Combined biological and membrane treatment of food-processing wastewater to achieve dry-ditch criteria: Pilot and full-scale performance

Bioresource Technology 97 (2006) 1–14

Combined biological and membrane treatment of food-processing wastewater to achieve dry-ditch criteria: Pilot and full-scale performance George Nakhla a

a,*

, Andrew Lugowski b, Javnika Patel b, Victor Rivest

c

Department of Chemical and Biochemical Engineering, University of Western Ontario, London, ON, Canada N6A 5B9 b Conestoga-Rovers & Associates, 651 Colby Street, Waterloo, ON, Canada N2V 1C2 c Sun-Brite Canning, P.O. Box 70, Ruthven, ON, Canada N0P 2G0 Received 6 December 2004; received in revised form 10 March 2005; accepted 10 March 2005 Available online 31 May 2005

Abstract This study tested the applicability of a submerged vacuum ultrafiltration membrane technology in combination with the biological treatment system to achieve dry-ditch criteria stipulated as follows: BOD5, TSS, NH3-N, and total phosphorous (TP) concentration not exceeding 10, 10, 1, and 0.5 mg/L respectively for the treatment of high strength food-processing wastewater. During the study, the biological system, operated at average hydraulic retention time of 5–6 days, achieved 95–96.5% BOD removal and 96–99% COD removal. The external membrane system ensured the achievability of the BOD and TSS criteria, with BOD and TSS concentrations in the permeate of 1–2 and 1–8 mg/L respectively. Nitrate, and nitrite concentrations increased during membrane filtration, while ammonia concentrations decreased. The most salient finding of this study is that, contrary to common belief, for industrial wastewaters, the filterability of the mixed liquor is influenced by the soluble organics, and may be low, thus necessitating operation of bioreactors at low mixed liquor solids. This study demonstrated that bioreactors operated at low SRTs and in combination with ultrafiltration can still achieve superior effluent quality that may meet reuse criteria at reasonable cost.
2005 Elsevier Ltd. All rights reserved. Keywords: Suspended solids; Biological treatment; Ultrafiltration membrane; Food-processing

1. Introduction Membrane technology has been used for the treatment of municipal and industrial wastewaters from tanneries (Krauth, 1996), textiles (Rozzi et al., 2000), chemical (Livingston et al., 1998; Greene et al., 2000), and food-processing (Mavrov and Be´lie`res, 2000; Cuperus, 1998; Mavrov et al., 1997) facilities. Membrane technology has been applied successfully to the foodprocessing industry as membrane bioreactors (Cantor et al., 2000) and physical separators (Kuemmel et al., *

Corresponding author. Tel.: +1 519 661 2111/85470; fax: +1 519 850 2921. E-mail address: [email protected] (G. Nakhla). 0960-8524/$ - see front matter
2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.03.034

2000; FakhruÕl-Razi and Noor, 1999). By far, most of the MBRs applied for municipal and industrial wastewater treatment utilize external membranes in contrast to the submerged vacuum membranes, which are employed in this study. The vacuum submerged membranes, used in this study were provided by Zenon Environmental (Oakville, Ontario, Canada), have a nominal pore opening of 0.036 lm, molecular cutoff point of 300 kilodaltons, and operate at low vacuum pressures (less than 0.3 atm). The main advantages of these membranes include lower energy consumption, better effluent quality, better retention of microbes and viruses, and less fouling due to continuous cleaning of the membranes by air. Due to operation at high solids retention times (SRTs) and consequently high mixed

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liquor suspended solids (MLSS) concentrations, membrane bioreactors have been reported to enhance biodegradation of non-readily biodegradable compounds including high molecular weight compounds (Reemtsma et al., 2002; Schro¨der, 2002). However, even when used for solids separation only, biological activity on membranes cannot be ruled out due to the formation of a biofilm on the membrane surface. Despite the very brief contact time in the membrane unit, the very high biosolids concentrations will inevitably influence effluent quality. While MBRs have recently elicited significant interest, biological activity during membrane separation and its impact on effluent quality has not received attention. While the operation of membrane bioreactors at high SRTs and consequently high MLSS concentrations has been thoroughly reported in the literature, the performance of membranes at low SRTs has not been thoroughly investigated. Chaize and Huyard (1990) ran a pilot plant MBR with external ultrafiltration membranes on municipal wastewaters at hydraulic retention times (HRT) of 2–8 h, long SRT of 50–100 days, and high biomass concentrations of 8–10 g/L, and achieved virtually complete removal of organics and nitrification. Similarly Muller et al. (1995) have operated an MBR system with external ultrafiltration on domestic wastewater at biomass concentrations of 40–50 g/L and reported greater than 90% removal of carbon and complete nitrification. On the other hand, Ng and Hermanowicz (2003) have operated an MBR system employing submerged pressure membranes on synthetic

