Use of a ceramic membrane bioreactor (CMBR) to treat wastewater at Guilin University of Technology

In China, the number of universities increased to 2, 762 by 2011, almost 2.7 times the number in 1998, to accommodate the increasing numbers of students (China Education Annuals 2011). For example, the number of campuses for Guilin University of Technology (GUT) went from two to four, increasing the campus surface area from 5.3 × 10⁵ m² to 2.7 × 10⁷ m². Green areas occupy over 50% of the surface area. The annual irrigation water needed for the green areas is estimated to be 7.3 × 10⁴ m³. Tap water is often used for this irrigation. However, as there is a lack of potable water in most Chinese cities, this use is wasteful. Every day, 8 × 10⁶ m³/d of wastewater is generated from the university campuses. Therefore, it is becoming increasingly important to recycle this wastewater.

Recently, attention has been increasing towards the use of membrane bioreactor (MBR) technology for wastewater recycling due to its higher efficiency in pollutant removal, compact size, and lower energy consumption (Melin et al. 2006; Chon et al. 2015; Judd 2015; Khalid et al. 2015; Kim et al. 2015). The material of the ultra-filtrating membrane, the most important part of the MBR technology, has evolved from polypropylene to polyethylene (PE) and, finally, to polyvinylidene fluoride (PVDF) (Tatsuki & Kenji 1999; Jeffrey et al. 2000). Currently, PVDF membranes are widely used because of economic considerations. A PVDF membrane generally has a hollow fibrous or flat-sheet structure. A hollow fibrous membrane can easily break up during operation, with the fracture not easily found, causing a deterioration in water quality. A ceramic membrane is a type of solid membrane. MBR technology based on a ceramic membrane has a very low running cost. The membrane can be recycled, which corresponds with the national industrial policy of promoting a circular economy featuring energy conservation and emission reduction. The ceramic membrane can be used as the recycling membrane module in the treatment of industrial and municipal wastewater. Compared to conventional organic membranes, the advantages of a flat ceramic membrane are: (1) High permeability performance: the water permeability can be increased by backwashing, (2) Energy-saving and cost reducing: the aeration consumption due to membrane cleaning can be reduced by 50%, (3) Durability: the ceramic membrane has a long operating life and potentially can withstand a pH range of 1–14, (4) Easy maintenance and management: the membrane could be backwashed automatically, and (5) No pollution: the ceramic membrane is made of an environmentally-friendly material that will not create secondary pollution.

This paper describes how a flat-sheet ceramic membrane, which can be produced at a low cost, was used as a membrane module to get a higher permeate flux. The newly-developed ceramic membrane bioreactor (CMBR) process, which consisted of a 1 mm screen, raw water tank, and submerged flat-sheet ceramic membrane module, was evaluated for the treatment of campus wastewater.

The campus wastewater used in the system was from the primary sedimentation tank of the wastewater treatment plant. First, the wastewater was pumped into the clean screening equipment by a sinking pump, and then it went through the 1 mm screen into the raw water tank. The screen removed the large particles and solid impurities before the wastewater entered the raw water tank. The raw water tank served as a storage pool for the wastewater that was used in the system.

The GUT campus wastewater included domestic water from the student dormitories, and water from public use. The water from the student dormitories included drinking water and undrinkable water for washing. The undrinkable water included water used for irrigation, firefighting, and cleaning. Different departments and buildings have different water utilization structures, and the amount of water used for the same purposes is also different. The amounts of water used and discharged by different parts of the GUT Yanshan campus are listed in Table 1.

In most Chinese universities, cooking is forbidden in student dormitories because of the risk of natural gas leakage. Therefore, only personal hygiene and laundry washing generate wastewater from the dormitories. Three canteens serve all the university students at GUT Yanshan campus. Leftover food was collected in the trash. Tableware were washed by the canteen workers. Oil separation units were used to separate the oil from the wastewater according to Chinese law. The average concentration of oil in the inlet of the aeration chamber was 12.53 mg/L during dry weather and 7.67 mg/L during storm water conditions. Unlike municipal wastewater (Jeffrey et al. 2000), nitrogen concentrations were relatively high throughout the test period (average: TN = 76 mg/L). Table 2 shows the concentrations of relevant parameters measured at the inlet of to the CMBR throughout the test period.

