Wastewater systems in the US are sized based on the maximum number of gallons per day they can treat.
A 300-room hotel, for instance, might require a 50,000 gallon-per-day system. Depending on soil loading rate*, that system might need a 2 acre drip field for effluent disposal.
Here are some factors that determine how many gallons per day your community septic or other wastewater system must be able to handle:
Capacity in gallons per day is determined by state and local design specifications.
These regulatory agencies calculate required treatment capacity in terms of maximum gallons used per person per day or maximum flow per bedroom per day, etc.
Commercial wastewater systems use more complex formulas that take their specific usage into account. The hotel mentioned above might need to account for 75 gallons per bed per day but might also have a restaurant and a bar attached for which another 12 gallons per seat per meal would have to be added.
Design criteria must also assume the level of pollution present within wastewater from different sources. Very dirty wastewater takes longer to treat which means systems must have higher capacity than what is released to give the system the time needed.
Here is an example of a design criteria matrix from an actual state regulatory agency:
Design criteria tables such as the one above provide a starting point to determine size, but in most cases, regulatory agencies grant variances based on actual flow and treatment level.
We at Aqua Tech will research the design criteria required for your project and budget around them. As the build gets closer, we reevaluate your treatment needs and work with civil engineers and regulatory authorities to ensure regulatory compliance without excess expense.
Bottom line: Use this table to get a rough estimate. When you’re ready, let’s talk and get more specific.
*Soils differ in how much moisture they can absorb per hour. Very dense soil might only be able to absorb one tenth of a gallon per square foot every hour while porous soil can absorb almost a full gallon per square foot. Soil absorption per hour is called its “loading rate.” The higher the loading rate the smaller the drip field needed.
Wastewater treatment uses natural biological processes to protect the environment from contaminants in sewage.
Wastewater poses several threats to the environment. Micro organisms which digest the suspended organic matter (Total Suspended Solids – TSS) in sewage use up the dissolved oxygen (DO) present in the water. The rate of this digestion can be measured as Carbonaceous Biological Oxygen Demand (CBOD). Water with high CBOD can deplete dissolved oxygen in waterways thereby suffocating wildlife.
All wastewater treatment from septic tanks to municipal systems use gravity to settle out most solids. After settling, smaller organic particles remain suspended in the effluent. The settled wastewater then moves into biological treatment which increases the density of micro organisms in an oxygen rich environment. When done properly, biological treatment can neutralize the oxygen depleting effects of wastewater.
Wastewater treatment also removes chemical pollutants.
One byproduct of human metabolism, ammonia, can poison watersheds through untreated sewage. Beneficial bacteria naturally occurring in wastewater use DO to convert toxic ammonia into the nutrient, nitrate. That’s good, but not quite good enough. When nitrate along with another nutrient, phosphorus, enters the environment, they can cause plant overgrowth that chokes waterways. Conveniently, other wastewater bacteria turn nitrate into nitrogen gas and mineralize phosphorus which settles out of the resulting effluent.
These bacteria multiply into a slime layer called, “biofilm” in the biological reactor. Advanced treatment systems achieve high biofilm density by giving it a lot of surface area (media) to grow upon. The greater the surface area, the higher the treatment level.
“Advanced” doesn’t necessarily imply a particular type of technology so much as it refers to a degree of treatment. If the effluent leaving a system meets stringent criteria it’s said to have undergone advanced treatment.
Longer answer – Advanced treatment systems:
Use new technologies to accelerate biological consumption of organic contaminants in wastewater.
Drip irrigation systems are an efficient and proven technology many communities use to recycle and dispose of treated wastewater. The effluent is applied to the soil slowly and uniformly from a network of narrow tubing, placed in the ground at shallow depths of 6 to 12 inches in the plant root zone.
Because water is such a precious commodity, recycling wastewater can have both economic and environmental benefits for communities. Reusing wastewater to irrigate land can help protect surface water resources by preventing pollution and by conserving potable water for other uses. This is particularly important where community water supply sources rely on wells. The more water that is pumped from wells and discharged as effluent into a stream or other surface water, the less will be available to recharge aquifer or ground water sources upon which future well water supplies rely.
Another benefit of applying wastewater to the land is that the soil provides additional treatment through naturally occurring physical, biological and chemical processes. Irrigating with wastewater also adds nutrients and minerals to soil that are good for plants and it helps to recharge valuable groundwater resources.