wastewater with average COD of 400 mg/L at HRTs of 3–6 h and SRT of 0.25–5 days, and average mixed liquor suspended solids concentration of 346–2300 mg/L and reported complete removal of carbon and nitrification at an SRT of 5 days. Cicek et al. (2001) have shown that the filterability of wastewater by membranes is influenced not only by suspended solids but also by soluble products. Thus, the performance of membranes in industrial wastes applications may differ markedly from municipal wastewater treatment. Furthermore, the achievability of strict surface discharge criteria with ultrafiltration membrane bioreactors has not solicited much attention since this feat is readily accomplished by nanofiltration and reverse osmosis systems, despite the high energy requirements. The primary objective of this study was to test the applicability of a submerged vacuum ultrafiltration membrane in combination with biological treatment in the food-processing industry to achieve stringent dry-ditch criteria. A detailed pilot study was undertaken at a food-processing facility to investigate the achievability of effluent discharge criteria of 5-day biochemical oxygen demand (BOD5), total suspended solids (TSS), ammonia nitrogen (NH3-N), and total phosphorous (TP) concentration not exceeding 10, 10, 1, and 0.5 mg/L respectively from the treatment of wastewaters generated primarily during the off-season from canning of various products, predominantly beans. Preliminary data from full-scale operations is also included. This study demonstrated that biological activity occurred in membrane filtration systems even at relatively low influent biomass concentrations.

UF UNIT BLOWER/PUMP HOUSE Permeate

Concentrate Waste Activated Sludge

PUMP HOUSE

ANOXIC TANK

AERATION TANK

FINAL CLARIFIER

Final Effluent (Direct Discharge)

SCREEN

Internal Recirculation Influent

PRIMARY CLARIFIER

FACULTATIVE LAGOON SYSTEM NOTE: Future Additions

Fig. 1. Process flow diagram of treatment system including pilot-scale membrane.

SPRAY IRRIGATION FIELD

G. Nakhla et al. / Bioresource Technology 97 (2006) 1–14

3

447 ± 409 (24) 488 ± 429 (24) 68.5 ± 37.0 (33) 20.6 ± 13.0 (36) 0.026 ± 0.044 (38) 1.5 ± 2.0 (38) 3.0 ± 2.0 (39) 1254 ± 291 (2) 1965 ± 996 (23) (n): Represents the number of data points.

3120 ± 1619 (38) 1448 ± 475 (22)

920 ± 326 (22)

188–2374 1.4–170.0 0.5–55.0 0.002–0.212 0.1–6.5 0.2–9.0 1048–1460

Mean ± SD (n)

Soluble

270–3540 591–7450

Total Soluble

288–1400

Total

234–2256 Range (min–max)

TSS (mg/L) PO4 (mg/L) PO4-P (mg/L) NO2 (mg/L) NO3 (mg/L) NH3 (mg/L) TKN (mg/L) COD (mg/L) BOD (mg/L)

Table 1 Wastewater characterization

The ultimate wastewater treatment system to be implemented at the site includes primary clarification to remove suspended solids, anoxic/oxic biological treatment for removal of organics followed by secondary clarification for solid/liquid separation prior to discharge to a dry-ditch. The system has been built in phases and the existing facilities include primary clarification, a 480,365 US gal aeration tank and a secondary clarifier. Agitation and oxygen in the aeration tank are provided by a diffused aeration system consisting of 284 medium-bubble diffusers and two 100-hp blowers. Dissolved oxygen (DO) concentrations in the aeration tank varied from 2 to 4 mg/L. Due to the lack of sludge recirculation systems, daily disposal of the entire influent wastewater quality was exercised. Settled sludge was hauled directly from the aeration tank to the local wastewater treatment plant. The total volume of settled sludge hauled was equal to the daily wastewater flow. Mixing in the aeration tank was stopped for 8 h, during which time the supernatant was pumped out of the aeration tank and trucked to the local wastewater treatment plant. Due to the lack of full-scale solid/liquid separation process, the biological system was operated as a fed sequencing batch reactor as opposed to an activated sludge, i.e. the SRT was not equal to the hydraulic retention time (HRT) of approximately 5–6 days, based on an average wastewater flow rate of 60,000–75,000 US gpd, due to allocation of 120,091 US gal of the aeration tank capacity for storage of wastewater in case of emergency. The pilot-scale membrane system used in this study was provided by Zenon Environmental Inc. of Oakville, Ontario and utilized a 180 US gal aerated tank to accommodate the 500 ft2 ZeeWeed hollow fiber, vacuum filtration membrane. The membrane tank was continuously aerated to reduce fouling and cleaning requirements, and as a result the DO concentration in the permeate typically ranged from 2 to 3 mg/L. Due to visible quantities of fine colloidal material in the raw wastewater, the selected membrane opening was 0.04 lm to remove colloidal material, bacteria, and viruses, thus rendering the effluent amenable for recycle back to the production facility to be used for cleaning and other operations. The membrane filtration system was operated at four distinct permeate flows: 2.0, 2.5, 3.5, and 3.8 US gpm corresponding to fluxes of 5.8, 7.2, 10, and 11 gpd/ft2. However, due to the close flows of 3.5 and 3.8 US gpm, the data has been combined. Thus the system operation is divided into three operating periods: OP-1 at 2.0 US gpm which lasted for 25 days, OP-2 at 2.5 US gpm which lasted for 9 days, and OP-3 at permeate flows was 3.5–3.8 US gpm, which

VSS (mg/L)

