(Continued from May 2004, U.S. Water News)
By Chris Fox Carter & Burgess, Inc.
U.S. Water News Online
"Oh the times, they are a changing," sang Bob Dylan. And change is the only real constant we all face.
During the past quarter century, technology in general has changed more rapidly than any other time in history. In the personal computer industry, for example, purchase a state-of-the-art computer and within weeks it is replaced by new technology that is faster and smarter. Within industries and across industries, technology has almost outpaced itself.
The wastewater industry is no different in the need to advance existing technologies and embrace new technologies that make the systems work more effectively and efficiently.
"The need for ever-changing technology in the wastewater industry stems from past biological systems that did a good job at removing particulate matter that we knew existed. But instrumentation just kept getting better and better at detecting more and more trace compounds that we didn't know existed or couldn't detect in earlier years," said Robert McMillon, former president of the Water Environment Federation (WEF). "Now, as the need to protect ever shrinking water supplies increase and stricter permit limits that require higher water quality standards occur, the technology needs to rise to the challenge."
Some solutions may come from the increasing use of membrane treatment technology and biological system enhancements such as biological nutrient removal (BNR) and sequencing batch reactors (SBR) coupled with energy efficiency innovations.
Biological nutrient removal
Biological nutrient removal (BNR) was talked about in other industries in the 1980s but did not become of interest in the wastewater industry until the late 1990s. In earlier years, the technology was not very refined, not very reliable, and not very well understood. Today BNR technology has resurfaced because it is now well understood in wastewater applications with regard to design criteria, process, operation, maintenance, and computer controls, says Al Petrasek, Carter & Burgess principal and senior project manager.
BNR uses naturally occurring micro-organisms, with oxygen rather than chemicals, to remove such nutrients as nitrogen and phosphorus from the wastewater stream.
Raw or untreated wastewater contains ammonia, which is toxic to fish. Ammonia degrades to nitrates, which removes the oxygen from the stream, therefore, killing animal and plant life. Nitrates also become fertilizers promoting algae growth. As algae die and decompose, algae create a high oxygen demand, which creates low dissolved oxygen in the water. The BNR denitrification process can convert nitrate into nitrogen gas bubbles that are harmless and the wastewater effluent has no deleterious effects on receiving waters, Petrasek explains.
One of the driving forces behind BNR technology is tightening permit limits, says Ray Hamilton, public works unit manager in Denver. If permit limits for phosphorous are 1mgd/L, BNR alone can very effectively meet these parameters. If the permit limit is .5 mgd/L or less, then it is most economical to use BNR to treat phosphorous down to the 1 mgd/L limit and then use chemical precipitation to get to the lower limits, Hamilton continues. There can still be a significant chemical cost reduction using the BNR method.
"In plants with low alkalinity in the wastewater, BNR is advantageous in the denitrification step because it reduces oxygen required for treatment and restores alkalinity previously destroyed in the nitrification stage," says Hamilton.
Using BNR, fewer chemicals are added to the treatment process, which reduces the amount of sludge produced and increases sludge quality, therefore positively impacting sludge disposal or reuse issues.
Sequencing batch reactors
According to the United States Environmental Protection Agency (EPA), "fill-and-draw batch processes" like sequencing batch reactors (SBR) were introduced in the 1920s. The SBR and conventional activated sludge systems are almost the same. A 1983 EPA report describes the SBR as "no more than an activated sludge system which operates in time rather than space."
SBR technology advances and equipment improvements with regard to the aeration devices and control systems, have recently re-ignited interest in the technology. In one unit, an SBR can undertake equalization, biological treatment, and secondary clarification with a timed, controlled sequence. A single-unit SBR can also include primary clarification. Each of these processes occurs in separate units of a conventional activated sludge system.
In a single chamber SBR, there are multiple biological environments that can be created and maintained to nurture each specific type of bacteria for optimum performance at the right time and right stage of the process, Hamilton says. For example, "one type of bacteria absorbs excess phosphorous in the aerobic condition when aeration occurs, but the bacteria do nothing at all in the absence of oxygen or in the anaerobic stage."
Bacteria quality and quantity is key. With conventional systems, the bacteria are settled in the clarifier and pumped back to the head of the plant. The bacteria are recycled to maintain sufficient quantities. In SBRs, the bacteria continue to live and work in the unit as needed, when needed. These bacteria stay in the system longer and, therefore, are stronger and more effective.
