Features

March 2007, Vol. 19, No.3

Prerequisite Pretreatment

Stable long-term operation of membrane bioreactors demands adequate pretreatment, particularly fine screening

Prerequisite Pretreatment.jpg Pierre Côté, David Brink, and Ali Adnan

Constructed in the 1970s and 1980s, the first generation of membrane bioreactor (MBR) technology incorporated large-diameter tubular membranes and primarily were used to treat small-scale industrial effluents containing little trash. Pretreatment for MBRs first became an issue in the 1990s, when hollow-fiber and plate-immersed membranes were introduced to municipal wastewater.


Today, municipal MBRs with capacities up to 50,000 m3/d (13 mgd) are in operation, and much larger systems are being constructed, designed, or planned. Current research aimed at reducing MBR costs will result in more densely packed membranes and reduced air-scour requirements for membranes. Because of these trends, pretreatment methods adequate to protect the membranes at the core of an MBR will become increasingly important. Read full article (login required)
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Let Them Wear Cake
A thin cake layer on the membranes can improve membrane bioreactor performance
Eugenio Giraldo and Mark LeChevallier

While developing a membrane filtration model, researchers discovered a correlation between aeration rate, cake layer, and transmembrane pressure (TMP). They used this information to develop an unusual operating strategy that can increase a membrane’s sustained peak permeate flux and minimize its long-term internal fouling.

This mathematical model of membrane filtration in a membrane bioreactor (MBR) incorporates the following fouling processes:

mass transport of particles to the membrane surface because of convective flux,
back transport of retained particles because of Brownian and shear-induced motion,
dynamic membrane fouling because of blocked (surface or internal) pores,
cake formation and growth,
dynamic cake filtration of soluble microbial products and exopolymers,
cake compression because of frictional pressure losses, and
clean membrane resistivity.
Each process is incorporated in a set of simultaneous differential equations that are solved numerically using the Euler method. The calculations were programmed in Visual Basic for Excel.

The resistance-in-series model is used to calculate TMP over time based on mixed liquor suspended solids (MLSS), soluble microbial products (SMPs), membrane flux, and clean membrane characteristics. Model results include an estimation of cake depth and growth over time, related fouling, and the variation in membrane resistance over time because of fouling related to surface blockage and internal pore plugging. Read full article (login required)  

 

Let Them Wear Cake 

A thin cake layer on the membranes can improve membrane bioreactor performance

Let Them Wear Cake.jpg Eugenio Giraldo and Mark LeChevallier

While developing a membrane filtration model, researchers discovered a correlation between aeration rate, cake layer, and transmembrane pressure (TMP). They used this information to develop an unusual operating strategy that can increase a membrane’s sustained peak permeate flux and minimize its long-term internal fouling.

This mathematical model of membrane filtration in a membrane bioreactor (MBR) incorporates the following fouling processes:

mass transport of particles to the membrane surface because of convective flux,
back transport of retained particles because of Brownian and shear-induced motion,
dynamic membrane fouling because of blocked (surface or internal) pores,
cake formation and growth,
dynamic cake filtration of soluble microbial products and exopolymers,
cake compression because of frictional pressure losses, and
clean membrane resistivity.
Each process is incorporated in a set of simultaneous differential equations that are solved numerically using the Euler method. The calculations were programmed in Visual Basic for Excel.

The resistance-in-series model is used to calculate TMP over time based on mixed liquor suspended solids (MLSS), soluble microbial products (SMPs), membrane flux, and clean membrane characteristics. Model results include an estimation of cake depth and growth over time, related fouling, and the variation in membrane resistance over time because of fouling related to surface blockage and internal pore plugging.  Read full article (login required) 

 

Just Like New 

Liners give pipes a new lease on life. Here's what to do when they don't behave

justlikenew.jpg Salam Khan and Christoph Dobson

Pipe liners are a cost-effective method for rehabilitating water, sewer, and gas lines. Many types of liners, including cured-in-place pipe (CIPP), fold-and-form polyvinyl chloride (PVC), deformed-and-reformed high-density polyethylene (HDPE), and spiral-wound PVC liner, are used to restore the structural integrity of deteriorated pipes and reduce infiltration and inflow.


