Desalination, 98 (1994) 413-421 Elsevier Science B.V. Amsterdam - Printed in The Netherlands

413

Process performance comparison of membrane introduced anaerobic digestion using food industry waste water

Yoshinori Yushina and Jun Hasegawa

chiyoda Corporation, Environmental Technology Center, Moriya-cho 3-13, Kanagawa-ku, Yokohama 211 (Japan)

SUMMARY

Three different methane fermentation processes were evaluated using the soybean processing waste water. These processes are as follows: Process A- acidification and methane reactors; Process B - acidification and methane reactors followed membrane; Process C - acidification reactor, membrane and methane reactor. Three performance data were compared from the points of gas production under identical organic loading and operating conditions. Process C is excellent and recommendable when a waste water enriched organic SS is treated because a membrane application between acidification and methane reactors allows a clear two-phase separation in substrate degradation.

INTRODUCTION

The methane fermentation process is considered one of the major biological waste water treatment processes characterizing energy recovery from substrates contained in waste water. From a kinetic viewpoint,

OOll-9164/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved. SSD10011-9164(94)00167-7

414

methane fermentation is described as three steps involving: (1) hydrolysis of complex organic material, (2) acid production, (3) conversion of acetate or hydrogen to methane. The overall anaerobic conversion of organic matters to the end-products; CH,, CO, and NH, proceeds simultaneously within a reactor. Otherwise, according to reactor operating conditions, methane fermentation is classified under the acidogenic and methanogenic phases. While a major drawback in methane fermentation was long hydraulic retention time (HRT), much work for the upgrade of methane fermentation efficiency has been carried out.

Such efforts have been made to improve methane fermentation efficien- cy; for example, (1) a two-phase fermentation process in which acidogenic and methanogenic phases can operate under optimal conditions [L]; (2) accumulation of high microbial concentration in a reactor using microbial carriers [2]; (3) USAB reactor contributed by microbial pellet and granule formation [3]; and (4) membrane introduced reactor [4].

A membrane reactor of aerobic waste water treatment plants has been widely utilized in water reclamation systems. On the other hand, the application of membranes to the methane fermentation process is just beginning. A great advantage of membrane application in the methane fermentation process might upgrade efficiency due to methanogen accumu- lation. However, few studies have been reported regarding when a mem- brane should be introduced in a two-phase methane fermentation process. Further, there are few reports on quantitative analysis of improved perfor- mance between a conventional two-phase fermentation process and a membrane combined two-phase fermentation process.

This study demonstrates two-phase methane fermentation combined with a membrane separation unit for further improvement of efficiency in gas production and treated water qualities. The purpose was to obtain compari- son performances in terms of three different methane fermentation process- es: Process A - acidification and methane reactors; Process B - acidifica- tion and methane reactors followed membrane; Process C - acidification reactor, membrane and methane reactor.

Since a membrane was introduced between two reactors, it was of great interest whether perfect two-phase separation occurred in the process. A series of continuous field tests was carried out in a soybean processing factory employing a pilot scale plant consisting of fixed bed reactors.

415

EXPERIMENTAL

Apparatus

A flow sheet of the pilot plant and the specifications of the main apparatus are shown in Fig. 1 and Table I. The reactor is divided into two chambers, acidification and methane reactors. Both reactors equip microbial carriers made of polyvinylidene chloride nonwoven fabrics. Circulation pumps provide up-flow in each reactor. Gas produced from each reactor passes through a desulfurization tower. A membrane used is of an external pressure capillary type whose molecular weight cut-off approximates 15,000. Five modules of ultrafiltration (UF) membrane ensured a constant effluent water quality with merry-go-round usage.

TABLE I

Main specifications of reactor and membrane

Item Parameter Snecification

Acidification

Bioreactor

Size, mm

Empty bed volume, m3 Packed bed volume, m3 Packing ratio, % Carrier material Carrier size, mm

Methane

Bioreactor

Size, mm

Empty bed volume, m3 Packed bed volume, m3 Packing ratio Module material Module size, mm

Membrane Type

Shape, mm Materials

Mw cut-off Module shape, mm Packing rate, % Membrane area, m2 Feed tank volume, m3

900W x 750L x 1950H

1.0 0.54 54 Polyvinylidene chloride 55L X 55 (cylindrical)

900W x 950L x 3020H

2.0 1.60 80 Polyvinylidene chloride 450 W x 430L x 500H

External pressure; capillary 1.35 od x 0.80 id Polysulfone and poly- vinylalcohol

Approx. 15,000 1OOid x 1OOOL Approx. 60 Approx. lO/module 1.0

416

Anaerobic Eioreactor

Two-phase Type

Membrane Module

Fig. 1. Process flow sheet of the pilot plant.

Procedures

Table II shows the typical characteristics of waste water discharged from the soybean processing factory. A specific feature of waste water lies in 60% protein content of total organic matter and relatively high VSS concentration stemming from fine soybean particles. The temperature of waste water was controlled at about 30°C.

The pH was adjusted to 6.0 in an acidification reactor and 7.5 in the methane reactor. The HRT ranged from 3.3-3.5/h in the acidification reactor and 6.7-7.0/h in the methane reactor.

TABLE II

Characteristics of waste water (mg 1-l)

BOD 1000

COD 1629

TOC 670

vss 693

Protein 544

Carbohydrate 234

Lipid 23

417

Process A

;c,;idt;cation tk tbane Reactor - -

Process EJ 7

Acidification We thane Rex tor Membrane - Reactor - -

a I Process C

Acidification Reactor

1

Membrane He thane Reactor

Fig. 2. Three different methane fermentation processes.

