Considerations When Designing a Wastewater Treatment Plant
Consider this: a city decided to build a wastewater treatment plant so that they can get rid of septic tanks throughout the city, which are overflowing or contaminating the groundwater aquifer by percolation and seepage. The current situation is so bad that potential industries and commercial enterprises are rethinking if they should consider moving into town where adequate facilities for the wastewater generated by them do not exist. Instead, they consider another location for its operations. Every day, the city is in the news that its wastewater overflows are contaminating the environment and polluting the once-pristine stream running in the middle of town that had provided recreational facilities to its citizens. The citizens are up in arms for its poor record on environment.
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Now come a series of questions. What type of plant should it be? What technologies should the plant use? What should be the capacity of the plant? From where should the money for its design and construction come? Would it require legislation for passing user fees and how will the citizens accept these charges? The list goes on and on. Perhaps, a good, experienced and competent consulting firm can assist the city in addressing all these issues.
But a big question still remains. Where should the plant be located? Everyone will say NIMBY i.e. NOT IN MY BACKYARD. Everyone associates a wastewater treatment plant with odors, chemicals with possible health effects, noise caused by machinery, traffic resulting from trucks moving in and out, tanks sticking up and being visually obtrusive, plain concrete and/or steel structures, and overall unaesthetic appearance. All these issues, if not addressed properly, will undoubtedly degrade the neighborhood and adversely affect property values. True or False?
All these concerns can be reasonably addressed by a competent and environmentally conscious consultant team. The team needs to have good and competent process engineers; architects; landscape architects, civil, structural, mechanical engineers; planners; environmentalists; public- relations experts; and civic-minded professionals on board. A modern treatment plant can be and should be a good neighbor to the adjoining residences and enterprises-commercial as well as industrial if applicable.
This paper contains some examples where most of these concerns existed, but were addressed to the satisfaction of concerned neighbors.
Choosing a Plant Name
No one wants to have his or her home near a wastewater treatment plant, regardless of how well the plant is designed and is aesthetically acceptable. But calling it by a different name could mitigate that concern. Consider naming it a water reclamation plant, water conservation plant, water recycling plant or water factory instead. This could mitigate, if not eliminate, that concern. Sometimes, it could simply be a perception issue, which can be addressed by giving the facility a different catchy name. Most plants in the U.S. today are renaming their facilities with those listed earlier. A plant in Orange County, California, for instance named its plant Water Factory 21.
Designing the Layout
A good layout can often be helpful in public acceptance of the project. Consider the following:
- Locate the plant downwind of residences and other concerned neighbors.
- Keep some buffer between residences and the nearest plant facility (say 500 ft.).
- Build odorous facilities farthest from residences (i.e. headworks).
- Cover and/or house the odor causing facilities, provide necessary ventilation and air scrubbing.
Selecting the Appropriate Wastewater Treatment Processes
Selection of treatment processing, among other factors, should consider which processes are less amenable to generation of odors. For example, membrane bioreactors (MBR), by virtue of their smaller footprint, may be a better treatment process than conventional activated sludge. In small size plants, with MBRs, it may be possible to eliminate primary clarifiers, which often generate more odors than other processes at the plant. A similar criterion can be used in the selection of sludge handling processes. This may involve eliminating gravity sludge thickeners, or using centrifuges or screw processes with integral covers instead of belt presses, as an example.
Controlling & Mitigating Odors
This is often a major concern. How can a wastewater treatment plant not cause odors? This is what the citizens have always believed, and they probably are right. They have likely visited some old plant designed at a time when either the odor was not an issue (remote location perhaps) or the technologies to confine and treat odors were not available.
Now, wastewater professionals understand both the chemistry of odors as well as the means to confine and capture odorants and treat them. There are several technologies that have been developed—both proprietary and non-proprietary—that can reduce odors to levels barely discernable by humans. Wastewater professionals now fully understand the chemistry of odors—such as sulfides, mercaptans etc.—and therefore know how to treat them biologically and/or with chemicals.