2.1. Description of existing treatment system

126–2242

2. Methods

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lasted for 42 days, following complete start-up and commissioning of the full-scale biological system and pilotscale membrane unit. The maximum capacity of the pilot unit was 5 US gpm corresponding to 7205 US gpd or 7.5% of the total wastewater flow. A process flow diagram of the system including the pilot-scale membrane system is shown in Fig. 1. The membranes were cleaned in accordance with the procedures specified by Zenon Environmental (Burlington, Ontario). Two types of cleanings were performed: maintenance clean and a recovery clean. A maintenance clean is a preventative cleaning, that is performed on a regular basis, and basically involves 10 cycles each comprised of stopping the permeate while continuing to aerate for 5–10 min, backpulsing (BP) with a 1000 mg/L sodium hypochlorite solution at 12 gallons/ft2/days for 30 s, and relaxing for 4.5 min. A recovery clean is performed when the suction required to permeate through the membrane reaches 7–9 psi, and involves alternate soaking in concentrated solutions of sodium hypochlorite (1000 mg/L) to remove organic debris, and 2 g/L of citric acid to remove inorganics. Two recovery cleans were performed on day 13 and day 53, while weekly maintenance cleans were initiated on day 42. The full-scale membrane system designed for a permeate flow of 82.3 US gpm (120,000 US gpd) consisted of forty-eight (48) 500-ft2 membrane cassettes for a total

area of 24,000 ft2, submerged in a 12 0 (L) · 7.8 0 (W) · 8.67 0 (side water depth) aerated epoxy coated carbon steel tank. At full operating capacity, with a permeate to concentrate flow ratio of 1:1, the HRT in the membrane tank was about 36 min only. The full-scale system was commissioned and fully operational by day 687 from the start of the pilot plant. 2.2. Wastewater characteristics Wastewater generation and quality from the foodprocessing facilities varies considerably throughout the year. Peak wastewater flows at approximately 480,365 US gal are generated during the tomato canning season extending from August to October while much lower wastewater flows at approximately 60,000–75,000 US gal result from the processing of other vegetable products during the rest of the year. The organic matter concentrations during tomato canning are higher than during the rest of the year. The results presented here pertain to the treatment of wastewater from processing of vegetable products exclusive of tomato canning wastewater. Table 1 presents the variations in primary effluent (influent to biological system) quality. BOD total mostly ranged from 234 to 2256 mg/L while COD varied more widely between 591 and 7450 mg/L. Phosphorus ranged

Table 2 Pilot–plant operational conditions Permeate flow (US gpm)

2.0 2.5 3.5–3.8

Operation (days)

25 9 42

Aeration tank HRT (h)

215 185 187

Average F/M ratio in aeration tank BOD g BOD/g MLVSS/day

COD g COD/g MLVSS/day

0.10 0.10 0.31

0.29 0.25 0.35

MBR HRT (h)

0.74 0.59 0.42

Average F/M ratio in MBR BODs g BODs/g VSS/day

CODs g CODs/g VSS/day

BOD g BOD/g VSS/day

COD g COD/g VSS/day

1.26 0.55 0.65

1.28 1.06 NA

6.31 7.20 7.15

13.7 14.5 NA

NA: Not available.

Table 3 Overall pilot plant system performance Permeate flow

Aeration tank

Membrane

BODT % removal

BODS % removal

CODT % removal

CODS % removal

BOD % removal

COD % removal

Operating period 1 Range (min–max) Mean ± SD (n)

41–72 59 ± 11 (7)

49–95 84 ± 18 (6)

13–89 68 ± 24 (10)

94–98 96 ± 2 (5)

48–99 75 ± 27 (4)

44–69 56 ± 13 (3)

Operating period 2 Range (min–max) Mean ± SD (n)

41–68 54 ± 19 (2)

93–97 95 ± 3 (2)

64–65 64 ± 1 (2)

96–97 97 ± 0 (2)

96–97 96 ± 1 (2)

49–78 63 ± 21 (2)

Operating period 3 Range (min–max) Mean ± SD (n)

74–90 85 ± 7 (4)

83–96 91 ± 6 (4)

13–94 55 ± 29 (13)

86–98 93 ± 5 (11)

25–96 62 ± 31 (4)

13–75 51 ± 22 (8)

(n): Represents the number of data points.

G. Nakhla et al. / Bioresource Technology 97 (2006) 1–14

from 0.5 to 55 mg/L depending on the production process with an average of 20 mg/L, which was satisfactory for maintaining biological activity. As presented in Table 1, highly fluctuating chemical characteristics of wastewater required a combined biological and membrane treatment system to operate over a wide range of suspended solids concentrations, as well as to achieve stringent effluent quality objectives.

and analyzed for COD (total and soluble), NH4-N, ni trate nitrogen ðNO 3 -NÞ, nitrite nitrogen ðNO2 -NÞ and 3 phosphate phosphorus ðPO4 -PÞ concentrations. Grab samples of the aforementioned three waste streams were collected twice a week, and sent off-site for analysis of the 5-day BOD (total and soluble), total suspended solids (TSS), and volatile suspended solids (VSS) concentrations. Weekly grab samples from the membrane tank were collected and analyzed off-site for TSS and VSS. For the full-scale system, the same streams were analyzed and for the same parameters as the pilot unit, albeit at a lower frequency. Off-site analysis was done once a week and parameters determined on-site, i.e.  3 COD, NH4-N, NO 3 -N, NO2 -N, and PO4 -P were analyzed twice a week. For the membrane pilot plant, the permeate flow, the reject flow, vacuum before and after

