Treating the Cause, Not the Symptom

April 12, 2006

About the author: Dennis Livingston is MBR product manager at Enviroquip, Inc. He can be reached at 512/834- 6019 or by e-mail at dennis.livingston@enviroquip. com. Hiren Trivedi is technical manager, biological processes at Enviroquip, Inc. He can be reached at 512/834-6015 or by e-mail at [email protected].

undefined

Since the early 1990s, submerged membrane bioreactor (MBR) technologies have been reshaping wastewater treatment industries around the world and creating new reuse opportunities through decentralization and plant upgrades. In fact, in the U.S. alone, there are now hundreds of operating MBR plants and more than 4,000 installations worldwide. With the development of MBR technology over the last decade, it has become apparent that an overall system approach is necessary to design and operate a successful MBR plant.

Assuming adequate pretreatment, this type of approach accounts for the interdependency between control strategies, plant hydraulics and biological processes. This interdependency is referred to as biohydraulics. To understand the principles of biohydraulics, it is important to note that all submerged membranes have a biofilm that must be properly managed using a variety of design and operational techniques.

A biofilm creates a dense dynamic membrane that allows for enhanced nutrient removal and degradation of refractory organics, and more importantly, prevents reversible and irreversible fouling. Enviroquip, Inc. has developed proactive design and operational strategies to prevent both types of fouling by managing biofilm properties. This approach deals with the causes of biofouling, rather than the effects, and gives operators better tools to run MBR systems.

Proactive biofilm management

Until recently, MBR suppliers focused on the effects of biofilm formation and not the causes. In other words, the question was, How can operators react to changing biofilm conditions and eliminate them through physical/chemical methods such as backpulsing and chemical cleaning?

Instead, Enviroquip has focused on changing the question to, How do I manage fouling conditions to prevent irreversible fouling in the first place?

Aside from temperature, EPS/SMP, colloidal particle size distribution and particle diversity can all be addressed through design and operational choices.

It is widely believed that extracellular polymeric substances (EPS) and soluble microbial products (SMP) are the main culprits that cause reversible and irreversible biofouling. At a short solids retention time (SRT), polysaccharides, which are secreted by microbes in an effort to stabilize their environment and to aid in flocculation, can combine to form colloidal material that subsequently blocks biofilm pores and increases filtration resistance.

In fact, using hollow fiber membranes, researchers at the University of Berkeley observed that flux rates declined roughly 800% faster as F:M ratios were increased from roughly 0.5 day-1 to greater than 1.5 day-1. These results are consistent with a 2004 UNESCO survey evaluating the cause of MBR system problems in North America for hollow fiber systems, which pointed out that an SRT of <20 days may have accelerated biofouling. At the other end of the spectrum, microbes begin to lyse if the SRT is too high (50+ days) and generate SMP or protienaceous EPS, which are also known foulants.

Although SMP concentrations increase at long SRT, there is evidence that average particle size also increases at higher mixed liquor suspended solids (MLSS) and at long SRT. Particle size is important because it determines the rate at which particles migrate away from biofilm due to lift forces induced by air scouring. In other words, bigger (heavier) particles move faster back into bulk solution (mixed liquor) at a constant cross-flow velocity induced by air scour. Considering EPS/SMP data and similar information regarding particle size distribution as a function of SRT, it appears that the optimum SRT range is 12 to 50+ days.

SRT control through sludge management is the best way to keep EPS concentrations down and to maximize air-scouring efficiency. Adding coagulants, however, can bind up free EPS and agglomerate colloidal material to increase average particle size. During a recent pilot study using a hollow-fiber MBR system, stable TMP values maintained for weeks with a 25-mg/L dose of aluminum sulfate (alum) increased within 48 hours after alum dosing was terminated. Within 12 days, the average TMP increased by 61% (in spite of backpulsing and daily chemicalcleans) and did not fully recover after resumption of alum dosing. It was speculated that colloidal polysaccharides, which formed after alum dosing was terminated, may have caused irreversible fouling.

