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BACKGROUND/GENERAL
Septic tanks are excellent for remote installations such as rural homes and very small business operations that do not generate large volumes of wastewater, or wastewaters that have high concentrations of solids, heavy metals, etc. They are not designed to handle toxic wastes, paints, solvents, etc.
When small businesses or many homes are constructed close together, wastewater treatment facilities are required to collect and then treat the wastewater that is generated. What type of facility that is designed and constructed depends heavily on the cost and the amount of land available, the topography of the community, the constituents in the wastewater, the concentration of the organics and suspended solids in the wastewater, the financial resources of the community constructing the wastewater collection system (sewers) and the treatment facilities, and the treatment goals/discharge permit parameters.
Small communities in rural environments usually construct wastewater treatment ponds, as the land is usually less expensive, they are usually less expensive to construct and operate, and the treatment goals have usually been modest, such as not requiring high water quality parameters for water reuse. As the small communities grow, land becomes more expensive, and/or there is increased demands placed on the water quality produced by the treatment plants, communities move from ponds into advanced wastewater treatment options.
The more advanced treatment processes are "usually" more expensive to construct and operate. There has been a rough rule of thumb that if it costs a community "$X amount of money" to remove about 86% of the BOD and Suspended Solids from a wastewater stream, it will cost double that (2 times $X amount) to increase the removal rate to 94/96%. If it desired to remove about 98%-99% then the community triples the cost that it just doubled! (We now have about six times the "base 86% removal cost!)
Communities that currently have septic tanks only, and are converting to collection systems and a treatment plant, will find large costs associated with the transformation. Those communities who have been "gradually converting" by installing wastewater collection piping to convey new construction wastewaters being generated to ponds, etc., will have a much lower construction and operation cost than those that convert from septic tanks to advanced treatment facilities. These treatment systems that are "evolving" are able to use past expenditures for pipes and treatment which in turn help to minimize present and future costs.
The type of wastewater treatment plant, and the specific treatment processes that are selected, are most often a blended function of what the community currently has for wastewater treatment and that can still be used, what their financial ability to afford is, what the cost of land and availability of contractors is, what the new wastewater discharge permit and water quality goals are. To some extent, the treatment processes selected also depend on what the design engineering firms have had experience in designing, what the generally accepted types of treatment processes are for that region, and what the community feels it can properly and safely operate. Some treatment process types are more costly to install, but may have lower annual operating costs; or there may be those with lower construction costs with higher annual operating costs. These "financials" may be a factor for an agency in selecting a process that they can afford to spend more on now, and have lower operating costs; or conversely, spend the amount they have now to construct a process, and assume higher annual operating costs later.
I have personally seen instances where designers have even taken into account the education and experience of the wastewater treatment operators when selecting a treatment process. Some treatment processes are easier to operate than others. (An example is comparing the less difficult "trickling filter-solids contact" secondary process with a "conventional activated sludge" secondary treatment process that is a wee bit more difficult to properly operate.) They may also consider future discharge permit requirements and the ability of the staff to meet these new challenges with different operational schemes rather than new construction.
A community may also be justified in not wishing to design and construct a "new treatment process" as it does not want to be involved as part of the "continuing research and development" of that new process of piece of equipment. I know of excellent water and wastewater managers who have "mantras" that say something like "I need to see four of these installed within 150 miles that have been working successfully for more than 10 years for it to be installed here." Other excellent managers may be more open to newer technology. (It probably depends a lot on how successful they have personally been with "new and innovative technology!")
OVERVIEW
The goal of the wastewater treatment facility is to remove and reduce undesirable materials from the water, prior to being discharged into the environment. Based on the size of the facility, the tasks necessary to achieve this goal, may be broken down into different work groups.
Administration / Plant Operations: Most states require that state certified treatment plant operators are on duty 24 hours a day to monitor and control the treatment process. Some plants are now fully automated and have a state-of-the-art instrumentation systems. These systems reduce labor costs, and provide operators with accurate real-time information to control their facilities. The operators are usually highly trained, skilled individuals who are educated at the college level. Operators welcome the opportunity to provide tours for individuals or groups in their communities. These tours can be easily arranged by calling the water or wastewater plant near you!
Electrical/Instrumentation/Mechanical: Several states now have state certification in these trades, due to the complexity and critical nature of these job functions. The water and wastewater facilities of today are complex and challenging processes, requiring skilled individuals to maintain and repair them.
Water Quality Assurance & Laboratory: Most of the medium to large facilities operate state certified labs. The laboratory performs water quality and solids analysis for process control of each unit process, and for insuring/documenting compliance testing for the plant’s National Pollution Discharge Elimination System (NPDES) permit.
THE TREATMENT FACILITY
The wastewater flows from our homes and businesses through the sanitary sewer system, at a flow rate of about 2 feet per second. This velocity is necessary to keep the sand and other materials moving toward the treatment facility. When the wastewater reaches the treatment facility, there are quite a few processes and variations of each process, that can be employed in this endeavor, as stated above. One of the common threads throughout all of the facilities, is that they start removing the largest debris, and then remove or reduce the progressively “smaller” constituents in the water as it passes through the facility.
Generally speaking, a community will construct a wastewater treatment plant that will have Influent Pumps to lift the raw wastewater up from the wastewater collection pipes in the ground to a Preliminary Treatment Process. Preliminary Treatment involves removing large rags, sticks, a canoe paddle, bundles of newspapers, three tires and rims (we are still waiting for the fourth one so we can have a spare set for the old Chevy Vega!). The incoming wastewater may be “prechlorinated” or “aerated” to freshen the wastewater, control bacterial decomposition of the wastewater, and to reduce odors in the preliminary and primary process steps. The water then flows to the grit chamber where it slows down to approximately 1 foot per second velocity to allow grit, sand, and other heavier particles to settle out. This gritty material is then dewatered and either sent to the landfill, or is carried to an incinerator and burned. Sand, egg shells, “action figures” such as GI Joe, and even a high school class ring (which was returned to it’s owner 20 years later!)…have all been removed in our careers….and more!