Fig. 2 gives a block diagram of the three different processes used in this study. The continuous run for Process A was initially conducted for approximately 6 months. In the second step, Process B continued for approximately 100 days. In the final step a membrane was replaced between the acidification and methane reactors. The continuous run for Process C proceeded for approximately 6 months. Thus, a series of continuous runs for three processes used the identical experimental condi- tions .

Analysis and measurement

Flow rates of water and gas, temperatures, pHs, ORPs and methane content were continuously recorded. The water quality analysis in terms of TOC, BOD, COD, SS, VFA and ammonia was executed routinely. An automatic TOC measurement monitored hourly variation of raw and treated waters.

RESULTS AND DISCUSSION

Perjbrmance of Process A

The continuous run for Process A was carried out to compare perfor- mances of two membrane introduced processes with a conventional two- phase process. Performance data from organic loading of approximately

418

3 kg COD/m3/d was chosen for comparison. Average data are summarized in Table III.

Process A shows a higher gas production and methane content in the methane reactor than that in the acidification reactor. One cubic meter of waste water produces 308 Nl of methane. in treated water SS and COD concentrations were relatively high and acetic acid (HAc) of 38 mg/l was still residual. The COD removal percentage (51.8%) that should theoreti- cally be greater than the methane conversion resulted in the opposite phenomenon. This might be attributed to high SS concentration discharge in the treated water due to slough off of attached SS (microbes and soybean particles) on carriers during the period. Because the acclimation run (COD loading 3.0 kg/m3/d) prior to this test provided a good material balance showing COD removal 67.8%) methane conversion 54.9% and SS concen- trations in raw and treated water - 608 mg/l and 238 mg/l, respectively - all the data in Process A, except SS and COD, might be considered possible to use for a process performance comparison.

Petiormance of Process B

The purpose of Process B was to obtain clarified water and enhanced biogas production from the recovery of methanogen with a membrane. Average data for Process B are also given in Table III. The resultant COD loading was 3.25 kg/m3/d. COD removal (77.7%) was appreciably improved by decreasing free SS in the treated water. In terms of soluble organic acids, higher acetic acid (HAc) and propionic acid (HPr) concentra- tions were found residual in the treated water. Pertaining to gas production, methane conversion (68.9%) showed, contrary to expectations, a lower value than that of Process A.

Since the membrane separated perfectly solids and liquids, the decrease in methane conversion seemed to be derived from an accumulation of soybean suspended solids in the methane reactor. SEM proved the presence of mixture, microbes and soybean SS. Accumulation of SS in the methane reactor might cause simultaneous reaction of solubilization, acidification and methane production. The simultaneous reaction led deterioration of methanogenic activity. The two-phase separation was found difficult when a membrane was introduced in the final step.

TA

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raw

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aste

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ater

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CO

D

(kg)

.

420

Per$ormance of Process C

A membrane was replaced between acidification and methane reactors in Process C. The purpose of this process was to enclose soybean particles in the acidification reactor and to promote solubilization of particles therein. Accordingly, the methane reactor received a permeate containing organic acids. The average data for Process C from Table III show the resultant COD loading 3.0 kg/m3/d. Process C revealed a remarkable improvement in terms of COD removal (92.4%) and methane conversion (83.4%).

100 mg/l of COD, 26 mg/l of HAc and 16 mg/l of HPr in the treated water gave the lowest concentrations among the three processes. Further- more, 4 mg/l of SS was a low value which could explain a firm attachment to methanogen on nonwoven fabric carriers. Unlike the performances of Process A and B, the acidification reactor provided an increase in gas production. Methane production from the acidification reactor was 53 % of the total value. Hence, the acidification reactor introduced a membrane system permitting symbiosis of acid-forming and methane-forming bacteria. Since the membrane permeate was free of SS, ideal methane fermentation seemed to take place in the second reactor. SEM observation disclosed a lot of methanothrix-like bacteria attached to the carriers.

CONCLUSIONS

Introduction of a membrane in different locations caused a great difference in performance. Process B showed a clear blockage of SS in the treated water. However, there was no remarkable improvement in terms of gas production and soluble organic matter in the treated water. Process C gave noteworthy improvement in results compared with Process A. Concerning two-phase separation, Process B did not attain it and inversely enhanced a heterogeneous microbial formation in the methane reactor. Process C, despite a membrane application between acidification and methane reactors, performed no obvious two-phase separation either. However, it allowed a clear two-phase separation in substrate degradation processes. In conclusion, among the three processes, Process C is excellent and is recommended when waste water enriched with organic SS is treated.

421

ACKNOWLEDGEMENT

This study is a part of Research and Development for New Waste Water Treatment Systems (Aqua-Renaissance ‘90 Project), supported by NED0 (New Energy and Industrial Technology Development Organization). The authors would like to express their appreciation to NED0 and Mr. H. Satoh, Mr. T. Koyama, Mr. M. Sohma, Mr. I. Masuhara, Mr. J. Fujiya, Mr. H. Nomura and Mr. I. Abe for their collaboration in analytical and field work.

REFERENCES

1 T.C. Zhang and T. Noike, Water Sci. and Technology, 23 (1991) 1157.

2 J.B. Coulter, S. Soneda and M.B. Ettinger, Sewage and Industrial Waste, 29 (1957) 468. 3 W. Ruppel, M. Biedron, B. Thornton and R.J. Swintek, Food Processing, September

(1982) 65. 4 S. Kumura, Proc., 15th Biennial Conference of the International Association on Water

Pollution Research and Control, Pergamon Press, 1990, pp. 1573-1582.