Although all types of linings have been installed in the field successfully, problems occasionally occur during and after installation. Such problems include an insufficiently cured soft liner, bottom “lift up,” ribs, pinholes, pokeholes, delamination, wrinkles, blockages (because the liner shifted over openings), severed tie-ins at manholes, and nonadvancement during inversions.

Following are five case studies of lining problems in the field and how they were solved. They involve CIPP and deformed-and-reformed HDPE liners.  Read full article (login required)  

 

Aeration Innovation
 

By switching to floating-brush rotor aerators, the City of Eden, N.C., has improved treatment plant performance and cut energy costs

aerationinnovation.jpg Larry W. Moore and Charles Van Zandt

Until recently, activated sludge typically was aerated by diffused aeration systems or vertical aerators manufactured by several different companies. Oxidation ditches, meanwhile, generally were aerated using fixed horizontal rotor aerators, usually brush-type or disk-type units.

Although the fixed units generally have performed well, shortcomings associated with the units have prompted some users to replace them with floating-brush rotor (FBR) technology.

For example, in certain cases, the blades on some older fixed units have separated from the shaft upon extended use, substantially reducing aeration capacity and mixing in the oxidation ditch. Some older disk-type units have failed because of structural fatigue, causing disks to crack and break. Maintenance associated with the units also occasionally proved troublesome to treatment plant operators. For example, some units have failed to maintain adequate channel velocities, allowing significant quantities of biomass to accumulate on the bottom of oxidation ditches.

Another concern is that fixed units do not operate at optimum depth, as surface water levels in oxidation ditches fluctuate a few inches during the day because of diurnal variations in influent wastewater flow rates. In some cases, vertical aerators in extended-aeration activated sludge basins have failed to meet oxygen requirements and inadequately mixed the mixed liquor.  Read full article (login required)  

 

Operations Forum Features

Cold and Clean 

Antarctic researchers depend on this facility to protect them and the local environment

coldandclean.jpg Kam P. Law, Robert J. Kulchawik, David R. Zenz, Tony B. Bouchard, and Arthur Jung

Imagine being responsible for wastewater treatment at the South Pole. The National Science Foundation (NSF; Arlington, Va.) is.

NSF’s Office of Polar Programs operates three bases in
Antarctica. Its primary scientific research base, the McMurdo Research Station, is situated on the Ross Sea roughly 2200 mi (3540 km) south of New Zealand.

For years, McMurdo’s wastewater was simply macerated and then discharged into the Ross Sea. This practice met the requirements of the Antarctica Conservation Act, but subsequent research has shown that the effluent could be problematic for the local environment. So in the late 1990s, NSF proposed adding a wastewater treatment facility that met 1972 Clean Water Act standards (even though this law does not apply to Antarctica).  Read full article (login required)   

 

10 Steps to Preparedness

No wastewater utility’s emergency response plan is complete unless it addresses these 10 steps
 

10steps.jpg Jack W. Moyer

For many public works agencies, including wastewater utilities, Hurricane Katrina in 2005 served as a wake-up call regarding the need for emergency preparedness. Although drinking water systems and other infrastructure have received much of the subsequent focus, wastewater systems also have been shown to be vulnerable, resulting
in significant service interruptions and environmental damage when they fail.

Despite the many improvements in emergency preparedness made by wastewater utilities in recent years, additional opportunities for improvement remain. This article describes 10 relatively easy and inexpensive steps that wastewater utilities can take to improve their disaster preparedness.  Read full article (login required)  

 

From ‘Sow’s Ear’ to Silk Purse

A Virginia treatment plant revamped its solids-handling facilities to great effect

fromsowsear.jpg James N. Struve, Alan L. Stone, and Mark Prentice

Almost every major treatment facility has a “sow’s ear,” that is, a process component or major piece of equipment that has been abandoned partially or totally.

The in-vessel composting facility at the Henrico County Water Reclamation Facility (Richmond, Va.) caught fire several times during startup and never performed as planned. In 1993, the process and equipment were abandoned and most of the building housing it was mothballed. Henrico County opted to
apply Class B biosolids to agricultural lands instead of producing compost that would meet Class A standards.

When a new and significant capital improvement program began, county staff decided to use the existing structure to recoup some of the expense of building the compost building. In so doing, the county ultimately transformed its sow’s ear into the proverbial silk purse.  Read full article (login required)