Rami Messalem(1), Asher Brenner(2), Semion Shandalov(2), Gideon Oron(2) ,Ora Kedem(1), and David Wolf(1)
1 - The Institutes for Applied Research, Ben-Gurion University ofthe Negev, Israel
2 - The Jacob Blaustein Jnstitute for Desert Research, Ben-Gurion University of the Negev, Sde Boker Campus, Israel

It has long been recognized that the shortage of water in Israel and its neighboring countries should be solved by a careful water resource management program that incorporates advanced technologies. Desalination of seawater and brackish groundwater together with reclamation and reuse of municipal wastewater are the main strategies that have been put forward for investigation and application. The present research is a part of the national effort to study and develop advanced secondary and tertiary treatment schemes incorporating mechanical biological treatment processes (mainly activated sludge systems) and membrane filtration that will be applied in wastewater reclamation plants. This paper summarizes the findings obtained from the operation of a pilot-scale system for treating typical municipal wastewater. The sequencing batch reactor (SBR) process has been shown to be an efficient biological treatment stage, producing effluents with low biological oxygen demand (BOD) and low total suspended solids (TSS). The effluents may be further refined by a filtration stage that gives high-quality effluents. Membrane separation processes, such as microfiltration, are emerging technologies that may be used for the efficient removal of colloidally dispersed, turbidity-causing particles. The expected benefits include the possibility of producing an effluent suitable for unrestricted irrigation, including food crop irrigation, and a very considerable improvement in disinfection of the product. Microorganisms present in domestic sewage, such as Giardia and Cyptosporidium, constitute a major problem in the reuse of water and wastewater. Because of the limited removal of these microorganisms by conventional sand filtration systems and their resistance to disinfection, a microfiltration stage may be essential for their removal if the effluent is to be recycled.

Our set-up comprises a two-phase system, consisting of SBR and a microfiltration units. Both these units have already been operated successfally as pilot plants on a stand-alone basis. The experimental system used for pilot-scale testing of secondary biological treatment and tertiary filtration comprised an SBR followed by a microfiltration unit. The SBR was selected for the activated sludge treatment process because of its convenient and flexible operation and its reported success in the treatment of both domestic and industrial. The SBR tank consisted of a cylindrical PVC vessel with a diameter of 1.4 m and a total water volume of 2.7m3. The feed solution comprised raw municipal wastewater pumped from the main sewage pumping station in Beer-Sheva, Israel. The SBR was operated in four cycles each day to treat a total of 3.6 m3 day-1, resulting in a hydraulic residence time (HRT) of 18 h. The organic load (on a BOD/MLVSS basis) was 0.15 day-1, giving a solids retention time (S RT) of eight days.

The self-cleaning, continuous microfiltration system (Memtec, Australia) was adapted into the two-stage pilot plant. At the heart of the microfiltration system are hollow-fiber microporous membranes, approximately 0.5 mm in diameter, which are encapsulated into a bundle to form a filter module. The nominal filtration area of the unit is 4 m2. The system can treat between 4 and 5 m2, of sewage per day, at a nominal rate of 80 gpd/ft2, i.e. about 500 1/h. A proprietary feature of the Memtec system is its air-operated backwash: raw feed (via the feed pump) and compressed air are used to backwash the unit whenever there is an increase in the resistance to flow in terms of transmembrane pressure (TMP). Air is introduced into the filtrate side of the system and released through the walls of the hollow fiber. Accumulated solids are flushed from the membrane surface by means of the feed water and gas backwash. The sequence of backwashing involves a pulse of air (1-2s) at 6OOkPa across the membrane, followed by a liquid flow of the feed side. The filtration rate and the frequency of backwash depend on the characteristics of the waste stream coming from the SBR. During normal operation, the feed passes from the outside of the membrane (from the module shell) into the center (lumen) and exits as filtrate. The system can be operated either in cross-flow configuration, or in dead-end flow mode. When it is operated in cross-flow mode, the effluent is recirculated a number of times in the unit, according to the rate of the cross-flow. In dead-end mode, all the effluent passes through the membrane, in a single pass. Typical system feed pressure is 25-35 psi (170 to 240 kPa). The normal operating differential pressure for the membrane is 5-30 psi (35 to 210 kPa), with average initial differential pressure loss of 5-8 psi (35-55 kPa).

