kaldnes stage in operation. The Kaldnes stage was completely out of operation between January 26th and February 12th and it can be seen in the figure that the sludge volume index in the activated sludge plant immediately increases during this period. Up till the beginning of March the Kaldnes reactor is periodically out of operation due to some mechanical problems.

E. DALENTOFf and P. THULIN

128

600 - , - - - - - - - - - - - - - - - - - - - - - - - - - - - ,

E =-

500

---------------

400

~--------------

>

200

~

1:1

-

-

-------------------------------------

.5 CI) E 300 ~ '0 CD

C)

1:1 ~

en

100 0-1+H-H+t+H+t+H+t+1+t++11+t++1tH+i++++!+H+I+H+I+H+I+++++++++++++++H+t+H+t+H+t+H+t+1tlt+1+tI+1+tI+1+tI+1+tI+1ttH 1996-01-02 1996-01-22 1996-02-11 1996-03-02 1996-03-22 1996-04-11 Date

Figure 3. SVI in the activated sludge stage with and without the Kaldnes stage in operation.

Installation of a full scale plant The full scale plant was taken into operation in the middle of January this year. The biomass established rapidly on the suspended carrier and seeding was not necessary. After about one month 24-hour flow was introduced to the plant. The flow was at the beginning approximately 40% of full flow and was incrementally increased to 100% flow over a few days. To speed up the biomass growth in the activated sludge tank part of the inlet flow was taken directly to this stage. The development of the biomass was closely followed by microscopic viewing and these investigations showed continuous improvements of the biomass. In Figure 4 the total COD-reduction after the biological treatment and after tertiary treatment is shown. It can be seen in the figure that the performance of the plant was rapidly improved during the first week. The load to the Kaldnes stage was 10-15 kg soluble COD/m 3·d resulting in a soluble COD reduction over this stage of 50%. The load to the activated sludge stage was at the beginning 0.1 kg BOD.sfkg TSS·d, but was increased during the period to 0.4 kg BODs/kg TSS·d. The surface load of the final clarifier was about 0.55 m 3/m 2·h. 100 90 0~

80

C

70

0 ;:;

u

60

1:1

50

0

40

ii

30

~

f c•

.. 0

0 I-

---------

r---....--::T:"""C"""'---.-

Major plant upset ---------------------------dueloto~clnfluenf---------------'---------'

20 10 O-H--+-+-+-+-+-+-t-H-+--t--+-+-+-+-+-+-t-H-+-I-+-+-+-+-+-+-+-H-+-II-++-+--I-~ 1996-02-20 1996-03-01 1996-03-21 1996-03-11 1996-03-31 Date

Figure 4. The reduction of total COD after the biological treatment and after tertiary treatment.