Some facilities have a septic tank receiving station for accepting residential septic tank sludge which may or may not be stored and then metered into the facility at this preliminary treatment process. From Preliminary Treatment the wastewater then goes to the primary treatment process.
Primary Treatment: The wastewater then flows into large tanks called primary clarifiers. It takes the wastewater approximately two (2) hours to pass through the basins. As the wastewater passes through the tanks, suspended and settleable solids (we call this sludge) settle to the bottom, and grease and oil rises to the surface ( we call this scum). The sludge is pumped from the bottom of the tanks, and the scum is skimmed off and pumped to the digesters for treatment. In this process, 25 to 35% (up to 50% in some communities) of the sugars, starches, and carbohydrates we call biochemical oxygen demand (BOD) has been taken out of the wastewater, and 40 to 60% of the suspended solids have been removed. The wastewater then goes to the secondary treatment process to remove the dissolved or colloidal “BOD” ( food) and suspended solids that will not settle out in a tank, and are still in the water.
Secondary treatment: Microorganisms are utilized in secondary treatment to remove the colloidal and dissolved pollutants from the wastewater. There are two principle methods of bringing the microbes and the wastewater together: a "suspended growth" method or a "fixed-media" method. [Which type of secondary process is selected depends on many local design, cost, and water quality goals.] In the suspended growth method, the wastewater flows into large aeration basins where it is mixed with the microbes, termed “activated sludge”. This solution of microbes and wastewater is mixed with oxygen to provide an aerobic (oxygen rich environment) for the microorganisms to tie up and consume the “food stuff” contaminants. The microbes then reproduce, and settle out in the next down stream tank, the secondary clarifier. In essence we take the “sugars, starches, etc”, convert them into microbes, and settle them out and removing the microbes from the wastewater. Simply stated: The "sugars, starches, and carbohydrates" are dissolved in the wastewater and will not settle out, so we use bugs to eat them, which then makes more bugs, and then we can settle out the bugs!]
The fixed-film media method involves passing the wastewater over a special plastic media, slag, rocks, or other uniform media that has a zoogleal film of microbes on it. The microbes tie up and consume the organics, etc., making more microbes in the process. The microbes then slough off of the media, and settle in the downstream clarifier, like in the activated sludge process. An improved fixed-film media method is known as "trickling-filter/solids contact."
Secondary Clarifiers: As stated above, the water then flows into large clarifiers where the microorganisms settle to the bottom. In the activated sludge process, some microbes are removed in a process called "wasting" by pumping them to a dewatering/thickening process, and then to the anaerobic digesters or to aerobic digesters. Another portion of these settled microbes is continually returned to the aeration basins to provide the “seed microbes” which perpetuates the never-ending treatment process.
Filtration (tertiary treatment): Prior to disinfection, some facilities have filtration units which are designed to remove any remaining suspended particles. These are usually very small floc particles consisting of microbes from the secondary treatment process that did not settle out in the secondary clarifiers. Removing these biological small flocs prior to the disinfection process will enhance the operation of the disinfection process.
Disinfection: After we have completed the secondary treatment process the wastewater has been now been "cleaned pretty well." There are usually no particles to be seen, it is usually clear (especially in nitrified secondary treatment processes) and now passes to the disinfection process where a disinfectant like sodium hypochlorite, or chlorine is added to kill any remaining pathogenic (disease-causing ) organisms. It is important that the water be as free from particles in the water as affordable, as the particles interfere with the disinfection process by sheltering microbes from the disinfection action. Some facilities utilize ozone, or Ultra-violet Light (UV) to disinfect the wastewater. Where chlorine is utilized to disinfect the wastewater, most facilities utilize sulfur dioxide, sodium bisulfite, or other chemicals to stop the disinfecting action. If the disinfectant is not neutralized, then the disinfection process continues in the receiving waters, possibly harming microbes, small aquatic animals and plants, fish and shell fish.
Effluent: The treated wastewater is then discharged through an “outfall” pipe into the facilities receiving waters. This may be a creek, river, lake or other body of water.
Solids Handling: The primary sedimentation sludge and scum, along with the “waste activated sludge” or fixed-film media microbes produced by the secondary process units is usually thickened and pumped to an aerobic digester or to an anaerobic digester. Aerobic digesters use blowers to introduce air into the thickened sludge so that aerobic microorganisms may further oxidize and reduce its organic content. In an anaerobic digester there it is often heated in an anaerobic (no oxygen) state at a temperature of 95 degrees Fahrenheit. This reduces and stabilizes the solids and microbes further. After approximately three weeks, or so, the solids are ready to pass through to the solids dewatering process. The excess water is then removed from the microbes and stabilized solids by solids dewatering equipment like belt press’s or by high-speed centrifuges. After dewatering, some biosolids are then utilized in compost operations to make garden compost and soil amendments, or are spread in farm fields at agronomic rates like other fertilizers. In some facilities, the sludge is incinerated in multiple-hearth gas-fired incinerators at temperatures about 1,500 degrees Fahrenheit, which reduces the volume by approximately 75% and destroys all pathogenic microorganisms. The waste gases from the furnace pass through scrubbers and an electrostatic precipitator to protect air quality. They usually dispose of the ash in a landfill.
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