High Loaded Synthetic Hazardous Wastewater Treatment Using Lab-Scale Submerged Ceramic Membrane Bioreactor

Submerged ceramic membrane bioreactors (SCMBRs) are more efficient combinations of traditional activated hazardous sludge and new membrane separation processes in wastewater treatment. Suspended solids are separated from hazardous effluent using microfilter ceramic membranes in SCMBRs. A high loaded wastewater was treated using an SCMBR employing a homemade tubular ceramic membrane in laboratory scale. Hydraulic Retention Time (HRT) was 32 h and COD range was varied from 2000 to 5000 mg/l. COD removal was evaluated to be more than 90% after a week and the lab scale SCMBR showed desired performance for the wastewater treatment. Mixed Liquor Suspended Solid (MLSS) was increased from 2000 to 4000 mg/L during the SCMBR operation time.

treatment were also reported [32][33][34]. Many parameters affecting the MBR performance like solid retention time (SRT), the behavior of the accumulated Soluble Microbial Product (SMP), aeration (nutrition and membrane fouling reduction), were studied by other researchers. The aim of this research was to characterize an SCMBR in terms of COD reduction at high organic loading rate and variable biomass concentration.

Materials and methods 2.1 Membrane preparation
Mullite microfilters were prepared from micronized kaolin clay, dedicated by Iran China Clay Industries Company with composition reported in Table 1. Kaolin clay was suspended in distilled water using Stuart stirrer (SS20) and the suspension was filtered using a 270 mesh sieve (Damavand test sieves Ltd., particle size < 53 μm). The suspension was then allowed to be settled inside its container and the upper clear water was removed periodically. Finally, a homogeneous mud was obtained and tubularly extrudates were extruded. For uniform drying and avoiding mechanical stress when tubes were dried, the wet extrudates were placed inside an oven at 100 °C for a few hours. After that, the dried extrudates were placed inside a furnace (Zohouri Furnace Industries) to be sintered with a heating rate of 2 °C/min at 1225 °C for 3 h. Finally, ceramic tubular membranes of 9 mm ID, 14 mm OD and 25 cm length were obtained. The sintered membranes were characterized with XRD and mercury porosimetry, as shown in Figs. 1 and 2.
Free silica was removed from the sintered membranes by leaching with 20 wt.% aqueous caustic soda solution at 80 °C for 5 h. Removal of the free silica results formation of in microporous tubular ceramic membranes with high porosity. After washing with the aqueous caustic soda solution, membranes were washed with distilled water for 12 h at 80 °C in order to remove the caustic soda.

SCMBR set up
A schematic diagram of a laboratory scale experimental system is shown in Fig. 3. The set up consisted of aerated bioreactor with working volume of 10 L and a dispensing pump transferring feed from the feed tank to the aerated bioreactor. Three ceramic microfilters (ID = 9 mm, OD = 14 mm, Length = 25 cm) were connected to a collector where a plastic tube connected the collector to the permeate tank. The permeate tank was vacuumed employing a vacuum pump (BCV vacuum pumps, P 2-S). Vacuum pressure was indicated using an analog pressure gauge. A pH meter (Lab-215, palintest Inc.) and a dissolved oxygen (DO) probe (HACH, Germany) were also installed in the SCMBR to monitor pH and DO online. SCMBR operation temperature and steady-state continuous flow rate were held at 32 °C and 3 mL/min (resulting in hydraulic retention time (HRT) of 32 h), respectively. Other operational parameters are given in Table 2.

Synthetic influent
Activated sludge used was supplied from Tehran refinery with initial Mixed MLSS of 2000 mg/L. The SCMBR was operated for 25 days while the activated sludge was fed with glucose continuously to reach MLSS to more than 4000 mg/L. Under aeration condition, DO was kept around 4-5 mg/L. Glucose, (NH 4 ) 2 SO 4 , and KH 2 PO 4 were used as source of carbon, nitrogen and phosphorous respectively, making COD:N:P to be 100:5:1. COD of the feed was changed in the range of 2000-5000 mg/L. Table 3 shows the synthetic wastewater components. The range of COD was selected based on some investigated industrial wastewaters with COD concentrations more than 2000 mg/L. These high COD loadings have high risks of discharging to the environment and should be treated before discharging. Landfill and dairy wastewaters are high COD loading wastewaters. As a result, the synthetic wastewater was selected with a composition close to these industries.

