By Claude Goguen, P.E., LEED AP
Editor’s Note: This is the last part of a three-part series that is intended to educate wastewater structure manufacturers about the nature of septic tank influent, what happens to wastewater in a typical tank and what else can be done to increase that level of treatment. The series also offers suggestions on ways to increase the level of serviceability by offering different types of systems and treatment options for a wider range of needs. Click here for the second article in the series.
When we started this series, our intent was to show how precasters that manufacture water and wastewater systems could expand their product lines to serve a more diverse market. First, we focused on what happens in a septic tank. Second, we discussed advanced treatment and how members can incorporate different systems and processes in their tanks to address specific challenges. Now, with this last article in the series, we think big and look at how members can benefit from another emerging market in decentralized wastewater treatment – larger-capacity systems that can serve multiple residences or large facilities such as schools, campgrounds, rest areas and factories.
Why decentralized cluster systems?
According to the 2015 U.S. Census Bureau’s American Housing Survey, approximately 20% of all households and 16% of new housing units are served by individual decentralized systems. Once considered a temporary installation until the centralized sewer could be connected, decentralized wastewater treatment has become a safe, affordable and sustainable long-term wastewater treatment option.
Centralized wastewater treatment systems are beneficial in large, densely populated areas but are not used much in smaller communities since the resources to operate and maintain these systems properly can often surpass local capacities. As a result, some systems are poorly maintained and result in discharges. According to the U.S. Environmental Protection Agency, small communities using centralized sewers to treat and discharge wastewater account for most noncompliance violations.
The investment needed to install new or upgraded services can be much more attainable when considering decentralized treatment. The smaller piping keeps costs low and makes installation easier. These systems are also generally affordable to operate, maintain and replace since relatively simple equipment can be used.
Since decentralized systems treat wastewater close to the source and often use passive treatment, such as soil dispersal, these systems may offer substantial savings in energy costs. A 2002 Electric Power Research Institute report stated that at least 4% of energy use in the U.S. is associated directly with water transport and treatment. On a community level, this can translate to about 25% of a community’s energy use. Plus, having a decentralized system promotes local business and job opportunities.
Regarding treatment, decentralized systems can produce effluent quality that is equal to or higher than centralized treatment, as it possesses the same advanced treatment technologies as centralized systems. Since decentralized systems use the treatment capacity of the soil, they achieve high-quality treatment at a lower cost than other options.
Another reason for the upward trend in community or cluster systems is the desire to avoid individual treatment systems. Some developers prefer to use a shared system in a subdivision to allow for smaller lot sizes and reduced infrastructure costs. Also, transferring maintenance responsibility from homeowners to a third-party management entity avoids common issues.
How does it work?
Clustered systems can be as simple as a conventional subsurface disposal field shared by two lots and served by individual septic tanks, or as complex as a neighborhood collection treatment and disposal system that is comparable in size and scope to a small municipal sewer system. Larger systems can also be installed to serve industrial or institutional facilities or clusters of businesses.
Cluster systems transport wastewater via alternative sewers to either a conventional treatment plant or a pretreatment facility followed by soil absorption of the effluent.
There are different types of systems that can be used, based on conditions and scope. The piping can be pressurized, gravity or vacuum.
Pressure systems can use a grinder pump that shreds sewage at each home prior to pumping. Another system that uses pressure piping is known as a septic tank effluent pumping system. A STEP system uses individual septic tanks at each residence that remove solids and grease. Then it pumps the effluent into a pump chamber and finally to a central location for soil-based treatment.
A septic tank system that uses gravity to flow from each tank to a central location is sometimes called a septic tank effluent gravity system. These systems can coexist with STEP systems within the same decentralized sewer.
Vacuum systems rely on suction created at a central pumping station. Small holding tanks may be used at each home. When a certain level is reached, a valve is opened allowing the effluent to be transported to a central location for treatment. This type of system works best in flat or gently rolling terrain.
Package wastewater treatment plants
When supplying residential septic tanks, the typical capacity range is 750-to-2,500 gallons. What if a prospective client needs to treat 500,000 gallons per day, or perhaps 1 million gallons per day? Having the capability to supply complete package systems can quickly open new markets.
These large systems can include a series of tanks, modular multi-cell tanks or post-tensioned rectangular or circular tanks.
The ability to not only supply these structures, but to also design the system based on the effluent limits can make your company a valuable resource for developers and contractors. Effluent parameters and limitations such as biological oxygen demand, suspended solids, nitrogen, phosphorous, dissolved oxygen, chlorine residual, coliform levels and many others may need to be addressed.
Many of these package systems include components such as:
- Flow comminution and screening pretreatment
- Influent lift station
- Trash trap
- Activated sludge aeration
- Secondary clarification
- Activated sludge return
- Scum removal
- Froth control
- Rapid sand filtration
- Ultraviolet disinfection
- Chlorination and dechlorination
- Post aeration and sludge dewatering
When supplying large systems, the effluent can contain materials that require more than conventional residential wastewater treatment. Waste from a commercial kitchen may be more involved and would require a grease interceptor. A large school that has a working garage would require an oil separator. These components are not uncommon for large package systems.
How producers take advantage of this growing market?
For cluster or community systems, producers may already possess the means to produce and furnish structures for this type of infrastructure. Individual septic tanks used to perform initial separation are similar to conventional tanks. Some systems may require smaller holding tanks and/or pump tanks. Some manufacturers have found ways to incorporate the pump system within the settlement tank. When considering package wastewater systems, upgrades in manufacturing may be required to produce specific components. In addition, this type of product would require partnerships with suppliers of advanced treatment systems and other technologies so the final product and service are turnkey. The key is to make it easy for developers to choose your product. That means being aware of the types of systems allowed by your authority having jurisdiction, knowing what contractors and developers prefer and supplying a quality, durable product.
Claude Coguen, P.E., LEED AP, is NPCA’s director of sustainability and technical education.
U.S. Environmental Protection Agency Office of Wastewater Management Decentralized Program,
Small Community Wastewater Cluster Systems – Purdue University Cooperative Extension Service,