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The use of ADS biodiffusers for residential wastewater treatment is far more cost-effective for non-urban areas than centralized wastewater treatment plants.
By Daniel P. Duffy
Suppose you live in a small community on the outskirts of a big city or out in the country, which is now growing as more and more families discover your town and what a nice place it is to live in. Growth will bring with it opportunities and headaches as your community tries to preserve what made it popular in the first place in the face of ever-growing demands on local utilities and infrastructure. Prior to experiencing rapid growth, most rural communities can get by easily on diffused septic systems. In fact, individual septic fields are far more cost-effective for non-urban areas than centralized wastewater treatment plants.
Suppose further that your community would like to preserve as much of its current layout as possible while still being able to provide services to newcomers. Instead of floating long-term bonds to pay for a new, large, central wastewater treatment facility, suppose instead that each new residential development and business park could utilize its own septic system to treat its effluent. Will standard septic fields with their relatively large area requirements, high maintenance needs, and tendency to fail in a most unpleasant fashion provide what your community’s looking for? Probably not. This is one of the reasons why large urban areas tend to utilize centralized treatment facilities. But combine the standard septic system with a highly efficient recirculating sand filter and construct them both out of easily installed and inexpensive biodiffuser chambers, and a system of diffused treatment works becomes attractive. This article will examine the design and operation of each of these system components and take a look at their successful use by a small community in Tennessee.
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Photo: ADS Hancor Inc. |
| The bottom of the 2- to 3-foot-wide trench is filled 4 to 6 feet deep with gravel. |
Standard Septic Field Design
Until the development of the biodiffusers, the standard onsite wastewater treatment system consisted of either a basic septic field or a more advanced recirculating sand filter. A septic system is an underground series of pipes connected to a central distribution tank. The pipes have perforated walls arranged in parallel at regular spacing intervals to form a belowground drainage field. Effluent from homes and businesses flows into the central tank and then flows from the tank either directly to the pipes or through one solid-walled pipe connected to a distribution box, which in turn is connected to the drain field pipes.
The volume of the central tank depends on the amount of expected effluent. As a rule of thumb, this can be estimated by the number of bedrooms in a house or by the square footage of a commercial building. For most small to moderately sized septic systems, the volume of the tank will be at least 50% more than the expected daily effluent flow. Within the tanks, incoming effluent settles out into three zones. In the upper zone, lighter materials such as grease, oils, and fats form a layer of scum. In the middle zone is the water from which the lighter and heavier materials have been separated. This water layer contains suspended solids, live bacteria, and diluted chemicals such as nitrogen and phosphorus. The lower zone consists of heavier organic solids that settle out and form a layer of sludge. Each of these layers (scum, water, and sludge) is managed differently.
The scum and water flow out of the central tank via gravity discharge to the perforated pipe drainage field. Natural filtering soils and their associated soil bacteria that lie below the drainage field provide the treatment. The force behind the flow is the displacement of existing water and scum by incoming effluent. As the water is displaced, the scum and water already in the tank flow out of a discharge pipe located near the top of the tank but at the opposite end from the inflow pipe where wastewater enters the tank. From the discharge pipe, the scum and water evenly flow to the perforated pipes of the drainage field. Meanwhile, back at the tank, the sludge accumulates over time until it has to be manually removed and disposed of.
These perforated pipes vary in diameter from 4 inches to 8 inches and can be made of polyvinyl chloride (PVC), high-density polyethylene (HDPE), or corrugated acrylonitrile butadiene styrene (ABS). They are set in 2- to 3-foot-wide trenches dug to depths of 4 feet to 6 feet. The depth of the trench varies with soil permeability. The bottom 2 feet to 3 feet of the trench are first backfilled with gravel. The perforated pipes are set on top of this gravel backfill and further bedded with another 3 inches of gravel. The pipe is then buried with soil backfill taken from the trench excavation. To prevent the downward migration of finer natural soils into the void spaces of the larger gravel, the pipe and gravel are often covered with a geotextile filter. With pipes and trenches to a flow gradient of at least 2%, the septic field pipes allow for gravity flow of the scum and water down the pipes and out into the soil.