According to the EPA, an SBR can include "1) idle, 2) fill, 3) react, 4) settle, and 5) draw" steps. A major technology improvement in the SRB, according to Hamilton, allows effluent to be drawn out of the system even when the effluent or water level is at a variable, or non-consistent, level.
During the "fill step," static fill, mixed fill, or aerated fill can be used, depending on the facility and its goals. "Static fill" is when influent is mixed with existing biomass and "no mixing or aeration" has occurred when the mixing begins. "Static fill" can be compared to using 'selector' compartments in a conventional activated sludge system," according to the EPA. "Mixed fill" is mixing influent organics and biomass. "Aerated Fill" consisted of aerating the SBR contents to start an aerobic reaction.
During the "react stage," the biological reactions are finished so that nitrification or denitrification and phosphorous removal can be accomplished.
During the settling stage, no influent or effluent can get in to disturb settling. Gentle mixing and settling time can produce a clearer effluent than a conventional system.
In the "draw step," a decanter is used to remove treated effluent out of the top of the tank while the bacteria settle at the bottom.
SBR have been used successfully in both municipal and industrial wastewater applications. SBRs can provide possible facility cost savings due to omission of clarifier construction and other conventional process equipment purchase. But SBRs also require operator training to set up and maintain SBR unit timing and control systems, which can become complicated.
Membrane treatment technology
Basic membrane technology first began about 30 years ago in water systems. At that time, membranes did not provide quality or reliability for wastewater applications, according to Al Petrasek, Carter & Burgess principal and senior project manager. Costs associated with initial membrane technology were extremely high. The combination of low quality performance and high cost made the initial technology unattractive.
As membrane technology advanced, however, it became much more appealing. Advances dramatically improved performance and reliability in wastewater applications that require a very high quality effluent while dramatically reducing utilization costs. Other improvements include membranes with reduced thickness, improved stability with pH and temperature extremes, and better compatibility with chlorine and other oxidants.
Membranes provide a separation process using a wide variety of molecular sizes or pores to fit specific filtering or demineralization needs. Membranes not only filter out most suspended solids but can also remove potentially dangerous bacteria. In a membrane plant, ultraviolet light can be used to effectively kill bacteria for disinfection. Membrane treatment technology completely eliminates the need for secondary clarifiers.
Membranes, as first designed, worked like a "soda straw" with pressure applied to the inside to force fluids outside, says Hamilton. Pressure was applied to the inlet side of the membrane and caused the liquids to flow outside, but solids would "plug up the straw" making it an operation and maintenance nightmare in a wastewater application, Hamilton says.
"Membranes were an easy design in clean water applications but because of the 'inside-outside' operation, small particles in solids would plug up the 'soda straw' making it unworkable in wastewater treatment. The advent of 'outside-inside' technology made the wastewater application attractive," Hamilton says. "The membrane works better because pressure is applied to the outside of the 'straw' and pushes the liquid from the outside to the inside through the membrane so the solids stay outside. If solids plug up the outside of the membrane and reduce the flux rate, a backwash technique can refresh the membrane for continued use." When considering membrane treatment technology, it is important to note that membrane technology is geared at specifics and not standardized; equipment can be specifically designed to meet a facility's specific effluent quality needs.
Currently, membrane treatment technology can be a more costly design feature than other filtration processes, but as more manufacturers enter the marketplace, the competition will reduce costs, says Hamilton. But in the design process, "It is important to consider the membrane's design life, which is usually between seven to ten years, as part of the process economics," Petrasek contends.
With that said, "membranes for wastewater treatment are a technology advance that is getting the most attention right now as the most promising addition to increasing effluent quality," Petrasek says. McMillon predicts that "membrane use will increase and cost will decrease, especially as the need for direct reuse and blended water supplies increase."
Energy efficient technology
Anoxic/Oxic System Design. Anoxic/oxic systems are biological nitrogen removal processes. These systems can lower power costs because bacteria is oxidized in the nitrate molecule and used or rather re-used. Therefore, there is no need for a wastewater facility to purchase oxygen to put into the wastewater process.
During this anoxic/oxic process, basically a wastewater treatment facility is using the oxygen that is available as a byproduct of the treatment process itself. Without anoxic facilities, or in a conventional process, the oxygen produced is discharged with the effluent.
The energy savings and cost savings will vary by facility.
Vertical Loop Reactors. A vertical loop reactor is a variation of the BNR process that incorporates an anoxic zone into the process. The mixing efficiency of the vertical loop reactor lowers energy costs. Similar design criteria, advantages, and disadvantages exist with the addition of a vertical loop reactor as with the basic BNR technology.
