Membrane bioreactor
Membrane bioreactors are combinations of membrane processes like microfiltration or ultrafiltration with a biological wastewater treatment process, the activated sludge process. These technologies are now widely used for municipal and industrial wastewater treatment. The two basic membrane bioreactor configurations are the submerged membrane bioreactor and the side stream membrane bioreactor. In the submerged configuration, the membrane is located inside the biological reactor and submerged in the wastewater, while in a side stream membrane bioreactor, the membrane is located outside the reactor as an additional step after biological treatment.
Overview
has prompted efforts to reuse waste water once it has been properly treated, known as "water reclamation". Among the treatment technologies available to reclaim wastewater, membrane processes stand out for their capacity to retain solids and salts and even to disinfect water, producing water suitable for reuse in irrigation and other applications.A semipermeable membrane is a material that allows the selective flow of certain substances.
In the case of water purification or regeneration, the aim is to allow the water to flow through the membrane whilst retaining undesirable particles on the originating side. By varying the type of membrane, it is possible to get better pollutant retention of different kinds. Some of the required characteristics in a membrane for wastewater treatment are chemical and mechanical resistance for five years of operation and capacity to operate stably over a wide pH range.
There are two main types of membrane materials available on the market: organic-based polymeric membranes and ceramic membranes. Polymeric membranes are the most commonly used materials in water and wastewater treatment. In particular, polyvinylidene difluoride is the most prevalent material due to its long lifetime and chemical and mechanical resistance.
Image:MBR Schematic.jpg|thumb|Simple schematic describing the MBR process
When used with domestic wastewater, membrane bioreactor processes can produce effluent of high enough quality for discharge into the oceans, surfaces, brackish bodies, or urban irrigation waterways. Other advantages of membrane bioreactors over conventional processes include reduced footprints and simpler retrofitting.
It is possible to operate membrane bioreactor processes at higher mixed liquor suspended solids concentrations compared to conventional settlement separation systems, thus reducing the reactor volume to achieve the same loading rate.
Image:MBRvsASP Schematic.jpg|thumb|Schematic of conventional activated sludge process and external membrane bioreactor
Recent technical innovation and significant membrane cost reduction have enabled membrane bioreactors to become an established process option to treat wastewater. Membrane bioreactors have become an attractive option for the treatment and reuse of industrial and municipal wastewater, as evidenced by their consistently rising numbers and capacity. The current membrane bioreactor market was estimated to be worth around US$216 million in 2006 and US$838.2 million in 2011, grounding projections that the market for membrane bioreactors was growing at an average rate of 22.4% and would reach a market size of US$3.44 billion in 2018.
The global membrane bioreactor market is expected to grow in the near future due to various driving forces, for instance increasing scarcity of water worldwide which makes wastewater reclamation more profitable; this will likely be further aggravated by continuing climate change. Growing environmental concerns over industrial wastewater disposal along with declining freshwater resources across developing economies also account for increasing demand for membrane bioreactor technology. Population growth, urbanization, and industrialization will further complicate the business outlook.
However, high initial investments and operational expenditure may hamper the global membrane bioreactor market. In addition, technological limitations, particularly the recurrent costs of membrane fouling, are likely to hinder production adoption. Ongoing research and development progress toward increasing output and minimizing sludge formation are anticipated to fuel industry growth.
Membrane bioreactors can be used to reduce the footprint of an activated sludge sewage treatment system by removing some of the liquid components of the mixed liquor. This leaves a concentrated waste product that is then treated using the activated sludge process.
Recent studies show the opportunity to use nanomaterials for the realization of more efficient and sustainable membrane bioreactors for wastewater treatment.
History and basic operating parameters
Membrane bioreactors were introduced in the late 1960s, shortly after commercial-scale ultrafiltration and microfiltration membranes became available. The original designs were introduced by Dorr-Oliver Inc. and combined the use of an activated sludge bioreactor with a cross-flow membrane filtration loop. The flat sheet membranes used in this process were polymeric and featured pore sizes ranging from 0.003 to 0.01 μm. Although the idea of replacing the settling tank of the conventional activated sludge process was attractive, it was difficult to justify the use of such a process because of the high cost of membranes, the low economic value of the product and sometimes rapid losses of performance due to membrane fouling. As a result, the initial design focus was on the attainment of high fluxes, and it was, therefore, necessary to pump the mixed liquor and its suspended solids at high cross-flow velocity at significant energy demand to reduce fouling. Because of the poor economics of the first-generation devices, they only found applications in niche areas with special needs such as isolated trailer parks or ski resorts.The next breakthrough for the membrane bioreactor came in 1989 with the introduction of submerged membrane bioreactor configurations. Until then, membrane bioreactors were designed with a separation device located external to the reactor and relied on high trans-membrane pressure to maintain filtration. The submerged configuration takes advantage of coarse bubble aeration to produce mixing and limit fouling. The energy demand of the submerged system can be up to 2 orders of magnitude lower than that of the side stream systems and submerged systems operate at a lower flux, demanding more membrane area. In submerged configurations, aeration is considered as one of the major parameters in process performance both hydraulic and biological. Aeration maintains solids in suspension, scours the membrane surface, and provides oxygen to the biomass, leading to better biodegradability and cell synthesis. Submerged membrane bioreactor systems became preferred to side stream configurations, especially for domestic wastewater treatment.
