Cultural eutrophication, the process of accelerated chemical nutrient concentration (primarily nitrogen and phosphorus) in waterways, poses a major problem to affected water supplies, ecosystems and their supply chain constituents. Not only does eutrophication contribute to excessive plant growth and eco-decay, but further ramifications of this human-borne phenomenon include diminished oxygen and severe reductions in water quality, fish and other animal populations.
A major cause of cultural eutrophication continues to be the discharge of nutrients from WWTPs. As such, there has been an ongoing effort to control the discharge of nutrients from U.S. wastewater plants. One of the tools that designers now have to significantly reduce nutrients in the effluent discharge is the membrane bioreactor (MBR).
Enviroquip, a division of Eimco Water Technologies, provides MBR systems that use Kubota membranes, which reduce both phosphorus and nitrogen to levels lower than that at which eutrophication can thrive. Typical MBR effluent values are provided in Table 1.
The Enviroquip MBR process is based on the Modified Ludzack-Ettinger (MLE) system, incorporating the Kubota flat-plate membranes to eliminate secondary clarifiers and the higher MLSS in the MBR to host a higher population of nitrifiers. The Kubota membranes were developed exclusively to meet the unique requirements for treating wastewater. When the application of MBRs is used with the Bardenpho process configuration, both biological nitrogen and phosphorus removal (chemical polishing of the phosphorus) is accomplished.
Design
During a four-month demonstration in Michigan, an Enviroquip-designed MBR was able to consistently reduce the total phosphorus (TP) in a waste stream from 5.2 mg/L to an average value of less than 0.03 mg/L. During this test, 12 of the 43 samples taken indicated that TP was below the level of detection.
The rugged construction of the membranes allowed the plant to treat wastewater during freezing conditions when the average water temperature was 10.9?C and the lowest 6.2?C. The membranes suffered no damage when the plant froze up during a particularly cold period. The simplicity of the Enviroquip system was demonstrated when it required only one maintenance cleaning during the four-month period. Once the plant was stabilized, the TP values remained quite low, as shown in Table 2. The Michigan demonstration plan was specifically designed for the removal of phosphorus.
During the test period, the system operated at a constant controlled flux of 10 gal per sq ft per day (gfd). The system was designed to operate at a target MLSS of 15,000 mg/L. The actual MLSS values varied between less than 6,940 mg/L at startup to more than 26,180 mg/L. The average MLSS was 16,304 mg/L. Although the high MLSS may have impacted the biological oxygen demand (BOD) removal and nitrification due to its impact on the dissolved oxygen, hydraulic performance was unaffected.
As shown in Table 3, the removal of phosphorus with metal salt has the added benefit of reducing BOD to nearly nondetectable limits.
Mechanics
A unique benefit of the MBR system is its ability to remove small particles. The Kubota membranes have an effective operating pore size of less than 0.1 micron. The most common process for the removal of phosphorus is to use metal salt, primarily iron or aluminum, for chemical precipitation. Although the majority of the precipitants were fairly heavy and would settle in traditional clarification systems, a significant amount of phosphorus-formed colloidal salts would otherwise not be captured by conventional technology. Conversely, the small, effective pore size of the membranes and their resulting capture and retention rates within the MBR resulted in larger forming particles collected with the waste activated sludge.
The effective 0.1-micron membrane pore size is the result of biofilm formation. Biofilm is a complex aggregation of microorganisms characterized by surface attachment and structural heterogeneity resulting in a protective and adhesive matrix. Any surface that is placed in a wastewater medium is subject to the formation of a biofilm. Many cross-flow MBR systems are designed to minimize the formation of this biofilm due to its propensity to plug the membrane pores. Hollow-fiber membrane systems reduce this biofilm formation by using air to scour the fibers and constantly flex them while in operation; however, this flexing weakens the fibers and ultimately shortens the membrane life.
The Enviroquip system also uses air to scour the membranes, but instead of weakening the membrane fibers, the design takes advantage of biofilm properties. The biofilm acts as a protective coating on the membranes and polishes the wastewater before it permeates through the membrane. Enviroquip has incorporated a “relax cycle” in the scouring system to allow the biofilm to reform and reduce the possibility of creating an excessive pressure drop, also known as trans-membrane pressure (TMP). The low TMP significantly reduces the wear and tear on the Kubota membranes, which results in a longer membrane life than any major membrane manufacturer.
Conclusion
MBRs currently represent the state of the art in wastewater treatment technology in regard to nutrient removal from wastewater discharge. Their application will play a vital role in the removal of eutrophication contributors from America’s waterways. MBRs serve to prevent environmental catastrophes from occurring and can help reverse the effects of clogged waterways already wreaking havoc on our ecosystems.
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