WEFTEC® 2004
PHYSICAL, CHEMICAL AND BIOLOGICAL PROPERTIES OF SUBMERGED MEMBRANE BIOREACTOR AND CONVENTIONAL ACTIVATED SLUDGES Rion P. Merlo, R. Shane Trussell, Slawomir W. Hermanowicz, and David Jenkins Department of Civil and Environmental Engineering University of California at Berkeley
[email protected] ABSTRACT A pilot-scale submerged membrane bioreactor (SMBR) and two bench-scale conventional activated sludge (CAS) reactors were operated on municipal primary effluent over a range of mean cell residence times (MCRTs) from 2-10 d. The reactors had different turbulence levels. The root mean square velocity gradient (G) of the SMBR was 632 s-1; for the CAS reactors the G values were 72 s-1 and 250 s-1. The sludges from all systems were analyzed for particle size distribution (PSD), colloidal material, extracellular polymer substances (EPS) and filamentous microorganisms. Capillary suction time (CST) and time to filter (TTF) analyses were also performed. The SMBR sludges had the highest amount of small particles and higher levels of colloidal material than the CAS sludges. This was attributed to its higher G value and the use of a membrane for solids-liquid separation. The SMBR sludge contained the higher levels of total filamentous organisms attributable largely to its higher nocardioform level. This resulted from more efficient foam trapping by the SMBR. The normalized CST values of the SMBR sludge were lower than for the CAS sludges. This was attributed to its lower EPS content. There was no significant difference between the normalized TTF values of the SMBR and the CAS sludges. This was attributed to the offsetting effects of colloidal material and EPS contents. KEYWORDS Submerged membrane bioreactor (SMBR), particle size distribution (PSD), extracellular polymeric substances (EPS), filamentous microorganisms, nocardioform organisms, capillary suction time (CST), time to filter (TTF) BACKGROUND The membrane bioreactor (MBR) process uses a membrane (either an ultrafilter or a microfilter) to perform solids-liquid separation in place of the gravity clarifier used in typical conventional activated sludge (CAS) systems. The membrane can be submerged in the reactor and operated under vacuum, (submerged MBR, SMBR), or external to the reactor and operated under pressure, (external MBR, EMBR). The MBR process can achieve high biochemical oxygen demand (BOD) removal efficiencies (>95%) and virtually complete total suspended solids (TSS) removals when treating domestic wastewater (Cote et al., 1998, Fan et al., 1996, Rosenberger et al., 2002, Yamamoto et al., 1989) making it an attractive treatment option when water reclamation or stringent effluent discharge requirements exist. MBRs are capable of all of the unit processes of biological nutrient removal (BNR) including nitrification (Fan et al., 2000,
Copyright ©2004 Water Environment Federation. All Rights Reserved.
WEFTEC® 2004
Trouve et al., 1994), denitrification (Adham et al., 2000, Cote et al., 1997, Murakami et al., 2000) and enhanced biological phosphorus removal (EBPR) (Adam et al., 2002). In the SMBR, membrane fouling is controlled by using coarse bubble aeration of the membrane fibers to provide the cross-flow needed to prevent solids accumulation (Ueda et al., 1997). The use of the coarse bubble aeration for fouling control and a membrane for solids-liquid separation may influence the SMBR sludge properties. An understanding of sludge properties is needed to predict the behaviour of waste SMBR sludge thickening, digestion and dewatering processes. In this study, the physical, chemical and biological characteristics of CAS and SMBR sludges grown on municipal wastewater were evaluated. Properties evaluated were: particle size distribution (PSD), colloidal material content, extracellular polymeric substances (EPS) concentration, filamentous bacteria content, capillary suction time (CST) and time to filter (TTF). MATERIALS AND METHODS Wastewater characteristics All reactors were fed with primary effluent from the Southeast Water Pollution Control Plant (SEP), San Francisco, CA. The SMBR pilot plant was fed continuously through a line that took primary effluent directly from the SEP primary clarifier effluent channel. The CAS reactors were fed in the same way during the 10-d MCRT testing period. At all other times, the CAS reactors were fed from a tank that was filled and emptied daily to minimize seeding the reactors with filamentous organisms that grow on piping walls (Gabb et al., 1989). Reactor influent characteristics are presented in Table 1. Measurement of turbulence The root mean square velocity gradient (G) has been used as a measure of turbulence in several activated sludge studies (Das et al., 1993, Parker 1970). The G value for the SMBR and CAS reactors was calculated from:
G≅
Qair Hγ w Vµ
where: G Qair H γw µ V
(1) = = = = = =
root mean square velocity, s-1 air flow, m3/s depth of water column, m specific weight of water, N/m3 viscosity of water, Pa⋅s reactor volume, m3
Copyright ©2004 Water Environment Federation. All Rights Reserved.
