Submerged Hollow Fiber Membrane Research Articles

Start-up of the Anammox process in a membrane bioreactor

The start-up of an Anammox process was studied in a membrane sequencing batch reactor (MSBR) in which a submerged hollow fibre membrane module was used to retain the biomass. The reactor was seed with Anammox biomass and fed using the Van de Graaf medium. During a first operating stage, salt precipitation was observed and interfered with microbial activity and caused a decrease of the nitrogen removal rate of the reactor from 100 to only 10 mgl(-1) per day. Salt precipitation was avoided by diminishing adequately the Ca and P concentrations of the Van de Graaf medium during the last operating stage. This action increased quickly the activity of the system, and nitrogen removal rate reached up to 710 mgl(-1) per day with almost full nitrite removal. Sporadic flotation of the sludge was observed in the MSBR. The use of the membrane avoided biomass wash-out from the system. Moreover, a surprising fact was that Anammox biomass did not grow in flocs in the MSBR, but in granules. This fact showed that this kind of microorganisms have a trend to grow in aggregates. Results indicated that the use of the MSBR could be a suitable system for nitrogen removal by using the Anammox reaction.

Defining critical flux in submerged membranes: Influence of length-distributed flux

Flux can vary along the fiber length in submerged hollow-fiber membranes depending upon the axial gradients of both pressure and foulant layer build-up. However, the measurement of flux is necessarily length-averaged because it is determined by the flow rate exiting the end of the fiber. The length-averaged flux below which no foulant accumulates in a specified filtration time is defined as the critical flux. Critical flux is shown in this work to be a relative rather than absolute value. It depends on the fiber length, observation time, aeration rate and the compressibility of the particles. Fouling will occur in full-scale if the critical flux test is established in tests with fibers that are much shorter than in full-scale and/or with a filtration time that is shorter than in full-scale.

The effect of hydrodynamic conditions and system configurations on the permeate flux in a submerged hollow fiber membrane system

The present study was undertaken to investigate the effect of the hydrodynamic conditions and system configurations on the permeate flux in a submerged hollow fiber membrane system. Specifically, the study was aimed at establishing the magnitude of the contribution from single-phase (i.e. water only) and dual-phase (i.e. air sparged) cross-flow, as well as that from the lateral sway in multi-fiber modules, on the permeate flux in a submerged hollow fiber membrane system. The reduction in the permeate flux over time could be characterized by an initial short period of fast permeate flux decline, followed by a longer period of slower decline. The reduction in the permeate flux could be effectively modeled using three parameters: a pseudo-steady-state permeate flux, an initial rapid permeate flux decline coefficient and a long term slow permeate flux decline constant. Single-phase cross-flow had a limited effect on the pseudo-steady-state permeate flux. However, dual-phase cross-flow (i.e. with air sparging) significantly enhanced the pseudo-steady-state permeate flux. The pseudo-steady-state permeate flux for dual-phase cross-flow was 20–60% higher than that for single-phase cross-flow. The pseudo-steady-state permeate flux increased with the extent of air sparging for dual-phase cross-flow. However, a plateau was observed above which an incremental increase in air sparging intensity was not accompanied by an increase in the pseudo-steady-state permeate flux. For dual-phase cross-flow, when promoting lateral sway in a multi-fiber module, it was possible to further increase the pseudo-steady-state permeate flux by up to approximately 40%, compared to that for a single fiber module. Also, when promoting lateral sway in a multi-fiber module, a high pseudo-steady-state permeate flux could be achieved at a much lower bulk cross-flow velocity (i.e. air sparging intensity) compared to that for a single fiber module. The results suggest that promoting lateral sway in a multi-fiber module for dual-phase cross-flow resulted in the physical contact between the membrane fibers, which enhanced the permeate flux by mechanically eroding the foulant layer that formed on the membrane surface. No consistently significant trend in the initial rapid permeate flux decline coefficient was observed for the experimental conditions investigated. However, the long term slow permeate flux decline coefficient was observed to be proportional to the inverse of the pseudo-steady-state permeate flux. These results suggest that the long term permeate flux decline coefficient is governed by a time dependent mechanism, such as the adsorption of the fouling material to the inside of the membrane pores.