2.3. Sampling and monitoring program The pilot system was commissioned on April 26, 2002 and the study lasted for 3 months. Daily grab samples of the primary clarifier effluent (influent to biological treatment), aeration tank mixed liquor (influent to membrane system), and ZeeWeed permeate were collected

10000

5

Influent Aeration Tank Aeration Tank - Soluble

Above MAC Aeration Tank: 15/15 Permeate: 5/17

Permeate

Log BOD (mg/L)

1000

100

MAC = 10 mg/L 10

1

0.1 0

20

40

60

80

100

120

Time (days)

(a) 10000

Influent Aeration Tank Aeration Tank - Soluble Permeate

Above MAC Aeration Tank Soluble: 2/4 Permeate: 0/4

Log BOD (mg/L)

1000

100

MAC = 10 mg/L 10

1 0

(b)

2

4

6

8

10

12

14

Time (days)

Fig. 2. Temporal variation of influent, aeration tank and permeate BOD: (a) pilot data; (b) full scale data.

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backwash, ZeeWeed tank temperature, and pH were recorded several times a day. Due to the high concentrations of colloidal matter in the wastewater, the system was based on a permeate to concentrate ratio of 1:1. 2.4. Testing methodology  COD (total and soluble), NH4-N, NO 3 -N, NO2 -N 3 and PO4 -P concentrations were measured on-site using a Hach DR 2500 system. Standards with known concentrations of the various analytes were run with all analysis, and tests were repeated when the accuracy of standards determinations was below 95%. The chemical analyses for BOD (total and soluble), TSS, and VSS concentrations were carried out off-site at certified laboratories in accordance with the Standard Methods SM5210 B, SM2540 B, and SM2540 E respectively (APHA, 1985). It must be asserted that all BOD results reported here are 5-day BOD. Samples were transported to the off-site laboratory in ice-packed coolers, and stored at the lab in a cold room for a maximum period of 1 day prior to analysis. Standard deviations listed in the various tables were calculated using all the pertinent analytical data using the statistical formulae in Microsoft Excel, and reflect the variability of the water quality, not the accuracy of the analytical tests.

3. Results and discussion 3.1. Organics removal A summary of the operational conditions for the three operational period OP-1 to OP-3 during the pilot

plant testing is presented in Table 2. It is apparent that, despite the long HRT in the aeration tank, it operated at loadings similar to conventional activated sludge systems with average food-to-microorganisms (F/M) ratios ranging from 0.1 to 0.3 g BOD/g VSS-d. For the membrane tank, the HRT was based on the total influent flow rate from the aeration system and not based on the permeate flow. Due to the low HRT in the membrane tank, and despite the excellent removal of BOD in the aeration tank, the soluble organic loadings were still high in the 0.55–1.26 g BOD/g VSS-d range, typical of high rate systems. It should be noted that ammonia nitrogen in the raw wastewater (Table 1) was extremely low relative to organic matter with the BOD:NH3-N ratio, based on average observed concentrations of 483:1, thus indicating a severe nitrogen deficiency. This deficiency was corrected by addition of 751 kg of dry urea (40% nitrogen) over the course of the study to both the influent as well as the aeration tank. This translates to an average nitrogen concentration based on a flow of 75,000 US gpd and 76 days of net operation of 10 mg/L, thus reducing the BOD:NH3-N ratio to a more typical 144:1. A summary of the removals of total and soluble BOD and COD in the full-scale aeration tank and across the pilot-scale membrane during the pilot plant study is depicted in Table 3. BOD and COD removals by the membrane system were based on influent soluble BOD and COD. Total BOD removal efficiency in the biological system ranged from 41% to 90% averaging at 65% while total COD removal efficiencies were generally similar varying from 13% to 94% with an average of 62%. Removal of soluble organics in the biological treatment system on the other hand was much higher, averaging

10000

Log COD (mg/L)

1000

100

10 Influent Aeration Tank Aeration Tank - Soluble Permeate 1 0

10

20

30

40

50

60

70

80

Time (days)

Fig. 3. Temporal variation of influent, aeration tank and permeate COD.

90

100

0.5–20 3.7 ± 5.4 (12) 46–75 64 ± 11 (5) 94–100 98 ± 2.1 (7) 13–77 37 ± 21 (8) 0.1–2.2 1.3 ± 0.7 (7) 32–196 88 ± 47 (12) 20–170 86 ± 47 (11) 211–2151 974 ± 574 (15) 156–698 404 ± 195 (8) 270–3470 1568 ± 1106 (12) 288–1400 990 ± 331 (11) (n): Represents the number of data points.

591–4370 2199 ± 1153 (16) 234–2178 1523 ± 552 (11) 3.5–4.0 3.8 ± 0.1 (30) Operating period 3 Range (min–max) Mean ± SD (n)

3–14 9±8 (2) 49–78 63 ± 21 (2) 96–97 96 ± 0.7 (2) 22–36 30 ± 7 (3) 1.0–2.1 1.6 ± 0.8 (2) 70–99 85 ± 21 (2) 24–65 45 ± 29 (2) 1030–1300 1165 ± 191 (2) 437–705 571 ± 190 (2) 2110–2750 2350 ± 349 (3) 860–939 900 ± 56 (2) 2920–3610 3287 ± 347 (3) 1191–1347 1269 ± 110 (2) 2.5–2.5 2.5 ± 0.0 (8) Operating period 2 Range (min–max) Mean ± SD (n)