COD_Cr, SV30, SS, BOD₅, oil: Filtered COD_Cr (1 μm) was measured by the closed reflux colorimetric method (APHA 1995). SS and volatile suspended solids in effluent and sludge samples were measured in accordance with Standard Methods (Wenjie et al. 2009, 2011). Oil was measured by infrared spectrophotometry (ET1200, EURO-TECH). Total nitrogen (TN) was determined by the persulfate method (Jin et al. 2015; Wenjie et al. 2014, 2015) using the UV spectrophotometric screening method (Yue & Wenjie 2016) for quantification of TN as NO₃-N (the oxidization product of the persulfate digestion). The pH was measured by using a pH meter (9010, Jenco, USA), and dissolved oxygen (DO) was measured by using a DO meter (6010, Jenco, USA).

The total sludge content was estimated as mixed-liquor suspended solids (MLSS). For determination of MLSS, a sludge sample of known volume was washed twice by centrifugation (1,000× g for 15 min), decanted and re-suspended in deionized water, dried to a constant weight at 105 °C, and then cooled to room temperature using desiccation prior to weighing.

As shown in the Figure 3(b), the effluent BOD₅ remained almost stable within an acceptable inflow BOD₅ of 113.57–252.1 mg/L. The mean percentage of the BOD₅ removal rate was 94.5%, and the mean value of the BOD₅ concentration in the effluent was closer to 0 mg/L.

Transmembrane pressure (TMP) is an important indicator of how much a membrane is clogged by particles. In this study, backwashing was adopted to prevent the clogging of the membrane. During the operation period, the membrane was backwashed for 1 h each week to maintain its functionality.

SV30 increased quickly at the initial stage of operation, and the value remained above 95% for most of the test. There was essentially no change in the SV30 value after the sludge was discharged from the system.

Along with the increases in the sludge concentration, the volume of the sludge floc also increased, as did the viscosity (Figure 7). On day 90, DO was increased to inhibit the growth of filamentous bacteria. This caused the viscosity to decrease and mostly level off at a relatively low value.

The results show that the CMBR had stable removal efficiencies for organic compounds. Affected by the temperature and concentrations of the sludge, the COD_Cr of at the outlet in winter was higher than that in summer, with mean values of 44.8 mg/L and 34.3 mg/L, respectively. The outlet was always free of solids. Thus, except for the coliform group, all effluent values met the standards specified in Table 3 for the use of treated water for irrigation. The filtrate tank is a possible the pollutant source. Therefore, disinfection should be carried out before used.

In addition, no oil accumulation was detected in the aeration tank (Figure 8). It can be deduced that oil-degrading microorganisms were successfully cultivated in the CMBR process. The CMBR process ran with a stable flux of about 33 L/m² h, with the TMP maintained at −10 kPa until the end of the experiment period. To our knowledge, this is the highest flux observed in wastewater treatment to date.

The CMBR plant construction total investment composed of infrastructure cost, design cost, and equipment expenses, was US$85,937. The operation costs, including electricity, chemical costs, and labor costs, are estimated to be about US$0.056/m³. Once the treated water is substituted appropriately for potable water, investment recovery takes only 3 years.

As shown in Table 4, the flat-sheet type of ceramic membrane used in this study had more advantages that that of the other type of organic membranes (PVDF and PE) (Meng et al. 2017). For domestic wastewater treatment, high fluxes of up to 42 L/m² h can be achieved with this type of membrane, whereas only 25 L/m² h and 17 L/m² h fluxes are possible for flat-sheet type organic membranes and hollow-fiber type membranes, respectively. Long life, about two times longer than organic membranes, is another merit for flat-sheet ceramic membranes. The operating cost and maintenance are also superior. The footprint of a flat-sheet ceramic membrane is higher than that of a flat-sheet organic membranes but lower than that of a hollow-fiber type organic membrane. The flat-sheet ceramic membrane can be recycled after used. Therefore, flat-sheet ceramic membranes, as demonstrated in this study, are a good prospect for use in the MBR process.

The CMBR process has proven to be a robust system, capable of producing high quality service water from campus wastewater. Even under difficult local conditions, such as varying inlet concentrations, the quality requirements for the use of treated wastewater were met throughout the whole test period after disinfection. Fresh water demand can be reduced significantly if this service water is used for irrigation. Fluxes above 33 L/m² h, with MLSS at about 12,000 mg/L, were maintained. By using the CMBR process, treated water meets the Chinese national standards for landscaping irrigation.

This research was supported by Guilin Scientific Research and Technology Development Program (No. 2016012303), Guangxi Scientific Experiment Center of Mining, Metallurgy and Environment (KH2012ZD004), the project of high level innovation team and outstanding scholar in Guangxi colleges and universities (002401013001).