A novel process, inclined-plates hydrolytic tank (IHT) and membrane bioreactor (MBR), was used to treat domestic sewage continuously. In this study, the effects of carriers' addition on operational performances of IHT-MBR were studied at the hydraulic retention time of 5.4 h and the recycling rate of 200%. Experimental results indicated the removal efficiencies of chemical oxygen demand, total nitrogen and total phosphorus reached 86.8%, 82.9% and 89.6%, respectively, corresponding trans-membrane pressure decreased to 1.50 kPa/d at a packing ratio of 20%. Simultaneously, the scanning electron microscope and soluble microbial products analysis demonstrated that high nutrient removal and low membrane fouling were attributed to the attached growth of microorganisms on carriers. The bioattachment and adsorption of carriers not only decreased the soluble proteins and polysaccharide in MBR, but also provided good living environments for denitrifying bacteria and phosphorus-accumulating bacteria.
The discharge of wastewater with high nitrogen and phosphorus is the main reason for water eutrophication. In this way, biological nutrient removal (BNR) processes have been widely used in existing wastewater treatment plants (WWTPs) to protect receiving waters (Lee et al. 2015). However, the discharge standards of WWTPs in China become more and more rigorous, and the demands for novel and advanced technologies for wastewater treatment are more urgent.
Membrane bioreactor (MBR) is considered as a perfect integration of a biological process and membrane separation process, which is widely used in municipal and industry wastewater treatment (Jensen et al. 2015; Leyva-Díaz et al. 2016). Recently, some biological processes such as moving bed biofilm reactor, inclined-plates and anaerobic-anoxic-oxic configuration are combined with MBR, enhancing the simultaneous organic and nutrient removal of municipal wastewater (Falahti-Marvast & Karimi-Jashni 2015; Reboleiro-Rivas et al. 2015). Xing et al. (2006) discovered that the application of inclined-plates effectively controlled the concentration of mixed liquor suspended solids (MLSS) in MBR, and the average removals of chemical oxygen demand (COD) and total nitrogen (TN) reached 92.1% and 71.7%, respectively.
MBR has some advantages such as an excellent effluent, process control and small footprint (Drews 2010). However, membrane fouling increases the operational costs (high energy and chemical costs) and shortens the life of the membrane, and as a result, it becomes a key obstacle that restricts its widespread application in wastewater treatment (Le-Clech et al. 2006). In general, membrane fouling is correlated with cake layer caused by sludge particle deposition on the membrane surface, the gel layer caused by adhesion of macromolecules to the membrane surface, and pore clogging caused by small molecules (Huang et al. 2008). Some studies demonstrated that the cake layer was the main contributor to membrane fouling in MBR (Lee et al. 2003), and the lower MLSS resulted in less membrane fouling (Xing et al. 2006).
Therefore, a number of methods have been undertaken to control membrane fouling, among which, adding suspended carrier with specific characteristics into MBR is a promising technology (Wei et al. 2006; Jin et al. 2013). The dissolved organic substances considered as important contributors to membrane fouling can be adsorbed to the carriers, leading to the reduction of suspended solid concentration in MBR (Leiknes & Ødegaard 2007). It was also reported that effective carrier dose played a very important role in mitigating membrane fouling in MBR (Huang et al. 2008). However, the impacts of carriers on both membrane fouling and BNR in inclined-plates hydrolytic tank (IHT) and MBR has not been discussed.
The objectives of this study were to investigate the effects of carriers packing ratios on the nutrient removal performance and the membrane fouling in IHT-MBR process. The COD, TN, and total phosphorus (TP) removal performances in IHT and MBR were investigated, respectively. Also, the trans-membrane pressure (TMP) was applied to analyze membrane fouling at different carriers packing ratios. Especially, the scanning electron microscope (SEM) and soluble microbial products (SMP) analysis were used to explore the mechanism for good nutrient removal and membrane fouling mitigation.
EXPERIMENTAL MATERIALS AND METHODS
Experimental setup and operation
The carriers used in this study were made of polyether and polyester with a cubic shape (16 mm × 16 mm × 16 mm). Carriers with packing ratios of 20% and 40% were respectively added into MBR to investigate nutrient removal performances and membrane fouling. The IHT-MBR process at different packing ratio continuously operated for 30 days at room temperature.
The domestic sewage was obtained from the residential area of Harbin Institute of Technology. The influent qualities of IHT-MBR were summarized as follows: COD 212.42–322.28 mg/L, ammonia nitrogen (NH4+-N) 35.68–47.78 mg/L, TN 40.67–55.71 mg/L, TP 3.22–5.99 mg/L, pH 6.95–7.40, and alkalinity 211.2–389.7 mgCaCO3/L.
Influent and effluent samples of IHT and MBR were analyzed every other day. The COD, NH4+-N, TN, TP and MLSS concentrations were determined according to Standard Methods (APHA 2005). DO was monitored using WTW Handheld Dissolved Oxygen Analyzer (JPB-607, Shanghai, China). SMP were collected and extracted based on the reference (Zhang et al. 2015), and the concentrations of proteins (PN) and polysaccharide (PS) in SMP were analyzed, respectively. The PN concentrations were measured by the Lowry method with the bovine serum albumin as the standard (Lowry et al. 1951), and the PS concentrations were analyzed by the phenol-sulfuric method with the glucose as the standard (Herbert et al. 1971).
RESULTS AND DISCUSSION
Overall removal performances
TN and TP removal in IHT and MBR
A combined process of IHT-MBR was applied to treat domestic sewage. The nutrients were effectively removed, and the membrane fouling was remitted by adding carriers into MBR at the HRT of 5.4 h and the recycling rate of 200%. The optimal packing ratio was determined to be 20%, which provided a good living environment for denitrifying bacteria and PAOs. Simultaneously, the SEM analysis indicated carriers were beneficial for attached growth of micrograms, which decreased the MLSS in the supernatant of MBR. Also, more PN were absorbed on the surfaces of carriers contributing to the mitigation of membrane fouling.
This work was financially supported by the National High Technology Research and Development Program of China (863 Program) (No. 2012AA063503), and Natural Science Foundation of Jiangsu Province (No. BK20160937).