1–1 1 ± 0.1 (2) 44–69 56 ± 18 (2) 98–99 99 ± 0.6 (2) 23–35 28 ± 6 (3) 1.3–1.8 1.5 ± 0.3 (3) 62–90 80 ± 15 (3) 72–196 134 ± 88 (2) 720–800 760 ± 40 (3) 542–699 614 ± 79 (3) 2410–3210 2900 ± 429 (3) 1083–1354 1210 ± 136 (3) 3190–6540 4903 ± 1676 (3) 1428–2256 1770 ± 432 (3) 2.0–2.0 2.0 ± 0.0 (9)

BODS (mg/L) CODT (mg/L)

7

Operating period 1 Range (min–max) Mean ± SD (n)

COD % removal BOD % removal COD (mg/L) BOD (mg/L)

Permeate

CODS (mg/L) BODS (mg/L) CODT (mg/L)

Aeration tank

BODT (mg/L) BODT (mg/L)

CODS (mg/L) Influent

Permeate (US gpm)

Table 4 Summary of membrane operational and performance data—BOD and COD (total and soluble)

at 88% and 94% based on BOD and COD respectively. Approximately 2/3 of the organics were in soluble form and the organics were moderately biodegradable as reflected by a BOD5-to-COD ratio of around 0.4:1. The difference between total and soluble organics removal in the aeration tank is attributed to the slower biodegradation kinetics of particulate organics relative to the soluble organics, and the absence of full-scale solid–liquid separation, i.e. secondary clarification, which would have increased particulates removal. It must be emphasized that the organic removal efficiencies are remarkable considering that the biological system was run at low SRT and consequently low MLSS of mostly approximately 1000–1500 mg/L despite the long HRT. The diurnal variation of BOD, COD respectively, in the influent, aeration tank, and permeate together with the anticipated discharge criteria is illustrated in Figs. 2 and 3. BOD results from the full-scale operation are also included in Fig. 2. As shown in Fig. 2, 100% and 50% of the aeration tank effluent samples taken for soluble BOD analysis during the pilot study and full-scale study, respectively, exceeded the effluent BOD criteria of 10 mg/L. During the pilot study, the final effluent BOD criteria was exceeded in five out of 17 samples, four of them during the early commissioning and start-up phase, i.e. the system readily met the criterion for 94% of the time producing BOD of 1–2 mg/L, as shown in Table 4. Thus, in any of the three operating conditions listed in Table 4, the membrane would easily meet the 10 mg/L criterion, as confirmed by the permeate BOD data of Table 4 as well as the results of the fullscale operation. As depicted in Fig. 3, despite the highly fluctuating influent COD, and the aeration tank soluble COD varying from 30 to 200 mg/L, the permeate COD was stable at 30–50 mg/L. It is noteworthy that the typical effluent soluble COD from municipal wastewater treatment plants, is 30–50 mg/L, which constitutes the inert soluble COD component of municipal waste (IAWQ, 1995). Thus, the achievability of a similar effluent COD concentration in the context of high strength wastes is remarkable. Details of the performance of the membrane system during the pilot study are presented in Table 4. The membrane was operated at three different flow rates of 2.0, 2.5, and 3.5–3.8 US gpm. It should be noted that the BOD and COD removals calculated for the membrane were based on the soluble fractions of the influent BOD and COD to the system. At a flow rate of 2.5 US gpm corresponding to a flux of 7.2 gpd/ft2, the average BOD and COD removals achieved were 96% and 63% respectively. Effluent BOD and COD concentrations in the permeate at 2–2.5 US gpm were mostly in the 1.3– 1.8 mg/L while effluent CODs were in the range of 23– 35 mg/L. At 3.5–3.8 US gpm or 10 gpd/ft2, average permeate BOD and COD concentrations increased

TSS (mg/L)

G. Nakhla et al. / Bioresource Technology 97 (2006) 1–14

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G. Nakhla et al. / Bioresource Technology 97 (2006) 1–14

and COD in the aeration tank effluent that is filterable through 0.04 lm filter, the predominance of physical separation vis-a`-vis enhanced biodegradation cannot be precisely delineated. While the short hydraulic contact time in the membrane tank of 50–90 min may not be conducive to the biodegradation of slowly biodegradable organics escaping a 5–6-day HRT biological treatment system, the relatively much higher biomass concentrations and different microbial groups (Cicek et al., 2001) may have induced biodegradation.

sharply to 5.5 and 68.5 mg/L. At the low flows, the membrane filtration affected removal of approximately 43 mg BOD/L and approximately 50 mg COD/L. Since the soluble BOD and COD concentrations in the influent were based on filtration through 0.45 lm while the membrane opening is 0.04 lm, it is possible that physical separation of particles in the 0.04–0.45 lm range was the primary removal mechanism. Alternatively, enhanced biodegradation of slowly biodegradable waste constituents that escaped biological treatment in the aeration tank may have contributed to the organics removal. Oxygen uptake rates in the membrane tank averaged 0.64 ± 0.07 mg/L min, and it is thus estimated that the average COD removed by biodegradation in the membrane tank at an average HRT of 50–90 min is 32–57 mg/L. However, in the absence of the fraction of BOD

3.2. Suspended solids removal The temporal variation of the influent, aeration tank effluent, and permeate total suspended solids concentrations both during the pilot study and full-scale operation

10000

Log TSS (mg/L)

1000

100

MAC = 10 mg/L 10

1 Influent Aeration Tank Permeate

Above MAC Aeration Tank: 26/26 Permeate: 3/19 0.1 0

20

40

60

(a)

80

100

120

Time (days)

10000

Influent Aeration Tank Permeate

Log TSS (mg/L)

1000

100

MAC = 10 mg/L

10

1

0.1 Above MAC Aeration Tank: 4/4 Permeate: 0/4 0.01 0

(b)

2

4

6

8

10

12

14

Time (days)

Fig. 4. Temporal variation of influent, aeration tank and permeate TSS: (a) pilot data; (b) full scale data.