Analytical methods
COD and MLSS were measured according to the procedures described in Standard Methods. COD was measured by spectrometry (Palintest, England). For measuring MLSS, three samples were taken each time and the average value was reported. Fig. 4 shows flux decline of the ceramic membranes under constant vacuum pressure during 2 h. As observed, increasing transmembrane pressure (TMP) leads to higher membrane flux due to higher driving force, however, increasing TMP results in sharper initial flux decline due to the more membrane pore blockage and the membranes surface fouling. As observed, the membrane flux decline can be divided into two stages, a sharp decay stage and a pseudo-steady state stage. As observed, for a TMP of 0.2 bar, initial sharp decay is not the case indicating there is no serious membrane pore blockage and membrane surface fouling can be removed by means of aeration. In the case of other studied TMPs, at the early period of filtration, the flux reduces sharply due to the rapid membrane pore blockage and the significant biomass and dust deposition. In these cases, aeration is not sufficient to keep the flux constant and flux decline occurs due to surface fouling and pore blockage. Flux decline rates of fluxes, become much slower after 20 min. In fact, the fluxes approach to their pseudo-steady stage values. Steady state flux of the SCMBR at 32 °C and with MLSS of 2500 mg/L was compared with that of distilled water at different TMPs, as shown in Fig. 5. It can be seen that by increasing TMP from 0.2 to 0.4 bar, distilled water flux changes from 6 to 12 L/m 2 h, while steady state flux of the SCMBR varies in the range of 4 to 5 L/m 2 h showing the fact that pore blockage in the SCMBR reduces the flux and increasing TMP cannot recover it totally. Generally, it can be mentioned that permeation flux of submerged membranes which is lower than 30 l/m 2 h [35,36] is lower than that of external membranes in a cross-flow mode which is upper than 30 l/m 2 h [37,38].

Steady state flux and MLSS growth during SCMBR operation
The SCMBR was operated without any chemical cleaning during the experimental period. As mentioned above, steady state flux of the ceramic membranes was kept constant at 4 -5 L/m 2 h by means of TMP adjustment resulting in HRT of 32 h during the experiment. Unlike the adjusted flux, the membrane permeability was reduced because of the membrane pore blocking caused in the operation period. Increasing TMP in order to keep the flux constant, results in more the membrane surface fouling and the membrane pore blockage and this causes the membrane permeability to decrease within the experimental period. The flux and permeability of the SCMBR is shown in Fig. 6. Due to the feed loading in the range of 0.4-1 (kg COD/kg MLSS. day), MLSS was increased from 2000 to 4000 mg/L. MLSS growth in the SCMBR during the experimental period is shown in Fig. 7.

COD removal
Organic removal in the SCMBR was very high and stable during the operational period. The effluent COD of the SCMBR was varied in the range of 0-250 mg/L, and the average value is around 100 mg/L. The influent and effluent COD values illustrated in Fig. 8. The influent COD fluctuated from 2000 to 5000 mg/L; however, the effluent COD was maintained at a low and steady level. Taking 25 days of the experiment, an averaged COD removal efficiency of 95% was achieved after 4 days. After that, the effluent COD was not influenced by the influent COD variation.

Conclusions
Application of homemade ceramic microfilters in an SCMBR was investigated. The results showed that no significant flux decline is observed at low TMP, however increasing TMP results in significant reduction of the membrane permeability. The results also showed that the ceramic membranes have high potential for separation of treated water from activated sludge in the SCMBR. The designed SCMBR exhibited 95% COD removal during 4 days, while MLSS was increased from 2000 to 4000 mg/L.