Once the scum and water leave the pipes, they drain downward into the gravel placed below the pipes and then down and out into the soil adjacent to the trenches. The gravel layer provides both a filtration mechanism and structural support to the trench. The bacteria, chemicals, and other contaminants are removed by the gravel’s filtration action and the absorption properties of the natural soil. This is why such systems are referred to as soil absorption systems. Over time, a layer of biological scum grows along the sides and bottoms of the trenches. This scum provides the system’s primary treatment medium. The permeability of the adjacent soil determines the overall size of the drainage field, with septic systems set in clays needed for larger septic fields to compensate for the clay’s low permeability. Conversely, coarse-grained soils with high permeability can support smaller septic fields while having equivalent treatment capacity.
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Photo: ADS Hancor Inc. |
| PVC, HDPE, and ABS are among the types of pipe laid in the trenches. |
Recirculating Sand Filters
Recirculating sand filters (RSFs) represent an advancement over standard soil absorption system designs. The main problem with conventional septic systems is clogging failure at high effluent loads. Most septic fields are designed by rule of thumb based on average site values and typical design parameters. However, large systems tend to have greater variations in both quality and quantity of the incoming effluent. Furthermore, large systems require larger fields inevitably located in areas having differing soil characteristics. For these reasons, an RSF is typically required for systems managing effluent flows of 2,000 gallons and 10,000 gallons per day, especially in soil environments with high nitrogen sensitivity (many of the effluent contaminants also act as fertilizers).
An RSF provides a more controlled treatment environment and provides a better treatment process than standard septic systems. The recirculating sand filter modifies the conventional soil absorption system to include a recirculating tank as well as a septic tank. Primary treatment still occurs in the septic tank. This tank is set at an elevation higher than that of the other system components to allow for gravity flow out of the tank.
After primary treatment (the separation of sludge from the waste and scum components of the effluent), the effluent flows from the septic tank to the recirculation tank. Unlike septic fields, RSFs do not rely exclusively on gravity flow. Controlled amounts of effluent are circulated from the recirculation tank via a system of pressure flow pipes to discharge onto the top surface-specified sand media bed. The top surface of the sand layer is usually set at a higher elevation than the recirculation tanks. The applied pressure ensures a widespread and uniform distribution of the effluent. The flow rates are controlled by a timer and floating switch that regulates the frequency and amount of effluent being pumped.
From the top of the sand media bed, the effluent trickles down and through the sand. Biological and physical treatment of the effluent occurs on the surface of the sand particles. The sand layer that the effluent passes through is usually 2 feet thick. This treatment results in a reduction of the effluent’s pollution levels. Once it passes through the sand layer, the effluent is collected by a series of pipes installed along the bottom of the sand layer. These pipes drain the effluent back to the recirculation tanks usually by gravity flow (taking advantage of the sand layer’s elevation head) or in pressure pipes after the effluent is collected in a central sump and removed by a submersible extraction pump. Once back in the recirculation tank, the collected and partially treated effluent is mixed with additional raw effluent from the septic tank for a return trip to the sand filter.
The return pipe transmitting filtrate from the bottom of the sand filter back to the recirculation tank is equipped with a downturned tee connection, which is further connected to a pipe leg. The pipe leg opening is equipped with a ball float valve consisting of a ball that floats upward as the depth of water in the recirculation tank increases. As the water depth increases and the ball rises, the opening is closed off, preventing further inflow back into the recirculating tank. The filtrate from the sand layer then flows past the tank to the leachate field. This simple mechanism automatically prevents the recirculation tank from overflowing while regulating the number of times that filtrated effluent passes through the sand filter. This self-regulation of flow and depth is one of the main attractions of the recirculation tank design. Other configurations can be used to control rates of recirculation and frequency of discharge.
As with the standard septic design, the final end point for the effluent is a septic field soil absorption system. What determines the effectiveness of a recirculating sand filter is its recirculating ratio. This is defined as the ratio of total flow rate through the sand filter to the incoming wastewater flow. Most systems have ratios from 3:1 to 5:1. This in effect means that every gallon of wastewater entering the system passes through the sand filter three to five times before it makes its way to its final disposition in the leach field. The high recirculation ratio reduces the potential for odors by ensuring that the bulk of the liquid passing through the sand filter is previously filtered wastewater.