0.2–21.8 7.4 ± 6.0 (16) 0.3–97.3 27.6 ± 23.3 (16) 0.1–3.4 1.0 ± 1.0 (13) 0.2–26.9 11.7 ± 8.2 (16) 0.3–73.7 22.5 ± 18.6 (16) 0.1–5.4 1.3 ± 1.6 (11) 3.5–41.3 22.5 ± 11.1 (13) (n): Represents the number of data points.

0.2–9.0 2.9 ± 2.6 (17) Operating period 3 Range (min–max) 3.5–4.0 Mean ± SD (n) 3.8 ± 0.1 (30)

0.2–4.4 0.9 ± 1.4 (16)

4.2–19.6 10.2 ± 8.3 (3) 1.3–22.6 10.5 ± 11.0 (3) 2.3–47.1 23.3 ± 22.5 (3) 5.8–21.6 11.4 ± 8.9 (3) 1.0–13.0 6.4 ± 6.1 (3) 2.7–41.6 21.0 ± 19.5 (3) 6.6–21.0 15.7 ± 8.0 (3) 2.8–4.7 3.5 ± 1.0 (3) Operating period 2 Range (min–max) 2.5–2.5 Mean ± SD (n) 2.5 ± 0.0 (8)

0.1–6.5 2.3 ± 3.6 (3)

7.4–51.9 33.9 ± 19.3 (4) 2.3–32.5 17.9 ± 13.5 (4) 0.1–3.4 1.1 ± 1.6 (4) 6.4–53.9 32.5 ± 20.6 (4) 2.9–19.4 14.4 ± 7.7 (4) 0.4–3.7 1.9 ± 1.7 (4) 22.1–55.0 41.1 ± 15.2 (4) 0.3–0.5 0.4 ± 0.1 (4) 2.2–8.1 4.5 ± 2.6 (4)

NO3-N + NO2-N (mg/L)

9

Operating period 1 Range (min–max) 2.0–2.0 Mean ± SD (n) 2.0 ± 0.0 (9)

NO3-N + NO2-N (mg/L) NH3-N (mg/L)

Permeate

PO4-P (mg/L) NH3-N (mg/L)

NO3-N + NO2-N (mg/L) Aeration tank

NH3-N (mg/L)

PO4-P (mg/L) Influent

The performance of the combined biological and pilot-scale membrane system with respect to nitrogen and phosphorous removal is presented in Table 5 while the diurnal variations of the two aforementioned parameters are depicted graphically in Figs. 5 and 6. The adequacy of soluble phosphorous and deficiency of soluble nitrogen is noteworthy. No chemicals to remove phosphorous were added during the pilot study. Nitrogen deficiency was corrected by addition of urea as elaborated upon earlier. As shown in Fig. 5, during the pilot study the system exceeded the 1 mg/L NH4-N criteria in 13 of 28 samples. Two out of 13 samples that exceeded the NH4-N criterion occurred during the early commissioning and start-up phase. In the aeration tank effluent, 50% of the samples exceeded the effluent NH4-N criterion. Considering that the biological system

Permeate (US gpm)

3.3. Nutrient removal

Table 5 Summary of membrane operational and performance data—nitrogen and phosphorus

is depicted in Fig. 4. The effluent TSS criterion of 10 mg/ L was met in 85% of the samples during the pilot and 100% of the samples during full-scale operation (Fig. 4). Only three exceedances occurred, two being slightly higher than 10, i.e. 11 and 14 mg/L. Additionally, two out of the three exceedances of the TSS criteria occurred during the early commissioning and start-up phase of the pilot study. This data clearly demonstrates the achievability of the effluent TSS criterion of 10 mg/L. While the mixed liquor suspended solids concentration in the aeration tank varied mildly, from 1000 to 2000 mg/L, the final effluent TSS concentrations were mostly in the 1–2 mg/L during stable operation of the membrane system. It is note-worthy however that the performance of the membrane with respect to TSS removal at much higher influent TSS, i.e. 4000–6000 mg/L has not been tested in this study. Sludge yields in the aeration tank during the entire operating period of the pilot plant study were derived from charts of cumulative sludge produced (as VSS) versus cumulative COD/BOD removals. The calculated yields were 0.09 g VSS/g COD (R2 = 0.89), 0.15 g VSS/ g SCOD (R2 = 0.89), 0.215 g VSS/g BOD (R2 = 0.84), and 0.363 g VSS/g SBOD (R2 = 0.88). Thus, it is apparent that the system biological sludge yield is relatively low compared with the 0.5 g VSS/g BOD observed in municipal wastewater treatment. This low yield is attributed to the relatively low F/M ratio that the system operated at. It must be asserted that the volatile fraction of the aeration tank MLSS during the pilot study was 0.85, i.e. MLVSS was 85% of the MLSS. This atypically high ratio is attributed to two factors: the low concentration of influent inorganic suspended solids, as depicted in Table 1, and the low operational sludge age. The high volatile fraction of mixed liquor suspended solids, combined with the low sludge yield indicates that particulate organics were biodegraded in the system.

