Accommodating Growth

The Bullhead City, Ariz., Community Development and Engineering Department, under the guidance of City Engineer/Community Development Director Pawan Agrawal, is going to be completing several large projects in the Bullhead City area in the near future.

“The Section 18 Waste Water Treatment Plant Expansion is one of these projects,” Agrawal said. “The original Section 18 Wastewater Treatment Plant, an extended aeration treatment plant, was built in the early 1990s and is no longer able to support the city’s growth.”

Servicing growth

There has been a great deal of research done to find the best process/product for the expansion, as it is very important for Bullhead City to protect water quality as well as the Colorado River water. Also, because this is a desert region, reuse water quality is very important. Reuse water helps the city by allowing it to use water allocations for citizens and reuse on parks, golf courses, etc. In addition, reuse quality influences groundwater; therefore, the city wants to use the best-quality reuse water possible. Other factors considered when expanding the plant were cost, size of plant, odor control, impact on citizens, etc.

After completing all the research, it was found that using a newer membrane bioreactor (MBR) system process would be much better for Bullhead City. In April 2006, the Section 18 Waste Water Treatment Plant contract with CDM Constructors was approved by council to expand the plant to 2 mgd using the MBR process.

Compared to other local projects using the older technology, which produces lower-quality reuse water and uses more space, CDM is going to provide a better product for a comparable price. The expansion was set to break ground in late April 2006 and will be completed in October 2007. This expansion will be one of the steps needed to close an old plant on Silver Creek.

Benefits of MBR process

The membrane is an extremely effective solids separation device. The high removal efficiency results in very high effluent quality. The reactor volume is greatly reduced when compared with an activated sludge plant basin (extended aeration). There is no requirement for final settlement tanks, and MBRs offer bacterial removal without the need for complicated UV systems.

MBRs are more automated, making them ideal for decentralized treatment because they are simpler to operate. Less sludge is generated than with conventional wastewater treatment systems, and MBRs eliminate the backwash step, resulting in a simple system.

MBRs come of age

MBRs combine the activated sludge found in high throughput sewerage treatment plants with membrane filtration. In addition to removing biodegradable organics, suspended solids, and inorganic nutrients (such as nitrogen and phosphorus), MBRs retain particulate and slow-growing organisms (thereby treating more slowly biodegraded organics) and remove a very high percentage of pathogens (thereby reducing chemical disinfection requirements).

An MBR is a combination of the activated sludge process, a wastewater treatment process characterized by a suspended growth of biomass, with a micro- or ultra-filtration membrane system that rejects particles. The membrane system replaces the traditional gravity sedimentation unit (clarifier) in the activated sludge process. The turbidity and suspended solids concentration of the effluent is far lower than in conventional treatment. All biomass is retained and becomes returned activated sludge. Biological growth leaves the system as waste activated sludge.

The engineering principles underlying MBRs are familiar enough to ensure reliability. Because MBRs combine two familiar technologies—activated sludge and membrane filtration—significant engineering expertise can be applied to MBR design and operation. Several studies already have applied activated sludge-related biology to MBRs. MBRs have been used in enough applications to verify successful performance and identify critical design and operating factors. Membrane-manufacturing capacity is expanding, so unit costs are declining. The long-term trend is a “virtuous cycle” in which declining costs spur more demand, which spurs further cost reductions.

How MBRs work

The microfilters are submerged in a basin. A vacuum is applied downstream of the membranes to allow for the solid/liquid separation process to occur. The membranes eliminate the need for a secondary clarifier because they act as an absolute barrier. Air is introduced into the system to scour the membranes and drive the biological treatment.

The influent enters the bioreactor, where it is brought into contact with the biomass. The mixture is pumped from the bioreactor and filtered through the membrane. The permeate is discharged from the system while the entire biomass is returned to the bioreactor. Excess sludge is pumped out in order to maintain a constant sludge age, and the membrane is regularly cleaned by backwashing with occasional chemical washing. The entire biomass is confined within the system, providing both perfect control of the residence time for the microorganism in the reactor (sludge age) and the disinfection of the effluent.

Rusti Holguin is project administrator for Bullhead City. She can be reached at 928/763-0132 or by e-mail at rusti@bullheadcity.com.

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