Residential developments with low building density required by septic drain fields contribute to an undesirable sprawl and limit land available for playgrounds, hiking trails and other open space amenities. Spray systems, while superior to septic, can also limit land use since they produce aerosols which require large buffer zones.
Community sewers that use drip irrigation consolidate undersoil treatment into one region of the subdivision. This region can provide a visually appealing common area for the development. Achieving higher land use densities with desirable open spaces are important and shared goals of land use planners, environmentalists and developers alike.
Soil reuse systems require less monitoring and thus lower operating costs when compared to surface discharge.
Additionally, subsurface discharge expedites acquisition of state and county permits by addressing potential concerns of downstream property owners removing any reason for them to contest approval.
Beneficial reuse through drip irrigation is just another way we’re equipping responsible growth. Click the button below to see how we can equip you.
One of the biggest challenges to implementing comprehensive land use plans is how to accommodate new development in locally designated growth areas that do not have public sewers. Many rural and suburbanized towns in the US face this question.
They want to direct growth to the most suitable areas of town – near existing services, such as fire stations and schools, for example – but have no prospect of gaining access to public sewer lines. New development must rely on soils, usually on a lot by lot basis, to handle wastewater. The conventional wisdom says that means low densities of development, negating the effectiveness of a growth area. However, towns and counties without public sewer systems have options that they may not realize.
Additionally, watersheds in the United States reflect tremendous diversity of climatic conditions, geology, soils, and other factors that influence water flow, flora and fauna. There is equally great variation in historical experience, cultural expression, institutional arrangements, laws, policies and attitudes. With regards to wastewater issues, it would be a mistake to impose a standard model from the federal level to address the needs on a local level. Correspondingly, centralized sewer systems are aging, frequently underfunded with respect to replacement costs and expensive to maintain. In addition, centralized sewer strategies are increasingly challenged by environmental and social considerations such as inter-basin transfer issues, aquifer depletion, nutrient loading and urban sprawl.
The new emerging civic agenda of smart growth, community preservation, open space planning, ecologically sound economic development, resource conservation, and watershed management demands that we rethink what constitutes assets and liabilities. With a capacity of roughly 200,000 gallons per day, these off-grid plants can be constructed at a cost of well under $2,000 per home. These are economic, environmental and quality of life issues and they do not lend themselves to single purpose solutions. They require local community based consideration within the context of flexible multipurpose planning.
Statistics have shown us that within the U.S., twenty-five percent of existing residential real estate and forty-seven percent of new construction are served by onsite treatment systems. Many of these systems are acknowledged to be inadequate with respect to soil absorption, nutrient removal, resource protection and public health. Ironically, despite these statistics and EPA policy changes, most regulatory codes as well as most municipal and commercial planning continue to consider onsite systems to be temporary solutions awaiting a centralized sewer hookup.
Looking beyond the traditional assumption that wastewater is simply a matter of safe disposal and the public health; the real contemporary wastewater issues are the economic and environmental issues in which the public has a primary interest:
Drinking water quality
Deterioration of recreational water resources and other natural systems services
Economic development in small and rural communities
Beyond just disposal, decentralized wastewater management has the potential to contribute to the formation of an infrastructure to sustain watershed integrity. Decentralized wastewater treatment serves the “watershed agenda” and the principles of “community preservation” and “sustainable development.”
When approaches to the larger wastewater issues are successfully accomplished everyone benefits:
Local communities win open space zoning, water quality and supply protection, increased development capacity and an expanding tax base.
Natural systems are sustained through prudent zoning and reduction of non-point pollution.
Developers win additional lots for development and higher margins typically associated with conservation subdivision design and municipal infrastructure.
Regulatory agencies win because they gain partners in compliance management such as the municipality and perhaps a watershed authority.
Citizens and homeowners win because property values are enhanced as schools, healthcare providers, and retail outlets crop up around the new infrastructure which decentralized systems provide.
There are no major obstacles to a decentralized infrastructure for wastewater treatment.
New technologies in a properly managed context provide the opportunity for a land based watershed initiative that could significantly reduce small flow point source discharges such as those associated with onsite treatment systems. A decentralized wastewater management infrastructure should include:
Clustered, performance-based, decentralized wastewater management systems
Industrial & commercial pretreatment prior to discharge to existing sewage treatment systems
Wastewater reuse systems
Estimates suggest that this infrastructure is achievable with technologies that require 50% to 70% less space with corresponding reductions in cost of 40% to 50%. For citizens in small and rural communities these reductions represent opportunities to preserve water quality, to stimulate economic development and job formation and to restore property values. Essentially, we are shifting from large sewage collection systems and centralized treatment plants to small and decentralized management systems. Keep in mind also that this is not an alternative to centralized sewer. Rather, it is a complimentary adjunct to the existing infrastructure.