Kaldnes suspended carrier process

129

In the middle of March a major plant upset took place. Immediately before the plant upset the treatment efficiency was> 90% COD reduced and the operation of the plant was stable. From one day to the other about 70% of the biomass was killed resulting in deflocculation of the sludge. The treatment efficiency decreased remarkably because of decreased biological activity and sludge escaping from the final clarifier. The only explanation found to this plant upset was something toxic occurring in the influent water. The plant recovered very quickly in spite of the fact that the biomass in the plant was immature. COD reductions> 90% were achieved after one week. After a short period of operation after the upset the plant was shut down for two weeks and was then restarted. After only a few days of operation the plant performed well reaching> 90% total COD-reductions. The three studies presented show only a few ways to use the Kaldnes process, more variants can be considered. It is shown that the Kaldnes process in series with an activated sludge stage provides an efficient and stable process for treatment of wastewaters from secondary fiber mills. In addition, the Kaldnes process is ideal for upgrading existing processes. To make a comparison of the three process alternatives K-K, K-AS and AS, as presented in Figure 1, the operating costs and sludge productions have been calculated for three treatment plants with the same investment costs. The model wastewater used in the calculations came from a secondary fiber mill with a CODIBOD ratio of 2. To obtain the same investment cost for a Kaldnes process and an activated sludge process the Kaldnes stages must be highly loaded, typically the volume of the Kaldnes reactor is about 1/7 of that of an activated sludge stage. The design base has been a total organic load for the K-K alternative of 12 kg COD/m 3 ·d (25 kg COD/m 3·d for the first stage). In the K-AS alternative the organic load is 25 kg COD/m 3·d for the Kaldnes stage and the expected soluble COD-and BOD-reductions are 50 and 60%, respectively. The load to the subsequent AS stage is 0.25 kg BOD/kg VSS·d. Finally, for the AS process the design BOD load is 0.25 kg /kg VSS·d. In the Kaldnes stages the aeration is carried out by coarse bubble aerators and for the activated sludge reactors the aeration systems used are fine bubble aerators. The final clarifiers have been designed for a surface load of 0.5 m 3/m 2·h in all three cases. In Table 2 the operating costs and the sludge productions for the three process alternatives are presented. The calculations of the operating costs include nitrogen, phosphorus, defoamer for the Kaldnes stages, energy cost and chemicals (ferric chloride and polymer) for precipitation of the sludge produced in the K-K process to be able to efficiently separate the sludge in the final clarifier. The calculations for the operating costs are based on Swedish prices and might be subject to adjustments depending on local prices. Table 2. Comparison of operating costs and sludge production for different process combinations for a treatment plant at a secondary fiber mill AS K-AS K-K 1 1 1 Investment costs 0.85 1 1.31 Operating cost 0.75 1 1.22 Sludge production As can be seen from these calculations the operating cost and the sludge production are highest for the K-K process and lowest for the activated sludge process. CONCLUSIONS There are many advantages of the Kaldnes suspended carrier process compared with the conventional activated sludge process, e.g. the process is insensitive to load variations and other disturbanc~s ~d ~o sludge recirculation is needed. However, the relatively high investment cost of this process puts a lmut to Its use. To make the investment cost of the Kaldnes process comparable to the activated sludge process it must be highly loaded, typically up to 25 kg COD/m 3·d. With a highly loaded suspended carrier process the treatment efficiency is diminished, partly due to excess sludge in the effluent being difficult to separate. For

E. DALENTOFf and P. THULIN

130

special cases, e.g. space limitations, special wastewaters or need for a very stable and robust process, a higher investment cost is often motivated and a lower loaded Kaldnes process can come into question. The Kaldnes process in series with an activated sludge process provides an efficient process for treatment of wastewater from secondary fiber mills. With this combination process the stability of the Kaldnes process is taken advantage of, and at the same time the subsequent activated sludge stage lowers the sludge production and flocculates the sludge, making it easy to separate in the final clarifier. The compactness and flexibility of the Kaldnes process also makes it ideal for upgrading of existing plants. In comparing a K-K process, a K-AS process and an AS process with similar investment costs the operating cost for the K-K process is about 30% higher compared to that of the K-AS process. In addition, the treatment efficiency for the K-K process is lower even if chemical precipitaion of the effluent is carried out. The sludge production is about 20% higher. For the activated sludge process the operating cost is 15% lower compared to the K-AS process and the sludge production is 25% lower. ACKNOWLEDGEMENTS The authors wish to thank our colleagues Mr Jorgen Wennmo for carrying out the pilot plant experiments, Mr Vlf Richter at Purac Leuna GmbH who has worked with the upgrading of the treatment plant and Mr Robert Proctor at Anglian Water International for contributing with experiences and data from the commissioning of the full scale plant. Without their devoted work this paper could not have been produced. REFERENCES Broch-Due, A, Andersen, R. and Kristoffersen, O. (1994). Pilot plant experience with an aerobic moving bed biofilm reactor for treatment of NSSC wastewater. Wat. Sci. Tech., 29(5-6),295-301. Hem, L. J., Rusten, B., Broch-Due, A, Mattsson, E. and Westrum, T. (1994). Treatment of forest industry wastewaters in moving bed biofilm reactors. Proceedings 49th Annual Purdue University Waste Conference. Rusten, B., Mattsson E., Broch-Due, A. and Westrum, T. (1994). Treatment of pulp and paper industry wastewater in novel moving bed biofilm reactors. Wat. Sci. Tech., 30(3),161-171. Rusten, B., 0degaard, H., Rusten, B. and Lundar, A (1992). Treatment of dairy wastewater in a novel moving bed biofilm reactor. Wat. Sci. Tech., 29(3-4), 703-711. Strehler, A. and WeIander, T. (1994). A novel method for biological treatment of bleached kraft mill wastewaters. Wat. Sci. Tech., 29(5-6), 283-294.