Joining the Mainstream

As effluent discharge standards become more stringent, advanced treatment alternatives such as membrane bioreactor technology are gaining acceptance with municipal and industrial wastewater treatment plants

• By T. David Chinn, PE
• Dec 01, 2004

For more than 30 years the Clean Water Act (CWA) has established programs and requirements to protect the quality of U.S. rivers, lakes, and coastal waters. In that time, this regulatory framework has succeeded in doubling the number of water bodies in the United State that are considered swimmable and fishable. Today, according to the U.S. Environmental Protection Agency (EPA) more than two thirds of water bodies are regarded as healthy compared to only one third in 1972 when the CWA was first created by Congress.

CWA Drives Treatment Technologies
There are a number of regulatory programs that have developed as a result of the original CWA and it subsequent amendments, such as the Total Maximum Daily Load (TMDL) program and the National Pollutant Discharge Elimination System (NPDES). Each of these programs works to limit the amount of pollution that can be discharged into a body of water from point and non-point pollution sources.

The CWA, and its various programs, has also been credited with spurring the development of advanced wastewater treatment technologies to help wastewater treatment plants comply with increasingly stringent effluent discharge standards. Membrane bioreactor (MBR) technology is one example of a proven advanced treatment solution that is rapidly gaining acceptance as the best available technology for municipal and industrial wastewater treatment plants (WWTP) to achieve the highest level of effluent quality in discharge or reuse applications.

NPDES Permits Required by All Dischargers
The best available technology (BAT) is a key consideration when setting NPDES effluent limits for point source dischargers. Every public or private facility that discharges pollutants into a receiving body of water must obtain an NPDES permit that will specify the control technology, the effluent quality, and the deadline to meet the requirements. Each source also is required to monitor effluent quality, maintain discharge records, and renew permits every five years.

Three levels of technology-based standards are controlled by NPDES permits. The first level addresses conventional pollutants that are biodegradable, such as five-day biochemical oxygen demand (BOD₅), total suspended solids (TSS), pH, and fecal coliform. The second level addresses non-conventional pollutants such as ammonia, nitrogen, phosphorus, and chemical oxygen demand (COD); and the third level, toxic or priority pollutants, includes heavy metals, pesticides, and other organic chemicals.

While NPDES permits are making great progress in resolving large-scale water quality issues, they are limited in scope to municipal and industrial wastewater treatment systems. There are many smaller sources that are much more difficult to control, such as stormwater sources and agricultural operations, that contribute a great deal of non-point pollutants to surrounding water bodies. As a result, EPA data shows that over 40 percent -- more than 20,000 -- of assessed river segments, lakes, and estuaries with established TMDLs still suffer from excessive pollution caused by sediments, excess nutrients, and dangerous microorganisms.

TMDL Sets the Standard
The TMDL program is a watershed planning approach designed to restore water quality in vulnerable water bodies. Developed by EPA, the TMDL program assigns limits to the amount of pollutants that a water body can receive from point and non-point pollution sources and still meet water quality standards for designated uses. These uses may include drinking water, recreational use, irrigation, industrial water use, and protection of aquatic life.

Developing a TMDL for a body of water is an extremely difficult task and requires the cooperation of all stakeholders to identify all pollution sources and establish the maximum allowable pollutant load. The implementation of TMDL regulations is also challenging since it is difficult to apply fixes to non-point pollution sources that are sometimes not easy to identify. As a result, point source dischargers may be subject to more stringent discharge standards to ensure that an affected water body can meet the set TMDL. Many water bodies in the United States still do not have established TMDLs, but EPA is planning to create TMDLs for thousands of these areas in the upcoming years.

Advanced Treatment Technology for Compliance
Although the standards for NPDES and TMDL are set by EPA, the implementation of these programs falls to the individual states. This enables each state to customize the program to address the specific challenges that its water bodies face and to strengthen the regulations if required. States are free to set more stringent standards than those set by EPA; however, a reduction of effluent requirements is not permitted.

Advanced treatment technologies are becoming the preferred wastewater treatment choice to meet today's stringent effluent requirements and also to prepare plants for even tighter requirements in the future. MBR technology is at the forefront of advanced treatment alternatives, providing extremely high-quality effluent in compact plants that significantly outperform conventional wastewater treatment options.

MBRs -- Providing High-quality Effluent
MBRs combine biological treatment with immersed, hollow-fiber, ultrafiltration (UF) membranes to produce near drinking-water-quality effluent. This effluent is of such high quality that it can be safely discharged to the most sensitive environments, reused for non-potable applications such as irrigation and industrial processes, or used for indirect potable applications such as groundwater recharge.

Typical MBR Effluent

Since the MBR process provides solids separation by filtration rather than settling, primary and secondary clarifiers can be eliminated -- a feature that significantly reduces the size of the plant. The microscopic pores in the hollow-fiber membranes have nominal pore size of 0.04 micrometers (ìm) and form a physical barrier against suspended solids, inorganic nutrients such as phosporus and nitrogen, and pathogenic organisms including Giardia and Cryptosporidium. MBRs meet, or even exceed, the world's most stringent regulations for non-potable water reuse, including California's Title 22 standard.

