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 can be treated in up to three stages generally known as primary, secondary, and tertiary treatment. Here’s what’s involved in each of these stages:
In this stage, heavy solids and grease are separated from the raw sewage through gravity and buoyancy respectively. A conventional septic tank is an example of primary treatment.
The wastewater that leaves a septic tank or other primary treatment apparatus is still pretty contaminated with suspended solids and toxic chemicals such as ammonia. Secondary treatment systems use oxygen to facilitate natural digestion of contaminants by micro organisms already present in the wastewater. All municipal systems use secondary treatment.
Even though much cleaner, water leaving secondary treatment can still pose somewhat of a threat to the environment. To ensure complete protection of aquifers and watersheds, wastewater effluent can enter a third treatment stage. Tertiary treatment usually involves some sort of natural or chemical filtration/sanitization. Examples of tertiary treatment are constructed wetlands or drip irrigation fields.
Our systems use all three stages of wastewater treatment to equip you for responsible growth. Let us show you how!
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).