How Biodiffuser Design Works
The use of biodiffusers takes the art and science of onsite effluent treatment to the next level by further improving on the recirculating sand filter design and the operation of its associated septic system. This new kid on the block is a leach field constructed of manually installed, open-bottomed, plastic segments, which are assembled to form leaching chambers. The chambers are assembled on an excavation floor and covered with clean backfill. Neither the floor nor the backfill is compacted. Effluent escaping the plastic chamber segments flows out of the bottom and louvered sidewalls keep backfill out of the chambers. No filter stone or aggregate is required. The only equipment needed for installation is a backhoe, a level, and a rake.
A single person could perform the installation all by himself. This is because the segment’s post and dome joint configurations require neither screws nor nails to fit them together. The open ends are covered with matching end caps. If the bottom of the excavation floor has varying slopes and contours, their articulated joints allow the chambers to tightly follow its ups and downs. The segments come in varying widths to match the width of the trenching or excavation.
Effluent enters the starting end of the chamber formed by linked segments. As it trickles into the chamber it flows down to the incompacted soil floor of the chamber and flows along the bottom, distributing itself through the chamber. The effluent exits the chamber through the open bottom or through the louvered slots along the sides. These escape routes greatly increase the chamber’s potential contact treatment area. This allows for the use of biodiffusers that are half the size of standard gravel trench septic systems with equivalent treatment capabilities.
There are several configurations that can be used when constructing biodiffusers’ leaching chambers. Each segment of the biodiffusers is assembled from multiple chamber parts. Then flow pipes that carry the effluent to each segment connect each segment. Two or more segments whose trench bottoms have been excavated to differing floor elevations that have been connected in series (one segment after another in a sequential chain) by flow pipes is referred to as a serial system. Similar to the serial system is the series system where each chamber segment is connected in line but is set at the same elevation. A parallel trench system is similar to the series configuration but various chamber segments are connected to a single diffuser box, which distributes incoming effluent to individual pipes flowing parallel to each other toward the chamber segments.
Each of these configurations is placed in excavated trenches dug parallel to each other. The last configuration utilizes area excavation to create a broad flat floor for the installation of the chambers and is referred to as the bed installation. The bed installation utilizes a parallel pipe distribution network, but the parallel chamber segments are installed adjacent to each other on the same excavated level floor. Instead of backfilling with native soil, the excavation can be backfilled with coarse sand or fine gravel. No low-permeability materials such as clay or silt are used to backfill the bed.
In general, an RSF greatly improves the rate of biochemical oxygen demand (BOD) removal with removal rates as high as 95%. Significant removal rates are also achieved for total suspended solids (TSS) and nitrogen. The modular design of the system allows for easy expansion. No chemical additives are required and the entire system can be installed in a area one-fifth the size needed for a simple soil absorption system. However, costs tend to be higher and the system requires mechanical pumps and electronic controls.
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Photo: ADS Hancor Inc. |
| Each segment of biodiffusers is assembled from multiple parts. |
The Project Design Implementation
Until the development of decentralized sewage treatment (DST) systems, most homeowners relied on either sewage treatment plants (STPs) or traditional septic systems to manage household effluent. STPs were inflexible in responding to continued development or excessive rainfalls that result in flows that exceed the system’s capacity. Traditional septic fields often lacked capacity and were prone to failure due to sludge buildup or clogging.
Both of these potential drawbacks were avoided in the design of the DST for the Lewis Downs subdivision in Rutherford County, TN. The design engineer, Jeremy Reed of SEC Inc., utilized this approach to provide an onsite tertiary treatment process for the development. The site is located 4 miles from the nearest gravity flow sanitary sewer main. Utilizing the DST design approach resulted in increased yield, permanent green space, and a highly dependable system.
The system includes a watertight septic tank at each residence to remove settleable solids and sludge from the effluent streams. Each tank is serviced by a septic tank effluent pump (STEP). Float sensors control these pumps. The STEPs pump the effluent from the tanks to a common force main servicing the subdivision.
The force main transmits the effluent to an RSF. Most of the effluent’s treatment is performed by the RSF. The RSF’s recirculating ratio is five times and utilizes HDPE biodiffuser chambers at the bottom of the filter to provide the required storage volume. After recirculation, the effluent passes through a disc filter and ultraviolet light for final disinfection prior to being pumped to a subsurface drip irrigation system. This drip system covers approximately 13.4 acres. The final stage of treatment is provided by the native soil. The last stage of treatment is minimal since the effluent leaves the sand filter as polished water. The entire treatment process from the RSF to the drip system is controlled by a “smart” panel, which allows for remote monitoring of error messages or high flow readings.
