Water, Wastewater Treatment Technologies Evolving
By Frank Rogalla, Terry Johnson and Bruce Long
Many people must resist the temptations of seven sins, but water and wastewater utility managers must also successfully address a number of significant sources of stress:
General public sentiment pressing for the best qualities possible and reduced risk of exposure to all bacteriological contaminants and chemical releases.
In Wastewater Treatment:- Ever-increasing levels of nutrient control, in addition to tighter limits on conventional pollutants, not only in particularly sensitive streams, but even in estuaries, salt water sounds and bays.
- A greater burden on treatment facilities to handle increased flows and loads from the reduction of combined and/or sanitary sewer overflows from collection systems.
- A growing number of rules governing solids separation (enhanced particulate capture and mandatory filtration) and disinfection byproducts.
- The multi-barrier concept to ensure adequate removal and inactivation of microorganisms in addition to selected elements such as arsenic and radon.
- More onerous, site-specific criteria, such as environmental conditions or nuisance control in highly populated areas where development has encroached upon once-remote treatment sites.
- Adaptation of existing structures and assets within limited plant space.
- Challenges from contract operations firms to become more fiscally responsible and competitive, requiring utilities to do more with the same or even less and to reduce operations staff through increased automation.
Water treatment technologies are evolving to meet the changing demands of this new century. Many water treatment technology trends have been observed for some time in Europe, where supranational directives forced the enhancement of national standards for water quality and wastewater discharges. With a population roughly equivalent to that of the U.S. compacted into approximately one-fourth as much space and a historic location of infrastructure in central settings, Europe has already faced the challenge of upgrading treatment plants despite limited room for expansion.
Let's explore a few of the most significant technology trends that have evolved in response to such stresses and constraints.
High-Rate Solids SeparationIn wastewater treatment, peak flows are often associated with low pollutant concentrations that only slightly exceed discharge limits. In such instances, efficiently removing suspended matter results in quality that is, either on its own or when blended with the full-treatment flow, in compliance with discharge criteria.
Storage of these peak flows is often impractical, requiring significant site space and high operation costs for cleaning storage facilities, posing higher odor generation potential, and adding to full treatment sizing requirements. These peak flows can be treated by adding coagulant chemicals to the wastewater for gravity solids separation either in traditional primary settlers or in specific storm separators.
Where treatment site space is most critical, various ballasted flocculation processes can be applied, using high chemical coagulant dosages and settling ballast to form solids that settle rapidly in plate or tube settlers. A portion of the settled sludge or the recovered ballast is recycled to the incoming wastewater to seed the process. This process has an extremely small footprint. Rapid startup and automation capabilities also have contributed to the growing acceptance of ballasted flocculation.
An example is under development in Lawrence, KS, where sanitary sewer overflows will be treated with a ballasted flocculation system of 40 mgd peak capacity. About 50 plants worldwide use this technology, with the largest units treating peak flow of 500 mgd. The economic viability of this process increases if it can be installed for multiple duties, such as primary treatment or tertiary treatment for phosphorus control, during normal flows.
In potable water treatment, efficient coagulant and solids separation enhances the removal of particulate matter, including microbial contaminants and disinfection byproduct precursors adsorbed on chemical flocs. As expansion of existing plants becomes more difficult, high-rate processes such as ballasted flocculation and dissolved air flotation become more attractive. Such processes enable retrofit of existing tanks while freeing up volume for adding capacity or treatment steps, such as ozonation, in the same footprint as the present plant.
More than 30 plants using ballasted floc settling to treat surface water are in operation or under construction in North America, doubling the number of worldwide applications of this process. In Passaic Valley, NJ, the ability of ballasted flocculation to consistently achieve settled water turbidities below 1 NTU at overflow rates up to 45 gpm/ft2 was demonstrated on a pilot for a 120 mgd plant upgrade project.
Once a process mainly used in northern Europe, flotation is now applied in the US on similar scale with a 75 mgd plant in Greenville, SC. The next step in compact treatment processes combining flotation and filtration in one basin is already operated in various European plants, on a 50 mgd scale in Walton and Felindre in the UK, while a plant in Lever, Portugal, has twice that flow. Compared to conventional flotation and filtration, COCODAFF (Counter Current Dissolved Air Flotation/Filtration) can reduce a footprint by more than half.
Nutrient ControlWastewater treatment facilities increasingly are required to limit their discharges of phosphorus in addition to nitrogen. Biological nutrient removal (BNR) processes have proliferated over the past 20 years, with most of this technology developing out of work conducted in South Africa by Dr. James Barnard. Continued efforts to make BNR processes more efficient, reliable, and adaptable to a higher range of wastewater characteristics have begun to include fixed-film systems to reduce footprint and increase reliability.Fixed-Film Biological Treatment ProcessesTwo wastewater biofilm treatment processes are emerging in the United States. The first, commonly referred to as integrated fixed-film activated sludge (IFAS), uses plastic media retained in aeration basins to increase capacity of conventional activated sludge. With IFAS, effective biomass within the bioreactor is augmented through growth on the media. This process enhancement allows retrofit of existing basins with media to achieve nitrification in approximately half the aeration volume otherwise needed for the same level of treatment.When nearly all of the biomass is resident on the media with only the wastage, or slough, discharged in the reactor effluent, the process is generally referred to as a moving bed bioreactor (MBBR). It resembles a more traditional fixed-film biological treatment process, except that the media is submerged and aeration is required to support the process.
IFAS is gaining acceptance as an emerging process: Broomfield, CO, is upgrading its 5.4 mgd carbonaceous activated sludge plant to 8 mgd of nitrification by retrofitting its existing aeration basins with media.