Vertical Tube Reactors. "Vertical tube reactors are a very new alternative technology, using very sound engineering theory for sludge treatment," says Petrasek. "Engineering evaluations show the technology does work and holds promise as an up-and-coming method for sludge treatment."
A vertical tube reactor begins with a hole drilled straight into the ground about 6,000 feet deep. A pipe is inserted into the center of the hole and sludge is pumped in to the pipe. Over time, the sludge flows back up around the pipe, but still remains in the hole. While the sludge achieves a "critical pressure and temperature" all the organic material is burned up without the use of chemicals, energy or horizontal surface space.
Energy demand and usage
About 70 to 80 percent of energy used in wastewater treatment is in the aeration process. If energy use can be lowered in this process, costs can be dramatically impacted overall.
In new facility design, for example, changing from coarse bubble aeration to fine bubble aeration and increasing the tank depth can lower power consumption in this process by about 50 percent or more. Also, if improvements can be made to the dissolved oxygen control system so that it supplies only the amount of air really needed to effectively and efficiently operated the aerators, this fine-tuning can reduce energy usage and costs, Petrasek said.
With regard to energy efficiency overall, it is very important to examine the "control demand charge" of equipment with an electric service review. For example, it is financially beneficial to make sure that all equipment is operating at 100 percent efficiency because any less operation efficiency requires additional power to be used. "A good example is a 10 horse power (hp) motor that is operating at 100 percent efficiency will use only x amount of energy. If that same motor is operating at only 50 percent efficiency, then it may require twice as much energy to get it to function at the rated 10 hp," says Petrasek.
Wastewater technology today
Today's wastewater treatment challenges are to improve or develop technologies that address the changing issues of society, while keeping an eye on the advances these technologies may lead to in the future. Some technology advances will lead away from chemical additives and back to using naturally occurring bacteria as well as recycling what is produced naturally in the process, such as oxygen and methane. Therefore, technologies like membranes and BNR will thrive. Other technology changes will lead toward more complicated systems controls and computers, as is the case with SBRs, which means the industry will need to keep personnel trained to operate and maintain the more complicated systems. As the philosopher Heraclitus said "Nothing endures but change."
For more information, please contact Chris Fox, Carter& Burgess, Inc., 777 Main St., Fort Worth, Texas 76102-5304, 817)735-6212, fax (817)735-2890, foxch@c-b.com, www.c-b.com.
Sidebars:
The Peninsula Water Reclamation Plant Dallas, Texas
Carter & Burgess designed innovative processes for high operational flexibility and increased ability to meet strict effluent limits for the Peninsula Water Reclamation Plant in Dallas, Texas. Located in a residential neighborhood, stringent effluent water quality criteria including nitrogen and phosphorus limits, and sensitivity to odors were special considerations. To address the nitrogen and phosphorus limits, innovative biological nutrient removal (BNR) with anoxic/oxic and anaerobic cells were added. BNR is an environmentally friendly process with no chemical additives. The plant discharges into a receiving stream that merges with a major drinking water source, so high quality effluent was required, which BNR can provide. Because of the plant's proximity to a residential neighborhood, ultraviolet (UV) disinfection is used instead of conventional gaseous chlorine as a risk management strategy, Tertiary filters are used to achieve a very high clarity effluent. Belt press dewatering equipment is planned to minimize odor and to render biosolids safe for disposal at an approved landfill.
Hot Sulphur Springs Wastewater Treatment Plant Colorado
When the existing treatment plant at Hot Sulphur Springs encountered compliance issues for over accumulation of solids in the aerated lagoons, they commissioned Carter & Burgess to design an advanced facultative pond system for the Hot Sulphur Springs Wastewater Treatment Plant. The technology utilized is an accelerated treatment process from the conventional lagoon system. There are five cells in the process. The first cell is anaerobic, called a fermentation cell, which acts as a primary clarifier and digester to reduce organic. The next three cells are aerobic cells that provide aerated treatment. The fifth cell is a deep, still cell for settling biological solids before discharging the effluent. A portion of the highly oxidated water from cell 3 is recirculated back to cell 1 to provide an aerobic cap in the fermentation cell to prevent odor issues. A PVC liner was used in this pond system because the substrate material is a very permeable river rock.
For more information, please contact Chris Fox, Carter& Burgess, Inc., 777 Main St., Fort Worth, TX 76102, 817)735-6212, fax (817)735-2890, foxch@c-b.com, www.c-b.com.
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