Biological Scrubbers
Most modern plants are opting for biological scrubbers due to the facts that no chemicals are required thus reducing the carbon foot print of their plant; they are not as tall, thus have lesser visual and obtrusive impact; and they can scrub nearly all odor-causing compounds—whatever their origin—if properly designed with adequate residence time unique to the nature of odorants to be removed. In fact, these scrubbers can be designed to be aesthetically pleasing—even underground or sticking a couple of feet above ground and mildly landscaped at the top. The media can be compost, wood chips, bark, peat, lava rock, or any combination of the above materials. For sensitive neighborhoods, these scrubbers can be followed by an adsorption scrubber using activated carbon as the adsorption media for final polishing.
Chemical Scrubbers
However, owners and operators of some plants still prefer chemical scrubbers due to their past experience. These scrubbers use sodium hydroxide and sodium hypochlorite or other oxidants sprayed from the top through nozzles into foul air moving upwards through tall towers filled with some inert media to provide sufficient contact surfaces between the chemicals and the foul air. The chemicals oxidize hydrogen sulfide and other odorous compounds producing innocuous byproducts. If the owners prefer chemical scrubbers, they usually will be tall (10 to 15 ft.), but can be hidden behind an architecturally designed wall facing the neighbors. In exceptional cases, the wall can have a nice mural painted on it to enhance appearance. [See Figure 1]
Confining, Capturing & Transporting Odors to Scrubbers
The other issue, of course, is how to confine, capture and transport odors to the site of the scrubber(s). That is easily done by enclosing and/or covering the odor-causing processes, withdrawing the foul air by fans, and transporting it to the scrubbers in high-density polyethylene ducts, which are laid underground or aboveground.
There are several plants where these technologies have been used successfully in the U.S and abroad.
Masking Agents
Another remedy, which has been used at some plants, is to use a masking agent, often a fragrant chemical or a perfume, that is pumped via a small diameter piping system- usually half inch to 1 inch diameter in size – often attached to the fence or the boundary wall, around the periphery of the entire property or only around a portion of the property which faces the concerned neighbors. The perfume is pumped and sprayed through suitably designed and spaced nozzles when the odors are prominent – say on windy and hot days.
Public Relations for Wastewater Treatment Plants
Today, it is as important to have a good public relations program for acceptance of the project by citizens as are other elements of the project—technical, financial etc. The consultant or the owner should have an experienced and competent public relations (PR) firm on the team. This firm should understand the technologies that the plant will use, the odor causing potential of these technologies, and ways in which this important issue was addressed by other plants in the past.
Concerned citizens should be given tours of these plants by members of the PR team so that they are comfortable with the technologies being proposed. Their comments should be heard and addressed in the design and layout of the plant. In other words, these citizens should become a part of the selling team that promotes the need, location, design and other elements of the plant to other citizens, interested groups or skeptics in the community, who were not able to attend tours.
Board and agency members, who are somewhat skeptical about the project, should also be invited to attend these tours so their concerns also are assuaged. Everyone should be encouraged to ask questions during these site tours and raise concerns. Never show by words, actions or body language that you know it all, and insist the concerns of citizens are invalid or stupid. Questions are never stupid; answers can be and should not be.
This also should not be a one time undertaking. The visits should be arranged as often as required with the same or different groups of people and could include members of the Rotary Club, Lion Club, school board and others who likely can impact the success and outcome of the project.
All these ideas were successfully used in the more than $3 billion dollars Clean Water Program in the city of San Diego, California in mid 1990s. The program involved construction of three green-field water reclamation plants, several miles of reclaimed water distribution system to provide reclaimed water for landscaping and other industrial uses, and pumping stations.
Build a 3D Model of the Plant
To promote good public relations and improve public perception and image of the project, it will be wise to build a table-size 3D model of the entire plant with all the facilities shown on it. It should be to scale so that the interrelationships of different components of the plant are the same as they would be on the full -scale project when built. The model should be prominently displayed in an appropriate place and used to explain different features of the plant to the visitors and concerned citizens.