WEFTEC® 2004
The viscosity and the specific weight of sludge were assumed to be the same as water at 20ºC (10-3 Pa⋅s and 9800 N/m3, respectively). For the intermittently-aerated SMBR membrane tank, Qair was the air flow during aeration. Reactor Operation The reactors were operated at several MCRTs in the range of 2-10 d. Data collection for a given MCRT commenced only after at least 3 MCRT values at the target MCRT had elapsed. The dissolved oxygen (DO) concentration was always ≥2 mg/L in all reactors. For the SMBR, the MLSS concentration was held constant at 8 ± 2 g/L and the hydraulic retention time (HRT) was varied from 1.1 to 3.6 h. The HRT of the CAS reactors was held constant at 7.1 h, and the MLSS concentrations were varied from 0.7-3.0 g/L. SMBR pilot plant The SMBR was a pilot-scale ZenoGem (Zenon Environmental Services Inc., Burlington, ONT, Canada), submerged membrane system (Figure 1) containing one full-scale membrane module (60.4 m2 surface area Zenon OCP ultrafilter, 0.035 µm nominal pore size). Solids are retained on the outside of the membrane, and permeate passes through the membrane into the interior of the hollow fibers. The SMBR consisted of an aeration tank and a membrane tank. Influent wastewater was pumped through a 3-mm screen to the aeration tank (757 L working volume), which was aerated through membrane diffusers fed with either compressed air or pure oxygen at a flow rate of 85 L/min. The membrane was submerged in the membrane tank (833 L working volume). Mixed liquor was pumped from the bottom of the aeration tank to the bottom of the membrane tank using either a high-shear centrifugal pump (Teel Model 1P70, W.W. Grainger Inc., Northbrook, IL) or a low-shear, lobe pump (Boerger LLC, Minneapolis, MN). When using the centrifugal pump, the SMBR is said to be under “higher shearing conditions” (SMBR H); when the lobe pump was in service, the SMBR is said to be under “lower shearing conditions” (SMBR L). Return mixed liquor flowed by gravity from the surface of the membrane tank back to the aeration tank surface. A vacuum pump downstream of the membrane provided transmembrane pressure to maintain a constant permeate flux of 30 L/m2⋅h. Intermittent (10 s on/10 s off) coarse bubble aeration at a flow of 850 L/min (G = 632 s-1) was used to control membrane fouling. The permeate vacuum pump was automatically turned off for 30 s every 9 min. A portion of the membrane permeate could be returned to the membrane tank so that HRT could be controlled without changing either the tank volume or membrane flux. Waste SMBR sludge was removed continuously from the membrane tank using a peristaltic pump. Bench-scale CAS reactors The CAS reactors consisted of an aeration basin (10-L working volume) and a gravity clarifier (4-L working volume). Mixed liquor flowed by gravity from an aeration basin surface overflow to the gravity clarifier. Secondary effluent flowed over an unbaffled weir; settled sludge was continuously pumped back to the aeration basin. Waste sludge was removed from the aeration basin once per day at the 10, 5 and 4-d MCRT conditions and more frequently at the 3 and 2-d MCRT conditions. CAS A (G = 250 s-1) used coarse bubble aeration at a flow rate of 4.7 L/min. CAS B (G = 72 s-1) used fine bubble aeration at a flow rate of 0.5 L/min.
Copyright ©2004 Water Environment Federation. All Rights Reserved.
WEFTEC® 2004
Particle size distribution (PSD) A sludge sample (5 mL for CAS reactors and 1 mL for the SMBR) was removed from each reactor with a wide-mouth pipette and placed in 1 L of particle-free SMBR effluent. The diluted sample was gently mixed with a magnetic stir bar to disperse the sludge evenly. The mounting and Methylene Blue staining procedures were adapted from Parker (1970). Samples were viewed under a light microscope at 200x magnification for the SMBR sludge, 100x for the CAS A sludge and 50x for the CAS B sludge. Digital photographs were taken of 20 random fields and stored as TIF files (2048 by 1536 pixels, 24 bits/pixel) for image analysis. Each image was converted into an 8 bit gray image by extracting the green component of the original RGB color image then further scaled down to 1024 by 768 pixels to enhance contrast. The particles deposited on the filter appeared as dark objects on a lighter background. A suitable image threshold was selected from the maximum of the second derivative of the grayscale image histogram. After selecting a threshold, the selected objects were eroded and dilated to separate individual particles, and their geometric features were measured. Only particles completely visible in each image were counted and analyzed. Image processing was performed using Mocha software (Jandel Scientific, San Rafael, CA). Initial file conversion employed a procedure written in IDL (Research Systems Inc., Boulder, CO). Frequency distributions were reported as number of particles, and characteristic length was reported as the square root of the projected area. Other definitions of characteristic length (such as geometric mean of maximum and minimum lengths, feret diameter) were also considered. They yielded identical trends. The lower particle size detection limit of the PSD analysis was 2 µm for CAS B sludge and 1 µm for the SMBR and CAS A sludges. Colloidal material Particles smaller than the detection limit of the PSD analysis (colloidal material) were assessed by turbidity measurements. Mixed liquor from each reactor was centrifuged for 2 min at 1000g and supernatant turbidity was measured using a Hach 2100N Turbidimeter (Hach Company, Loveland, CO) following Wilen et al. (2000). This method measured the turbidity due to particles with diameters of approximately