Enviroquip has been working with the Nalco Corp. to develop and integrate a specially formulated polymer, known as a flux enhancer, to sustain high fluxes at cold temperatures. MPE50 is a long chain cationic polymer that bridges between negatively charged floc particles to reduce free EPS concentrations and decrease the number of small particles, e.g. colloids, that can clog biofilm and/or membrane pores. The product is integrated into Enviroquip MBR systems to increase maximum monthly flows 30 to 50%, and in certain cases, decrease air scour requirements.

Hydraulics

Topics seemingly unrelated to biohydraulics, such as flow splitting, gate types and hydraulic profiles, often determine the success of an MBR plant, irrespective of the membrane technology selected.

Proper flow splitting is essential to the successful operation of any MBR system and is dependent on the plant layout and flow control methods. Without proper flow splitting, there is a potential for preferential flow to one or more basins that can cause a significant imbalance in MLSS. Even with the proper plant layout, the type of gate used in between basins can be an issue.

For example, using sluice gates instead of weir gates can trap nuisance organisms such as Microthrix parvicella in isolated areas and create foaming conditions at long SRT conditions common to MBR operation. Also, submerging return points can mask flow-splitting problems and uneven flow distribution.

Conventional wisdom says that internal recycle should be pumped back from MBRs to the head of the plant to reduce pumping costs. However, this strategy saves only a small amount of energy (<1%) at the expense of increased plant maintenance and operational complexity.

Enviroquip standard MBR design practice reverses the typical hydraulic profile in order to gain precise control of the recycle rate and partially equalize flows upstream of the MBRs. Constant water level in the MBR generally improves overall performance.

Operational controls

To successfully run any MBR system, it is very important to monitor and control biological conditions while sustaining high permeability, defined as flux over TMP (gpd/psi). Generally, this is done by maintaining a target MLSS that corresponds to an approximate SRT, and in some cases (Enviroquip UNR), using online biomonitoring. In order to optimize SRT, systems must be designed with the flexibility to operate over a range of MLSS concentrations.

For example, Enviroquip allows for MLSS concentrations up to 18,000 mg/L without derating hydraulic capacity. Even with such flexibility, physical/chemical methods to address both reversible and irreversible fouling are necessary.

The relaxation and aeration strategies employed at a given plant can have a tremendous impact on operation. For example, Enviroquip has developed unique strategies, such as proportional aeration and enhanced relaxation, which can reduce energy costs by up to 30% while increasing sustainable fluxes. Whatever the strategy, it is evident that the industry trend is to move away from complicated and potentially counterproductive backpulsing, and toward variations on relaxation.

Whatever the means of fouling control, it is generally a good idea to keep TMP as low as possible to avoid collapsing biofilm and minimize the rate of irreversible fouling. Enviroquip generally employs permeability control automation and TMP interlocks to avoid high TMP operation and to reduce CIP requirements.

Understanding biohydraulics is the key to successfully designing and operating an MBR system. Moreover, taking a proactive approach to biofilm management, using the concepts grounded in biohydraulics, can significantly reduce MBR system costs and maintenance requirements.

Sponsored Recommendations

2024 Manufacturing Trends Unpacking AI, Workforce, and Cybersecurity

April 25, 2024
The world of manufacturing is changing, and Generative AI is one of the many change agents. The 2024 State of Smart Manufacturing Report takes a deep dive into how Generative ...

State of Smart Manufacturing Report Series

April 25, 2024
The world of manufacturing is changing, and Generative AI is one of the many change agents. The 2024 State of Smart Manufacturing Report takes a deep dive into how Generative ...

ArmorBlock 5000: Boost Automation Efficiency

April 25, 2024
Discover the transformative benefits of leveraging a scalable On-Machine I/O to improve flexibility, enhance reliability and streamline operations.

Blower Package Integration

March 20, 2024
See how an integrated blower package can save you time, money, and energy, in a wastewater treatment system. With package integration, you have a completely integrated blower ...