SBR biological treatment of the raw municipal wastewater gave consistent and highly efficient removal of organic matter with good clarification efficiency. The micro filtration unit was operated under the cross-flow mode. Air backwash at 600 kPa across the membrane was carried out automatically, every 20-30 min, and chemical cleaning was applied periodically, every two weeks, with an alkaline detergent solution containing 1-2% NaOH. Filtrate flow rates of 80-180 L.m-2. h-1 were consistently obtained during the operating period, when the cross-flows applied varied from 0.10 to 1.25 m3. H-1 per module. A decline of the flow rate, at high cross-flow (1.25 m3. H-1 per module), after 10 hours of operation was observed, and the experiment was stopped. This may be caused by compaction of the layer cake that could not easily removed by normal backwash. Only after repeated air backwash followed by intensive chemical cleaning, the original flux could be restored.

The microfiltration system efficiently removed all suspended matter, thus reducing the total BOD and COD values (see table below). Total bacterial counts for MF permeate showed 6-log removal of coliforms and fecal coliforms, and the turbidity decreased from 7-10 to 0.1 NTU. These values are well below the requirements for unrestricted water reuse for agriculture.

Further treatment of the effluent by reverse-osmosis to remove dissolved salts was carried out. SDI (silt density index) measurements of the filtrate right after passing through the microfiltration system were performed, with average values of 0.8 min-1 as compared to 1.2 min-1 for tap water were obtained. The RO unit comprised a spiral wound module with a TFC polyamide membrane of 0.85 m2 and 99% rejection, (Hydranautics, USA). Several batches of 500 litres MF effluent were filtered through the RO unit, after adjusting the pH to 5, with concentrated sulfaric acid. The permeate flux remained constant for each run, under the same operating conditions. It varied from 30 to 40 l.m-2.h-1, when the pressure applied was changed from 7.5 to 12 bars, to achieve different concentration factors. Specific flow rates were calculated for 5 to 15-fold concentrations and found practically constant, about 4 l.m-2.h-1.bar '. This is a strong indication that there is no fouling of the RO membrane, due to pretreatment of the effluent by microfiltrati on.

Performance of SBR & microfiltration treatment processes

	Influent	SBR effluent	MF effluent
pH	8.1+0.2*	8.0+0.2	7.8+0.2
Conductivity, mS/cm	1.7+0.1	1.5+0.1	1.5+0.1
TDS,mg/L	1158	1022	970
Alkalinity, mg/L CaCO3	352+50	215+14	212+20
BOD total, mg/L	245+13	13.6+7.7	3.7+1.5
BOD filtered, mg/L	94+2.9	3.6+1.2	3.7+1.5
CODtotal,mg/L	800+154	77.51+6.5	38+6.1
COD filtered, mg/L	312+22	44.2+10.1	38+6.1
DOC, mg/L	28.4+8.1	12.6+1.7	12.1+1.9
NH4, mgN/L	40.3+18.2	3.7+4.9	3.7+5.0
N03,mgNIL	<1	41.8+10.9	41.4+11.3
N02, mgN/L	<1	3.2+2.9	3.5+3.2
P04, mgP/L	44.8+23.9	34.6+18.8	6.8+17.1
TSS,mg/L	318+78	17.5+9.8	0
Turbidity, NTU	>100	7.2+3.5	0.1+0.03
Total coliforms, cfa/mL	25*106	8.0*105	60
Fecal coliforms, cfu/mL	5.4*106	2.4*105	22
Totalcount,cfa/mL	8.3*106	6.0*105	387
* Standard deviations (when sufficient data were available)

The overall results of SBR/microfiltration treatment of the influent indicate the feasibility of this operation. SBR treatment of raw municipal wastewater produced an effluent containing BOD <20 mg/L and TSS <20 mg/L. Further treatment by microfiltration resulted in BOD<5 mg/L, TSS<1 mg/L and turbidity<0.2 NTU. Bacterial counts showed 6-log removal of coliforms and fecal coliforms. These results indicate that this treatment layout is indeed capable of producing an effluent that meets or even surpasses the requirements for unrestricted water reuse for agriculture. After disinfection, such effluents can easily meet the Israeli requirements for unlimited crop irrigation.