Assess the Aesthetics & Neighborhood Compatibility
The plant should not degrade the neighborhood. This can be accomplished in a number of ways.
- The buildings associated with the project should match the buildings in the surrounding neighborhood—both architecturally and in materials selection.
- Sometimes, the fencing can be an eye sore. It could be made of wrought iron or other suitable material compatible with the surrounding community. A plant designed by this author used white picket fencing to give a barn-like appearance to the plant which blended with both the existing and planned community around the plant. At this plant—the Rancho Las Virgenes Solids Handling Facility, owned and operated by the Las Virgenes Municipal Water District in the Greater Los Angeles area—buildings were designed with clerestories, cupolas and dormers to blend with the neighborhood. Roofs of the buildings were pitched, sloped and shaped to give character when viewed from homes situated above the plant.
At the Rancho Las Virgenes, tanks were buried to the extent possible to reduce visual impact. High profile equipment such as odor control towers received siding and special treatment to give the appearance of farm silos. Consider also:
- Natural terrain should not be scarred to the extent possible, if the existing terrain is pleasing. Structures built in the project should blend in.
- Too often, treatment facilities are laid out in straight lines and pushed together, resulting in the appearance of an industrial facility. This should be avoided. Structures, if possible, should be off-set, rotated, and separated to provide a more open appearance.
The bottom line is to be conscious and appreciative of the needs and concerns of the neighbors, and to build facilities that are compatible with their homes and businesses.
Review Landscaping & Area Lighting
The importance of good landscaping and area lighting cannot be overemphasized. Lighting should be soft and must not produce glare to the residents at night. Use bollards with lights along walkways about 3 feet in height. Light should point downward, not outward. Neighbors want to sleep at night and not be bothered by looking at bright lights.
Similarly, landscaping, featuring trees and natural vegetation, should be used liberally to soften the appearance of the treatment facility. Another plant designed by this author used all the surplus earthwork generated by excavation to build an 8- to 10-foot tall berm around the entire project property and landscape it with drought-resistant native bushes, shrubs and plants in Bakersfield, California. This satisfied the existing neighbors and the planned community nearby.
A plant located in the Los Angeles area, owned and operated by the city of Los Angeles—the Tillman Water Reclamation Plant—houses a beautiful Japanese Garden on its property (below), which is regularly visited by tourists and has become a sought-after place for holding wedding ceremonies and receptions. Recycled water produced at the plant is used for irrigation.
Another advantage of the plants is that they mask the residual odors if they still prevail despite all the odor control facilities incorporated in design. Additionally, they enhance the appearance and neighborhood.
Noise & Decibel Ratings
This is another concern that the plant design must address. Equipment with low decibel ratings should be selected (e.g. high speed turbo bowers instead of positive displacement or multi-stage centrifugal blowers). The buildings which house such equipment (blowers, pumps etc.) can have acoustic treatment done to the walls. In addition, many of these equipment come with their own acoustically designed enclosures which could be used on the project if possible.
Out of all the perceived and real nuisances, noise probably is the easiest to handle. Trips to the existing plants, discussed above, can mitigate this concern.
Conclusions
Public acceptance of the Water Reclamation Plants is extremely important. If not handled properly, an otherwise good project can be derailed by citizens protesting and/or litigating. Thus, this issue must be given high, if not the highest, priority by the owners of the plant, engineers, consultants, architects and other engineering disciplines involved in designing these projects.
This paper deals with issues that often concern the public. The paper also provides insights with examples how these issues were successfully handled in other projects.
All communities are different. And so would be their concerns. But this paper highlights remedies that can be applied- perhaps with modifications unique to each project.
About the author:
Madan Arora Ph.D., P.E., BCEE is a technical director at Parsons. Arora can be reached at [email protected].