38 Civil Engineering May 2008

In recent years, membrane bioreactor (mbr) technology has gained in pop-ularity as a means of treating domestic wastewater, in part because of increas-ingly stringent discharge requirements. Capable of producing high-quality efflu-ent, the membrane units in these sys-tems are almost always coupled aerobic reactors. However, significant amounts of energy are required to inject oxygen into the reactors to support the micro-organisms that help cleanse the waste-water, and air must be blown into the mbrs to keep the membranes free of solids. Amid mounting concerns about future water scarcity and rising energy costs, researchers in the united Kingdom are preparing to pilot test a 52,800 gpd (200 m/d) anaerobic mbr. Because they require less energy to operate and gener-ate less sludge than their aerobic counter-parts, anaerobic mbrs could reduce energy demands related to wastewater treatment and decrease the costs associated with sludge disposal.

On February 28 the Royal Soci-ety, the united Kingdoms national academy of science, awarded the Brian Mercer Award for Innovation to David Stuckey, Ph.D., a professor of biochemical engineering in Impe-rial College Londons chemical engi-neering department. Stuckey and Alan Hu, a graduate student in the same department, have developed what they call a submerged anaerobic membrane bioreactor (sambr) for treating dilute wastewater. As part of his award from the Royal Society, Stuckey received a grant of 250,000 (u.S.$494,000) to fund a yearlong test of a sambr pilot plant using actual screened sewage. To be hosted by Anglian Water at its Cam-bridge treatment facilitythe Water Innovation Centerthe field-scale test will be designed by the consulting engineering firm Black and Veatch, which has its headquarters in Kansas City, Missouri.

Thus far Stuckey and Hu have tested the sambr process only under labora-tory conditions using three 0.8 gal (3 L) reactors outfitted with two dif-ferent types of submerged mem-branes, each having a pore size of 0.4 m. Seeded with sludge from a conventional sewage sludge digester, the reactors received a synthetic waste-water stream having a chemical oxy-gen demand (cod) concentration of approximately 460 mg/L. Biogas gen-erated during the treatment process was collected and returned to the reactors at a point beneath the membrane units. In this way, the resulting bubbles helped to clean the membranes. The studys findings indicated that, on average, the sambrs removed 93 percent of the cod when operating at a hydraulic reten-tion time of three hours, according to an article by Stuckey and Hu in the February 2006 issue of asces Journal of Environmental Engineering.

As concerns about supplies of both energy and water mount, a critical nexus is emerging that will force waste-water treatment facilities to seek ways to use less energy while simultaneously producing more high-quality water for reuse, Stuckey says. Energy is becom-ing a major issue for treatment facili-ties, he notes. I cant imagine that in twenty years time we wont have treat-ment plants that are far more efficient. What is more, wastewater facilities, particularly in the united Kingdom, increasingly face challenges related to the disposal of the biosolids that result from the treatment process. The less sludge you have to dispose of, the bet-ter, Stuckey says.

As it turns out, the sambr process may be able to address problems related to energy consumption and sludge dis-posal as well. Compared with aerobic systems, anaerobic mbrs produce sig-nificantly less sludge, Stuckey says, because microorganisms in anaerobic

systems convert most of the potential energy in the wastewater into methane. By contrast, microorganisms in aero-bic systems use the potential energy to produce new cells. In fact, anaero-bic systems typically produce more energyin the form of methanethan they consume. If captured and converted into electricity, the meth-ane could be used to power the sambr process, greatly reducing the amount of energy needed to treat wastewater. All of these factors seem to push the technology toward using anaerobic processes, Stuckey says.

Having worked extensively with aerobic mbrs, engineers at Black and Veatch were well aware of the technol-ogys ability to produce high-quality effluent that could be recycled, says Frank Rogalla, a global practice and technology leader in Black and Veatchs office in Redhill, united Kingdom. But the conventional mbr has a very high energy need, Rogalla notes. We wanted to come up with a more sustainable approach. Because of the sambrs potential for producing energy while generating less sludge, Black and Veatch opted to conduct an indepen-dent evaluation of the technology at the bench-scale level.