The next key steps in membrane bioreactor development were the acceptance of modest fluxes and the idea to use two-phase flow to control fouling. The lower operating cost obtained with the submerged configuration along with the steady decrease in the membrane cost led to an exponential increase in membrane bioreactor plant installations from the mid-1990s. Since then, further improvements in membrane bioreactor design and operation have been introduced and incorporated into larger plants. While earlier devices were operated at solid retention times as high as 100 days with mixed liquor suspended solids up to 30 g/L, the recent trend is to apply lower solid retention times, resulting in more manageable suspended solids levels. Thanks to these new operating conditions, the oxygen transfer and the pumping cost in the reactors have tended to decrease and the overall maintenance has been simplified. There is now a range of membrane bioreactor systems available commercially, most of which use submerged membranes although some side stream modules are available; these side stream systems also use two-phase flow for fouling control. Typical hydraulic retention times range between 3 and 10 hours. For the most part, hollow fiber and flat sheet membrane configurations are utilized in membrane bioreactor applications.
Despite the more favorable energy usage of submerged membranes, there continued to be a market for the side stream configuration, particularly in smaller flow industrial applications. For ease of maintenance, side stream configurations can be installed on a lower level in a plant building, and thus membrane replacement can be undertaken without specialized lifting equipment. As a result, research and development has continued to improve the side stream configurations, and this has culminated in recent years with the development of low energy systems which incorporate more sophisticated control of the operating parameters coupled with periodic backwashes, which enable sustainable operation at energy usage as low as 0.3 kWh/m3 of product.
Configurations
Internal/submerged/''immersed''
In the immersed Membrane Bioreactor configuration, the filtration element is installed in either the main bioreactor vessel or in a separate tank. The modules are positioned above the aeration system, fulfilling two functions, the supply of oxygen and the cleaning of the membranes. The membranes can be a flat sheet or tubular or a combination of both and can incorporate an online backwash system which reduces membrane surface fouling by pumping membrane permeate back through the membrane. In systems where the membranes are in a separate tank from the bioreactor, individual trains of membranes can be isolated to undertake cleaning regimes incorporating membrane soaks, however, the biomass must be continuously pumped back to the main reactor to limit mixed liquor suspended solids concentration increases. Additional aeration is also required to provide air scouring to reduce fouling. Where the membranes are installed in the main reactor, membrane modules are removed from the vessel and transferred to an offline cleaning tank. Usually, the internal/submerged configuration is used for larger-scale lower strength applications. To optimize the reactor volume and minimize the production of sludge, submerged membrane bioreactor systems typically operate with mixed liquor suspended solids concentrations comprised between 12000 mg/L and 20000 mg/L, hence they offer good flexibility in the selection of the design Sludge retention time. It is mandatory to take into account that an excessively high content of mixed liquor suspended solids may render the aeration system less effective; the classical solution to this optimization problem is to ensure a concentration of mixed liquor suspended solids which approaches 10.000 mg/L to guarantee a good mass transfer of oxygen with a good permeation flux. This type of solution is widely accepted in larger-scale units, where the internal/submerged configuration is typically used, because of the higher relative cost of the membrane compared to the additional tank volume required.Immersed MBR has been the preferred configuration due to its low energy consumption level, high biodegradation efficiency, and low fouling rate compared to side stream membrane bioreactors. In addition, iMBR systems can handle higher suspended solids concentrations, while traditional systems work only with suspended solids concentrations between 2.5 and 3.5, iMBR can handle concentrations between 4 and 12 g/L, an increase in range of 300%. This type of configuration is adopted in industrial sectors including textile, food & beverage, oil & gas, mining, power generation, pulp & paper.