Pilot study on the effects of flocculation pretreatment on membrane fouling of membrane bioreactor treating wastewater from dairy cattle farms

Abstract: A cost-effective pretreatment method is highly demanding for the high-strength wastewater from dairy cattle farms in modern agricultural industry. In this study, a pilot test was conducted to investigate the effects of the flocculation on the membrane fouling of submerged membrane bioreactor (MBR). Normally, the wastewater from dairy cattle farm cannot be directly used as the influent of MBR, due to it contains high organic matters. Therefore, the flocculated and diluted raw wastewater can serve as the influent of MBR, where the chemical oxygen demand (COD) contents were similar. Two specific stages can be selected to operate sequentially in membrane fouling. The mechanism was also proposed in terms of transmembrane pressure, particle size, extracellular polymeric substances, and soluble microbial products inside MBR process tank during the two stages. The results showed that the transmembrane pressure in the first stage increased rapidly with time, when the flocculated effluent was used in the second stage, the membrane fouling rate decreased by 47% and the maintenance cleaning interval was 16 d in comparison with the interval of 10 d in the first stage. The concentration of extracellular polymeric substances (EPS) and soluble microbial products (SMP) of MBR in treating raw wastewater in the first stage were (4.76 ± 1.67) g/L and (3.94 ± 1.41) g/L, while the corresponding values of EPS and SMP in treating flocculated effluent were (3.97 ± 1.58) g/L and (2.23 ± 1.12) g/L, respectively. The SMP concentration in the first stage increased rapidly after the operation of the membrane bioreactor, but the SMP concentration in the second stage increased first and then decreased with time, and basically returned to its initial concentration level on the 30th day into the experiment. During both operation stages of MBR, the EPS concentration gradually increased with the running time. The peak volume percentages of the maximum particle size (MPS) in both stages increased first and then declined, and the maximum values in the first and second stage appeared on 16 d and 23 d, respectively. Both EPS concentration and SMP concentration decreased with the increase of particle size in the first stage, while in the second stage, EPS concentration increased with the increase of particle size, but there was no change rule between SMP concentration and particle size. Dispersed distribution of MPS in the first stage was observed and MPS turned to decrease after 16 days, while the MPS in the second stage remained relatively stable, and the maximum volume percentage of MPS increased from 3.57% to 5.95%. The COD removal rate of two influent waters by MBR can reach more than 90%, and the NH3-N (ammonia nitrogen) removal rate was close to 90%. The TP (total phosphorus) treatment effect of flocculated effluent was higher than that of raw sewage. The concentration of NPAM residue in flocculating effluent was analyzed, and the concentration ranged from 2 to 6 mg/L. When the flocculated effluent entered the process tank as MBR influent, the residual flocculant was not easy to permeate through the membrane module, and subsequently accumulated in the MBR process tank, where the macromolecule NPAM can combine the soluble constituents with the activated sludge flocs to form floc particles larger than the membrane pores, indicating effective to inhibit the clogging of membrane pores and the forming of gel layer, thereby to mitigate the membrane fouling. In Flocculation pretreatment, the EPS and SMP concentrations of liquid inside MBR process tank decreased with the concentration of SMP protein decreased significantly (P<0.05), but the particle size of liquid inside MBR process tank increased significantly (P<0.05), indicating beneficial for MBR fouling mitigation. The cost of dairy wastewater flocculation was estimated to be 0.08 Yuan/m3. The combination of flocculation and MBR can be expected to be a promising technology for high-strength wastewater treatment in dairy cattle farms.