The design flow was 78,000 gallons per day (gpd). The BOD5 effluent averages 4.0 milligrams per liter with an efficiency of 98% (which compares favorably with the nearby city of Murfreesboro’s STP, which operates at BOD5 of 4.3 milligrams per liter with an efficiency of 98%). The nitrate nitrogen effluent averages 0.21 milligram per liter with an efficiency of 99.6%. The TSS are below detectable levels. The fecal coliform count averages 2.3 per 100 milligrams. Comparing the effluent, it is easily seen that the DST used is as effective, if not more, than the traditional tertiary STP. A standard septic field would have been able to manage effluent from 180 lots, while this design can handle 242 lots.
Biodiffusers and Clustered/Decentralized Wastewater Treatment Systems
In addition to its greater waste removal and effluent treatment efficiencies, the RSF greatly reduces the need for maintenance compared to standard soil adsorptive systems. Though requiring greater capital costs up front, the RSF mitigates management and maintenance problems inherent in regular septic systems while greatly reducing the chances of system failure, thereby reducing long-term costs.
Overall costs are reduced and long-term planning is simplified when using these biodiffusers and sand recirculation systems as “cluster” units for large-scale applications. Preventative, incremental planning is made possible by the ability to assess each service expansion, in effect allowing the treatment system to grow with its community instead of constructing a single large treatment facility. Homeowners and businesses that already use a septic system can continue to do so without abandoning it (and throwing away the money already invested) when a centralized sewer system comes online. Cluster systems added in increments as a community grows have less impact on the surrounding environment and watersheds by reducing the amount of water transferred compared to centralized systems. Treatment methods can be tailored to individual needs. Septic systems, especially those augmented with recirculating sand filters, remain the most cost-effective systems for low-population-density rural areas. Using clusters of these systems allows for cost-effective, targeted growth as the community increases in population.
Performance Advantages and Cost Savings
What does such a system cost? The initial capital costs vary somewhat from location to location. The need for pretreatment will also affect the startup costs (though this is more applicable to businesses and industries than to residences). The recirculation tank and its pumping system can cost between $10,000 and $20,000 depending on the size, required pumping head, and flow rate, etc. A sand filter with its sand and associated piping can cost between $10,000 and $40,000. Additional costs such as engineering and contingencies can vary between $2,000 and $10,000 and $4,000 and $8,000, respectively. The cost of land is often minimal, since most of the facilities and neighborhoods utilizing these systems have collocated them on their own properties. Total capital costs can therefore vary with size and location from $26,000 to $78,000. Such relatively small capital costs (compared to the overall budget of a small town) reduce the need for long-term financing and interest charges. Such investments in infrastructure could even appear as line items in a town’s annual budget.
Annual operating and maintenance cost are usually harder to quantify. Labor can run as high as $20 per hour but the number of hours required for operations and maintenance will vary, so this could be a cost for part-time labor only (note that unlike large centralized systems, these cluster units will not require significant management organization or significant staffing to operate). Electrical power to run the recirculating pumps will vary also, but they too can be replaced with solar panels as an independent energy source. Removal and disposal of sludge accumulated in the septic tank will vary with flow rates and effluent characteristics, but $0.10 a gallon is a typical cost with most medium-size systems generating enough sludge to cost $50 to $100 per year.
As mentioned previously, these systems provide distinct advantages to growing municipalities. In addition to the financial advantages, the use of cluster systems incrementally added with each new neighborhood or business allows for better and more accurate planning of a community’s utility needs. There is no need to spend money on an oversized central wastewater system that is built in anticipation of future growth. Growth and construction can go hand in hand. Engineers will appreciate the more accurate sizing of these systems based on known effluent quantities. The simplicity of design will also be an attractive feature. Contractors will be able to quickly install minimal-sized systems quickly and efficiently without the need to manage multiple subcontractors, navigate complicated contractual obligations, or jump through multiple regulatory hoops. Use of these systems opens the way for small businesses to compete in their community’s utility market.
Daniel P. Duffy, P.E., is an environmental engineer employed by URS Corporation in Akron, OH.
OW - July/August 2007 |