Optimising the operation of a MBR pilot plant by quantitative analysis of the membrane fouling mechanism

In order to optimize some operational conditions of MBR systems, a MBR pilot plant equipped with a submerged hollow fibre membrane module was employed in this study. The pilot MBR was fed with real municipal wastewater and the filtration flux, backwashing interval, aeration frequency and temperature were varied. A filtration flux below 25 I/m2h is generally recommended, at below this flux, the MBR operated at sub-critical flux conditions, the filter cake was minimized and membrane fouling was mainly attributed to the membrane pore blocking. Moreover, the membrane fouling, at below 25 I/m2h, was more reversible to backwashing; above this value, backwashing became less efficient to clean the membrane. Less frequent backwashing (e.g. 600 s filtration/45 s backwashing) decreased the amount of fouling irreversible to backwashing and its performance was superior to that of frequent backwashing (e.g. 200 s filtration/15 s backwashing). The MBR suffered more fouling at low temperature conditions (e.g. at 13-14 degrees C) than at high temperature conditions (e.g. at 17-18 degrees C). A conceptual model was built up and successfully interpreted this temperature effect.

Treatment of hospital wastewater using a submerged membrane bioreactor

Performance of a submerged hollow fiber membrane bioreactor (MBR) for treatment of hospital wastewater was investigated. The removal efficiency for COD, NH 4 +–N, and turbidity was 80, 93 and 83% respectively with the average effluent quality of COD <25 mg/l, NH 4 +–N <1.5 mg/l and turbidity <3 NTU. Escherichia coli removal was over 98%. The effluent had no colour and no odour. The transmembrane pressure increased slowly during 6 months operation. No membrane cleaning operation was used and no sludge was discharged during the 6-month operation period.

Application of a PAC-Membrane Hybrid System for Removal of Organics from Secondary Sewage Effluent: Experiments and Modelling

As world supplies of clean, fresh water come under increasing pressure and the need for water reuse rises, membrane technology is becoming increasingly important as a possible solution. However, membrane fouling is a major obstacle to the successful operation of the membrane process in wastewater treatment. In this study, a submerged hollow-fiber membrane with powdered activated carbon (PAC) adsorption was investigated for the removal of organics from secondary sewage effluent from a sewage treatment plant. The use of PAC in the membrane system was found to be very effective, not only in removing refractory organics, but also in reducing membrane clogging. A simple mathematical model was developed to predict the effluent quality [in terms of total organic carbon (TOC)] of the submerged membrane–adsorption hybrid system.

Submerged hollow fibre membrane module — design options and operational considerations

A new membrane module concept, the submerged hollow fibre membrane, has been widely accepted for the wastewater membrane bioreactor. This paper focuses on operational considerations and design aspect of the submerged hollow fibre membrane module. Experiments with both crossflow and ‘dead-end’ submerged systems have demonstrated that bubbling is effective in enhancing filtration performance, but the effect of gas flow rate is constrained because of the limited effect of bubbling on turbulence in two-phase flow. The experimental results also indicated that module configuration exerts crucial effects on the performance of the system. A better filtration performance can be obtained with vertical axial fibre orientation, small fibre diameters, and a loose fibre bundle. A model describing the filtration behavior of the submerged hollow fibre at steady state shows that when the maximum initial flux is lower than the critical flux, the flux distribution along the fibre can be estimated according to a dimensionless parameter x = 4 LR i − 3 2 R m − 1 2 which includes fibre length and radius and hydraulic resistance. When the average imposed flux is lower than the critical flux but the maximum local initial flux is higher than the critical flux, a steady state can be achieved after an initial deposition over some of the fibre length. The filtration resistance caused by the initial deposition becomes significant when the fibre radius is smaller than 0.2 mm, particularly with long fibre lengths and high ratios of average imposed flux to critical flux. The simulation also suggested that the optimal combination of fibre radius and length goes to small radius and short length.