PO4-P (mg/L)

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full-scale operation. For all three operating permeate flow rates during the pilot study, shown in Table 5, the membrane did not meet the 0.5 mg/L TP criterion.

operated at very low SRTs and low biological solids concentrations that are not conducive to nitrification, the inadequacy of the nitrification is not surprising. Furthermore, since the waste is nitrogen deficient, more controlled addition of urea may alone be sufficient to meet the criteria. Thus, based on the pilot study results, for the three flowrates 2.5, 2.0, 3.5–3.8 US gpm based on 3-, 4-, 14-sample averages, respectively, the membrane will not meet the 1 mg/L NH4-N criterion, as shown in Table 5. However, the results of the full-scale system indicate that the combined biological and membrane system met the criterion in all five samples (Fig. 5). As anticipated, phosphorus removal without chemical addition was not adequate for meeting TP criterion of less than 0.5 mg/L. The effluent TP criterion was exceeded in 86% of the samples. A chemical addition system is needed for phosphorus removal during the

3.4. Membrane system performance The performance of the pilot-scale membrane filtration system with respect to nitrogen and phosphorus removals is presented in Table 5. Orthophosphate concentrations decreased during membrane filtration at the three operational conditions. At fluxes of 7.2 and 10–11 gpd/ft2, based on average conditions, 1.2 and 3.4 mg/L of PO3 4 -P were removed. Interestingly, nitrates and nitrites concentrations in the permeate increased, while ammonia concentrations decreased. The relationship between ammonia and nitrates concentrations in the permeate, and the influent to the membrane

100

Log NH3-N (mg/L)

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MAC = 1 mg/L

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Above MAC Aeration Tank: 15/32 Permeate: 13/28 0.01 0

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40

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80

100

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Above MAC Aeration Tank: 4/5 Permeate: 0/5 0.01 0

(b)

2

4

6

8

10

12

14

16

Time (days)

Fig. 5. Temporal variation of influent, aeration tank and permeate NH3-N: (a) pilot data; (b) full scale data.

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11

100

1

3-

PO 4 -P (mg/L)

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MAC = 0.5 mg/L

0.1 Above MAC Influent: 35/36 Aeration Tank: 36/38 Permeate: 24/28

Influent Aeration Tank Permeate

0.01 0

10

20

30

40

50

60

70

80

90

100

Time (days)

Fig. 6. Temporal variation of influent, aeration tank and permeate PO3 4 -P.

system both during the pilot study as well as during the full-scale operation is depicted in Figs. 7 and 8. It is evident that strong statistical correlations, as reflected by R2 values of 0.81 and 0.94 respectively were observed. Changes to these two soluble species are not anticipated to occur during physical separation since ultrafilters retain only particles with molecular weights of 1000– 1,000,000 (Scott, 1997) and the Zenon filter had a molecular cut-off point of 300 kilodaltons. The data of Fig. 7 indicates that in the range of 0.1–10 mg/L influent ammonia concentration, ammonia decreased by about 24% during membrane filtration. The results of the full-scale operation were also similar with permeate ammonia concentrations of less than 0.1 mg/L at influent concentrations of up to 2 mg/L. The concentration of nitrates and nitrites in the permeate, during the pilot

study, increased by about 15% (Fig. 8). The relatively good agreement between the full-scale data and the pilot study correlation confirms the trend of substantial increase in nitrates and nitrites during membrane filtration. Thus, nitrification was achieved in the membrane tank, despite the very short HRT of 0.4–0.75 h. This is consistent with the findings of Muller et al. (1995) who observed up to 86% conversion of influent nitrogen in domestic wastewater to nitrates in an MBR operating at MLSS concentrations of 10–40 g/L. It is interesting to note that nitrification in the membrane tank occurred rapidly, after 20 days following start-up of the pilot system. While the change in ammonia concentration of only 24% may not be significant considering the observed concentrations of about 1 mg/L, the change in nitrates (Fig. 8) and nitrites (Table 5) is significant.

10 9

Membrane NH3-N (mg/L)

8 7 6 y = 0.7646x R2 = 0.8107

5 4 3 2

Pilot Data Full Scale Data

1 0 0

2

4

6

8

10

Aeration Tank NH3 (mg/L)

Fig. 7. Relationship between permeate NH3-N and aeration tank NH3-N.

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Membrane NO3 (mg/L)

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80 y = 1.1983x R2 = 0.9377 60

40 Pilot Data Full Scale Data 20

0 0

10

20

30

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50

Aeration Tank NO3 (mg/L)

60

70

80

 Fig. 8. Relationship between permeate NO 3 and aeration tank NO3 .

3.5. Membrane cleaning

Before BP Vacuum (" Hg)

The temporal variation of vacuum prior to BP and aeration tank MLSS concentrations is illustrated in Fig. 9. It should be noted that due to employment of a 1:1 permeate to retentate ratio, the ambient solids concentrations in the membrane tank were essentially twice the aeration tank or influent MLSS. It is conspicuous

14.0

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Fig. 9. Temporal variance of before BP vacuum and MLSS.