Moreover, the decentralized solution is coming from local community and watershed needs. It is not coming from the bureaucracy. It is essentially good old bottom-up American pragmatism. It is important, therefore, that the general population becomes informed about the benefits of the decentralized approach. We must find a suitable mechanism to accelerate the progress to support watershed management. If we can not find such a mechanism, we run the risk of letting the limited existing strategies (centralized and onsite) dominate the next 20 to 30 year cycle.
These collection or conveyance systems often represent the major portion of the total capital cost associated with any wastewater system, so careful consideration should be made to avoid extraneous expense while also ensuring reliability and environmental compliance.
Let us help you design a system that takes everything into account.
Several places around the US are currently experiencing a construction boom and we’re delighted to be a part of it. Here’s a mixed use system that our engineers have just designed.
This particular system was designed to treat residential and commercial wastewater at the same time. Notice that the effluent (outflow) discharges at ground level. This is a septic system with no leach field!
Here’s the secret:
This private wastewater treatment plant removes nearly all of the Biological Oxygen Demand (BOD), Total Suspended Solids (TSS), and Total Nitrogen (TN).
The BioTank uses floating and fixed film processes in which microorganisms attach themselves to a highly permeable media that is submerged in the wastewater. This allows for the absorption of organic and inorganic matter into the slime layer where treatment is realized. Designed properly, this filter is self-purging.
Hydraulic dosing and secondary sludge airlift pump systems are set at pre-determined rates to minimize maintenance and enhance treatment. The self-purging biological filter is designed by Aqua Tech Systems to accommodate influent characteristics and achieve effluent requirements. Oxygen is introduced to the system via an oil-less compressor and membrane aeration equipment.
Wastewater is pumped from the influent pump chamber to mechanical equipment or directly into the first baffled compartment of the BioTank. Alternatively, primarily settled or prescreened wastewater is pumped from an equalization basin to the BioTank. Wastewater flows by gravity through each treatment compartment of the BioTank and effluent is discharged over a weir.
As flow enters each aerobic compartment dissolved oxygen is transferred to the wastewater via compressor and membrane aeration module. Each compartment has an independent and fully adjustable air regulation valve. In the aerobic modules the compressor acts as a mixer to enhance treatment and prevent the short-circuiting of wastewater through the plant.
In the BioTank, the organic material in the wastewater is reduced by a population of microorganisms that attach to the filter media and form a biological slime layer. In the outer portion of the slime layer treatment is accomplished by aerobic microorganisms. As the microorganisms multiply the biological film thickens and diffused oxygen is consumed before penetrating the full depth of the slime layer. Consequently the film develops aerobic, anoxic and anaerobic zones.
Absent oxygen and a sufficient external organic source for all cell carbon the microorganisms near the media surface lose their ability to cling to the media. The wastewater flowing over the media washes the slime layer off the media and a new slime layer begins to form. The process of losing the slime layer is called “sloughing” and it is primarily a function of organic and hydraulic loading on the filter. This natural process allows a properly designed media bed to be self-purging and maintenance free.
Any excess sloughed biomass is transferred with the wastewater flow to the final clarifier as sludge. These secondary sludges are periodically pumped back to the primary tank or sludge holding tank for eventual removal or further treatment.
The BioTank treatment plants may also be supplied with bar racks or screens, grit chambers, flow meters, chemical dosing equipment, UV disinfection modules and sludge dewatering systems.
To put the BioTank to work for you, click the button below to schedule a consult.
Our STEP system creates a pressurized, small pipe influent delivery structure to the treatment plant which eliminates the need for the expensive piping and lift stations that gravity systems require. This means that developers can cut their cost as well as defer some of that reduced cost of the community wastewater system until lots are sold. Since each home shoulders some of the load associated with wastewater treatment, the initial cost and maintenance can be distributed to the homeowners as well.
Beyond reduction in development cost, STEP technology further enhances effluent water quality by leaving the majority of the solids at the point of origin where they can degrade through anaerobic processes. The effluent leaving the tank at the home then becomes the influent which the next component in the Aqua Tech system will treat through aerobic processes.