Performance of a bioreactor with submerged membranes for aerobic treatment of municipal waste water

Aerobic treatment of municipal waste water in a membrane bioreactor was studied for 535 d. Apart from sampling, sludge was retained completely by a submerged hollow fibre membrane with a pore-size of 0.2 μm. The pilot plant comprised an anoxic zone to enable denitrification. The maximum liquid hold-up of the plant was 3.9 m 3. In this study the reactor performance and the stability of the process and the membrane capacity were investigated. A stable flux of 18 l m −2 h −1 could be realised with a mean transmembrane pressure difference of 0.3 bar with air-bubbling and backflushing the membrane and cleaning it in place every two months for one or two hours. For about 140 d, a flux of 27 l m −2 h −1 was achieved, but cleaning became necessary more often. The hydraulic retention time (HRT) varied between 10.4 and 15.6 h. Accordingly the volumetric loading rate was between 1.1 and 1.7 kg COD m −3 d −1. No inoculum was used. The mixed liquor suspended solids (MLSS) concentration gradually increased to 18–20 g MLSS l −1. The feed to microorganism ( F/ M) ratio varied according to the operation conditions but decreased against a value of 0.07 kg COD kg −1 MLSS d −1. Treatment performance was very stable and on a high level. The COD was reduced by 95%. Nitrification was complete and up to 82% of the total nitrogen could be denitrified.

Nitrogen Removal in Household Wastewater Treatment Using an Intermittently Aerated Membrane Bioreactor

Membrane bioreactor using a submerged hollow fiber membrane was applied in laboratory scale conditions to treat household wastewater including toilet-flushing water. The bioreactor was aerated intermittently to alternate anoxic/oxic conditions while membrane filtration occurred during the aeration period to take advantage of the air bubbles for fouling control. After being operated for about 150 days, the initial flux and suction pressure were maintained almost constant at 0.01m hr−1 and 4-6 kPa, respectively, indicating fouling control by air bubbling was very effective. With 10-15 hour hydraulic retention time (HRT), and a very long solid retention time (SRT), 97% of Total chemical oxygen demand (TCOD) and 100% suspended solids (SS) could be removed. On average, removal efficiencies for total nitrogen (TN) and total phosphorus (TP) were 83% and 55% respectively. Ammonia and coliform bacteria were completely removed. Due to the long SRT and sufficient oxygen supply, fast and complete nitrification was accomplished regardless of operational mode, and denitrification was the rate-limiting step. Results from track study revealed that the initial specific denitrification rate (SDNR) varied between 0.6 and 1.8 mg g−1VSS hr−1. Endogenous SDNR was found to be 0.35-0.51 mg g−1VSS hr−1. Nitrogen removal was controlled mainly by the value of BOD/TN of the influent. However, the role of endogenous denitrification was relatively significant under the high mixed liquor volatile suspended solid (MLVSS) condition, making the system more robust to the fluctuation of external carbon supply. The anoxic/oxic cycle of 60/90 minutes with an hydraulic retentin time (HRT) of 10 hrs appears to be an appropriate choice for the process.

Treatment of household wastewater using an intermittently aerated membrane bioreactor

An intermittently aerated membrane bioreactor using a submerged hollow fiber membrane was applied in laboratory scale to treat household wastewater including toilet flushing water. Nitrogen removal was optimized with intermittent aeration of the reactor and adjusting the time lengths of anoxic and aerobic phases of a cycle. Membrane filtration occurred during the aeration period to take advantage of the air bubbles for fouling control. With 8–15 h HRT and a very long SRT, 96% of TCOD and 100% SS could be removed. On average, the removal efficiency for TN was 83%. No significant accumulation of inert matter was observed after 4 and 5 months of operations without sludge wasting. Fast and complete nitrification was accomplished regardless of the operation cycle mode, and denitrification appears to be the rate-limiting step. Results from track studies revealed that the specific denitrification rate (SDNR) varied between 0.6 and 1.8 mg/gVSS·h. Endogenous SDNR was determined in a separate batch experiment and found to be 0.56 mg/gVSS·h. A linear relationship between the measured SDNR and the influent BOD/TN value was found. Based on this relationship, a rule-of-thumb method to determine the optimum time length for the anoxic phase was proposed. Intermittently aerated MBR appears to have advantages over other biological nitrogen removal processes in that denitrification can be enhanced under high MLVSS conditions. Endogenous denitrification may play a significant role, making the system more robust to the fluctuation or the limitation of external carbon supply.