The second fixed-film process emerging in the US is the biological aerated filter (BAF). This process uses submerged granular media to simultaneously support biological growth and act as filtration medium, eliminating the need for clarifiers. If aerated, the filter can be used for carbon removal and nitrification. In an unaerated mode and in the presence of sufficient carbon source, this technology provides denitrification. BAF can be installed in separate stages for carbon removal, nitrification, and denitrification. Some suppliers design for simultaneous reactions in the same unit.
BAFs are extremely space-efficient and achieve reliable treatment, and easy filter automation makes them attractive if remote operation is desired. The first large-scale installation with a capacity of 14 mgd has been operating in Roanoke, VA, for two years, and similar size plants in Minnesota are starting up. Two large plants with peak flows close to 100 mgd are now under construction in Binghamton and Syracuse, NY, corresponding to hundreds of installations with different BAF systems in the UK, German-speaking countries, France, Scandinavia, and Japan.
BAF currently is being considered in Hong Kong for pilot demonstration at a 500 mgd wastewater treatment plant.
Aerated biofilters are already applied as drinking water pretreatment for nitrification before flotation in the 65 mgd Tai Po plant. Biologically active filtration is also applied in drinking water to reduce the biodegradable matter which might lead to regrowth of bacteria in distribution systems. Typically without aeration, but using ozone to enhance pollutant bioavailability and supply oxygen, downstream contactors of activated carbon or anthracite provide growth surfaces for biomass.
In contact times of 5 to 10 minutes, a significant amount of biodegradable matter can be removed, lowering the necessary disinfection dosage and reducing the formation of halogenated byproducts. The world's largest plant with ozone/biofiltration is currently being completed at Southern Nevada Water Authority's Alfred Merrit Smith water treatment plant in Las Vegas, with a daily flow of up to 600 mgd from Lake Mead.
Membrane ProcessesMembrane processes generally refer to a broad range of membrane types: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). The first two are simple solids separation techniques, with the latter two providing molecular separation of organics or even salts. A large array of experience exists in applying all of these membrane processes in pressure vessels, as shown in the water reuse campus in Scottsdale, AZ. There, the largest of more than 20 municipal reuse facilities in the US treats up to 10 mgd of raw sewage to potable quality for aquifer recharge trough MF and RO operated in series.Submerged membranes, driven by basin-head and vacuum pumping downstream, are emerging as an extremely viable treatment process, cutting down on infrastructure cost through reuse of existing basins.
In wastewater, the membrane bioreactor (MBR) process applies micro- or ultrafiltration membranes that are located directly in the aeration tank for the separation of mixed liquor solids from treated effluent. When applied in this manner, membranes replace at least two steps of conventional facilities: final clarifiers and effluent filtration, up to advanced tertiary quality. Depending upon the actual membranes employed, bacteriological control is also achieved, reducing and sometimes eliminating the need for further disinfection.
MBR is becoming more viable as membrane costs continue to decrease, competition develops, and higher discharge quality is imposed. Because it is a compact process occupying as little as 15 to 30 percent of more traditional, comparable processes and requiring minimal plant site and therefore less costly odor control it is a good-neighbor technology. Further, the process can be retrofitted into existing basins, automated quite easily, and is extremely reliable in that its performance is not subject to the more fickle gravity sedimentation techniques commonly used.
Similar arguments push the application of membranes in potable water: reduced footprint, easy automation, high quality of treated water, and operational reliability. And less chemical is required because of the physical barriers of the membranes, resulting in more effective removal of organics as well as microorganisms.
The largest submerged membrane system for drinking water clarification is currently under design in Singapore, where the Choa Chu Kang Waterworks is being upgraded to a capacity close to 100 mgd.
DisinfectionPublic concerns over the risk of chlorine gas release and the confirmed effectiveness of ultraviolet (UV) irradiation, especially for Cryptosporidium, is driving many utilities to convert their primary disinfection system to UV. With UV disinfection increasing in popularity, improvements and innovations in UV are continuously hitting the market. The technologies are evolving from low-pressure, low-intensity applications towards high-intensity and medium-pressure systems. Each equipment design has its own set of advantages and disadvantages regarding system configuration, investment and operating costs, and manpower requirements.As pressure is brought upon utilities to reduce risks of chemical releases and control more refractory pathogens and cysts, UV will be installed in more facilities. Continued improvements in these systems, including self-cleaning methods, make UV more attractive. In Dublin, Ireland, a compact wastewater treatment facility with a peak flow of 250 mgd is currently being equipped with UV disinfection, making it the largest in the Northern hemisphere. A handful of installations in North America have a capacity above 100 mgd, with Jacksonville, FL, treating 150 mgd, currently one of the largest installations in the US.
ConclusionPressure to do more with less has led to the search for treatment technologies that require less space, are easy to enclose to avoid air emissions, can be highly automated for operations control, reduce initial investment as well as operation cost, achieve high levels of treatment efficiency, result in low discharge of contaminants, and are very reliable. These driving forces have affected innovation in all conventional stages of water and wastewater treatment as well as residuals management, resulting in rapid relief of significant stresses. WW/About the Authors: Frank A. Rogalla leads Black & Veatch's global team of water and wastewater process and technology specialists, known as Proteus. A Fulbright Scholar, Rogalla is internationally recognized as a treatment expert who has worked with a broad range of client organizations spanning four continents over the past two decades. Proteus team member Terry L. Johnson, Ph.D., P.E., has served as the Black & Veatch Director of Wastewater Treatment Technology for the past 16 years. His 31 years of experience include numerous wastewater treatment and biosolids management concept designs, research on related technologies, and innovations. Bruce Long, also a prominent Proteus team member, has been the Director of Water Treatment Technology for Black & Veatch for 10 years. He has more than 30 years of experience in the development, evaluation, and application of emerging water treatment technologies.