An MBR system typically includes pretreatment for trash removal, membrane cassettes, bioreactor, permeate pumps, blowers for process and membrane scouring, and clean-in place equipment for membrane maintenance. Hollow-fiber membranes are immersed directly into the aerobic chamber of a bioreactor and typically operate at mixed liquor suspended solids (MLSS) concentrations of 8,000 to 15,000 milligrams per liter (mg/l). This enables the system to function with a relatively small bioreactor volume and considerably reduces capital and operating costs since less land, fewer components, and a smaller physical plant is required. An MBR facility can occupy as little as one quarter of the space that a conventional plant requires.

A Closer Look at One of the MBR Systems Available

Many small and large communities are considering MBRs to retrofit or replace aging conventional wastewater treatment plants to ensure that they can meet current and future effluent requirements, specifically those that are required by existing or future TMDL programs. Municipal and industrial users also are examining MBRs as a means to implement water reuse programs.

Wastewater Becoming New, Viable Water Source
Once destined only for the sewer, today wastewater is starting to go places -- namely back into the communities that it came from. Water recycling is becoming increasingly important for many drought-prone and water-short areas of the United States that are struggling to provide a consistent, reliable supply of potable water to residents and businesses. Municipalities throughout the country are conserving potable water supplies by substituting recycled water for applications where humans do not directly consume or come into prolonged contact with the water.

Recycling water makes sense for many communities considering that less than 10 percent of potable water is used for domestic purposes. Moreover, since recycled water is produced by a community, its ownership cannot be disputed in the same way that water from a river or lake can. Recycled water is immune to political debate, withdrawl disputes, and drought, and it can provide a stable, long-term new water source that can ease water concerns for communities throughout the United States.

New EPA Guidelines for Water Reuse
In September 2004, EPA released revised guidelines for water reuse. Although these guidelines, originally established in 1992, do not impose water recycling and are not enforceable by government agencies, many states have used the EPA guidelines to develop their own water reuse programs. As of November 2002, 25 states had created comprehensive regulations for water reuse that cover a broad spectrum of applications ranging from irrigation and toilet flushing to indirect potable reuse. Each state also has many reuse categories that allow for restricted and unrestricted urban, recreational, and agricultural reuse. Other categories include environmental and industrial reuse and groundwater recharge.

Municipal water reuse programs not only reduce TMDL to the watershed and create a new water source, but they also can create a new revenue stream for the community through selling recycled water as a substitute for potable water. Many municipalities are building separate water distribution systems to deliver reuse water to agricultural and recreational sites.

Industrial users also can benefit from on site MBR technology to pretreat wastewater prior to release to the municipal WWTP or to create recycled water that can be reused in manufacturing processes. On site wastewater treatment and recycling with MBR technology can provide significant cost savings to industrial water users since they can avoid paying treatment fees to municipal WWTPs and reduce TMDLs to surrounding surface water bodies.

Looking Forward
As the U.S. population continues to grow and water bodies come under increasing pressure from rising point and non-point pollution sources, municipal and industrial WWTPs will face more stringent effluent discharge standards. EPA has committed to establishing thousands of new TMDLs for water bodies throughout the country; an effort that will undoubtedly require both small and large communities and industries to look toward advanced treatment technologies to meet revised effluent requirements.

MBR technology can provide municipal and industrial sites throughout the United States with cost-effective and easy to operate WWTPs that will provide near drinking-water-quality effluent. By selecting MBR technology, prudent wastewater treatment managers will not only ensure that their plants can meet today's effluent and reuse standards, but also standards that are coming in the foreseeable future.

---

ZeeWeed: A Closer Look at One of the MBR Systems Available
ZENON's ZeeWeed® MBRs are serving many communities throughout the United States such as American Canyon, Calif. 2.5 million gallons per day (MGD) peak flow; Cauley Creek, Ga. (3.6 MGD peak flow); and Traverse City, Mich. (17 MGD peak flow). All of these plants mostly treat municipal wastewater, although industrial wastewater makes up a portion of the waste stream. In each case, MBR technology was selected for its ease of use, compact footprint, and high-quality effluent that meets or exceeds regulatory requirements. The City of American Canyon, Calif., estimates it will save up to 1.85 MGD of potable water once its ZeeWeed MBR facility and its reuse distribution network is fully functional.

ZeeWeed incorporates a modular architecture and is inherently scalable to meet the increasing needs of a growing community. Thousands of hollow-fiber membranes hang loosely in cassettes that are immersed in process trains. The size and number of trains can be adjusted to meet a community's needs and more can be added on an as-needed basis. The systems are highly automated and can be configured to monitor permeate production and automatically perform backpulse cleaning, membrane integrity testing, and periodic maintenance cleaning. This ease of operation, combined with continuously declining system costs and improved performance, has made MBRs a viable option for conventional wastewater treatment technology.

---

Typical MBR Effluent
Parameter	Effluent
BOD5	< 2 mg/l
TSS	<2 mg/l
Ammonia	<1 mg/l
Nitrogen	< 10 mg/l+
Phosphorus	< 0.1 mg/l++
Turbidity	< 0.1 NTU
SDI	< 3
+ with anoxic zone ++ with coagulant

This article originally appeared in the 12/01/2004 issue of Environmental Protection.