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from Fig. 9 that generally BP started to increase, as the concentration of MLSS increased. However attempts to directly correlate BP vacuum with MLSS concentrations, as depicted in Fig. 10 clearly indicate that the correlation is not very strong. This implies that the filterability of the mixed liquor is not solely related to MLSS, i.e. soluble products affect filterability. This is consistent with the observations of Cicek et al. (2003), who provided evidence that the soluble fraction of mixed liquor, i.e. smaller size solutes, proteins, and sugars have a great impact on filtration performance in MBRs. For this particular waste, the BP vacuum of 8 to 9 psi, prescribed by the supplier as a trigger for recovery cleans, would be achieved at MLSS concentrations

The increase in nitrates plus nitrites across the membrane varied from 20% to 40%. The implications of this for denitrifying membrane bioreactors may be serious as it clearly manifests the potential for secondary nitrification.

G. Nakhla et al. / Bioresource Technology 97 (2006) 1–14

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Before BP Vacuum (" Hg)

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1000

1500

2000

2500

3000

3500

MLSS (mg/L)

Fig. 10. Relationship between before BP vacuum and MLSS.

of about 3000 mg/L, which translates to 6000 mg/L in the membrane tank. This is much lower than the typical concentration of 12,000 mg/L. Since one of the fundamental premises for using membranes, is the ability of the MBR process to operate at high MLSS concentrations, the filterability of this particular waste may constrain this kinetic advantage of MBRs over conventional activated sludge systems. Under such circumstances, contrary to common practice, it is only possible to operate the bioreactor at relatively low MLSS concentrations in the 2000–3000 mg/L range. Thus the achievability of stringent soluble organics and nutrients criteria at low biomass concentrations is critical, and therefore the evidence provided in this study to the aforementioned effect is highly pertinent.

4. Summary and conclusions

• Ammonia decreased, while nitrates, and nitrites increased across the membrane, indicating that biologically mediated nitrification occurred in the membrane tank despite the short hydraulic retention time of 50–90 min. • For this particular wastewater, the filterability of the biological sludges was fairly low and did not correlate well with biomass concentrations. Accordingly, operation of the bioreactor at low MLSS concentrations in the range of 2000–4000 mg/L, may be required to sustain satisfactory membrane performance without significantly increasing cleaning frequency. • Contrary to common applications of membranes wherein high biomass concentrations are employed, in this case the combination of biological treatment at low biomass concentrations in conjunction with membrane separation yielded excellent performance meeting stringent criteria.

Based on the results of this pilot study, the following conclusions can be drawn: Acknowledgements • The membrane ultrafiltration system has achieved complete removal of suspended solids and colloidal matter, thus facilitating compliance with the dryditch discharge criteria. • Approximately 90–99% of the soluble BOD as well as 40–50% of the soluble COD were removed by the membrane. • Membrane flux was relatively low with satisfactory performance achieved at 5.8–7.2 US gpd/ft2. • Orthophosphates removal across the membrane was minimal varying between 0.5 and 2.6 mg/L, thus necessitating chemical addition to meet the phosphorus criteria.

The analytical assistance provided by the Leamington Water Pollution Control Plant and Ontario Clean Water Agency (OCWA) is greatly appreciated.

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Livingston, A.G., Arcangeli, J., Boam, A.T., Zhang, S., Marangon, M., Freitas dos Santos, L.M., 1998. Extractive membrane bioreactors for detoxification of chemical industry wastes: process development. J. Membrane Sci. 151 (1), 29–44. Mavrov, V., Be´lie`res, E., 2000. Reduction of water consumption and wastewater quantities in the food industry by water recycling using membrane processes. Desalination 131, 75–86. Mavrov, V., Fa¨hnrich, A., Chmiel, H., 1997. Treatment of lowcontaminated waste water from the food industry to produce water of drinking quality for reuse. Desalination 113, 197–203. Muller, E.B., Stouthamer, A.H., van Verseveld, H.W., Eikelboom, D.H., 1995. Aerobic domestic waste water treatment in a pilot plant with complete sludge retention by cross-flow filtration. Water Res. 29 (4), 1179–1189. Ng, H.Y., Hermanowicz, S.W., 2003. Membrane bioreactor performance at short mean cell residence times. Proc of WEFTEC 2003 Conf., Los Angeles, CA. Reemtsma, T., Zywicki, B., Stueber, M., Kloepfer, A., Jekel, M., 2002. Removal of sulfur-organic polar micropollutants in a membrane bioreactor treating industrial wastewater. Environ. Sci. Technol. 36, 1102–1106. Rozzi, A., Malpei, F., Bianchi, R., Mattioli, D., 2000. Pilot-scale membrane bioreactor and reverse osmosis studies for direct reuse of secondary textile effluents. Water Sci. Technol. 41 (10–11), 189–195. Schro¨der, H., 2002. Mass spectrometric monitoring of the degradation and elimination efficiency for hardly eliminable and hardly biodegradable polar compounds by membrane bioreactors. Water Sci. Technol. 46 (3), 57–64. Scott, K., 1997. Handbook of Industrial Membranes, first ed. Elsevier Advanced Technology.