The evaluation, which was con-ducted in the united Kingdom by a doctoral student at Cranfield univer-sity from mid-2006 to early 2007, was designed to test certain myths about anaerobic reactors, Rogalla says. For example, anaerobic treatment processes are commonly thought to work only on highly concentrated, warm waste-water. Thats why anaerobic treat-ment up to now has been limited to industrial applications that have warm substrates or are located in warm cli-mates, Rogalla says. However, during the bench-scale test, we found by con-centrating our biomass, thanks to the membrane, we could work at colder

TEChNoLoGy

Study Will Evaluate Anaerobic MBRs Potential For Treating Wastewater

May 2008 Civil Engineering 39

temperatures with dilute sewage and still get excellent cod removal and low sludge yields, he says.

The test involved a hybrid anaero-bic mbr of 10.6 gal (40 L) that was equipped with submerged membranes as well as side stream membranes, that is, membranes located outside of the mbr. Continuously fed raw munici-pal sewage for 120 days, the mbr was operated at three temperatures: 95F (35C), 72F (22C), and 54F (12C). Influent to the system contained cod concentrations between 250 and 600 mg/L. The system achieved its highest rate of cod removal (97 per-cent) while operating at 95F (35C). However, the mbr produced stable effluent with cod concentrations below

90 mg/L at temperatures as low as 54F (12C). Contrary to expectations, the membranes fouled less than membranes typically do on aerobic mbrs, Rogalla says, requiring less energy for scouring the membranes. Although the results are promising, Rogalla acknowledges that a more comprehensive evaluation of the process is needed to make sure it really works as well as we think.

Scheduled to begin in August, the test at Anglian Waters facility will involve operating the 52,800 gpd (200 m/d) sambr at various loading conditions to see how it performs in the face of such factors as differing organic loading rates and hydraulic retention times. More-over, the prototype facility will test four types of membranes provided by dif-

ferent manufacturers. By operating the process at full scale, the researchers hope to optimize its design while developing rigorous mass balances and energy balances to carefully track all inputs and outputs, Stuckey says.

Thanks to the grant from the Royal Society, the project team has enough money to pay for an operator to run the system for one year and a doctoral stu-dent to evaluate laboratory results for three years. If the results are positive, the research team hopes to obtain addi-tional funding to extend the operation of the pilot project, Stuckey says. ulti-mately, he notes, the goal is to install and test the sambr technology at an even larger treatment facility.

Jay Landers

TEChNoLoGy

hoRIzoNTaL dRILLING may LoCaTE NaTuRaL GaS RESERVES

Horizontal directional drilling may be used to reach natural gas trapped in portions of black shale difficult to reach in northern Appalachia, signifi-cantly boosting the nations known natural gas reserves, according to a team of researchers from Pennsylvania State university and the State university of New york (suny) at Fredonia. The team, led by Terry Engelder, Ph.D, a professor of geosciences at Penn State, and Gary Lash, Ph.D, a professor of geosciences at suny Fredo-nia, has estimated that a layer of shale known as the Marcellus Formation that extends from southern New york through western Pennsylvania, West Virginia, and eastern Ohio may contain 168 to 516 trillion cu ft (4.75 to 14.6 trillion m) of natural gas. The natural gas industry has been aware of the ability of the Marcel-lus shale to store natural gas but has found it difficult using traditional (vertical) drilling techniques to find and reach the fractures in the shale that trap the gas. The researchers believe that horizontal drilling, though more expensive than vertical drilling, can encounter a large series of these fractures sequentially, thus expos-ing multiple pockets of gas. The united States currently produces roughly 30 trillion cu ft (850 billion m) of gas per year; the research team estimates that the Marcellus

Formation could produce as much as 50 trillion cu ft (1.4 trillion m) per year.

Pennsylvania State university

CaNadIaN RESEaRCh CENTER SEEkS BEST WayS of REINfoRCING maSoNRy

The isis Canada Research Network (the acronym denoting intelligent sensing for innovative struc-tures) is working with Public Works and Government Services Canada to evaluate the many different methods currently being used to seismically retrofit the masonry walls of historically important buildings across the nation. The team will evaluate the performance of various seis-mic upgrade methods in the laboratory and determine which ones are the most effective. Researchers will exam-ine the inherent seismic capacity of the walls, the perfor-mance of traditional masonry anchors, the performance of the walls during freeze-thaw cycles, and the perfor-mance of masonry walls that have been reinforced. They will also test such anchorage assemblies as cementitious, epoxy, and mechanical anchors, including steel and fiber- reinforced polymers, to determine their effectiveness and will develop a structural health monitoring system that can be used universally on masonry walls in buildings of historical importance.

isis Canada Research Network

RESEaRCh BRIEfS