Document Type : Research Paper
Currently, treated industrial wastewater is discharged to the environment in most industrial towns in Iran. It is, however, a potential water resource for produce industrial process water. For reach this reuse application, further treatment would be needed. Nowadays membrane separation processes are becoming quite popular in wastewater treatment and reclamation, since they combine process stability with an excellent effluent quality. One of this membrane processes for water reuse and reclamation is using reverse osmosis (RO) that is increasingly being used in all over the world. RO relies on pressure differential to force a solution (usually water) through a membrane that retains the solute on one side and allows the pure solvent to pass to the other side.
MBR is a process in which conventional biological system is coupled with the membrane process (microfiltration, MF or ultrafiltration, UF).
Due to the shortage of water resources in the Shokouhieh industrial town (located in Qom province, Iran) reclamation and reuse of industrial wastewater treatment plant effluent using RO modules were put on the agenda. Effluents of this WWTP were not being adequately treated by biological treatment and there are biodegradable organic matters in effluent of wastewater treatment plant. This research has focused on the evaluation of the pilot scale operation and monitor of an MBR system to advance treatment of an industrial wastewater in order to produce water with appreciate quality as RO feed water. In other words this study has discussed the feasibility of RO pretreatment for water reuse from industrial wastewater treatment effluent (before disinfection) with operation of a MBR pilot. The removal of certain pollution parameters such as chemical oxygen demand (COD) and suspended solids (SS) were monitored and Silt Density Index (SDI) analyses were performed on the MBR effluents to determine the fouling potential of MBR effluent as RO influent.
Materials and methods
Actual wastewater used in this study was taken from an industrial wastewater treatment plant of Shokouhieh, Qom, Iran. This plant receives and treats the wastewater from different factories such as welding, dairy, beverage, metal finishing, … Due to poor design this existing treatment system is not effective in removing the all organic load of influent wastewater. So there is significant amount of biodegradable organic matters in effluent. The wastewater samples as MBR feed wastewater were collected from outlet of sand filters in plastic containers and were delivered to the laboratory where pilot is operated there.
Continuous operation of a pilot scale ultrafiltration membrane bioreactor system was carried out in this study. The bioreactor was made of Plexiglass with total volume of 32 liters. A flat sheet membrane ultrafilter was placed in the center of bioreactor.
Membrane operated at a constant flow rate of 4 L/hr using a prestaltic pump. Air blower was used to provide required sufficient air during operating the MBR. Also pilot was equipped with control instruments for measuring temperature, dissolved oxygen (DO), pH and wastewater level.
Membrane bioreactor was operated continuously, corresponding to an 8-hour hydraulic retention time (HRT) and the duration of operation was 30 days. Prior to use, membrane was washed with tap water until a steady pure water permeate flux was obtained. The MLSS temperature in the bioreactor was kept constant at 22–27 °C. Transmembrane pressure (TMP) was continuously recorded using an analogue pressure gage. Chemical cleaning of the membrane module was not carried out during the operation. No biomass was initially removed from the reactor to allow the biomass concentration build up in the system to about 2000 mg/L. After that Daily withdrawal of mixed liquor was conducted from the reactor in order to maintain the predetermined SRT (25 day) and to control an excessive increase of organic matter and solid concentrations in the bioreactor. Most analytical techniques used in this research followed the standard methods described by APHA. Data in this paper was averaged by at least 2 experiment results at each process
Results and discussion
During this study, it was detected that the MBR had MLSS in the range of 1600–2300 mg/L. Because of the extend order of magnitudes of the concentration values, the concentration measurements are plotted on a logarithmic scale. Results shows excellent solids separation achieved by the UF membrane. Removal of SS reached greater than 98% resulting in the MBR permeate with SS levels below 3 mg/L.
Also as can be seen from results, the inlet COD varied from 178 to 320 with the average COD concentration of the influent 220 mg/L whereas COD concentration in Effluent varied between 41 and 51 and Average elimination rate was higher than 75%. It means MBR system produced excellent removal of organic constituents and it was capable of achieving a high removal of COD and can effectively decrease the COD. Some previous studies reported more than 90% of COD removal which is higher than results of this study. Lower COD removal in this study may relate to less organic material concentration in this bioreactor.
For investigation of membrane fouling, the change of TMP with time in the MBR was monitored. TMP increased and went up slowly in exponential manner due to the fouling of the UF membrane. TMP reached 58 kPa on the 13th day of operation which was the fastest fouled MBR. In this stage particle, colloidal, biological and organic matters rapidly accumulated onto the membrane, and formed a cake which was probably compressible, leading to a rapid increase in the TMP. Some of these foulants are easily removed through physical wash by water, thus called reversible fouling. There is another fouling that is not readily removable from the membrane surface and requires use of chemical cleaning. As was mentioned before, for remove fouling in this study membrane was soaked in a 250 mg/L NaOCl solution and afterwards with 4000 mg/L citric acid solution for at least 4 hours. Then membrane was cleaned with tap water. However, it still remains a bit clogging of the membrane pores that are not washed away and caused pore blocking. During operation of MBR and several cleaning of membrane, pore blocking increases. Thus, as was shown, the time interval between the membrane washing is reduced during operation and cleaning of membrane repeats in a shorter duration (10 and 7 days).
As mentioned before, if RO process feed directly with filtrate wastewater without any pre-treatment it will show a significant increase in process pressure. In this study, the permeate SDI was below 3 for most of the time, although there was a slight increase and fluctuation during the testing periods. The average measured value was 2.21, with the tendency to increase with increasing duration of operation.
In this study, we presented the possibility and applicability of MBR for RO pretreatment and reclaim effluent in an industrial wastewater treatment plant. The MBR pilot was evaluated in terms of effluent quality. In general, it can be concluded that MBR can produce high permeate quality and is capable to be a very efficient method for RO pretreatment. Product permeate from MBR with average SDI less than 3 indicate that by using MBR pretreatment for RO system, it can be anticipated that the rate of membrane fouling reduce and the life of RO membrane modules extend. Also effluent water from the MBR has a high quality according to SS and COD removal during operation.